Basic Anatomy of a Neuron
- Soma or cell body
- Dendrites -- relatively short, branch-like structures
extending from the soma.
- Axon -- relatively long fiber extending from the soma.
It usually branches near the end furthest from the soma.
- Terminal buttons -- endings that terminate the axon
Functions of the neuron and its parts
- You can think of a neuron as a device for transmitting
information from one place to another and for doing computations
on that information.
- Signals are received by the neuron at specialized sites
called synapses, located on the dendrites and cell body.
- Signals are conducted by the axon from the cell body to the
- Signals are transmitted from the terminal buttons to the
receiving cell across a synapse (if the receiving cell is a neuron)
or a neuromuscular junction (if the receiving cell is a
The Neural Impulse
The neuron transmits signals in the form of neural impulses that
race at about 100 meters/second from the base of the axon where it
connects to the soma to the terminal buttons. In brief, here's how
- The neuron normally maintains a resting potential of about
-70 to -90 millivolts (mV). (By contrast, a single cell in a flashlight
has a charge of about 1500 mV or 1.5 volts.) The negative sign
indicates that the interior of the cell is negative with respect to
- If the resting potential is temporarily reduced to a threshold
value of perhaps -50 mV, this sets in motion a chain of events that
begins at the base of the axon. Positively charged sodium ions
rush in, rapidly causing the local charge at the base of the axon to
reverse (to perhaps +30 mV). Positively charged potassium ions in the
axon then rush out, bringing the charge back to negative. The ions
are then pumped back across the membrane to their starting positions.
- The narrow region of positive charge causes the next segment of
axon to trigger, starting the same process there, and so on; the
region of positive charge moves rapidly down the axon. This is the
action potential or neural impulse.
The Synapse and the Neuromuscular Junction
- Each terminal button joins to either another neuron, a gland cell,
or a muscle cell. The connection to another neuron, which takes place
at a dendrite or on the soma, is termed a synapse. When the
terminal button connects to a muscle cell, the connection is termed a
- The base of the terminal button connects to a portion of the
membrane of the receiving cell via tiny fibers, leaving a small,
fluid-filled gap between the two membranes.
- The terminal button contains numerous small bags called
vesicles. Each vesicle contains about 10,000 molecules of a
special chemical called a neurotransmitter substance.
- The surface of the receiving cell's membrane contains molecules
called receptor protiens.
- When an action potential arrives at the terminal button, the
vesicles move toward the synaptic gap. Those that reach the surface
burst open, spilling their contents into the gap where they quickly
diffuse to the other side and attach to the receptor protiens of the
- The receptor protiens then change their shape, allowing ions
(charged atoms) to enter the receiving cell. An influx of positive
ions (e.g., sodium ions) would tend to reduce the negative charge
inside the receiving cell (resting potential). If this occurs
in enough places, the receiving cell will reach threshold and fire
its own action potential. Or, in the case of the neuromuscular
junction, the receiving muscle cell will contract.
- Finally, the neurotransmitter is removed from the receptor
protiens, terminating the action. One way this happens is when a
special enzyme breaks down the neurotransmitter.
How Drugs Affect the Synapse or Neuromuscular Junction
Most psychoactive drugs do their work by affecting events at the synapse. There are three main
ways in which drugs can affect the synapse or neuromuscular junction:
- Blocking -- the drug can occupy the receptor sites for a neurotransmitter but fail to
activate the receptor. This is like placing chewing gum in a lock -- even if you have the right key,
it can't get into the lock and open it.
- Activating -- the drug can occupy the receptor sites for a neurotransmitter and activate
the receptors like the "real thing."
- Prolonging the action -- by interfering with the mechanism that removes the neurotransmitter
from the receptors. The natural neurotransmitter then continues to stimulate the receiving cell for
a longer-than-normal time. One way this happens is by inhibiting the enzyme that normally breaks down
Excitatory versus Inhibitory Synapses
Synapses can be either excitatory or inhibitory.
- Excitatory synapses -- activation of the synapse causes
the receiving neuron to become depolarized (the negative charge
is reduced toward zero). This makes it more likely that the neuron
will fire its own action potential.
- Inhibitory synapses -- activation of the synapse causes
the receiving nturon to become hyperpolarized (the negative
charge of the receiving neuron is increased). This makes it less
likely that the neuron will reach threshold and fire an action
- A given synapse is always either excitatory or inhibitory.
- Whether a synapse is excitatory or inhibitory depends on the
type of neurotransmitter it uses and on the nature of the receptor
Some Principles of Operation
- All or none principle -- When a neuron fires an impulse,
it is always the same: you either get a full impulse or none at all.
- Frequency coding -- the summed strength of all inputs to
a neuron (excitatory and inhibitory) determine the frequency (number
of impulses per second) of firing. Thus the intensity of stimulation
is coded (represented) by the frequency of firing.
- "Doctrine of specific nerve energies" -- how a stream of
neural impulses is interpreted by the brain depends not on the source
of these impulses, but on where in the brain they are analyzed. Thus
if impulses coming in from the auditory nerve from the ears were to
arrive at the visual cortex rather than the auditory cortex, one
would experience patterns of light rather than sounds.