What is the meaning of epi? Of mys? How do these word roots relate to the role and position

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1 What is the meaning of epi? Of mys? How do these word roots relate to the role and position of the epimysium? Blood vessel---"7""s"', fe--- Muscle fiber (cell) Perimysium --"+t Fascicle (wrapped by perimysium) Endomysium (between fibers) --'--:--h--- Bone FG U R E 6. 1 Connective tissue wrappings of skeletal muscle. in the way their fibers are arranged. Many are spindle-shaped as just described, but in others, the fibers are arranged in a fan shape or a circle, as described on p Smooth Muscle Smooth muscle has no striations and is involuntary, which means that we cannot consciously control it. Found mainly in the walls of hollow visceral organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels substances along a definite tract, or pathway, within the body. We can best describe smooth muscle using the terms visceral, nonstriated, and involuntary. "epsnw e )19110)0 uodn LiJeeLJSe S iumssund» elj1 'epetuu = saw pue episeo ')19110 'uodn = d3

2 , -,, -,, Dark Light Nucleus (A) band () band (a) Segment of a muscle fiber (cell) Thin (actin) filament-il~jl!~~jii~\i{f'ii ~~~~'Hi' Thick (myosin) filament FG J\'r--- Sarcolemma a skel (a) A ~:- One ~ (b) E~ =- a rnvc" Myofibril pattersarcor-= - myof:::- and -,, the sa':-_ --",- -,,,,,-,, H zone ~,,-, -, -,,, rj (b) Myofibril or fibril (complex organelle composed of bundles of myofilaments) Thin (actin) filament #X:x:x:x:x:>o< band A band band ~Sarcomere~ J M line Thick (myosin) filament -<><,.."..,>E M Thick filament Bare zone Thin filament,, (d) Myofilament structure (within one sarcomere)

3 , -,, -,, Dark Light Nucleus (A) band () band (a) Segment of a muscle fiber (cell) Thin (actin) filament-il~jl!~~jii~\i{f'ii ~~~~'Hi' Thick (myosin) filament FG J\'r--- Sarcolemma a skel (a) A ~:- One ~ (b) E~ =- a rnvc" Myofibril pattersarcor-= - myof:::- and -,, the sa':-_ --",- -,,,,,-,, H zone ~,,-, -, -,,, rj (b) Myofibril or fibril (complex organelle composed of bundles of myofilaments) Thin (actin) filament #X:x:x:x:x:>o< band A band band ~Sarcomere~ J M line Thick (myosin) filament -<><,.."..,>E M Thick filament Bare zone Thin filament,, (d) Myofilament structure (within one sarcomere)

4 Axon terminals at neuromuscular junctions Muscle -- Motor neuron cell bodies Muscle Muscle fibers Branching axon to motor unit (b) (a) FG U R E 6. 4 Motor units. Each motor unit consists of a motor neuron and all the muscle fibers it activates. (a) Portions of two motor units are shown. The motor neurons reside in the spinal cord, and their axons extend to the muscle. Within the muscle, each axon divides into a number of axon terminals, distributed to muscle fibers scattered throughout the muscle. (b) Photo of a portion of a motor unit (BOX). part of the sarcolemma. f enough acetylcholine is released, the sarcolemma at that point becomes temporarily more permeable to sodium ions ( a +), which rush into the muscle cell, and to potassium ions (K+), which diffuse out of the cell. However, more a + enters than K+ leaves. This gives the cell interior an excess of positive ions, which reverses the electrical conditions of the sarcolemma and opens more channels that allow Na + entry only. This "upset" generates an electrical current called an action potential. Once begun, the action potential is unstoppable it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other. The result is contraction of the muscle cell. t should be mentioned that while the: potential is occurring, acetylcholine, whicr the process, is broken down to acetic choline by enzymes (acetylcholinesterase. - present on the sarcolemma (see Figure this reason, a single nerve impulse produ one contraction. This prevents continued tion of the muscle cell in the absence tional nerve impulses. The muscle eel, until stimulated by the next round 0: choline release. This series of events is explained n: on pp in the discussion of nerve ogy, but perhaps it would be helpful to this to some common event, such as

