This is a neuron, which has four main parts. The dendrites receive information.

The
cell body processes and integrates that information. The axon carries the information
along long distances from one part of the neuron to another. And the axon terminal
transmits the information to the next cell in the chain. A bundle of axons travelling
together is called a nerve. Nerves can be very long, as they often need to transmit
information over long distances.

As we just saw, the dendrites are the part of the neuron that receives incoming
signals. Based on the strength of this incoming stimulation the neuron must decide
whether to pass that signal along or not. If the stimulation is strong enough, the
signal is transmitted along the entire length of the axon in a phenomenon called an
action potential. When this happens, we say the neuron fires. Transmission of a
neuronal signal is entirely dependent on the movement of ions, or charged particles.
Various ions, including sodium, potassium, and chloride, are unequally distributed
between the inside and the outside of the cell. The presence and movement of these
ions is not only important when a neuron fires but also at rest.

To start, let's think about the positively-charged sodium and potassium ions. When a
neuron is not sending a signal, it is considered to be at rest. In a typical neuron in its
resting state, the concentration of sodium ions is higher outside the cell than inside.

The relative concentration of potassium ions is the opposite, with more ions inside
the cell than outside. This ionic separation occurs right at the cell membrane
and creates a chemical gradient across the membrane. Because ions are charged
particles, we also need to consider their charge when thinking about their
distribution across the membrane. At rest, there are more positively charged ions
outside the cell relative to the inside.

This creates a difference in charge across the membrane, which is called an

electrical gradient. Together with the chemical gradient we already mentioned, we
refer to this ionic imbalance as the electrochemical gradient. The difference in total
charge inside and outside of the cell is called the membrane potential. At rest,
when no signals are being transmitted, the neuronal membrane has a resting
potential of approximately minus 70 millivolts. This means that the inside of the cell
is approximately 70 millivolts less positive than the outside.

Both the chemical and electrical gradients we just discussed contribute to

establishing this potential. While the inside of the cell has a net negative charge
and the outside of the cell has a net positive charge, the charges line up at the
membrane. And the bulk solution on either side is actually electrically neutral. The
resting-membrane potential is the point where the cell has achieved
electrochemical equilibrium. This means that the concentration gradient and the
electro gradient for each ion is equal and opposite. Ions cannot simply move
across the membrane at will. Instead, they need a protein embedded in the
membrane to facilitate their movement. Most ions cross the membrane through a
structure called an ion channel. Ions move through channels by passive diffusion
along their concentration gradient. Some ion channels are always open, but many
require signals to tell them to open or close. For example, voltage-gated channels
only open when the membrane potential reaches a certain value. On the other
hand, ligand-gated ion channels are triggered to open when they are bound by a
specific molecule. Mechanically-gated ion channels open in response to physical
forces, such as changes in length or changes in pressure.

Most ion channels are selectively permeable, meaning that they only allow one, or
a small subset of ions, to pass through. Voltage-gated ion channels, for example,
typically only allow a single ion to cross the membrane when they open. This
means that we need separate channels for each ion, i.e. voltage-gated sodium
channels, as well as voltage-gated potassium channels.

As ions move through a channel and cross from one side of the cell membrane to
the other, they cause the membrane potential of the cell to move away from its
resting potential. If the resulting change in membrane potential is small, we call
this a graded potential. Graded potentials can vary in size, can be either positive
or negative, are transient, and typically do not result from the opening of
voltage-gated ion channels. When ion channels open and a graded potential
occurs, the neuron moves quickly to reset its membrane potential to resting
values.

This is accomplished primarily by the use of the sodium-potassium pump, which

uses the energy generated by ATP hydrolysis, to actively transport ions across the
membrane against their concentration gradient. In other words, sodium is
transported to the outside of the cell, where its concentration is higher, and
potassium is transported back into the cell, where its concentration is higher. One
cycle of this pump transports three sodium ions outside the cell and brings two
potassium ions inside the cell. This unbalanced charge transfer contributes to the
separation of charge across the membrane and also to the ionic concentrations we
see at rest, thus, restoring the chemical and electrical gradients to their resting
levels.

Maintaining these ionic balance in neurons is so important that this process can
account for 20% to 40% of the brain's total energy use. Only when the resting
membrane potential and ion distributions are maintained at precise levels, will the
neuron be poised and ready to fire an action potential.

When the outside stimulation is large enough to bring the membrane potential in
the neuron body up from minus 70 millivolts to the threshold voltage of minus 55
millivolts is higher, this triggers an action potential at the axon hillock, which then
travels down the axon. Voltage-gated sodium channels have three states– open,
closed, and inactivated. At rest, the sodium channel is closed. Once the cell
membrane reaches the threshold voltage, the channel changes to an open
position and sodium rushes into the cell because of the electrochemical gradient.
As positive-sodium ions enter the cell, the membrane potential becomes less
negative and more positive as it approaches 0 millivolts. This is called
depolarization. Eventually, the voltage gradient goes to zero and beyond 0 up to a
positive 30 millivolts. This is called an overshoot. As the membrane potential
becomes positive, the sodium channel inactivation gate shuts, making the channel
inactivated. This stops the flow of sodium ions into the cell.

The change in membrane potential also opens the voltage-gated potassium

channels, though they open and close more slowly. Because of the
potassium-electrochemical gradient, potassium ions flow out of the cell, making it
less positive and eventually negative. This process is called repolarization.
Because the potassium channels are a little slow to close, for a brief period, the
membrane potential is hyperpolarized. It's more negative than the resting potential.
During hyper-polarisation, the potassium channels close. Throughout all of this, the
sodium-potassium pump is still working.

The pump restores the chemical gradients by putting the sodium and potassium
back in place. And the pump re-establishes the potential gradient by moving more
sodium ions out than potassium ions in. This returns the membrane potential back to
its resting potential.

During repolarization, the inactivated sodium channels won't respond to any stimulus
at all. During this time, the neuron is in its absolute refractory period, the period of
time when a nerve cannot fire another action potential, no matter how strongly it's
stimulated. The absolute refractory period prevents action potentials from happening
again too quickly and prevents action potential from travelling backwards along the
axon. During hyperpolarization the sodium channels are closed and the inactivation
gate opens. There is no change in sodium flow, but now they could be opened again.
This is called the relative-refractory period. Because, while the sodium channels
could open, it would take a larger than usual stimulus to reach threshold, because
the cell is hyperpolarized due to the potassium still leaving the cell. The amplitude of
the action potential for a particular neuron, that is, the maximum voltage in one
neuron during an action potential, never changes.

An action potential doesn't get bigger with a bigger stimulus. It's all or nothing. It
either happens, or it doesn't happen. What can change is the frequency of the action
potential. A neuron might fire many more times per second in response to, say, an
intense pain and less frequently in response to a gentle breeze. Some axons
transmit action potentials faster than others.One variable that increases conduction
velocity is the presence of myelin sheaths around axons.
Myelin speeds up transmission through a process called saltatory conduction, in
which the action potential signal appears to jump along the part of the axon covered
by the sheath.

In the peripheral nervous system, the sheaths are formed from glial cells known as
Schwann cells. There are small gaps between Schwann cells called the nodes of
Ranvier. The action potential appears to jump from node to node, speeding the
transmission. In the central nervous system, the sheets are made by cells known as
oligodendrocytes.
To review, with no stimulus, the membrane is at its resting potential.A small
stimulus causes a graded potential. And a stimulus above the threshold creates an
action potential, and the neuron fires.