HHSC 105 Chapter 10 Muscle Tissue and Organization: What did you learn and Content Review.

What are the properties of muscle tissue?

The properties of muscle tissue are excitability, contractility, elasticity, and extensibility.

What proteins are in thick filaments and thin filaments?

Thick filaments are composed of myosin only. The proteins in thin filaments are actin, tropomyosin, and troponin.

Describe the structure of intercalated discs in cardiac muscle cells, and briefly discuss their functions.

Intercalated discs are complex junctions where cardiac muscle fibers joint together at their ends. Intercalated discs appear as thick dark lines between cells in histological sections. At these junctions, the sarcolemmas of adjacent cardiac muscle fibers interlock through desmosomes and gap junctions. Desmosomes hold the adjacent membranes together and the gap junctions facilitate ion passage between cells. This free movement of ions between cells allows each cardiocyte to transmit an electrical impulse and directly stimulate its neighbors.

Give at least four criteria used to determine how muscles are named

Muscles can be named according to (1) orientation of muscle fibers, (2) muscle attachments, (3) specific body regions, (4) muscle shape, (5) muscle size, (6) muscle heads/tendons of origin, (7) muscle function/movement, and (8) muscle position at body surface.

Compare and contrast myofibrils and myofilaments

Myofibrils are small, cylindrical structures located within the sarcoplasm of a skeletal muscle. Myofibrils consist of bundles of thick and thin protein filaments, generally called myofilaments.

What is the function of calcium ions in skeletal muscle

Calcium ions bind to subunits of troponin causing a change in the conformation of the troponin molecule and move the tropomyosin off of the active sites for myosin on the G-actin molecule. Consequently, cross-bridges form between the thick and thin filaments, and a contraction begins.

Explain why athletes who excel at short sprints probably have fewer slow fibers in their leg muscles.

Slow fibers contract more slowly than fast fibers, often taking two or three times as long to contract after stimulation. These fibers are specialized to continue contracting for extended periods of time. Therefore, an athlete who excels at sprinting would benefit from having more fast fibers than slow because sprinting is not a long-term aerobic activity. A successful sprinter would have more fast fibers due to the powerful contractions they produce because of their large number of sarcomeres.

What anatomic changes occur in a muscle fiber when it undergoes hypertrophy

In hypertrophy, each muscle fiber develops more myofibrils, and each myofibril contains a larger number of myofilaments. This results in increased fiber diameter.

Explain why the ratio of motor neurons to muscle fibers is greater in muscles that control eye movement than in postural muscles of the leg.

A motor unit is composed of a single motor neuron and all of the muscle fibers it controls. The movements of an eye require very precise control thus, each motor unit must be small. In contrast, there is much less precision of movement needed in the postural muscles of the leg, so one motor neuron will innervate many more muscle fibers

Describe in your own words the events that occur during skeletal muscle contraction.

A muscle impulse travels down the transverse tubules, causing calcium ions to be released from the terminal cisternae. When calcium ions enter the sarcoplasm, some bind to a subunit of troponin. This causes a change in the conformation of the troponin, resulting in the movement of tropomyosin molecules off active sites on G-actin molecules. Myosin crossbridges form between myosin and actin. The myosin head pivots toward the center of the sarcomere pulling the actin. When ATP then binds to the myosin heads, they detach from the actin and resume their original orientation toward the edge of the thick filament.

Describe the structural relationship between a nerve fiber and a muscle fiber at the neuromuscular junction.

A neuromuscular junction is the site where the membrane of a motor neuron axon and the sarcolemma of the skeletal muscle fiber meet. The ends of the nerve fibers are called synaptic knobs. Each synaptic knob overlies a region of the skeletal muscle fiber membrane called the motor end plate. Chemical communication between the synaptic knob of the neuron and the motor end plate of the skeletal muscle fiber occurs at the neuromuscular junction.

Contrast an agonist and a synergist.

An agonist, also called a prime mover, is a muscle that contracts to produce a particular movement, such as extension of the forearm. The triceps brachii is an agonist of the forearm causing extension. A muscle that assists the prime mover in performing its action is a synergist. The contraction of a synergist usually either contributes to tension exerted close to the insertion of the muscle or stabilizes the point of origin. Usually, synergists are most useful at the start of a movement when the agonist is stretched and cannot exert much power. Synergists may also assist an agonist by preventing movement at a joint and thereby stabilizing the origin of the agonist. These synergistic muscles are called fixators.

Describe the structural relationships among the following in a resting skeletal muscle fiber: A band, H zone, M line.

At the ends of a muscle, the connective tissue layers merge to form a fibrous tendon. A tendon attaches a muscle to bone, skin or another muscle. Tendons usually have a thick cord or cable-like structure. An aponeurosis also attaches a muscle to bone, skin or another muscle. However, its structure is that of a thin, flattened sheet.

Describe the structure of tendons and aponeuoroses, and discuss their purpose in the body.

At the ends of a muscle, the connective tissue layers merge to form a fibrous tendon. A tendon attaches a muscle to bone, skin or another muscle. Tendons usually have a thick cord or cable-like structure. An aponeurosis also attaches a muscle to bone, skin or another muscle. However, its structure is that of a thin, flattened sheet.

Briefly describe the differences between muscle atrophy and muscle hypertrophy. What might cause each condition?

