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Chapter 12: Muscle: Mechanisms of Contraction and Neural Control


Chapter Summary

Chapter 12: Muscle: Mechanisms of Contraction and Neural Control

Structure and Actions of Skeletal Muscles

I. Skeletal muscles are attached to bones by tendons.

A. Skeletal muscles are composed of separate cells, or fibers, that are attached in parallel to the tendons.

B. Individual muscle fibers are covered by the endomysium; bundles of fibers, called fascicles, are covered by the perimysium; and the entire muscle is covered by the epimysium.

C. Skeletal muscles are striated.

1. The dark striations are called A bands, and the light regions are called I bands.

2. Z lines are located in the middle of each I band.

II. Muscles in vitro can exhibit twitch, summation, and tetanus.

A. The rapid contraction and relaxation of muscle fibers is called a twitch.

B. A whole muscle also produces a twitch in response to a single electrical pulse in vitro.

1. The stronger the electric shock, the stronger the muscle twitch, whole muscles can produce graded contractions.

2. The graded contraction of whole muscles is due to different numbers of fibers participating in the contraction.

C. The summation of fiber twitches can occur so rapidly that the muscle produces a smooth, sustained contraction known as tetanus.

D. When a muscle exerts tension without shortening, the contraction is termed isometric; when shortening does occur, the contraction is isotonic.

III. The contraction of muscle fibers in vivo is stimulated by somatic motor neurons.

A. Each somatic motor axon branches to innervate a number of muscle fibers.

B. The motor neuron and the muscle fibers it innervates are called a motor unit.

1. When a muscle is composed of many motor units (such as in the hand), there is fine control of muscle contraction.

2. The large muscles of the leg have relatively few motor units, which are correspondingly large in size.

3. Sustained contractions are produced by the asynchronous stimulation of different motor units.

Mechanisms of Contraction

I. Skeletal muscle cells, or fibers, contain structures called myofibrils.

A. Each myofibril is striated with dark (A) and light (I) bands; there are Z lines in the middle of each I band.

B. The A bands contain thick filaments, composed primarily of myosin

1. The edges of each A band also contain thin filaments, which overlap with the thick filaments.

2. The central regions of the A bands contain only thick filaments, these regions are the H bands.

C. The I bands contain only thin filaments, composed primarily of actin.

D. Thin filaments are composed of globular actin subunits known as G-actin. A protein known as tropomyosin is also located at intervals in the thin filaments. Another protein, troponin, is attached to the tropomyosin.

II. Myosin cross bridges extend out from the thick filaments to the thin filaments.

A. At rest, the cross bridges are not attached to actin.

1. The cross bridge heads function as ATPase enzymes.

2. ATP is split into ADP and Pi, activating the cross bridge.

B. When the activated cross bridges attach to actin, they undergo a power stroke and in the process release ADP and Pi.

C. At the end of a power stroke, the cross bridge bonds to a new ATP.

1. This allows the cross bridge to detach from actin and repeat the cycle.

2. Rigor mortis is caused by the inability of cross bridges to detach from actin because of a lack of ATP.

III. The activity of the cross bridges causes the thin filaments to slide toward the centers of the sarcomeres.

A. The filaments slide, they do not shorten, during muscle contraction.

B. The lengths of the H and I bands decrease, whereas the A bands stay the same length during contraction.

IV. When a muscle is at rest, the Ca2+ concentration of the sarcoplasm is very low and cross bridges are prevented from attaching to actin.

A. The Ca2+ is actively transported into the sarcoplasmic reticulum.

B. The sarcoplasmic reticulum is a modified endoplasmic reticulum that surrounds the myofibrils.

V. Action potentials are conducted by transverse tubules into the muscle fiber.

A. Transverse tubules are invaginations of the cell membrane that almost touch the sarcoplasmic reticulum.

B. Action potentials in the transverse tubules stimulate the release of Ca2+ from the sarcoplasmic reticulum.

VI. When action potentials cease, Ca2+ is removed from the sarcoplasm and stored in the sarcoplasmic reticulum.

Neural Control of Skeletal Muscles

I. The somatic motor neurons that innervate the muscles are called lower motor neurons.

A. Alpha motoneurons innervate the ordinary, or extrafusal, muscle fibers. These are the fibers that produce muscle shortening during contraction.

B. Gamma motoneurons innervate the intrafusal fibers of the muscle spindles.

II. Muscle spindles function as length detectors in muscles.

A. Spindles consist of several intrafusal fibers wrapped together. These spindle fibers are in parallel with the extrafusal fibers.

B. Stretching of the muscle stretches the spindles, which excites sensory endings in the spindle apparatus.

1. Impulses in the sensory neurons travel into the spinal cord in the dorsal roots of spinal nerves.

2. The sensory neuron makes a synapse directly with an alpha motoneuron within the spinal cord, which produces a monosynaptic reflex.