5 -\----- Myelinated axon of motor neuron Action potential Axon terminal Nucleus --~= (a) -: erminalof --:or neuron : ondrion ao ic cleft _- e Axon terminal r-fusing synaptic vesicle i!r -r: ACh molecules Acetic acid :=. nation :3 sarco- ~ at _ -'T]uscular :- n o o AChE Binding of ACh to receptor opens Na+/K+ channel (b) (c) J R E 6. 5 The neuromuscular junction. (a) Axon terminal of, -::or neuron forming a neuromuscular junction with a muscle fiber. --:: axon terminal contains vesicles filled with the neurotransmitter -- :::noline (ACh), which is released when the nerve impulse reaches - =- on terminal. The sarcolemma is highly invaginated (folded) =-::~ to the synaptic cleft, and acetylcholine receptors are present -:-e folds. (c) Acetylcholine diffuses across the synaptic cleft and ::-es to ACh receptors on the sarcolemma, initiating changes in the ca condition of the sarcolemma. under a small dry twig (Figure 6.6). The n of the twig by the flame can be como the change in membrane permeability ows sodium ions into the cell. When that the twig becomes hot enough (when sodium ions have entered the cell), the _ -ill suddenly burst into flame, and the flame ume the twig (the action potential will be conducted along the entire length of the sarcolemma). The events that return the cell to its resting state include (1) diffusion of potassium ions CK+) out of the cell and (2) operation of the sodiumpotassium pump, the active transport mechanism that moves the sodium and potassium ions back to their initial positions.

6 Protein complex (c) Myosin myofilament (a) n a relaxed muscle cell, the regulatory proteins forming part of the actin myofilaments prevent myosin binding (see a). When an action potential sweeps along its sarcolemma and a muscle cell is excited, calcium ions (Ca 2 +) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum). Myosin binding site Ca ~ The free myosin heads are "cocked," much like a set mousetrap. The physical attachment of myosin to actin "springs the trap," causing the myosin heads to snap (pivot) toward the center of the sarcomere. Because actin and myosin are still firmly bound to each other when this happens, the thin filaments are slightly pulled toward the center of the sarcomere (see c). ATP provides the energy needed to release and recook each myosin head so that it is ready to take another "step" and attach to a binding site farther along the thin filament. When the action potential ends and calcium ions are reabsorbed into the SR storage areas, the regulatory proteins resume their original shape and position, and again block myosin binding to the thin filaments. Since myosin now has nothing to attach to, the muscle cell relaxes and settles back to its original length. (b) Upper part of thick filament only FG U R E 6. 8 Schematic representation of contraction mechanism: The sliding filament theory. The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, they change both their shape and their position on the thin ilaments. This action exposes myosin binding sites on the actin, to which the myosin heads can attach see b), and the myosin heads immediately begin eeking out binding sites. ctin ("the ground"), so that 'the thin filaments canot slide backward as this cycle repeats again and [gain during contraction. otice that the myofilanents themselves do not shorten during contracion they simply slide past each other. The attachment of the myosin cross bridges to tin requires calcium ions (Ca 2 +). So where does e calcium come from? As indicated in Figure 6.5b, ction potentials (black arrows) pass deep into the niscle cell along membranous tubules (T tubules) tat fold inward from the sarcolemma. nside the cell, the action potentials stimulate the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions trigger the binding of myosin to actin initiating filament sliding. This sliding process and the precise role of calcium are depicted in Figure 6.8. When the action potential ends, calcium ions are immediately reabsorbed into the SR storage areas, and the muscle cell relaxes and settles back to its original length. This whole series of events takes a few thousandths of a second.

