Spinal cord reflexes are neuronal circuits extending from peripheral receptors through the spinal cord to effectors. Reflexes are classified by the origin and termination of the receptor and effector. Major reflexes include the monosynaptic stretch reflex, flexor withdrawal reflex, reciprocal innervation, crossed extensor reflex, extensor thrust reflex, scratch reflex, and panniculus reflex. These reflexes maintain posture, remove the body from harm, and protect skin from irritants.

1. SPINAL CORD REFLEXES
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2. Student Preparation
Textbook of Medical Physiology
Guyton and Hall, Ch. 54
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3. Definition - Spinal reflex arc
• A neuronal circuit extending from a
peripheral receptor through the spinal
cord to a peripheral effector.
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4. Classification of reflex arcs
• According to origin of the receptor and
termination of the effector
– Somatosomatic reflex - the reflex
originates and terminates in the body wall
(soma) – in contrast to the viscera. An
example of this reflex would be the
withdrawal reflex. A nociceptive stimulus
applied to sensory receptors of the skin of
a limb results in skeletal muscle movement
which removes the limb away from the
source of the stimulus.
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5. Somatosomatic reflex
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6. Classification of reflex arcs
cont.
• According to origin of the receptor and
termination of the effector
– Somatovisceral reflex - the reflex
originates in the body wall and terminates
in a visceral effector. Example - a painful
(nociceptive) stimulus applied to the limb of
a dog results in urination.
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7. Classification of reflex arcs
cont.
• According to origin of the receptor and
termination of the effector
– Viscerovisceral reflex - the reflex
originates and terminates in a visceral
structure. Example - gastric distention
results in increased intestinal peristalsis.
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8. Classification of reflex arcs
cont.
• According to origin of the receptor and
termination of the effector
– Axonal reflex - Sensory activity ascends a
single branch of a peripheral nerve to a
bifurcation, and then passes back to the
same tissue, causing a neuroeffector
response without entering the spinal cord.
Example – a painful stimulus to the skin
results in vasodilation via the axonal reflex.
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9. Classical signs of infection:
• Calor – heat
• Dolor – pain
• Rubor – redness
• Tumor - swelling
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10. Classification of reflex arcs
cont.
• According to origin of the receptor and
termination of the effector
– Ganglionic reflex - does not involve the
spinal cord. The reflex originates in a
peripheral organ, synapses in an
autonomic ganglion and terminates in a
peripheral effector.
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11. Classification of reflex arcs
According to origin of the receptor
and termination of the effector
• Viscerovisceral
reflex
• Axonal Reflex
• Ganglionic reflex
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12. Classification of reflex arcs
• According to destination of interneuron
in the spinal cord
– Intrasegmental reflex - all central reflex
activity occurs within a single spinal cord
segment. Example - dorsal root ganglion
afferent enters C7, synapses with an
interneuron within C7, which synapses on
a C7 motor neuron, and its axon exits C7.
– Intersegmental reflex- central reflex
activity involves more than one spinal cord
segment. 12

13. Classification of reflex arcs
• According to destination of interneuron
in the spinal cord
– Ipsilateral reflex - central reflex activity
remains on the same side of the cord.
– Contralateral reflex - afferent input enters
the cord on one side and motor output
exits the cord on the opposite side.
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14. Classification of reflex arcs
• According to number of synapses
– Monosynaptic reflex - reflex contains one
central synapse. This results in minimal
delay within the spinal cord and rapid
response of the effector organ in reaction
to the sensory stimulus.
– Di (bi)-synaptic reflex - two synapses
within the spinal cord.
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15. Classification of reflex arcs
• According to number of synapses
– Polysynaptic reflex - multiple synapses
within the spinal cord. Although
(simplistically speaking) reflexes may be
isolated as mono- or di-synaptic, all
reflexes have polysynaptic components.
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16. Functional components of a
typical reflex arc
• Sensory receptor -
detects some physical
or chemical
phenomena and
transduces it into
electrical impulses
• Dorsal root ganglion
(cell body of the
afferent neuron)
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17. Functional components of a
typical reflex arc cont.
• Interneuron (internuncial
or intercalated neuron) - a
neuron inserted between
the afferent and efferent
neurons – zero, one or
multiple
• Ventral horn motor neuron
(efferent neuron)
• Effector organ - muscle or
gland
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18. Examples – Major
Reflexes
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19. Monosynaptic reflex
• Synonyms - stretch reflex, myotatic reflex,
muscle spindle reflex
• Stimulus - stretch of the primary or secondary
receptor of the muscle spindle
• Receptor - muscle spindle, Group Ia or II
afferents
• Number of synapses - one
• Purpose - reflex contraction of the stretched
muscle thus opposing stretch beyond a
certain length
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20. Monosynaptic reflex
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21. Monosynaptic
reflex
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22. Flexor reflex
• Synonyms - withdrawal reflex,
nociceptive reflex, pain reflex
• Stimulus - noxious stimulus to an
extremity
• Receptor - class II, III, or IV afferents
• Number of synapses - multisynaptic
• Purpose - to remove affected part from
danger
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23. Local sign
• The response to the stimulus by the
body will be determined by the location
of the stimulus on the body – i.e., a
noxious stimulus to the hind limb does
not result in withdrawal of a forelimb.
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24. Reciprocal innervation
• Neuronal circuits that allow inhibition
(reciprocal inhibition) or facilitation
(reciprocal excitation) of neurons either
opposing or aiding the reflex response.
Example - muscle spindle reflex in
biceps brachii will (through reciprocal
innervation) produce reciprocal
inhibition of neurons to triceps brachii
and reciprocal excitation of neurons to
brachialis muscle.
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25. Reciprocal inhibition is required with
the monosynaptic reflex 25

26. Polysynaptic
flexor reflex
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27. Crossed extensor reflex
• Not a separate reflex, but is accessory
to, or part of, the flexor reflex
• Synonyms - none
• Stimulus, receptors, number of
synapses same as flexor reflex
• Purpose - contraction of extensor
muscles of contralateral limb to support
weight
• After discharge - contraction outlasts
stimulus 27

28. Crossed-extensor reflex
Flexor reflex
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29. Extensor thrust reflex
• Synonym - none
• Stimulus - pressure applied to the foot
pads
• Receptors - tactile receptors and
muscle spindles; group I, II, III, IV
afferent fibers
• Number of synapses - multisynaptic
• Purpose - maintenance of posture
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30. Scratch reflex
• Synonyms - none
• Stimulus - irritating stimulus on the skin
of the dorsal or lateral surfaces of the
thorax and neck
• Receptors - combination of tactile and
pain receptors; group II, III, IV afferent
fibers
• Number of synapses - multisynaptic
• Purpose - to remove source of irritant 30

31. 31

32. Panniculus reflex
• Synonym – cutaneous reflex
• Stimulus – local stimulus to the skin,
causing contraction of the cutaneous
trunci muscle in the same segment
• Receptors – pain and touch receptors,
group III, IV afferent fibers
• Number of synapses – multisynaptic
• Purpose – to remove irritating stimuli –
biting flies, etc. 32

33. Panniculus Reflex
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34. Canine Dermatomes – Skin
Areas Innervated By Spinal
Nerves
Shingles in
humans –
Herpes zoster