Note: This page is part of the section
about the structure and function of different Tissue
Types, which is related to the section about Histology
and Cells (incl. structure
of animal cells, cell
division, mitosis, meiosis).
This "Tissue Types" section is included to complete description
of the knowledge of "Histology - The Cell" required by
some courses in First-Level Anatomy and Physiology. To read about
other tissue types see the list of on the left.
Nervous tissue consist of two main types of cells:
neurons
and neuroglia.
Nerve cells,
or neurones (also written "neurons") transmit
nerve impulses that move information around the
body. Neuroglia
are also known simply as "glia" and have various
functions in support of nerve cells but do not transmit
nerve impulses themselves. More information about
both of these types of cells follows below:
Neurons
Neurons
(nerve cells) are sensitive to various types
of stimuli such as heat/cold, light/dark,
pressure.
They transmit electrical nerve impulses
thereby moving information around the body.
There are several types of neurons, including
motor neurones,
sensory neurones
and relay
neurones.
Each neurone consists of an enlarged part
called the cell body (or
"perikaryon")
which contains of nucleus of the cell and
many "processes" called dendrites
that extend away from the cell body and
are important because they receive the (electrical
signals called) nerve impulses.
For more detailed information parts of
neurones and their structures and functions
see the page about neurones.
Neuroglia / Glia
Neuroglia are sometimes known as simply "Glia".
They are not sensitive to stimuli and so do not
generate or conduct nerve impulses. "Glia"
or "Neuroglia" are therefore sometimes
referred to as the "non-nervous cells of
the nervous system".
Their functions within the nervous system depend
on the specific type of neuroglia:
Types
of Neuroglial Cell:
Astroglial
Cell
Astrocyte
Astroglial cells,
also known as "astrocytes" are
found throughout the Central Nervous System
(CNS), that is in the tissues of the Brain
and Spinal Cord.
Structure:
An astrocyte is a star-shaped cell that
has many processes extending from its cell
body into the surrounding network of nerve
fibres.
Function(s):
Supply of nutrients to neurons.
Removes excess neurotransmitters
Maintains appropriate balance of Ca2+
and K+ ions (which are important
in passing nerve impulses at synapses).
Helps migration of neurons during brain
development.
Aids formation of the blood-brain barrier.
(Possibly participating in information
storage processes)
Ependymal Cell
Ependymal cells are found in the Central
Nervous System (CNS),
that is in the tissues of the Brain and
Spinal Cord.
Structure:
Function(s):
Protection: Forms lining of the ventricles
of the brain and central canal of the
spinal cord.
Forms cerebrospinal fluid (CSF).
Aids circulation of cerebrospinal fluid
(CSF).
Ependymal cells form the extremely thin
membrane called the ependyma
that lines the ventricles of the brain and
choroid plexuses.
Microglial Cell
Microglial cells are sometimes known simply
as "microglia" and are found in
the Central Nervous System (CNS), that is
in the tissues of the Brain and Spinal Cord.
Structure:
Microglia are small glial cells.
Function(s):
Protects CNS neurons from disease -
e.g. by clearing away debris and dead
cells. Therefore some texts describe microglia
has having a "mainly scavenging"
function.
Microglia may be compared with macrophages
(which are large scavenger cells, not
specific to the nervous system).
Oligodendrocyte
Oligodendrocyte cells are found in the Central
Nervous System (CNS), that is in the tissues
of the Brain and Spinal Cord.
Structure:
Oligodendrocytes differ from astrocytes
(listed higher-up this page) by
having fewer and thinner processes and no
gap junctions.
Function(s):
Forms supporting network around CNS
neurons.
Some texts describe oligodendrocytes
as the CNS "equivalent"
of Schwann Cells in the PNS because
they produce myelin sheath around several
adjacent axons of CNS neurons.
Schwann Cell
Schwann cells are found in the Peripheral
Nervous System (PNS), that is around the
nerves of the extremeties of the body, e.g.
located in the skin.
Structure:
Function(s):
Forms part of the Myelin sheath (protective
covering) around a PNS neuron.
That is, Schwann cells wrap around the
axon of the neurone, forming areas in
which myelin forms - resulting in the
formation of a "myelin sheath"
around the axon - between the axon and
the Schwann cell(s). This is important
because myelinated nerves conduct impulses
more quickly than non-myelinated nerves.
Also helps with regeneration of PNS
axons.
For more details see the diagram and description
of a Motor Neuron - recall that the
neurone (nerve cell) transmits electrical
impulses, in comparison with the neuroglia
cell (Schwann Cell), which does not
transmit nerve impulses but is, nevertleless,
a specialised cell of the nervous system.
Compare Oligodendrocytes and Schwann Cells:
Both Oligodendrocytes
and Schwann Cells facilitate the formation of
myelin around the axons of nerve cells. However,
Schwann Cells
are found in the Peripheral Nervous System (PNS),
and form myelin around a portion of a single
axon only. Oligodendrocytes
are found in the Central Nervous System (CNS),
and each oligodendrocyte can form myelin around
several axons.
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