Overview of Muscle Tissues
Types of Muscle Tissue
Skeletal muscle is associated with the bony skeleton, and consists of large cells that bear striations and are controlled voluntarily.
Cardiac muscle occurs only in the heart, and consists of small cells that are striated and under involuntary control.
Smooth muscle is found in the walls of hollow organs, and consists of small elongated cells that are not striated and are under involuntary control.
Functional Characteristics of Muscle Tissue (p. 280)
Excitability, or irritability, is the ability to receive and respond to a stimulus.
Contractility is the ability to contract forcibly when stimulated.
Extensibility is the ability to be stretched.
Elasticity is the ability to resume the cells’ original length once stretched.
Muscle Functions (pp. 280-281; Table 9.3)
Muscles produce movement by acting on the bones of the skeleton, pumping blood, or propelling substances throughout hollow organ systems.
Muscles aid in maintaining posture by adjusting the position of the body with respect to gravity.
Muscles stabilize joints by exerting tension around the joint.
Muscles generate heat as a function of their cellular metabolic processes.
Skeletal Muscle (pp. 281-309; Figs. 9.1-9.23; Tables 9.1-9.3)
Gross Anatomy of Skeletal Muscle (pp. 281-284; Figs. 9.1-9.2; Tables 9.1, 9.3)
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.
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.
Direct - epimysium fused to periosteum or perichondrium.
Indirect - epimysium extends as a tendon or sheetlike aponeurosis attached to periosteum, perichondrium, or the fascia of other muscles.
Microscopic Anatomy of a Skeletal Muscle Fiber (pp. 284-288; Figs. 9.3-9.6; Tables 9.1, 9.3)
Skeletal muscle fibers are long cylindrical cells with multiple nuclei beneath the sarcolemma.
Myofibrils account for roughly 80% of cellular volume, and are the contractile elements of the muscle cell.
Myofibrils consist of repeating units called sarcomeres (the contractile unit of the myofibril), which have overlapping myofilaments connected to Z discs at either end of the sarcomere.
The myofilaments that make up the myofibrils consist of thick (myosin) and thin (actin) filaments.
The Z disc is primarily composed of the protein alpha actinin and connected to Z discs of adjacent myofibrils by intermediate filaments composed of desmin.
The elastic filament titin anchors the thick filaments to Z discs and runs within the thick filaments to the M line.
- Holds thick filaments in place
- Helps muscle spring back into shape after contraction or stretching
Dystrophin links thin filaments to the sarcolemma
Nebulin, myomesin, and vimentin are other proteins that bind filaments or sarcomeres together.
Striations are due to a repeating series of dark A bands (anisotropic, polarize visible light) and light I bands (isotropic, don't polarize visible light).
A bands - where thick and thin filaments overlap
I bands - along Z lines, where only thin filaments are present
Ultrastructure and Molecular Composition of the Myofilaments
Thick filaments are composed of bundles of myosin molecules, which have a head joined to a tail by a flexible hinge region.
Thin filaments are composed of strands of f-actin, each f-actin filament is composed of g-actin subunits.
Tropomyosin and troponin are regulatory proteins present in thin filaments.
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.
The sliding filament model of muscle contraction states that during contraction, the thin filaments slide past the thick filaments.
The thick filaments pull the thin filaments toward the center of the sarcomere.
Overlap between the myofilaments increases as the thin filaments slide they pull the Z discs toward each other and the sarcomere shortens.
All of the sarcomeres along the myofibril shorten (and all of the myofibrils within the muscle fiber shorten) which causes the entire muscle to shorten - this is muscle contraction.
Physiology of a Skeletal Muscle Fiber (pp. 288-294; Figs. 9.7-9.11; Table 9.3)
The neuromuscular junction is a connection between an axon terminal and a muscle fiber where stimulation of the muscle cell to contract occurs.
The neuromuscular junction consists of the plasma membrane of the motor neuron axon terminal, the synaptic cleft, and the motor endplate.
The motor endplate is part of the sarcolemma where chemically regulated ion channels that respond to neural stimulation are found. Junctional folds increase the surface area at the motor endplate.
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.
An action potential, or wave of depolarization of significant strength, opens voltage regulated Ca++ channels in the axon terminal.
Ca++ influx into the axon stimulates fusion of synaptic vesicles with the axon terminal plasma membrane and the release of neurotransmitter (Ach) in the synaptic cleft.
Ach diffuses across the synaptic cleft, binds to receptors on the motor endplate, and opens chemically-regulated ion channels in the sarcolemma.
Ach is broken down by acetylcholine esterase, which terminates stimulation of the sarcolemma
When acetylcholine binding with receptors opens chemically-regulated ion channels in the sarcolemma Na+ ions enter the cell faster than K+ ions exit, which makes the membrane potential slightly less negative (depolarizes the membrane).
Positively charged ions move across the inside of the sarcolemma into more negative areas - this is a wave of depolarization. The depolarization can be measured (just like a resting membrane potential) and is referred to as a graded local potential, or in this specific case, an endplate potential.
Generation of an action potential across the sarcolemma occurs in response to the wave of depolarization reaching a voltage regulated Na+ channel with sufficient strength to open it.
The degree of depolarization required to open a voltage regulated Na+ channel is called threshold (typically 15 - 20 mV above the resting membrane potential).
The influx of Na+ through voltage regulated channels opens voltage regulated K+ channels.
As K+ leaves the cell it becomes repolarized and can be stimulated again.
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.
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. Note the latent period, the period of contraction, and the period of relaxation on the myogram.
There are three kinds of graded muscle responses: wave summation, multiple motor unit summation (recruitment), and treppe.
Wave summation is generated by increasing the frequency of the stimulus.
