Physiology Of Muscle

PHYSIOLOGY OF MUSCLE Muscle cells are “excitable cells”. Like neurons, action potentials can be produced in response to chemical, electrical, or mechanical stimuli. Due to the presence of contractile elements, muscle response is able to generate a force, which could be used in the performance of many activities, for example, movement of the body parts, mixing of foods, elimination of wastes, etc. A. Types of muscles: There are three types of muscle which differ anatomically and physiologically from one other. 1. Skeletal muscle - cross striations are a very prominent feature - muscle fibers are distinctly separate - normal contractions is in response to nervous stimulation - under voluntary control 2. Cardiac muscle - is also striated - functionally responding as a single muscle fiber - contracts automatically and rhythmically even in the absence of its nerve supply - involuntary 3. Smooth muscle - unstriated - may posses some automaticity and rhythmicity - involuntary B. Muscle contraction: Muscle contraction is a mechanical change, characterized by shortening or development of tension and which is associated with electrical, chemical and thermal changes, all of which are reversible. 1

1. Electrical changes: Excitation of the skeletal muscle fiber. a) Electrical properties at rest are similar to nerve fibers: - concentration of K+ higher inside the cell - cell wall, at rest, is highly permeable to K+ but only slightly permeable to Na+ - concentration of Na+ higher outside cell - resting membrane potential is about 90 mv, inside negative b) Excitation of the muscle, the action potential. - the nerve impulse causes release of acethylcholine at the endings - at the end plate, an end plate potential is created. If its magnitude reaches the firing level, an action potential is produced which moves along the muscle membrane in both directions. The action potential is transmitted rapidly to innermost muscle fibers thru the transverse tubular system. The whole process is termed neuromuscular transmission c) Electrical properties during activity - the action potential has an overshoot of up to +30 mv. - duration of an action potential 2-4 millisecond - rate of conduction through muscle 5 m/sec - the electrical response always occurs before and is completed by the time the mechanical response begin 2. Mechanical changes Depolarization of the muscle fibers results in contraction. The transformation of electrical to mechanical activity involves chemical reactions and the process is called excitationcontraction coupling. a) The muscle twitch The response to a single action potential starts about 2 msec. after the beginning of depolarization of muscle membrane

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Stages: (i) Latent period The interval between the depolarization of the muscle fiber and the beginning of muscle contraction. (ii)

Period of Contraction The muscle fiber contracts and shortens or develop tensions. The interval between the beginning of the electrical response to the peak of the tension curve is sometime called the contraction time.

(iii)

Period of relaxation There is reduction in the developed tension of force.

. The duration of the muscle twitch varies, depending on the specie and the type of muscle. It maybe as short as 7.5 msec. in “fast” muscle such as those concerned with fine, rapid and precise movements. It maybe as long as 100 msec. – 200 msec. in “slow” muscle like those concerned with maintenance of posture. b) Types of muscle contraction (i) Isometric contraction Contraction without a decrease in the length of the whole muscle. The muscle exerts tension but does not shorten. No work is done. Example: Contraction of muscles maintaining equilibrium and when one holds on object. (ii)

Isotonic contraction

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posture

and

The muscle shortens under a constant load. Weight is carried through a distance and thus work is performed (positive work) If, during the contraction, the tension developed is less than the opposing force, the muscle is actually stretched or lengthened (negative work) ( c) Mechanism of shortening ( i) Molecular background - A myofibril is made up of thick and thin filaments which contain contractile proteins longitudinally arranged. - Each thick filament is surrounded by six thin filaments which are equally spared in a hexagonal manner - The thick filament is a bundle of myosin molecules which are rod-shaped and possesses a double head at one end. The myosin rods are parallel to each other while their head are oriented towards both ends and the filament. - The thin filament contains the other contractile proteins – actin, troponin and troypomyosin - Stands of globular actin molecules form a double helix. Within the groove of the double helix are found the tropponin-tropomyosin complex. This complex is made up of strings of tropomyosin molecules with troponin molecules located at internvals along it. Each string of tropomyosin extends over 7 molecules of actin. - There are at least there (3) known components of troponin: Troponin I, interaction inhibitors; Troponin T, binds the troponin to tropomyosin; Troponin C, Calcium binding component. ( ii)

