1. Neuromuscular Transmission synaptic 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 Junction The 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 , Alberts et al., Molecular Biology of the Cell
Fig 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. 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.

8. 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.

9. 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)

10. 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

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

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