Whole Skeletal Muscle Anatomy (176) Skeletal muscle with its associated connective tissue, constitutes approximately 40% of body weight. Skeletal muscle is so named because many of the muscles are attached to the skeletal system. However, some skeletal muscle attaches to the skin or connective tissue sheets. Skeletal muscle is also called striated muscle because transverse bands, or striations, can be seen in the muscle under the microscope. Individual skeletal muscles, such as the biceps brachii, are complete organs. Recall from chapter 1 that an organ consists of two or more tissue types working together for a common function. A skeletal muscle consists of skeletal muscle tissue, nervous tissue, connective tissue, and adipose tissue. For our purposes, we will start our discussion of skeletal muscle anatomy and physiology at the whole muscle (organ) level and conclude at the cellular level. Each muscle cell is called a muscle fiber. Connective Tissue Coverings (176) Each skeletal muscle is surrounded by several connective tissue layers that support the muscle during contraction. A skeletal muscle has three layers of connective tissue: (1) the epimysium, (2) the perimysium, and (3) the endomysium. The epimysium (ep-ih-MIS-ee-um; mys, muscle) forms a connective tissue sheath that surrounds each skeletal muscle. The protein fibers of the epimysium gradually merge with a layer of connective tissue between adjacent muscles and connecting the skin to superficial muscles. These outer layers of connective tissue separate the muscles from nearby structures. The perimysium (PER-i-MIS-ee-um, PER-i-MIZ-ee-um) subdivides each whole muscle into numerous, visible bundles of muscle fibers (cells) called fascicles (FAS-i-kuhls). The perimysium is a loose connective tissue serving as passageways for blood vessels and nerves that supply each fascicle. The endomysium (EN-doh-MIS-eeum) is a delicate layer of connective tissue that separates the individual muscle fibers within each fascicle. The endomysium serves as passageways for nerve fibers and blood vessels that supply each separate muscle fiber. The protein fibers of the three layers of connective tissue blend into one another and merge at the ends of most muscles to form tendons, which attach muscle to bone Action Potentials (183) An action potential occurs when the excitable cell is stimulated. The action potential is a reversal of the resting membrane potential such that the inside of the cell membrane becomes positively charged compared with the outside. This charge reversal occurs because ion channels open when a cell is stimulated. The diffusion of ions through these channels changes the charge across the cell membrane and produces an action potential. An action potential lasts from approximately 1 millisecond to a few milliseconds, and it has two phases: (1) depolarization and (2) repolarization. Figure 7.7 shows the changes that occur in the membrane potential during an action potential. ❶ Before a neuron or a muscle fiber is stimulated, the gated Na+ and +ion channels are closed. ❷ When the cell is stimulated, ligand-gated Na+ channels open and Na+ diffuses into the cell. The positively charged Na+ makes the inside of the cell membrane depolarized (more positive). If the depolarization causes the membrane potential to reach threshold, an action potential is triggered. Threshold is the membrane potential at which gated Na+ channels open. The depolarization phase of the action potential is a brief period during which further depolarization occurs and the inside of the cell becomes even more positively charged. ❸ As the inside of the cell becomes positive, this voltage change causes additional permeability changes in the cell membrane, which stop depolarization and start repolarization. The repolarization phase is the return of the membrane potential to its resting value. It occurs when ligand-gated Na+ channels close and gated K+ channels open. When K+ moves out of the cell, the inside of the cell membrane becomes more negative and the outside becomes more positive. The action potential ends, and the resting membrane potential is reestablished by the sodium-potassium pump. Next, we consider the specific communication between a motor neuron and the skeletal muscle fiber. Muscle Relaxation (189) Muscle relaxation occurs when acetylcholine is no longer released at the neuromuscular junction. The lack of action potentials along the sarcolemma stops Ca2+ release from the sarcoplasmic reticulum and Ca2+ is actively transported back into the sarcoplasmic reticulum. As the Ca2+ concentration decreases in the sarcoplasm, the Ca2+ diffuses away from the troponin molecules and tropomyosin again blocks the attachment sites on the acting molecules. As a consequence, cross-bridges cannot re-form and the muscle relaxes. Thus, energy is needed for both muscle contraction and relaxation. Three major ATP-dependent events are required for muscle relaxation: 1. After an action potential has occurred in the muscle fiber, the sodium-potassium pump must actively transport Na+ out of the muscle fiber and K+ into the muscle fiber to return to and maintain resting membrane potential. 