Neuromuscular Transmissionsynaptic transmission between a somatic motor neuron and a skeletal muscle fiber

Somatic

Fig. 11.5

• 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

Neuromuscular JunctionThe anatomical structure of the

neuromuscular junction is called the motor end plate.

Fig. 11.6

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

The Nicotinic Cholinergic Receptor

Fig. 4-18 Ganong

Fig. 15-15, Alberts et al., Molecular Biology of the Cell

Na+ and Ca++

ACh

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.

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

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)

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:

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

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

Fig. 10-17, Alberts et al.,Molecular Biology of the Cell

Protein Anchored to Membrane via a Glycolipid