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MUSCLE Three types of muscles based on morphological and functional difference Skeletal muscle Cardiac muscle Smooth muscle Composed of bundles of very long cylindrical multinucleated cells that has cross-striations. Composed of elongated or branched individual cells that run parallel to each other. Also have crossstriations. Consists of collections of fusiform cells, no cross-striations. Contraction is quick, forceful and usually under voluntary control. Contraction is involuntary, vigorous and rhythmic. Contraction is slow and involuntary Sarcomere Sarcomere is a basic functional unit of myofibril. Myofibrils show a pattern of alternate light bands and dark bands known as (I band and A band). Each sarcomere contains two types of filaments: thick and thin filaments. Structure of sarcomere: In the central region of the sarcomere are the thick filaments and they appear as dark band. This band is termed the A band. The thick filaments contain the protein myosin. The dark band is supported in centre by M line. The thin filaments contain the protein myosin and are attached at either end of the sarcomere at a structure known as the Z line. The limits of a sarcomere are defined by two successive Z lines. The thin filaments extend from the Z lines toward the center of the sarcomere where they overlap with the thick filaments. The I band represents the region between the ends of the A bands of two adjoining sarcomeres. In this region, only thin filaments are present. The H zone corresponds to the space between the ends of the thin filaments, where only thick filaments are present. Arrangement of sarcomere: The thick and thin filaments are arranged hexagonally with respect to one another. Each thick filament is surrounded by six thin filaments; each thin filament is surrounded by three thick filaments. The thin filaments of 2 adjacent sarcomeres show a polarity which can be observed by interacting them with isolated myosin heads with decorate the thin filaments at specific angles. The decoration takes place in the ration of one head per monomer. Why would muscle exhibit constant volume contraction? Every time the muscle contracts, sliding of filaments occur and there is a change in the angels at the zigzag regions of the Z-line. Consequently, the muscle bulged out. The sliding filament theory Muscle contraction occurs as the result of the sliding of the thick and thin filaments past one another. The length of individual filaments remains unchanged. The myosin cross bridges produce the sliding of the filaments. When bound to the actin filaments, the movement of cross bridge causes the sliding of thin and thick filaments past each other. Events between nerve action potential and contraction and relaxation of a muscle fiber The action potential in the axon causes the release of the neurotransmitter known as acetylcholine from the axons. Once released, acetylcholine binds to the receptor sites on the motor end plate. Acetylcholine increases the permeability of motor end plate to sodium and potassium ions, causing the depolarization of the end plate. The electrical signal spreads along the membrane down the T-tubules. Eventually, this will result in the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum. At low Ca2+, TN-I binds strongly to actin, tropomyosin is in a blocking position preventing the attachment of myosin heads to the actin monomers. When Ca2+ is released from the sarcoplasmic reticulum, it binds to TN-C, strengthens the linkage of TN-C to TN-I, and weakens the TN-I actin bond. The TN-I moves off the actin and carries the tropomyosin from its blocking position to one nearer the groove. The muscle contraction cycles begins. The Ca2+ is re-sequestered in the sarcoplasmic reticulum, the troponin-tropomyosin moves back and the muscle relaxes. Muscle contraction cycle: A myosin head unit with a bound ATP is in the rotated position and unattached to actin. A series of reaction takes place during which the myosin head is rotated to the cocked position relative to actin. This is probably a charged M-ATP* complex. In this state, the M-ATP* complex binds to actin in a rapid process. With actin bound, myosin is able to split ATP and ATP leaves the complex as ADP and P. The myosin head rotates while attached and generates a tension stroke. In the rotated state, another ATP attaches in a fast reaction. The actin dissociates leaving a myosin ATP complex. This cycle repeats again. Rigor Mortis Rigor Mortis is a phenomenon in which the muscles of the body become very stiff and rigid after death. It results directly from the loss of ATP in the dead muscles cells because the actin-myosin complex will not dissociate of no ATP is bind to the myosin head. Role of ATP in muscle contraction Muscle at rest: ATP + creatine -> ADP + phosphocreatine Working muscle: Phosphocreatine + ADP (in the presence of creatine kinase) -> Creatine + ATP Myosin head conformational change Dissociation of actin-myosin complex Active transport of Ca1+ Maintenance of Na+ and K+ gradients across membrane Single twitch and summation The