Central Nervous System Overview

The central nervous system (CNS) is composed of the brain and spinal cord.
The brain resides within the skull, while the spinal cord occupies the vertebral column.

The brain is divided into three main regions: the forebrain, midbrain, and hindbrain.

Three layers of meninges surround the brain and spinal cord: dura mater, arachnoid mater, and pia mater.
Cerebrospinal fluid (CSF) resides in the subarachnoid space between the dura and arachnoid mater.

CSF originates in the lateral ventricles of the cerebral hemispheres.
It flows through the third ventricle between the diencephalon, then through the cerebral aqueduct of the midbrain to the fourth ventricle of the hindbrain.
From the fourth ventricle, CSF enters the subarachnoid space surrounding the brain and spinal cord via four openings in the roof of the fourth ventricle.

The cerebrum is the largest part of the brain.
It is divided into two cerebral hemispheres by a medial longitudinal fissure.
Each cerebral hemisphere has three surfaces: superolateral, medial, and inferior surfaces.
The surface features sulci (grooves) and gyri (ridges) to increase its surface area.
Each cerebral hemisphere contains three main sulci: central, lateral, and parieto-occipital sulci.
These sulci divide the hemisphere into four lobes: frontal lobe, parietal lobe, temporal lobe, and occipital lobe.
The frontal lobe houses motor gyri responsible for body movement.
The parietal lobe contains sensory areas.
The temporal lobe contains the auditory area.
The occipital lobe contains the visual area.
The superficial part of the cerebral hemisphere is composed of nerve cell bodies (gray matter) forming the cerebral cortex.
The deeper part is composed of nerve cell axons (white matter).

The brainstem is the part of the brain connecting the cerebrum and spinal cord.
It consists of three parts: midbrain, pons, and medulla oblongata.
The midbrain lies just below the diencephalon and contains a canal called the cerebral aqueduct.
The pons is located between the midbrain and medulla oblongata.
The medulla oblongata continues downward with the spinal cord at the foramen magnum.

The cerebellum is part of the brain located behind the pons and medulla oblongata.
It is separated from them by the fourth ventricle.

The spinal cord is a column of nerve tissue that begins at the base of the brain as a continuation of the medulla oblongata.
It descends through the vertebral canal of the vertebral column, ending at the intervertebral disk between the first and second lumbar vertebrae.
It is enveloped by dura mater, arachnoid mater, and pia mater.
The spinal cord is divided into five cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal segments.

The spinal cord consists of an H-shaped central gray matter (nerve cell bodies) surrounded by peripheral white matter (nerve cell axons).

Thirty-one pairs of spinal nerves arise from the spinal cord segments and exit through the intervertebral foramina.
Each nerve is connected to its spinal cord segment by a ventral (anterior, motor) root and a dorsal (posterior, sensory) root.
Ventral roots convey motor information through efferent nerve fibers.
Dorsal roots transmit sensory information through afferent nerve fibers and contain a collection of nerve cell bodies called the dorsal root ganglion.
Ventral and dorsal roots unite to form a mixed nerve trunk.
The mixed nerve trunk divides outside the intervertebral foramina into dorsal and ventral rami.

Twelve cranial nerves arise from the brain and thirty-one pairs of spinal nerves arise from the spinal cord.
Nerve fibers can be either somatic (innervating skin, skeletal muscle, tendons, and joints) or autonomic (innervating viscera, heart, blood vessels, and glands).

The autonomic nervous system is further divided into sympathetic and parasympathetic branches.

Originates in the thoracolumbar region (T1-L2 or L3).
Short preganglionic fibers.
Long postganglionic fibers.
Sympathetic ganglia are arranged in a longitudinal chain along the sides of the vertebral column.

Originates in the craniosacral region: cranial nerves 3, 7, 9, and 10; sacral nerves 2, 3, and 4.
Long preganglionic fibers.
Short postganglionic fibers.
Parasympathetic ganglia in the head and neck; terminal ganglia close to the organ.