CM

Survival depends on the body’s ability to maintain a constant body temperature.

Contraction of muscle fibers produces most of the heat required to maintain normal body temperature.

A fever is an elevated body temperature and is often a sign of illness.

Hypothermia is a body temperature below the setpoint.

Note that fatigue caused by repeated muscle stimulation without adequate periods of rest results in reduced strength of muscle contraction.

Repeated muscular contraction depletes cellular ATP stores and outstrips the ability of the blood supply to replenish oxygen and nutrients.

Contraction in the absence of adequate oxygen produces lactic acid, which contributes to muscle soreness.

Be sure you also recall the meaning of the terms, excess post-exercise consumption or recovery oxygen uptake . They are used to describe the metabolic efforts required to burn excess lactic acid that may accumulate during prolonged periods of exercise.

This increased metabolism helps restore energy and oxygen reserves to setpoint or resting levels.

Remember that other body systems have a role in movement as well.

Muscle functioning is influenced by many other parts of the body. Respiratory, circulatory, nervous, muscular, and skeletal systems play essential roles in producing normal movements. Pathological conditions in other body organ systems can dramatically affect movement. Examples include multiple sclerosis, brain hemorrhage, and spinal cord injury.

Interaction between the nervous system and the muscular system is essential for body movement. Stimulation of a muscle by a nerve impulse is required before a muscle can shorten and produce movement.

A motor neuron is a nerve cell that transmits an impulse to a muscle, causing contraction. A single motor neuron, with the muscle fibers it innervates and thus controls, is called a motor unit.

The neuromuscular junction (abbreviated NMJ) is the point of contact between a nerve ending and a muscle fiber. Chemicals called neurotransmitters cross the NMJ to trigger contraction in muscle. Acetylcholine is the neurotransmitter operating at each neuromuscular junction.

Next, we’ll review information about a muscle stimulus.

A muscle will contract only if an applied stimulus reaches a certain level of intensity. A threshold stimulus is the minimal level of stimulation required to cause a muscle fiber to contract.

Once stimulated by a threshold stimulus, a muscle fiber will contract completely, a response called all or none.

Different muscle fibers in a muscle are controlled by different motor units having different threshold-stimulus levels. Although individual muscle fibers always respond all or none to a threshold stimulus, the muscle as a whole does not. Different motor units responding to different threshold stimuli permit a muscle as a whole to execute contractions of graded force.

The types of skeletal muscle contraction include twitch, tetanic, isometric, and isotonic.

Twitch contractions are laboratory phenomena, not typical muscle activity; they are a single contraction of muscle fibers caused by a single threshold stimulus.

Tetanic contractions are sustained muscular contractions caused by stimuli hitting a muscle in rapid succession.

Isometric contractions do not produce movement; the muscle as a whole does not shorten (The word part “iso-“ means “same” and “metric” refers to measure or length; that is, why isometric contractions describe contractions where the muscle does not shorten.) Although no movement occurs, tension within the muscle increases.

Isotonic contractions produce movement at a joint because the muscle changes length. Most types of body movements (such as walking and running) are produced by isotonic contractions. Isotonic contractions can be divided into concentric and eccentric contractions.

In concentric contractions, the muscle shortens and the insertion end of the muscle moves toward the point of origin.

In eccentric contractions, the muscle lengthens under tension, thus moving the insertion away from the origin.

Exercise, if regular and properly practiced, improves muscle tone and posture, results in more efficient heart and lung functioning, and reduces fatigue.

Muscles physically change in relation to the amount of work they normally do. Prolonged inactivity causes disuse atrophy (think “use it or lose it”).

Regular exercise increases muscle size, called hypertrophy.

Strength training is exercise involving contraction of muscles against challenging resistance. It increases the numbers of myofilaments in each muscle fiber, and as a result, the total mass of the muscle increases. Strength training does not increase the number of muscle fibers.

Endurance training is exercise that increases a muscle’s ability to sustain moderate exercise over a long period; it is sometimes called aerobic training. It allows more efficient delivery of oxygen and nutrients to a muscle via increased blood flow. Endurance training does not usually result in muscle hypertrophy.

Next, we’ll review the movements produced by skeletal muscle contractions.

Angular movements include flexion, extension, abduction, and adduction.

Flexion decreases an angle.

Extension increases an angle.

Abduction moves a body part away from the midline.

Adduction moves a body part toward the midline.

Circular movements include rotation, circumduction, supination, and pronation.

Rotation is movement around a longitudinal axis.

Circumduction occurs when the distal end of a body part is moved in a circle.

Supination and pronation are hand positions that result from twisting of the forearm. To recall which movement is which term, form your hands into a soup bowl. Your forearms are now supinated.

Special movements, those not easily described with general terms, include dorsiflexion, plantar flexion, inversion, and eversion.

Dorsiflexion and plantar flexion are foot movements that result in upward and downward ankle movement.

Inversion and eversion are sideways foot movements.

Be sure to review the skeletal muscle groups and the specific muscles in each group.

Muscles of the head and neck include the facial muscles, muscles of mastication, and the sternocleidomastoid and trapezius.

Be sure you can identify these facial muscles: the orbicularis oculi, orbicularis oris, and zygomaticus.

Muscles of mastication include the masseter and temporal muscles.

The sternocleidomastoid muscle, named for its origins and insertion, flexes the head.

The trapezius muscle elevates the shoulders and extends the head.

Muscles that move the upper extremities include the pectoralis major, which flexes the arm; the latissimus dorsi, which extends the arm; the deltoid, which abducts the arm; the biceps brachii, which flexes the forearm; and the triceps brachii, which extends the forearm.

Muscles of the trunk include the abdominal and respiratory muscles.

Be sure you can distinguish these abdominal muscles from each other: the rectus abdominis, external oblique, internal oblique, and the transversus abdominis.

Respiratory muscles include the intercostal muscles, and the dome-shaped diaphragm.

The last muscle group to review are the muscles that move the lower extremities.

The iliopsoas flexes the thigh, and the gluteus maximus extends the thigh.

Adductor muscles adduct the thighs.

The three hamstring muscles flex the leg; they are the semimembranosus, semitendinosus, and biceps femoris.

The four quadriceps femoris muscles extend the leg.

There is one rectus femoris in the quadriceps, and there are three vastus muscles.

The tibialis anterior dorsiflexes the foot while the gastrocnemius plantar flexes the foot. The fibularis group or peroneus group of muscles also flexes the foot.

This concludes the audio review of chapter 8.