Neurology: Proprioception & Reflexes

Proprioception

• Proprioception is the provision of feedback relative to the length and contraction of muscles, tension in tendons, the position of joints and orientation of the body during movement (Nolte, 2008)

• Process of sensation begins in a sensory receptor –specialised cell or dendrites of sensory neuron

• Proprioceptors are specialised cells located in muscles, tendons, joints, skin, fascia and inner ear

• They send a continuous stream of nerve impulses to the brain related to the position of different body parts, and make adjustments to ensure balance and co-ordination (Tortora & Derrickson, 2008)

Proprioception • Muscle spindles are proprioceptors located in the

muscle belly that monitor muscle length

• Their fibres are interspersed among the regular skeletal muscle fibres

• These proprioceptors are sensitive to the degree and rate of stretch in the muscle and prevent over-stretching

• The brain regulates muscle spindle sensitivity and adjusts how vigorously the spindles respond to stretching, thus setting an overall level of muscle tone

• They are responsible for the stretch reflex

(Tortora & Derrickson, 2008)

Proprioception

• Golgi tendon organs are proprioceptors found in the musculotendinous junction and monitor muscle tension

• They protect tendons and associated muscles from damage due to excessive tension

• Responsible for the tendon reflex

(Tortora & Derrickson, 2008)

Proprioception • Joint Kinesthetic receptors are found in the

capsule/ligaments of synovial joints

– Ruffini corpuscles

– Pacinian corpuscles

– Free nerve endings

• Respond to pressure and acceleration /deceleration of joints during movement

• Ligaments contain proprioceptors similar to tendon organs→ if strain placed on joint is too excessive- muscles relax/antagonists contract to prevent damage to joint

(Tortora & Derrickson, 2008)

Involuntary Movement

• Reflexes are fast, predictable, automatic responses to changes in the environment (Tortora & Derrickson, 2008)

–Cranial reflexes; blinking

–Autonomic (visceral) reflexes; heart rate

– Somatic reflexes; stretch reflex in skeletal muscle

• The pathway followed by nerve impulses that produce a reflex is a reflex arc (Nolte, 2008)

Reflex Arc

• A reflex arc includes five functional components;

1. Sensory receptor

– Dendrite of sensory neuron serves as sensory receptor and responds to stimulus

2. Sensory neuron

– The nerve impulse travels along the axon of the sensory neuron to the axon terminal in the gray matter of the spinal cord

(Tortora & Derrickson, 2008)

Reflex Arc

3. Integrating centre

– Integration occurs in the gray matter of the CNS

– A simple reflex has a single synapse between a sensory neuron and a lower motor neuron, known as;

• Monosynaptic Reflex Arc

– A reflex with more than two types of neurons (sensory, inter- and motor neuron)and more than one synapse is known as;

• Polysynaptic Reflex Arc

(Tortora & Derrickson, 2008)

Reflex Arc

4. Motor Neuron

– Impulses triggered by the integrating centre travel along a lower motor neuron to the part of the body that will respond

5. Effector

– the part of the body that responds to the motor nerve impulse is the effector.

– Its action is called a reflex

(Tortora & Derrickson, 2008)

Reflex Arc

Reflexes

• Reflexes enable the body to make extremely rapid adjustments to maintain homeostasis (Nolte, 2008)

• Reflexes are normally predictable and can be used as a diagnostic tool for the nervous system (Tortora & Derrickson, 2008)

• There are two important somatic spinal reflexes;

– Stretch reflex

– Tendon reflex

The Stretch Reflex

• Muscle spindles responsible for stretch reflex monitor muscle length

• If a muscle spindle senses over-stretching of muscle fibres, it sends a message to CNS and causes muscle contraction (& antagonist relaxation)

• The stretch reflex causes muscle contraction in response to muscle stretch

(Tortora & Derrickson, 2008)

The Stretch Reflex • The stretch reflex is a monosynaptic reflex arc

• Muscle spindles are stimulated by stretch in a muscle

• A nerve impulse is transmitted along the sensory neuron to the gray matter

• The sensory neuron synapses with a lower motor neuron in the anterior gray horn

• A nerve impulse travels to the muscle and causes a contraction

(Waugh & Grant, 2006)

Stretch Reflex • The sensory neuron also synapses with an

inhibitory interneuron in the gray matter

• This interneuron synapses with and inhibits a lower motor neuron that normally excites the antagonistic muscles

• When the stretched muscles contract during a stretch reflex, antagonistic muscles that oppose the contraction relax

(Waugh & Grant, 2006)

The Stretch Reflex

The Tendon Reflex

• Golgi tendon organs responsible for tendon reflex monitor muscle tension

• If golgi tendon organ senses increased tension, it sends message to CNS and causes muscle relaxation (& antagonist contraction)

• The tendon reflex causes muscle relaxation in response to muscle tension

(Tortora & Derrickson, 2008)

The Tendon Reflex • The tendon reflex is a polysynaptic reflex arc

• Golgi tendon organs are stimulated by tension in a tendon (Waugh & Grant, 2006)

• A nerve impulse is transmitted along the sensory neuron to the gray matter

• The sensory neuron activates an inhibitory interneuron that synapses with a lower motor neuron in the anterior gray horn

• This inhibits the motor neuron, which generates fewer impulses

• The muscle attached to the same tendon relaxes and relieves excess tension (Tortora & Derrickson, 2008)

The Tendon Reflex • The sensory neuron also synapses with an

excitatory interneuron in the gray matter

• This interneuron synapses with a lower motor neuron controlling antagonistic muscles

• The tendon reflex causes relaxation in the muscles attached to the tendon and contraction in the antagonist muscle

(Tortora & Derrickson, 2008)

Tendon Reflex

References

• Tortora, G., Derrickson, B., 2008. Principles of Anatomy and Physiology. 12th Edition. John Wiley & Sons

• Waugh, A., Grant, A, 2006. Ross and Wilson: Anatomy and Physiology in Health and Illness. 10th Edition. Churchill Livingstone

• Nolte, J., 2008. The Human Brain: An introduction to it’s functional anatomy. 6th Edition. Mosby Inc.