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November 2009

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i. Abstract ii. Content Part I: Proprioception

1. Introduction to proprioception 2. Proprioceptors

3. Sensory integration at the spinal cord level 4. Ascending spinal tracts conveying proprioceptive information

5. Conscious proprioception

Part II: Control of posture

6. Introduction to control of posture and movement

7. Control of posture and movement at the spinal cord level

8. Control of posture and movement at the brain stem level

9. Control of posture and movement at the cerebral cortex level

10. Control of posture and movement at the associate areas level

11. Summary

Proprioception (the sense of position of body parts) is an important sensation without which we would not be able to locate our body parts. Posture may be defined as the position adopted by the individual within his or her environment. We can think of proprioception as the sensory segment of information processing and posture control as the motor segment. Naturally, control of posture is dependent on proprioception. Abnormalities in the process of proprioception or posture control would lead to inability to react to changes in the external environment crucial to our survival. In the first part of this Module will discuss in detail the process of proprioception starting from the receptors involved, the pathways of transmission of sensory information, and the processing (integration) of information to produce proprioception. In the second part of the Module, we will discuss the process of posture control both voluntarily and involuntarily.

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iii. Checklist of topics and activities in this module Content page Comments on

mastery i Abstract ii Content iii Checklist of topics and activities in this

module

iv Learning resources v Background knowledge vi Terms to know vii Objectives viii Learning activities

1 Introduction to proprioception Activity 1: Significance of proprioception and posture

control

2 Proprioceptors Activity 2. Levels of motor control Activity 3. Joint receptors Activity 4: Proprioceptors

3 Sensory integration at the spinal cord level Activity 5: Sensory ntegration at spinal cord

4 Ascending spinal tracts conveying proprioceptive information

Activity 6: Ascending spinal tracts conveying proprioceptive information

5 Conscious proprioception Activity 7. Conscious proprioception

6 Introduction to control of posture and movement

6.1. Significance of posture control Activity 8: Postural reflex Activity 9: Connections to the motor neuron 6.2. Organization of motor output Activity 10: Sequence of events that lead to movement Activity 11: Postural Reflexes 6.3. Descending tracts Activity 12: Descending tracts 6.4. Control of axial and distal muscles Activity 13: Control of axial and distal muscles

7 Control of posture and movement at the spinal cord level

Activity 14: Control of posture and movement at the spinal cord level

8 Control of posture and movement at the brain stem level

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Activity 15: Control of posture and movement at the brain stem level

9 Control of posture and movement at the cerebral cortex level

Activity 16. Cortical motor areas Activity 17. Motor homunculus

10 Control of posture and movement at the associate areas level

ix Summary x Conclusion iv. Learning resources

• Boron and Boulpaep, • Guyton & Hall, Ch • Ganong, • Marieb, Ch. • Tortora & Grabowski, • Vander, Sherman & Luciano,. • Supplementary materials provided

Web Animation:

v. Background knowledge To complete this module successfully, you should have the following background: • The anatomy and histology of the nervous system (relevant General Anatomy

modules). • The whole Introduction to Medical Physiology modules vi. Terms to know Please add other terms that you feel are relevant to your understanding of this module.

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vii. Objectives

Objectives from the American Physiological Society

Medial and Lateral System Control of Movement NEU 87. Draw a “box” diagram of motor control systems, including cerebral cortex, basal ganglia, cerebellum, thalamus, brainstem motor nuclei, and spinal cord. Indicate with arrows the flow of information among these structures and, ultimately, to the alpha and gamma motor neurons.

NEU 88. Draw a cross section of the spinal cord and discuss the organization of the sensory and motor components of gray matter. Describe the somatotopic arrangement of motor neuron pools.

NEU 89. List the medial and lateral motor systems. Describe their origin, pathway, and termination within the spinal cord. Compare their functions in motor control.

NEU 90. Describe the effects of lesions in medial and lateral systems.

Cerebellum and Basal Ganglia NEU 91. Describe the roles of the cerebellum in the regulation of skilled movement.

NEU 92. List three functional divisions of the cerebellum, detailing the input and output connections of each. Be able to differentiate the functions of each and their integration with lateral and medial motor systems.

NEU 93. Draw and label the circuitry of the cerebellar cortex, assign the functional role of each neuron type and give its synaptic action (excitatory/inhibitory). Be able to describe how this circuit functions as a timing mechanism and how it produces synergy in opposing muscle groups.

After studying the materials in this module, the students should be able to:

1. Define proprioception and discuss the significance of conscious and non-conscious proprioception.

2. Describe the functions of the proprioceptors. 3. Draw and describe the process of integration of sensory information at the spinal cord

level. 4. Draw and describe the ascending spinal tracts conveying proprioceptive information. 5. Compare and contrast the processes that lead to conscious and non-conscious

proprioception. 6. Define posture and describe the significance of posture control for survival. 7. Describe the organization of the motor output and the descending tracts involved. 8. Describe the control of posture and movement at the

a. spinal cord level b. brain stem level c. cerebral cortex level d. associate areas level

Please set up more specific objectives after you have thoroughly studied the material in this module to help yourself in your revision later on. Write notes that meet the requirement of the new objectives.

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NEU 94. On the basis of input-output organization, somatotopic organization, and overall function, predict the neurological disturbances that can result from disease or damage in different regions of the cerebellum.

NEU 95. Contrast the spinal proprioceptive pathways to the cerebellum with those to the cortex.

NEU 96. List and describe the major interconnections between components of the basal ganglia and the motor cortex. Identify the neurotransmitters determining the flow of information in the system.

NEU 97. Describe the overall function of the basal ganglia in movement control and initiation in association with medial and lateral motor systems.

NEU 98. List the appropriate signs of rigidity, dyskinesias, akinesia, and tremor for Parkinsonism, chorea, hemiballism, and athetosis. Assign a likely lesion site or chemical system defect for each clinical syndrome.

NEU 99. Describe the rationale for treatment of Parkinsonism with anticholinergic drugs, L-DOPA, or transplantation of catecholamine-producing cells.

Cerebral Cortex NEU 100. Describe the medial to lateral, rostral to caudal, and surface to white matter organizations of the primary motor cortex and the premotor cortex. Draw those regions on a sketch of the brain and also locate the supplementary motor cortex.

NEU 101. Compare the effects of electrical stimulation of motor cortex and premotor cortex, relating the expected results to the control of voluntary movement.

NEU 102. Describe the origin, course, and termination of the pyramidal tract.

NEU 103. Compare the consequences of upper motor neuron loss to lower motor neuron loss. Describe the consequences of pyramidal tract transection.

NEU 104. Draw a “flow diagram” for the brain regions involved in planning, initiating, and properly executing a skilled voluntary movement.

NEU 105. Identify Brodmann areas for visual, auditory, somatic sensory, motor, and speech areas.

