Topic 3B: Muscular system

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Function: provides structural support, maintains body posture, helps move blood to the circulatory system, regulates openings, and helps with thermoregulation (shivering reflex)

Categories:

smooth: involuntary, associated with multiple tissue and organ systems. Non-striated, single nuclei

cardiac: involuntary, found only in the heart. Striated, contain discs that synchronize movements of the myocytes

skeletal: mostly movement is voluntary, helps maintain posture subconsciously. Skeletal myocytes are elongated and tubular, have striations

Muscle cells: develop from myoblasts derived from the mesoderm. # of myocytes remains constant throughout life, because muscle cells do not undergo mitosis. Instead they increase in size, which is known as hypertrophy. This is possible via increase of muscle fiber diameter; also muscle fibers can gain more sarcomeres and mitochondria, without dividing​​​​​​​

Cardiomyocytes: unique in that they contract on their own rhythm without external stimulation

Myosin is an ATPase and provides energy for power stroke via ATP hydrolysis

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Muscles:

Fasiculus: muscle fibers bound into one to form a muscle

Myocytes: made of multiple myofibrils bound together by sarcolemma (plasma membrane that interacts with the nervous system via the transverse tubule system).

have many mitochondria to produce ATP required for contraction, and contain high quantities of the protein myoglobin which acts as a reserve for oxygen

have sarcoplasm (cytoplasm). This system provides channels for ion flow and anchor points for sarcomeres

Myofibrils: bundles of muscle fibers, which are composed of myofilaments. Sarcomeres are lined end to end to form a myofibril

Each myofibril is surrounded by the sarcoplasmic reticulum (modified ER, filled with Ca2+ ions)

Myofilaments:

thin filaments: made of actin fibers, have thin diameter, associated with troponin and tropomyosin

thick filaments: made of myosin fibers, thick diameter, have protruding heads

sarcomeres: units of organized overlapping actin and myosin fibers, resulting in characteristic striations, smallest unit of the contractile apparatus, the distance between 2 consecutive Z lines

A band: area where thick and thin filaments overlap, includes the H zone, size remains constant during contraction, contains entirety of myosin filament

I band: has only thin filaments, excludes areas where actin overlaps with myosin, does not contain entire actin filament, only where it is not overlapped, shortens during contraction

H zone: has only thick filaments, excludes areas where myosin overlaps with actin, does not contain entire myosin filament, only where it is not overlapped, shortens during contraction

Z line: separates 1 sarcomere from another, actin filament attachment site

titin: a huge protein that helps maintain muscle elasticity, stretches from Z disc to M line, can be considered a muscular spring that helps maintain the integrity of the sarcomere.

It also affects the length-tension curve beyond just the actin-myosin overlap, but is not directly involved in muscle contraction

it spans 1/2 sarcomere

M line: center of sarcomere

shortening of sarcomeres results in muscle contraction

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Contraction of skeletal muscles: Responsible for voluntary movement

Contractile Velocity:

Type I/Slow twitch fibers, aka red: Have more mitochondria and more myoglobin, giving them more endurance because they make ATP from aerobic cellular respiration. Low amount of force, slow speed of contraction, however able to contract for longer periods of time. AKA red fibers because myoglobin gives them a red color

Type II/Fast twitch fibers, aka white: designed for a fast rate of contraction, however lack stamina because anaerobic cellular respiration is the preferred method of energy accumulation in these fibers

Type II A: fast contractile velocity, resistant to fatigue

Type II B: low myoglobin content, contract rapidly, generate large amount of force, fatigue quickly

skeletal muscle cells have multiple nuclei as a result of cell fusion.

Muscular hypertrophy occurs as a result of increase in myofibrils; cells themselves do not divide, but increase in size as a result of various stimuli (weight training, steroids, etc.)

Innervated by the somatic branch of the PNS.

sliding filament model and skeletal muscle contraction:

Neuromuscular junction: sarcolemma forms synapse with axon terminals of a motor neuron from the somatic branch of the PNS

release of ACh from motor neuron and binds to receptors to sarcolemma, causes an action potential resulting in shortening of the sarcomeres

T-tubules: infoldings of sarcolemma that facilitate uniform contraction, by allowing quick spread of action potential

motor unit: the muscle fibers influenced by by a single neuromuscular junction

Henneman's size principle: the smaller the motor unit, the more precise the muscle action, so motor units are recruited in order of size from smaller to larger as contraction increases

Action potential from the motor neuron causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.

