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Anatomical Organization of the Nervous System Pt 3

Anatomical Organization of the Nervous System Pt 3 -...

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Anatomical Organization of the Nervous System-Part 3 Jim Sharp DVM, PhD Reflexes A reflex is an involuntary response to a stimulus. The most common reflex that we are familiar with is the “tendon-tap” reflex that the doctor administers when we have an exam. This type of reflex is a stretch reflex . When the tendon is tapped the muscle lengthens, which activates special sensors in the muscle called muscle spindles . This will activate a sensory (afferent) neuron that directly synapses with a lower motor neuron. The action potential along the motor neuron results in the release of the neurotransmitter, acetylcholine, into the synaptic cleft at the motor end plate (myoneural junction). Acetylcholine binds with its receptors on the sarcolemma and results in the depolarization of the muscle cell. This opens voltage gated Ca 2+ channnels and causes the muscle to contract. The stretch reflex is an important postural reflex that alters tension in the muscle in order to maintain constant length. The sensor organ for stretch reflexes is the muscle spindle. The muscle spindles are made of special muscle fibers termed intrafusal fibers and are located near the tendinous insertions of muscles. Muscle spindles tend to be very numerous in muscles that require highly skilled movements, such as muscles of the hand. Muscle spindles consist of a fusiform capsule whose connective tissue bends in with tissue of the surrounding muscle. Generally, there are 2 to 14 intrafusal fibers
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per muscle spindle. The center portion of intrafusal fibers lack actin and myosin, but contain an aggregation of nuclei. There are two kinds of intrafusal fibers, nuclear bag fibers and nuclear chain fibers. Nuclear bag fibers have an expanded central region while chain fibers do not. Extrafusal fibers are normal muscle fibers that surround the muscle spindle. Sensory nerve fibers spiral around the central portions of both types of intrafusal fibers of the muscle spindle in what is termed as an annulospiral ending . There are also secondary sensory endings at a distance from the mid-region of the intrafusal fiber. Nuclear bag fibers respond to active, dynamic stretch while the nuclear chain fibers will respond to a static stretch. When a muscle is stretched, the intrafusal fibers of the muscle spindle are stretched as well. The annulospiral ending is distorted resulting in a membrane depolarization and the afferent neuron is stimulated. The activated afferent nerve synapses directly on a large motor neuron (alpha motor neuron) supplying the stretched muscle. A reflex will be initiated that results in the activation of the motor neurons innervating the extrafusal myofibers of the stretched muscle. The muscle spindle also has motor (efferent) innervation from small motor (gamma) neurons. Cells bodies of gamma motor neurons reside in the ventral grey horn of the spinal cord. Primary motor neurons (upper motor neurons) descending from the brain will synapse on these gamma motor neurons and affect their activity or discharge levels. The gamma motor neurons provide “tone” to the intrafusal fibers of the muscle
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