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Somatic

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.

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Somatic

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  1. Neuromuscular Transmissionsynaptic transmission between a somatic motor neuron and a skeletal muscle fiber Somatic Fig. 11.5

  2. 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

  3. Neuromuscular JunctionThe anatomical structure of the neuromuscular junction is called the motor end plate. Fig. 11.6

  4. 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

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

  6. 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

  7. 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.

  8. Fig. 11.8 Excitation – Contraction Coupling  - DHP receptor    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. ryanodine receptors (not shown)

  9. 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)

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

  11. 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

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

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