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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