Motor system1 reflexes
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Transcript of Motor system1 reflexes
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Reflexes
Prof. Vajira Weerasinghe
Dept of Physiology
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What is a reflex?
• Response to a stimulus
• Stimulus Response
Task:Write down 3 reflexes .
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What is a reflex?
Stimulus
Effector organ
Response
Centralconnections
Efferent nerve
Afferent nerveReceptor
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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• non contractile fibres present inside the
muscle spindle• lie parallel to extrafusal fibres• contains stretch receptors .
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Extrafusal fibre
Intrafusal fibre
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Contractileareas Stretch
receptor
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Nerve supply
Sensory to intrafusal fibre:Ia afferentII afferent
Motor:to extrafusal fibre
motor neuronto intrafusal fibre
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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StretchReflex
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Stretch Reflex - Knee Jerk
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– 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
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Ia afferent fibre
II afferent fibrenuclear bag fibre
nuclear chain fibre
motorneuron
motorneuron
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Importance of stretch reflex
• detects muscle length and changes in muscle length .
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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 stimulatedstretch 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
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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 .
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motor neuron
Undue stretchGolgi tendon organ
musclerelaxation
Ib afferent nerveinhibitory
interneuron
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motor neuron
Undue stretchGolgi tendon organ
musclerelaxation
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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Withdrawal Reflex
• Stimulus:– cutaneous stimulation (usually noxious)
• Response:– withdrawal of the hand
• Polysynaptic reflex .
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Withdrawal Reflex
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musclecontraction
cutaneousreceptors
polysynaptic
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musclecontraction
cutaneousreceptors
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Withdrawal Reflex
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Reciprocal innervation
• inside the spinal cord– anatagonistic muscles are reciprocally
innervated– stimulation of flexor muscles – inhibition of extensor muscles .
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
• upper motor neuron lesion causes reduced or loss of these reflexes
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Flexor plantar reflex
• Scratching the sole of foot
• Plantar flexion
• Normal response
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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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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)
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Other primitive reflexes
• Moro reflex; startle reaction
• Walking/stepping reflex
• Sucking reflex
• Tonic neck reflex
• Palmar grasp reflex
video
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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