Chapter 8 Summary

CHAPTER 8 SUMMARY

INTRODUCTION

•Muscles evolved for a variety of animal movements: Purposeful locomotory movement of the whole body or parts of the body; manipulation of external objects; propulsion of contents through various hollow internal organs; emptying the contents of certain organs to the external environment; production of heat as a metabolic by-product; and production of sound.

•Muscles are categorized as striated (skeletal, cardiac) and unstriated (smooth)

SKELETAL MUSCLE

Structure of Skeletal Muscle

•Almost all living cells possess rudimentary intracellular machinery for producing movement.

•Thespeed of contraction of muscle fibers is accomplished, in part, byreapportionment of key intracellular structures in addition to shifts in the isoforms in the kinetic rates of certain proteins

•Muscle cells are specialized for contraction. There are three types of muscle found in vertebrates: skeletal, smooth, and cardiac. Invertebrates lack smooth muscle. Skeletal muscles are made up of bundles of long, cylindrical muscle cells known as muscle fibers, wrapped in connective tissue. Muscle fibers are packed with myofibrils, with each myofibril consisting of alternating, slightly overlapping stacked sets of thick and thin filaments. This arrangement leads to a skeletal muscle fiber’s striated microscopic appearance. Thick filaments are composed of the protein myosin. Cross-bridges made up of the myosin molecules’ globular heads project from each thick filament.

•Thin filaments are composed primarily of the protein actin, which has the ability to bind and interact with the myosin cross-bridges to bring about contraction. However, two other proteins, tropomyosin and troponin, lie across the surface of the thin filament to prevent this cross-bridge interaction in the resting state.

•Myosin filaments are linked to the Z lines by the gigantic, elastic protein titin, which functions as a locomotory spring assisting the return of the sarcomere to its relaxed conformation.

Molecular Basis of Skeletal Muscle Contraction

•Excitation of a skeletal muscle fiber by its motor neuron brings about contraction through a series of events that results in the thin filaments sliding closer together between the thick filaments. This sliding- filament mechanism of muscle contraction is switched on by the release of Ca++ from the lateral sacs of the sarcoplasmic reticulum.

•Calcium release occurs in response to the spread of a muscle fiber action potential into the central portions of the fiber by means of the T tubules. Released Ca++ binds to the troponin-tropomyosin complex of the thin filament, causing a slight repositioning of the complex to uncover actin’s cross-bridge binding sites. After the exposed actin attaches to a myosin cross-bridge, the molecular interaction between actin and myosin releases the energy within the myosin head that was stored from the prior splitting of ATP by the myosin ATPase site. This released energy powers cross-bridge stroking. During a power stroke, an activated cross-bridge bends toward the center of the thick filament, “rowing” in the thin filament to which it is attached. With the addition of a fresh ATP molecule to the myosin cross-bridge, myosin and actin detach, the cross-bridge returns to its original shape, and the cycle is repeated.

•Repeated cycles of cross-bridge activity slide the thin filaments inward step by step. When there is no longer a local action potential, the lateral sacs actively take up the Ca++, troponin and tropomyosin slip back into their blocking position, and relaxation occurs. The entire contractile response lasts about 100 times longer than the action potential.

Skeletal Muscle Mechanics

•Gradation of whole-muscle contraction can be accomplished by (1) varying the number of muscle fibers contracting within the muscle and (2) varying the tension developed by each contracting fiber. The greater the number of active muscle fibers, the greater the whole muscle tension. The number of fibers contracting depends on (1) the size of the muscle (the number of muscle fibers present); (2) the extent of motor unit recruitment (how many motor neurons supplying the muscle are active); and (3) the size of each motor unit (how many muscle fibers are activated simultaneously by a single motor neuron).

•Also, the greater the tension developed by each contracting fiber, the stronger the contraction of the whole muscle. Two readily variable factors having an effect on the fiber tension are (1) the frequency of stimulation, which determines the extent of twitch summation, and (2) the length of the fiber before the onset of contraction. Twitch summation refers to the increase in tension accompanying repetitive stimulation of the muscle fiber. After undergoing an action potential, the muscle cell membrane recovers from its refractory period and is able to be restimulated again while some contractile activity triggered by the first action potential still remains. As a result, the contractile responses (twitches) induced by the two rapidly successive action potentials are able to sum, increasing the tension developed by the fiber.

•If the muscle fiber is stimulated so rapidly that it does not have a chance to start relaxing between stimuli, a smooth, sustained maximal (maximal for the fiber at that length) contraction known as tetanus takes place.

•The tension developed upon a tetanic contraction also depends on the length of the fiber at the onset of contraction. At the optimal length (lo), which is the resting muscle length, there is maximal opportunity for cross-bridge interaction due to optimal overlap of thick and thin filaments; thus, the greatest tension can be developed. At lengths shorter or longer than lo, less tension can be developed upon contraction, primarily because a portion of the cross-bridges are unable to participate.

