Nerve Cell Physiology
A nerve cell is called a neuron and it transmits its messages by the axon.
The nucleus of a neuron is located in the cell body. Extending out from the
cell body are processes called dendrites and axons. These processes serve to conduct impulses (with dendrites
conducting impulses toward the cell body and axons conducting impulses away from
the cell body).
Neurons can respond to stimuli and conduct impulses because a membrane
potential is established across the cell membrane. In other words, there is an
unequal distribution of ions (charged atoms) on the two sides of a nerve cell
membrane. This is illustrated with a voltmeter:
With one electrode placed inside a neuron and another outside, the
voltmeter is 'measuring' the difference in the distribution of ions on the
inside versus the outside. And, in this example, the voltmeter reads -70 mV (mV
= millivolts). In other words, the inside of the neuron is slightly negative
relative to the outside. This difference is referred to as the Resting Membrane
Potential. How is this potential established?
Establishment of the Resting Membrane Potential
Membranes are polarized or, in other words, exhibit a
RESTING MEMBRANE POTENTIAL. This means that there is an unequal distribution
of ions (atoms with a positive or negative charge) on the two sides of the nerve
cell membrane. This POTENTIAL generally measures about 70 millivolts (with the
INSIDE of the membrane negative with respect to the outside). So, the RESTING
MEMBRANE POTENTIAL is expressed as -70 mV, and the minus means that the inside
is negative relative to (or compared to) the outside. It is called a RESTING
potential because it occurs when a membrane is not being stimulated or
conducting impulses (in other words, it's resting).
What factors contribute to this membrane potential?
Two ions are responsible: sodium (Na+) and potassium
(K+). An unequal distribution of these two ions occurs on
the two sides of a nerve cell membrane because carriers actively transport these
two ions: sodium from the inside to the outside and potassium from the outside
to the inside. AS A RESULT of this active transport mechanism (commonly referred
to as the
SODIUM - POTASSIUM PUMP), there is a higher concentration of sodium on the
outside than the inside and a higher concentration of potassium on the inside
than the outside.
The nerve cell membrane also contains special passageways for these two ions
that are commonly referred to as GATES or CHANNELS.
Thus, there are SODIUM GATES and POTASSIUM GATES. These gates represent the only
way that these ions can pass through the nerve cell membrane. IN A RESTING NERVE
CELL MEMBRANE, all the sodium gates are closed and some of the potassium gates
are open. AS A RESULT, sodium cannot diffuse through the membrane & largely
remains outside the membrane. HOWEVER, some potassium ions are able to diffuse
OVERALL, THEREFORE, there are lots of positively charged potassium ions just
inside the membrane and lots of positively charged sodium ions PLUS some
potassium ions on the outside. THIS MEANS THAT THERE ARE MORE POSITIVE CHARGES
ON THE OUTSIDE THAN ON THE INSIDE. In other words, there is an unequal
distribution of ions or a resting membrane potential. This potential will be
maintained until the membrane is disturbed or stimulated. Then, if it's a
sufficiently strong stimulus, an action potential will occur.
An action potential is a very rapid change in membrane potential that occurs
when a nerve cell membrane is stimulated. Specifically, the membrane potential
goes from the resting potential (typically -70 mV) to some positive value
(typically about +30 mV) in a very short period of time (just a few
What causes this change in potential to occur? The
causes the sodium gates (or channels) to open and, because there's more
sodium on the outside than the inside of the membrane, sodium then diffuses
rapidly into the nerve cell. All these positively-charged sodiums rushing in
causes the membrane potential to become positive (the inside of the membrane is
now positive relative to the outside). The sodium channels open only briefly,
then close again.
Please go to next page of nerve physiology