MUSCLES - Notes and Objectives

Objective 1. Describe the 3 types of muscles. Three types of muscle tissue: skeletal, cardiac, smooth Differ in cell structure, body location, function, and the means by which they are activated to contract. Similar in that all are elongated and contraction depends on 2 kinds of myofilaments.

Skeletal Muscle Tissue - Packaged into skeletal muscle that attach to and cover the bony skeleton. Visually obvious bands -- striations. Voluntary control - subject to conscious control. Can contract rapidly and vigorously but tire easily. Can exert tremendous power. Remarkably adaptable.

Cardiac Muscle Tissue - Found only in the heart. Striated and Involuntary. Gap junctions allow rapid communication between cells which synchronizes cell contractions.

Smooth Muscle Tissue in walls of hollow organs -- stomach, bladder, resp. tract, etc. Non-striated. Involuntary control. Contractions are slow and sustained. Found in sheets of alternating circular and longitudinal layers; synchronized contraction of the sheets give rise to a wave of contraction along the hollow organ - peristalsis.

Objective 2. List the functions of muscle.

1. Movement - Just about all movement results from muscle contraction. Skeletal muscles are responsible for all locomotion and manipulation. Skeletal muscles enable the body to respond quickly to changes in the external environment.

2. Maintenance of Posture - Muscles function almost continuously making small adjustments to maintain an erect or seated posture despite the effects of gravity.

3. Joint Stability - Skeletal muscles help stabilize joints with poor reinforcement or noncomplementary articular surfaces -- shoulder, knee.

4. Heat Generation - Generate heat as the muscles contract. Vitally important in maintenance of normal body temperature. Skeletal muscle are most responsible.

Objective 3. Describe the functional characteristics of muscle.

a. Excitability - Ability to receive and respond to a stimulus -- stimulus is usually a chemical - neurotransmitter, hormone, or pH change. The response is the generation and transmission of an electrical current (action potentials) that signals the muscle to contract.

b. Contractility - Ability to shorten and thicken forcibly. Unique to muscles.

c. Extensibility - Ability to stretch or extend. Muscle fibers shorten when contracting; they can be stretched beyond their resting length when relaxed.

d. Elasticity - Ability of muscle fibers to recoil and resume their resting length after being stretched.

Objective 4. Describe the gross anatomy of muscle.

A muscle fiber refers to a single muscle cell. Each muscle is a discrete organ composed of hundreds to thousands of fibers, nerves, blood vessels, and connective tissue wrappings.

Connective Tissue Wrappings - Within an intact muscle, fibers are wrapped by several different layers of connective tissue. Each muscle fiber is surrounded by a fine sheath of connective tissue (C.T.) -endomysium. Several ensheathed muscle fibers are bound together side by side into larger bundles - fascicles. Each fascicle is bound by a C. T. sheath - perimysium. The fascicles are bound together by a coarser wrap of dense fibrous C.T. - epimysium - which surrounds the entire muscle. External to the epimysium is the deep fascia, a still coarser sheet of connective tissue that binds muscles into functional groups and extends to wrap other structures as well. All these C.T. sheaths are continuous with one another as well as with the tendons that join muscles to bones or other muscles. All three coverings may extend beyond the muscle fibers as a tendon. When the C.T. elements extend as a broad, flat layer, the tendon is called an aponeurosis. Muscle fibers are soft and fragile. The C.T. reinforces the muscles as a whole and provides muscle tissue with its natural elasticity.

Nerve and Blood Supply - Muscle activity is absolutely dependent on its nerve and rich blood supply. Those neurons that stimulate muscle to contract are called motor neurons. Contraction uses large amounts of ATP and therefore needs large amounts of nutrients and oxygen. Waste products from the reactions must also be eliminated. Prolonged muscle action depends on a rich blood supply to deliver nutrients and oxygen and remove wastes and heat. Each muscle fiber (cell) is in close contact with one or more capillaries.

Attachments - Most muscles at joints are attached to bones in at least 2 places. When a muscle contracts the movable bone (insertion) moves toward the immovable or less movable bone(origin). Muscle attachments may be direct or indirect. Direct or fleshy attachments: the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage. Indirect:muscle fascia extends beyond the muscle as a tendon (ropelike) or a flat broad aponeurosis and anchors the muscle to bone. Indirect is more common.

Arrangement of Fascicles (p. 290)

Parallel: long axis of the fascicles run with the longitudinal axis of the muscle - strap like or fusiform - expanded belly (biceps).

Pennate (feather): fascicles are short and attached obliquely to a central tendon running the length of the muscle.Fascicles insert into only 1 side of the tendon -unipennate. If inserted into the tendon from opposite sides - bipennate.

