1 Dr W Kolbinger, Sensory Systems (2009)

Sensory Systems

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Lecture Outline

• Common Plan of Sensory Systems • Four Sensory Receptor Classes • Three Basic Processes in a Sensory Receptor • Encoding of Four Stimulus Attributes • Convergence, Divergence and Lateral Inhibition

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Common Plan of Sensory Systems Perception Behavior

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Pathway

Stimulus

Receptor

Sensory receptors

First-order sensory afferent neurons

Second-order sensory afferent neurons

Third-order sensory afferent neurons

Fourth-order sensory afferent neurons

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Sensory Systems and their Receptors

Sensory System Sensory Receptor Class

Somatosensory System (touch, vibration, proprioception, pain and temperature)

• NociceptorsVisual System

Vestibular System

Auditory System

Olfactory System

Gustatory System

• Mechanoreceptors• Thermoreceptors• Chemoreceptors

• Photoreceptors

• Mechanoreceptors

•Mechanoreceptors•Chemoreceptors •Chemoreceptors

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Examples of Sensory Receptors• Sensory receptor neuron(somatosensory and

olfactory systems• • • • Sensory receptor cell(visual, taste, and auditory

systems

1. Sensory receptors

A: Free nerve endings (pain, temperature)

B: Pacinian corpuscle (pressure)

C: Meissner’s corpuscle (touch)

D: Muscle spindle (stretch)

A

B C

D

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Three Basic Processes

• Cell body• transduction site• synaptic Terminal

• “graded potentials”

1) Receptor potential

2) Action potentials

3) Transmitter release

in Different Componentsof a Sensory Receptor

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Encoding of Four Stimulus Attributes

Attributes Encoding

• Modality labeled lines

• Intensity Amplitude of graded receptor potentials Frequency code of action potentials

• Duration Mechanisms depending on receptor adaptation

• Location Concept of receptive fields and other mechanisms

Sensory Transduction and Receptor Potentials1. The environmental stimulus interacts with the sensory receptor and causes a change in its properties

2. receptor potential or generator potential.

3.receptor potentials are graded in amplitude

Encoding of Stimulus Intensity

Encoding of Stimulus Intensity

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Encoding of Duration: Different Strategies

• Slowly adapting receptors – remain active for the

duration of a stimulus

• Rapidly adapting receptors – are active only

during times of changes (on/off)

slowly adapting receptors are better in constantly monitoringlevels of stimulation, whereas rapidly adapting receptors are most sensitive to changes, not to constant stimulation.

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A receptive field defines an area of the body that when stimulated results in a change in firing rate of a sensory neuron

Encoding of Location:Receptive Field of a Sensory Receptor

1

2

1

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Stimulation Recordings

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• Convergence • Divergence

Convergence and Divergence

convergence answers thequestion “Where does the information come from?”divergence answers the question “Where does the information go to?”

Receptive fields can be excitatory or inhibitory. The areas of inhibition contribute to a phenomenon called lateral inhibition, and aid in the precise localization of the stimulus by defining its boundaries and providing a contrasting border

lateral inhibition

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Lecture Outline

• Five Modalities and their Receptors • Different Fibers for Different Receptors • Segmental Spinal Nerves and Dermatomes • Pathways for Different Modalities • Clinical Correlations •

Somatosensory System

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Modalities of the Somatosensory System

• Touch (discriminative touch) • Vibration • Proprioception • • Pain (nociception) • Temperature

most of the somatic sensory modalities refer to sensations of the skin, proprioception refers to sensory receptors originating in the skeleto-muscular system.

All receptors of the somatic sensory system are pseudo-unipolar neurons. Their sensory endings can be found in skin, or in (or close to) the muscle. Their fibers run in peripheral nerves and their cell bodies are located in ganglia (dorsal root ganglia in case the fibers run in spinal nerves or in cranial nerve ganglia).

Touch is transduced byMerkel’s disks (discriminative touch) and Ruffini’s endings (skin stretch).

Vibration is transducedby Meissner’s corpuscles (for lower frequencies of about 50 Hz) and Pacinian corpuscles (for higher frequencies of about 300 Hz).

Pain (pricking pain by rapidly adapting mechano-sensitive or thermo-sensitivereceptors, burning pain by slowly adapting polymodal receptors) andTemperature (cold receptors and warm receptors) are transduced by free nerve endings.

Muscle spindles are embedded in extrafusal fibers of the working musculature of the muscle.

The primary receptor of a muscle spindle is a rapidly adapting receptor with a Ia("one a") afferent fiber. It carries sensory information of muscle stretch. Thisreceptor forms the afferent limb of the myotatic reflex (deep tendon reflex),The secondary receptor of a muscle spindle is a slowly adapting receptor carrying sensory information of muscle length. It uses a class II ("two") afferent fiber.

Golgi tendon organs arepositioned close to the border between muscle and tendon.Their Ib afferent fibers form the afferent limb of the reverse myotatic reflex(inverse myotatic reflex).

