Holes Anatomy CH 8 muscle Assessment

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.

zygomaticus

draws the corner of the mouth upward

platysma

extends over the neck from the chest to the face

temporalis

inserted on the coronoid process of the mandible

tibialis anterior

inverts the foot

List the major parts of a skeletal muscle fiber, and describe the function of each part.

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. 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. It is open to the outside of the sarcoplasm at both ends and contains extracellular fluid. Each T-tubule likes between two enlarged portions of the S.R., called cisternae, near the regions where the actin and myosin filaments overlap. With the S.R., the T-tubules activate the muscle contraction mechanism.

Distinguish between tetanic contraction 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.

Describe the difference between a tendon and an aponeurosis.

Aponeurosis are broad fibrous sheets which may attach to bone or to the coverings of adjacent muscles. Tendons are cord-like and attach muscle to bone.

Compare smooth and skeletal muscle 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.

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.

Describe how connective tissue associates with skeletal muscle.

Fascia blends with the epimysium, a layer of connective tissue that closely surrounds each skeletal muscle. Other layers of connective tissue, called perimysium, extend inward from the epimysium and separate the muscle tissue into small sections. Each muscle fiber within a fascicle (fasciculus) lies within a layer of connective tissue in the form of a thin covering called endomysium.

Explain how muscles may become fatigued.

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 skeletal muscle stimulation produces 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 multiunit and visceral smooth muscle fibers.

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.

List the major events of muscle fiber contraction and relaxation.

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 muscles obtain oxygen.

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.

compare contraction mechanisms of cardiac and skeletal muscle fibers.

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.

The three types of muscle tissue are

Skeletal, cardio, and smooth

Describe how ATP and creatine phosphate interact.

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.

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 are 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 muscle's 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.

Define agonist, antagonist, and synergist.

The prime mover is the muscle that is primarily responsible for a certain movement. An agonist is synonymous with prime mover. A synergist is a muscle that aids the prime mover in its action to move a part. An antagonist is a muscle that acts against the prime mover. For instance, if the prime mover raises the arm, the antagonist lowers it. The combination of both the prime mover and antagonist working at the same time will cause the part to remain rigid. This aids in maintaining posture, balance, and locomotion.

Explain how skeletal muscle function affects the maintenance of body temperature.

Two-thirds of the energy released in cellular respiration is lost as heat. Muscle accounts for most of the total body mass so when muscles are active, large amounts of heat are produced. This is circulated throughout the body by the blood thereby supporting the maintenance of body temperature.

Describe how an 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.

biceps femoris

a hamstring muscle

rectus femoris

a member of the quadriceps group

gastrocnemius

a plantar flexor of the foot

deltoid

abducts the arm

gluteus medius

abducts the thigh

gracilis

adducts the thigh

A neurotransmitter: a. binds actin filaments, causing them to slide b. diffuses across a synapse from a neuron to a muscle cell c. carries ATP across a synapse d. travels across a synapse from a muscle cell to a neuron

b. diffuses across a synaptic cleft from a neuron to muscle cell

Which of the following describes the addition of muscle fibers to take part in a contraction? a. summation b. recruitment c. tetany d. twitch

b. recruitment

splenius capitis

can pull the head into an upright position

rhomboid major

can raise and adduct the scapula

orbicularis oris

closes lips

buccinator

compresses the cheeks

external oblique

compresses the contents of the abdominal cavity

epicranius

consists of two parts—the frontalis and the occipitalis

gluteus maximus

largest muscle in the body

triceps brachii

primary extensor of the elbow

pectoralis major

pulls the arm forward and across the chest

latissimus dorsi

pulls the shoulder back and downward

teres minor

rotates the arm laterally

pronator teres

rotates the arm medially

brachialis

strongest flexor of the elbow

biceps brachii

strongest supinator of the forearm