Anatomy and Physiology: Muscle Tissue

Define isotonic and isometric contraction.

- Isotonic contraction An isotonic contraction is a contraction that occurs when the tension in the muscle remains the same but the muscle shortens. An example of an isotonic contraction is when you lift a textbook from a table. - Isometric contraction An isometric contraction is a contraction that occurs when tension is applied to a muscle but it does not shorten. An example of an isometric contraction is when you carry a box of books.

Define muscle tone and note how it normally works in body posture maintenance.

A sustained partial contraction of portions of a relaxed skeletal muscle results in a firmness known as muscle tone. At any given moment, a few muscle fibres within a muscle are contracted, while most are relaxed. This small amount of contraction is essential for maintaining posture.

Define a twitch and discuss its component parts.

A twitch contraction is a brief contraction of all the muscle fibres in a motor unit in response to a single action potential. A twitch contraction includes three periods: latent, contraction, and relaxation.

Describe the microscopic anatomy of a skeletal muscle fibre.

Skeletal muscle fibres arise from myoblasts. A few myoblasts persist in mature skeletal muscle as satellite cells. Skeletal muscle consists of fibres (cells) covered by a sarcolemma. T tubules are tiny invaginations of the sarcolemma that quickly spread the muscle action potential to all parts of the muscle fibre. Sarcoplasm is the muscle cell cytoplasm, which contains a large amount of glycogen for energy production and myoglobin for oxygen storage. Each fibre contains myofibrils that consist of thin and thick filaments (myofilaments). The sarcoplasmic reticulum encircles each myofibril. It is similar to smooth endoplasmic reticulum in nonmuscle cells; and in the relaxed muscle, it stores calcium ions. Myofibrils are composed of thick and thin filaments arranged in units called sarcomeres. Sarcomeres are the basic functional units of a myofibril, and show distinct dark (A band) and light (I band) areas. A Z disc passes through the centre of the I band.

Summarize the principal characteristics of the three types of muscle.

Table 8.1 summarizes the principal characteristics of the three types of muscle.

List the ways skeletal muscles are named.

Table 8.2 summarizes the characteristics used to name skeletal muscles. Review this list to better understand how skeletal muscle is named on the basis of direction, size, shape, action, number, location, origin, and insertion.

What is the purpose of tendons and aponeuroses with respect to muscle contraction in skeletal muscle?

Tendons and aponeuroses attach muscle to bone or muscle to other muscle.

Identify the epimysium, perimysium and edomysium.

epimysium—covers the entire muscle perimysium—covers the fasciculi endomysium—covers individual muscle fibres

List the major structural and functional differences among the three types of muscle tissue.

The major structural and functional differences among the three types of muscle tissue are the following: Skeletal muscle tissue is primarily attached to bones. It is striated and voluntary. Cardiac muscle tissue forms the wall of the heart. It is striated and involuntary. Smooth (visceral) muscle tissue is located in the viscera. It is nonstriated (smooth) and involuntary.

Describe the structural and functional characteristics that allow skeletal muscle to be classified into three main types.

All skeletal muscle fibres are not identical in structure or function. Colour varies according to the content of myoglobin. Fibre diameter varies as do the cell's allocations of mitochondria, blood capillaries, and sarcoplasmic reticulum. Contraction velocity and resistance to fatigue also differ between fibres. On the basis of structure and function, skeletal muscle fibres are classified as follows: slow oxidative (lots of myoglobin) fast oxidative-glycolytic (mixture) fast glycolytic fibres (appear white, less myoglobin)

Describe the structure and function of cardiac muscle.

Cardiac muscle tissue is found only in the heart wall. Its fibres are arranged similarly to skeletal muscle fibres. Cardiac muscle fibres connect to adjacent fibres by intercalated discs that contain desmosomes and gap junctions. Cardiac muscle contractions last longer than the skeletal muscle twitch due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid. Cardiac muscle fibres contract when stimulated by their own autorhythmic fibres. This continuous rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.

Describe the connective tissue coverings which support skeletal muscle.

Connective tissue coverings extend from the deep fascia as follows: epimysium—covers the entire muscle perimysium—covers the fasciculi endomysium—covers individual muscle fibres

Identify the contractile elements of skeletal muscle.

Contractile proteins generate force during contraction. Myosin, the main component of thick filaments, functions as a motor protein. Motor proteins push or pull their cargo to achieve movement by converting energy from ATP into the mechanical energy of motion or force. Actin, the main component of thin filaments, have myosin binding sites where myosin "heads" attach to produce the sliding together of the filaments. The regulatory proteins tropomyosin and troponin are a part of the thin filament. In relaxed muscle, tropomyosin, which is in held in place by troponin, blocks the myosin-binding sites on actin to prevent myosin from binding to actin. During contraction, myosin heads pull on the actin and shorten the muscle cell. This process is the sliding-filament mechanism.

Outline the steps involved in the sliding filament mechanism of muscle contraction.