Multiple motor unit summation or recruitment is generated by increasing the strength of the stimulus (increasing the number of motor neurons firing).
Treppe is its own thing - the response occurs with the frequency and strength of stimulus held constant.
Muscle Response to Increased Frequency of Stimulation: Wave Summation
Muscle Response to Stronger Stimuli: Multiple Motor Unit Summation (Recruitment)
Recruitment of Motor Neurons: The Size Principle
Treppe
Muscle tone is the phenomenon of muscles exhibiting slight contraction, even when at rest, which keeps muscles firm, healthy, and ready to respond.
Isotonic contractions result in movement occurring at the joint and a change in the length of muscles (the force remains constant).
Concentric isotonic contractions - The muscle shortens as it moves the load
Eccentric isotonic contractions - The muslce lengthens as it resists the load
Isometric contractions result in increases in muscle tension, but no lengthening or shortening of the muscle occurs.
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.
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 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.
Force of Muscle Contraction (pp. 304-305; Figs. 9.21-9.22)
As the number of muscle fibers stimulated increases, force of contraction increases.
Large muscle fibers generate more force than smaller muscle fibers.
As the rate of stimulation increases, contractions sum up, ultimately producing tetanus and generating more force.
There is an optimal length-tension relationship when the muscle is slightly stretched and there is slight overlap between the myofibrils.
Velocity and Duration of Muscle Contraction (pp. 305-307; Fig. 9.23; Tables 9.2-9.3)
There are three muscle fiber types: slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers.
Muscle fiber type is a genetically determined trait, with varying percentages of each fiber type in every muscle, determined by specific function of a given muscle.
As load increases, the slower the velocity and shorter the duration of contraction.
Recruitment of additional motor units increases velocity and duration of contraction.
Effect of Exercise on Muscles (pp. 307-309)
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.
Smooth Muscle (pp. 309-313; Figs. 9.24-9.26; Table 9.3)
Microscopic Structure of Smooth Muscle Fibers (pp. 309-311; Figs. 9.24-9.25; Table 9.3)
Smooth muscle cells are small, spindle-shaped cells with one central nucleus, and lack the coarse connective tissue coverings of skeletal muscle.
Smooth muscle cells are usually arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer.
Contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called peristalsis, which propels substances through the organs.
Smooth muscle lacks neuromuscular junctions, but have varicosities instead, numerous bulbous swellings that release neurotransmitters to a wide synaptic cleft.
Smooth muscle cells have a less developed sarcoplasmic reticulum, sequestering large amounts of calcium in extracellular fluid within caveolae in the cell membrane.
Smooth muscle has no striations, no sarcomeres, a lower ratio of thick to thin filaments when compared to skeletal muscle, and has tropomyosin but no troponin.
Smooth muscle fibers contain longitudinal bundles of noncontractile intermediate filaments anchored to the sarcolemma and suurounding tissues via dense bodies.
Contraction of Smooth Muscle (pp. 311-316; Figs. 9.26-9.27; Table 9.3)
Mechanism and Characteristics of Contraction
Smooth muscle fibers exhibit slow, synchronized contractions due to electrical coupling by gap junctions.
Like skeletal muscle, actin and myosin interact by the sliding filament mechanism. The final trigger for contraction is a rise in intracellular calcium level, and the process is energized by ATP.
During excitation-contraction coupling, calcium ions enter the cell from the extracellular space, bind to calmodulin, and activate myosin light chain kinase, powering the cross-bridging cycle.
Smooth muscle contracts more slowly and consumes less ATP than skeletal muscle.
Regulation of Contraction
Neural Regulation:
Autonomic nerve endings release either acetylcholine or norepinephrine.
May see action potentials generated by neurotransmitter binding or may see graded local potentials, depending on the type of muscle.
May see excitation of certain groups of smooth muscle cells or inhibition of others by the same neurotransmitter, depending on the receptor subtype present on the surface of the cells.
Hormones and Local Factors:
Smooth muscle may have no nerve supply and only depolarize spontaneously or in response to chemicals binding to receptors linked to G-proteins.
Some smooth muscle will respond both to neural and chemical stimuli.
Lack of oxygen, histamine, excess carbon dioxide, or low pH, may act as signals for contraction without stimulating an action potential by affecting Ca++ entry into the sarcoplasm.
Special Features of Smooth Muscle Contraction
Smooth muscle initially contracts when stretched, but contraction is brief, and then the cells relax to accommodate the stretch.
Smooth muscle stretches more and generates more tension when stretched than skeletal muscle.
Hyperplasia, an increase in cell number through division, is possible in addition to hypertrophy, an increase in individual cell size.
Types of Smooth Muscle (p. 316)
Single-unit smooth muscle, called visceral muscle, is the most common type of smooth muscle. It contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials.
Multiunit smooth muscle is located in large airways to the lungs, large arteries, arrector pili muscles in hair follicles, and the iris of the eye. It consists of cells that are structurally independent of each other, has motor units, and is capable of graded contractions.
Developmental Aspects of Muscles (pp. 316-320)
Nearly all muscle tissue develops from specialized mesodermal cells called myoblasts.
Skeletal muscle fibers form through the fusion of several myoblasts, and are actively contracting by week 7 of fetal development.
Myoblasts of cardiac and smooth muscle do not fuse but form gap junctions at a very early stage.
Muscular development in infants is mostly reflexive at birth, and progresses in a head-to-toe and proximal-to-distal direction.
Women have relatively less muscle mass than men due to the effects of the male sex hormone testosterone, which accounts for the difference in strength between the sexes.
Muscular dystrophy is one of the few disorders that muscles experience, and is characterized by atrophy and degeneration of muscle tissue. Enlargement of muscles is due to fat and connective tissue deposit.
