At the present time there are about ten molecules that are known to be neurotransmitters.  Dozens more are good candidates and the list of possibilities is growing.  Acetylcholine is the most common neurotransmitter.  This chemical triggers the contraction of a muscle cell at the junction of the motor neuron and a skeletal muscle cell. 

     Biogenetic amines are derived from amino acids.  Biogenetic amines commonly function as neurotransmitters within the central nervous system.  Dopamine and serotonin are biogenetic amines that affect sleep, mood, attention, and learning.  An imbalance of these transmitters can be associated with mental illness.  For example Schizophrenia is tied to an excess of dopamine.  Psychoactive drugs like LDS induce hallucinations by binding the receptors for certain neurotransmitters.  Gamma amino butyric acid (GABA) is believed to be the transmitter at inhibitory synapses in the brain. The brain has hundreds times more GABA than any other neurotransmitter.

     The peripheral nervous system is composed of two systems: the afferent nervous system and the efferent nervous system.  The afferent nervous system is the part of the PNS that deals with senses, that is it brings information into the brain from the sensory receptors.  The efferent nervous system is the part of the PNS that deals with carrying the signals from the CNS to the body.

     Voluntary contractions of the skeletal muscles are controlled by the somatic nervous system.  These motor neurons mainly respond to external stimuli and a conscious decision is made to move the muscles.  Reflexes are automatic reactions to a stimulus.  In most cases reflexes are controlled by the spinal cord and the brain stem.

     The nervous system that controls smooth and cardiac muscle is the autonomic nervous system.  This system regulates the internal environment and it is generally involuntary.  The autonomic nervous system contains two subdivisions: the sympathetic and the parasympathetic nervous systems.  The Sympathetic nervous system and the parasympathetic nervous system have opposite effects when they act upon the same organ.  The parasympathetic nervous system enhances activities and is able to conserve energy.  Conversely the sympathetic nervous system increases energy expenditures and prepares the individual for action.