Seeley's A&P Chapter 9 Muscular System

Z Disk

A Z disk is a filamentous network of protein forming a disklike structure for the attachment of actin myofilaments. The arrangementof the actin myofilaments and myosin myofilaments gives the myofibril a banded, or striated, appearance when viewed longi-tudinally. Each isotropic band, or I band, includes a Z disk and extends from each side of the Z disk to the ends of the myosin myofilaments. When seen in longitudinal and cross sections, the I band on each side of the Z disk consists only of actin myofilaments. ( sacromeres extend from Z disks)

Nerves and Blood Vessels

Abundant nerves and blood vessels extend to skeletal muscles. Motor neurons are specialized nerve cells that stimu-late muscles to contract. Their cell bodies are located in the brain and spinal cord, and their axons extend to skeletal muscle fibers through nerves. At the level of the perimysium, the axons of motor neurons branch repeatedly, each branch projecting toward the center of one muscle fiber. The contact points between the axons and the muscle fibers, called synapses or neuromuscular junctions, are described later in the chapter. Each motor neuron innervates more than one muscle fiber, and every muscle fiber receives a branch of an axon. However, most whole muscles are innervated by more than one neuron.An artery and either one or two veins extend together with a nerve through the connective tissue layers of skeletal muscles. Numerous branches of the arteries supply the extensive capillary beds surrounding the muscle fibers, and blood is carried away from the capillary beds by branches of the veins.

Actin and Myosin Myofilaments

Each actin myofilament is composed of two strands of fibrous actin (F actin), a series of tropomyosin molecules, and a series of troponin molecules. The two strands of F actin are coiled to form a double helix, which extends the length of the actin myofilament. Each F actin strand is a polymer of approximately 200 small, globular units called globular actin (G actin) monomers. Each G actin monomer has an active site, to which myosin molecules can bind during muscle contraction. Tropomyosin is an elongated protein that winds along the groove of the F actin double helix. Each tropomyosin molecule is suffi-ciently long to cover seven G actin active sites. Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that has a binding site for Ca2+. The tropo-nin molecules are spaced between the ends of the tropomyosin molecules in the groove between the F actin strands. The complex of tropomyosin and troponin regulates the interaction between active sites on G actin and myosin.Myosin myofilaments are composed of many elongated myosin molecules shaped like golf clubs. Each myosin mol-ecule consists of two myosin heavy chains wound together to form a rod portion lying parallel to the myosin myofilament and two myosin heads that extend laterally. Four light myosin chains are attached to the heads of each myosin molecule. Each myosin myofilament consists of about 300 myosin molecules arranged so that about 150 of them have their heads projecting toward each end. The centers of the myosin myofilaments consist of only the rod portions of the myosin molecules. The myosin heads have three important properties: (1) The heads can bind to active sites on the actin molecules to form cross-bridges (2) the heads are attached to the rod portion by a hinge region that can bend and straighten during contraction; and (3) the heads are ATPase enzymes, which break down aden osine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Anisotropic Band

Each anisotropic band, or A band, extends the length of the myosin myofilaments within a sarcomere. The actin and myosin myofilaments overlap for some distance at both ends of the A band. In a cross section of the A band where actin and myosin myofilaments overlap, each myosin myofilament is visibly surrounded by six actin myofilaments. In the center of each A band is a smaller band called the H zone, where the actin and myosin myofilaments do not overlap and only myosin myofilaments are present. A dark line, called the M line, is in the middle of the H zone and consists of delicate filaments that attach to the center of the myosin myofilaments. The M line helps hold the myosin myofilaments in place, similar to the way the Z disk holds actin myofilaments in place. The numerous myofibrils are oriented within each muscle fiber so that A bands and I bands of parallel myofibrils are aligned and thus produce the striated pattern seen through a microscope.