NEU 106. Identify the cortical areas that receive projections from the following thalamic nuclei: ventral lateral, dorsomedial, pulvinar, medial geniculate, lateral geniculate, ventral posterolateral, and posteromedial.

NEU 107. Describe the cortical areas important for language.

NEU 108. Describe the cortical area important for spatial relations.

NEU 109. Describe the functions of the prefrontal association cortex.

NEU 110. Define and explain the physiological basis of evoked potentials and the electroencephalogram (EEG). List the main clinical uses of each.

NEU 111. Describe the primary types of rhythms that make up the EEG and the behavioral states that correlate with each.

NEU 112. Describe the origin of spontaneous electrical activity of the cerebral cortex.

NEU 113. Distinguish EEG activity from evoked potentials and the uses of evoked potentials.

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viii. Learning Activities Part I: Proprioception 1. Introduction to proprioception Definition: Proprioception= sense of position of body parts Kinesthesia = sense of movement of body parts

In Module 18 we discussed the flow of information from receptor to control centre to effector in accomplishing various reflexes. In such cases, the stimuli could come from the internal or external environment (please give examples), but the control centre does not involve the conscious part of the brain (where are the control centres for reflexes?). We have also previously agreed that external environmental stimuli are perceived consciously (sight, smell, hearing, taste, touch) and internal environmental stimuli (blood pressure, PO2, etc) are not.

In contrast, the sense of position of our body parts (proprioception) could be consciously perceived or nonconsciously sensed. It is consciously perceived when we put our thoughts into it but normally we are not conscious about the positions of our body parts. For example, we know where our limbs are even with our eyes are closed and we can control their movement consciously when we think about them, but they normally assume positions (for appropriate postures or even for the act of walking) without much conscious control at all. Regardless of whether we are conscious of the positions and control of our body parts or not, afferent information about them is being sent to the control centre all the time (mostly to the subcortical areas), and efferent information is continually sent to the effectors to adjust our posture appropriately. This is how we assume our minute-to-minute positions of our body.

How is information that brings about proprioception generated? How is the information processed? What is the significance of the sensory information? How do we go about controlling our minute-to-minute posture? What is the nature of the efferent information? We’ll try to address these issues in this module.

Proprioception provides feedback solely on the status of the body internally. It is the sense that indicates whether the body is moving with required effort, as well as where the various parts of the body are located in relation to each other. Thus, proprioception correctly describes afferent information arising from internal peripheral areas of the body that contribute to postural control, joint stability, and several conscious sensations. Proprioceptive information concerning the status of the joint and associated structures is essential for neuromuscular control. Proprioception is conveyed to all levels of the central nervous system, where it provides a unique sensory component to optimize motor control. In humans, a distinction is made between conscious proprioception and nonconscious proprioception:

• Conscious proprioception is communicated by the posterior column-medial lemniscus pathway to the cerebrum. Give examples.

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• Nonconscious proprioception is communicated primarily via the dorsal spinocerebellar tract, to the cerebellum. Give examples.

This module will be basically divided into 2 main parts: 1. Proprioception, and 2. Posture control. We’ll focus on the afferent segment of proprioception and the efferent segment of posture control. We’ll start off with how afferent inputs are generated by the proprioceptors for perception to take place.

Activity 1: Significance of proprioception and posture control • What is the significance of the phenomena that external environmental stimuli are perceived

consciously (sight, smell, hearing, taste, touch) and internal environmental stimuli (blood pressure, PO2, etc) are not?

• Compare and contrast between proprioception and kinesthesia. Give examples. • What are proprioceptors? Give examples. What would stimulate proprioceptors? What is the

significance of proprioception? • What happens to the afferent information coming from proprioceptors? What is the significance

that most of the afferent information coming from proprioceptors are not consciously perceived? • What happens to the efferent information resulting from integration of afferent information coming

from proprioceptors? • What are the effectors for controlling body posture and movement? Why is body posture

important? Explain how it is controlled consciously and nonconsciously. • Draw a model that shows the relationship between proprioceptors, control centres and effectors.

Identify the control centres and effectors. Identify the ascending and descending pathways in the model.

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2. Proprioceptors

The awareness of the orientation of the body in space (proprioception) and the direction, extent, and rate of movement of the limbs (kinesthesia) depend in part upon information derived from sensory receptors in the joints (joint receptors, Fig. 2), tendons (Golgi tendon organ, Fig 4), and muscles (muscle spindle, Fig 3). Information from these proprioceptors is normally integrated with that arising from vestibular receptors (which signal gravitational acceleration and changes in velocity of movements of the head), as well as from visual, auditory, and tactile receptors. Sensory information from certain proprioceptors, particularly those in muscles and tendons, need not reach consciousness, but can be used by the motor system as feedback to guide postural adjustments and control of well-practiced or semiautomatic movements such as those involved in walking.

Although visual and vestibular input contributes, the peripheral mechanoreceptors are the most important from a clinical orthopaedic perspective (Fig. 1). Afferent pathways (dotted lines) convey input to the 3 levels of motor control areas (spinal cord, brain stem and cerebral cortex) and associated areas such as the cerebellum. Activation of motor neurons may occur in direct response to peripheral sensory input (reflexes) or from descending motor commands, both of which may be modulated or regulated by the associate areas (gray lines). Efferent pathways from each of the motor control levels (solid lines) converge upon the alpha and gamma motor neurons located in the ventral aspects of the spinal cord. The contractions by the extrafusal and intrafusal muscle fibers cause new stimuli to be presented to the peripheral mechanoreceptors. In summary, information for proprioception and kinesthesia is derived from proprioceptors, and is normally integrated with that arising from

a. vestibular receptors (which signal gravitational acceleration and changes in velocity of movements of the head),

b. visual, auditory, and tactile receptors

Activity 2. Levels of motor control • Where do afferent

information for proprioception come from? Name the proprioceptors. What is the information from these proprioceptors integrated with?

• What are the three levels

of motor control? • How are motor neurons

activated? • Discuss the function of α

and γ motor neurons. Fig. 1. Levels of motor control

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Joint Receptors. Joint receptors are found within the connective tissue, capsule and ligaments of joints (Fig. 2). The encapsulated endings resemble the Ruffini and Pacinian corpuscles and the Golgi tendon organs.

The joint 1° afferents respond to changes in the angle, direction, and velocity of movement in a joint. The responses are predominantly rapidly adapting with few joint 1° afferents signaling the resting (static) position of the joint. It has been suggested that information from muscles, tendons, skin and joints are combined to provide estimates of joint position and movement. For example, when the hip joint is replaced — removing all joint receptors — the ability to detect the position of the thigh relative to the pelvis is not lost.

Free Nerve Endi ngs . As mentioned above, free nerve endings of 1° afferents are abundant in muscles, tendons, joints, and ligaments. These free nerve endings are considered to be the somatosensory receptors for pain resulting from muscle, tendon, joint, or ligament damage and are not considered to be part of the proprioceptive system.

