Anatomy Chapter 9 Questions

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.

A neurotransmitter ____________

diffuses across a synaptic cleft from a neuron to muscle cell

List three outcomes of muscle actions

1. Movement 2. Muscle actions provide muscle tone. 3. Propel body fluids and food 4. Distribute heat

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.

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

A muscle fiber is a single, multinucleated cell, which contracts when stimulated. It is a thick, elongated cylinder with rounded ends that may extend the entire length of the muscle. Its component parts are: a. Sarcolemma—the specific name for a muscle fiber's cell membrane. b. Sarcoplasm—the specific name for a muscle fiber's cytoplasm. It contains the many small, oval nuclei and mitochondria. c. Myofibrils—the numerous, threadlike proteins that lie parallel to one another and are contained in the sarcoplasm. The arrangement of the two proteins below produces the striations seen in the skeletal muscle. 1. Myosin—the primary protein of the myofibril. It is a thick protein filament running longitudinally within the muscle fiber. 2. Actin—the thin protein filaments arranged intertwined within the myosin filaments. The alternating light and dark striations are named for their positions within the fiber. 1. A bands—the primary location of the myosin filaments that produces the dark striations. 2. I bands—the primary location of the actin filaments that produces the light striations. 3. Z lines—the attachment point of the actin filaments at the ends of I bands. They are arranged so that those of adjacent myofibrils are side by side.

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.

synergist

A synergist is a muscle that aids the prime mover in its action to move a part.

Distinguish between concentric and eccentric contractions and muscle tone

A tetanic contraction (tetany) results when a sustained forceful contraction lacks even partial relaxation. Muscle tone (tonus) 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.

Distinguish 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.

agonist

An agonist is synonymous with prime mover.

antagonst

An antagonist is a muscle that acts against the prime mover. For instance, if the prime mover raises the arm, the antagonist lowers it.

Compare the characteristics of skeletal and smooth muscle fiber 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. The differences between their contractions are: a. Skeletal muscles use acetylcholine as the neurotransmitter. Smooth muscle uses acetylcholine and norepinephrine as its neurotransmitters. b. Several hormones that cause either contractions or alter the amount of response to the neurotransmitters affect smooth muscles. c. 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. d. Unlike skeletal muscle, smooth muscle fibers can change length without changes in tautness. As a result, as the hollow organs become filled, the muscles can stretch without internal pressure changing.

Concentric vs Eccentric

Concentric = Shortening occurs If a person lifts an object, the muscles remain taut, their attached ends pull closer together, and the object is moved. The muscle contracts with force greater than resistance and shortens. Eccentric = lengthening occurs The muscle contracts with less force than resistance and lengthens. Laying a book down on a table is an example.

Distinguish among deep fascia, subcutaneus 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. Subserous fascia is the portion that forms the connective tissue layer of the serous membranes covering organs in various body cavities and lining those cavities.

Describe a Neuromuscular junction

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.

Distinguish between fast and slow twitch muscle fibers

Fast-contracting muscles (white muscles) 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. A slow-contracting muscle 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.

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.

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 muscle fiber contraction

Muscle fiber contraction is a complex process involving a number of cell parts and chemical substances that result in the sliding movement of the actin and myosin filaments and causes a contraction. 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. Although the process is not completely understood, the sliding filament theory suggests that the myosin cross-bridge attaches to an actin active site and bends slightly, pulling the actin with it. It releases its attachment, straightens, and combines with another active site further down the actin filament, causing the sarcomere to shorten. When the nerve impulse reaches the distal end of its branch, acetylcholine is released into the gap. The acetylcholine diffuses rapidly across the motor end plate and combines with protein receptors in the sarcolemma. This causes a muscle impulse to be generated and pass in all directions over the entire sarcolemma, and through the T-tubules deep into the fiber. The S.R., which contains a high concentration of calcium ions, becomes more permeable and allows the ions to diffuse into the sarcoplasm. When a high enough concentration is present in the sarcoplasm, the linkages between the actin and myosin filaments occur and contraction takes place. The calcium ions are moved quickly back into the S.R. by an active transport system (calcium pump). When enough calcium ions have been removed from the sarcoplasm, the muscle relaxes. At the same time, the acetylcholine is rapidly decomposed by the enzyme cholinesterase. This prevents a single nerve impulse from causing a sustained contraction.

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. Myoglobin is similar to hemoglobin in its oxygen capacity, and reduces the muscle's need for continuous blood supply during contraction.

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

Skeletal muscle can contract individually. Cardiac muscle works in a network. When one portion of the cardiac muscle network is stimulated, the impulse travels to the other fibers in the network. It then contracts as a unit. It is self-exciting and rhythmic, and the entire network responds in an all-or-none manner. Cardiac muscle occurs only in the heart. It is made of striated muscle fibers joined end-to-end, forming three-dimensional networks. The internal components are the same, except that the cisternae of the cardiac fibers are not as well developed and contain fewer calcium ions than skeletal muscle fibers. The T-tubules of the cardiac muscle fibers are more developed and release more calcium ions, which comes from the extracellular fluid, in response to stimuli. This enables the cardiac muscle fibers to maintain contractions for a longer period of time. The cardiac muscle cells are joined to each other at the ends by means of intercalated disks. These disks help hold adjacent cells together and transmit the force of the contraction from cell to cell. The disks are also low in electrical resistance, so the muscle impulse travels from cell to cell rapidly. Cardiac muscle is self-exciting, rhythmic, and the entire network responds in an all-or-none manner.

Describe the difference between a tendon and an aponeurosis

Tendon: projection of connective tissue beyond the ends of the muscle that attaches to bone Aponeurosis: broad fibrous sheet of connective tissue that connects muscles to adjacent muscles.

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. The cross-bridges of myosin contain the enzyme ATPase that causes ATP to decompose into ADP and phosphate, thereby releasing energy. The primary source of regeneration of ATP from ADP is creatine phosphate. Creatine phosphate contains high-energy phosphate bonds and is four to six times more abundant in the muscle fibers than ATP. Creatine phosphate cannot directly supply energy to the muscle fiber. Instead, it acts as a storehouse of energy for the ADP. In the mitochondria, the enzyme creatine phosphokinase creates creatine phosphate to be used for ATP synthesis. The creatine phosphate, in turn, converts ADP into ATP by resupplying the phosphate molecule.

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

The nerve fibers of a motor neuron are highly branched. Each of these branches is connected to the motor end plate of a single muscle fiber. When the motor neuron is stimulated, the impulse is carried to all of the muscle fibers attached to its branches. In this way a single motor neuron controls the contractions of many muscle fibers. All the muscle fibers attached to this motor neuron and the motor neuron itself constitute a motor unit. The fewer muscle fibers in the motor unit, the finer the movements that can be produced.

Distinguish between a muscles origin and its insertion

The origin of a muscle is the end of the muscle attached to an immovable part. The insertion is the end of a muscle attached to a moveable part. When a muscle contracts, the insertion is pulled toward the origin.

prime mover

The prime mover is the muscle that is primarily responsible for a certain movement.

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.

Which of the following describes addtion of muscle fibers to take part in a contraction

b. recruitment