Nervous System
Physiology
By
Dr. SHAHAB SHAIKH
PhD MD MBBS
Lecture – 2: Spinal Cord Reflexes
••••••••••••••••••••••••••••••••••
Faculty of Medicine
Al Maarefa Colleges of Science & Technology
SPINAL CORD
• It is the elongated part of the Central nervous
system
• Begins as Continuation of Medulla Oblongata
at Foramen Magnum and extends up to
upper border of L2 vertebra
• It contains white matter on the outside mainly
and grey matter in the central region
• Many Interneurons in the grey matter of
spinal cord are Centers for spinal reflexes
• Function:
– Conduit for Information passage To & fro Brain
– Center for some reflexes
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TRANSVERS SECTION OF SPINAL CORD
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REFLEXES
• Reflex : It is an automatic response to a
stimuli that occurs without conscious effort
• Reflexes in the body may be …
– Simple or Basic
– Conditioned or Acquired
• Reflexes Important in maintaining homeotsasis
• Examples
a.
b.
c.
Reflexes carry out the automatic actions of swallowing,
sneezing, coughing, vomiting.
Reflexes maintain balance and posture; e.g., spinal
reflexes control trunk and limb muscles.
Brain reflexes involve reflex center in brainstem; e.g.,
reflexes for eye movement.
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SPINAL REFLEX
• Reflex : It is an automatic response to a
stimuli that occurs without conscious effort
• Components of reflex arc are:
–
–
–
–
–
Sensory Receptor
Afferent pathway
Center
Efferent pathway
Effector organ
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REFLEX
Receptor responds to the stimulus and produces action
potential (AP)
↓
AP is taken by afferent pathway to the integrating center
(usually CNS)
↓
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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Classification of reflexes
 Monosynaptic reflex
 Also called as stretch reflex or tendon jerk
 e.g. Bicep jerk ,triceps jerk, knee jerk, ankle jerk
 Polysynaptic reflex
 e.g. Withdrawal reflex, Abdominal reflex, Plantar reflex
 Visceral reflex
 e.g. Micturation, defecation reflex
Jendressik Phenomenon
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Stretch Reflex [monosynaptic]
 Sudden stretch to a muscle leads to contraction of
that muscle is known as stretch reflex.
 Stretch Reflex is a basic spinal reflex. Example
Knee jerk
• The 5 components of Stretch Reflex
–
–
–
–
–
Sensory receptor – Muscle Spindle in skeletal muscle
Afferent pathway – 1a fibers
Center – spinal cord
Efferent fibers – α-motor neuron
Effector organ – skeletal muscle contraction
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Types of muscle fibers
I.
Extrafusal muscle fibers
–
–
Takes part in muscle contraction
Supplied by α - motor neuron
II. Intrafusal muscle fibers
–
–
–
Also called as Muscle Spindle
Receptors for stretch reflex
supplied by γ - motor neuron
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Muscle spindle
• These are the receptors of
the muscles
• Consist of two types of fibers
– 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
• Detect both dynamic and
static changes in muscles
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Muscle spindle
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Types of Nerve Fibers
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Types of motor neuron
• α - motor neuron
– Supply Extrafusal fibers
• γ - motor neuron
– Supply Intrafusal fibers
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Stretch Reflex (Knee Jerk)
KNEE JERK – MONOSYNAPTIC REFLEX
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Stretch Reflex (Knee Jerk)
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Polysynaptic Reflex
• In polysynaptic reflex, there are many synapses
between inter-neurons in the reflex pathway
• Eg. Withdrawal reflex:
– When a person touches a hot stove or pin prick, a
withdrawal reflex occurs from the painful stimulus
– Receptor- pain and Temperature Receptors stimulated
– Afferent impulse travels via Sensory nerves
– Efferent fibers to Biceps to contract ( flexion 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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Poly synaptic reflex
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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REFLEXES
Jendressik
Phenomenon
(Reinforcement)
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Jendressik
Phenomenon
(Reinforcement)
Superficial
Reflexes
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Plantar Reflex
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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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Polysynaptic Reflexes
1. Golgi tendon Reflex
2. Crossed Extensor Reflex
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Golgi Tendon Organs
• Golgi tendon organ is present in the tendon and is
Composed of Nerve fiber endings that wind between
collagen fibers inside connective tissue capsule that make
up the tendon.
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Golgi Tendon Reflex
• If muscle is stretched by striking the tendon Free nerve
endings are pinched in the tendon and they fire
• Activation of Golgi tendon organs Inhibits alpha motor
neurons and decreases muscle contraction
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Fig 13.6 – Muscle reflexes
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Silverthorn 2nd Ed
Crossed Extensor Reflex
Painful Stimuli
Flexion & withdrawal of stimulated limb
Extension of opposite limb
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Crossed Extensor Reflex
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Crossed Extensor Reflex
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Clinical Importance of reflexes
• To test the integrity of reflex arc.
• Localization of neurological lesion.
– Deep tendon reflexes are absent in lower motor
neuron lesion.
– They become exaggerated in upper motor neuron
lesions.
– Pendular jerks are observed in cerebellar lesions.
• Identifying the type of lesion.
• Monitoring the progress of neurological
deficit.
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References
• Human physiology by Lauralee Sherwood,
8th edition
• Text Book Of Physiology by Guyton & Hall,
11th edition
THANK YOU
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