25 - Chapter 25 Synaptic function in the CNS reflexes a...

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Chapter 25. Synaptic function in the CNS: reflexes Simple reflexes have some basic neuronal requirements and properties that are illustrated in Figure 25-1. The simple two- neuronal reflex arc, shown in Figure 25-1a, exhibits the following components: a receptor , in this case a muscle spindle, which generates APs an afferent neuron , transmitting sensory information to the CNS an afferent cell body , located in the dorsal root ganglion a synapse , in the spinal gray matter an efferent neuron , exiting via a ventral nerve root an effector , for example, a neuromuscular junction, innervating the target muscle fibers Understand that while this requirement (and the reflex depicted in Figure 25-1a) describes the minimal path between receptor and effector , the single synapse shown is a gross oversimplification of the complexity of inputs coming to a single efferent neuron (see Chapter 21). Figure 25-1. Components of simple reflexes. The components of the simple monosynaptic reflex are shown in part a. Afferent traffic (APs) originates at a receptor (a), it passes along an afferent (sensory) neuron (b) with its soma (c), and enters the dorsal horn of the gray matter. After synapsing at (d), motor traffic passes through the efferent neuron (e) to supply some effector structure (f). In this case, the effector is an array of motor end plates on certain fibers of a skeletal muscle. This shows the minimal path between a receptor and effector and would be exampled by the reflex involved in the knee jerk when the ligament/tendon just distal to patella is tapped. In b. , the afferent neuron (b) synapses on an internuncial neuron (g) and on an ascending neuron (h) at distinct loci. The basic reflex, from receptor to effector, now involves two synapses (each labeled (d)), and is termed bisynaptic . These reflexes are described more fully in the text. 2007 version – page 195
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Note that the soma of the motor neuron lies close to the synapse within the ventral horn of the spinal gray matter. In this example, the receptive and motor components lie within the same tissue, a skeletal muscle, and the reflex could serve to protect the muscle from undue stretching. During stretching of this muscle (or its tendon), activation of the reflex would elicit a reactive contraction, or shortening of the very same muscle. The reaction could protect the muscle fibers or the tendons from being torn. The synapse is the simple site of integration, whereby sensory information leads to the effector response, being carried
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25 - Chapter 25 Synaptic function in the CNS reflexes a...

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