Muscle tissue

Mesodermal in origin, muscle has several functions: supply force for movement, restrain movement, proper posture, act on viscera (internal organs) for peristalsis (moving food down digestive tract), give body shape, form sphincters, (such as in esophagus, between stomach and intestine, large and small intestine, in anus), in sheets of muscles, affect air flow in and out of lungs, line blood vessels and play vital role in circulation.

Secondary roles: heat production (shivering a specialized heat production to supplement metabolism).

Muscles co-opted to other non-original functions: sharks detect electrical field created by fish muscles. Some fish formed electric organs, create current strong enough to repel predators or stun prey. Other fish can use field as “radar” to see things and communicate with other animals. (Evolved independently in different groups).

Different classifications: by color, (red or white) location, nature of nervous system control (voluntary or involuntary), embryonic origin, or by general microscopic appearance (striated, smooth, and cardiac.)

Striated muscle (or skeletal muscle): under voluntary control. Individual cells called fibers, grouped into fascicle. Myofibrils founding one cell made of even smaller myofilaments. Each striated cell very long and multi-nucleated. Fibers joined end to end to form longer composite fibers. Sarcomeres: repeating units make up myofibrils. Two kinds of myofilaments, thick kind made up of myosin and thin of actin. Striations visible in light microscope, smaller part only with electron microscope.

Cardiac muscle: occurs only in heart. Light banding visible under light microscope. Each band short, principally mononucleate (occasionally dinucleate) often branched, joined together with intercollated discs. Involuntary. Waves of contraction spread through intercollated discs. Initiated by nerve stimulation or can originate in the heart itself (useful in heart transplants.)

Smooth muscle: no striations visible with light microscope. Almost entirely visceral function: digestion, sphincters, urogenital tracts, piloerectory muscles (make hairs stand up), lungs. Non-voluntary control. Slow and sustained action. Each cell mononucleate, short, fusiform (spindly) in shape, cells usually uniform in size.

Striated muscle contraction: Muscle broken into units called fascicles, in units of myofibrils. Repeating units called sarcomeres, consisting of two kinds of myofilaments:
1. thick, myosin filament
2. thin, actin filament.

Sarcomere: Thick and thin filaments interspersed in ordered grid.

Sliding filament theory: thick and thin filaments move past each other in opposite direction, shortening length. Longer muscles contract more rapidly than short ones (see cell bio for details).

Myosin molecule: two polypeptides twisted together with two globular heads at end.

Myosin filament: many slender myosin molecules together.

Actin filament: chain of actin single, tropomyosin strands with repeated globular troponin, and with actin. All play role in muscle contraction. Myocin heads have sites that bind to actin. Actin filaments have many regular sites that can bind to myosin.

Troponin has four sites:
1. one to bind myosin
2. one for actin
3. one for tropomyocin
4. one for calcium ions

Nerve signal reaches muscle, triggers release of chemical signal called neurotransmitter, that diffuses across cell membrane (sarcolimic reticulum) and binds to receptors in it. Receptor is acetylcholine, ACH. When there is enough nerve signal, the message travels through t-line to sarcoplasmic reticulum to release calcium ions.

Lacking calcium, tropomyosin site blocked. In calcium, myosin binding sites exposed and heads bind to actin molecules, delivering force to move fibers in relation to each other. Myocin head then interacts with ATP to get “recocked”, if myosin still exposed then it fires again and results in further muscle contration. If there is no further nerve signal, sarcoplasmic reticulum sequesters Ca+ ions again and no recocking occurs.

Quirari (or curare): known from movies, used in South America, blocks acetylcholine receptors in cell and causes skeletal paralysis. Victim dies of asphyxiation because he can’t breathe.

Duchenne's muscular dystrophy: degeneration of sarcolema, plasma membrane of muscle cell unable to release signal and quickly atrophies.

Fast and slow twitch fibers: vertebrate muscle fiber. Terms relative within one group of animals. Differences related to differences in enervation, type of myocin, and actin activation.

Two parts of force generated by muscle: 1. active component 2. elastic component (energy stored in muscle when stretched by gravity or another force. Stored in muscle elastic tissue around tendons. Especially important in limb oscillation, like running, or trunk twisting, like fish swimming. Up to 90% of stored elastic energy can be recovered.)

How does a muscle match its power to its job? Two ways:
1. rate modulation, derived from frequency of nervous stimulation of muscle, force increases as frequency of stimulation increases up to point of tetanus.
2. selective involvement of motor units, a given neuron enervates a fixed number of muscle cells, (a motor unit), and force is increased by recruiting more motor units. Motor units may be small, such as in eye, or larger, like in leg muscle.

How do muscles grow stronger?
1. add more myofilaments, increases cross sectional area by up to 50%, more little ratchets working
2. proliferation in blood vessels and connective tissue around muscle

Muscle strength is relative to cross sectional area, not length. Not always feasible to add more cross sectional area.

Pinnate fibers: oriented obliquely (Y-shaped) to minimize muscle mass, in certain circumstances, like calf muscle. Spreads muscle out.

Velocity of shortening greater in long muscle than short. Why? Contraction tied to relation between fibers, and to total length of muscle. Both long and short muscles reach same percentage of contraction in same unit time, but distance covered by the longer muscle is greater.

Synergist muscles: muscles work together to produce motion in same general direction. Bicep shares work with brachialis.

Antagonist muscles: muscles that oppose each other. Bicep pulls forearm in, triceps pulls it back out.

Origin vs. insertion: origin is the end of the muscle that more fixed in its attachment to the body. The more movable end called insertion.

Fixators: muscles that act to stabilize a joint or lever system. Like upper arm when you clench your fist hard.

Flexors and extensors: applied mainly to limbs. Flexor bends one part relative to another about limb, extensor straightens it.

Adductor and abductor: adductor draws a limb toward the ventral surface. Abductor moves limb away from ventral surface. (Adduct: drawn toward; abduct: carry away).