Cardiac Muscle Involuntary –heart only Contracts & relaxes continuously throughout life –Contracts without nervous stimulation! –A piece of cardiac muscle can even continue to contract for a while outside the body! Striated - alternating light and dark bands Each cell has 1 nucleus

Smooth Muscle Involuntary –internal organs, blood vessels Controlled by ANS (& partly by endocrine system) –Many can function without nervous stimulation Smooth muscle is not striated Each cell has 1 nucleus

Skeletal Muscle Muscle fiber= skeletal muscle cell –Skeletal muscles- skeletal muscle fibers, connective tissues, blood vessels & nerves VoluntaryStriated Skeletal muscle cells can develop multiple nuclei –Cells don’t divide after mitosis, when muscles mitosis, when muscles grow, they are not getting grow, they are not getting more muscle cells, they are more muscle cells, they are getting more nuclei, getting more nuclei, mitochondria, & mitochondria, &cytoskeleton!

Muscle Contraction Muscle fibers are arranged into bundles that contract in response to a nerve impulse. Muscle tone- constant slight contraction in muscles –posture Sliding filament theory- when thin filaments slide over thick filaments, you get muscle contraction

Muscle Filaments Filaments- small units that make up myofibrils –Myofibrils= cytoskeleton (majority of muscle cell) Thick filaments- contain protein called myosin –Rods & heads Thin filaments- contain protein called actin –Globular, also contain tropomyosin & troponin

Sliding Filament Contraction Thick & thin filaments arrange in units called sarcomeres –Striations are from alternating thick & thin filaments When muscle is relaxed, no thin filaments are in the center of the sarcomeres, when it contracts, they move into the center Sarcomere Shortening (499.0K)

Membrane Potential Resting potential- partial negative charge inside cell rest –More K + inside & Na + outside When muscle cell is stimulated, what opens? –Na + channels just like neuron! –Depolarization- change of charge –Repolarization- return to resting charge Membrane Outside the cell Cytoplasm Na + K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ - K+K+ K+K+ K+K+ K+K+ Action potential Action potential

Motor Unit Axons on motor neurons branch out & form junctions with muscle fiber –A synapse called the Neuromuscular junction Motor unit- single motor neuron & all muscle fibers associated with it Acetylcholine- neurotransmitter released by axon to excite muscle cells –Binds to Na + channels in muscle fibers to begin action potential (just like in nerve cells!) –Impulse in muscle cell membrane causes the release of Ca 2+ Excitation-Contraction CouplingExcitation-Contraction Coupling 2

Why do muscles need calcium? It makes the filaments slide! Ca 2+ binds to the thin filament (to troponin), exposing active site on actin proteins Myosin attaches to active sites to form “cross- bridges” Energy from ATP in the thick filament moves the myosin head, the movement causes the actin filament to slide over top ATP binds to myosin head, actin filament is released Actin Myosin Bridge