Spinal cord reflexes

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SPINAL CORD REFLEXES

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Student Preparation
Textbook of Medical Physiology
Guyton and Hall, Ch. 54

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Definition - Spinal reflex arc
•A neuronal circuit extending from a
peripheral receptor through the spinal
cord to a peripheral effector.

4. 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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Somatosomatic reflex

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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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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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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.

9. 9
Classical signs of infection:
• Calor – heat
• Dolor – pain
• Rubor – redness
• Tumor - swelling

10. 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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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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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.

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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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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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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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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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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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Examples – Major
Reflexes

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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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Monosynaptic reflex

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Monosynaptic
reflex

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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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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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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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Reciprocal inhibition is required with
the monosynaptic reflex

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Polysynaptic
flexor reflex

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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

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Crossed-extensor reflex
Flexor reflex

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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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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

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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
33

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