It consists of:
- Lower motor neuron which extends from the brain stem or spinal cord via a long axon to the skeletal muscle
- The neuromuscular junction or motor end plate
- A skeletal muscle cell
Where the motor end plate and muscle join is the neuromuscular junction - a synapse. Signals transmitted down the axon to the motor end plate cause the release of acetylcholine (a neurotransmitter) into the synaptic space. Acetylcholine interacts with the muscle cell causing contraction of muscle fibers. The acetylcholine must be immediately broken down by the enzyme cholinesterase before another signal can be transmitted.
You can find an excellent review of the physiology of muscle contraction here.
See an animation of the actin myosin crossbridge in action here.
Muscle fibers are either type 1 or type 2 (A and B). Go here to read a good review on muscle fiber types and training.