Convergent: broad origin - fascicles converge toward a sin,gle tendon -essentially triangular (pectoralis major).

Circular: fascicles arranged in concentric rings. Surround external body openings which they close by contracting sphincters.

Pattern of fascicle arrangement determines a muscles range of motion and power. Skeletal muscle fibers shorten to about 70% of their resting length when contracted - the longer and more nearly parallel the fibers are to the long axis, the greater the muscles range of motion. Parallel fascicles provide the greatest degree of shortening -- great range of motion, but usually not very powerful. Muscle power depends on the total number of muscle cells in the muscle.

Objective 5. Describe the microscopic anatomy of muscle. Typical skeletal muscle composed of hundreds to thousands of very long, cylindrical cells called muscle fibers. The sarcolemma is a muscle fibers plasma membrane, and it surrounds the fiber's cytoplasm or sarcoplasm. Each fiber has many oval nuclei just under the sarcolemma, conveniently out of the way of the contractile elements. The mitochondria lie in rows throughout the muscle fiber, close to the muscle proteins that use ATP. Sarcoplasm - similar to cytoplasm of other cells and contains large amounts of stored glycogen.

Myofibrils: Each muscle fiber is seen to contain large numbers of rodlike myofibrils. Myofibrils are the contractile elements of skeletal muscle. Contain three types of smaller structures called filaments:
1. thick

. thin
3. elastic

The thick and thin filaments overlap one another. The pattern of their overlap causes the cross-striations seen in muscle fibers.Filaments inside a myofibril do not extend the entire length of a muscle fiber. They are arranged in compartments called sarcomeres.

Narrow plate-shaped regions (Z lines) separate one sarcomere from the next. Within a sarcomere, the darker area (A band) extends from one end to the other of the thick filaments and includes portions of the thin filaments where they overlap the thick filaments. A lighter, less dense area (I band) contains the rest of the thin filaments but no thick filaments. The Z line passes through the center of each I band. Alternating dark A bands and light I bands give the muscle fiber its striated appearance. A narrow H zone in the center of each A band contains thick but not thin filaments. (p. 252)

Ultrastructure: The two contractile proteins in muscle are myosin and actin. Thick filaments - Each thick filament is composed of myosin molecules. Each myosin molecule is distinctive in structure - has a rodlike shaft/tail with 2 globular heads, somewhat like a golf club with 2 heads. Myosin molecules are bundled together so their tails form the central part of the filament, and point toward the M line.Their globular heads project outward and in opposite directions from all around the shaft in a spiraling fashion. The myosin heads (cross bridges) extend out toward the thin filaments. (p. 253)

Thin filaments - extend from anchoring points within the Z lines. Each thin filament is composed of actin, tropomyosin, and troponin molecules. Actin is the contractile protein. Two regulatory proteins are also present, tropomyosin and troponin. Individual actin molecules join to form an actin filament that is twisted into a helix. On each actin molecule, is a myosin binding site, for cross bridge attachment. In relaxed muscle, tropomyosin covers the myosin-binding sites on actin, blocking cross bridge attachment. (p. 253)

The third component of the sarcomere is the elastic filament which anchors thick filaments to the Z line, and helps stabilize the position of the thick filaments.

Sarcoplasmic Reticulum and T tubules (p. 254) SR inside each muscle cell is a fluid filled system that encircles each myofibril. It is a membrane system and is similar to smooth endoplasmic reticulum. In a relaxed muscle fiber, the SR stores calcium. Release of calcium into the sarcoplasm around the thick and thin filaments triggers muscle contraction. Major role of S.R. is to regulate intracellular levels of calcium.

Transverse tubules (T - tubule) - are tunnel-like infoldings of the sarcolemma that protrude deep into the cell interior. The T tubules are open to the outside of the fiber and filled with extracellular fluid. Thousands of T-tubules form the T -system. T-tubules conduct nerve stimulus deep into the cell to every sarcomere. T-tubules also provide inlets to bring extra-cellular fluid (glucose, oxygen, ions) into close contact with deeper parts of the muscle cell.

Objective 6. Describe the mechanism of contraction in a skeletal muscle fiber. When a muscle cell contracts, its individual sacromeres shorten and the distance between successive Z lines is reduced. As the length of the sarcomeres decreases, the myofibrils shorten resulting in shortening of the muscle fiber.

