Neurons send various instructions via centrifugal projections (i.e., axons) and receive signals from other neurons via dendrites, both of which constitute a coordinated neural network performing brain functions through the transfer of information. The cerebral cortex includes neurons with morphologies of varying size and appearance, and similar neurons are horizontally lined up as they are grouped into six main layers; beginning from the external surface, these are the molecular, external granular, external pyramidal, internal granular, internal pyramidal, and polymorphic layers. In general, neurons in the cerebral cortex and other nuclei are broadly divided into projection neurons, which project axons toward other nuclei; intrinsic neurons, which project to other neurons within the nuclei; and neurons, which perform both the functions. These neurons control communication between (projection) or within (intrinsic) the nucleus to transfer and control complex nerve information.
Glial cells are broadly divided into macroglia and microglia. Macroglia include astrocytes, oligodendroglia, and ependymocytes. Astrocytes guide neuronal migration during brain formation, encapsulate the neuronal cell body (soma) and synapse, and control the metabolism of neurotransmitters released in the synaptic cleft. They also have “feet” that encircle cells forming the walls of vascular structures and the brain surface and help form the basal lamina. In addition, astrocytes often change their morphology to produce fiber components in response to tissue damage, called gliosis. Oligodendrocytes form the myelin sheath surrounding the nerve fiber (axon); in the peripheral nervous system, this role is performed by Schwann cells. Microglia are responsible for immune system activity and scavenging of damaged and/or destroyed substances.
Mesenchymal cells are responsible for blood supply in blood vessels, meninges (dura mater, pia mater, and leptomeninges), and choroid plexus; cerebrospinal fluid circulation; and overall protection of the brain.