Skeletal Muscle Fibers

Each skeletal muscle fiber is a single, long, cylindrical cell con-taining several nuclei, which are located around its periphery, near the plasma membrane. A single fiber can extend from one end of a muscle to the other. In most muscles, the fibers range from approximately 1 mm to about 4 cm in length and from 10 μm to 100 μm in diameter. Large muscles contain a large percentage of large-diameter fibers, whereas small, delicate muscles contain a large percentage of small-diameter fibers. However, any given muscle contains a mixture of small- and large-diameter fibers. As seen in a longitudinal section, alternating light and dark bands give the muscle fiber a striated, or striped, appearance

Skeletal Muscle Structure

Each skeletal muscle is a complete organ consisting of cells, called skeletal muscle fibers, associated with smaller amounts of connective tissue, blood vessels, and nerves. The connective tissue fibers that surround a muscle and its internal components extend beyond the center of the muscle to become tendons, which connect muscles to bones or to the dermis of the skin.A muscle is composed of numerous visible bundles called muscle fasciculi. Each fasciculus is surrounded by another, heavier connective tissue layer called the perimyseum. The entire muscle is surrounded by a layer of connective tissue called the epimysium. The epimysium is composed of dense collage-nous connective tissue. Fascia is a general term for con-nective tissue sheets within the body. Muscular fascia (formerly deep fascia), located superficial to the epimysium, separates and compartmentalizes individual muscles or groups of muscles. It consists of dense irregular collagenous connective tissue

Resting Membrane Potential

Electrically excitable cells, like most cells, are polarized. That is, the inside of most plasma membranes is negatively charged compared with the outside. Thus, a voltage difference, or electrical charge difference, exists across each plasma membrane. This charge difference across the plasma membrane of an unstimulated cell is called the resting membrane potential. Action potentials cannot be produced without a resting membrane potential. The resting membrane potential is the result of three factors: (1) The concentration of K+ inside the plasma membrane is higher than that outside the plasma membrane; (2) the concen-tration of Na+ outside the plasma membrane is higher than that inside the plasma membrane; and (3) the plasma membrane is more permeable to K+ than to Na+. Because the concentration gradient for an ion determines whether that ion enters or leaves the cell after its ion channel opens, when voltage-gated Na+ channels open, Na+ moves through the channels into the cell. In a similar fashion, when gated K+ channels open, K+ moves out of the cell. Since excitable cells have many K+ leak ion channels, K+ moves out of the cell faster than Na+ moves into the cell. In addition, negatively charged molecules, such as proteins, are "trapped" inside the cell because the plasma membrane is impermeable to them. For these reasons, the inside of the plasma membrane is more negatively charged than the outside

Smooth Muscle

Location: Walls of hollow organs, blood vessels, eyes, glands, and skin Cell Shape:Spindle-shaped (15-200 μm in length, 5-8 μm in diameter) Nucleus:Single, centrally located Special Cell-to-Cell Attachments:Gap junctions join some visceral smooth muscle cells together Striations: No Control:Involuntary Capable of Spontaneous Contraction: Yes (some smooth muscle) Function: Moving food through the digestive tract, emptying the urinary bladder, regulating blood vessel diameter, changing pupil size, contracting many gland ducts, moving hair, and having many other functions

Skeletal Muscle

Location:Attached to bones Cell Shape:Very long and cylindrical (1 mm-4 cm, or as much as 30 cm, in length, 10 µm-100 μm in diameter) Nucleus: Multiple nuclei peripherally present Striations:Yes Special Cell-to Cell Attachments:No Capable of Spontaneous Contraction: No Function:Body Movement. The primary function of skeletal muscle cells is to generate force by contracting, or shortening. A parallel arrangement of myofila-ments in a sarcomere allows them to interact, which causes muscle contraction

Cardiac Muscle

Location:Heart Cell Shape:Cylindrical and branched (100-500 u.m. in length, 12-20 u.m. in diameter) Nucleous: Single, centrally located Special Cell-to-Cell Attachments: intercalated disks join cells to one another Capable of Spontaneous Contraction: Yes Control:Involuntary Striations: Yes Function: Pumping blood; Contractions provide the major force for propelling blood through blood vessels

