The Central Nervous System

This page outlines the basic physiology of the central nervous system, including the brain and spinal cord. Separate pages describe the nervous system in general, sensation, control of skeletal muscle and control of internal organs.

1. The central nervous system CNS is responsible for integrating sensory information and responding accordingly. It consists of two main components:
   1. The spinal cord serves as a conduit for signals between the brain and the rest of the body. It also controls simple musculoskeletal reflexes without input from the brain.
   2. The brain is responsible for integrating most sensory information and coordinating body function, both consciously and unconsciously. Complex functions such as thinking and feeling as well as regulation of homeostasis are attributable to different parts of the brain.
2. The brain and spinal cord share some key anatomic features:
   1. Living nervous tissue has the consistency of jelly and requires special protection from physical damage. The entire CNS is encased in bone. The brain is within the cranium, while the spinal cord runs within a canal through the vertebrae.
   2. Within its bony case, the entire CNS is bathed in a cerebrospinal fluid (CSF), a colorless fluid produced by special structures in the brain. CSF provides a special chemical environment for nervous tissue, as well as an additional buffer against physical damage.
   3. The special chemical environment of nervous tissue is maintained by the relatively impermeable membranes of capillaries in the CNS. This feature is known as the blood-brain barrier.
   4. There are two general types of tissue in the CNS:
      1. Gray matter consists of nerve cell bodies, dendrites, and axons. Neurons in gray matter organize either in layers, as in the cerebral cortex, or as clusters called nuclei.
      2. White matter consists mostly of axons, causing it to look white due to the myelin sheathing of the axons.
3. In the early embryo, the CNS forms as a relatively uniform tube. The major regions of the brain develop as enlargements at the head end of this tube:
   1. The medulla oblongata appears as a swelling at the upper end of the spinal cord. Besides being a conduit for fibers running between the spinal cord and higher regions of the brain, it contains control centers for involuntary functions such as blood pressure, breathing, swallowing and vomiting.
   2. Just above the medulla are the pons and cerebellum. The pons relays information between higher regions of the brain and the cerebellum, which processes sensory information and helps coordinate movement.
   3. The next segment, the midbrain, is primarily responsible for eye movement.
   4. Above the midbrain lies the diencephalon, which is composed of two major parts:
      1. The thalamus processes and integrates all sensory information going to the higher regions of the brain.
      2. The hypothalamus is critical for homeostasis, the maintenance of the body's internal environment. It influences nervous control of all internal organs and also serves as the master regulator of endocrine function by its control over the pituitary gland.
   5. The highest region of the brain is the cerebrum, which includes both the cerebral cortex that is visible on the outside of the brain as well as other internal structures. The cerebrum is responsible for conscious sensation and voluntary movement, as well as advanced functions such as thinking, learning and emotion.