Nervous System Organization

The human nervous system can be described in terms of three (or more) main parts, which could also be called or sub-systems, divisions or sub-divisions of the nervous system. These include:

• The Central Nervous System
  (sometimes referred to by its initials "CNS") which consists of the nervous tissues in the brain and spinal cord.

• The Peripheral Nervous System
  (sometimes referred to by its initials "PNS") which consists of the nervous tissues in the rest of the body including e.g. the limbs.

• The Autonomic Nervous System
  (sometimes referred to by its initials "ANS") which consists of nervous tissues that are not under concious control.

Nervous tissue, including neuroglia as well as neurons, can also be classified in other ways, such as according to function e.g. the Somatic Nervous System ("SNS") includes somatic and special sensory receptors and neurons and somatic motor neurons, and, in some cases, according to location e.g. the Enteric Nervous System ("ENS") is specific to the nervous tissue in the gastrointestinal tract (also called the GI tract, and the alimentary canal) and the associated plexuses*.

* The myenteric plexuses are located between the inner and outer layers of the muscularis externa, and the submucosal plexuses are located in the submucosa. The muscularis externa and the submucosa are layers of the stomach wall.

The relationships between the main parts of the nervous system, including the CNS, PNS, ANS, SNS and ENS, can be represented on a diagram of the sub-systems of the nervous system.

Diagram of the Organization of the Nervous System

Above: Diagram of the organization of the human nervous system

As shown above, many cells within the nervous system can be classified as part of more than one of the sub-systems within the nervous system. For example, motor neurons located within the inner-layers of the stomach (so within the GI tract) are enteric motor neurons so part of the ENS, and are also part of the PNS because they are not located within the brain or spinal cord (i.e. the CNS).

The main subsystems of the nervous system shown in the diagram. Descriptions in physiology textbooks often mention more specific parts of the nervous system, e.g. particular structures, and distinguish between the sympathetic and parasympathetic divisions of the ANS.

Brief Notes about each of the main parts of the Nervous System:

• Central Nervous System (CNS)
  - consists of the brain and the spinal cord ... more about parts of the CNS.
• Peripheral Nervous System (PNS, or sometimes PeNS to avoid confusion with the PSNS)
  - connects CNS to the limbs and organs of the body
  - consists of the nerves and ganglia outside of the brain and spinal cord
• Somatic Nervous System (SNS sometimes used but risks confusion with the Sympathetic Nervous System, alternatively SoNS)
  - associated with control of movement by skeletal muscle using via spinal nerves, cranial nerves and association nerves.
• Autonomic Nervous System (ANS, also called the Visceral Nervous System and the Involuntary Nervous System)
  - operates unconsciously and controls the body's visceral functions
  • Parasympathetic Nervous System (PSNS, but preferably write out in full to avoid confusion with the PNS)
    - regulates internal organs and glands (unconsciously).
    - includes control of salivation, lacrimation (tears), digestion, defecation, micturation and sexual arousal.
  • Sympathetic Nervous System (write out in full to avoid confusion with the Somatic Nervous System)
    - maintains homeostasis
    - activates the body's 'fight-or-flight' response
• Enteric Nervous System (ENS, also called the Intrinsic Nervous System)
  - controls the gastrointestinal system which includes the stomach and intestines (small intestine and large intestine).