The Central Nervous System
The brain and spinal cord comprise the central nervous system. The adult human brain weighs approximately 3 pounds (1.4 kilograms). There are two major parts of the brain:
- The higher and larger forebrain (the cerebrum)
- The lower and smaller brain stem
The Cerebrum
The cerebrum is the uppermost and largest part of the brain. It is the most highly developed section of the brain. There are several components to the cerebrum:
The Cerebral Cortex. The cortex is the outermost layer of the cerebrum. It is made of gray and white matter:
- Gray matter is a thin sheet of nerve cells that cover the surface of the brain.
- White matter is a bundle of insulated nerve fibers that underlies the cortex and makes up the core of the cerebral hemispheres.
The Hemispheres. The two hemispheres control higher brain functions, such as memory, learning, decision making, and processing input from the senses. They are each divided into four lobes, which regulate different brain functions:
- Frontal lobe: This is the brain's "gatekeeper." It controls higher motor functions, including speech, and governs concentration, attention, inhibition, judgment, and personality traits.
- Parietal lobe: Processes information from the senses and controls walking, posture, and head and eye movements.
- Occipital lobe: Responsible for interpreting visual input from the eyes.
- Temporal lobe: Responsible for interpreting auditory input from the ears. Also regulates how language is interpreted and retrieves information for memory storage.
The Basal Ganglia. The basal ganglia are clusters of gray matter within each of the lobes. They are important for coordinating voluntary muscle movement, balance, and posture.
The Limbic System. The limbic system is located deep in the cerebrum and controls interpretation of smell, instinctive behavior, emotions, and drives.
Brain Stem
The brain stem is responsible for all vital functions. It is divided into the following areas, which are responsible for specific functions:
- Medulla: sleep, breathing, heartbeat, digestion, activation of higher forebrain functions
- Pons: sleep, breathing, motor control, activation of higher forebrain functions
- Cerebellum: movement coordination
- Midbrain: walking, posture, head, eye movement
- Hypothalamus: body temperature, appetite, sexual behavior, reproductive hormones
- Thalamus: communication with higher forebrain
The Spinal Cord
The spinal cord extends out of the base of the skull through the vertebrae of the spinal column. It is continuous with the brain. Thirty-one pairs of nerves extend from the sides of the spinal cord to other parts of the body (the peripheral nervous system).
The Meninges and Cerebrospinal Fluid
The meninges are three membranes that enclose the brain and spinal cord. They contain cerebrospinal fluid, which protects the central nervous system from pressure and injury.
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