Question

List & explain the events that transpire from the time an efferent (motor) neuron is stimulated until a motor unit contracts; indicate where each event occurs

Answer 1

Converting electrical signal into chemical signal .
Every neuron receives impulse in form of action potential (Electrical signal).

Motor unit consist of single motor neuron and all the muscle fibres it innervate .

Neuromuscular Junction

*The synapse that is formed between axons of an alpha-motor neuron acting as presynaptic neuron and a skeletal muscle fiber is called the neuromuscular junction (NMJ).

*The terminals of alpha-motor neurons contain vesicles containing acetylcholine (Ach), thus the synaptic transmission at the neuromuscular junction a type of cholinergic transmission.

*Normal RMP ( Resting membrane potential ) of neuron is -70 .The presynaptic membrane have Voltage sensitive sodium channels whose activation cause and entry of NA+ sodium inside and depolariation .So when membrane potential reaches -55 mv ,Action potential fires by opening sodium channels and allow entry of sodium channels inside and Membrane potential changes to -40 mv .At -40 mv opens voltage gated calcium channels .

Steps in Neuromuscular transmission
1.)The action potential travels down the motor neuron and depolarizes the presynaptic membrane.

2)This depolarization opens voltage-gated Ca2+ channels in the presynaptic membrane, resulting in Ca2+ influx into the presynaptic terminal.

3) The rise in Ca2+ causes synaptic vesicles to release Acetylcholine. The amount of neurotransmitter release is directly related to the rise in cytosolic Ca2+, i.e., the more Ca2+ enters , more Acetylcholine released.

4. Ach binds to a nicotinic receptor located on the muscle membrane (NM receptor). The NM receptor is a non-selective monovalent cation channel (both Na+ and K+ can move). Now Na+ has a much greater net force depolarization occurs. This depolarization is called an end-plate potential (EPP). The magnitude of the EPP is directly related to the amount of Ach released.

5. The resulting depolarization opens fast Na+ channels on the muscle membrane (sarcolemma) causing an action potential to occur in the sarcolemma. An action potential in the motor neuron cause release of enough Ach to cause End Plate Potential that is at least threshold for the action potential in the skeletal muscle cell which occurs under normal circumstances . There is a one-to-one relationship between an action potential in the motor neuron and an action potential in the skeletal muscle cell.

6. The actions of Ach are terminated by acetylcholinesterase (AchE), that breaks Acetylcholine into choline and acetate.
Acetylcholinesterase located on postsynaptic membrane . Choline is taken back into the presynaptic terminal (reuptake), hence providing substrate for re-synthesis of Acetycholine .