Image of a Nerve Cell
nerve cells of the central and peripheral nervous systems
are called neurons. Most neurons have three
parts; an axon, a cell body or soma, and dendrites. All neurons have one soma
and one axon, but while some neurons have many dendrites,
others have none.
Neurons vary in
size; the smallest have a diameter of 5 microns while the
largest are approximately 100 microns in width. (A micron is
one one-thousandth of a millimeter).
soma of a neuron contains the cell's
nucleus and cytoplasm, a jelly-like substance
that surrounds the nucleus.
Chromosomes, which consist of
molecules of DNA (deoxyribonucleic
are found in the nucleus of the neuron. RNA (ribonucleic acid) molecules are also located
within the nucleus.
Nissl Bodies or Nissl Substances, which also contain RNA,
and Golgi Apparati are found in the
DNA forms the
genetic code that determines the cell's function. As DNA
cannot pass through the nuclear membrane, its commands are
carried to the cytoplasm by messenger RNA which can travel
outside of the nucleus. These RNA molecules link up with the
Nissl substances, connecting to them as a key would fit into
a lock. This combination causes the cell to
work, or use glucose. The waste products
generated by this process are removed from the cell by the
The axon allows
the neuron to send messages to other nerve cells. Each
neuron has only one axon, but this may have numerous
branches which connect the cell to many other.
Axons vary in
length; the axons of some pyramidal cells in the precentral
gyrus are long enough to travel all the way down to the end
of the spinal cord but other axons are very short.
Cells can be
classified based on the length of their axons.
Type I neurons have long axons while those
of Golgi Type II cells are short.
Axons arise from
an area on the cell body of the neuron called the axon
hillock. Most axons form many branches as they extend away
from the soma. At its end, each axonal branch divides into a
number of telodendria. The boutones terminaux or
passage, which contain neurotransmitters,
are located on the telodendria.
of the axons in the central and peripheral nervous systems
are covered at regular intervals with a fatty insulating
substance called myelin. The segments of the axon which lie
between areas of myelin and are therefore in direct contact
with extracellular fluid are called the Nodes of
increases the speed with which an axon can transmit
messages. The neural impulse travels by a process known as
saltatory conduction, jumping from
one unmyelinated segment to the next. This means that the
impulse does not have to be propagated through the entire
area of the axon.
dendrites of a neuron receive
messages from the axons of other nerve cells. There are two
types of dendrites, apical dendrites and basilar dendrites.
dendrites have stalks filled with cytoplasm
that appear to be part of the soma of the neuron to which
they are attached. Most of these dendrites are found in the
dendrites do not have a stalk. They are more
numerous than apical dendrites.
relevant information about nerve
Image of a Nerve Cell
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Transmission of Neural Messages
messages conducted along axons are electrochemical in
types of ions, or electrically charged atoms, are
involved in the transmission of neural impulses;
chloride ions (Cl-), sodium ions
(Na+), potassium ions
and organic anions
When a neuron is
at rest, there are high concentrations of A- and K+ within
the cell, while most Na+ and Cl- ions are located outside
its membrane. The resting potential
the neuron is -70 millivolts, meaning that the electrical
charge of the cell is slightly negative in comparison to
that of the extracellular fluid surrounding it.
is due to the selective permeability of the neural membrane.
Cl- and K+ can pass through the membrane, but Cl- does not
enter the cell in great quantities because both it and the
interior of the neuron are negatively charged. Organic
anions cannot move through the membrane due to their large
size. As Na+ is positively charged and the interior of the
nerve cell has a negative electrical potential, these ions
should pass into the cell. However, sodium cannot readily
pass through the membrane and most of the Na+ ions that do
enter the cell are extruded by the sodium-potassium
This is the name given to a group of molecules located in
the cellular membrane which push Na+ out of the cell and
draw K+ ions inside.
potential of a neuron changes when messages are received
from other nerve cells. Inhibitory impulses cause the
electrical charge of the neuron to become even more
negative, decreasing its ability to fire, or send messages
to other nerve cells. When excitatory messages are received,
however, the permeability of the cell membrane changes,
allowing Na+ ions to enter the neuron. This influx of
positively charged ions causes the cell's electrical
potential to temporarily become positive, peaking at +40
millivolts. This change of the electrical potential of the
cell from negative to positive is the action
potential, which ultimately causes the cell to
When the charge of
the cell reaches its positive peak, K+ ions are forced out
of the neuron because they are positively charged. The exit
of these ions causes the potential of the cell to become
negative again, temporarily dipping below -70 millivolts
before returning to resting potential.
After the cell
fires, it goes through a refractory
period during which it will not fire again.
The refractory period may be divided into two phases, the
period and the relative refractory
absolute refractory period, the neuron will not fire again,
no matter how strong the excitatory messages that it
The cell will fire
during the relative refractory period, but only if it
receives a very strong stimulus.
neuron fires, it communicates with other nerve cells through
the release of chemicals called neurotransmitters.
are found in the boutons terminaux located on the
teledendria of the axons. They are stored in circular or
oval-shaped capsules called synaptic
When an excitatory
impulse of sufficient strength reaches the teledendria, the
synaptic vesicles fuse with the axonal membrane and open,
spilling the chemicals they contain into the extracellular
fluid. The neurotransmitters then travel across a small
space called the synaptic cleft to attach to the neuron
that will receive their message.
After a message
has been sent, excess quantities of the neurotransmitter
that remain in the synaptic cleft must be cleared away in
order to allow further communication between the cells. In
some cases, the left-over chemicals are recycled; they are
picked up and repackaged in new synaptic vesicles to be used
in future transmissions. Other types of neurotransmitters
are destroyed by enzymes when they remain in the synaptic
is one neurotransmitter that has been well-studied. It is
the major neurotransmitter of the peripheral nervous system
and is also present in the central nervous system. It
carries messages controlling voluntary muscle movement, as
the nerve fibers located in muscles and in the spinal and
cranial nerves are acetylcholinergic. After messages have
been transmitted, ACh is broken down in the synaptic cleft
by an enzyme called acetylcholinesterase.
supply of acetylcholine, whether due to excess quantities of
acetylcholinesterase or resulting from inadequate synthesis
of the chemical, causes Myasthenia
Gravis. In this disorder, the strength of
neural impulses is attenuated, causes the voluntary movement
of muscles, including those involved in articulation,
voicing and respiration, to be weakened.
is distinguished from other disorders like ALS by the
administration of a derivative of curare. This drug will
temporarily improve the strength of someone suffering from
Myasthenia Gravis, but will have no effect in cases of ALS.
should not be confused with Myasthenia
Laryngis, which is a localized weakness in
the larynx and is not the result of a neurological
neurotransmitters include two groups of chemicals called the
monoamines and the
peptides. The monoamines, which
include dopamine, norepinephrin, and serotonin, are all synthesized from
proteins called amino acids. The peptides, including
enkephalin, endorphins, and substance
are large molecules which may be involved in blocking
sensations of pain.
cells in the rest of the body are injured, they can
regenerate and repair themselves. Neurons, on the other
hand, do not have this capability. If an axon is damaged,
the soma of the cell may also degenerate. Also, when a cell
body is injured or an axon is severed from the
Wallerian Degeneration, or death of the axon occurs.
When a neuron
dies, it is ingested by the support cells in the
nervous system. This process is called phagocytosis.