Although 4 types of receptors are dispersed throughout ligamentous and capsular tissues, Ruffini receptors are the most frequently described. They are considered to behave as both static and dynamic receptors based on their low-threshold, slow-adapting characteristics. In contrast, the low-threshold, rapidly adapting characteristics of Pacinian corpuscles cause them to be exclusively classified as dynamic receptors. Also present in these tissues are Golgi tendon organ-like endings and free nerve endings. Golgi tendon organs (GTOs, Fig. 4) are mechanoreceptors located within musculotendinous tissue Through each GTO passes a small bundle of muscle tendon fibers destined to attach to muscle fibers. This series arrangement, coupled with the very low threshold and high dynamic sensitivity exhibited by the sensory endings, enables GTOs to provide the CNS with feedback concerning muscle tension. GTOs function primarily in signaling active muscle tension (tension developed during contraction) rather than passive tension (tension developed during inactive muscle stretching).

Fig. 2. There are four main types of joint receptors.

Activity 3. Joint receptors • Describe the four types of joint receptors.

(Hint: Type I receptors resemble Ruffini endings in the skin. Type II receptors take the form of flattened Pacinian corpuscles. Type III receptors resemble the Golgi tendon organs and are tension-sensing receptors. Type IV receptors are unmyelinated nerve endings that resemble pain fiber terminals).

• Describe the flow of afferent information

into the spinal cord and the ascending tracts that are involved with the transmission of the afferent signals from the receptors.

• How do different parts of the brain react to

the information transmitted by the receptors?

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Muscle spindles (Fig. 3). As a whole, muscle spindles are responsible for conveying information regarding muscle length and rate of changes in length. Muscle spindles consist of specialized afferent nerve endings that are wrapped around modified muscle fibers (intrafusal fibers), several of which are enclosed in a connective tissue capsule. There are different types of intrafusal fibers: some are mainly sensitive to changes in muscle length, whereas others are more sensitive to the rate of change in muscle length. Although the central areas of the intrafusal muscle fibers lack contractile elements, the peripheral areas contain contractile elements, which are innervated independent of extrafusal (skeletal) muscle fibers via the gamma motor neurons (γ MNs). Activation of the peripheral contractile elements stretches the central regions containing the sensory receptors from both ends. This results in an increase in the firing rates of the sensory ending and an increase in the sensitivity of the muscle spindle to length changes. At the spinal level, various peripheral receptors, such as skin receptors, articular receptors, and chemoreceptors, strongly influence the activity of the γ-MN system and, therefore, the muscle spindle in providing afferent information.

Table 1. Proprioceptors in muscle, tendon and joints

Receptor T ype Sensat ion S ignals Adaptat ion Musc le Spind le

Encapsulated annulospiral and flower spray endings

Muscle stretch Muscle length & velocity

Rapid initial transient and slow sustained

Musc le: Golgi Tendon Organ

Encapsulated collagen Muscle tension Muscle contraction

Slow

Jo in t : Pac ini an

Encapsulated & layered Joint Movement

Direction & velocity

Rapid

Jo in t : Ruf f i ni

Encapsulated collagen Joint pressure Pressure & Angle

Slow

Jo in t : Golgi Organ

Encapsulated collagen Joint torque Twisting force Slow

Fig. 3. Muscle spindles. Fig. 4. Golgi tendon organ.

Fig. 4. Golgi tendon organ

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Activity 4: Proprioceptors • Draw diagrams to depict proprioceptors in the muscle, tendon and joints. Describe how these

receptors are activated. • Describe how afferent information is carried to the CNS. Discuss how it affects motor control at

the level of the spinal cord, brain stem and cerebral cortex. • Explain how information from other receptors (special senses, tactile senses) affects afferent

and efferent information with regard to postural control.

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3. Sensory integration at the spinal cord level Many of the axons conveying proprioceptive information bifurcate once they enter the dorsal horn of the spinal cord to synapse with interneurons. The essence of afferent integration at the spinal cord level lies with the interneurons and the neurons connecting with higher CNS levels. Control over these neurons via descending commands from the brain stem and cortex provides these centers with the ability to filter the sensory input that will be conveyed via the ascending tracts. In other words, the supraspinal CNS regions modulate the sensory information from the periphery that enters the ascending tracts. Integration at the spinal cord level is also influenced by the final common input to the α-motor neuron (Fig. 3). This hypothesis resides on the strong influence that the muscle, skin, and joint afferents and descending pathways have over gamma neuron activation. As mentioned previously, the peripheral regions of intrafusal muscle fibers contain contractile elements innervated by γ MNs, with the level of activation directly controlling muscle spindle sensitivity. Any of the signals barraging the γ-MN pools alter their level of activation, and, therefore, influence the input arising from the muscle spindles. Thus, the afferent signals from muscle spindles are hypothesized to be a function of muscle length changes superimposed on the integrated peripheral receptor and descending pathway information. In this manner, the γ-MN system may be considered a “premotor neuronal integrative system” that conducts “polymodal feedback” to the CNS (Fig. 3).

Activity 5: Sensory ntegration at spinal cord • Explain the essence of

integration at the level of the spinal cord. How are sensory inputs that are to be conveyed via the ascending tract filtered?

• Explain the role of γ motor

neurons on the flow of proprioceptive information towards the higher centres and the efferent information to the α-motor neurons.

Fig. 5. Integration f information in the spinal cord.

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4. Ascending spinal tracts conveying proprioceptive information Most proprioceptive information travels to higher CNS levels through either the dorsal lateral tracts or the spinocerebellar tracts. The 2 dorsal lateral tracts are located in the posterior region of the spinal cord and ultimately convey the signals to the somatosensory cortex. Although the majority of the sensations traveling in this tract are touch, pressure, and vibration, various amounts of the conscious appreciation of position and kinesthetic sensations have also been attributed to this tract. The spinocerebellar tracts are characterized by the fastest transmission velocities in the CNS. As their name suggests, the spinocerebellar tracts terminate in various areas of the cerebellum, where the signals may be processed and integrated with other afferent and descending information. In contrast to the conscious sensory appreciation associated with the dorsal lateral tracts, the spinocerebellar tracts are believed to be responsible for “nonconscious proprioception” (i.e. limb position, joint angles, and muscle tension and length) used for reflexive, automatic, and voluntary activities. In addition to relaying peripheral afferent information, parts of these tracts are associated with transmitting an efferent copy of motor neuron drive back to the higher CNS levels.

Activity 6: Ascending spinal tracts conveying proprioceptive information • Draw a cross section of the spinal cord

showing the incoming fibres from various proprioceptors (muscle spindle, Golgi tendon organ and joint receptors). Locate the synapse between the first and second order neurons. Locate the position of the tracts.

• Compare and contrast between the dorsal

lateral tracts and spinocerebellar tracts. What is the significance of conscious and nonconscious proprioception?

