neuromuscular transmission synaptic transmission between a somatic motor neuron and a skeletal...
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Neuromuscular Transmissionsynaptic transmission between a somatic motor neuron and a skeletal muscle fiber
Somatic
Fig. 11.5
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• 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
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Neuromuscular JunctionThe anatomical structure of the
neuromuscular junction is called the motor end plate.
Fig. 11.6
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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.
Neuromuscular Transmission
Fig. 11.7
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The Nicotinic Cholinergic Receptor
Fig. 4-18 Ganong
Fig. 15-15, Alberts et al., Molecular Biology of the Cell
Na+ and Ca++
ACh
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Neuromuscular Transmission
Fig. 11.7
(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.
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Excitation – Contraction
Coupling
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.
Fig. 11.8
- DHP receptorDHP receptor
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Excitation – Contraction
Coupling
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.
- DHP receptorDHP receptor
Fig. 11.8
ryanodine receptors (not shown)
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Direct-coupling Model
terminal cistern of SR
(SR)
DHP receptor
Alberts, et al, Molecular Biology of the Cell (cf. Fig. 3-8 Ganong)
Triad Structures
ryanodineryanodinereceptorreceptor
ryanodine receptors:
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Excitation – Contraction
Coupling
Calcium binds to troponin, and allows actin and myosin to interact.
Filaments slide; the muscle contracts.
- DHP receptorDHP receptor
Fig. 11.8
ryanodine receptors
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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
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Fig. 10-17, Alberts et al.,Molecular Biology of the Cell
Protein Anchored to Membrane via a Glycolipid