Porth's Essentials of Pathophysiology, 4e

884

Nervous System

U N I T 1 0

The Stretch Reflex

U N D E R S T A N D I N G

Muscle tone is controlled by the stretch reflex, which monitors changes in muscle length.The activity of the stretch reflex can be divided into three steps: (1) activation of the stretch receptors, (2) integration of the reflex in the spinal cord, and (3) regulation of reflex sensitivity by higher centers in the brain.Testing the (4) knee-jerk reflex provides a means of assessing the stretch reflex.

Spinal Reflex Centers. Afferent impulses from the Ia sensory fiber of the muscle spindle are transmit- ted to the spinal cord, where they synapse with α motor neurons of the stretched muscle to form a mono- synaptic reflex arc. They are “mono- synaptic” because only one synapse separates the primary sensory input from the motor neuron output. The reflex muscle contraction that fol- lows resists further stretching of the muscle. As this spinal reflex activ- ity is occurring, impulses providing information on muscle length are transmitted to higher centers in the brain. It is the coordinated activ- ity of all the monosynaptic reflexes supplying the extrafusal fibers in a skeletal muscle that provides the muscle tone needed for organized movement. Stretch Reflex Receptors. Skeletal muscle is composed of two types of muscle fibers: a large num- ber of extrafusal fibers, which con- trol muscle movement, and a smaller number of intrafusal fibers, which control muscle tone. The intrafusal fibers are encapsulated in sheaths, forming a muscle spindle that runs parallel to the extrafusal fibers. Each intrafusal fiber is innervated by a large Ia sensory nerve fiber, which encircles the central noncontractile portion of the fiber to form the so- called annulospiral ending. Because the spindles are oriented parallel to the extrafusal muscle fibers, stretch- ing of the extrafusal fibers also stretches the spindle fibers and stim- ulates the receptive endings of the Ia afferent neuron. 1 2

Muscle spindle

Intrafusal muscle fiber

Primary sensory (Ia) nerve fiber

Extrafusal muscle fiber

Ascending fibers to brain centers

Ia afferent

Homonymous muscle

α motor neuron