Several types of cells support an action potential, such as plant cells, muscle cells, and the specialized cells of the heart (in which occurs
the cardiac action potential ).

However, the main
excitable cell is the neuron , which also has the simplest mechanism for the action potential.


Neurons are electrically excitable cells composed, in general, of one or more dendrites , a single soma, a single axon and one or more
axon terminals. The dendrite is one of the two types of synapses, the other being the axon terminal boutons.

Dendrites form protrusions in
response to the axon terminal boutons. These protrusions, or spines, are designed to capture
the neurotransmitters released by the presynaptic neuron.


They have a high
concentration of ligand activated channels. It is, therefore, here where synapses from two
neurons communicate with one another. These spines have a thin neck connecting a bulbous
protrusion to the main dendrite.

This ensures that changes occurring inside the spine are less likely to affect the neighbouring spines. The
dendritic spine can, therefore, with rare exception (see LTP ), act as an independent unit.


The dendrites then connect onto the soma. The soma houses the nucleus , which acts as the regulator for the neuron.

Unlike the spines, the surface of the soma is populated by voltage
activated ion channels. These channels help transmit the signals generated by the dendrites.

Emerging out from the soma is the axon hillock . This region is characterized by having an
incredibly high concentration of voltage-activated sodium channels.


In general, it is
considered to be the spike initiation zone for action potentials.

Multiple signals generated
at the spines, and transmitted by the soma all converge here. Immediately after the axon
hillock is the axon . This is a thin tubular protrusion traveling away from the soma.


The axon is insulated by a myelin sheath. Myelin is composed of either Schwann cells (in the
peripheral nervous system) or oligodendrocytes
(in the central nervous system), types of glial cells . Although glial cells are not involved with
the transmission of electrical signals, they communicate and provide important biochemical support to neurons.


(Note: I some how have a strange feeling, a connection between “Soma”-“action potential” & Veda)

Exploring the dimensions inner

Illustration of the major elements in chemical synaptic transmission. An electrochemical wave called an action potential travels along the axon of a neuron. When the action potential reaches the presynaptic terminal, it provokes the release of a small quantity of neurotransmitter molecules, which bind to chemical receptor molecules located in the membrane of another neuron, the postsynaptic neuron, on the opposite side of the synaptic cleft.


Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse.

Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of the synapse.

Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may also follow graded electrical potentials.

Low level “baseline” release also occurs without electrical stimulation.

Neurotransmitters are synthesized from plentiful and simple precursors, such as amino acids, which are readily available from the diet and which require only a small number of biosynthetic steps to convert.

Ramón y Cajal (1852–1934): Discovered a ‘ 20 to 40’ nm gap between neurons, known today
as the synaptic cleft.

Neurons – when we go fathoms

Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move the squid’s tail. How giant is this axon? It can be up to 1 mm in diameter – easy to see with the naked eye.

Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are “electrically-charged” — when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable.
Resting Membrane Potential

When a neuron is not sending a signal, it is “at rest.” When a neuron is at rest, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane.

Action Potential

In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) – this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a “spike” or an “impulse” for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire…for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell – all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired – this is the “ALL OR NONE” principle.

Action potentials are caused by an exchange of ions across the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.


the figure shows an action potential recorded from a pyramidal neuron in the CA1 region of a rat hippocampus117, illustrating commonly measured parameters. The action potential was elicited by the injection of just-suprathreshold depolarizing current (purple). Use of a brief (1 ms) injection has the advantage that the spike and the afterpotentials are not directly influenced by the current injection. The response to a subthreshold current injection is also shown (red). Resting potential (Vrest) is typically in the range of -85 mV to -60 mV in pyramidal neurons. Voltage threshold (Vthresh) is the most negative voltage that must be achieved by the current injection for all-or-none firing to occur (in this neuron it is about -53 mV, a typical value). Threshold is less well defined for spontaneously firing neurons, especially in isolated cell bodies where transition from gradual interspike depolarization to spike is typically less abrupt than in slice recordings, and for such cases threshold is more easily estimated from phase-plane plots (Fig.2). The upstroke (also called the depolarizing phase or rising phase) of the action potential typically reaches a maximum velocity at a voltage near 0 mV. Overshoot is defined as peak relative to 0 mV. Spike height is defined as the peak relative to either resting potential or (more commonly) the most negative voltage reached during the afterhyperpolarization (AHP) immediately after the spike. Spike width is most commonly measured as the width at half-maximal spike amplitude, as illustrated. This measurement is sometimes referred to, confusingly, as ‘half-width’ or ‘half-duration’; ‘half-height width’ would be clearer. As is typical for pyramidal neurons, the repolarizing phase (also called ‘falling phase’ or ‘downstroke’) has a much slower velocity than the rising phase. Figure modified, with permission, from Ref. 117 © (1987) Cambridge Univ. Press.