Atrophy is a reduction in muscle size, tone, and power. If a skeletal muscle experiences markedly reduced stimulation, it loses both muscle mass and tone. The muscle becomes flaccid its fibers decrease in size and become weaker. Causes of muscle atrophy include any situation or condition that causes a muscle to experience little or no use, such as wearing a cast or paralysis. Hypertrophy, in contrast, is an increase in muscle cell size. The repetitive, exhaustive stimulation of muscle fibers results in an increased number of mitochondria, larger glycogen reserves, and an increased ability to produce ATP. Ultimately, each muscle cell develops more myofibrils, and each myofibril contains a larger number of myofilaments. Repeated stimulation of muscles to produce near-maximal tension, such as experienced during weight lifting, causes muscle hypertrophy.

Describe how thick and thin filaments interact during a muscle contraction

During a muscle contraction, the heads of the myosin molecules bind to active sites on G-actin molecules and then the heads swing toward the center of the A band, pulling the attached thin filaments toward the center of the A band and shortening the muscle fiber.

Describe the difference between isometric and isotonic contractions, and give an example of each.

During an isometric contraction, the length of the muscle does not change because the tension produced by this contracting muscle never exceeds the resistance, so the muscle does not shorten. In an isotonic contraction, the tension produced exceeds the resistance, and the muscle fibers shorten, resulting in movement. In the arm muscles, an isometric contraction occurs when a person tries to lift a weight that is too heavy, while an isotonic allows a person to lift a book.

Explain what the following names tell us about muscles: longus, extensor, triceps, rectus, superficialis.

Longus means "long". Extensor indicates that the muscle causes an extension at a joint. Triceps muscles have three muscular head origins. Rectus means "straight". Superficialis muscles are found towards body surfaces.

Describe the changes that occur in the length of thick and thin filaments, I bands, and the sarcomere during muscle contraction.

Neither the thick nor thin filaments change length. The I band becomes narrower as the thin filaments slide past the thick filaments. The sarcomere shortens as the Z-discs move toward each other.

Describe smooth muscle cell structure. What are dense bodies?

Smooth muscle is composed of short muscle fibers that have a fusiform shape and a single, centrally located nucleus. Although smooth muscle fibers have both thick and thin filaments, they are not precisely aligned, so no striations or sarcomeres are visible. Dense bodies are small concentrations of protein scattered throughout the sarcoplasm of the smooth muscle fiber and on the inner face of the sarcolemma. Thin filaments are attached to dense bodies by elements of the cytoskeleton.

What are the four main patterns of skeletal muscle fiber organization?

The four main patterns of skeletal muscle fiber organization are circular muscles, convergent muscles, parallel muscles, and pennate muscles.

The gluteus maximus muscle gets its name from which categories for naming muscles?

The gluteus maximus gets its name from (1) the gluteal region of the body (buttocks) = gluteus and from (2) the size of the muscle = maximus (largest).

Describe the difference between the origin and insertion of a skeletal muscle. Where is each typically located?

The origin is the stationary or less movable attachment of a muscle, whereas the insertion is the more movable attachment. In the limbs, the origin typically lies proximal to the insertion. Usually, the insertion is pulled toward the origin.

Compare the roles of the three concentric layers of connective tissue wrappings in the organization of skeletal muscle

The three connective tissue layers are the inner endomysium, the central perimysium, and the outer epimysium. The endomysium is the innermost layer and it surrounds each individual muscle fiber. This thin sheath of loose connective tissue and reticular fibers binds together the neighboring fibers and supports the capillaries that supply these fibers. The perimysium surrounds each fascicle. It is a layer of dense irregular connective tissue that supports and contains neurovascular bundles that branch to supply each individual fascicle. The epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle. Often the epimysium integrates or blends with the deep fascia between adjacent muscles.

List and describe the connective tissue components of skeletal muscle

The three connective tissue layers are the outer epimysium, the central perimysium, and the inner endomysium. The epimysium is a layer of dense irregular connective tissue that surrounds the whole skeletal muscle. The perimysium is a connective tissue layer that surrounds each fascicle. The endomysium is the innermost connective tissue layer, and is a thin sheath of delicate loose connective tissue that surrounds each muscle fiber.

List the three types of skeletal muscle fibers, and describe the anatomic and functional characteristics of each.

The three types of skeletal muscle fibers are slow (Type I, slow oxidative), intermediate (Type IIa, fast aerobic), and fast (Type IIb, fast anaerobic). Slow fibers are usually about half the diameter of fast fibers, contract more slowly, and are specialized to continue contracting for extended periods of time. Their vascular supply is extensive, thus, they receive more nutrients and oxygen. Slow fibers are also called red fibers because they contain the red oxygen binding pigment myoglobin. Additionally, they have a relatively large number of mitochondria, permitting these fibers to produce a greater amount of ATP than fast-fiber muscles making the cell less dependent on anaerobic metabolism. Intermediate fibers exhibit properties that are somewhere between those of fast fibers and slow fibers. The intermediate fibers contract faster than the slow fibers and slower than the fast fibers. Histologically, they resemble fast fibers, but they have a greater resistance to fatigue. Fast fibers have a large diameter, and they contain large glycogen reserves, densely packed myofibrils, and relatively few mitochondria. They are also called white fibers because they lack myoglobin. Fast fiber muscles produce powerful contractions because they contain a large number of sarcomeres. These contractions use vast quantities of ATP but they use anaerobic mechanisms to make their ATP so their prolonged activity causes the fast fibers to rapidly fatigue.