Motor system

Reflexes

Prof. Vajira Weerasinghe

Professor of Physiology

Faculty of Medicine

Motor Functions

• 1. Voluntary Functions

– Voluntary movement

• 2. Involuntary Functions

– Reflexes

What is a reflex?

• Response to a stimulus

• Involuntary, without significant

involvement of the brain

• Stimulus Response

Task:

Write down 3 reflexes

What is a reflex?

Stimulus

Effector organ

Response

Central

connections

Efferent nerve

Afferent nerveReceptor

Higher centre

control

Stretch reflex

• This is a basic reflex present in the

spinal cord

• Stimulus: muscle stretch

• Response: contraction of the muscle

• Receptors: stretch receptors located

in the muscle spindle

skeletal muscle

• two types of muscle fibres

– extrafusal

• normally contracting fibres

– Intrafusal

• fibres present inside the muscle spindle

• lie parallel to extrafusal fibres

• either end of the fibre contractile

• central part contains

stretch receptors

Extrafusal

fibre

Intrafusal

fibre

Contractile

areas Stretch

receptor

Nerve supply

Sensory to intrafusal fibre:

Ia afferent

II afferent

Motor:

to extrafusal fibre

A motor neuron

to intrafusal fibre

A motor neuron

Ia afferent nerve

motor neuronone

synapse

muscle

stretchmuscle

contraction

Stretch reflex

• When a muscle is stretched

• stretch receptors in the intrafusal fibres

are stimulated

• via type Ia afferent impulse is transmitted

to the spinal cord

• motor neuron is stimulated

• muscle is contracted

• Monosynaptic

• Neurotransmitter is glutamate

Stretch

Reflex

Stretch Reflex - Knee Jerk

– nuclear bag fibre

• primary (Ia) afferent

– supplies annulospiral ending in the centre

– provide information on muscle length and velocity

(phasic response) fast stretch reflex

– nuclear chain fibre

• primary (Ia) and secondary (II) afferent

– supplies flower spray ending

– monitor the length of the muscle (tonic response) –

slow stretch reflex

Two types of intrafusal fibres

Ia afferent fibre

II afferent fibrenuclear bag fibre

nuclear chain fibre

motor

neuron

motor

neuron

Importance of stretch reflex

• detects muscle length and changes

in muscle length

• Phasic stretch reflex– Stretching the quadriceps muscle quickly (e.g. by tapping

the patellar tendon) evokes a discharge in the primary afferent (Ia) fibres

– These form monosynaptic excitatory connections with motor neurons supplying physiological extensors of the knee, which contract briefly

• Tonic stretch reflex– Passive bending of the joint elicits a discharge from the

group II afferents that increases the tone of physiological extensor (antigravity) muscles

– Tonic stretch reflex is important for maintaining erect body posture

motor neuron

• cell body is located in the anterior

horn

• motor neuron travels through the

motor nerve

• supplies the intrafusal fibres

(contractile elements at either end)

motor neuron

motor

neuron

motor neuron

• When motor neuron is active

– extrafusal fibres are contracted

– muscle contracts

• when motor neuron is active

– intrafusal fibres are contracted

– stretch receptors are stimulated

– stretch reflex is activated

– impulses will travel through Ia

afferents

– alpha motor neuron is activated

– muscle contracts

at restmuscle

stretched

active motor

neuron

IaIa

Ia afferents are stimulated

stretch reflex is initiated

motor neuron activity

• active all the time - mild contraction

• Maintain the sensitivity of the muscle

spindle to stretch

• modified by the descending pathways

• descending excitatory and inhibitory

influences

• sum effect is generally inhibitory in nature

Alpha gamma co-activation

• gamma motoneurons are activated in parallel with alpha motoneurons to maintain the firing of spindle afferents when the extrafusal muscles shorten

• Activity from brain centres often causes simultaneous contraction of both extra- and intrafusal fibres, thereby ensuring that the spindle is sensitive to stretch at all muscle lengths

Inverse stretch reflex

• When the muscle is strongly

stretched -> muscle is relaxed

• Golgi tendon organs are stimulated

• Via type Ib afferents impulse is

transmitted to the spinal cord

• inhibitory interneuron is stimulated

• motor neuron is inhibited

• muscle is relaxed

motor neuron

Undue stretch

Golgi tendon organ

muscle

relaxation

Ib afferent nerveinhibitory

interneuron

motor neuron

Undue stretch

Golgi tendon organ

muscle

relaxation

Ib afferent nerveinhibitory

interneuron

Inverse Stretch Reflex

Importance of inverse

stretch reflex

• detects muscle tension

Deep tendon reflexes (DTR)

• Biceps jerk

• Triceps jerk

• Supinator jerk

• Knee jerk

• Ankle jerk

• Jaw jerk

• reflex level

• biceps jerk C56

• supinator jerk C56

• triceps jerk C78

• knee jerk L34

• ankle jerk S12

Spinal cord level of stretch

reflexes (tendon jerks)

Superficial reflexes

• Withdrawal reflex

• Superficial abdominal reflex

• Flexor plantar reflex

Withdrawal Reflex

• Stimulus:

– cutaneous stimulation (usually noxious)

• Response:

– withdrawal of the hand

• Polysynaptic reflex

Withdrawal Reflex

muscle

contraction

cutaneous

receptors

polysynaptic

muscle

contraction

cutaneous

receptors

Withdrawal Reflex

• Elbow flexion, extension

• Wrist flexion, extension

• Forearm supination, pronation

• Shoulder adduction, abduction

• Hip flexion, extension

• Thigh adduction, abduction

• Knee flexion, extension

• Ankle dorsiflexion, plantar flexion

Reciprocal innervation

• inside the spinal cord

– Agonist and antagonistic muscles are

reciprocally innervated

– stimulation of flexor muscles

– inhibition of extensor muscles

– excitatory neurotransmitter is glutamate

– inhibitory neurotransmitter is glycine

flexor

extensor

+++

----

Reciprocal Innervation

Withdrawal ReflexFlexor & Crossed extensor reflex

Withdrawal Reflex

Superficial abdominal

reflexes

• light scratch of the abdominal skin

• brisk unilateral contraction of the

abdominal wall

Flexor plantar reflex

• Scratching the sole of foot

• Plantar flexion

• Normal response

Primitive reflexes

Primitive reflexes

• These are reflexes present in

newborn babies but disappear as the

child develops

• They were evolutionarily primitive in

origin

• In adults these reflexes are inhibited

by the higher centres

Other primitive reflexes

• Moro reflex: startle reaction

• Walking/stepping reflex

• Sucking reflex

• Tonic neck reflex

• Palmar grasp reflex

Babinski sign• when outer border of the sole of the foot is

scratched

• upward movement of big toe (dorsiflexion)

• fanning out of other toes

• also called extensor plantar reflex

• feature of • upper motor neuron lesion

• seen in infants during 1st year of life (because of

immature corticospinal tract)

positive Babinski sign

Clinical Importance of reflexes

(tendon jerks)

• Locate a lesion in the motor system

• To differentiate upper motor neuron

lesion from a lower motor neuron

lesion