Muscle and Muscle Tissue
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Muscle and Muscle Tissue Make up about half of total body mass Exerts force by converting chemical energy, ATP, to mechanical energy Muscle tissue is classified based on Shape Number and position of nuclei Presence of striations Voluntary or involuntary control Functional Characteristics of Muscle Tissue Excitability Ability to receive and respond to stimuli Contractility Ability to shorten after adequate stimulation Extensibility Ability to stretch Elasticity Ability to return to its original length after contraction Functions of Muscle Movement Skeletal Cardiac Smooth Maintain posture Stabilize joints Generate heat Types of Muscle Tissue Skeletal muscle Cells are elongated Multinucleated Nuclei are peripherally placed Striated Voluntary They attach to and cover the bony skeleton Cardiac Cells branch Single centrally placed nucleus Striated Involuntary Located in the heart (Tab 9.3) Smooth Spindle shaped fibers Centrally placed nucleus Non-striated Involuntary Located in the walls of hollow visceral organs Types of Smooth Muscle Smooth muscles in different organs differ in: Fiber arrangement and organization Responsiveness to stimuli Innervation Categorized into two main categories both of which are innervated by the ANS and respond to hormonal controls Single-unit smooth muscle Multiunit smooth muscle Single-Unit Smooth Muscle Also called visceral muscle Cell contract rhythmically and as a single unit Cells are connected to each other via gap junctions and are arranged in opposing sheets Exhibit spontaneous action potentials (stress-relaxation response) Multi-unit Smooth Muscle Muscles fibers are independent of each other (Gap junctions rare) Richly innervated and each nerve forms a motor unit with a number of muscle fibers (spontaneous depolarizations are infrequent) Responds to neural stimulation with graded contractions Examples: Arrector pili muscles, eye muscles that adjust pupil size, muscles in large airways and large arteries Gross Anatomy of Skeletal Muscle Each muscle has a nerve and blood supply that allows neural control and ensures adequate nutrient delivery and waste removal. Connective tissue sheaths are found at various structural levels of each muscle: endomysium surrounds each muscle fiber, perimysium surrounds groups of muscle fibers, and epimysium surrounds whole muscles. (Fig 9.2) Attachments span joints and cause movement to occur from the movable bone (the muscle’s insertion) toward the less movable bone (the muscle’s origin) Muscle attachments may be direct or indirect. Microscopic Anatomy of Skeletal Muscle Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma. Myofibrils account for roughly 80% of cellular volume, and contain the contractile elements of the muscle cell. (Tab 9.1) Striations are due to a repeating series of dark A bands and light I bands. Myofilaments make up the myofibrils, and consist of thick and thin filaments. (Fig 9.3) Ultrastructure and Molecular Composition of the Myofilaments There are two types of myofilaments in muscle cells: thick filaments composed of bundles of myosin, and thin filaments composed of strands of actin. Tropomyosin and troponin are regulatory proteins present in thin filaments. (Fig 9.4) The sarcoplasmic reticulum is a smooth endoplasmic reticulum surrounding each myofibril. T tubules are infoldings of the sarcolemma that conduct electrical impulses from the surface of the cell to the terminal cisternae. (Fig 9.5) Sliding Filament Theory The sliding filament model of muscle contraction states that during contraction, the thin filaments slide past the thick filaments. Overlap between the myofilaments increases and the sarcomere shortens Physiology of a Skeletal Muscle Fiber The neuromuscular junction is a connection between an axon terminal and a muscle fiber that is the route of electrical stimulation of the muscle cell. A nerve impulse causes the release of acetylcholine to the synaptic cleft, which binds to receptors on the motor end plate, triggering a series of electrical events on the sarcolemma. (Fig 9.7) Generation of an Action Potential Across the Sarcolemma Like plasma and nerve cell membranes the sarcolemma is polarized The potential difference between the extracellular space and the intracellular space is called the Resting Potential Resting Potential Potential difference is the result of an unequal distribution of ions between the inside and the outside of the cell Typical resting potential (Nerve) is -70mV meaning the inside of the cell is more negative than the outside Difference is due to selective ionic permeability of the cell membrane It is maintained by Na+/K+ pump K+ ion concentration is higher inside than outside Negatively charged proteins are trapped inside the cell Resting membrane potential is created because the membrane is selectively permeable K+ ions K+ ions diffuse down its concentration gradient Na+ is not allowed to enter the cell thus the cell remains polarized Transmission of action potentials lead to disruption of the ionic gradients which must then be restored by the Na+/K+ pump that uses ATP to transport 