Reflexes

Prof. Vajira Weerasinghe

Dept of Physiology

What is a reflex?

• Response to a stimulus

• Stimulus Response

Task:Write down 3 reflexes .

What is a reflex?

Stimulus

Effector organ

Response

Centralconnections

Efferent nerve

Afferent nerveReceptor

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• non contractile fibres present inside the

muscle spindle• lie parallel to extrafusal fibres• contains stretch receptors .

Extrafusal fibre

Intrafusal fibre

Contractileareas Stretch

receptor

Nerve supply

Sensory to intrafusal fibre:Ia afferentII afferent

Motor:to extrafusal fibre

motor neuronto intrafusal fibre

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

StretchReflex

Stretch Reflex - Knee Jerk

– nuclear bag fibre• primary (Ia) afferent

– supplies annulospiral ending in the centre

– nuclear chain fibre• primary (Ia) and secondary (II) afferent

– supplies flower spray ending .

two types of intrafusal fibres

Ia afferent fibre

II afferent fibrenuclear bag fibre

nuclear chain fibre

motorneuron

motorneuron

Importance of stretch reflex

• detects muscle length and changes in muscle length .

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 stimulatedstretch 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

Inverse stretch reflex

• When the muscle is strongly stretched• 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 stretchGolgi tendon organ

musclerelaxation

Ib afferent nerveinhibitory

interneuron

motor neuron

Undue stretchGolgi tendon organ

musclerelaxation

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)

Withdrawal Reflex

• Stimulus:– cutaneous stimulation (usually noxious)

• Response:– withdrawal of the hand

• Polysynaptic reflex .

Withdrawal Reflex

musclecontraction

cutaneousreceptors

polysynaptic

musclecontraction

cutaneousreceptors

Withdrawal Reflex

Reciprocal innervation

• inside the spinal cord– anatagonistic muscles are reciprocally

innervated– stimulation of flexor muscles – inhibition of extensor muscles .

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

• upper motor neuron lesion causes reduced or loss of these reflexes

Flexor plantar reflex

• Scratching the sole of foot

• Plantar flexion

• Normal response

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

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

scratched

• upward movement of big toe

• 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

Other primitive reflexes

• Moro reflex; startle reaction

• Walking/stepping reflex

• Sucking reflex

• Tonic neck reflex

• Palmar grasp reflex

video

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