Chapter 9

Define threshold stimulus.

A muscle fiber remains unresponsive until a certain amount of stimulus is applied. This minimal strength required is called the threshold stimulus.

Describe the neuromuscular junction.

(short answer) Site where the nerve fiber communicates with the muscle fiber Each skeletal muscle fiber is connected to a fiber from a nerve cell called a motor neuron. The muscle fiber contracts only when stimulated by this specific fiber. At the connection point between the nerve fiber and the muscle fiber, the muscle fiber's sarcolemma is tightly coiled and heavily concentrated with mitochondria. This region is known as the motor end plate. The branches of the motor nerve fiber project into recesses (synaptic clefts) of the motor end plates and the distal ends are filled with mitochondria and synaptic vesicles that store chemicals called neurotransmitters. This entire region is known as a neuromuscular junction.

Explain the benefits of exercise for maintaining muscular health while aging.

-can lead to new muscle. -Helps keep blood pressure at healthy levels. -Aerobic exercise improves oxygen levels and endurance.

Describe three aging-related changes in the muscular system.

1) Myoglobin, ATP, and Creatine decline. 2) Gradually muscles become smaller and drier. 3) By 80 half of muscle has atrophied.

Identify the major parts of a skeletal muscle fiber and describe the functions of each.

A MUSCLE FIBER is a single, multinucleated cell, which contracts when stimulated...components are. a. Sarcolemma— name for a muscle fiber's cell membrane b. Sarcoplasm—the name for a muscle fiber's cytoplasm. c. Myofibrils—the numerous, threadlike proteins that lie parallel to one another and are contained in the sarcoplasm. d. Sarcomere—the segment of the myofibril between two successive Z lines. This regular arrangement causes the muscle fiber to appear striated. e. Sarcoplasmic reticulum (S.R.)—the specific name for the endoplasmic reticulum. The network of membranous channels that surround each myofibril run parallel to them. f. Transverse tubules (T-tubules)—the invaginations of the fiber's sarcoplasm that extend inward and pass completely through the fiber.

Describe a lever, and explain how its parts may be arranged to form first-class, second-class, and third-class levers.

A lever has four basic components: a rigid bar or rod, a fulcrum or pivot, an object that is moved against resistance, and a force that supplies energy. -First-class levers: resistance-fulcrum-force, for example, scissors. -Second-class levers: fulcrum-resistance-force, for example, wheelbarrow. -Third-class levers: resistance-force-fulcrum, for example, tweezers.

Describe the neural control of skeletal muscle contraction.

A neurotransmitter is a chemical stored in the synaptic vesicles, which, when stimulated by a nerve impulse, is released into the gap at the motor end plate and stimulates the fiber to contract.

Describe the difference between a tendon and an aponeurosis.

A tendon is a projection of connective tissue beyond the ends of the muscle that attaches to bone. An aponeurosis is a broad fibrous sheet of connective tissue that connects muscles to adjacent muscles.

Distinguish between a complete tetanic contraction and muscle tone.

A tetanic contraction results when a sustained forceful contraction lacks even partial relaxation. Muscle tone... -Is a response to nerve impulses originating repeatedly from the spinal cord, and traveling to small numbers of muscle fibers within a muscle. -Muscle tone is responsible for maintaining posture. -Muscle tone appears to be a conscious phenomenon because when the person is rendered unconscious, the body will collapse.

13. Distinguish between a twitch and a sustained contraction.

A twitch is the contractile response of a single muscle fiber to a muscle impulse. During a sustained contraction, many action potentials are triggered in a motor neuron, resulting in a contraction of increasing strength.

What is recruitment of motor units?

An increase in the number of activated motor units is called multiple motor unit summation (recruitment). As stimulation intensity increases recruitment continues until all units are activated in the muscle.

Compare the characteristics of skeletal and smooth muscle cell contractions.

Both smooth and skeletal muscle contractions involve the reactions of actin and myosin, are triggered by membrane impulses and the release of calcium ions, and use energy from ATP molecules. - Skeletal muscles use acetylcholine as the neurotransmitter. Smooth muscle uses acetylcholine and norepinephrine as its neurotransmitters. -Smooth muscle is slower to contract and relax than skeletal muscle. However, smooth muscle can maintain a forceful contraction for a longer period of time with a given amount of ATP.

Distinguish among deep fascia, subcutaneous fascia, and subserous fascia.

Deep Fascia = is the portion of the network of the fasciae that surrounds and penetrates the muscles. Subcutaneous fascia = is the portion that lies just beneath the skin forming the subcutaneous layer. Subsereous fascia = is the portion that forms the connective tissue layer of the serous membranes covering organs in various body cavities and lining those cavities.

Distinguish between fast- and slow-twitch muscle fibers.

Fast twitch (white) = contain less myoglobin and have a poorer blood supply in relation to slow-contracting muscles (red muscles). They have fewer mitochondria and a reduced respiratory capacity. They do have a better-developed sarcoplasmic reticulum and a higher ATPase activity. This allows them to contract rapidly. Slow twitch (red) = has a lot of myoglobin and a well-developed blood supply. They have many mitochondria to carry on aerobic respiration. As a result, they can generate the necessary ATP needed for contraction. They contract for long periods of time prior to muscle fatigue becoming a factor.

