Anatomy of the Nervous System

Structures of the Nervous System

• The nervous system consists of two main interconnected divisions: the central nervous system (CNS) and the peripheral nervous system (PNS)
• The CNS includes the brain and spinal cord
• The PNS includes nerves, ganglia, enteric plexuses, and sensory receptors

The Brain

• The brain is enclosed and protected by the skull in the cranial cavity
• It contains approximately 85 billion neurons
• The brain consists of three main parts: forebrain, midbrain, and hindbrain
• The forebrain consists of two cerebral hemispheres and diencephalons
• The cerebral hemispheres have four lobes: frontal, parietal, temporal, and occipital
• Each lobe has specific functions, such as motor control, sensory reception, and speech processing

Cavities of the Central Nervous System

• The CNS has two types of cavities: ventricles and central canals
• Ventricles are wide cavities that include the lateral ventricles, third ventricle, and fourth ventricle
• Central canals are narrow cavities that include the central canal of the midbrain and the central canal of the spinal cord

The Spinal Cord

• The spinal cord is enclosed and protected by the bones of the vertebral column
• It is approximately 45 cm in length and has 31 segments
• Each segment gives origin to a pair of spinal nerves
• The spinal cord is covered by three meninges: pia mater, arachnoid mater, and dura mater

The Peripheral Nervous System

• The PNS includes cranial nerves and their branches, spinal nerves and their branches, and sensory receptors
• Cranial nerves are 12 pairs of nerves that emerge from the brain and are distributed in the head and neck
• Spinal nerves are 31 pairs of nerves that emerge from the spinal cord and are distributed in the rest of the body
• Sensory receptors are structures that monitor changes in the internal and external environment

Autonomic Nervous System

• The autonomic nervous system (ANS) has sensory and motor components
• The ANS is divided into two main subdivisions: the sympathetic nervous system and the parasympathetic nervous system
• The sympathetic nervous system causes dilatation of the eye pupil, increase heart rate, dilatation of bronchi, and vasoconstriction of arteries
• The parasympathetic nervous system causes constriction of the pupil, decrease heart rate, constriction of the bronchi, and does not supply the arteries
• The enteric nervous system (ENS) is a third subdivision of the ANS that consists of neurons in the gastrointestinal tract that function independently of the ANS and CNS to some extent