Overview of Muscle Tissues

• Muscle is one of the four primary tissue types of the body, and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

• All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane.

• A muscle can return to its original length when relaxed due to a quality of muscle tissue called elasticity.

• Muscle tissue also has the quality of extensibility; it can stretch or extend.

• Contractility allows muscle tissue to pull on its attachment points and shorten with force.

• Skeletal muscle fibers are multinucleated structures that compose the skeletal muscle.

• Cardiac muscle fibers each have one to two nuclei and are physically and electrically connected to each other so that the entire heart contracts as one unit (called a syncytium).

• Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber (which has only a single nucleus) has a uniform, nonstriated appearance (resulting in the name smooth muscle).

Skeletal Muscle

• The best-known feature of skeletal muscle is its ability to contract and cause movement.

• Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture.

• Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium.

• Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium.

• In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones.

Skeletal Muscle Fibers

• Some other terminology associated with muscle fibers is rooted in the Greek sarco, which means “flesh.”

• The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specializedsmooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) is called the sarcoplasmic reticulum (SR).

• The functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins.

The Sarcomere

• Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere.

• Because the actin and its troponin-tropomyosin complex (projecting from the Z-discs toward the center of the sarcomere) form strands that are thinner than the myosin, it is called the thin filament of the sarcomere.

• Because the myosin strands and their multiple heads(projecting from the center of the sarcomere, toward but not all to way to, the Z-discs) have more mass and are thicker, they are called the thick filament of the sarcomere.

The Neuromuscular Junction

• Another specialization of the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ).

• This is where the muscle fiber first responds to signaling by the motor neuron.

Excitation-Contraction Coupling

• This is achieved by opening and closing specialized proteins in the membrane called ion channels.

• Although the term excitation-contraction coupling confuses or scares some students, it comes down to this: for a skeletal muscle fiber to contract, its membrane must first be “excited”—in other words, it must be stimulated to fire an action potential.

• Signaling begins when a neuronal action potential travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ.

• At the NMJ, the axon terminal releases a chemical messenger, or neurotransmitter, called acetylcholine (ACh).

• The ACh molecules diffuse across a minute space called the synaptic cleft and bind to ACh receptors located within the motor end-plate of the sarcolemma on the other side of the synapse.

• As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open.

• For the action potential to reach the membrane of the SR, there are periodic invaginations in the sarcolemma, called T-tubules (“T” stands for “transverse”).

• The arrangement of a T-tubule with the membranes of SR on either side is called a triad.

• The triad surrounds the cylindrical structure called a myofibril, which contains actin and myosin.

Sources of ATP

• Creatine phosphate is a molecule that can store energy in its phosphate bonds.

• Glycolysis is an anaerobic (non-oxygen-dependent) process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate.

• The breakdown of one glucose molecule produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or when oxygen levels are low, converted to lactic acid.

• If oxygen is not available, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue.

• Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP.

• Intense muscle activity results in an oxygen debt, which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction.

Nervous System Control of Muscle Tension

• In isotonic contractions, where the tension in the muscle stays constant, a load is moved as the length of the muscle changes (shortens).

• A concentric contraction involves the muscle shortening to move a load.

• An eccentric contraction occurs as the muscle tension diminishes and the muscle lengthens.

• An isometric contraction occurs as the muscle produces tension without changing the angle of a skeletal joint.

Motor Units

• The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit.

• This increasing activation of motor units produces an increase in muscle contraction known as recruitment.

The Frequency of Motor Neuron Stimulation

• A single action potential from a motor neuron will produce a single contraction in the muscle fibers of its motor unit. This isolated contraction is called a twitch.

• The tension produced by a single twitch can be measured by a myogram, an instrument that measures the amount of tension produced over time.

• The first phase is the latent period, during which the action potential is being propagated along the sarcolemma and Ca++ ions are released from the SR.The contraction phase occurs next. The last phase is the relaxation phase, when tension decreases as contraction stops.

• Normal muscle contraction is more sustained, and it can be modified by input from the nervous system to produce varying amounts of force; this is called a graded muscle response.

• If the fibers are stimulated while a previous twitch is still occurring, the second twitch will be stronger.

• This response is called wave summation, because the excitation-contraction coupling effects of successive motor neuron signaling is summed, or added together.

• If the stimulus frequency is so high that the relaxation phase disappears completely, contractions become continuous in a process called complete tetanus.

Treppe

• The muscle tension increases in a graded manner that to some looks like a set of stairs.

• This tension increase is called treppe, a condition where muscle contractions become more efficient.

Muscle Tone

• Skeletal muscles are rarely completely relaxed, or flaccid. Even if a muscle is not producing movement, it is contracted a small amount to maintain its contractile proteins and produce muscle tone.

• The absence of the low-level contractions that lead to muscle tone is referred to as hypotonia or atrophy, and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis.

Types of Muscle Fibers

• Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP.

• Fast oxidative (FO) fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibers.

• Fast glycolytic (FG) fibers have fast contractions and primarily use anaerobic glycolysis.

Exercise and Muscle Performance

• Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases.

• The reverse, when structural proteins are lost and muscle mass decreases, is called atrophy.

• Age-related muscle atrophy is called sarcopenia.

Endurance Exercise

• Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions.

• The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods.

• The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen and remove metabolic waste.

Cardiac Muscle Tissue

• Cardiac muscle tissue is only found in the heart.

• Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system.

• An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

• A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting.

• This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate.

Smooth Muscle

• Smooth muscle (so-named because the cells do not have striations) is present in the walls of hollow organs like the urinary bladder, uterus, stomach, intestines, and in the walls of passageways, such as the arteries and veins of the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems.

• A dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma.

• Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli.

• This can happen as a subset of cross-bridges between myosin heads and actin, called latch-bridges, keep the thick and thin filaments linked together for a prolonged period, and without the need for ATP.

• A varicosity releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells.

• A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle.

• This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle.

• Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response.

Hyperplasia in Smooth Muscle

• Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size.

• Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia.

Development and Regeneration of Muscle Tissue

• Paraxial mesodermal cells adjacent to the neural tube form blocks of cells called somites.

• A myoblast is a muscle-forming stem cell that migrates to different regions in the body and then fuse(s) to form a syncytium, or myotube.

• A satellite cell is similar to a myoblast because it is a type of stem cell; however, satellite cells are incorporated into muscle cells and facilitate the protein synthesis required for repair and growth.

• If a cell is damaged to a greater extent than can be repaired by satellite cells, the muscle fibers are replaced by scar tissue in a process called fibrosis.

• Smooth muscle tissue can regenerate from a type of stem cell called a pericyte, which is found in some small blood vessels.