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Muscular System

Overview of Muscle Tissues

1. Skeletal muscle forms the muscle's attached to the skeleton, which move the limbs and other body parts. Its cells are long, striated, and multinucleate. Skeletal muscle fibers are subject to voluntary control. Connective tissue coverings (endomysium, perimysium, and epimysium) enclose and protect the muscle fibers and increase the strength of skeletal muscles. Skeletal muscles make up the muscular system (Figure 6.1).

2. Smooth muscle cells are uninucleate, spindle-shaped, and arranged in opposing layers in the walls of hollow organs. When they contract, substances (food, urine, a baby) are moved along internal pathways. Smooth muscle control is involuntary (Figure 6.2).

3. Cardiac muscle cells are striated, branching cells that fit closely together and are arranged in spiral bundles in the heart. Their contraction pumps blood through the blood vessels. Control is involuntary (Figure 6.3).

4. The sole function of muscle tissue is to contract or shorten. As it contracts, it causes movement, maintains posture, stabilizes joints, and generates heat.

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Microscopic Anatomy of Skeletal Muscle

1. The multinucleate cylindrical skeletal muscle fibers are packed with unique organelles called myofibrils (Figure 6.4). The banding pattern (striations) of the myofibrils and the cell as a whole reflects the regular arrangement of thin (actin-containing) and thick (myosin) filaments within the sarcomeres, the contractile units composing the myofibrils (Figure 6.5).

2. Each myofibril is loosely enclosed by a specialized ER, called the sarcoplasmic reticulum (SR), which plays an important role in storing and releasing calcium ions. Calcium ions are the final "trigger" for muscle fiber contraction.

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Skeletal Muscle Activity

1. All skeletal muscle cells are stimulated by motor neurons. When the neuron releases a neurotransmitter (acetylcholine), the permeability of the sarcolemma changes, allowing sodium ions to enter the muscle cell. This produces an electrical current (action potential), which flows across the entire sarcolemma, resulting in release of calcium ions from the SR (Figure 6.6).

2. Calcium binds to regulatory proteins on the thin filaments and exposes myosin binding sites, allowing the myosin heads on the thick filaments to attach. The attached heads pivot, sliding the thin filaments toward the center of the sarcomere, and contraction occurs. ATP provides the energy for the sliding process, which continues as long as ionic calcium is present (Figure 6.7).

3. Although individual muscle cells contract completely when adequately stimulated, a muscle (an organ) responds to stimuli to different degrees, i.e., it exhibits graded responses.

4. Most skeletal muscle contractions are tetanic (smooth and sustained) because rapid nerve impulses are reaching the muscle, and the muscle cannot relax completely between contractions. The strength of muscle contraction reflects the relative number of muscle cells contracting (more = stronger).

5. ATP, the immediate source of energy for muscle contraction, is stored in small amounts in muscle fibers and is quickly used up. ATP is regenerated via three routes. From the fastest to the slowest, these are via a coupled reaction of creatine phosphate with ADP, via anaerobic glycolysis and lactic acid formation, and via aerobic respiration. Only aerobic respiration requires oxygen.

6. If muscle activity is strenuous and prolonged, muscle fatigue occurs due to an accumulation of lactic acid in the muscle and a decrease in its energy (ATP) supply. After exercise, the oxygen debt is repaid by rapid deep breathing.

7. Muscle contractions are isotonic (the muscle shortens and movement occurs) (Figure 6.8) or isometric (the muscle does not shorten, but its tension increases) (Figure 6.9).

8. Muscle tone keeps muscles healthy and ready to react. It is a result of a staggered series of nerve impulses delivered to different cells within the muscle. If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed, and atrophies.

9. Inactive muscles atrophy. Muscles challenged by resistance exercise to respond (almost) beyond their ability increase in size and strength. Muscles subjected to regular aerobic exercise become more efficient and stronger and can work longer without tiring. Aerobic exercise also benefits other body organ systems.

10. Are you exercising at maximum efficiency? Is your health at risk due to your weight? Answer these questions by calculating your Target Heart Rate and your Body Mass Index.

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Muscle Movements, Types, and Names

1. All muscles are attached to bones at two points. The origin is the immovable attachment; the insertion is the movable bony attachment. When contraction occurs, the insertion moves toward the origin (Figure 6.10).

2. Body movements include flexion, extension, abduction, adduction, circumduction, rotation, pronation, supination, inversion, eversion, dorsiflexion, and plantar flexion.

3. On the basis of their general functions in the body, muscles are classified as prime movers, antagonists, synergists, and fixators.

4. Muscles are named according to several criteria, including muscle size, shape, number and location of origins, bones associated with, and action of, the muscle.

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Gross Anatomy of Skeletal Muscles

1. Muscles of the head fall into two groups. The muscles of facial expression include the frontalis, orbicularis oris and oculi, and zygomaticus. The chewing muscles are the masseter, temporalis, and buccinator, which is also a muscle of facial expression (Figure 6.11).

2. Muscles of the trunk and neck move the head, shoulder girdle, and trunk and form the abdominal girdle. Anterior neck and trunk muscles include the sternocleidomastoid, pectoralis major, intercostals, rectus abdominis, external and internal obliques, and transversus abdominis. Posterior trunk and neck muscles include the trapezius, latissimus dorsi, and deltoid. Deep muscles of the back are the erector spinae muscles (Figure 6.12).

3. Muscles of the upper limb include muscles that cause movement at the shoulder joint, elbow, and hand. Muscles causing movement at the elbow include the biceps brachii and triceps brachii.

4. Muscles of the lower extremity cause movement at the hip, knee, and foot. They include the iliopsoas, gluteus maximus and medius, adductors, quadriceps and hamstring groups, gastrocnemius, anterior tibialis, and peroneus muscles (Figure 6.13).

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Developmental Aspects of the Muscular System

1. Increasing muscular control reflects the maturation of the nervous system. Muscle control is achieved in a cephalic/caudal and proximal/distal direction.

2. To remain healthy, muscles must be regularly exercised. Without exercise, they atrophy; with extremely vigorous exercise, they hypertrophy.

3. As we age, muscle mass decreases and the muscles become more sinewy. Exercise helps to retain muscle mass and strength.