Proprioception Reflexes 2xn31

Neurology: Proprioception & Reflexes

Proprioception • Proprioception is the provision of relative to the length and contraction of muscles, tension in tendons, the position of ts 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, ts, 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 overstretching • 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 •

t Kinesthetic receptors are found in the capsule/ligaments of synovial ts – Ruffini corpuscles – Pacinian corpuscles – Free nerve endings • Respond to pressure and acceleration /deceleration of ts during movement • Ligaments contain proprioceptors similar to tendon organs→ if strain placed on t is too excessivemuscles relax/antagonists contract to prevent damage to t (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.