Neuron Structure & Excitation Impulse
If you are coming to this piece from the “Overview of Muscle Contraction” article, then you will know that electrochemical signaling is the communication port by which the nervous system carries messages throughout the body. As such, it is important to understand how exactly the body can go about fulfilling this role in regards to muscle contraction.
If you think of the nervous system as a series of people, in a long line, holding hands and relaying a message by how hard each person squeezes another’s hand, then you have a basic understanding of how the nervous system operates in regards to messaging. We will use this analogy in several related articles, so keep it in mind.
Now, if we replace each “person” in line with the term neuron then we can effectively relate.
We zoom in with our mind’s eye and focus on just one neuron in our chain; this neuron is built a particular way by our body so as to maximize efficiency. The neuron (or nerve cell) is made up of three distinct parts:
- the Dendrites
- the Soma
- the Axon
Each of these play a vital role in carrying the message we have to propel to the next neuron.
This section is relatively straightforward. Dendrites tend to be numerous, unlike the axon (explained later) and their main function is to accept information from the neurons around them. Attached to the soma, they are given signals that they relay to the information centers within the soma.
The soma is also known as the cell body. It houses several pieces that make the neuron function efficiently. Like many other cells of our body, the soma houses organelles (nucleus, endoplasmic reticulum, mitochondria, etc) and again, like many other cells, is the relative middle point between the dendrite and the axon. However, specific to the soma is one feature called the axon hillock.
The axon hillock is, in a manner of speaking, the “decision point” at which the neuron either continues the message (the nerve impulse) it receives down the line or the stopping point.
The axon hillock is a type of control point that is dependent on a particular set of factors. As the dendrites accept signals from surrounding neurons, these signals are measured at the axon hillock.
EPSP vs IPSP
At the axon hillock, a series of incoming signals from the dendrites are measured; this measurement is based off a summation of Excitatory Post-Synaptic Potentials (EPSP) and Inhibitory Post-Synaptic Potentials (IPSP).
As the terms imply, each has a specific impact. While the EPSP increases the chance of signal continuation, the IPSP decreases the chance of signal continuation. Once these signals are measured at the axon hillock, the “decision” is made on how to proceed.
How are EPSP and IPSP measured?
There are two ways by which these signals are measured at the hillock.
1. Spatial Summation: Simply, this is the mathematical amount of EPSP signals subtracted by the IPSP signals.
If you have 3 EPSPs and 2 IPSPs, that leaves you with 1 EPSP, so the neuron will continue the message.
2. Temporal Summation: Simply, this is the frequency by which each signal is received.
If you receive 3 EPSPs per second and 4 IPSPs per second, you have a higher frequency of IPSPs and the message is discontinued.
This is where things inevitably get really intense, because if we assume a situation in which the signal (nerve impulse) is continued (EPSP overcoming IPSP), we need to then pull out our more powerful microscope and check out the molecular happenings of the cell.
The Action Potential
In our instance, EPSP reigns supreme and this means some significant changes for the neuron. You may be familiar with the fact that our neurons communicate via an electrochemical system; well, this is just one aspect of that complex electrochemical system – the resting and action potential.
While un-stimulated, the neuron remains in a resting potential state. From an electrical stand point, the cell remains at a negative state during resting potential; specifically, it remains at a -70mV electrical charge. From a chemical standpoint, the -70mV is achieved by having a lower concentration of K+ (potassium) ions on the inside of the nerve cell than Na+ (sodium) which is by large majority on the outside of the cell.
This is all possible due to the cell’s membrane holding Na+ at bay while keeping K+ inside. The imbalance is held by the active transport system of the sodium-potassium pump keeping the concentration within a strict range.
So, we understand the resting potential; the point at which no excitation has occurred. Now when we introduce an electrical neural impulse from the aforementioned overcoming nature of EPSP over IPSP, the chemical gradient changes. The gradient changes by introducing a flood of extracellular Na+ ions through the cell membrane and pushing some intercellular K+ ions out of the cell. Because this is an action that moves with the gradient, it is achieved by facilitated diffusion.
So, as the ionic makeup of the inside and outside of the cell change, the electrical component also changes. From -70mV, the resting potential spikes to +30mV, is creating, at its height, an action potential.
