The central nervous system (CNS) is formed by the brain and spinal cord.
These elements are enclosed within the skull and spinal vertebral canal.
They are covered by the meninges, the dura, arachnoid and pia. Cerebrospinal
fluid flows over the surface and fills the chambers (ventricles, central
canal of the spinal cord). Two primary cell types make up the CNS - the
neurons, and the glia.
Neurons are stellate cells with projections extending from the cell
body. The projections come into contact with other neurons or outside the
CNS, with other cell types such as muscle. There are many types of neuron
based on the size and shape of the cell body and the arrangement of the
processes. A silver stain developed by Golgi was used for many years to
assign neurons to categories. Based on their staining neurons could be
seen to be unipolar, bipolar or multipolar. Most of the neurons within
the CNS are multipolar. The processes extending from the cell body are
either axons or dendrites. Neurons usually have only one axon but many
dendrites.
The neuronal cell body
Neurons are unique in the body with respect to cell size. Although the
cell body may only be about 50um in diameter, the axon may be several feet
long. This feature of neurons is related to the organization of the organelles
in the cell body. Neurons have an extensive cytoplasm and surface membrane
to maintain, and must therefore have the machinery to synthesize the components.
It is not surprising therefore that the most prominent organelle is the
rough endoplasmic reticulum. RER is formed into large bodies in the neuronal
cell cytoplasm. By light microscopy these bodies are clearly visible and
are termed Nissl bodies, after the discoverer. The cell bodies of neurons
also contain extensive neurofilaments and microtubules. These organelles
are involved in the transport of materials synthesized in the cell body
out into the processes. Other organelles present are common to most cells.
The nucleus of many neurons shows a nucleolus, a dense ball of chromatin.
The axon
Each neuronal cell body gives rise to a single axon (although there may be two). The axon projects through the neuropil (the mass of cell processes) or through a peripheral nerve to make contact with other cells. The axon contains cytoplasm, microtubules and neurofilaments but no Nissl substance. There is therefore little ability in the axon to synthesize protein and the requirement for protein substrate must be met by axoplasmic transport from the cell body. Within the CNS axons terminate at synapses on neuronal cell processes or cell bodies. At a synapse the axon is expanded and contains vesicles containing neurotransmitter substances. The neurotransmitters are stored within vesicles. The transmitters are released when the membrane of the vesicle fuses with the presynaptic membrane at the synapse. The vesicle membrane is not released and may be reused. The vesicles may store a range of transmitter molecules either separately or within the same vesicle. Molecules released from synaptic vesicles can bind with either the pre-synaptic or the post-synaptic membrane, and may inhibit or enhance the activity of other transmitter molecules.
Synapses
are described
according to the two elements involved: axo-axonic, axo-dendritic, or axosomatic. The pre-
and post-synaptic membranes may show increased density, indicating the
active site. Some axons terminate close to blood vessels. The axon terminals
in this case release their products as neurosecretions. Examples of these
axons are found in the posterior lobe of the pituitary gland, where oxytocin
and vasopressin are released. The spread of depolarisation from the cell
body to the axon terminal begins at the axon hillock, at the base of the
axon. The rate of conduction of the impulse depends on axon diameter and
the presence of myelin. The rate of conduction of the action potential
increases as the diameter of the axon increases. The presence of a myelin
sheath around the axon also markedly increases the speed of conduction.
The dendrite
Most neurons have many dendrites emanating from the cell body. In some
neurons the dendrites are organised as apical or basal giving the cell
a kind of polarity. The denditic cytoplasm is a reflection of the cytoplasm
of the cell body. Dendrites are therefore able to synthesise much of what
is synthesised in the cell body. The surface of dendrites is specialised
to receive axon terminals. In most cells, dendritic spines are present
over the entire surface, e.g. about one million spines are present on the
dendrites of each cerebellar Purkinje cell.
Neuronal organization
Neurons are organized within the CNS as sheets, or as nuclei. In the
brain, the cerebral hemispheres have layers of neurons (the grey matter)
close to the surface. In the middle of the brain, neurons are grouped into
nuclei in the parts of the diencephalon. In the spinal cord the neurons
form a central core with the fibres (the white matter) on the outside.
The neuroglia, or nerve glue, have a close relationship with the neurons
and act to support the neurons both structurally and functionally.
Astrocytes
Two types of astrocytes are found in the CNS. Fibrous astrocytes form
a type of scaffolding throughout the grey matter. Their cell bodies give
off many thin processes which pass through and contribute to the neuropil.
Protoplasmic astrocytes have shorter, thicker processes and are usually
found encircling blood vessels where they form a continuous covering. Both
types of astrocyte have extensive cytoplasmic filaments, the glial fibrillary
acid protein cytoskeleton. Both types of astrocyte are able to divide in
response to injury. In the presence of appropriate stimuli, astrocytes
become phagocytic. Although these cells are not the blood brain barrier,
they induce the endothelial cells of the blood vessels to form tight junctions
with each other, forming the functional blood brain barrier.
Oligodendrocytes
The oligodendroglia form the myelin sheaths around central axons. Each cell is able to send out processes which wrap around up to twelve axons. Each process myelinates a short section of axon. Nodes of Ranvier are the short naked axon segments between neighbouring myelinated sections. Myelin is formed by an oligodendrocyte process wrapping around an axon until several layers have formed. As the process of myelination develops, the cytoplasm is squeezed out of the layers until the cell membranes touch. By the fusion of inner and outer leaflets of the cell thick and thin lies alternate in the pattern characteristic of myelin.
In some places the cytoplasms does
not get completely squeezed out, leaving swellings called incisures of
Schmidt-Lantermann. Myelin insulates the parts of the axon it covers. This
encourages saltatory conduction in which the impulse jumps from node to
node, increasing the speed of conduction many times over. Myelin of axons
can be removed and replaced. In some disease processes myelin is destroyed
and impulse conduction fails.
Microglia
In addition to those large, or macroglial, cells just described, small
or, microglial cells are also present. These cells are quiescent under
normal circumstances. With age they slowly accumulate pigment. In some
disease or injury states the microglia become active and proliferate. The
active cells are phagocytic and engulf cellular debris.
Some neurons exist in ganglia, outside of the central nervous system.
The neurons in the ganglia have the same features as those within the
central nervous system, except for their relationship to glial cells. Satellite
cells usually surround the cell bodies of ganglion cells. Myelination of
axons is provided by Schwann cells.
In peripheral nerves the peripheral processes of neurons are formed into bundles by connective tissue. Each axon may be myelinated or unmyelinated. Myelin is produced by Schwann cells, which unlike oligodendrocytes myelinate only one section of one axon. The connective tissue is formed by fibroblasts and lies between axons as the endoneurium, as the wrapping around bundles of fibres - the perineurium. Epineurium forms the connective tissue which bundles all of the fibre bundles into a nerve. Since two types of tissue are present in peripheral nerve, there are basement membranes between them. Schwann cells are able to divide in response to injury and become phagocytic. Injury to peripheral axons may be followed by regrowth in which axon sprouts are able to grow back through the connective tissue skeleton of the nerve.
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