mechanical response of a muscle fiber to a single electrical signal is known as twitch. Muscle fibers like nerve fibers have a refractory period after one stimulus, during which they will not respond to a second stimulus. The refractory period in skeletal muscle is so short that muscle can respond to second stimulus while still contracting in response to first. The result of this is the summation of contractions, which leads to a greater than normal shortening of muscle fibre. Tetanus Tetanus tension is the maximum tension builds up when all the muscle fibers are contracting at their maximally rate. Tetanus is the results of muscle contraction in response to a string of action potentials that fire to the motor end plate. There are two types of tetanus that based on the rate at which the electrical signal is delivered to a muscle fiber after the refractory period. If the muscle is stimulated at a lower rate, at this point, it will partially relax, causing a wave in contractions. This is known as incomplete tetanus or unfused tetanus. However, if the muscle is stimulated at a higher rate and it cannot relax at all, causing it to be contracted all the time. This is known as completed or fused tetanus. Tetanus ends when muscle fatigue. When muscle is fatigue, it is unable to contract anymore. This is primarily induced by the accumulation of lactic acid. Control of muscle tension The total tension that a muscle can develop depends on two factors: The number of muscle fibers contracting at a given time. The tension developed by each fiber. The number of muscle fibers that are contracting at any given time depends upon the number of motor units being stimulated. Each motor neuron innervates several muscle fibers, forming a motor unit. Stimulation of a motor neuron produces a contraction in all muscle fibers in the motor unit. One cannot simultaneously fire all the motor units in a muscle. The motor neurons fire in an asynchronous pattern. o Prevents fatigue of the muscle. o Maintaining a nearly constant tension in the muscle. Both Parkinson ‘s disease and normal shivering inhibit the subcortical centers in the brain, and movement is now dominated by the local feedback loops from the stretch receptors, and thus the motor neurons firing tend to be synchronous and oscillatory. The number of muscle fibers associated with a single motor neuron varies. In muscles of the hand, for example, which are able to produce delicate movements, the individual motor units is small. In the muscle of leg, for example, each motor unit contains hundreds of muscle fibers. The smaller the number of fibers in a motor unit, the more delicate the movement obtainable and the larger is the area of the motor cortex associated with that muscle. Antagonistic muscle Muscles can only exert a pull but not a push. For this reason, muscles are typically arranged in antagonistic pairs. When one piece of muscle within the pair contract, the other piece would have to stretch. Eg: Biceps and triceps. Relaxation of muscle has done nothing with the contraction of triceps. However, contraction of triceps stretched the relaxed biceps. In an arm-wrestling event, we should keep our arms bended at as small an angle as possible. This is because the length of the biceps would have been shorten, meaning that there s already a great overlap between thick and thin filaments within sarcomeres at the start of the competition. Isotonic and isometric contraction Moderate exercise increases the diameter of muscle cells, thus enlarging and strengthening the gross muscle being exercised. Isometric contraction: If one exercise by pushing against an immovable object or by opposing antagonistic muscle to each other, the resulting contraction does not actually shorten the muscle. However, sarcomeres shorten, generating force, but elastic elements stretch, along muscle length remain the same. Isotonic contraction: It the exercise involves movement, as in weight lifting, it is said to be isotonic, for though a muscle does shorten during such exercise, its tension does not greatly increase. Sarcomeres shorten more, but, since elastic elements are already stretched, the entire muscle must shorten. Different types of muscles with different ATPase activity and differing speeds of contraction Fast muscle low oxidation Fast muscle high oxidation Slow muscle high oxidation Low mitochondria Many mitochondria Many mitochondria White or pink, few oxidative enzymes. No myoglobin. Darker pink oxidative enzymes. Low myoglobin. Dark red oxidative enzymes. High myoglobin. Uses only glucose Uses glucose, fat and protein Uses glucose, fat and protein Easily fatigue; lactic acid formed Easily fatigue; lactic acid formed Long lasting; little lactic acid formed Fast ATPase Intermediate ATPase Slow ATPase Muscle number and size Although muscle cells enlarge with body growth and exercise, they do not increase in number, so that as they die they cannot be replaced. Thus, the number of muscle cells in all muscles drastically decreases in old ages. Part of this decrease is no doubt due to minor injuries that occurs in the course of a life time.