8: Skeletal muscle 1

• A lot of heat lost from skeletal muscles comes from the breakdown of ATP.
• The primary role of cardiac muscle is to generate the pressure that drives blood flow through the circulatory system.
• Smooth muscles have the primary role of generating pressure in hollw organs or regulating intenrla movement, like flow through the blood vessels.
• Myofibers, or muscle fibers, are muscle cells. These are made up of myofibrils, which are made up of repeating units called sarcomeres.
• The neurotransmitter released at neuromuscular junctions is acetylcholine, which is taken up by nicotinic N1 type receptors (N-AChR) that open channels permeable to Na+ and K+.
• Acetylcholine released in the neuromuscular junction, after binding to its receptor, is broken down by the enzyme acetylcholinesterase located on the motor end plate. Only choline is taken back up by the presynaptic motor neuron to be recycled.
• Myofibril structure: thick filaments are made up of myosin, while thin filaments are made up of actins plus the regulatory proteins of troponin and tropomyosin.
• Action potentials propagate along the muscle fiber plasma membrane down openings of T-tubules to the myofibrils, where it can affect Ca2+ release from segments of the sarcoplasmic reticulum.
• DHP is the voltage sensor located in the T-tubule membrane, which undergoes a conformational change when action potential arrives. When this happens, the RyR receptor channel located on the SR membrane is "pulled" open to release Ca2+ into the cytosol.
• When Ca2+ concentrations are high in the cytosol, it will bind to troponin on the thin filament. This changes interaction between actin and tropomyosin, causing tropomyosin to to move out of blocking position, allowing myosin to bind to actin and begin cross-bridge cycling.
• Myosin on the thick filament have an anti-parallel arrangement that allows for a sliding filament mechanism, where Z lines of the sarcomere are pulled together. This is known as contraction.
• Relaxation of the muscle means that Ca2+ must be taken back up into the sarcoplasmic reticulum. This is performed by SERCA, which is a Ca2+ ATPase that is always pumping back into the SR.