•The two primary types of muscle contraction—isometric (constant length) and isotonic (constant tension)—depend on the relationship between muscle tension and the load. If tension is less than the load, the muscle cannot shorten and lift the object but remains at constant length, producing an isometric contraction. In an isotonic contraction, the tension exceeds the load so the muscle can shorten and lift the object, maintaining constant tension throughout the period of shortening.

•The bones or exoskelton, muscles, and joints form lever systems. The most common type amplifies the velocity and distance of muscle shortening to increase the speed and range of motion of the body part moved by the muscle. This increased maneuverability is accomplished at the expense of the muscle having to exert considerably more force than the load.

Skeletal Muscle Metabolism and Fiber Types

•Three biochemical pathways furnish the ATP needed for muscle contraction: (1) the transfer of high-energy phosphates from stored creatine phosphate to ADP, providing the first source of ATP at the onset of exercise; (2) oxidative phosphorylation, which efficiently extracts large amounts of ATP from nutrient molecules if sufficient O2 is available to support this system; and (3) glycolysis, which can synthesize ATP in the absence of O2 but uses large amounts of stored glycogen and produces lactic acid in the process.

•There are three types of muscle fibers, classified by the pathways they use for ATP synthesis (oxidative or glycolytic) and the rapidity with which they split ATP and subsequently contract (slow twitch or fast twitch): slow-oxidative fibers, fast-oxidative fibers, and fast-glycolytic fibers.

•Most vertebrate muscles contain a mixture of all three fiber types; the type of activity for which the muscle is specialized largely determines the percentage of each type.

Control of Motor Movement

•Control of any motor movement depends on the level of activity in the presynaptic inputs that converge on the motor neurons supplying various muscles. These inputs come from three sources: (1) spinal-reflex pathways, which originate with afferent neurons; (2) the corticospinal motor system, which originates at the primary motor cortex and is concerned primarily with discrete, intricate movements of the hands; and (3) the multineuronal motor system, which originates in the brain stem and is mostly involved with postural adjustments and involuntary movements of the trunk and limbs. The final motor output from the brain stem is influenced by the cerebellum, basal nuclei, and cerebral cortex.

•Establishment and adjustment of motor commands depend on continuous afferent input, especially feedback about changes in muscle length (monitored by muscle spindles) and muscle tension (monitored by Golgi tendon organs).

•The most significant space as well as energy saving solution for flight was the evolution of asynchronous muscle contractions in some insects. Contraction of asynchronous flight muscle is normally triggered by stretch and deactivated by “shortening deactivation” both in the presence of elevated myoplasmic Ca++concentrations.

SMOOTH MUSCLE

•The thick and thin filaments of smooth muscle are not arranged in an orderly pattern, so the fibers are not striated. Cytosolic Ca++, which enters from the extracellular fluid as well as being released from sparse intracellular stores, activates cross-bridge cycling by initiating a series of biochemical reactions that result in phosphorylation of the myosin cross-bridges to enable them to bind with actin. Multiunit smooth muscle is neurogenic, requiring stimulation of individual muscle fibers by its autonomic nerve supply to trigger contraction.

•Single-unit smooth muscle is myogenic; it is able to initiate its own contraction without any external influence as a result of spontaneous depolarizations to threshold potential brought about by automatic shifts in ionic fluxes. Once an action potential is initiated within a single-unit smooth muscle cell, this electrical activity spreads by means of gap junctions to the surrounding cells within the functional syncytium, so that the entire sheet becomes excited and contracts as a unit. The autonomic nervous system as well as hormones and local metabolites can modify the rate and strength of the self-induced contractions.

•Smooth muscle contractions are energy efficient, enabling this type of muscle to economically sustain long-term contractions without fatigue, particularly in “latch” types in vertebrates and “catch” types in invertebrates, such as bivalve adductor muscles. This economy, coupled with the fact that single-unit smooth muscle is able to exist at a variety of lengths with little change in tension, makes single-unit smooth muscle ideally suited for its task of forming the walls of distensible hollow organs.

CARDIAC MUSCLE

•Cardiac muscle is found only in the heart. It has highly organized striated fibers like skeletal muscle. Like single-unit smooth muscle, some cardiac muscle fibers are capable of generating action potentials, which are spread throughout the heart with the aid of gap junctions.

• The expression of myosin genes change with activity and other environmental factors related to muscle use, in concert with changes in muscle fiber types (see main text). In human muscles repeatedly loaded, as in weight training, the genes of fast IIx fibers stop transcribing the IIx myosin and begin transcribing the IIa myosin, as the fibers convert from IIx type to IIa type.(In rodents subjected to the equivalent of weight training, fibers convert from IIb to IIx.) In contrast, in sedentary people, much of the limb muscle converts to type IIx (which fits the way their muscles are used: quiet most of the time, with intermittent short bursts of usage).

• Smooth muscle appears to be different. There is one smooth-muscle myosin gene in mammals, but it can yield up to four different types of myosin proteins: SM1A, SM1B, SM2A, and SM2B. This is accomplished by the process of alternative RNA splicing in the nucleus, a process in which the same parent RNA transcribed from one gene can be cut and spliced during intron removal to yield different mRNAs.

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