Explain how a muscle may become fatigued, and how a person's physical condition may affect tolerance to fatigue.

If a muscle is exercised strenuously for a long period, it may lose its ability to contract. This is called muscle fatigue. This condition may result from an interruption in a muscle's blood supply or from the depletion of acetylcholine in the motor nerve fibers. The most common cause of muscle fatigue is due to the accumulation of lactic acid from anaerobic respiration. The lactic acid causes factors, such as pH, to change so that the muscle fibers no longer respond. Occasionally, a muscle becomes fatigued and develops a cramp simultaneously. A cramp is a painful condition in which the muscle contracts spasmodically, but does not completely relax. This may be caused by a lack of ATP. A person who exercises can stimulate new capillaries to grow within the muscles, supplying more oxygen and nutrients to the muscle fibers. This will allow more aerobic respiration to take place, thereby decreasing dependence on anaerobic respiration, which results in less lactic acid buildup.

Explain how a skeletal muscle can be stimulated to produce a sustained contraction.

If a muscle is exposed to a series of stimuli increasing in frequency, a point is reached where the muscle is unable to complete its relaxation period before the next stimulus arrives. This stacking of twitches causes a sustained contraction.

Distinguish between isotonic and isometric contractions.

Isotonic occurs when muscles shorten (concentric). Isometric when tension occurs but muscles do not move. Yoga pose.

Explain how limb movements function as levers.

Limb movements function as levers in that they contain all four basic components: a rigid bar, a pivot, an object that is moved against resistance, and a force that supplies energy.

Define motor unit and explain how the number of fibers within a unit affects muscular contractions.

Motor unit = motor neuron and the muscle fibers it controls. The fewer the muscle fibers = the more precise the movements.

Distinguish between multiunit smooth muscle and visceral smooth muscle.

Multiunit smooth muscle features fibers that are somewhat disorganized and occur as separate fibers rather than in sheets. It can be found in the iris of the eyes and in the walls of blood vessels. Multiunit smooth muscle tissue contracts only after stimulation by motor nerve impulses. Visceral smooth muscle is composed of sheets of spindle-shaped cells in close contact with one another. This type, which is more common, is found in the walls of hollow, visceral organs such as the stomach, intestines, urinary bladder, and uterus. The fibers of visceral smooth muscle are capable of stimulating each other. So, when one fiber is stimulated, the impulse may excite adjacent fibers that, in turn, may excite others. Visceral smooth muscle fibers also display rhythmicity—a pattern of repeated contractions. These two features of visceral smooth muscle are largely responsible for peristalsis.

Identify the major events that occur during skeletal muscle fiber contraction.

Muscle fiber contraction is a process involving of cell parts and chemical substances that result in the sliding movement of the actin and myosin filaments and causes a contraction. (complex) A myosin filament is composed of protein strands with globular ends called cross-bridges that extend outward along the length of the filament. The actin filaments have ADP molecules attached to its surface that serve as active sites for linking the cross-bridges of the myosin filaments.

Describe how connective tissue is part of the structure of a skeletal muscle.

Muscle fibers are grouped together in fascicles that are surrounded by a layer of connective tissue called the endomysium. The fascicles are bundled together and surrounded by another layer of connective tissue called the perimysium, which also fills the spaces between the fascicles. Several layers of fibrous connective tissue called the epimysium, which surrounds the entire muscle, then cover the perimysium.

Describe how oxygen is supplied to skeletal muscle.

Oxygen is carried from the lungs by hemoglobin in the blood. When the hemoglobin reaches the muscle, the oxygen is transferred to the myoglobin in the muscle fiber.

Define peristalsis and explain its function.

Peristalsis consists of alternating contractions and relaxations of the longitudinal and circular muscle fibers. It functions to force the contents of a tube along its length

Compare the characteristics of skeletal and cardiac muscle cell contractions.

Skeletal muscle: - can contract individually. Cardiac muscle: -works in a network. - occurs only in the heart.

Explain how ATP and creatine phosphate function in skeletal muscle fiber contraction.

The basic energy source for muscle contraction comes from ATP molecules supplied by the mitochondria. Creatine phosphate cannot directly supply energy to the muscle fiber. Instead, it acts as a storehouse of energy for the ADP.

Describe how oxygen debt may develop.

When skeletal muscles have been used where they have exceeded their oxygen reserves, anaerobic respiration must take over. Anaerobic respiration changes glucose into pyruvic acid, and due to the lack of oxygen, the pyruvic acid is converted into lactic acid. The lactic acid diffuses out of the muscles and is taken to the liver by the blood. The liver can change lactic acid back into glucose: however, this conversion also requires the use of ATP. During strenuous exercise, primarily the muscles and not the liver use the oxygen, so the lactic acid accumulates. Oxygen debt then, is defined as the amount of oxygen needed by the liver to convert the lactic acid back into glucose, plus the amount needed by the muscles to resynthesize ATP and creatine phosphate and return them to their original concentrations. Because the conversion of lactic acid into glucose is a slow process, it may take several hours to repay the oxygen debt.