Fig. 6. Ascending tracts.

Fig. 7. Ascending tracts.

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5. Conscious proprioception As mentioned previously, proprioception information for conscious appreciation travels via the dorsal lateral tracts, but the contributions to these tracts from muscle and joint mechanoreceptors remaining largely unknown. Thus, projections to the cortical sensory areas and conscious perceptions after direct receptor stimulation is the second necessity in determining the predominant source of conscious proprioception (Figure 4). Cortical projections have been reported from joint (both capsular and ligament) afferents, muscle spindles, and GTOs. It has been demonstrated that mechanical stimulation of cutaneous receptors elicited kinesthetic sensations. While direct stimulation of a single muscle spindle afferent failed to elicit movement perception, stimulation of several muscle spindles through vibration and isolated traction has been reported to evoke conscious movement sensations. The failure of joint and cutaneous afferents anesthesia to disrupt conscious kinesthesia and JPS provides further support for the importance of muscle receptors in conscious proprioception. In summary, proprioception is the awareness of the orientation of the body in space. The afferent information from proprioceptors is integrated with information from vestibular receptors (which signal gravitational acceleration and changes in velocity of movements of the head), as well as from visual, auditory, and tactile receptors). This could be consciously perceived or non-consciously processed to produce appropriate motor functions i.e. posture control. We’ll now proceed to discussion on control of posture and movement.

Fig. 8. The role of the articular mechanoreceptors in sensorimotor control over dynamic joint stability and conscious appreciation of proprioception. Dotted lines represent roles that are still controversial.

Activity 7. Conscious proprioception • List the receptors involved in

conscious proprioception. • Describe the transmission of

afferent signals from receptors to specific parts of cerebral cortex where conscious prpprioception takes place.

• Compare and contrast

between conscious perception and non-conscious perception.

• How is conscious perception

related to dynamic joint stability?

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Part II: Control of Posture 6. Introduction to control of posture and movement 6.1. Significance of posture control

So far we have discussed the afferent segment involved in proprioception. The significance of proprioception is so that we are able to adjust body posture and body movement appropriately, either voluntarily or involuntarily or both. We’ll now look at the efferent segment i.e. how our movement and body positions are adjusted to suit the activities that we do.

Posture may be defined as the position adopted by the individual within his or her environment. Muscles that help us manage our posture are mostly axial and antigravity muscles. These muscles, like most skeletal muscles are always in a partial state of contraction, so they exhibit some degree of tone. We are able to stand and sit comfortably for hours without feeling fatigue, because of these muscles, in particular their tone. The tone of these muscle and the maintenance of an upright posture, in turn involves postural reflexes which include the stretch reflex and the crossed-extensor reflex which have already been discussed (Module 18).

Along with the maintenance of upright posture is the maintenance of balance which is a complex process in the human because of our tall height, which must be balanced over the feet which have a small area. Adding to the difficulty of maintaining balance and posture is our high center of gravity, centered over the hips.

The postural reflexes are aided by afferent sensory information from the eyes, the vestibular apparatus, and the somatic receptors (proprioceptors) and the efferent response is to the skeletal muscles after integration in the brainstem and spinal cord. Control of posture involves active

Activity 8: Postural reflex • What is posture? What is postural reflex? Hint: relate the relevant receptor to control centre

to effector. • Compare and contrast between conscious and non-conscious control of posture. • Compare and contrast between axial muscles and distal muscles. Which motor pathways is

their contraction mediated by? Give examples. • What is the meaning of muscle tone? Why is it important? • Explain how stretch reflex and cross extensor reflex are involved in posture regulation. • What are the organs that have receptors that provide sensory inputs for postural reflexes?

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muscular resistance to displacement of the body. The crossed-extensor reflex is one example of a postural reflex. As one leg is flexed, the other is extended to support the added weight of the body. In addition, the positions of various parts of the body are shifted to move the center of gravity over the single, weight-bearing leg. This shift in the center of gravity is important in the stepping mechanism of walking.

The motor components of the sensorimotor system contributing to posture consist of a central axis and 2 associate areas. The central axis corresponds to the 3 levels of motor control, spinal cord, brain stem, and cerebral cortex, whereas the 2 associate areas, cerebellum and basal ganglia, are responsible for modulating and regulating the motor commands. Sensory information underlies the planning of all motor output and is conveyed to all 3 levels of motor control.

Activation of motor neurons may occur in direct response to peripheral sensory input (reflexes) or from descending commands initiated in the brain stem or cerebral cortex, or both. Independent of the initiating source, skeletal muscle activation occurs through signal convergence onto the motor neurons located in the spinal ventral horns i.e. the final common path (Module 13). Both types of motor neurons, alpha motor neurons (α MNs) controlling extrafusal muscle fibers (skeletal) and γ MNs controlling intrafusal muscle fibers (muscle spindles), exit the spinal ventral horns. The central axis areas are organized in both a hierarchic and parallel manner (Module 17). The hierarchic organization allows the lower motor areas to automatically control the details of common motor activities, while the higher centers can devote resources to controlling the more precise and dexterous motor activities. In addition, as mentioned earlier, higher levels can regulate the afferent information reaching them through inhibitory and facilitatory control over sensory relay nuclei. Through the parallel arrangement, each motor control center can directly issue independent contributory descending motor commands directly on the motor neurons.

The final common paths to skeletal muscle (the spinal motor neurons and homologous neurons in the motor nuclei of the cranial nerves), are bombarded by impulses from an immense array of pathways:

a. Each spinal motor neuron has many inputs from the same spinal segment (e.g: reflexes).

Fig. 9. Planning and execution of movement.

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b. Numerous supra-segmental inputs also converge on these cells from other spinal segments, the brainstem, and the cerebral cortex.

Some of these inputs end directly on the motor neurons, but many exert their effects via interneurons or via the -efferent system to the muscle spindles and back through the Ia afferent fibers to the spinal cord.

It is the integrated activity of these multiple inputs from spinal, medullary, midbrain, and cortical levels that regulates the posture of the body and makes coordinated movement possible. The inputs converging on the motor neurons subserve three semi-distinct functions:

• they bring about voluntary activity; • they adjust body posture to provide a stable background for movement; and • they coordinate the action of the various muscles to make movements smooth and precise.

In summary, the control of posture and movement involve the following activities:

• The patterns of voluntary activity are planned within the brain. • The commands are sent to the muscles primarily via the corticospinal and corticobulbar

systems. • Posture is continually adjusted not only before but also during movement by posture-

regulating systems. • Movement is smoothed and coordinated by the medial and intermediate portions of the

cerebellum (spinocerebellum) and its connections. • The basal ganglia and the lateral portions of the cerebellum (neocerebellum) are part of a

feedback circuit to the premotor and motor cortex that is concerned with planning and organizing voluntary movement.

