Muscle Structure and Action Potential

Muscle is composed of fascicles containing muscle fibers (multinucleated cells)
Myofibrils within muscle fibers are made up of myofilaments arranged into sarcomeres, the contractile units
Sarcomeres have A bands (darker, containing myosin), I bands (lighter, containing actin), H zone (lighter region within A band), and M line (center of H zone, composed of myomesin)
Z discs are at the center of I bands and define the boundaries of each sarcomere
Thick filaments (myosin) extend across the A band, connected at the M line
Thin filaments (actin) extend across the I band and into the A band
Elastic filaments (titin) span from the Z disc to the thick filament, acting as the core of the thick filament

Myosin has two globular heads with ATP and actin binding sites, essential for interaction with actin
Actin filaments twist together to form the backbone of thin filaments
Actin subunits have active sites for myosin binding, blocked by tropomyosin when a muscle is relaxed
Troponin, a complex of three polypeptides, binds to actin, tropomyosin, and calcium

Sarcoplasmic reticulum is a network of tubules surrounding each myofibril, storing and releasing calcium for muscle contraction
T tubules encircle each sarcomere at the A band – I band junction, helping signals reach every region of the muscle cell

Nerve impulse stimulates muscle fibers, causing myosin heads to interact with actin binding sites
Thick filaments pull thin filaments towards the center of the sarcomere, shortening the I bands and eliminating the H zone
A bands from adjacent sarcomeres get closer, shortening the entire muscle cell

Neuromuscular junction is the interface between the nervous system and a skeletal muscle
Axon terminal releases acetylcholine into the synaptic cleft
Acetylcholine binds to receptors on the sarcolemma, triggering sodium and potassium ion movement across the membrane
This initiates an action potential, which travels across the sarcolemma and down the T tubules

Action potential causes calcium ion release from the sarcoplasmic reticulum
Calcium binds to troponin, causing a conformational change that removes tropomyosin from the actin binding sites
This allows for cross-bridge cycling, where myosin heads bind to actin, pull, detach, and repeat, leading to contraction
As calcium levels deplete, troponin reverts to its original shape, tropomyosin blocks actin binding sites, and relaxation occurs

Muscle is made up of bundles of muscle fibers (cells) arranged into fascicles.
Each muscle fiber contains myofibrils - long protein bundles responsible for contraction.
Sarcomeres, the basic contractile units of a muscle fiber, are organized within myofibrils.
A bands are darker, containing myosin (thick filaments).
I bands are lighter, containing actin (thin filaments).
H zone is a lighter region within the A band.
M line is the center of the H zone, composed of myomesin.
Z discs define the boundaries of each sarcomere and are located at the center of each I band.
Titin, an elastic protein, spans from the Z disc to the thick filament, acting as the core of the thick filament.

Myosin, the thick filament, is a protein with two globular heads that bind to actin and ATP.
Actin, the thin filament, forms a twisted chain of protein subunits with active sites for myosin.
When a muscle is relaxed, the protein tropomyosin blocks the active sites on actin, preventing myosin binding.
Troponin, a complex of three polypeptides, binds to both actin and tropomyosin. It also binds to calcium, critical for muscle contraction.

Sarcoplasmic reticulum (SR) is a network of tubules surrounding each myofibril, storing and releasing calcium ions.
T tubules are invaginations of the sarcolemma (cell membrane) that encircle each sarcomere at the A band-I band junction, transmitting nerve impulses to the interior of the muscle fiber.

Nerve impulses stimulate muscle fibers, causing myosin heads to attach to actin binding sites.
Myosin heads pull the thin filaments towards the center of the sarcomere.
The I bands shorten and the H zone disappears as thin filaments slide over thick filaments.
A bands from adjacent sarcomeres get closer, shortening the entire muscle cell.

Neuromuscular junction is the point of contact between a motor neuron and a muscle fiber.
The axon terminal of the motor neuron releases acetylcholine (ACh) into the synaptic cleft.
ACh binds to receptors on the sarcolemma, triggering sodium and potassium ion movement across the membrane.
This generates an action potential that travels across the sarcolemma and down the T tubules.

The action potential in T tubules triggers the release of calcium from the sarcoplasmic reticulum.
Calcium binds to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites.
This allows for cross-bridge cycling:
- Myosin heads bind to actin.
- They pull the thin filaments towards the center of the sarcomere.
- They detach from actin.
- This cycle repeats, leading to muscle shortening.
As calcium levels decrease, troponin and tropomyosin return to their resting positions, actin binding sites are blocked, and the muscle relaxes.