Excitationcontraction

contraction

coupling;

the process

of

- When the call membrane is depolarized, the action potential is transmitted to the muscle fibers thru the 4

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-

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sarcotubular system (T-tubules and sarcoplasmic reticulum) Ca++ ions are released from the terminal cisterns (lateral saca) of the sarcoplasmic reticulums and diffuses to the reactive sites. Ca++ binds to troponin C Removal of inhibitory action of the troponintropomyosin complex on action Interaction between actin and myosin; myosin heads attach to actin reactive sites (formation of crossbridges) Breakdown of ATP and release of energy “Swiveling” or “rowing” movement of myosin heads attached to actin. After each rowing movement the myosin head detaches from the actin, swings back and binds with the next actin reactive site. Repeated attachment, swiveling and detachment of the myosin cause the sliding of the thick and thin filaments along and past each other thus causing shortening. This mechanism is the so-caled sliding filament theory of muscles contraction.

( iii) Relaxation of muscle - While the cell membrane repolarizes, Ca++ is actively sequestered back to the cisterns. ATP provides the energy for this. - Inhibitory action of troponin-tropomyosin on actin is re-established. - Myosin attachments to actin are broken and the filaments slide back to the pre-contraction position (d ) The motor unit An individual motor nerve fiber supplies more than one muscles Fiber. Each individual motor neuron and all the muscle fiber it supplies, is called the motor unit. The number of muscles fibers in the motors unit is variable depending on the function of the muscle. Muscles which are precisely controlled and rapidly reacting has smaller number of muscles in the motor unit, like the 5

muscles of the hand and eye, where the muscle fibers on the motor unit maybe as low as 3-6. Muscles concerned with coarser movements like those for maintainance of posture have more fibers in the motor unit, averaging 180 fibers. (3) Chemical Changes: (a) The immediate source of energy of muscle contraction is ATP. Hydrolysis of ATP to ADP is associated with the release of a great amount of energy. (b) ATP is resynthesized from ADP by the addition of a phosphate group. Energy from this reaction maybe derived from: (i)

(ii) (iii) (iv)

Hydrolysis of phosphoceatine to creatine and phosphates with liberation of energy Breakdown of glucose into pyruvic acid and into CO2 and water in the presence of oxygen (aerobio glypolysis) In the absence of oxygen, breakdown of glucose into pyruvic acid, then into lactic acid (anerobic glycolysis) Oxidation of free fatty acids into CO2 and water.

The energy liberated by the breakdown of carbohydrates is also used to resynthesize phosphocreatine. If muscular exertion is to great, the supply of oxygen, even though markedly increased, becomes insufficient for the aerobic resynthesis of ATP. Thus the energy provided by the anerobic breakdown of glucoses to lactic acid is availed of for resynthesis. The accumulation of lactic acid however reduces pH in the muscles tissues and inhibits the process of resynthesis. Thus the oxidative metabolism continues even after the exertion is over, in order to remove excess, lactic acid as well as to continue the resynthesis of energy stores. The additional amount of oxygen needed for these activities is the consequence of the disparity between aerobic resynthesis and the utilization of energy stores during the increased muscular effort. Hence, an oxygen debts, is said to have been incurred.

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4. Thermal changes The liberation of energy during muscle contraction is associated with the production of heat. (a) Resting heat - heat given off by the inactive muscle, it is due to the metabolic process of the resting muscle. (b) Initial heat – heat liberated during the active phase of contraction. It is made-up of the following: (i)

(ii)

Activation heat - heat production when the muscle contracts as in a muscle twitch. (in titanic contraction it is called maintenance heat) Hat of shortening - liberated when the muscle shortens. It is dependent upon the degree of shortening.

