Review of Key Concepts
Types and Characteristics of Muscular Tissue (p. 408)
- Muscular tissue has the properties of responsiveness, conductivity, contractility, extensibility, and elasticity.
- Skeletal muscle is voluntary striated muscle that is usually attached to one or more bones.
- A skeletal muscle cell, or muscle fiber, is a threadlike cell typically 100 ^m in diameter and 3 cm long.
Microscopic Anatomy of Skeletal Muscle (p. 409)
- A muscle fiber forms by the fusion of many stem cells called myoblasts, and is thus multinucleate.
- The sarcolemma (plasma membrane) exhibits tunnel-like infoldings called transverse (T) tubules that cross from one side of the cell to the other.
- The sarcoplasm (cytoplasm) is occupied mainly by protein bundles called myofibrils. Mitochondria, glycogen, and myoglobin are packed between the myofibrils.
- The fiber has an extensive sarcoplasmic reticulum (SR) that serves as a Ca2+ reservoir. On each side of a T tubule, the SR expands into a terminal cisterna.
- A myofibril is a bundle of two kinds of protein myofilaments called thick and thin filaments.
- Thick filaments are composed of bundles of myosin molecules, each of which has a filamentous tail and a globular head.
- Thin filaments are composed mainly of a double strand of actin, with a myosin-binding active site on each of its globular subunits. In the groove between the two actin strands are two regulatory proteins, tropomyosin and troponin.
- Elastic filaments composed of titin run through the core of a thick filament and attach to Z discs.
- Skeletal and cardiac muscle exhibit alternating light and dark bands, or striations, that result from the pattern of overlap between thick and thin filaments. The principal striations are a dark A band with a light H zone in the middle, and a light I band with a dark line, the Z disc, in the middle. 10. The functional unit of a muscle fiber is the sarcomere, which is a segment from one Z disc to the next.
The Nerve-Muscle Relationship (p. 412)
- Skeletal muscle contracts only when it is stimulated by a somatic motor nerve fiber.
- One somatic motor fiber branches at the end and innervates from 3 to 1,000 muscle fibers. The nerve fiber and its muscle fibers are called a motor unit. Small motor units (few muscle fibers per nerve fiber) are found in muscles where fine control of movement is important, and large motor units in muscles where strength is more important than precision.
- The point where a nerve fiber meets a muscle fiber is a type of synapse called the neuromuscular junction. It consists of the synaptic knob (a dilated tip of the nerve fiber) and a motor end plate (a folded depression in the sarcolemma). The gap between the knob and end plate is the synaptic cleft.
- Synaptic vesicles in the knob release a neurotransmitter called acetylcholine (ACh), which diffuses across the cleft and binds to ACh receptors on the end plate.
- An unstimulated nerve, muscle, or other cell has a difference in positive and negative charges on the two sides of its plasma membrane; it is polarized. The charge difference, called the resting membrane potential, is typically about —90 mV on a muscle fiber.
- When a nerve or muscle fiber is stimulated, a quick, self-propagating voltage shift called an action potential occurs. Action potentials form nerve signals and activate muscle contraction.
Behavior of Skeletal Muscle Fibers (p. 416)
- The first stage of muscle action is excitation. An arriving nerve signal triggers ACh release, ACh binds to receptors on the motor end plate and triggers a voltage change called an end-plate potential (EPP), and the EPP triggers action potentials in adjacent regions of the sarcolemma.
- The second stage is excitation-contraction coupling. Action potentials spread along the sarcolemma and down the T tubules, and trigger Ca2+ release from the terminal cisternae of the SR. Ca2+ binds to troponin of the thin filaments, and tropomyosin shifts position to expose the active sites on the actin.
- The third stage is contraction. A myosin head binds to an active site on actin, flexes, tugs the thin filament closer to the A band, then releases the actin and repeats the process. Each cycle of binding and release consumes one ATP.
- The fourth and final stage is relaxation. When nerve signals cease, ACh release ceases. The enzyme acetylcholinesterase degrades the ACh already present, halting stimulation of the muscle fiber. The SR pumps Ca2+ back into it for storage. In the absence of Ca2+, tropomyosin blocks the active sites of actin so myosin can no longer bind to them, and the muscle relaxes.
- Overly contracted and overly stretched muscle fibers respond poorly to stimulation. A muscle responds best when it is slightly contracted before it is stimulated, so that there is optimal overlap between the resting thick and thin filaments. This is the length-tension relationship. Muscle tone maintains an optimal resting length and readiness to respond.