3 Na+ out for every 2 K+ it transports in Action Potential When a muscle cell receives excitatory impulse of sufficient strength, depolarization occurs The inside of the cell becomes progressively less negative and an action potential is generated Voltage changes on the membrane result in the opening of voltage-gated ion channels (Fig 9.10) An action potential begins when voltage-gated Na+ channels open in response to depolarization Na+ ions rush down its electrochemical gradient into the cell The segment of the cell where this occurs is depolarized The Na+ channels then close Voltage gated K+ ion channels then open K+ ions rush out down its electrochemical gradient The cell is repolarized (returns to a more negative potential) Ionic concentrations of the resting state are restored by Na+/K+ ATPase An action potential is therefore a transient reversal of the resting membrane potential The inside of the cell may become more negative than normal after repolarization (hyperpolarization) Immediately after an action potential, it may become very difficult or impossible to initiate another action potential This period is referred to as the refractory period (relative and absolute refractory periods) An action potential is propagated along the entire sarcolemma All-or-none response An action potential with consistent size and duration is produced only when a threshold membrane potential is reached Physiology of a Skeletal Muscle Fiber Generation of an action potential across the sarcolemma occurs in response to acetylcholine binding with receptors on the motor end plate. It involves the influx of sodium ions, which makes the membrane potential slightly less negative. Excitation-contraction coupling is the sequence of events by which an action potential on the sarcolemma results in the sliding of the myofilaments. Ionic calcium in muscle contraction is kept at almost undetectable levels within the cell through the regulatory action of intracellular proteins. Muscle fiber contraction follows exposure of the myosin binding sites, and follows a series of events (Fig 9.10, 9.11, and 9.12) Contraction of a Skeletal Muscle A motor unit consists of a motor neuron and all the muscle fibers it innervates. It is smaller in muscles that exhibit fine control. The muscle twitch is the response of a muscle to a single action potential on its motor neuron. (Fig 9.13 & 9.14) There are three kinds of graded muscle responses: wave summation, multiple motor unit summation (recruitment), and treppe. (Fig 9.15 & 9.16) Muscle Tone A state of partial contraction exhibited by relaxed muscles Results from spinal reflexes that activate a group of motor units in response to stretch receptor activation in muscles and tendons Does not produce movements Keeps muscles healthy and firm so they can respond when stimulated Helps stabilize joints and maintain posture Isotonic Contraction Same tension Muscle contracts and shortens to move a load Concentric contractions occur when muscle contracts as it shortens (picking a book) Eccentric contractions (more forceful) occurs when the muscle contracts as it lengthens (Fig 9.19) Isometric Contractions Same length Occurs when a muscle tries to move a load that is greater than the force the muscle is able to generate Tension builds up in the muscle but it does not shorten Muscle Metabolism Muscles contain very little stored ATP, and consumed ATP is replenished rapidly through phosphorylation by creatine phosphate, glycolysis and anaerobic respiration, and aerobic respiration. Muscles will function aerobically as long as there is adequate oxygen, but when exercise demands exceed the ability of muscle metabolism to keep up with ATP demand, metabolism converts to anaerobic glycolysis. (Fig 9.20) Muscle fatigue is the physiological inability to contract due to the shortage of available ATP. Oxygen debt is the extra oxygen needed to replenish oxygen reserves, glycogen stores, ATP and creatine phosphate reserves, as well as conversion of lactic acid to pyruvic acid and then to glucose after vigorous muscle activity Heat production during muscle activity is considerable. It requires release of excess heat through homeostatic mechanisms such as sweating and radiation from the skin. Effect of Exercise on Muscles Aerobic, or endurance, exercise promotes an increase in capillary penetration, the number of mitochondria, and increased synthesis of myoglobin, leading to more efficient metabolism, but no hypertrophy. Resistance exercise, such as weight lifting or isometric exercise, promotes an increase in the number of mitochondria, myofilaments and myofibrils, and glycogen storage, leading to hypertrophied cells. Forces of Muscle Contraction Number of muscle fibers stimulated Size of the muscle fiber stimulated Frequency of stimulation Degree of Muscle stretch Length-tension relationship Velocity and Duration of Contraction Velocity and duration of contraction is influenced by: Fiber type Fast and slow fibers exist Difference in speed is dependent on the rate at which myosin ATPase splits ATP Load Recruitment (Fig 9.21) (See table 9.2 for comparison between fast and slow twitch muscle fibers)