6.2. Organization of motor output Motor output is of two types: reflexive or involuntary, and voluntary. Some would add as a subdivision of reflex responses rhythmic responses such as swallowing, chewing, scratching, and walking, which are largely involuntary but subject to voluntary adjustment and control. To move a limb, for example, the brain must

• plan a movement, • arrange appropriate motion at many different joints at the same time, and • adjust the motion by comparing plan with performance.

Activity 9: Connections to the motor neuron

• Draw an appropriate diagram to show the neuronal connections to the motor neuron.

• How are involuntary motor activity executed?

• How are patterns of voluntary activity planned, and how are the signals conveyed to the motor neurons to be executed by the skeletal muscles?

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The motor system "learns by doing", and performance improves with repetition. This involves synaptic plasticity (the ability of the synapse between two neurons to change in strength). Commands for voluntary movement originate in cortical association areas (Fig. 9). The movements are planned in this cortical area as well as in the basal ganglia and the lateral portions of the cerebellar hemispheres. The basal ganglia and cerebellum both funnel information to the premotor and motor cortex by way of the thalamus. Motor commands from the motor cortex are relayed in large part via the corticospinal tracts to the spinal cord and the corresponding corticobulbar tracts to motor neurons in the brainstem. However, collaterals from these pathways and a few direct connections from the motor cortex end on brainstem nuclei, which also project to motor neurons in the brainstem and spinal cord. These pathways can also mediate voluntary movement. Movement sets up alterations in sensory input from the special senses and from muscles, tendons, joints, and the skin. This feedback information, which adjusts and smoothes movement, is relayed directly to the motor cortex and to the spinocerebellum. The spinocerebellum projects in turn to the brainstem. The main brainstem pathways that are concerned with posture and coordination are the rubrospinal, reticulospinal, tectospinal, and vestibulospinal tracts and corresponding projections to motor neurons in the brainstem.

A. Rubrospinal Tract: Originates in the red nucleus. Fibers project to interneurons in the spinal cord which excite motoneurons of flexor muscles and inhibit motoneurons of extensor muscles to release the body from a postural stance.

B. Pontine (Medial) Reticulospinal Tract: Originates in the reticulospinal tracts of the pons and has a greater excitatory effect on extensor than on flexor motoneurons, thus allowing for an erect posture.

C. Medullary (Lateral) Reticulospinal Tract: This pathway originates in the medullary reticular formation and terminates on interneurons in the spinal cord. This tract has the opposite effect of the pontine reticulospinal tract in that it strongly inhibits extensors.

D. Lateral Vestibulospinal Tract: This tract originates in the lateral vestibular nucleus (Dieter's nucleus) and projects to motoneurons and interneurons. Stimulation of cells in the nucleus produces a powerful excitation of extensors and inhibition of flexors. This tract plays an important role in the control of antigravity muscles and the maintenance of posture.

Activity 10: Sequence of events that lead to movement With the aid of a diagram, discuss the sequence of events that take place for movement to occur i.e. starting from sensory input.

Activity 11: Postural Reflexes

• What is the meaning of postural reflex?

• What are the common stimuli involved in postural reflex?

• What is the nature of the afferent information involved in postural reflex?

• Where are the integration centres involved in postural reflexes?

• How are these stimuli integrated to form efferent information that goes down the descending pathways to the effector?

• How does the effector respond to the efferent information?

• Can postural reflexes be influenced by cortical inputs? Give examples?

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6.3. Descending tracts The nerve fibers that pass from the motor cortex to the cranial nerve nuclei form the corticobulbar tract. The nerve fibers that cross the midline in the medullary pyramids and form the lateral corticospinal tract make up about 80% of the fibers in the corticospinal pathway. The remaining 20% make up the anterior, or ventral, corticospinal tract (Fig. 10), which does not cross the midline until it reaches the level of the muscles it controls. At this point, its fibers end on interneurons that make contact with motor nerves on both sides of the body. The lateral corticospinal tract is concerned with skilled movements, and in humans its fibers end directly on the motor neurons.

Activity 12: Descending tracts • Compare and contrast between the

corticobulbar and corticospinal tracts. • Compare and contrast between lateral

corticospinal tract and ventral corticospinal tract.

• Mark on the grey matter of the precentral

gyrus the specific locations involved in the control of movement of specific parts of the body. Which locations have neurons that form the corticobulbar tract? Which locations have neurons that form the corticospinal tract?

• Draw on the diagram the corticobulbar tracts. • Draw a cross section of the spinal cord and

locate the white matter areas that house the lateral corticospinal tract and those that house the ventral corticospinal tract. Describe the synapse that the terminals of these neurons make. Which muscles do they finally affect?

• What are the functions of the interneurons

that synapse with the spinal nerve in the spinal cord?

Fig. 10. The corticospinal tracts.

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6.4. Control of axial and distal muscles Another important theme in motor control is that in the brainstem and spinal cord,

• medial or ventral pathways and neurons are concerned with the control of muscles of the trunk and proximal portions of the limbs

• lateral pathways are concerned with the control of muscles in the distal portions of the limbs.

The axial muscles are concerned with postural adjustments and gross movements, whereas the distal limb muscles are those that mediate fine, skilled movements. Thus, for example, the neurons in the medial portion of the ventral horn innervate the proximal limb muscles, particularly the flexors, whereas the lateral ventral horn neurons innervate the distal limb muscles. Similarly, the ventral corticospinal tract and the medial descending paths from the brainstem (the tectospinal, reticulospinal, and vestibulospinal tracts) are concerned with adjustments of proximal muscles and posture, whereas the lateral corticospinal tract and the rubrospinal tract are concerned with distal limb muscles and, particularly in the case of the lateral corticospinal tract, with skilled voluntary movements.

7. Control of posture and movement at the spinal cord level It should be apparent from our earlier discussion that the spinal cord plays an integral role in motor control, despite the gross anatomy suggesting it may only be a medley of conduction pathways. From the spinal cord arise direct motor responses to peripheral sensory information (reflexes) and elementary patterns of motor coordination (rhythmic and central pattern generators). As discussed earlier, very little afferent input and few descending commands synapse directly on motor neurons. Instead, most input terminates upon the interneurons located throughout all areas of cord gray matter. Even in the case of a simple monosynaptic reflex, such as the stretch reflex, birfurcations from the incoming afferent fiber arise. These bifurcations may convey the afferent information to a number of locations, including interneurons, higher motor centers, and other motor neurons (antagonistic). The bifurcations and interneuronal networks provide the basis for the spinal cord's efferent integrative functions. Reflexes may be elicited from the stimulation of cutaneous, muscle, and joint mechanoreceptors and may involve excitation of α-MNs, γ-MNs, or both. For many clinicians, the stretch reflex in response to rapid muscle lengthening provides the most familiar example. These reflexes, as well as the other reflexes attributed to the spinal cord neuronal circuitry, are more complex than simple direct input-output connections. Superimposed on even the simplest monosynaptic reflexes are influences from such sources as other afferent input, descending commands, or both.