2. ATP is required to detach the myosin heads from the attachment sites for the recovery stroke. 3. ATP is needed for the active transport of Ca2+ into the sarcoplasmic reticulum from the sarcoplasm. Because the return of Ca2+ into the sarcoplasmic reticulum is much slower than the diffusion of Ca2+ out of the sarcoplasmic reticulum, a muscle fiber takes at least twice as long to relax as it does to contract. Types of Muscle Contractions (190) There are two major types of muscle contractions: (1) isometric contractions and (2) isotonic contractions. The type of muscle contraction differs based on whether the muscle is able to shorten when it contracts. In isometric (eye-soh-MET-rik) contractions, the muscle does not shorten. This type of contraction increases the tension in the muscle, but the length of the muscle stays the same. Isometric contractions happen if you try to lift something that is far too heavy for you. They also happen when you stand still and your postural muscles hold your spine erect. The postural muscles don’t shorten because they are held in place by bones that don’t move when standing still. In isotonic (EYE-soh-TON-ik) contractions, the muscle shortens. This type of contraction increases the tension in the muscle and decreases the length of the muscle. Isotonic contractions happen any time you move your limbs in order to lift an object and move it. Because an isotonic contraction results in shortening of the muscle, an isotonic twitch may change its overall characteristics depending on how much force is needed to lift the weight. For example, if the weight is increased, the lag phase may increase because it takes the muscle longer to generate enough force to lift the weight. If the weight exceeds the amount of force the muscle can generate, the contraction becomes an isometric contraction. Thus, we see that a twitch is not representative of everyday movements, which are smooth with varying rates, but this is still a helpful way to study muscle contraction. The change in muscle contraction strength depends on two factors: (1) the amount of force in an individual muscle fiber, called summation, and (2) the amount of force in a whole muscle, called recruitment. Increasing force in a whole muscle depends on the total number of muscle fibers contracting. Keep in mind, twitch refers to the contraction of a single muscle fiber, a motor unit, or a whole muscle. Before we examine the responses of individual fibers, a definition of motor unit will be helpful. Anaerobic Respiration (193) Anaerobic (an-uh-ROH-bik) respiration does not require O2 and involves the breakdown of glucose to produce ATP and lactate. Anaerobic respiration produces only enough ATP to power muscle contractions for 30–40 seconds. It’s important to note that exercise is not usually exclusively limited to one type of ATP production, such as anaerobic respiration. Later we will discuss the blending of the four ATP production pathways under typical muscle contraction conditions. Anaerobic respiration produces far less ATP than other pathways, but can produce ATP in a matter of a few seconds. The first step of anaerobic respiration is the enzymatic pathway, called glycolysis (glye-KOHL-ih-sis). In glycolysis, one glucose molecule is broken down into two molecules of pyruvate, producing a net gain of two ATP molecules. In anaerobic respiration, the pyruvic acid is then converted to a molecule called lactate (figure 7.13c). Historically, ATP production in skeletal muscle was thought to be clearly delineated into purely aerobic activities versus purely anaerobic activities. It was also believed that the product of anaerobic respiration was principally lactic acid, which was considered to be a harmful waste product that must be removed from the body. However, it is now widely recognized that anaerobic respiration ultimately gives rise to lactic acid’s alternate chemical form, the conjugate base, lactate. Moreover, it is now known that lactate is a critical metabolic intermediate that is formed and used continuously even under fully aerobic conditions. Lactate is produced by skeletal muscle cells at all times, but particularly during exercise, and is subsequently broken down (65–70%) or used to make new glucose (30–35%). Thus, the aerobic and anaerobic mechanisms of ATP production are linked through lactate Aerobic Respiration (194) Aerobic (ai-ROH-bik) respiration requires O2 and breaks down glucose to produce ATP, CO 2, and H2O (figure 7.13d). The ATP from aerobic respiration supplies 95% of the total ATP required by a cell and provides enough ATP for hours of muscle contraction as long as O2 is readily available. Aerobic respiration occurs mostly in the mitochondria and is much more efficient than anaerobic respiration. With aerobic respiration pathways, the breakdown of a single glucose molecule produces up to 36 ATP, 18 times more than anaerobic respiration. The first step in aerobic respiration is also glycolysis. However, instead of converting the two pyruvic acid molecules into lactate, as in anaerobic respiration, the two pyruvic acid molecules are transported into the mitochondria where the bulk of the ATP is produced.