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Afferent Fiber Classification

A alpha A beta

A delta

C

I

II

III

IV

72-120 m/sec

36-72 m/sec

4-36 m/sec

0.4-2 m/sec

12-20 μm

6-12 μm

1-6 μm

0.2-1 μm

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Afferent Fiber Classificationand Somatosensory Modalities

12-20 μm

6-12 μm

1-6 μm

0.2-1 μm

Touch, Vibration

Pain, Temperature

Proprioception

• From skin: • From muscle:

Segmental Organization of Spinal Nerves and Dermatomes- segmental organization of the sensory innervation of the skin

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Pathway for Touch, Vibration, Proprioception:Dorsal Column / Medial Lemniscus System

Touch, vibration and proprioception are carried in a pathway called the dorsalcolumn/ medial lemniscus system

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Topographical Organization of the Spinal Cord

• Dorsal column

MidlineFrom leg

From trunk

From arm

Spatial Orientation of Signals from Different Parts of the Bodyin Somatosensory Area

• Somatosensory area has a high degree of localizationof the different parts of the body

Sensory Neurons: Two-Point Discrimination

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Pathway for Pain and Temperature:Anterolateral System

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Topographical Organization of the Spinal Cord

• ALS

MidlineFrom leg

From trunk

From arm

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Topographical Organization of the Spinal Cord

L T A

Right Left

LT

A

Touch, vibration, proprioception from left side of body

Pain, temperature from right side of body

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Lissauer’s Tract and the Anterolateral System

Right Left

Chief Complaint: Left Leg Paralysis, Right Leg NumbnessHistory:A 75 year old retired pastry maker was in good health until about one year ago, whenhe started to develop gait difficulties and numbness of his right leg. Recently, he alsoexperienced urinary urgency with occasional incontinence. Though he started takinglaxatives, he experienced problems with bowel movements. His wife reported that healso had stiffness in the legs bilaterally. At times, the left leg has unexpectedly not beenable to support his body weight for a brief period, causing him to stumble to maintainbalance. He also reported that in addition to the numbness in the right leg, he also hasa constant tingling feeling in the same limb, which he describes as “intolerable.”General Examination:Normal vital signs. Patient has no significant cardiovascular history. Abdomen was softand non-tender. No palpable abdominal masses. Digital rectal examination showedsignificantly reduced muscle tone in the external sphincter and weakness of voluntarycontraction. Prostate was felt to be enlarged with a highly nodular, irregular surface.Neurological examination:Patient was fully alert and oriented x 3. Cranial nerve exam was unremarkable. Uponmotor examination, the upper extremities had normal strength, bulk, and tone, and thereflexes were 2+ throughout the C5 to C8 spinal level. In the lower extremities,however, the muscle tone was increased in left leg and the left iliopsoas muscle wasweaker than the right (4/5). The muscle bulk in both legs was normal. Reflexes in theright leg were 2+, knee jerk on the left leg was 3+, and ankle jerk was 4+. Plantarresponse was extensor the left and flexor on the right. Finger to nose and heel to shintesting was normal. Pinprick testing and temperature sensation showed decreasedsensitivity on the right side of the trunk below the umbilicus. Vibration and joint positionsense was significantly reduced in the left leg and foot.

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Brown Sequard Syndrome

Sensory loss at level of lesion

Ipsi- lateral

Contra- lateral

Touch, Vibration, Proprioception

Pain, Temperature

Sensory loss below level of lesion

Ipsi- lateral

Contra- lateral

Touch, Vibration, Proprioception

Pain, Temperature

X

X

X

X

33 Dr W Kolbinger, Visual System (2009)

Visual System

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Lecture Outline

• Structures of the Eye

• Refraction and Image Formation

• Visual Acuity

• Autonomic Control of Pupil Diameter

• Clinical Correlations •

Anatomic Considerations

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The Ocular Fundus

Fovea

Macula

Optic disc

The optic disc region itself only contains axons of retinal ganglion cells, the output elements of the retina, but it lacks photoreceptors. As a consequence, the optic disc is responsible for the blind spot, a region inside the boundaries of the visual field, where we don’t receivevisual information.

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Optics of the Eye

Cornea refractive power: 42 D

Flat lens refractive power: 13 D

Rounded lens refractive power: 26 D

Plasticity: 13 D

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Accommodation

• Far Vision

Focus on the Retina

Flat lens refractive power: 13 D

Ciliary muscle relaxed

Suspensory ligaments tightened

Accommodation Adjusts the Refractive Power of the Eye

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Accommodation

• Near Vision

Rounded lens refractive power: 26 D

Focus on the Retina

Ciliary muscle constricted

Suspensory ligaments floppy

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• Near Vision

Blurred picture on the Retina

Flat lens

Presbyopia

The variability of the refractive power of the lens between far vision (13 D)and near vision (26 D) is called refractive plasticity. Unfortunately,the lens looses its elasticityduring aging, therebyreducing the ability to focuson near objects, a conditioncalled presbyopia.