16.2: Functional Divisions of the Nervous System

The nervous system, responsible for sensing, integrating, and responding to various stimuli, is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS has two functional divisions: the sensory or afferent division and the motor or efferent division.

The sensory division transmits information from sensory receptors in the body to the CNS. It provides the CNS with knowledge about somatic senses (such as tactile, thermal, pain, and proprioceptive sensations) and special senses (like smell, taste, vision, hearing, and equilibrium).

On the other hand, the motor division carries output from the CNS to effectors like muscles and glands. This division further splits into the somatic nervous system (SNS) and the autonomic nervous system (ANS).

The SNS directs output from the CNS to skeletal muscles. The ANS controls output from the CNS to smooth muscles, cardiac muscles, and glands. The ANS comprises two main branches: the sympathetic nervous system and the parasympathetic nervous system. These usually have opposing actions. The parasympathetic nervous system manages "rest-and-digest" activities, while the sympathetic nervous system supports exercise or emergency actions, also known as "fight-or-flight" responses.

Lastly, there is the enteric nervous system (ENS), a network of over 100 million neurons confined to the wall of the gastrointestinal tract. The ENS helps regulate the activity of the smooth muscle and glands of the GI tract and communicates with and is regulated by the other branches of the ANS.

The nervous system senses, integrates, and responds to various stimuli. Based on bodily control, the central nervous system cannot be divided functionally, but the peripheral nervous system, or PNS, has two functional divisions.

The sensory or afferent division consists of nerves that carry external information from sensory receptors to the CNS.

In contrast, the motor or efferent division comprises nerves that relay impulses from the CNS to effectors like muscles and glands.

The motor division is subdivided into the somatic and autonomic systems. The somatic branch controls skeletal muscle movement— both voluntary and specialized involuntary movements called reflexes.

The autonomic branch controls smooth and cardiac muscle and glands, subconsciously regulating physiological processes.

The autonomic branch consists of the sympathetic and parasympathetic divisions, which regulate opposing functions. For instance, the sympathetic division can increase the heart rate while the parasympathetic division slows it down.

The enteric nervous system, a third division of the autonomic branch, consists of a network of neurons that regulates movement and secretions of the gastrointestinal tract.