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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.
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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.
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Classical signs of infection:• Calor – heat• Dolor – pain• Rubor – redness• Tumor - swelling
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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 atypical 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 atypical 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 withthe monosynaptic reflex
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Polysynapticflexor 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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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.
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Panniculus Reflex
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Canine Dermatomes – Skin Areas Innervated By Spinal
NervesShingles inhumans – Herpes zoster