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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