Nervous System

The nervous system is a complex network of nerves and cells that carry messages between the brain and the body. This sophisticated system controls everything you do, from breathing and thinking to walking, feeling, and responding to environmental stimuli.

The nervous system consists of two parts in vertebrates: the central nervous system (CNS) and the peripheral nervous system (PNS).[1]https://pubmed.ncbi.nlm.nih.gov/31194336[2]https://pubmed.ncbi.nlm.nih.gov/11345004[3]https://www.ncbi.nlm.nih.gov/books/NBK442010 

The central nervous system includes the brain and spinal cord, while the peripheral nervous system consists primarily of nerves that pass information to and from the brain to the rest of the body. The PNS is also divided into three separate systems: the somatic, autonomic, and enteric nervous systems.[4]https://pubmed.ncbi.nlm.nih.gov/31194336[5]https://pubmed.ncbi.nlm.nih.gov/11345004[6]https://www.ncbi.nlm.nih.gov/books/NBK442010

At a cellular level, the nervous system consists of special “conductor” cells called neurons. Neurons work by sending and receiving electrochemical impulses to create synapses or release brain chemicals called neurotransmitters.[7]https://pubmed.ncbi.nlm.nih.gov/31194336[8]https://pubmed.ncbi.nlm.nih.gov/11345004[9]https://www.ncbi.nlm.nih.gov/books/NBK442010

Neurons exist all over the body to help voluntary and involuntary movements and functions such as the heartbeat, bowel movements, blinking, limb movement, widening and relaxation of throat muscles, and many other human functions.[10]https://pubmed.ncbi.nlm.nih.gov/31194336[11]https://pubmed.ncbi.nlm.nih.gov/11345004[12]https://www.ncbi.nlm.nih.gov/books/NBK442010

Components of the Central Nervous System

The CNS is divided into two main parts: the brain and the spinal cord. 

The Brain

The brain is a complex organ that consists of soft tissue (grey and white matter), nerve cells, non-neuronal cells, and small blood vessels. The brain has high water content and is largely made of fat (about 60%). From a neurological and structural perspective, the brain consists of four parts:

  • Brain stem
  • Cerebrum
  • Cerebellum
  • Diencephalon

The brain helps process thoughts, communicate, feel sensations, movements, and emotions, and respond to stimuli. The brain also consists of 12 cranial nerves that facilitate motor and sensory functions. The cranial nerves are:

  • Olfactory 
  • Optic
  • Oculomotor 
  • Trochlear 
  • Trigeminal 
  • Abducens 
  • Facial 
  • Vestibulocochlear 
  • Glossopharyngeal 
  • Vagus 
  • Accessory 
  • Hypoglossal 
  • Abducens 

The Spinal Cord 

The spinal cord is a long, cylinder-like structure extending from the brain down the spinal column and comprises 31 parts, with a pair of spinal nerves in each segment. The spinal cord transmits motor commands between the brain, the peripheral body, and sensory organs.[13]https://pubmed.ncbi.nlm.nih.gov/31194336[14]https://pubmed.ncbi.nlm.nih.gov/11345004[15]https://www.ncbi.nlm.nih.gov/books/NBK442010 

Components of the Peripheral Nervous System

The PNS is also divided into two central systems:

  • The somatic nervous system
  • The autonomic nervous system 

The Somatic Nervous System 

The somatic nervous system (SNS) is like a messenger system for limbs and organs farther away from the brain. It’s made up of nerve fibers that send sensations (sensory information) from the peripheral organs to the CNS. The SNS also contains motor nerve fibers that communicate messages to and from the brain to the skeletal muscles.[16]https://pubmed.ncbi.nlm.nih.gov/31194336[17]https://pubmed.ncbi.nlm.nih.gov/11345004[18]https://www.ncbi.nlm.nih.gov/books/NBK442010

The Autonomic Nervous System 

The autonomic nervous system (ANS) is divided into three parts: 

  • The sympathetic nervous system
  • The parasympathetic nervous system
  • The enteric nervous system

This part of the nervous system controls the inner organs and body processes that happen automatically —things like digestion, breathing, and the beating of your heart. Nerves in the ANS also activate the smooth involuntary muscles of internal organs and glands.[19]https://pubmed.ncbi.nlm.nih.gov/31194336[20]https://pubmed.ncbi.nlm.nih.gov/11345004[21]https://www.ncbi.nlm.nih.gov/books/NBK442010

The Enteric Nervous System

A part of the ANS, the enteric nervous system (ENS) is a complex network of nerve fibers that run through the gastrointestinal tract, gallbladder, pancreas, and other parts of the abdomen and controls gastrointestinal behavior independently of the CNS.[22]https://pubmed.ncbi.nlm.nih.gov/31194336[23]https://pubmed.ncbi.nlm.nih.gov/11345004[24]https://www.ncbi.nlm.nih.gov/books/NBK442010