During muscle contraction, myosin-cross-bridges pull on thin filaments, causing them to slide inward toward the H zone; Z discs come toward each other, and the sarcomere shortens, but the thick and thin filaments do not change in length. The sliding of filaments and the shortening of sarcomeres causes the shortening of the whole muscle fibre and ultimately the entire muscle. This process is called the sliding-filament mechanism. At the beginning of contraction, the sarcoplasmic reticulum releases calcium ions that bind to troponin and cause the troponin-tropomysium complex to uncover the myosin-binding sites on actin. When the binding sites are "free," the contraction cycle begins. The contraction cycle is a repeating sequence of events that cause the filaments to slide. It includes ATP hydrolysis, the attachment of myosin to actin to form cross bridges, the power stroke, and the detachment of myosin from actin. An increase in calcium ion concentration in the cytosol starts muscle contraction; a decrease stops it. The muscle action potential releases calcium ions from the sarcoplasmic reticulum that combine with troponin, causing it to pull on tropomyosin to change its orientation. Thus, this process exposes the myosin-binding sites on actin and enables the actin and myosin to bind together. The use of calcium ions to remove the contraction inhibitor, and the joining of actin and myosin constitute the excitation-contraction coupling—the steps that connect excitation (a muscle action potential propagation through the T tubules) to the contraction of the muscle fibre. Calcium ions are returned to the sarcoplasmic reticulum by means of active transport pumps.

Describe the four characteristics of muscle tissue.

Excitability: Muscle tissue has the capacity to respond to the stimulation of nervous impulses or hormones. Contractility: Muscle tissue has the capacity to shorten or contract with force. Extensibility: Muscle tissue can stretch beyond (within reason) resting length. Elasticity: Muscle tissue returns (recoils) to resting length after being stretched.

Describe the neuromuscular junction and motor unit.

Muscle action potentials arise at the neuromuscular junction (NMJ), the synapse between a somatic motor neuron and a skeletal muscle fibre. A motor unit is a nerve and the muscle fibres it stimulates. A synapse is a region of communication between two neurons or a neuron and a target cell. The two neurons or a neuron and a target cell are separated by a gap, or a synaptic cleft. Neurotransmitters bridge that gap. The neurotransmitter at a NMJ is acetylcholine (ACh). A nerve action potential elicits a muscle action potential through the release of acetylcholine, the activation of ACh receptors, the production of a muscle action potential, and the termination of ACh activity. VIEW FIG 8.7

What is the purpose of nerves and blood with respect to skeletal muscle?

Nerves (containing motor neurons) convey impulses for muscular contraction. Blood provides nutrients and oxygen for contraction.

Describe the structure and function of smooth muscle.

Smooth muscle tissue is nonstriated and involuntary, and is classified into two types: visceral (single unit) smooth muscle multiunit smooth muscle Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; its fibres are arranged in a network. Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibres operate singly rather than as a unit. The duration of the contraction and relaxation of smooth muscle is longer than in the skeletal muscle. In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in striated muscle, the regulator protein is troponin); calmodulin activates the enzyme myosin light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate. The prolonged presence of calcium ions in the cytosol of smooth muscle fibres provides for smooth muscle tone, a state of continued partial contraction. Smooth muscle fibres can stretch considerably without developing tension; this phenomenon is called the stress-relaxation response.

Describe the fascia which surrounds muscles. - Superficial fascia - Deep fascia

Superficial fascia (or subcutaneous layer) separates muscle from skin, functions to provide a pathway for nerves and blood vessels, stores fat, insulates, and protects muscles from trauma. Deep fascia, which lines the body wall and limbs and holds muscles with similar functions together, allows for the free movement of muscles; carries nerves, blood vessels, and lymph vessels; and fills the spaces between muscles.

Differentiate between deep and superficial fascia.

Superficial fascia (or subcutaneous layer) separates muscle from skin, functions to provide a pathway for nerves and blood vessels, stores fat, insulates, and protects muscles from trauma. Deep fascia, which lines the body wall and limbs and holds muscles with similar functions together, allows for the free movement of muscles; carries nerves, blood vessels, and lymph vessels; and fills the spaces between muscles.

Describe energy use in muscle cells, and list the three sources for ATP production in muscle cells.

Three sources for ATP production in muscle cells are as follows: 1. Creatine phosphate can power maximal muscle contraction for about 15 seconds, and is used for maximal short bursts of energy. Creatine phosphate is unique to muscle fibres. 2. Anaerobic cellular respiration (glycolysis) can provide enough energy for about 30-40 seconds. 3. Aerobic cellular respiration (reactions requiring oxygen) completes the oxidation of glucose via cellular respiration, and provides energy for prolonged activity. Muscle tissue has two sources of oxygen: diffusion from the blood and release by myoglobin inside muscle fibres. The inability of a muscle to maintain its strength of contraction or tension is called muscle fatigue; it occurs when a muscle cannot produce enough ATP to meet its needs.

Describe the five key functions of muscle.

Through a sustained alternation of contraction and relaxation, muscle performs the following five key functions: Produces body movements Stabilizes body positions Regulates organ volume Moves substances within the body Generates heat

Define wave summation and differentiate it from unfused and fused tetanus.

Wave summation is the increased strength of a contraction, which is the result of the application of a second stimulus before the muscle has completely relaxed after a previous stimulus. A sustained muscle contraction that permits partial relaxation between stimuli is called incomplete (unfused) tetanus. A sustained contraction that lacks even partial relaxation between stimuli is called complete (fused) tetanus.