Porth's Essentials of Pathophysiology, 4e

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Disorders of Neuromuscular Function

C h a p t e r 3 6

Knee

Hip

Trunk

Hand

Thalamus

Face

Tongue

Basal ganglia

Internal capsule

MIDBRAIN

Corticospinal tracts

Corticobulbar tract

LOWER MEDULLA

Pyramids

FIGURE 36-3. Upper motor neuron pathways: corticospinal (red) and corticobulbar tracts (blue). (Modified from Bickley LS. Bates’ Guide to Physical Examination and HistoryTaking. 8th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2003:543.)

SPINAL CORD

Lateral corticospinal tract

Lower motor neuron

Anterior corticospinal tract

If the skeletal muscles of the body are to perform nor- mally, the brain must be continually informed of the cur- rent state of contraction, and the muscles must exhibit healthy tone (resistance to active and passive stretch at rest). The first requirement depends on the transmission of information regarding the sense of body position, movement, and muscle tone to the CNS. Information from sensory afferent neurons is relayed to the cerebel- lum and cerebral cortex and is experienced as proprio- ception or the sense of body movement and position, independent of vision. To provide this information, the muscles and their tendons are supplied with two types of receptors: muscle spindles and Golgi tendon organs. The muscle spindles , which are distributed throughout the belly of a muscle, relay information about muscle length and rate of stretch. The Golgi tendon organs are found in muscle tendons and transmit information

about muscle tension or force of contraction at the junc- tion of the muscle and the tendon that attaches to bone. Muscle Spindle and the Stretch Reflex. The muscle spindles consist of a group of specialized miniature skel- etal muscle fibers called intrafusal fibers that are encased in a connective tissue capsule and attached to the extra- fusal fibers of a skeletal muscle. In the center of the receptor area, a large sensory neuron spirals around the intrafusal fiber, forming the so-called primary or annu- lospiral ending (see Understanding the Stretch Reflex and Muscle Tone). The intrafusal muscle fibers function as stretch recep- tors that increase their firing when the muscle is stretched and decrease their firing when the muscle is relaxed. 3 Axons of these spindle fiber neurons enter the spinal cord through several branches of the dorsal root. Some

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