Muscle-Bound Notes

General Notes

· Anything that says myo, mys or sarco is referring to muscles

· muscle cells are elongated, hence the name muscle fibers

· muscle contraction depends on myofilaments

· of the three muscle types, we will be concentrating on skeletal muscles

Functions of the Muscle System

· movement

· maintains posture

· joint stabilization

· heat generation

Functional Characteristics of Muscles : distinctive characteristics of muscle tissue that allow it to perform its duties

· excitability (irritability)

· contractility

· extensability

· elasticity

Gross Anatomy of a Skeletal Muscle

· each muscle fiber is surrounded by a fine layer of areolar (loose) tissue called the endomysium

· several endomysium-wrapped muscle fibers are joined together in a bundle called a fascicle which is

· bound together by a dense fibrous tissue sheath called the perimysium

· fascicles are bundled together into the muscle itself, held together by the dense fibrous epimysium.

· superficial to the epimysium is another sheath of connective tissue called the fascia or muscle belly.

· all of these sheaths are continuous with one another, so that when muscle fibers contract, the forces are transmitted throughout the muscle.

· each muscle fiber is supplied with a nerve ending to control its activity

· each muscle is served by an artery and one or more veins

· attachments

· most muscles span joints and are attached to bones in at least two places

· when a muscle contracts, the muscle's insertion moves toward the muscle's origin

· muscle attachments are either direct or indirect

· direct attachments (fleshy) : the epimysium is fused to the periosteum or perichondrium

· indirect attachments : the connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

· tendon : connects the muscle to a skeletal element

· aponeurosis : connects the muscle to the fascia of other muscles

Microscopic Anatomy of a Muscle Fiber

· each skeletal muscle cell is a long cylindrical, multinucleated cell, a syncytium of hundreds of embryonic cells

· the sarcolemma is the plasma membrane of a muscle fiber

· sarcoplasm is similar to cytoplasm, but it has higher amounts of stored glycogen and a protein called myoglobin which binds oxygen

· sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum of a muscle cell; it regulates the intracellular Ca2+ in the cell

· transverse (or "t") tubules are hollow, elongated tubes where the sarcolemma penetrates into the cell's interior; they help to evenly distribute the muscle impulse

· myofibrils are rodlike structures that are organized into sarcomeres

Sarcomeres and Muscle Contraction

· a sarcomere, or "unit of muscle," is a contractile unit and is defined as the region between two "z" lines

· a z-line is a midline interruption of an i-band; it is a coin-shaped protein that anchors thin (actin) filaments and connects each myofibril to the next

· the i-band is a light band in a striated muscle; it is light because only actin (thin) filaments are found in that area

· an a-band is the dark band in striated muscle; it is dark because of the thick filaments there

· the h-zone is a slightly lighter area in the middle of the a-band; it is lighter because it is the point where there is no overlap of thin and thick filaments; they are only visible in relaxed muscle for that reason

· in the middle of the h-zone is the m-line, which is a dark band where the thick filaments are joined

· the thin filament -- the actin filament -- is actually made up of three proteins: G-actin, tropomyosin and troponin

· G-actin contains the active sites to which the myosin heads will bind

· G-actin "beads" join together to form F-actin

· two strands of F-actin coil around each other

· two strands of tropomyosin coil around the two strands of F-actin; they block the active sites during relaxation

· troponin, a three-polypeptide complex, binds to actin and to tropomyosin, and helps to control the positioning of tropomyosin

· the thick filament -- the myosin filament -- has two distinctive features: the myosin tail and the myosin head or cross bridge

· the cross bridge is named for the fact that it links actin and myosin during contraction

· the myosin heads contain ATP binding sites and ATPase enzymes to generate the energy needed during contraction

Sliding Filament Mechanism

· when a muscle contracts, the individual sarcomeres shorten, but the filaments do not

· the sliding filament theory of contraction was first proposed in 1954 by Hugh Huxley

· he said that thin filaments slide past the thick ones so that they overlap to a greater degree during contraction

· during contraction the z-lines move closer together, the h-zone disappears, and the a-bands gets closer while unchanging in their length but the i-band gets smaller

· the cause of sliding filaments

· when intracellular levels of calcium are low, the muscle is relaxed and the active sites on the thin filaments are blocked

· when calcium ions become available, they bind to troponin, moving the tropomyosin molecule and exposing the active sites and the following occurs in rapid succession:

· cross bridges attach

· power stroke

· cross bridge detachment

· "cocking" of the myosin head

· sliding of the filaments occurs as long as there are calcium ions available

· as the SR pumps the calcium back, and the active sites are blocked, relaxation occurs

· the sliding filament mechanism is the best accepted model of muscle contraction, but there is still some controversy

· actin has ATPase activity and undergoes conformation changes, so what is their true role?

· how many ATP molecules are used? one per power stroke?

