Chapter summary

Overview of muscle tissues

Properties and functions of muscle

1. The properties of all muscle include excitability, contractility, extensibility, and elasticity.

2. The functions of muscle include production of movement of both internal and external body parts, maintaining posture, Stabilizingjoints, and generating heat.

Types of muscle tissue

1. Skeletal muscle attaches to bones, is striated, and is voluntarily controlled.

2. Cardiac muscle is found in the heart, is striated, and is involuntarily controlled.

3. Smooth muscle, located chiefly in the walls of hollow organs, is nonstriated, and is involun­tarily controlled.

Skeletal muscle

Gross anatomy of a skeletal muscle

1. Skeletal muscle, constituting approximately 40% of body weight, is made up of muscle cells called fibers.

2. Skeletal muscle is protected and strengthened by connective tissue coverings. From superficial to deep, these coverings are called the epimysium, perimysium, and endomysium.

3. Skeletal muscle attachments (origins/inser­tions) may be direct or indirect via tendons or aponeuroses.

Microscopic anatomy of a skeletal muscle fiber

1. Skeletal muscle fibers are long, striated, and multinucleated.

2. Most of their cell body is occupied by myofibrils, which are composed of thick and thin myofila­ments consisting primarily of myosin or actin, respectively. Their striated appearance results from alternating dark (A) and light (I) bands.

3. The functional unit of skeletal muscle is the sar­comere, which consists of alternating A- and I-bands. Myofibrils consist of chains of sarcomeres. The heads of myosin molecules form cross bridges that interact with the actin (thin) filaments.

4. The SR, comparable to the endoplasmic reticu­lum in other cells, is a system of membranous tubules surrounding each myofibril. It functions to release and actively sequester calcium ions.

5. T tubules are invaginations of the Sarcolemma that run between the terminal cisternae of the SR. These tubules allow the action potential to go deep into the muscle fiber and interact with the SR.

Excitation-coupling and sliding filament theory

1. A motor end-plate potential develops when ACh released by a nerve ending binds to ACh recep­tors (i.e., a nicotinic receptor) on the sarcolemma, causing depolarization of the muscle fiber. This results in the production of an action potential that is self-propagating and unstoppable.

2. In excitation-contraction coupling, the action potential is propagated down the T tubules and causes the release of calcium from the SR into the sarcoplasm.

3. Calcium then binds to troponin, causing a con­formational change in its shape, allowing tropo­myosin to move and uncover myosin-binding sites. This allows cross-bridges to form between the myosin head and thin filament. Myosin ATPase splits ATP, which energizes the power strokes.

4. The binding of a new molecule of ATP to the myosin head is necessary for cross-bridge detach­ment. If ATP is not present, a rigor complex forms in which the myosin head remains bound to the thin filament.

5. Cross bridge activity ends when calcium is pumped back into the SR.

Contraction of a skeletal muscle

1. A motor unit consists of one motor neuron and all the muscle cells it innervates.

2. When stimulated, a motor unit responds with a single brief twitch. A twitch has three phases including the latent period (release of calcium from SR), the period of contraction (the muscle tenses and may shorten), and the period of relax­ation (calcium is resequestered and the muscle resumes its resting length).

3. When stimulated rapidly, wave summation of twitches occurs giving a graded response re­sulting in unfused or fused tetanus. With strong stimulation, many motor units can be re­cruited in a process called multiple motor unit summation.

4. Isotonic contractions occur when a muscle short­ens (concentric contraction) or lengthens (eccen­tric contraction), thus moving a load.

Muscle metabolism

1. The energy source for muscle contraction is ATP, which can be produced through either aerobic and anaerobic metabolism of glucose, or from a coupled reaction transferring a phosphate group from creatine phosphate ADP to form ATR

2. When ATP is produced by nonaerobic metabo­lism, lactic acid accumulates and an oxygen debt occurs. Following such activity, ATP must be produced aerobically and used to regenerate cre­atine phosphate, and accumulated lactic acid must be oxidized and NAD⁺ regenerated from NADH.

3. Muscle contraction is only about 42% efficient, with the remainder of the energy being liberated as heat.

Smooth muscle

Structure of smooth muscle fibers

1. Smooth muscle fibers are spindle-shaped, have a single centrally located nucleus, and have no striations. They also have no T tubules, and the SR is not well organized.