Functions of the Muscular System

Most of the body's movements, from the beating of the heart to the running of a marathon, result from muscle contractions. As described in chapter 4, there are three types of muscle tissue: skeletal, smooth, and cardiac. Because skeletal muscle is the most abundant and most studied type, this chapter examines the physi-ology of skeletal muscle in greatest detail. Chapter 10 focuses on the anatomy of the skeletal muscle system. The following list sum-marizes the major functions of all three types of muscle: 1.Movement of the body.Most skeletal muscles are attached to bones and are responsible for the majority of body movements, including walking, running, chewing, and manipulating objects with the hands. 2. Maintenance of posture. Skeletal muscles constantly maintain tone, which keeps us sitting or standing erect. 3.Respiration. Skeletal muscles of the thorax carry out the move-ments necessary for respiration. 4.Production of body heat. When skeletal muscles contract, heat is given off as a by-product. This released heat is critical for maintaining body temperature. 5.Communication.Skeletal muscles are involved in all aspects of communication, including speaking, writing, typing, gesturing, and smiling or frowning. 6.Constriction of organs and vessels. The contraction of smooth muscle within the walls of internal organs and vessels causes those structures to constrict. This constriction can help propel and mix food and water in the digestive tract; remove materials from organs, such as the urinary bladder or sweat glands; and regulate blood flow through vessels. 7.Contraction of the heart. The contraction of cardiac muscle causes the heart to beat, propelling blood to all parts of the body

Histology of Muscle Fibers

Muscle contraction is much easier to understand when we consider the structure of a muscle fiber. The plasma membrane of a muscle fiber is called the sarcolemma. Two delicate connective tissue layers are located just outside the sarco-lemma. The deeper and thinner of the two is the external lamina. It consists mostly of reticular (collagen) fibers and is so thin that it cannot be distinguished from the sarcolemma when viewed under a light microscope. The second layer also consists mostly of reticular fibers, but it is a much thicker layer, called the endomysium. Along the surface of the sarcolemma are many tubelike invaginations of the sarcolemma, called transverse tubules, or T tubules. They occur at regular intervals along the muscle fiber and extend inward, connecting the extracellular environment with the interior of the muscle fiber . The T tubules are also associated with the highly organized smooth endoplasmic reticulum called the sarcoplasmic reticulum in skeletal muscle fibers. Other organelles, such as the numerous mitochondria and energy-storing glycogen granules, are packed into the cell and constitute the cytoplasm, which in muscles is called the sarcoplasm.The sarcoplasm also contains numerous myofibrils, which are bundles of protein filaments. Each myofibril is a threadlike structure, approximately 1-3 μm in diameter, that extends from one end of the muscle fiber to the other. A myofibril contains two kinds of protein filaments, called myofilaments. Actin myofilaments, or thin myofilaments, are approximately 8 nanometers (nm) in diameter and 1000 nm in length, whereas myosin myofilaments, or thick myofilaments, are approximately 12 nm in diameter and 1800 nm in length. The actin and myosin myofilaments form highly ordered units called sarcomeres, which are joined end to end to form the myofibrils

Origin of Muscle Fibers

Muscle fibers develop from less mature, multinucleated cells called myoblasts. The multiple nuclei result from the fusion of myoblast precursor cells, not from the division of nuclei within myoblasts. Myoblasts are converted to muscle fibers as contractile proteins accumulate within their cytoplasm. Shortly after the myoblasts form, nerves grow into the area and innervate the developing muscle fibers.The number of skeletal muscle fibers remains relatively con-stant after birth. Enlargement, or hypertrophy, of muscles after birth in children and adults results from an increase in the size of each muscle fiber, not from a substantial increase in the number of muscle fibers. Similarly, hypertrophy of muscles in response to exercise is due mainly to an increase in muscle fiber size, rather than an increase in number.

General Properties of Muscle

Muscle tissue is highly specialized. It has four major functional properties: contractility, excitability, extensibility, and elasticity. 1.Contractility is the ability of muscle to shorten forcefully. For example, lifting this textbook requires certain muscles to contract When muscle contracts, it either causes the structures to which it is attached to move or increases pressure inside a hollow organ or vessel. Although muscle shortens forcefully during contraction, it lengthens passively; that is, other forces cause it to lengthen, such as gravity, contraction of an opposing muscle, or the pressure of fluid in a hollow organ or vessel. 2.Excitability is the capacity of muscle to respond to a stimulus. Normally, the stimulus is from nerves that we consciously con-trol. For instance, if you decide to wave to a friend, the conscious decision to lift your arm is sent via nerves. Smooth muscle and cardiac muscle can contract without outside stimuli, but they also respond to stimulation by nerves and hormones. 3.Extensibility means a muscle can be stretched beyond its normal resting length and still be able to contract. If you stretch to reach a dropped pencil, your muscles are longer than they are normally but you can still retrieve the pencil. 4.Elasticity is the ability of muscle to recoil to its original resting length after it has been stretched. Taking a deep breath demon-strates elasticity because exhalation is simply the recoil of your respiratory muscles back to the resting position, similar to releasing a stretched rubberband.