Activity 13: Control of axial and distal muscles Draw a cross section of a spinal cord showing the positions of sensory and motor tracts. Identify the tracts that contain nerve fibres responsible for movements of axial muscles and distal limb muscles.

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8. Control of posture and movement at the brain stem level Despite being the most primitive part of the brain from a phylogenetic perspective, the brain stem contains major circuits that control postural equilibrium and many of the automatic and stereotyped movements of the body. In addition to being under direct cortical command and providing an indirect relay station from the cortex to the spinal cord, areas of the brain stem directly regulate and modulate motor activities based on the integration of sensory information from visual, vestibular, and somatosensory sources. Two main descending pathways, the medial and lateral pathways, extend from the brain stem to the spinal cord neural networks. The medial pathways influence the motor neurons innervating the axial and proximal muscles, while the lateral pathway controls the distal muscles of the extremities. In addition to controlling postural control, some axons comprising the medial pathways make excitatory and inhibitory (including suppression of spinal reflexes) synapses with the interneurons and motor neurons involved with movement and postural control. Through influences on the γ-MNs, parts of both the medial and lateral tracts assist in maintaining and modulating muscle tone.

Activity 14: Control of posture and movement at the spinal cord level • Draw a cross section of the spinal cord. Indicate the grey matter and the white matter. • Draw the incoming afferent fibre and outgoing lower motor neuron. • Draw the synapses on the lower motor neuron i.e. directly from the afferent fibre and the

descending fibes; indirectly via interneurons. Discuss the significance of the spinal interneurons. • Briefly describe central pattern generators (CPG). • Summarise how the spinal cord is involved in the control of posture and movement.

Activity 15: Control of posture and movement at the brain stem level

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9. Control of posture and movement at the cerebral cortex level In general, the motor cortex is responsible for initiating and controlling more complex and discrete voluntary movements. It is divided into 3 specialized and somatotopically organized areas, each of which project directly and indirectly (via the brain stem) onto interneurons and motor neurons located in the spinal cord.

1. the primary motor cortex, receives peripheral afferent information via several pathways and is responsible for encoding the muscles to be activated, the force the recruited muscles produce, and the direction of the movement.

2. the premotor area, also receives considerable sensory input; however, it is mainly involved with the organization and preparation of motor commands.

3. the supplemental motor area also plays an important role in programming complex sequences of movement that involve groups of muscles.

Activity 16. Cortical motor areas • Where do corticospinal and corticobulbar

system originate from? Hint: 30% come from the motor cortex, 30% come from the preemptor cortex, and 40% come from the somatic sensory area.

• What kind of signal information flow

through the corticobulbar tract? What are the tracts that flow down from the brainstem nuclei?

• What kind of signal information flow

through the corticospinal tract? Which information goes through the lateral corticospinal and which information goes through the ventral corticospinal tract?

Fig. 11. Medial (above) and lateral (below) views of the human cerebral cortex, showing the motor cortex (Brodmann's area 4) and other areas concerned with control of voluntary movement, along with the numbers assigned to the regions by Brodmann.

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The various parts of the body are represented in the precentral gyrus, with the feet at the top of the gyrus and the face at the bottom (Fig. 4). The facial area is represented bilaterally, but the rest of the representation is generally unilateral, the cortical motor area controlling the musculature on the opposite side of the body. The cortical representation of each body part is proportionate in size to the skill with which the part is used in fine, voluntary movement. The areas involved in speech and hand movements are especially large in the cortex; use of the pharynx, lips, and tongue to form words and of the fingers and apposable thumbs to manipulate the environment are activities in which humans are especially skilled. The major direct descending pathway from the motor cortex to the α MNs and γ MNs is the corticospinal tract. In addition to influencing motor functions directly, the corticospinal tract also affects motor activity indirectly through the descending brain stem pathways.

Activity 17. Motor homunculus • What does motor homunculus mean to

you? • The size of the various body parts

represented in the homunculus is proportionate to the cortical area devoted to them. Explain the significance of this phenomenon. Give examples.

• Point to the location on the homunculus

where initial information originates in order to move the a. toes, b. shoulder, c. little fingers, d. thumb, e. eyelid, f. lips, g. tongue, h. trunk.

• Describe the flow of information from

these cortical areas (i.e. via corticobulbar or corticospinal tract; lateral or ventral corticospinal) until it reaches the motor neurons that spread to the muscles of the desired locations.

Fig. 12. Motor humunculus

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10. Control of posture and movement at the associate areas level Although the 2 associate areas, the cerebellum and basal ganglia, cannot independently initiate motor activity, they are essential for the execution of coordinated motor control. The cerebellum, operating entirely at a subconscious level, plays a major role in both the planning and modification of motor activities though comparison of the intended movement with the outcome movement. This is accomplished through the continuous inflow of information from the motor control areas and the central and peripheral sensory areas. The cerebellum is divided into 3 functional divisions. The first division receives vestibular input, both directly and indirectly from the vestibular labyrinth (semicircular and otolith receptors) and, as might be surmised based on the input, is involved with postural equilibrium. The second cerebellar division is mainly responsible for the planning and initiation of movements, especially those requiring precise and rapid dexterous limb movements. This division receives input from both the sensory and motor cortices. It is the third division, the spinocerebellum, which receives the somatosensory information conveyed through the 4 ascending spinocerebellar tracts. In addition to the somatosensory input, this division of the cerebellum also receives input from the vestibular labyrinth and visual and auditory organs. The output from the spinocerebellum serves to adjust ongoing movements through influential connections on the medial and lateral descending tracts in the brain stem and cortex via projections on the vestibular nucleus, reticular formation, red nucleus, and motor cortex. In addition to controlling movements, the spinocerebellum also uses the somatosensory input for feedback regulation of muscle tone through regulation of static γ-MN drive to the muscle spindles. Lastly, the cerebellum also receives an efferent copy of the motor commands arriving at the ventral roots of the spinal cord. The cerebellum has also been implicated in motor learning. The basal ganglia consist of 5 subcortical nuclei (groups of nerve cells) located deep within the cerebral hemispheres. In contrast to the cerebellum, which has input and output connections with all 3 levels of motor control, the cerebral cortex is the only central axis component having input and output connections (via the thalamus) with the basal ganglia. With respect to motor control, the basal ganglia are believed to be involved with more higher-order, cognitive aspects of motor control. An additional distinction from the cerebellum is that the basal ganglia receive input from the entire cerebral cortex, not just those associated with sensory and motor function. The widespread input and output cortical connections suggest that they are involved with many functions other than motor control.