Calcium binds to troponin in the thin filaments.

This causes a conformational change of tropomyosin in thin filaments, which exposes the myosin binding sites on the actin fibers.

Myosin binds to the myosin binding sites on the actin forming cross bridges

With the use of ATP hydrolysis, the power stroke occurs, pulling thin filaments towards the center of the sarcomere.

ATP is hydrolyzed to ADP when the myosin head goes from the contracted to the relaxed position

The cycle restarts when another ATP molecule binds to the myosin head causing the release of the myosin head from the myosin binding site on the actin.

In order for contraction to continue to occur, the sarcolemma must be continuously stimulated by the motor neuron

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Cardiac muscle: found only in the myocardium of the heart.

cardiomyocytes: capable of coordinated contraction, via gap junctions of intercalated discs

ECC: excitation contraction coupling, converting an electrical stimulus (action potentials) from the neurons into a mechanical response that facilitates muscle movement

calcium-induced calcium release: influx of Ca2+ ions via binding of ions to calcium ion channels that open during depolarization, triggers release of ions into the cytoplasm.

Influx of Ca2+ ions sustains depolarization and makes it last longer. Creates a plateau phase--> cell's charge stays slightly positive before repolarization with K+ ion influx

this is different from skeletal muscle, which repolarizes immediately

has striations and contracts in the same way as skeletal muscle

intercalated discs: structures that connect adjacent cardiac muscle cells via desmosomes, adherens junctions, and gap junctions; unique to cardiac muscle

does not have multiple nuclei; at most has two nuclei per cell. Muscular hypertrophy can result in obstruction of blood flow

Innervated by the autonomic system

like smooth muscle can perform myogenic activity.

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Smooth muscle: Found in various parts of the body --> around blood vessels, the bladder, digestive tract, and reproductive tracts.

have single nuclei.

lacks striations because the actin and myosin found in the muscle fibers is not organized into sarcomeres.

The actin and myosin fibers slide over each other to move the muscle, and this movement is regulated by calcium and ATP.

Innervated by the autonomic branch of the PNS to produce involuntary contractions.

Sympathetic response: Stimulated by epinephrine and norepinephrine.

Parasympathetic response: stimulated by acetylcholine.

Exhibits myogenic activity (movement without nervous stimulation).

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Energy reserves in muscles and muscle fatigue:

ATP--> aerobic metabolism

slow twitch muscle fibers

creatine phosphateis:

created by transferring phosphate group from ATP to creatine during rest

this is a reversible reaction, ATP can quickly be regenerated

anaerobic glycolysis

fast twitch muscle fibers

muscle fatigue: occurs when muscle use overwhelms body's ability to deliver O2, slow twitch fibers also switch to anaerobic metabolism and produce lactic acid

this lactic acid must be metabolized, it is converted back to pyruvate so it can enter the citric acid cycle. This process requires oxygen and the amount required is equal to the O2 debt

oxygen debt: the difference between the amount of oxygen needed and the actual amount present

Length-tension curve: measures the optimum muscle length (and optimal actin-myosin overlap) for maximum muscle contraction and generation of the most force. Length of the filaments determines minimum and maximum overlap possible

describes the amount of tension produced by a muscle as a result of its length; the maximum force measured will be different as the muscle shortens (contracts) or lengthens (relaxes)

the greater the amount of cross-bridges, the greater amount of force produced. A muscle can form the most cross-bridges while at rest

****overall trend is that amount actin-myosin overlap is directly related to contractile strength of muscle, and too much and too little overlap both result in low contractile strength

force of a muscle contraction is dependent on:

increasing the # of motor units ---> this increases the % of muscle mass contracting, thereby increasing force

increasing the size of motor units ---> as above

increasing the # of action potentials ---> sustained contractions, possibility of summation effect (this is different from size of action potential; action potentials DO NOT have a size, they are all-or-nothing)