Types of Muscle Tissue

Skeletal Muscle, Smooth Muscle, and Cardiac Muscle. Skeletal muscle, with its associated connective tissue, constitutes about 40% of the body's weight and is responsible for locomotion, facial expressions, posture, respiratory functions, and many other body movements. The nervous system voluntarily, or consciously, controls the functions of the skeletal muscles. Smooth muscle is the most widely distributed type of muscle in the body. It is found in the walls of hollow organs and tubes, in the interior of the eye, and in the walls of blood vessels, among other areas. Smooth muscle performs a variety of functions, including pro-pelling urine through the urinary tract, mixing food in the stomach and the small intestine, dilating and constricting the pupil of the eye, and regulating the flow of blood through blood vessels. Cardiac muscle is found only in the heart, and its contractions provide the major force for moving blood through the circulatory system. Unlike skeletal muscle, cardiac muscle and many smooth muscles are autorhythmic; that is, they contract spontaneously at somewhat regular intervals, and nervous or hormonal stimulation is not always required for them to contract. Furthermore, unlike skel-etal muscle, smooth muscle and cardiac muscle are not consciously controlled by the nervous system. Rather, they are controlled invol-untarily, or unconsciously, by the autonomic nervous and endocrine systems

Actin Potentials

The nervous system controls the contraction of skeletal muscles through these axons. Electrical signals, called action potentials, travel from the brain or spinal cord along the axons to muscle fibers and cause them to contract

Ion Channels

The phospholipid bilayer interior is a hydrophobic environment, which inhibits the movement of charged particles, particularly ions, across the membrane; however, the basis of the electrical properties of skeletal muscle cells is the movement of ions across the membrane. There are two types of ion channels: Ligand-gated and voltage-gated. Ligand-gated ion channels open when a ligand, a chemical signal, binds to a receptor that is part of the ion channel. For example, the axons of neurons supplying skeletal muscle fibers release ligands, called neurotransmitters, which bind to ligand-gated Na+ channels in the membranes of the muscle fibers. As a result, the Na+ channels open, allowing Na+ to enter the cell. Voltage-gated ion channels are gated membrane channels that open and close in response to a particular membrane potential. When a neuron or muscle fiber is stimulated, the charge difference changes, and a particular change causes certain voltage-gated ion channels to open or close. The voltage-gated channels that play major roles in an action potential are voltage-gated Na+, K+, and Ca2+ channels. For example, open-ing voltage-gated Na+channels allows Na+ to cross the plasma membrane, whereas opening voltage-gated K+ channels allows K+ to cross and opening Ca2+ channels allows Ca2+ to cross.

Sliding Filament Model

The primary function of skeletal muscle cells is to generate force by contracting, or shortening. A parallel arrangement of myofila-ments in a sarcomere allows them to interact, which causes muscle contraction. This interaction is described by the SLIDING FILAMENT MODEL. It is the shortening of sarcomeres that is responsible for the contrac-tion of skeletal muscles. In a sarcomere, the actin and myosin myo-filaments slide past one another but remain the same length as when the muscle is at rest. When the myofilaments slide past each other and the sarcomeres shorten, the myofibrils also shorten because the myofibrils consist of sarcomeres joined end to end. The myofibrils extend the length of the muscle fibers, and when they shorten the muscle fibers shorten. Groups of muscle fibers make up a muscle fascicle, and several muscle fascicles make up a whole muscle. Therefore, when sarcomeres shorten, myofibrils, muscle fibers, muscle fascicles, and muscles shorten to produce muscle contraction.

Sacromeres

The sarcomere is the basic structural and functional unit of skeletal muscle because it is the smallest portion of skeletal muscle capable of contracting. Each sarcomere extends from one Z disk to an adjacent Z disk

Titin

Titin is one of the largest known proteins, consisting of a single chain of nearly 27,000 amino acids. It attaches to Z disks and extends along myosin myofilaments to the M line. The myosin myofilaments are attached to the titin molecules, which help hold them in position. Part of the titin molecule in the I band functions as a spring, allowing the sarcomere to stretch and recoil.