To carry out its functions, muscle tissue must have the properties of [1] responsiveness (excitability) , or sensitivity to stimulation; [2] conductivity , the ability to carry a wave of excitation; [3] contractility , the ability to shorten; [4] extensibility , the ability to lengthen; and [5] elasticity , the tendency to recoil after it is stretched. Unlike other muscle types, skeletal muscle is [6] voluntary , or subject to conscious control, and has [7] striations , visible alternating light and dark bands. The connective tissues of a muscle are called [8] series-elastic components.

A muscle fiber is densely packed with protein microfilaments called [9] myoblasts . The plasma membrane has tunnel-like extensions called [10] transverse (T) tubules that pass through the cell to the other side. The smooth endoplasmic reticulum of a muscle fiber is called its [11] sarcoplasmic reticulum , and it acts as a reservoir for [12] calcium ions. The thick filaments of muscle are composed of many molecules of the protein [13] myosin , whereas the thin filaments are composed mainly of [14] fibrous (F) actin but also of two regulatory proteins, [15] tropomyosin and the smaller [16] troponin . The thick filaments line up with each other and form a dark striation called the [17] A band , while the lighter striations called [18] I bands are occupied only by thin filaments. Each of these light striations is bisected by a dark line called the [19] Z disc . The distance from one of these to the next is one contractile unit of the muscle cell, called a [20] sarcomere .

A muscle fiber cannot contract unless it is stimulated by a [21] somatic motor fiber (two words). One of those nerve fibers and all muscle fibers supplied by its branches act in unison; thus they are called a [22] motor unit . The synapse formed by a nerve fiber and a muscle fiber is called the [23] neuromuscular junction . Its features include a swollen nerve fiber tip called the [24] synaptic knob , secretory organelles within that tip called [25] synaptic vesicles , and a depression in the muscle fiber called the [26] motor end plate . To stimulate the muscle, the nerve fiber releases the chemical [27] acetylcholine (ACh) , which diffuses to the muscle fiber and binds to receptors on its plasma membrane. The enzyme [28] acetylcholinesterase (AChE) subsequently degrades this chemical to stop the stimulation. Before a muscle fiber is stimulated, it has a voltage across its plasma membrane called the [29] resting membrane potential . Most of this voltage is due to the diffusion of [30] potassium (K+) ions out of the cell, while organic ions are retained in the cytoplasm.

In the [31] excitation phase of muscle action, acetylcholine binds to a receptor and produces an [32] end-plate potential across the sarcolemma. This excites voltage-gated ion channels adjacent to the motor end plate, creating [33] action potentials that spread out across the sarcolemma. This wave of excitation travels down the T tubules and opens calcium gates in dilated portions of the sarcoplasmic reticulum (SR) called the [34] terminal cisternae . In excitation-contraction coupling, calcium ions from the SR flood the cytosol and bind to [35] troponin . The protein [36] tropomyosin then moves away from the binding sites on the thin filaments. According to the [37] sliding filament theory, ATP binds to the heads of the [38] myosin molecules. The ATP is hydrolyzed to ADP and phosphate, and the head attaches to the thin filament. ADP and phosphate are then released, and the head flexes, pulling the actin filament along; this is called the [39] power stroke. When a fresh ATP binds to the head, it releases the actin and extends into its original position, ready to repeat the process. To stop the process, the nerve fiber must stop firing and releasing ACh; the enzyme [40] acetylcholinesterase must break down the ACh that is already present, and Ca²⁺ must be returned to the SR.

The minimum stimulus needed to make a muscle contract is called the [41] threshold . After a short delay called the [42] latent period , the muscle exhibits a quick contraction-relaxation cycle called a [43] twitch . These contractions are of uniform strength if the muscle is stimulated at a low frequency, but if the stimulus frequency rises, the muscle shows a phenomenon called [44] treppe . In this case, the muscle contracts a little more strongly each time, but fully relaxes between contractions. At a still higher frequency of stimulation, the muscle cannot relax completely between one stimulus and the next, so twitches build on each other and the muscle develops more and more tension. This phenomenon is called [45] temporal summation, or wave summation , and it produces a state of sustained partial contraction called [46] incomplete tetanus . At extremely high stimulus frequency, a muscle exhibits [47] complete tetanus —a continual, sustained contraction with no relaxation at all until it fatigues. When a muscle shortens while maintaining uniform tension, it exhibits [48] isotonic contraction. If a muscle develops tension while it is getting shorter, it shows concentric contraction. If it develops or maintains tension while getting longer, thus resisting the lengthening, it shows [49] eccentric contraction.

During brief, intense movements such as basketball maneuvers, a muscle may not receive oxygen fast enough for aerobic synthesis of ATP. However, it can convert ADP to ATP by borrowing phosphate groups from an energy-storage molecule called [50] creatine phosphate . When this supply is used up, muscle must fall back on another method of ATP synthesis called [51] anaerobic fermentation . This pathway doesn’t require oxygen. but it does generate a toxic waste product, [52] lactic acid . This lowers the pH of the sarcoplasm and contributes to muscle [53] fatigue . A person’s endurance depends in part on the fastest rate at which he or she can supply oxygen to the muscle tissues; this is called the [54] maximum oxygen uptake . After exercise is over, we tend to continue breathing heavily in order to "repay" an [55] oxygen debt . This includes replacing compound 50, oxidizing compound 52, and other purposes to which oxygen is put. The most fatigue-resistant muscle fibers, called [56] slow-twitch fibers, are important for posture. Faster-acting but more easily fatigued muscle fibers, called [57] fast-twitch fibers, are especially important in stop-and-go activity such as tennis. Muscular strength depends on such factors as the size of the muscles, fatigue, the arrangement of fiber bundles called the [58] fascicles , and the ability to activate additional motor units, called [59] multiple motor unit summation .

Cardiac muscle can be recognized from its striations, branched fibers, and [60] intercalated discs , which consist of mechanical and electrical linkages from cell to cell. Cardiac muscle is described as [61] autorhythmic because it beats at regular time intervals without needing stimulation by the nervous system. Its rhythm is set by a group of cells that form a [62] pacemaker in the heart. The high energy demand of cardiac muscle is reflected in its very large and numerous [63] mitochondria , which compose about 25% of each cardiac muscle cell.

In [64] single-unit smooth muscle, nerve fibers do not synapse with individual muscle fibers but release neurotransmitters in the general area of several fibers. Muscle fibers electrically stimulate each other and contract together. This produces such actions as [65] peristalsis , a wave of contraction involved in swallowing food. There are no Z discs in smooth muscle, but thin filaments attach to [66] dense bodies on the sarcolemma. Whereas skeletal muscle is stimulated by ACh, most smooth muscle is stimulated by a different neurotransmitter. Most calcium for smooth muscle contraction comes from the [67] extracellular fluid , not the sarcoplasmic reticulum, and it binds to [68] calmodulin , not to troponin. Partly because of its latch-bridge mechanism, smooth muscle can remain partially contracted for a prolonged period without nervous stimulation. This partial contraction is called [69] smooth muscle tone . The ability of an organ such as the stomach or urinary bladder to expand so much results partly from the [70] stress-relaxation, or receptive relaxation, response of smooth muscle—its tendency to relax when it is stretched. Unlike skeletal muscle, smooth muscle exhibits [71] plasticity , the ability to adjust its tension to the amount of stretch. Thus, the smooth muscle walls of the stomach and urinary bladder remain firm and partially contracted whether those organs are full or empty.