7 .:r... L...:Jvvllll,.l _.. c ~A o (Stimuli) t (a) Twitch t t (b) Summing of contractions t t t t (c) Unfused (incomplete) tetanus tttttttttttttttt (d) Fused (complete) tetaru.s FG U R E 6. 9 A whole muscle's response to different rates of stimulation. n (a), a single stimulus is delivered, and the muscle contracts and relaxes (a twitch contraction). n (b), stimuli are delivered more frequently, so the muscle does not have time to completely relax contraction force increases because effects of the individual twitches are summed. n (c), more complete fusion of the twitches (unfused tetanus) occurs as stimuli are delivered at a still faster rate. n (d), fused tetanus, a smooth continuous contraction without any evidence of relaxation, results from a very rapid rate of stimulation. (Points at which stimuli are delivered are indicated by red arrows. Tension [measured in gramsl on the vertical axis refers to the relative force of muscle contraction.) ~DD YOU GET T? 7. What chemical-atp or Ca2+-triggers sliding of the muscle filaments? 8. What ions enter the muscle cell during action potential generation? 9. Which is a cross-bridge attachment more similar to: a precise rowing team or a person pulling a bucket on a rope out of a well? Contraction of a Skeletal Muscle as a Whole Graded Responses For answers, see Appendix D. n skeletal muscles, the "all-ot-none" law of muscle physiology applies to the muscle cell, not to the whole muscle. t states that a muscle cell will contract to its fullest extent when it is stimulated adequately it never partially contracts. However, the whole muscle reacts to stimuli with graded responses, or different degrees of shortening. n general, graded muscle contractions can be produced two ways: (1) by changing the frequency of muscle stimulation and (2) by changing - number of muscle cells being stimulated at time. A muscle's response to each of these briefly described next. Muscle Response to ncreasingly Rapid Stimulati Although muscle twitches (single, brief, jer. contractions) sometimes result from certain ne vous system problems, this is not the way our m cles normally operate. n most types of mu activity, nerve impulses are delivered to the mu at a very rapid rate-so rapid that the muscle d not get a chance to relax completely betweestimuli. As a result, the effects of the succes. contractions are "summed" (added) together, ar the contractions of the muscle get stronger ar smoother. When the muscle is stimulated so ra~ idly that no evidence of relaxation is seen and t! contractions are completely smooth and sustain _ the muscle is said to be in fused, or comple tetanus (tet' ah-nus), * or in tetanic contracti ~ 'Tetanic contraction is normal and desirable and is quite different from the pathological condition of tetanus (commo called iocejaui), which is caused by a toxin made by bacteri Lockjaw causes muscles to go into uncontrollable spasms, finally causing respiratory arrest.

8 Which of these methods of ATP generation is commonlv used bv the leg muscles of a long-distance cyclist? ADP (a) Direct phosphorylation of ADP by reaction with creatine phosphate (CP) (b) Aerobic respiration (oxidative phosphorylation) (c) Anaerobic glycolysis and lactic acid formation Energy source CP Energy sources: glucose pyruvic Enerpy source: glucose acid free fatty acids from adipose tissue amino acids from protein catabolism Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 see Oxygen use: Required Products: 36 ATP per glucose, CO 2, H 2 0 Duration of energy provision: Hours Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: see FG U R E Methods of regenerating ATP during muscle activity. The fastest mechanism is (a) direct phosphorylation the slowest is (b) aerobic respiration. when it is unable to contract even though it is still being stimulated. Without rest, a working muscle begins to tire and contracts more weakly until it finally ceases reacting,and stops contracting. Muscle.fatigue is believed 'to result from the oxygen deficit that occurs during prolonged muscle activity: A person is not able to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need when they are working vigorously. Obviously, then, the work that a muscle can do and how long it can work- without becoming fatigued depend on how good its blood supply is. When.(q) iustueuoetu :JlqOJaealJl muscles lack oxygen, lactic acid begins to accu: late in the muscle via the anaerobic me char: described above. n addition, the muscle's _ supply starts to run low and ionic imbalance curs. Together these factors cause the musc - contract less and less effectively and finally to contracting altogether. True muscle fatigue, in which the muscle. entirely, rarely occurs in most of us becau e feel fatigued long before it happens and we si slow down or stop our activity. t does hap commonly in marathon runners. Many of u have literally collapsed when their muscle came fatigued and could no longer work. Oxygen deficit, which always occurs to extent during vigorous muscle activity, mu - "paid back" whether fatigue occurs or not. Du