Porth's Essentials of Pathophysiology, 4e

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Disorders of Neuromuscular Function

C h a p t e r 3 6

and Muscle Tone

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Brain Center Connections. Although a spinal reflex can func- tion independently, its sensitivity is adjusted by higher centers in the brain. Both types of muscle fibers are supplied with motor neurons— the extrafusal fibers with large alpha motor neurons, which produce mus- cle contraction; and the intrafusal fibers with smaller gamma motor neurons, which control the sensitiv- ity of the stretch reflex. Descending fibers of motor pathways synapse with both alpha and gamma motor neurons, and the impulses are sent simultaneously to the large extra- fusal fibers and to the intrafusal fibers to maintain muscle spindle tension (and sensitivity) during mus- cle contraction. The Knee-Jerk Reflex. The knee-jerk reflex that occurs when the knee is tapped with a reflex hammer tests for the intactness of the stretch reflex arc in the quadriceps muscle. Stretching of the extrafusal fibers by tapping with a reflex hammer leads to lengthening of the intrafusal fibers and increased firing of the type Ia afferent neuron. Impulses from the Ia fiber enter the dorsal horn of the spinal cord and make monosynaptic contact with the anterior horn alpha motor neuron supplying the extra- fusal fibers in the quadriceps mus- cle. The resultant reflex contraction (shortening) of the quadriceps mus- cle is responsible for the knee jerk. These muscle reflexes are called deep tendon reflexes (DTRs). They can be checked at the wrists, elbows, knees, and ankles as a means of assessing the components of the stretch reflex at different spinal cord segments.

Descending fibers from brain centers

Intrafusal muscle fiber

Ia afferent

Extrafusal muscle fiber

Gamma motor neuron

α motor neuron

4

Muscle spindle

Ia afferent

Quadriceps

Tendon of quadriceps

α motor neuron