( c ) Recovery heat or delayed heat Additional amount of hat liberated as a consequence of metabolic processes which returns the muscles into its original precontraction physical condition. (i) Heat of relaxation - liberated when isotonic muscle contraction is over and relaxation occurs. The heat is produced as a result of the structuring by the load C. Effects of multiple stimuli on muscle contraction. 1. Treppe (staircase” phenomenon) If a fresh skeletal muscle is stimulated successively with maximal stimuli at a frequency which allow the muscle to relax completely, the tension developed during each twitch increases until the tension becomes the same during each contraction. This phenomenon maybe some kind of “warning up” process the mechanism of which is not well understood. 2. Wave summation and tetanus

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A single maximal stimulus will cause the development maximal tension. If another stimulus, stronger than maximal, is applied, the twitch tension developed will still be of the same magnitude. But if two maximal stimuli are used successively with the second being applied while the muscle is still responding to the first stimulus, response greater than that produced by a single maximal stimulus, will be elicited. Several stimuli applied successively in this manner cause responses of increasing magnitude. The phenomenon is called wave summation. If the stimuli will applied at t a rapid rate, all contractile elements are maximally stimulated, all elastic component are stretched fully and a sustained contraction is produced. This is called tetanus. If the rate of the stimulation is slower, mechanical fusion may not occur. The individual contractions maybe discernible and the phenomenon is called incomplete tetanus. The rate of stimulation capable the producing tetanus varies. Examples: 350 stimuli/sec. for internal rectus 30 stimuli/sec. for sole us muscle The action potential developed in muscle membrane is always of the same magnitude (all-or-none). The activation of the contractile elements is not. It is dependent on the amount of Ca++ released from the cisterns. Successive stimuli produces action potentials which cause more and more Ca++ to be released, thus increasing the contractile response. D. Effect of increasing the intensity of the stimulus: A weak stimulus activates only a few motor units. As the intensity of the stimulus increase, more and more motor units are activated. At a certain intensity, more motor units are activated. Any further increase in stimulus intensity therefore will not increase anymore the magnitude of response. The process is called recruitment of motor units, also sometime called quantal summation. E. Length-Tension Relationship Muscle is made up of contractile elements and elastic tissues. If no stretch is initially applied on the muscle, the active tension produced by muscle contraction will be less since the pull merely stretches the elastic tissue (series-elastic components). If the muscle will be stretched passively and passive tension develops, the active tension produced by muscle 8

contraction, increases. The greater passive tension, the greater is the tension produced up to a certain point. Beyond which, any further passive stretching of the muscle reduce the active tension developed during contraction. The length of the muscle in which maximum active tension is developed when the muscle is stimulated is called the resting length. This is the normal length of the muscle in the body. At thus length, the muscle is lightly stretched and therefore, there is some degree of passive tension present. Proof of this, is the shortening of a muscle that occurs when the tendon is severed from its attachment. F.

Effect of Load on the Velocity of Shortening: Increasing the load on the muscle slows down the shortening and reduces the distance of load is moved. G.

Smooth Muscle: 1. Types: (a) Multi-unit smooth muscle (i) Characteristics: - found in ciliary muscles, iris pilomotors and the blood vessels - responses somewhat similar to skeletal muscle - activity controlled by discharge from motor nerves which is believe to release nor-epinephrine at the endings. - Time course of muscle twitch greatly prolonged - Summation and tetanus maybe produced (c) Visceral Smooth Muscle (i) Characteristics - found in the walls of hollow visceral organs like the uterus, ureters and intestines. - Individual fibers are in functional continuity (syncitial) - Innervated by the fibers of the automatic nervous system which are responsible for regulation, not the initiation of muscular activity. - Involuntary - Capable of persistent, sustained activity independent of extrinsic and intrinsic nerve activity. This property is termed “tonus”. 9

- Stretching to a new length initially increases tension but this tension diminishes as the new length is attained. This property of “elasticity” allows an increase in the volume of the organ without a very great rise in pressure. (ii)

Electrical properties. - distribution and permeability of K+ across the resting muscle membrane is similar to other cells, - resting potential, about 60 mv., is lower and less stable than that of skeletal muscle. - Shall fluctuations of the resting potential occurs an if such a change reaches threshold, an action potential is produced.

(Additional consideration or smooth muscle physiology will be taken up ender the different organ systems)

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