BIO2305 Muscle Physiology

Muscle accounts for nearly half of the body’s mass - Muscles have the ability to change chemical energy (ATP) into mechanical energy

Types of Muscle Tissure
3 types:
1. Skeletal - tissue is attached to bones, it is striated, voluntary control, multinucleated
2. Smooth - located in the walls of hollow internal structures, nonstriated, involuntary
3. Cardiac - forms most of the heart, it is striated, involuntary, autorhythmic

Histology Review
1. Sarcolemma - a muscle fiber plasma (cell) membrane

2. Sarcoplasm - muscle fiber cytoplasm, almost completely filled with contractile filaments called myofilaments. Sarcoplasm contains glycosomes (granules of glycogen) and the oxygen-binding protein called myoglobin

3. Sarcoplasmic reticulum - (endoplasmic reticulum) a network of tubes surrounding myofibrils, functions to reabsorb calcium ion during relaxation, release them to cause contraction.

4. Transverse tubules - tubules formed by invaginations of the sarcolemma and flanked by the sarcoplasmic reticulum, they carry action potentials deep into the muscle fiber. T tubules and SR provide tightly linked signals for muscle contraction. T tubules at each A band/I band junction - continuous with the sarcolemma. Conduct electrical impulses to the throughout cell (every sarcomere) - signals for the release of Ca2+ from adjacent terminal cisternae

T tubule proteins (Dihydropyridine) act as voltage sensors
SR foot proteins are (ryanodine) receptors that regulate Ca2+ release from the SR cisternae

5. Myofibril - bundle of thread like contractile elements consisting of myofilaments, 80% of the muscle volume, contain the contractile elements of skeletal muscle cells

6. Myofilaments - extremely fine thread like proteins, there are three types;
1. thick filaments (16nm) called myosin. Each myosin molecule (two interwoven polypeptide chains) has a rodlike tail and two globular heads
2. thin filaments (8nm) called actin;
3. elastic filaments.
During muscle contraction, the myosin Heads link the thick and thin filaments together, forming cross bridges. Actin provides active sites where myosin heads attach during contraction. Tropomyosin and Troponin are regulatory subunits bound to actin.

7. Sarcomere – smallest contractile unit of a muscle fiber, a compartment of myofibrils

The sarcomere is characterized by alternating light and dark bands or zones produced by the myofilaments
Z disc - a line that separates one sarcomere from another
M line - central line of the sarcomere where myosin filaments are anchored
H zone - the area where only myosin filaments are present Z M Z
I band - the area where only actin filaments are present
A band - includes overlapping myosin and actin filaments

I A H A I

For contraction to occur, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential, along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final stimulus for contraction

Molecular Basis of Skeletal Muscle Contraction (Sliding Filament Model)

Excitation-Contraction Coupling is the sequence of events linking the transmission of an action potential along the sarcolemma to muscle contraction (the sliding of myofilaments)

The sarcolemma, like other plasma membranes is polarized. There is a potential difference (voltage) across the membrane
When Ach binds to its receptors on the motor end plate, chemically (ligand) gated ion channels in the receptors open and allow Na+ and K+ to move across the membrane, resulting in a transient change in membrane potential - Depolarization
End plate potential - a local depolarization that creates and spreads an action potential across the sarcolemma

The AP lasts only 1-2 ms and ends before contraction occurs. The period between action