· rigor mortis illustrates how cross bridge detachment is ATP driven

· muscles stiffen 3-4 hours after death (due to release of calcium ions from the SR)

· peak rigidity is about 12 hours after death

· 48-60 hours after death, muscles have relaxed again -- because the actin and myosin are being broken down

· at death, ATP is no longer being made, and so when the actin and myosin bind, they can't be detached without ATP

· regulation of muscle contraction

· skeletal muscle cells are stimulated by motor neurons of the somatic nervous system which form a neuromuscular junction

· the neuromusclar junction is not a direct connection, rather it is a very close association separated by a synaptic cleft

· the motor end plate is a highly folded part of the sarcolemma that has a great number of ACh receptors in it.

· when a nerve impulse reaches the junction, ACh (acetylcholine) is released into the synaptic cleft, fusing with the receptors in the motor end plate, causing a muscle impulse, similar to a nerve impulse

· the muscle impulse is the result of depolarizing the sarcolemma

· the ACh is rapidly destroyed after being taken up by the receptors in order to allow another muscle impulse; acetylcholinesterase is responsible for this

· a shortage of acetylcholine receptors and blood containing antibodies to ACh receptors can result in myasthenia gravis

· curare, an arrowhead poison used by natives in South America, binds to ACh receptors and blocks ACh attachment, resulting in paralysis

· the muscle impulse moves along the sarcolemma, including down into the muscle itself through the t-tubules

· the impulse triggers the release of calcium ions from the SR into the sarcoplasm

· the released calcium ions bind to troponin, allowing the binding of cross bridges to the now-exposed actin active sites

· calcium is shuttled back into the SR through a continuously active ATP-dependent calcium pump

· when intracellular calcium levels drop too low to allow contraction, the tropomyosin blockage is reestablished and relaxation occurs

Skeletal Muscle Contraction

· the motor unit is one motor nerve and the hundreds of muscle fibers that it services

· when a motor neuron fires, all of the fibers it supplies are stimulated as above

· a motor unit may have as many as several hundred fibers associated with it, or as few as 4

· the more finely controlled muscles, such as those controlling the fingers or eyes, have fewer fibers in each motor unit

· the fibers of a motor unit are not clustered together, but spread throughout the muscle, so stimulation of a single motor unit causes a weak contraction of the entire muscle

· the response of a muscle to a single, brief, threshold stimulus is a muscle twitch

· a muscle twitch produces a brief spike on a myogram

· it may be strong or weak depending on the number of motor units involved

· the muscle contracts quickly and then relaxes

· this is not the way our muscles work, not normally

· graded responses -- variation in the degree of muscle contraction -- are how our muscles work

· two ways of grading muscle contraction in vivo : changing the speed of contraction and changing the number of motor units activated

· if two identical stimuli are delivered to a muscle in rapid succession, the second twitch will be stronger than the first, and on a myogram, it will appear to "ride" on the shoulders of the first contraction -- this is wave summation, and occurs when the muscle is not completely relaxed before being stimulated again

· if the stimulus is held constant and the relaxation time between twitches becomes shorter and shorter, resulting in a "disappearance" of relaxation on a myogram, the sustained contraction is called tetanus

· tetanus is a reflection of how muscle contraction typically works in the body because motor neurons deliver a volley of impulses rather than just one impulse at a time

· tetanus can not be sustained indefinitely -- eventually the muscle will become fatigued and unable to contract any longer

· recruitment is how a muscle can call more than one motor unit into contraction, resulting in multiple motor unit summation

· threshold stimulus is the stimulus that results in the first observable muscle contraction; maximal stimulus is the strongest stimulus that results in increasing contractile force and represents the point where all of the motor units are engaged

· muscle tone is the state of semi-contraction that all muscles are in; it keeps the muscles firm, healthy and ready to act, as well as stabilizing joints and maintain posture

· isotonic and isometric contractions

· sometimes a muscle contracts but does not shorten

· isotonic contraction involves shortening the muscle fiber and doing work (concentric); or lengthening the muscle fiber (eccentric)

· isometric contractions increase tension, but the muscle no longer changes length

· posture maintaining muscles work isometrically

· isometric contractions occur when a muscle attempts to move a load greater than the force it is able to develop

· muscle fatigue : the muscle is physiologically unable to contract; this results from a relative deficit of ATP or an excessive accumulation of lactic acid or ionic imbalances

· muscle fiber types

· red slow -- thin cells that contain slow-acting myosin ATPases; have a lot of myoglobin (hence their red color), abundant mitochondria, rich capillary supply; oxygen dependent, very fatigue resistant and have high endurance, but don't generate much power

· white fast -- large pale cells with little myoglobin, contain fast-acting myosin ATPases and contract rapidly; contain few mitochondria, large glycogenic reserves, tire quickly, but very powerful

· red fast -- also called intermediate fibers or pink fibers, have fast-acting myosin ATPases, and contract quickly, but are more like red slow in that they are oxygen dependent, have a high myoglobin content and fatigue slower than the white but faster than the red slow