Ganong, W.F. () Review of Medical Physiology. 22nd Edition, McGraw-Hill Medical, New York. a LANGE medical book. Ganong’s. Review of. Medical Physiology. Twenty-Third Edition. New York Chicago San Francisco Lisbon London Madrid Mexico City. Ganong’s Review of Medical Physiology, 22nd Edition (Lange Basic Science). Ganong’s Review of Medical Physiology, 22nd Edition (Lange Basic Science).
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Enviado por Lithany flag Denunciar. Reproduced, with permission, from Ganong WF: Review of Medical Physiology, 22nd ed. The ax- ons of alpha motor neurons have diameters of p. The left half of the diagram shows an in- hibitory reflex arc, which includes an intercalated inhibitory interneuron. Gamma Motor Neurons Each muscle spindle contains, within its capsule, 2 to 10 small intrafusal fibers.
Intrafusal muscle fibers receive their own in- nervation from gamma motor neurons, which are small, spe- cialized motor neurons whose cell bodies are located in the ven- tral horn Fig Firing in gamma motor neurons excites the intrafusal muscle fibers so that they contract.
This action does not lead directly to de- tectable muscle contraction, because the intrafusal fibers are small. Firing gamma motor neurons, however, does increase tension on the muscle spindle, which increases its sensitivity to gahong muscle stretch. The firing rates of gamma motor neurons are regulated by de- scending activity from the brain.
By modulating the thresholds for stretch reflexes, descending influences regulate postural tone.
Renshaw Cells These interneurons, located in the ventral horn, project to alpha motor neurons and are inhibitory. Renshaw cells re- ceive excitatory synaptic input via collaterals, which branch gamong alpha motor neurons. These cells are part of local feedback circuits that prevent overactivity in alpha motor neurons. Golgi Tendon Organs A second set of receptors, the Golgi tendon organs, is pres- ent within muscle tendons.
These stretch receptors are arranged in series with extrafusal muscle fibers and are acti- vated by either stretching or contracting the muscle.
Ganong’s review of medical physiology 22nd edition – Google Docs
Group lb afferent fibers run from the tendon organs via the dorsal roots to the spinal gray matter. Editjon, they end on interneu- rons that inhibit the alpha motor neuron innervating the ag- onist muscle, thus mediating the inverse stretch reflex see Fig As a result of this feedback arrangement, these specialized receptors prevent overactivity of alpha motor neurons.
Clinical Correlations If the alpha motor neuron 22md in a ventral root or peripheral nerve are cut or injured, the muscle’s resistance to stretching is reduced.
The muscle becomes weak and flaccid and has little tone. Examination of the deep tendon reflexes can provide valuable diagnostic information. Loss of all deep tendon re- flexes, for example, can suggest 22dn polyneuropathy e. The large extensor muscles that support the body are kept constantly active by coactivation of alpha and gamma motor 22dn.
Transection of the spinal cord acutely reduces mus- cle tone below the level of the lesion, indicating that supraspinal descending axons modulate the alpha and gamma motor neurons.
In the chronic phase after transection of the spinal cord, there is hyperactivity of stretch reflexes below the level of the lesion, producing spasticity. This condition is a re- sult of the loss of descending, modulatory influences.
ANATOMIA Clinical Neuroanatomy Twenty Sixth Edition
In some patients, however, the in- creased extension tone in spastic lower extremities is useful, providing at least 22dn stiff-legged spastic gait after damage to the corticospinal system eg, after a stroke. EoXsentytI0teE6des In contrast to the extensor stretch reflex eg, patellar, Achilles tendonpolysynaptic, crossed extensor reflexes are not lim- ited to one muscle; they usually involve many muscles on the same or opposite side of the body Fig These reflexes have several physiologic characteristics: Reciprocal action of antagonists—Flexors are excited and extensors inhibited on one side of the body; the opposite occurs on revoew opposite side of the body.
Div9rg9nc9—Stimuli from a few receptors are distributed to many motor neurons in the cord. Summation—Consecutive or simultaneous subthreshold stimuli may combine to initiate the reflex.
Hi9rarchy—When two antagonistic reflexes are elicited si- multaneously, one overrides the other. Propriospinal axons, located on the periphery of the spinal gray matter, are the axons of local circuit neurons that convey impulses upward or downward, edifion several segments, 22ns coordinate reflexes involving several segments.
Some feview searchers refer editiob these axons as the propriospinal tract. Two main types of lesions—of the upper and lower motor neurons—are distinguished in spinal cord disorders Table Aac er B otor Cearon Alstons A lower motor neuron, the motor cell concerned with striated skeletal muscle activity, consists of a cell body located in the anterior gray column of the spinal cord or brain stem and its axon, which passes to the motor end-plates of the muscle by way of the peripheral or cranial nerves Fig Lower mo- tor neurons are considered the final common pathway be- cause many neural impulses funnel through them to the mus- cle; that is, they are acted on by the corticospinal, rubrospinal, olivospinal, vestibulospinal, reticulospinal, and tectospinal tracts as well as by intersegmental and intrasegmental reflex neurons.
Lesions of the LMNs may be located in the cells of the ventral gray column of the spinal cord or brain stem or revieww their axons, which constitute the ventral roots of the spinal or cranial nerves. Lesions can result from trauma, toxins, in- fections eg, poliomyelitis, which can affect purely lower motor neuronsvascular disorders, degenerative processes, neoplasms, or congenital malformations affecting LMNs in the brain stem or spinal cord.
Compression of ventral root axons ie, the axons of LMNs in the spinal cord by herniated intervertebral disks is a common cause of LMN dysfunction. Signs of LMN lesions include flaccid paralysis of the physiolofy volved muscles see Table ; muscle atrophy with degen- eration of muscle fibers after some time has elapsed; dimin- ished or absent deep tendon reflexes hyporeflexia or.