Physical Structure

The major structures of the CNS are described below.

Cerebrum/Cerebral Hemispheres

The largest area of the brain is the cerebrum, which consists of the left and right cerebral hemispheres. Generally speaking, the right cerebral hemisphere controls the left side of the body and the left cerebral hemisphere controls the right side of the body. Research has shown that the right cerebral hemisphere is responsible for such functions as innovation, intuition, and creativity. The left cerebral hemisphere is associated with analytic thought, logic, and language.

Corpus Callosum

The corpus callosum is made of nerve fibers, deep in the brain, that connect the two halves of the cerebral hemispheres.

Cerebellum

The cerebellum is the second largest area of the brain. It is made up of two hemispheres, or halves, as well as a middle section. The cerebellum is connected to the brain stem.

Each hemisphere is comprised of four sections called lobes—frontal, parietal, temporal, and occipital. Each lobe controls a specific group of activities.

• Frontal—Movement, intelligence, reasoning, behavior, memory, personality, planning, decision making, judgment, initiative, inhibition, mood.
• Parietal—Intelligence, reasoning, telling right from left, language, reading, and sensation.
• Temporal—Speech, behavior, memory, hearing, vision, smell, and emotions.
• Occipital—Vision.

Brain Stem

The brain stem is the bottom-most portion of the brain, connecting the cerebrum with the spinal cord. The midbrain, pons, medulla oblongata, and reticular formation are all part of the brain stem.

Pons

The pons, a part of the brain stem, contains the origins of the fifth, sixth, seventh, and eighth cranial nerves.

Midbrain

The midbrain is the short portion of the brain stem between the pons and the cerebral hemispheres. The top of the midbrain is called the tectum (or tectal area). The third and fourth cranial nerves originate in the midbrain.

Medulla Oblongata

The medulla oblongata, a part of the brain stem, connects the brain with the spinal cord. It contains the origins of the ninth, tenth, eleventh, and twelfth cranial nerves.

Reticular Formation

The reticular formation is the central core of the brain stem. It connects with all parts of the brain and brain stem.

Ventricles

The ventricles are connected cavities (the lateral, third, and fourth ventricles) that contain cerebrospinal fluid (CSF). This clear, watery fluid is produced by the choroid plexus. It flows through the ventricles and the subarachnoid space as it bathes and cushions the brain and spinal cord.

There are two lateral ventricles, one in each cerebral hemisphere. The third ventricle is beneath the corpus callosum and surrounded by the thalamus. The fourth ventricle is an expansion of the central canal of the medulla oblongata.

Cranial Nerves

There are 12 pairs of cranial nerves. Their functions are listed in the diagram below.

Optic Chiasm

The optic chiasm is the area under the hypothalamus where each of the two optic nerves crosses over to the opposite side, forming an X shape.

Pituitary Gland

The pituitary gland is attached to, and receives messages from, the hypothalamus. The pituitary gland is made up of two lobes—the anterior and the posterior. Several hormones are produced by the pituitary gland, including prolactin, corticotropin, and growth hormone.

Hypothalamus

The hypothalamus makes up part of the wall of the third ventricle and is the base of the optic chiasm.

Pineal Gland

The pineal gland lies below the corpus callosum. It produces the hormone melatonin. Melatonin is believed to control the biological rhythm of the body.

Spinal Cord

The spinal cord is made up of neurons and their extensions (ie, nerve fibers). It begins in the medulla oblongata and continues through the hollow center of the vertebrae (the bones of the spine. The spinal cord is covered by meninges. CSF flows through the meninges.

Meninges

The meninges are three membranes that completely cover the brain and spinal cord. CSF flows in the space between two of the membranes. A tumor called meningioma can originate from the meninges.

Glial Tissue (Neuroglia)

Glia is the supportive tissue of the brain. The cells which make up this tissue are called glial cells. The most common glial cells are astrocytes and oligodendrocytes. Ependymal cells are another form of glia. The largest percentage of brain tumors originate from the glia.

Tentorium

The tentorium is a flap of meninges separating the cerebral hemispheres from the structures of the posterior fossa.

Posterior Fossa/Infratentorium

The tentorium separates the posterior fossa from the cerebral hemispheres. The area below the tentorium is called the infratentorium, or the posterior fossa. This area within the skull contains the cerebellum and the brain stem.

Supratentorium

The supratentorium is the area above the tentorium containing the cerebral hemispheres.