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ix. Summary It is impossible to separate postural adjustments from voluntary movement in any rigid way, but it is possible to differentiate a series of postural reflexes (Table 2) that not only maintain the body in an upright, balanced position but also provide the constant adjustments necessary to maintain a stable postural background for voluntary activity. These adjustments include maintained static reflexes and dynamic, short-term phasic reflexes. The former involve sustained contraction of the musculature, whereas the latter involve transient movements. Both are integrated at various levels in the CNS from the spinal cord to the cerebral cortex and are effected largely through various motor pathways. A major factor in postural control is variation in the threshold of the spinal stretch reflexes, which is caused in turn by changes in the excitability of motor neurons and, indirectly, by changes in the rate of discharge in the efferent neurons to muscle spindles.

Table 2. Postural reflexes

Reflex Stimulus Response Receptor Integrated In

Stretch reflexes Stretch Contraction of muscle Muscle spindles Spinal cord,

medulla

Positive supporting

(magnet) reaction

Contact with sole or palm Foot extended to support

body

Proprioceptors in

distal flexors

Spinal cord

Negative supporting

reaction

Stretch Release of positive

supporting reaction

Proprioceptors in

extensors

Spinal cord

Tonic labyrinthine

reflexes

Gravity Contraction of limb

extensor muscles

Otolithic organs Medulla

Tonic neck reflexes Head turned: Change in pattern of

extensor contraction:

Neck

proprioceptors

Medulla

(1) To side (1) Extension of limbs on

side to which head is

turned

(2) Up (2) Hind legs flex

(3) Down (3) Forelegs flex

Labyrinthine

righting reflexes

Gravity Head kept level Otolithic organs Midbrain

Neck righting

reflexes

Stretch of neck muscles Righting of thorax and

shoulders, then pelvis

Muscle spindles Midbrain

Body on head

righting reflexes

Pressure on side of body Righting of head Exteroceptors Midbrain

Body on body

righting reflexes

Pressure on side of body Righting of body even

when head held sideways

Exteroceptors Midbrain

Optical righting

reflexes

Visual cues Righting of head Eyes Cerebral

cortex

Placing reactions Various visual,

exteroceptive, and

proprioceptive cues

Foot placed on supporting

surface in position to

support body

Various Cerebral

cortex

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Hopping reactions Lateral displacement while

standing

Hops, maintaining limbs in

position to support body

Muscle spindles Cerebral

cortex

Brainstem Control of Posture:

Transections at different levels of the brainstem have been used to demonstrate the importance of brainstem centers in the control of posture. Isolation of centers below the transection from central influences above, reveals the regulatory functions of the intact centers.

A. Spinal Transection

If the spinal cord is cut, three events begin to happen.

1. Complete loss of voluntary movements. This results from the interruption of descending pathways from motor centers located in the brain and higher centers. Following spinal transection, there is a total paralysis of all muscles below the level of the lesion.

2. Loss of conscious sensation below the level of the lesion. Sensory information from the body regions below the transection cannot reach higher centers and those regions appear to be anesthetized.

B. Decerebrate Rigidity (Mid-Collicular Transection):

Two brainstem centers that are important for the maintenance of muscle tone in antigravity muscles (the extensors) are the pontine reticular formation (medial reticulospinal tract) and Dieter's nucleus (lateral vestibulospinal tract). Both centers have an excitatory influence on extensor muscles. Stimulation of cells in the pontine reticular formation has a very powerful excitatory effect on extensors, but its activity is normally inhibited by central (cortical) projections. If the spinal cord is cut above the level of the pontine reticular formation, (mid-collicular), the inhibitory influence is removed and there is an exaggerated activation of muscle tone in the extensors (antigravity muscles). This produces a rigid posture which is referred to as decerebrate rigidity. In patients with this condition, the arms and legs are extended, the back is arched, the head is dorsiflexed, and the feet are pointed with the toes curled. This stiff posture does not permit the joints to bend and the body is capable of standing upright. This is very different from spinal transection, where extensor muscle tone is abolished and the body becomes limp.

C. Decorticate Rigidity (Transection of the corticospinal fibers or internal capsule)

Interruption of the corticospinal tract (with the brainstem circuitry intact) produces decorticate rigidity. In this condition the extensors of the legs and the flexors of the arms contract steadily. One reason for this is that the rubrospinal tract in humans only projects as far as the cervical cord and may counteract vestibulospinal facilitation of arm but not leg extensors.

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ix. Conclusion

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APPENDIX

Spinal Cord Injury Glossary The spinal cord is the major collection of nerves which transmits motor and sensory information from the brain to and from the rest of the body. It is surrounded by a column of bony rings called vertebrae. The column of nerves that travel from the brain to the tail bone make up the spinal cord and the column of protective bones is the spine. An injury to the spine may cause the bones around the spinal cord to break and press against the spinal cord, which can damage the nerves, affecting movement and sensation. Damage to the spinal cord and nerves can also occur without damage to the bones. Nerves operate along a pathway (the spinal cord). When the path is broken, the messages cannot get through. This occurs when there is an injury or disease of the spinal cord. The amount of the loss of body function following injury or disease to the spinal cord depends on the level and "completeness" of the injury. The completeness of the injury refers to the amount of messages that are getting through the spinal cord. If someone is without any feeling or movement below their level of injury, then it is considered a complete injury. If someone has some feeling or movement well below their injury level, then it is an incomplete injury. In addition to movement and sensation, the spinal cord carries nerves signals that affect many other body systems such as skin, bowel, bladder, and breathing. So, following damage to the spinal cord, depending on the level and "completeness" of the injury, these body functions may not work the same as before. Other terms related to spinal cord and spinal cord injury: Anterior Cord Syndrome: Condition affecting anterior part of spinal cord which involves variable loss of motor function and sensitivity to pain and temperature, while proprioception is preserved. Autonomic Dysreflexia (going hyper): Uninhibited, sympathetic response below the level of injury, affecting persons with neurologic levels of T6 and above, resulting in abnormally high blood pressure with resulting headache, sweating above the level of injury, goosebumps and chills/fever. Considered a medical emergency if untreated. ASIA Impairment Scale: The American Spinal Injury Association’s (ASIA ) system of neurological classification is the most commonly accepted evaluation of impairment for spinal cord injury.

• A - Complete: No sensory or motor function is preserved in sacral segments S4-S5. • B - Incomplete: Sensory, but not motor, function is preserved below the neurologic level

and extends through sacral segments S4-S5. • C - Incomplete: Motor function is preserved below the neurologic level and most key

muscles below the neurologic level have muscle grade less than 3. • D - Incomplete: Motor function is preserved below the neurologic level and most key

muscles below the neurologic level have muscle grade greater than or equal to 3. • E - Normal: Sensory and motor functions are normal.