Behavior of Whole Muscles (p. 423)
1. A stimulus must be of at least threshold strength to make a muscle
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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
Chapter 11 Muscular Tissue 439
- After a short latent period, the muscle responds to a single stimulus with a brief contraction called a twitch.
- A single twitch does no useful work for the body. In recruitment, however, multiple motor units are activated at once to produce a stronger muscle contraction. In high-frequency stimulation, successive twitches become progressively stronger; this is called treppe when the muscle completely relaxes between twitches and incomplete tetanus when it relaxes only partially and each twitch "piggybacks" on the previous ones to achieve greater tension.
- In isometric contraction, a muscle develops tension without changing length; in isotonic contraction, it changes length while maintaining constant tension. In concentric contraction, a muscle maintains tension as it shortens; in eccentric contraction, it maintains tension as it lengthens.
Muscle Metabolism (p. 427)
- A muscle must have ATP in order to contract. It generates ATP by different mechanisms over the duration of a period of exercise.
- At the outset, muscle uses oxygen from its myoglobin to generate ATP by aerobic respiration.
- As the stored oxygen is depleted, muscle regenerates ATP from ADP by adding a phosphate (Pi) to it. It gets this Pi either from another ADP, using the enzyme myokinase to transfer the phosphate, or from creatine phosphate, using the enzyme creatine kinase to do so. This is the phosphagen system for regenerating ATP.
- Further into an exercise, as the phosphagen system is depleted, a muscle shifts to anaerobic fermentation (the glycogen-lactic acid system).
- Still later, the respiratory and circulatory systems may catch up with the demands of a muscle and deliver enough oxygen for aerobic respiration to meet the muscle's ATP demand.
- Muscle fatigue results from several factors: ATP and ACh depletion, loss of membrane excitability, lactic acid accumulation, and central nervous system mechanisms.
- The ability to maintain high-intensity exercise depends partly on one's maximum oxygen uptake, which varies with body size, age, sex, and physical condition.
- Prolonged exercise produces an oxygen debt that is "repaid" by continued heavy breathing after the exercise is over. The extra O2 breathed during this time goes mainly to restore oxygen reserves in the myoglobin and blood, replenish the phosphagen system, oxidize lactic acid, and meet the needs of a metabolic rate elevated by the high post-exercise body temperature.
- Slow oxidative muscle fibers are adapted for aerobic respiration and relatively resistant to fatigue, but produce relatively slow responses. Fast glycolytic muscle fibers respond more quickly but fatigue sooner. Intermediate fibers are relatively rare but combine fast responses with fatigue resistance.
- The strength of a muscle depends on its size, fascicle arrangement, size of its motor units, multiple motor unit summation, temporal summation of twitches, prestimulation length, and fatigue.
- Resistance exercise stimulates muscle growth and increases strength; endurance exercise increases fatigue resistance.
Cardiac and Smooth Muscle (p. 432)
- Cardiac muscle consists of relatively short, branched, striated cells joined physically and electrically by intercalated discs.
- Cardiac muscle is autorhythmic and thus contracts even without innervation.
- Cardiac muscle is rich in myoglobin, glycogen, and large mitochondria, uses aerobic respiration almost exclusively, and is very fatigue-resistant.
- Smooth muscle consists of short, fusiform, nonstriated cells.
- Smooth muscle has no T tubules and little sarcoplasmic reticulum; it gets Ca2+ from the extracellular fluid.
- In multiunit smooth muscle, each cell is separately innervated by an autonomic nerve fiber and contracts independently. In single-unit smooth muscle, the muscle cells are connected by gap junctions and respond as a unit. Nerve fibers do not synapse with any specific muscle cells in the latter type.
- In smooth muscle, Ca2+ binds to calmodulin rather than troponin. This activates a kinase, which phosphorylates myosin and triggers contraction.
- Smooth muscle lacks Z discs. Its myofilaments indirectly pull on dense bodies and cause the cell to contract in a twisting fashion.
- Smooth muscle has a latch-bridge mechanism that enables it to maintain tonic contraction with little ATP expenditure.
- Smooth muscle is not subject to the length-tension relationship. Its unusual ability to stretch and maintain responsiveness allows such organs as the stomach and urinary bladder to expand greatly without losing contractility.
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