The Motor Unit

When a nerve signal approaches the end of an axon, it spreads out over all of its terminal branches and stimulates all the muscle fibers supplied by them. Thus, these muscle fibers contract in unison. Since they behave as a single functional unit, one nerve fiber and all the muscle fibers innervated by it are called a motor unit. The muscle fibers of a single motor unit are not all clustered together but are dispersed throughout a muscle (fig. 11.6). Thus, when they are stimulated, they cause a weak contraction over a wide area—not just a localized twitch in one small region.

Earlier it was stated that a motor nerve fiber supplies about 200 muscle fibers, but this is just a representative number. Where fine control is needed, we have small motor units. In the muscles of eye movement, for example, there are only 3 to 6 muscle fibers per nerve fiber. Small motor units are not very strong, but they provide the fine degree of control needed for subtle movements. They also have small neurons that are easily stimulated. Where strength is more important than fine control, we have large motor units. The gastrocnemius muscle of the calf, for example, has about 1,000 muscle fibers per nerve fiber.

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 11 Muscular Tissue 413

Table 11.1 Structural Components of a Muscle Fiber

Term

Definition

General Structure and Contents of the Muscle Fiber

Sarcolemma

The plasma membrane of a muscle fiber

Sarcoplasm

The cytoplasm of a muscle fiber

Glycogen

An energy-storage polysaccharide abundant in muscle

Myoglobin

An oxygen-storing red pigment of muscle

T tubule

A tunnel-like extension of the sarcolemma extending from one side of the muscle fiber to the other; conveys electrical signals from the cell surface to its interior

Sarcoplasmic reticulum

The smooth ER of a muscle fiber; a Ca2+ reservoir

Terminal cisternae

The dilated ends of sarcoplasmic reticulum adjacent to a T tubule

Myofibrils

Myofibril

A bundle of protein microfilaments (myofilaments)

Myofilament

A threadlike complex of several hundred contractile protein molecules

Thick filament

A myofilament about 11 nm in diameter composed of bundled myosin molecules

Elastic filament

A myofilament about 1 nm in diameter composed of a giant protein, titin, that emerges from the core of a thick filament and links it to a Z disc

Thin filament

A myofilament about 5 to 6 nm in diameter composed of actin, troponin, and tropomyosin

Myosin

A protein with a long shaftlike tail and a globular head; constitutes the thick myofilament

F actin

A fibrous protein made of a long chain of G actin molecules twisted into a helix; main protein of the thin myofilament

G actin

A globular subunit of F actin with an active site for binding a myosin head

Regulatory proteins

Troponin and tropomyosin, proteins that do not directly engage in the sliding filament process of muscle contraction but regulate myosin-actin binding

Tropomyosin

A regulatory protein that lies in the groove of F actin and, in relaxed muscle, blocks the myosin-binding active sites

Troponin

A regulatory protein associated with tropomyosin that acts as a calcium receptor

Titin

A springy protein that forms the elastic filaments and Z discs

Striations and Sarcomeres

Striations

Alternating light and dark transverse bands across a myofibril

A band

Dark band formed by parallel thick filaments that partly overlap the thin filaments

H band

A lighter region in the middle of an A band that contains thick filaments only; thin filaments do not reach this far into the A band in relaxed muscle

I band

A light band composed of thin filaments only

Z disc

A protein disc to which thin filaments and elastic filaments are anchored at each end of a sarcomere; appears as a narrow dark line in the middle of the I band

Sarcomere

The distance from one Z disc to the next; the contractile unit of a muscle fiber

Large motor units are much stronger, but have larger neurons that are harder to stimulate, and they do not produce such fine control.

One advantage of having multiple motor units in a muscle is that they are able to "work in shifts." Muscle fibers fatigue when subjected to continual stimulation. If all of the fibers in one of your postural muscles fatigued at once, for example, you might collapse. To prevent this, other motor units take over while the fatigued ones rest, and the muscle as a whole can sustain long-term contraction. The role of motor units in muscular strength is discussed later in the chapter.

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