ABSTRACT
The brain is divided into two basic parts by the tentorium cerebelli (commonly just called the tentorium). This is a crescent-shaped lamina of dura mater, the concave anterior border of which is free. Above the tentorium lies the forebrain and below it the hindbrain, with the midbrain lying at their junction in the tentorial notch-the opening left by the free anterior border of the tentorium-which allows the fore-and hindbrains to connect. These divisions, together with the major intracerebral
Table 22.1 Main types and functions of neuroglial cells in the CNS
Astrocyte (astro=star, cyte=cell)
Star-shaped cell with numerous processes Protoplasmic astrocytes (with many dendrites) are found in the grey matter of the CNS and fibrous astrocytes (with fewer dendrites) in the white matter of the CNS
Attach neurons to blood vessels and as such are part of the blood-brain barrier Curl around nerve cells to provide supporting network in brain and spinal cord Metabolic and regulating functions for biochemical environment
Oligodendrocytes (oligo=few, dendro=tree)
Resemble astrocytes but, as their name suggests, they have fewer and less arborizing dendrites A single oligodendrocyte may be applied to several neurons
Support the CNS neurons physically by forming a semi-rigid connective tissue structure They produce the phospholipid myelin sheaths around axons in the CNS Represented in the peripheral nervous system as Schwann cells
Microglia (micro=small, glia=glue)
Small cells with few processes, which are derived from monocytes Also called brain macrophages May migrate to the site of injury
Phagocytose and destroy cell debris and infective organisms Essentially function as macrophages Regarded as part of the reticuloendothelial system
Ependyma or ependymocytes (ependyma=blouse or upper garment)
Epithelial cells in a single layer which range from squamous to columnar; many are ciliated
Form the lining of the ventricles and the central canal of the spinal cord Secrete substances into and absorb them from the CSF In some areas form specialized cells of the choroid plexus
The forebrain comprises the cerebral cortex, thalamus, and hypothalamus; the hindbrain consists of the cerebellum, pons, and medulla; the midbrain joins the two together. A clinically important grouping of structures, called the brain stem, comprises the midbrain, pons, and medulla. Its importance lies in the fact that, in the absence of brain-stem function, despite artificial maintenance of circulation and gas exchange, cardiac arrest inevitably results together with death of the whole organism. The subdivisions, components, and functions of the brain are summarized in Table 22.2.
