motor system reflexes for students
TRANSCRIPT
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Motor system
Reflexes
Prof. Vajira Weerasinghe
Professor of Physiology
Faculty of Medicine
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Motor Functions
• 1. Voluntary Functions
– Voluntary movement
• 2. Involuntary Functions
– Reflexes
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What is a reflex?
• Response to a stimulus
• Involuntary, without significant
involvement of the brain
• Stimulus Response
Task:
Write down 3 reflexes
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What is a reflex?
Stimulus
Effector organ
Response
Central
connections
Efferent nerve
Afferent nerveReceptor
Higher centre
control
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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
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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
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Extrafusal
fibre
Intrafusal
fibre
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Contractile
areas Stretch
receptor
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Nerve supply
Sensory to intrafusal fibre:
Ia afferent
II afferent
Motor:
to extrafusal fibre
A motor neuron
to intrafusal fibre
A motor neuron
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Ia afferent nerve
motor neuronone
synapse
muscle
stretchmuscle
contraction
Stretch reflex
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• 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
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Stretch
Reflex
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Stretch Reflex - Knee Jerk
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– 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
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Ia afferent fibre
II afferent fibrenuclear bag fibre
nuclear chain fibre
motor
neuron
motor
neuron
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Importance of stretch reflex
• detects muscle length and changes
in muscle length
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• 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
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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)
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motor neuron
motor
neuron
motor neuron
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• 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
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at restmuscle
stretched
active motor
neuron
IaIa
Ia afferents are stimulated
stretch reflex is initiated
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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
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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
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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
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motor neuron
Undue stretch
Golgi tendon organ
muscle
relaxation
Ib afferent nerveinhibitory
interneuron
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motor neuron
Undue stretch
Golgi tendon organ
muscle
relaxation
Ib afferent nerveinhibitory
interneuron
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Inverse Stretch Reflex
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Importance of inverse
stretch reflex
• detects muscle tension
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Deep tendon reflexes (DTR)
• Biceps jerk
• Triceps jerk
• Supinator jerk
• Knee jerk
• Ankle jerk
• Jaw jerk
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• 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)
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Superficial reflexes
• Withdrawal reflex
• Superficial abdominal reflex
• Flexor plantar reflex
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Withdrawal Reflex
• Stimulus:
– cutaneous stimulation (usually noxious)
• Response:
– withdrawal of the hand
• Polysynaptic reflex
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Withdrawal Reflex
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muscle
contraction
cutaneous
receptors
polysynaptic
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muscle
contraction
cutaneous
receptors
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Withdrawal Reflex
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• 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
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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
+++
----
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Reciprocal Innervation
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Withdrawal ReflexFlexor & Crossed extensor reflex
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Withdrawal Reflex
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Superficial abdominal
reflexes
• light scratch of the abdominal skin
• brisk unilateral contraction of the
abdominal wall
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Flexor plantar reflex
• Scratching the sole of foot
• Plantar flexion
• Normal response
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Primitive reflexes
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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
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Other primitive reflexes
• Moro reflex: startle reaction
• Walking/stepping reflex
• Sucking reflex
• Tonic neck reflex
• Palmar grasp reflex
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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)
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positive Babinski sign
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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