General Reactions of the Motor Unit

Chapter 27 - Peripheral Nerve and Skeletal Muscle

Douglas C. Anthony MD, PhD

Matthew P. Frosch MD, PhD

Umberto De Girolami MD

Normal

The functional unit of the neuromuscular system is the motor unit, which consists of (1) a lower motor neuron in the anterior horn of the spinal cord or cranial nerve motor nucleus in the

brain stem, (2) the axon of that neuron, and (3) the multiple muscle fibers it innervates ( Fig. 27-1 ). Lower motor neurons are distributed in the anterior horns of the spinal cord in

columns or groups; they are arranged somatotopically so that cells lying medially innervate proximal muscles and those lying laterally supply the distal musculature. The number of

muscle fibers within each unit varies considerably. Muscles with highly refined movements, such as the extrinsic muscles of the eye, have a high neuron-to-muscle-fiber ratio (1:10);

those with relatively coarse and stereotyped movements, such as calf muscles, have a much lower ratio (1:1800).[1]

NORMAL PERIPHERAL NERVE

The principal structural component of peripheral nerve is the nerve fiber (an axon with its Schwann cells and myelin sheath). A nerve consists of numerous fibers that are grouped

Figure 27-1Normal and abnormal motor units. Normal motor units: Two adjacent motor units are shown. Segmental demyelination: Random internodes of myelin are injured and are

remyelinated by multiple Schwann cells, while the axon and myocytes remain intact. Axonal degeneration: The axon and its myelin sheath undergo anterograde degeneration (shown for

the green neuron), with resulting denervation atrophy of the myocytes within its motor unit. Reinnervation of muscle: Sprouting of adjacent (red) uninjured motor axons leads to fiber

type grouping of myocytes, while the injured axon attempts axonal sprouting. Myopathy: Scattered myocytes of adjacent motor units are small (degenerated or regenerated), whereas the

neurons and nerve fibers are normal.

Figure 27-2Electron micrograph of myelinated (arrow) and unmyelinated (arrowhead) fibers in human sural nerve. One Schwann cell nucleus is present.

Figure 27-3Electron micrograph of skeletal muscle in the longitudinal plane. A nucleus is located at the top of the illustration and the sarcomeres of two myofibrils are located below.

The principal components of the sarcomere are identified, creating the pattern of cross-striations. (From Bloom W, Fawcett DW: A Textbook of Histology, 11th ed. Philadelphia, WB

Saunders, 1986.)

TABLE 27-1-- Muscle Fiber Types

Type 1 Type 2

Action Sustained force Sudden movements

Strength Weight-bearing Purposeful motion

Enzyme content NADH dark staining NADH light staining

ATPase at pH 4.2, dark staining ATPase at pH 4.2, light staining

ATPase at pH 9.4, light staining ATPase at pH 9.4, dark staining

Lipids Abundant Scant

Glycogen Scant Abundant

Ultrastructure Many mitochondria Few mitochondria

Wide Z-band Narrow Z-band

Physiology Slow-twitch Fast-twitch

Color Red White

Prototype Soleus (pigeon) Pectoral (pigeon)

consist of specialized muscle and nerve fibers, delimited by a connective tissue capsule.

The connective tissue sheath of muscles includes the endomysium, which surrounds individual muscle fibers; the perimysium, which groups muscle fibers into primary and secondary

bundles (fasciculi); and the epimysium, which envelops single muscles or large groups of fibers.

Pathology

General Reactions of the Motor Unit

The two main responses of peripheral nerve to injury are based on the target of the insult: either the Schwann cell or the axon. Diseases that affect primarily the Schwann cell lead to a

loss of myelin, referred to as segmental demyelination. In contrast, primary involvement of the neuron and its axon leads to axonal degeneration. In some diseases, axonal degeneration

may be followed by axonal regeneration and reinnervation of muscle. The two principal pathologic processes seen in skeletal muscle are denervation atrophy, which follows loss of

axons, and those due to a primary abnormality of the muscle fiber itself, referred to as myopathy. We now consider the general features of these processes.

Date: 2016-04-22; view: 923