Skeletal muscle microscopically

Skeletal Muscle Fiber Long, cylindrical in shape Multinucleate Larger than normal body cells Results from the fusion of many embryonic cells

The major players Sarcolemma – plasma membrane Sarcoplasm – like cytoplasm a. myoglobin – red pigment that stores oxygen b. Glycosomes – stored glycogen Sarcoplasmic reticulum- like the smooth ER and regulates ionic calcium levels T- tubules – connect sarcolemma to fiber interior

Sliding Filament Theory of Contraction Thin filaments slide past thick filaments Actin and myosin overlap to a greater degree Cross bridges generate tension Occurs in sarcomeres simultaneously Muscle fiber shortens

Physiology of Skeletal Muscle Fiber Excitation-contraction coupling Propagates an action potential – electrical current

Neuromuscular junction Axons travel to muscle fiber Each branch meets a muscle fiber Located midway along the fiber

Action potential Polarization – resting sarcolemma – inside membrane is negative compared to outside Depolarization – charge differentiation changes and the inside become more positive and the outside more negative due to K+ and Na+ exchange – Na+ goes in Repolarization – K+ goes out Refractory period – time before stimulation can occur again

Excitation Contraction Coupling Sequence of events that lead from action potential to the sliding myofilaments Latent period – between action potential and beginning of mechanical activity

So What Happened? 1.Axon of neuron meets with muscle fiber at neuromuscular junction 2.ACh is released across the synaptic cleft 3.ACh causes the flow of Na+ ions across the sarcolemma. 4.The cell becomes less negative and causes a potential voltage difference.

5.This causes an action potential-an elec- trical current that propagates along the Sarcolemma 6.Then K+ moves out of the cell and The charge difference is restored. 7.Eventually the Na+ and K+ channels Restore the concentrations of Na+ and K+ extracellularly and intracellularly

Polarized – resting state sarcolemma With more K+ intracellular and more Na+ extracellularly. Intra is more (-) Depolarized – less negative inside Repolarized – returned to more (-) inside Refractory period – a muscle cell cannot be stimulated again until repolarization is complete

The Units, the Twitch, the Responses, and the Tone

A motor unit is a motor neuron and the muscle fibers it supplies Some small – fine motor Some large – large motor

Twitch Wave summation – when one twitch is followed by a 2 nd twitch Incomplete tetanus- more contraction Complete tetanus – the plateau of the muscle contracation Response of a motor unit to a single action potential a.Latent period – no response seen on the myogram b.Contraction – cross bridges occur c.Relaxation – Ca+ moves back to sarcolemma and muscle returned to initial length