Muscle Stretch Reflex

The simplest manifestation of muscle spindle function is the muscle stretch reflex. Whenever a muscle is stretched suddenly, excitation of the spindles causes

Figure 54-4

Neuronal circuit of the stretch reflex.

Figure 54-5

Figure 54-4

Neuronal circuit of the stretch reflex.

reflex contraction of the large skeletal muscle fibers of the stretched muscle and also of closely allied synergistic muscles.

Neuronal Circuitry of the Stretch Reflex. Figure 54-4 demonstrates the basic circuit of the muscle spindle stretch reflex, showing a type Ia proprioceptor nerve fiber originating in a muscle spindle and entering a dorsal root of the spinal cord. A branch of this fiber then goes directly to the anterior horn of the cord gray matter and synapses with anterior motor neurons that send motor nerve fibers back to the same muscle from which the muscle spindle fiber originated. Thus, this is a monosynaptic pathway that allows a reflex signal to return with the shortest possible time delay back to the muscle after excitation of the spindle. Most type II fibers from the muscle spindle terminate on multiple interneurons in the cord gray matter, and these transmit delayed signals to the anterior motor neurons or serve other functions.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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