Sliding filament theory of contraction - contraction involves sliding of the thin filaments past the thick ones so that the extent of myofilament overlap increases. In a relaxed muscle fiber, the thick and thin filaments overlap only slightly, but during contraction the thin filaments penetrate more and more deeply into the central region of the A band. During contraction, myosin cross bridges pull on the thin filaments, causing them to slide inward. The sliding of the filaments and shortening of the sarcomeres cause shortening of the whole muscle fiber. Muscle contraction requires calcium and energy in the form of ATP. ATP attaches to binding sites on the myosin cross bridges. A portion of each myosin head acts as an ATPase, splitting ATP into ADP + P. This reaction transfers energy from ATP to the myosin head. Activated myosin heads spontaneously bind to the myosin-binding sites on actin when the calcium level rises and tropomyosin moves away from its blocking position (p. 257). The shape change that occurs when myosin binds to actin produces the power stroke of contraction. Once binding sites on actin are exposed, the following events occur in rapid succession:

Cross bridge attachment - activated myosin heads are strongly attracted to the exposed binding sites on actin.

Power stroke - myosin head pivots pulling on the thin filament, sliding it toward the center of the sarcomere. Energy requiring step.

Cross bridge detachment - as new ATP binds to the myosin head, the myosin cross bridge is released.

Cocking of the myosin head - splitting of ATP provides the energy needed to return the head to its high-energy upright or cocked position.

(p. 258) A single power stroke of all the cross bridges in a muscle results in a shortening of only about 1%. Since contracting muscles shorten 30 -35%, each myosin cross bridge must attach and detach many times during a single contraction. Only half of the myosin heads of a thick filament are actively exerting a pulling force at the same instant, the balance are seeking their next binding site. Sliding of thin filaments continues as long as the calcium signal and ATP are present.

NOTE: SUMMARY PICTURE - p. 262 (be able to label this on the test)

1. Nerve impulses reaches the axon terminal of motor neuron.
2. Acetylcholine is released and travels across synaptic cleft.
3. Acetylcholine binds to receptors on the motor end plate of the muscle cell, initiating an electrical signal to contract.
4. T tubules conduct the signal deep into the muscle cell.
5. sarcoplasmic reticulum adjacent to the T tubules receive the signal which stimulates release of calcium that has been stored in the sarcoplasmic reticulum.
6. Calcium binds to troponin which causes the tropomyosin-troponin complex to move away from blocking the myosin-binding site on the actin.
7. Myosin heads that have been activated by the hydrolysis of ATP binds to the actin and exerts power stroke which pulls the thin filament toward center of sarcomere.
8. When a new ATP binds, myosin head releases from the actin.
9. Hydrolysis of the new ATP re-cocks the myosin head for another round of attaching and pulling.

Objective 7. Describe the regulation of muscle contraction.

A motor neuron delivers the stimulus that causes a muscle fiber to contract. A motor neuron plus all the muscle fibers it stimulates is called a motor unit (p. 263). A single motor neuron makes contact with an average of 150 muscle fibers. All muscle fibers of a motor unit contract and relax together. Muscles that control precise movements, have as few as 2 - 3 muscle fibers per motor unit. Muscles responsible for powerful gross movements may contain as many as 2000 muscle fibers per motor unit. The total strength of a contraction is varied in part by adjusting the number of motor units that are activated.

Neuromuscular Junction: Neurons and muscle fibers make contact and communicate at specialized regions called synapses. At most synapses a small gap (synaptic cleft) separates the two cells. The cells do not physically touch. The cells communicate by releasing a chemical messenger called a neurotransmitter. The type of synapse formed between a motor neuron and a skeletal muscle fiber is the neuromuscular junction. The axon of a neuron branches into clusters of bulb-shaped axon terminals. The region of the muscle fiber membrane adjacent to the axon terminals is called the motor end plate. Neuromuscular junction includes both the axon terminals and the motor end plate of the muscle fiber. The distal end of an axon terminal contains many membrane-enclosed sacs (synaptic vesicles) which contain the neurotransmitter molecules (acetylcholine - ACh). A nerve impulse triggers the release of ACh from the vesicles, which then diffuses into the synaptic cleft. On the muscle side of the synaptic cleft, the motor end plate contains ACh receptors. Binding of ACh to its receptor opens a channel that passes small cations (sodium). The resulting change in membrane potential triggers a muscle action and initiates muscle contraction. Most skeletal muscle fibers have only one neuromuscular junction for each fiber located near the fiber's midpoint. The muscle action potential spreads from the center of the fiber toward both ends. This permits nearly simultaneous contraction of all parts of the fiber.


Two changes permit a muscle fiber to relax:

1. ACh is rapidly broken down by acetylcholinesterase, which is present in the synaptic cleft. When action potentials cease, no new ACh is released and acetylcholinesterase breaks down the ACh. This stops the generation of muscle action potentials, and calcium release channels in the SR close.

2. Calcium transport pumps rapidly remove calcium from the sarcoplasm into the SR. As calcium levels drop, the tropomyosin-troponin complex slides back over the myosin binding sites on actin, preventing further cross bridge binding.