Detailing the Action Potential
1. The Electrical
At any given time, the cell is receiving signals, but those signals need to be strong enough to break threshold. The threshold for any cell is an increase of, roughly, +20mV. If the electrical signal is strong enough to allow the potential to rise to or above that charge (~ -50mV), then full depolarization immediately occurs (-70mV to +30mV) to create an action potential. This is called the All or Nothing Principle.
The All or Nothing Principle is detailed in several physiological reactions, but in this case, once threshold is broken by any degree (by 40mV or barely reaching it, etc), the cell fulfills the action potential by depolarizing fully (+30mV total); it cannot depolarize partially.
If threshold is not met, and only a small stimulus is applied (for example, 3mV – well below the needed 20mV minimum), the cell creates a graded potential; a graded potential simply being a small jump in charge insufficient to break threshold and therefor insufficient to depolarize the cell to carry the neural impulse down the cell’s axon (discussed later).
Finally, once +30mV is reached (the action potential), the cell then repolarizes down to -70mV. The neuron then undergoes a short refractory period in which it can not be restimulated fully.
2. The Chemical
Now, when depolarization up to the action potential occurs, some things occur across the chemical gradient between the extracellular and intracellular areas of the nerve cell. As you have read, outside of the nerve cell are proportionally high amounts of Na+ ions while inside the cell are high amount of K+ ions. These are strictly regulated by having closed ion channels that make the cell membrane a wall between the two ions. Now, when the cell depolarizes completely, these ion channels that would otherwise be closed during the resting potential suddenly open allowing an exchange of ions.
Sodium (Na+) rushes inside the cell, which makes the cell become positive (+30mV) and potassium (K+) escapes the inside of the cell. Then, once repolarization occurs, the K+ and Na+ ions are pumped back into their respective sides of the cell membrane.
Why does any of this matter, however?
Well, the action potential matters, because it is imperative to nerve impulse transmission down the axon. If an action potential does not occur, there is no chance for the nerve to communicate with the muscle (if the nerve is an alpha neuron). If the muscle innervation is never stimulated, the muscle does not activate – it is as simple as that.
Myelin and Node de Ranvier
Although long, the axon is covered in an insulation called myelin. This insulation actually speeds up impulse conduction a significant amount. Unmyelinated axons have a conduction speed of .5-10 m/s, and while this is quick, by comparison to myelinated axons with a conduction velocity of 150 m/s it pales (1).
Because myelin functions better in separate pieces, the body creates a “bridge” system from one myelin piece to the next called the node de ranvier. This jump in impulse from one myelin sheath to another, across the node de ranvier is called saltatory conduction.
It is due to myelin, the nodes de ranvier, and saltatory conduction that we can boast blindingly quick reaction speeds, as well as the appropriate stimulation of muscle fibers.
Imagine the neuron has received an excitatory stimulus strong enough to depolarize the cell and create an action potential from the axon hillock onward, where does that impulse travel?
The impulse travels across something that branches off of the soma – this is called the axon. The axon tends to make up the large percentage of the cell as its often singular branch extends quite a ways. Now, this poses a problem, because the distance can be great and yet the stimulation, depolarization, action potential, and impulse take place in a few milliseconds. So, how is this possible?
The answer lies in the axon’s structure.
Finally, as the impulse moves down the axon, it quickly finds itself at the end of the axon, called the axon terminal.
The axon terminal is an extremely important part of the entire neural cell as it is there that the impulse is passed on to the next neural cell, or in this case, the muscle motor end plate in the neuromuscular junction (Read: Neuromuscular Junction & Impulse Transmission)
For now, all you need to know is the impulse that has travelled down the axon ends up at the axon terminal and another iteration of the electrochemical neural system takes place to carry the impulse to the motor endplate, the muscle fibers. So, going back to our analogy above, if we replace neurons with people again, you can now see how the person is the neuron, one hand representing the dendrite accepting the message, the body interpreting that message, and the other hand replacing the axon to hand off the message to the next person’s hand.
Here we are, having a detailed understanding of neural impulse conduction from the dendrites (the beginning of the nerve impulse in relation to a single cell), the soma (the cell body, but specifically the axon hillock – the stimulus reception station), down the myelinated axon to the axon terminal where the impulse will be passed off to the muscle for activation. We went over certain elements of the electrochemical system, excitation vs inhibitory summation, and velocity of myelin sheathed axons.
(1) Purves, D. (2001). Increased Conduction Velocity as a Result of Myelination. Retrieved May 29, 2015, from