NEURON
N
STRUCTURE OF A NEURON
A neuron (nerve cell) consists of a cell body and several
branching projections, which are called dendrites. Every
neuron has a filamentous projection called an axon (nerve
fibre). Axons vary greatly in length - from a fraction of a
centimetre to about a metre. An axon branches at its end
to form terminals, via which signals are transmitted to target
cells, such as the dendrites of other neurons, muscle cells,
or glands. Bundles of the axons of many neurons are known
as nerves or, when they are located within the brain or spinal
cord, as nerve tracts or pathways.
N o d e o f R a n v i e r
A x o n t e r m i n a l s
C o n n e c t in g t o t h e c e l l b o d y a n d t o
d e n d r i t e s o f t h e n e u r o n d e p i c t e d
h e r e a r e t h e t e r m i n a l s o f a n a x o n
c o m i n g f r o m a s e c o n d n e u r o n .
S y n a p s e
T h is i s a g a p b e t w e e n a n
a x o n t e r m in a l a n d t h e
r e c e i v i n g c e l l . S i g n a l s
p a s s a c r o s s t h e g a p b y
m e a n s o f c h e m i c a l s
c a l l e d n e u r o t r a n s m i t t e r s .
BASIC TYPES OF NEURON
Sensory neurons carry signals from
sense receptors along their axons
into the central nervous system
(CNS). Motor neurons carry signals
from the CNS to muscles or glands;
the axon terminals form a motor
endplate. Interneurons form all the
complex interconnecting electrical
circuitry within the CNS itself. For
each sensory neuron in the body,
there are about 10 motor neurons
and 99 interneurons.
Neuron in a cell culture
A sensory neuron from an
adult dorsal root ganglion.
When a neuron transmits (“fires”) a
nerve impulse, a chemical called a
n e u -
r o t r a n s m i t t e r
is released from the axon
terminals at
s y n a p s e s
(junctions with
other neurons). This neurotransmitter
may make a muscle cell contract, cause
an endocrine gland to release a hor-
mone, or affect an adjacent neuron.
Different stimuli excite different types
of neurons to fire. For example, physical
stimuli such as cold, pressure, or light of
a certain wavelength may excite sensory
neurons. Most neurons’ activity is con-
trolled by the effects of neurotransmitters
released from adjacent neurons. Certain
neurotransmitters
generate
a
sudden
change in the balance of electrical poten-
tial inside and outside the cell (an “action
potential”), which occurs at one point on
the cell’s membrane and flows at high
speed along it. Others stabilize neuronal
membranes, preventing an action poten-
tial. Thus,
a
neuron’s
firing
pattern
depends on the balance of excitatory and
inhibitory influences acting on it.
LIFESPAN
If the cell body of a neuron is damaged
or degenerates, the cell dies and is
never replaced. A baby is born with
the maximum number of neurons, and
this
number
decreases
continuously
throughout life. However, because peo-
ple are born with a very large number
of neurons, problems normally only
arise when disease, injury, or persistent
alcohol abuse affects the CNS, and dra-
matically increases the rate at w hich
neurons are lost.
If a peripheral nerve is damaged, its
individual nerve fibres have the ability
to
regenerate
themselves
(see
n e r v e
i n j u r y
;
n e u r o p a t h y ) .
542
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