2. Generally arranged in sheets, smooth muscle fibers lack well-organized connective tissue coverings.

3. Actin and myosin filaments are present, but they lack myofibrils and sarcomere.

4. Thick filaments are found throughout, as well as a network of intermediate filaments which are attached to the sarcolemma via dense bodies. When the thick filaments pull on the intermedi­ate filaments, this causes the cell to shorten.

5. There are two types of smooth muscle: visceral and multiunit. Visceral smooth muscle cells are coupled by gap junctions (electrical synapses) and can contract as a functional syncytium. Mul­tiunit smooth muscle cells lack gap junctions and therefore can only contract when stimulated by a neural signal.

Contraction of smooth muscle

1. Smooth muscle contraction depends on ATP and is initiated by calcium, mostly entering from the extracellular space. Calcium binds to calmodulin rather than to troponin (which is not present in smooth muscle fibers), which then causes phosphorylation of the myosin head to initiate contraction.

2. Smooth muscle can contract for longer periods of time than skeletal muscle. It uses low amounts of ATP and does not fatigue.

3. Neurotransmitters from the autonomic nervous system (i.e., norepinephrine and ACh) can inhibit or stimulate smooth muscle contraction. Smooth muscle contractions may also be initiated by pacemaker cells, hormones, local chemical factors, or mechanical stretch.

Muscle systems

Naming muscles

Generally, naming muscles takes into account a mus­cle's location, action, size, shape, direction of fibers, number of origins/bellies, and attachment site.

Arrangement of fascicles

Skeletal muscles are arranged in fascicles. Fascicles are sometimes found to be arranged in a straight line and are referred to as parallel muscles. If the arrange­ment is circular, the fascicles appear as concentric rings. In a convergent muscle, the fascicles converge to the tendon for insertion. Therefore, the muscle begins wide similar to the shape of a fan and then narrows. In pennate muscle the fascicles are short and attach obliquely to the tendon. In a unipennate muscle, the tendon runs along one side of the muscle whereas in multipennate, the tendon branches within the muscle.

Muscles as levers

1. A lever consists of a rigid structure (i.e., bone) that moves around a fixed point called the fulcrum. Levers provide a mechanical advan­tage, allowing a force to move a heavier load either further or faster.

2. In a first-class lever, the fulcrum is between the force and load as in an animal lifting its head (effort-fulcrum-load). This type of lever may operate at a mechanical advantage or disadvan­tage. In a second class lever, the force is applied at one end and the fulcrum is found at the other end, as in how the calf muscles work. In a third- class lever, the fulcrum and load are at either end of the lever. This is the most common lever found in skeletal muscle.

Muscle terminology

1. The origin of a muscle is its fixed attachment whereas its insertion is in the movable end.

2. The movements of the skeleton caused by muscle contraction involve flexion or extension, adduc­tion or abduction, protraction or retraction, ele­vation or depression, rotation, circumduction, pronation or supination, inversion or eversion.

3. An agonist muscle produces a movement, while its antagonist opposes that action.

4. Muscles can make three types of attachments. In a fleshy attachment, there is an apparent direct attachment of the muscle to the bone, although in actuality, the muscles are attached to the bones by very short tendons. In a tendinous attach­ment, the muscle is attached to the bone via dense connective tissue. In an aponeurotic attach­ment, there is a flat, tendinous sheet attaching the muscle as is seen in the abdominal wall.

Stay apparatus of the horse

The stay apparatus allows a horse to rest while standing, using little muscular activity or fatigue. The stay apparatus uses a system of tendons and ligaments to "lock" the lower portion of the leg, thus requiring minimal muscular effort to stand.

Review questions and answers are available ¾ online.

References

Constantinescu_z G.M. 2001. Guide to Regional Ruminant Anatomy Based on the Dissection of the Goat. Iowa State Press, Ames_z IA.

Getty, R. 1964. Atlas for Applied Veterinary Anatomy. Iowa State Press, Ames, IA.

Pasquini, C., T. Spurgeon, and S. Pasquini. 1995. Anatomy of Domestic Animals, 8th edition. SUDZ Publishing, Pilot Point, TX.