Brown-Séquard Syndrome: Involves a hemisection lesion of the cord, causing greater ipsilateral proprioceptive and motor loss with contralateral loss of sensitivity to pain and temperature. Cauda Equina Syndrome: Involves injury to the lumbosacral nerve roots in the spinal canal leading to areflexic bladder, bowel and lower limbs. Central Cord Syndrome: An injury involving the central part of the cervical region of the spinal cord resulting in greater weakness in the upper limbs than in the lower limbs with sacral sensory sparing.

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Complete Spinal Cord Injury: No sensory or motor function in sacral segments of spinal cord S4-S5. Incomplete Spinal Cord Injury: Sensory and/or motor function in sacral segments of spinal cord S4-S5. Paraplegia: Injury to the thoracic, lumbar or sacral level of the spinal cord, resulting in paralysis of the lower extremities Quadriplegia: Injuries at the cervical level of the spinal cord resulting in paralysis of the upper and lower extremities (more commonly referred to as tetraplegia). Sacral Sparing: Sensory or motor function at the anal mucocutaneous junction. The presence of either is considered sacral sparing. Spinal Shock: Often occurring soon after spinal cord injury, this is a loss of reflexes below the level of injury with associated loss of sensorimotor functions. This condition can last for several hours to days after initial injury. Tetraplegia: (The more commonly used term for quadriplegia) Injury to the spinal cord in the cervical region with resulting loss of muscle strength in all 4 extremities.

STANCE & GAIT

Watching the patient walk may be the most important part of the neurologic examination. Seeing how

the patient initiates a planned action, evaluating the ability to maintain balance while "repetitively

hurling oneself into space"—as the act of walking has been described, and analyzing an abnormal gait for

clues to the nature of the deficit all provide valuable information.

The minimal screening examination should include evaluation of the following: (1) "Normal" gait across

the room. (2) "Heel-walking" with ankles dorsiflexed. (3) "Toe-walking" on the balls of the feet with

heels elevated. (4) "Tandem" gait, in which the patient puts one foot directly in front of the other, heel

to toe, and walks an imaginary line.

Some characteristic stances and gaits are as follows.

Steppage Gait ("Foot Drop")

Steppage gait results from an inability to dorsiflex the foot. The patient compensates by exaggerated

elevation of the flexed hip and knee to allow the foot to clear the ground while walking. This abnormality

is usually the consequence of a peripheral nerve disorder such as a peroneal palsy or other neuropathy,

but occasionally it results from a radiculopathy or central lesion.

Cerebellar Gait

This is a wide-based, irregular, staggering, or reeling gait, as if drunk.

Sensory-Ataxic Gait

A wide-based, short, uneven gait characterized by high steps and slapping down of the feet is seen with

proprioceptive loss, as in tabes dorsalis. The eyes may remain "glued" to the ground.

Hemiplegic Gait

With the affected spastic leg extended and internally rotated and the foot in inversion and plantar

flexion, the leg circumducts at the hip to allow the foot to clear the floor.

Paraplegic Gait

A slow, stiff shuffling gait with the toes scraping and the legs "scissoring" because of increased adductor

tone associated with spasticity is seen in myelopathy or other bilateral corticospinal tract disease.

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Dystrophic Gait

Waddling and lordotic posture may result from pelvic muscle weakness.

Parkinsonian Gait

This consists of slow starting and short shuffling steps with a tendency to accelerate ("festinate") as if

chasing the center of gravity. The posture is stooped, turns are "en bloc" with the feet moving only in

tiny steps, and there is loss of normal associated movements, such as arm swinging, that help to

maintain balance.

Apraxic Gait

Apraxia consists of an inability to execute a learned motor program. Gait apraxia is loss of the ability to

walk and results from diffuse cerebral damage—more specifically, damage to the frontal lobe—despite

normal strength and coordination. The gait is similar to a parkinsonian gait, but if severe the patient will

simply stand, partially upright, unable to "remember" how to go about walking, the feet seeming to be

"glued to the floor." Alternatively, the patient will lift and lower the feet without advancing, as if drawn to

the floor by magnetic force.

Antalgic Gait

Antalgic gait is a response to pain—favoring one leg by putting as little weight as possible on it.

Choreic Gait

Choreic gait is described as lurching, "jerky twitching," and "dancing." Falls are surprisingly rare.

Causes of Peripheral Nervous System Disorders Site Type Examples

Motor neuron* Inherited Spinal muscular atrophy types I–IV

Acquired, acute Polio, infections by coxsackievirus and other

enteroviruses (rare)

Acquired, chronic Amyotrophic lateral sclerosis, paraneoplastic

syndrome, postpolio syndrome, progressive

bulbar palsy

Nerve root Acquired Herniated disk, infections, metastatic cancer,

neurofibroma, trauma

Plexus Acquired Acute brachial neuritis, diabetes mellitus,

hematoma, local tumors (eg, schwannoma),

metastatic cancer, neurofibromatosis (rare),

traction during birth, severe trauma

Peripheral nerve Hereditary Hereditary adult-onset neuropathies,

hereditary sensorimotor neuropathies,

hereditary sensory and autonomic

neuropathies

Infectious Hepatitis C, HIV infection, Lyme disease,

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syphilis

In undeveloped nations: Diphtheria, parasites

Inflammatory Chronic inflammatory demyelinating

polyradiculoneuropathy, Guillain-Barré

syndrome and variants, vasculitis

Metabolic Amyloidosis, diabetes mellitus, dysproteinemic

neuropathy, ethanol with undernutrition

(particularly deficiency of B vitamins), ICU

neuropathy, leukodystrophies (rare), renal

insufficiency

Neuromuscular

junction

— Botulism in infants, congenital myasthenia

(very rare), Eaton-Lambert syndrome,

myasthenia gravis, toxic neuromuscular

junction disorders (eg, due to nerve gas)

Muscle fiber Dystrophies Distal muscular dystrophy (late distal

hereditary myopathy; rare), Duchenne's

muscular dystrophy and related dystrophies,

fascioscapulohumeral muscular dystrophy,

limb-girdle muscular dystrophy,

oculopharyngeal dystrophy (rare)

Channelopathies

(myotonic)

Familial periodic paralysis, myotonia congenita

(Thomsen's disease), myotonic dystrophy

(Steinert's disease)

Congenital Central core disease, centronuclear myopathy,

nemaline myopathy (very rare)

Endocrine Acromegaly, Cushing's syndrome, diabetes

mellitus, hypothyroidism, thyrotoxic myopathy

Inflammatory Infection (viral more than bacterial),

polymyositis and dermatomyositis

Metabolic Acid maltase deficiency, carnitine deficiency,

glycogen storage and lipid storage diseases

(rare)

*Upper motor neuron disorders (eg, spinal muscular atrophies) technically involve the CNS because

the cell body of the motor neuron is located in the spinal cord.

Adapted from Tandan R, Bradley WA: Amyotrophic lateral sclerosis. Part I: Clinical features,

pathology and ethical issues in management. Annals of Neurology 18:271–280, 1985; used with

permission of Little, Brown and Company.