Lecture - 2
Dr. Zahoor Ali Shaikh
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What is Reflex?
-- It is a response that occurs automatically
without conscious effort.
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Components of reflex arc are Five:
1- Sensory Receptor
2- Afferent pathway
3- Center
4- Efferent pathway
5- Effector organ
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Components of reflex arc
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Receptor responds to the stimulus and
produces action potential (AP)
AP is taken by afferent pathway to the
integrating center (usually CNS)
Spinal cord and brain integrate basic
(automatic) reflexes
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Integrating center processes all the
information and makes the decision about
the response
Instruction from integrated center are sent
via efferent pathway to the Effector organ
(muscle or gland)
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Monosynaptic or stretch reflex or tendon jerk
eg. Bicep jerk ,tricep jerk, supinator jerk
knee jerk, ankle jerk
 Polysynaptic reflex
eg. Withdrawal reflex
Abdominal reflex
Plantar reflex
 Visceral reflex
eg. Micturation, defecation reflex
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Jendressik Phenomenon
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Sudden stretch to a muscle leads to contraction of
that muscle is known as stretch reflex.
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Stretch Reflex is basic (automatic) spinal
reflex. We will take example Knee jerk
 There are 5 components of Stretch Reflex
1- Sensory receptor – muscle spindle in
skeletal muscle
2- Afferent pathway – 1a fibers
3- Center – spinal cord
4- Efferent fibers – α motor neuron
5- Effector organ – skeletal muscle contraction
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Extrafusal muscle fibers ( cause muscle
contraction supplied by α - motor neuron)
Intrafusal muscle fibers or muscle spindle
(receptor for stretch reflex, supplied by
γ - motor neuron)
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Fig 13.3 – Sensory receptors in muscle
Silverthorn 2nd Ed
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MUSCLE SPINDLE (INTRAFUSAL FIBERS)
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MUSCLE EXTRAFUSAL FIBERS
MUSCLE INTRAFUSAL FIBERS
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Nuclear bag fiber
 Nuclear chain fiber
Sensory innervations of muscle spindle
 Ia – nuclear bag & nuclear chain fibers
 II – nuclear chain fibers
Motor supply – γ motor neuron
Dynamic & Static
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α
– motor neuron (supply Extrafusal
fibers )
 γ - motor neuron (supply Intrafusal
fibers)
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KNEE JERK – MONOSYNAPTIC REFLEX
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In polysynaptic reflex, there are many
synapses between inter-neurons in the reflex
pathway
We will study withdrawal reflex
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Withdrawal Reflex
When a person touches a hot stove or pin prick, a
withdrawal reflex occurs from the painful stimulus
 1. Receptor- pain and Temperature Receptors
stimulated
 2. Afferent impulse travels via Action potential
 3. Center– Spinal cord.( Excitatory interneuron
stimulated that stimulate efferent fibers).
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4 .Efferent fibers to Biceps to cotract (
flextion of arm ) and inhibitory neuron
stimulated to inhibit contraction of Triceps.
This type of connection causing stimulation
of nerve supply to one muscle and
simultaneous inhibition of the nerve to its
Antagonistic muscle is known as
RECIPROCAL INNERVATION.
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Withdrawal reflex – Polysynaptic reflex
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Important
 Afferent neuron stimulate also interneuron that
carry signals to the Brain ( Ascending tracts in the
spinal cord )
 Therefore Brain can modify the with drawl reflex.
How?
 By sending impulses via descending pathways to
the efferent motor neuron supplying the involved
muscles and prevent contracting of biceps inspite of
painful stimulus e.g. pin prick
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DIFFERENT
REFLEXES SHOWN
Jendressik
Phenomenon
(Reinforcement)
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Jendressik
Phenomenon
(Reinforcement)
Superficial
Reflexes
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TRANSVERS SECTION OF SPINAL CORD
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Dermatomes of
Spinal roots and
divisions of
trigeminal nerve
(V1 ,V2, V3)
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Cranial
Nerve
Spinal
Cord
Reflexes
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1- Golgi tendon Reflex
2- Crossed Extensor Reflex
We will discuss first Golgi endon Reflex
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Composed of:
 Nerve fiber endings that wind between collagen
fibers inside connective tissue capsule
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If muscle is stretched:
 Free nerve endings are pinched and they fire
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Activation of Golgi tendon organs:
 Inhibits alpha motor neurons and decreases
muscle contraction
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Fig 13.6 – Muscle reflexes
Silverthorn 2nd Ed
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Painful Stimuli
Flexion & withdrawal of stimulated limb
Extension of opposite limb
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POLYSYNAPTIC REFLEX
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To test the integrity of reflex arc.
Localization of neurological lesion.
Identifying the type of lesion.
Monitoring the progress of neurological
deficit.
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Deep tendon reflexes are absent in lower
motor neuron lesion.
The become exaggerated in upper motor
neuron lesions.
Pendular jerks are observed in cerebellar
lesions.
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