Objective 8. Describe the mechanism of contraction in a skeletal muscle.

Single muscle cells respond to stimulation in an all-or-none fashion. Skeletal muscle consisting of hugh nunbers of cells contracts with varying degrees of force and for different periods of time. The response of a muscle to a single brief stimulus is called a muscle twitch. It may be strong or weak, depending on the number of motor units activated. Muscle contractions are relatively long and smooth and vary in strength.

Tetanus: smooth, sustained contraction. Usual manner of muscle contraction in the body. Results from volleys of motor neuron impulses, rather than a single impulse. Prolonged tetanus leads to a situation in which the muscle is unable to contract - known as muscle fatigue.

Objective 9. escribe the features of skeletal muscle metabolism. As a muscle contracts the energy of ATP is directly coupled to contractile events and to the activity of the calcium pump. As long as ATP synthesis balances ATP use, muscles can respond to low frequency stimuli for long periods of time. Muscles store very limited amounts of ATP, and once contraction begins the reserves are soon exhausted. ATP must be regenerated continuously if contraction is to continue.

Once vigorous exercise starts, ATP stored in working muscles is depleted in about 6 seconds. A supplementary system for rapid regeneration of ATP then starts.The reaction that occurs couples ADP with a unique high energy compound stored in muscles - creatine phosphate. This results in the almost instantaneous transfer of energy and a phosphate group to ADP to form ATP. Substantial amounts of creatine phosphate are stored in muscles. Reserves of creatine phosphate are also quickly exhausted. Together, stored ATP and creatine phosphate provide for maximum muscle power for about 10-1 5 seconds. Creatine phosphate reserves are replenished during periods of inactivity.

Aerobic Respiration: Resting and slowly contracting muscles obtain the bulk of their ATP supply via aerobic respiration of fatty acids, but when muscles are actively contracting, glucose becomes the primary fuel source. Aerobic respiration occurs in the mitochondrion requires oxygen, and involves chemical reactions in which the bonds of fuel molecules are broken and the energy released is used to make ATP. Glucose is broken down entirely, yielding water, carbon dioxide, and large amounts of ATP.

Fermentation: Initial pathway of glucose respiration is called glycolysis. Glucose is broken down into two pyruvic acid molecules and some of the energy released is captured to form small amounts of ATP. This pathway does not use oxygen (anaerobic). As long as oxygen and glycose is present, pyruvic acid then enters the oxygen-requiring aerobic pathway within the mitochondria and reacts with oxygen to produce still more ATP as described above. Prolonged vigorous muscle activity depletes both oxygen and glucose. Under anerobic conditions most of the pyruvic acid produced is converted to lactic acid, rather than carbon dioxide and water. Most of the lactic acid diffuses out of the muscle into the blood stream. When oxygen is again available, the lactic acid is reconverted into pyruvic acid and oxidized or converted into glycogen.

The aerobic pathway is 20 times more efficient than the anaerobic pathway. However, the totally anaerobic pathway is 2 1/2 times faster in producing ATIP. Heavy breathing during exercise is triggered primarily by high levels of lactic acid in the blood which stimulates the respiratory center of the brain.

Objective 10. Describe the features of smooth muscle. Smooth muscle fibers are small, spindle shaped cells with one centrally located nucleus. No striations are visible. Thick and thin filaments are present but the proportion and organization of the myofilaments are different.

Smooth muscle cells are organized into sheets of closely apposed fibers.Such smooth muscle sheets occur in the walls of hollow organs. In most cases, at least two smooth muscle sheets are present and oriented at right angles to each other. The longitudinal layer, runs with the long axis of the organ, while the circular layer runs around the circumference of the organ. The cyclic contraction and relaxation of these opposing layers allows the lumen of the organ to alternately constrict and dilate - peristalsis.

In many cases, adjacent smooth muscle cells exhibit slow, synchronized contractions, the whole sheet responds in unison to a stimulus. This reflects electrical coupling of smooth muscle cells by gap junctions, which allows smooth muscles to transmit action potentials from cell to cell. Some smooth muscle fibers are "pacemaker cells" and once excited, they act as drummers to set the contractile pace for the entire sheet. Both the rate and intensity of smooth muscle contraction may be modified by neural and chemical stimuli. Contraction of smooth muscle is slow, sustained, and resistant to fatigue. Smooth muscle takes 30 times longer to contract and relax. It can maintain the same tension of contraction for prolonged periods at less than 1% of the energy cost. The efficient contraction of smooth muscle is extremely important to overall homeostasis. Smooth muscle in small arterioles and other visceral organs routinely maintains a moderate degree of contraction.