Neuromuscular Transmission synaptic transmission between a somatic motor neuron and a skeletal muscle fiber Somatic Fig. 11.5

Neuromuscular Junction The neuromuscular junction is a chemical synapse at which a nerve impulse triggers the excitation of skeletal muscle. motor neuron = presynaptic cell at the motor neuron: electrical signal  chemical signal skeletal muscle fiber = postsynaptic cell at the skeletal muscle fiber: chemical signal  electrical signal

Neuromuscular Junction The anatomical structure of the neuromuscular junction is called the motor end plate. Fig. 11.6

Neuromuscular Transmission  At the neuromuscular junction, the neurotransmitter released from the motor neuron is acetylcholine (ACh). & The ACh is received by a nicotinic cholinergic receptor. The nicotinic receptor is a cation channel, that allows Na+ to enter the cell; Na+ entry causes depolarization. Fig. 11.7

The Nicotinic Cholinergic Receptor Na+ and Ca++ ACh Fig. 15-15, Alberts et al., Molecular Biology of the Cell Fig. 4-18 Ganong

Neuromuscular Transmission  (cont’d) This depolarization, called an end plate potential, is an example of an excitatory postsynaptic potential (EPSP - see graded potentials later).  If the EPSP causes the area next to the motor end plate to depolarize to threshold, an action potential is generated. Fig. 11.7

Excitation – Contraction Coupling Fig. 11.8 Excitation – Contraction Coupling  - DHP receptor   The muscle action potential activates the T-tubules’ voltage sensors, the dihydropyridine (DHP) receptors. DHP is a prototypical calcium channel blocker. In cardiac and smooth muscle, the DHP receptor is a functional voltage-gated Ca++ channel. However, in skeletal muscle, the DHP receptor does not function as a Ca++ channel. It is only a voltage sensor.

Excitation – Contraction Coupling Fig. 11.8 Excitation – Contraction Coupling  - DHP receptor  ryanodine receptors (not shown)  The activated DHP receptors cause the calcium release channels (ryanodine receptors) of the SR to open. direct coupling? the favored model for skeletal muscle via a second messenger (e.g., calcium-induced calcium release)? the favored model for cardiac muscle Calcium enters the cytosol.

Direct-coupling Model Triad Structures Direct-coupling Model ryanodine receptors: terminal cistern of SR (SR) DHP receptor ryanodine receptor Alberts, et al, Molecular Biology of the Cell (cf. Fig. 3-8 Ganong)

Excitation – Contraction Coupling Fig. 11.8  - DHP receptor  Calcium binds to troponin, and allows actin and myosin to interact.  Filaments slide; the muscle contracts. ryanodine receptors

End of Excitation The ACh lasts only a short time because it is broken down by an acetylcholinesterase. The ACh-esterase is anchored to the postsynaptic membrane by a glycolipid. Fig. 11.10

Protein Anchored to Membrane via a Glycolipid Fig. 10-17, Alberts et al., Molecular Biology of the Cell