The locomotor stepping pattern, the complex sequence of activation of flexor and extensor muscles required for walking, is controlled by the spinal locomotor system. We only briefly touch on several relevant mechanisms.
#Patellas quiver meaning full#
When spinal inhibitory interneuron activity is reduced, for example due to impaired supraspinal control, alpha-motor neurons become hyperexcitable leading to abnormal involuntary movements.Ĭontrol of locomotion is extremely complex, and a full discussion of spinal control of gait is beyond the scope of this manuscript. The principal neurotransmitters of spinal interneurons are gamma-aminobutyric acid (GABA) and glycine. Spinal interneurons receive supraspinal descending influences from the motor cortex, brainstem and cerebellum via descending pathways (including corticospinal and rubrospinal pathways), helping to integrate and modulate movement. In another reflex loop, Golgi tendon organs via Ib afferent fibers, along with joint and cutaneous receptors, convey signals to Ib inhibitory interneurons (not shown). The end result is an ipsilateral limb withdrawal from the nociceptive stimuli, while the opposite limb prepares for support. The final output is excitation and inhibition (respectively) of the ipsilateral flexor and extensor muscles, and vice versa on the contralateral side.
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Cutaneous nociceptive receptors send afferent signals via Aδ fibers which synapse with multiple interneurons before finally synapsing with the alpha-motor neuron (Fig. The second major reflex loop is the flexion-withdrawal reflex. An opposite chain is reversed in the opposite limb to prepare for support By this mechanism, flexion of the ipsilateral agonist muscle withdraws the limb from the nociceptive stimuli. The interneurons connect the afferent and efferent signals, resulting in an excitatory signal to the ipsilateral flexor and contralateral extensor muscles, and an inhibitory signal to the ipsilateral extensor and contralateral flexor muscles. Cutaneous nociceptive receptors send afferent signals via Aδ fibers which synapse with multiple interneurons before finally synapsing with the alpha-motor neuron. The cutaneous or flexion-withdrawal reflex is a polysynaptic reflex. The cutaneous or flexion-withdrawal reflex. The alpha-motor neurons, inhibitory interneurons and Renshaw cells also receive supraspinal control via descending tracts such as corticospinal or rubrospinal pathways. It receives input from collateral axons of the alpha-motor neuron controlling agonist muscles, and sends inhibitory signals back to the same alpha-motor neuron and also inhibitory signals to the alpha-motor neuron controlling antagonist muscles. The Renshaw cell is the specialized inhibitory interneuron that functions in feedback control of the alpha-motor neurons in A. The feedback control of the muscle stretch reflex.
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Ia afferent fibers also synapse with Ia inhibitory interneurons, which in turn convey inhibitory signals to antagonist muscles, (reciprocal inhibition).
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The proximal axons then synapse directly with the alpha-motor neuron (a monosynaptic reflex), which in turn conveys an efferent signal to the corresponding extrafusal fibers of the agonist muscle leading to muscle contraction. The afferent signal of the muscle stretch reflex from the muscle spindle is conveyed via Ia afferent fibers which are axons of dorsal root ganglia. Major spinal reflexes include muscle stretch and cutaneous reflexes. The Renshaw cell, another type of spinal interneuron, functions in the feedback system by receiving the signal from the collateral axons of the alpha-motor neuron and sending an inhibitory signal to the same alpha-motor neuron and Ia inhibitory interneuron (Fig.
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Ia afferent fibers also convey signals to Ia inhibitory interneurons to inhibit the antagonist muscle, a phenomenon called reciprocal inhibition (Fig. In the muscle stretch reflex, the afferent signal comes from the muscle spindle via Ia afferent fibers, and an efferent signal is conveyed to the corresponding extrafusal muscle fiber, leading to muscle contraction (Fig. Relay circuits may be monosynaptic (such as the muscle stretch reflex), or polysynaptic, (such as the flexion-withdrawal reflex). Efferent signals originate in the anterior (ventral) horn cells or alpha-motor neurons. Afferent signals are conveyed from the axons of dorsal root ganglia to the spinal cord gray matter.