These neuronal functions range from insulating, nourishing,
and supporting neighboring neurons. Research of these cells is still
underway in hope that glias will contribute more importantly to the
information processing of the brain.
Astrocytes:
Out of all the different types of glias, the most numerous glia in
the brain are the astrocytes. These cells fill the
space between the neurons in the brain. The remaining space between
an astrocyte and a neuron is approximately
20 nm wide. The purpose of the astrocyte is to regulate
the chemical content of the remaining space or extracellular space.
Astrocytes probably influence whether an axon or
dendrite can grow or retract. They also have special proteins in their
membranes to remove many neurotransmitters from the synaptic cleft.
The synaptic cleft will be explained later in the "synapses"
section. Astrocytes also possess neurotransmitter receptors that can
trigger electrical and biochemical events inside the glial cell.
Other functions of the astrocyte include regulating the concentration
of potassium ions and controlling the contraction of several substances
that can interfere with the functions of the neuron.
Neurotransmitters are chemical substances that transmit
information across a synapse.
*Note that the little circles inside the astrocyte can represent
mitochondria, nucleus, or golgibodies.
Oligodendroglia and Schwann Cells:
Other glia cells are the oligodendroglia and schwann cells. These
glia cells are responsible for providing layers of membrane to insulate
the axons. These layers of wrapping that spiral around the axons of
the brain are called the myelin. It is also called
a myelin sheath because the axon fits into
the myelin like a sword in its scabbard. The myelin sheath is periodically
interrupted, leaving a region of short length where the axonal membrane
is exposed. This region is dubbed the Node of Ranvier.
In the picture below, the label that says "cytoplasm" is
actually the cytoplasm of the oligodendroglia.
Picture of Oligodendroglial cell supplying
the myelin for the axon.
The oligodendroglial cells and schwann cells
differ based on the location and some characteristics.
Oligodendroglial cells are found only in the central
nervous system, such as the brain and spinal cord. One of these cells
can contribute myelin to several axon.
Schwann cells are found only in the peripheral nervous
system which includes the parts outside the skull and vertebral column.
One schwann cell can contribute myelin to only a single axon.
The yellow bananas in the picture above are mitochondria
that move slowly through the axon.
Synapses
Introduction:
A synapse is a specialized junction where the axon
ends (axon terminal) and contacts another neuron or cell type. The
normal direction of the information flow is from the axon terminal
to the target neuron. Hence the the axon terminal is commonly called
the presynaptic and the target neuron as the postsynaptic.
Types/Forms of Synapses:
There are currently two types/forms of synapses: electrical and
chemical. Electrical synapses occur at specialized sites
called gap junctions. The pre- and postsynaptic membranes are separated
by special proteins called connexons. Connexons allow ions to pass
from the cytoplasm of a cell to the cytoplasm of another cell. This
process gives off an electronically coupled effect.
The rest of this section will now focus on the chemical synapses.
Parts of Chemical Synapses and Synaptic
Transmission:
Parts
of a Chemical Synapse:
The presynaptic and postsynaptic membranes are separated
by a synaptic cleft. The synaptic cleft is filled of
a matrix of fibrous extracellular protein that attaches itself to the
pre- and postsynaptic membranes. It is essentially the space between
the presynaptic and postsynaptic membranes.
The presynaptic side of the synapse is called the presynaptic
element. This is usually referred to as the axon terminal,
The presynaptic element contains synaptic vesicles
and secretory granules.
Synaptic vesicles are numerous small
bubbles of membrane in the axon terminal that measure about 50 nm in
diameter.
Secretory granules are vesicles that
are larger than the synaptic vesicles measuring about 100 nm in diameter.
They act just the same as synaptic vesicles except they contain soluble
protein that appears dark in an electron microscope. They are sometimes
named large, dense-core vesicles.
Membrane differentiations are dense accumulations
of protein in the membranes on either side of the synaptic cleft. They
are usually categorized as active zones or postsynaptic
density.
On the presynaptic side, active zones
are the proteins jutting into the cytoplasms of the terminal along the
intercellular face of the membrane. Active zones resemble more like
pyramids and are the actual sites of neurotransmitter release. Synaptic
vesicles are clustered in the cytoplasm adjacent to the active zones.
On the postsynaptic side, the postsynaptic density
are the proteins spanning the thickness of the postsynaptic membrane.
It contains neutrontransmitter receptors, which convert
intercellular chemical signals into intracellular
signals in the postsynaptic cell.
Synaptic
Transmission:
Synaptic transmission in the human nervous system is chemical.
Synaptic transmission is the process of transfering
information at the synapse from one neuron to another.
At almost all chemical synapses, electrical impulses
going down the axon is converted to a chemical signal that crosses the
synaptic cleft. This chemical signal is converted to
an electrical signal when it reaches the postsynaptic membrane. This
chemical signal is called the neurotransmitter and
it is stored in and released from the synaptic vesicles
within the axon terminal.
This process of converting electrical-to-chemical-to-electrical
information is responsible for the brain's ablity to memorize, learn,
and compute. Dysfunctions of the synaptic transmission
usually account for mental disorders that people exhibit.
Central Nervous System (CNS) Synapses:
In the central nervous system or CNS, different types
of chemical synaptic synapses exist. They can be distinguished by which
part of the neuron is postsynaptic to the axon terminal. In other words,
how two neurons hook up with each other.
Synaptic Arrangements in the CNS (Image not
Availiable) (Copyrighted (1996) by Neuroscience:
Exploring the Brain)
(A) axodendritic synapse (B) axomatic synapse
(C) axoaxonic synapse.
There are three main types of synaptic arrangements in
the CNS: axodendritic synapse,
(A) Axodendritic Synapse is when the
axon of one neuron attaches itself to an extending branch or dendrite
of another neuron. (Designated as 'a' on the picture above)
(B) Axomatic Synapse is when the axon
of a neuron attaches itself to the body of another neuron. (Designated
as 'b' on the picture above)
(C) Axoaxonic Synapse is when the axon
of a neuron attaches itself to another axon of a neuron attaching itself
to an entirely different neuron. (Designated as 'c' on the picture above)
