Brain Anatomy

The brain and spinal cord make up the central nervous system. The spinal cord attaches to the brain through an opening at the base of the skull—the magnum foramen—and extends all the way down to the lumbar spine (lower back). The peripheral nervous system includes the 12 pairs of cranial nerves and 31 pairs of spinal nerves, all of which branch into the rest of the body.

Brain Cells

Brain tissue is composed of several different types of cells. These include neurons and specialized cells called glial cells, such as astrocytes, oligodendrocytes, ependymal cells, and microglial. Brain tumor types are determined by the type of cell in which the tumor originated.

Astrocytes—Astrocytes are small star-shaped glial cells. This cell type is one of the few types that maintain their ability to reproduce in the mature brain, and it is thought that they might be susceptible to agents that alter cellular replication. This may explain why most primary tumors in the central nervous system have an astrocytic origin.

Oligodendrocytes—Oligodendrocytes are highy specialized glial cells that form the myelin insulation, or myelin sheath, around the axons in nerve cells.

Ependymocytes—Ependymocytes are the glial cells that make up the ependyma, the membrane that lines the ventricles of the brain and the central canal of the spinal cord.

Microglial—Microglial cells function chiefly as scavengers. When central nervous system tissue is damaged, microglial cells migrate to the site where they proliferate and devour the cellular debris.

Regions of the Brain

The brain is composed of several different regions, each of which performs unique functions. The major regions include the brainstem, the midbrain, and the forebrain. Symptoms caused by a brain tumor depend on the tumor's size and location in the brain.

Brainstem—This is the most primitive region of the brain. It consists of the medulla (controls breathing, blood pressure, heart rate, and digestion) and the cerebellum (coordinates muscle movement and speech).

Midbrain—The structures contained in the midbrain link the brainstem to the thalamus for information relay and to the hypothalamus for regulation of drives and actions. The hypothalamus is part of the limbic system, which consists of several interconnected structures involved in hormone production, hunger, thirst, emotional reactions, biological rhythmns, and the coordination of complex physical activities.

Aggressive behavior is linked to the amygdala (uh-MIG-de-la). The hippocampus, located next to the amygdala, plays a crucial role in processing various forms of information into long-term memory. Damage to the hippocampus produces global retrograde amnesia, that is, the inability to retain new information.

Forebrain—The cerebrum covers the brainstem and has a walnutlike configuration of left and right hemispheres. The extremely convoluted surface of the hemispheres, called the cortex, is a complicated structure associated with the "higher" functions of the mind—thought, reasoning, sensation, and motion.

Right and Left Hemispheres of the Brain

The two hemispheres are in continual communication with each other, acting as independent parallel processors with complementary functions. The area associated with language, however, appears only in the left hemisphere.

In most people, the left hemisphere is dominant over the right in deciding which response to make. In individuals with normal hemispheric dominance, the left hemisphere, which manages the right side of the body, controls language and general cognitive functions, and appears to be most closely associated with the conscious self.

The right hemisphere controls the left half of the body and manages nonverbal processes such as attention, pattern recognition, line orientation, and the detection of complex auditory tones.

The left and right hemispheres each are divided into four lobes: occipital lobe, parietal lobe, frontal lobe, and temporal lobe.

  • The occipital lobe is involved in understanding visual images and the meaning of the written word.
  • The parietal lobe receives and interprets sensations of pain, pressure, temperature, touch, size, shape, and body part awareness.
  • The frontal lobe is most closely associated with controlling responses to input from the rest of the system. They are responsible for voluntary movement, emotion, planning and execution of behavior, intellect, memory, speech, and writing.
  • The temporal lobe is involved in understanding sounds and spoken words, as well as emotion and memory.

Ventricles and Cerebrospinal Fluid

There are four chambers in the brain, the ventricles, that contain structures (called the choroid plexus) that produce cerebrospinal fluid. The cerebrospinal fluid flows through the ventricles and bathes the brain and spinal cord.

Meninges

The meninges is the thin, protective membranous covering that suspends the brain within the skull and prevents the brain from pressing against the skull's inner surface. The meninges is a laminated, or layered, structure. The outermost layer is the dura mater ("tough mother" in Latin), a fibrous membrane that stabilizes the CNS within the skull and the spinal column.

The middle layer, the arachnoid mater, contains cerebrospinal fluid, arteries, veins, and cranial nerve roots. It also secures the dura to the innermost layer, the pia mater, a delicate membrane that clings to the contours of the brain, thereby distributing the forces that suspend the brain within the skull.

Cranial Nerves

There are 12 pairs of cranial nerves that control sensory and motor activities:

I Smell
II Vision
III Eye movement
IV Eye movement
V Facial sensation
VI Eye movement
VII Face movement
VIII Hearing and balance
IX Taste and gag reflex
X Involuntary muscles: heart, stomach, intestines, throat, chest
XI Voluntary muscles of the neck
XII Tongue movement

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 30 Jul 1999

Last Modified: 02 Sep 2015