Actin and Myosin Myofilament Structure and Neuromuscular Junction Structure

Actin myofilament

Consists of fibrous actin and G actin monomers

Fibrous actin

Two strands forming a double helix extending the length of the myofilament

G actin monomers

Individual proteins with active sites for myosin binding

Tropomyosin

Protein winding along the groove of the F actin double helix, covering active sites in relaxed muscle

Troponin

Composed of three subunits, regulates interaction between actin and myosin

Myosin myofilament

Composed of elongated myosin molecules with two globular heads and a rod-like portion

Myosin heads

Can bind to active sites on actin to form cross-bridges, attached to the rod portion by a hinge region, and are ATPase enzymes

Neuromuscular junction

Synapse between motor neuron and muscle fiber

Motor neurons

Carry electrical signals (action potentials) to stimulate muscle fiber action potentials and contraction

Axon terminals

Enlarged terminals of motor neurons resting in an invagination of the sarcolemma

Synaptic vesicles

Contain acetylcholine (ACh), a neurotransmitter that diffuses across the synaptic cleft

Synaptic cleft

Space between the presynaptic terminal and muscle fiber

Motor end plate

Folds of sarcolemma beneath the axon terminal, holding ACh receptors

ACh receptors

Receptors on the motor end plate that can be stimulated or inhibited by acetylcholine

Ligand-gated ion channels

Ion channels on the muscle fiber that can be activated by neurotransmitters