3. The alpha motoneuron stimulates the extrafusal muscle fibers to contract, thus relieving the stretch. This is called the stretch reflex.

C. The activity of gamma motoneurons tightens the spindles, thus making them more sensitive to stretch and better able to monitor the length of the muscle, even during muscle shortening.

III. The Golgi tendon organs monitor the tension that the muscle exerts on its tendons.

A. As the tension increases, sensory neurons from Golgi tendon organs inhibit the activity of alpha motoneurons.

B. This is a disynaptic reflex, because the sensory neurons synapse with interneurons, which in turn make inhibitory synapses with motoneurons.

IV. A crossed-extensor reflex occurs when a foot steps on a tack.

A. Sensory input from the injured foot causes stimulation of flexor muscles and inhibition of the antagonistic extensor muscles.

B. The sensory input also crosses the spinal cord to cause stimulation of extensor and inhibition of flexor muscles in the contralateral leg.

V. Most of the fibers of descending tracts synapse with spinal interneurons, which in turn synapse with the lower motor neurons.

A. Alpha and gamma motoneurons are usually stimulated at the same time, or coactivated.

B. The stimulation of gamma motoneurons keeps the muscle spindles under tension and sensitive to stretch.

C. Upper motor neurons, primarily in the basal nuclei, also exert inhibitory effects on gamma motoneurons.

VI. Neurons in the brain that affect the lower motor neurons are called upper motor neurons.

A. The fibers of neurons in the precentral gyrus, or motor cortex, descend to the lower motor neurons as the lateral and ventral corticospinal tracts.

1. Most of these fibers cross to the contralateral side in the brain stem, forming structures called the pyramids; this system is therefore called the pyramidal system.

2. The left side of the brain thus controls the musculature on the right side, and vice versa.

B. Other descending motor tracts are part of the extrapyramidal system.

1. The neurons of the extrapyramidal system make numerous synapses in different areas of the brain, including the midbrain, brain stem, basal nuclei, and cerebellum.

2. Damage to the cerebellum produces intention tremor and degeneration of dopaminergic neurons in the basal nuclei produces Parkinson's disease.

Energy Requirements of Skeletal Muscles

I. Aerobic cell respiration is ultimately required for the production of ATP needed for cross-bridge activity.

A. Resting muscles and muscles performing light exercise obtain most of their energy from fatty acids.

B. During moderate exercise, just below the lactate threshold, energy is obtained about equally from fatty acids and glucose.

C. Glucose, from the muscle’s stored glycogen and from blood plasma, becomes an increasingly important energy source during heavy exercise.

D. New ATP cam be quickly produced from the combination of ADP with phosphate derived from phosphocreatine.

E. Muscle fibers are of three types.

1. Slow-twitch red fibers are adapted for aerobic respiration and are resistant to fatigue.

2. Fast-twitch white fibers are adapted for anaerobic respiration.

3. Intermediate fibers are fast-twitch but adapted for aerobic respiration.

II. Muscle fatigue may be caused by a number of mechanisms.

A. Fatigue during sustained maximal contraction may be produced by the accumulation of extracellular K+ as a result of high levels of nerve activity.

B. Fatigue during moderate exercise is primarily a result of anaerobic respiration by fast-twitch fibers.

1. The production of lactic acid lowers the intracellular pH, which inhibits glycolysis and decreases ATP concentrations.

2. Decreased ATP inhibits excitation-contraction coupling, possibly due to a cellular loss of Ca2+.

III. Physical training affects the characteristics of the muscle fibers.

A. Endurance training increases the aerobic capacity of all muscle fiber types, so that their reliance on anaerobic respiration, and thus their susceptibility to fatigue, is reduced.

B. Resistance training causes hypertrophy of the muscle fibers due to an increase in the size and number of myofibrils.

Cardiac and Smooth Muscle

I. Cardiac muscle is striated and contains sarcomeres.

A. In contrast to skeletal muscle, which require neural stimulation to contract, action potentials in the heart originate in myocardial cells; stimulation by neurons is not required.

B. Also unlike the situation in skeletal muscles, action potentials can cross from one myocardial cell to another.

II. Smooth muscle cells lack sarcomeres and are not striated.

A. Smooth muscle cells contain myosin and actin, but these are not arranged in sarcomeres.

B. Myosin filaments are very long, and because of this, smooth muscle cells can contract even when they are greatly stretched.

C. When stimulated by graded depolarizations, Ca2+ enters smooth muscle cells and combines with calmodulin. This activates an enzyme that phosphorylates myosin cross bridges.

D. Unlike the situation in striated muscles, phosphorylation of cross bridges is required for their bonding to actin.

E. Depending upon their neural regulation, smooth muscles can be classified as single-unit or multiunit.

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