[3] The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors.

Innervation takes place at a neuromuscular junction and twitches can become superimposed as a result of summation or a tetanic contraction.

Essentially, motor neurons, also known as motoneurons, are made up of a variety of intricate, finely tuned circuits found throughout the body that innervate effector muscles and glands to enable both voluntary and involuntary motions.

While lower motor neurons start in the spinal cord and go to innervate muscles and glands all throughout the body, upper motor neurons originate in the cerebral cortex and travel to the brain stem or spinal cord.

The axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis (the basal plate).

There are 13 Hox transcription factors and along with the signals, determine whether a motor neuron will be more rostral or caudal in character.

[12] Corticomotorneurons project from the primary cortex directly onto motor neurons in the ventral horn of the spinal cord.

[14] Nerve tracts are bundles of axons as white matter, that carry action potentials to their effectors.

The CNS activates alpha motor neurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching.

These are also known as branchial motor neurons, which are involved in facial expression, mastication, phonation, and swallowing.

In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch.

A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.

[5] The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction.

Upon adequate stimulation, the motor neuron releases a flood of acetylcholine (Ach) neurotransmitters from synaptic vesicles bound to the plasma membrane of the axon terminals.

[20] In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory.