Neuronal Transmission Within the Neuron

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Neuronal Transmission within the Neuron

[Sreelakshmi S., 1214256]

Introduction

The neuron, or nerve cell, is the basic structural and functional unit of the human

nervous system. It is an electrically excitable cell whose function is to transmit information

and signals across the body, which it does through electrical as well as chemical signals.

Neurotransmission, i.e. the process of communication of information via the neurons, is

typically understood as the transmission of nerve impulses across a synapse, which is from

one neuron to another. However, before the impulse can pass from one axon ending to thedendrites of another cell across a synapse, the impulse needs to first travel through the length

of the pre-synaptic neuron itself. While neurotransmission across synapses is typically

chemical in nature, where neurotransmitters are released and diffused to the receptors of

another neuron which they bind and activate, the neuronal transmission that takes place

within the neuron is electrical in nature. This internal process within the nerve cell can be

understood as different and distinct from that of chemical neurotransmission.



To understand the electrical transmission within the neuron, it is first important to

understand two things: the first is the basic structure of the neuron, and the second is the

nature of an electrical change.

Structure of the Neuron

In brief, the neuron or nerve cell consists of dendrites, a soma or cell body, an axon,

and axon terminals. The dendrites have spines, which are smaller protrusions, and the main

function of the dendrites is to receive synaptic signals from other neurons that they are in

contact with. The soma is basically the cell body, and contains the nucleus which is the life-

force of the nerve cell. Emerging out from the soma is the axon hillock, which connects the

soma to the axon, and contains a high concentration of voltage-activated sodium channels.

This is the zone considered to be the spike initiation zone for action potentials, which play a

crucial part in electrical transmission within the neuron. Multiple signals received by the

various dendrites and then transmitted by the soma all converge here, from where they travel

electrically through the axon, which is the next connected segment. The axon is myelinated

for insulation, and this myelin layer is made of glial cells, which are either Schwann cells or

oligodendrocytes, with the former found in the peripheral and the latter in the central nervous

system. There are, however, unmyelinated patches of cell membrane along the length of theaxon placed at regular intervals, known as the nodes of Ranvier, which act as mini axon

hillocks, and work to boost signal transmission and prevent significant signal decay. Towards

the end of the axon are present the axon terminals, where the myelinated insulation is lost and

the axon ending branches out. These axon endings contain pockets of neurotransmitters

enclosed within what are called synaptic vesicles, ready to be released into the synapse for

the next neuron upon stimulation. At this point, the axon terminals become pre-synaptic, and

the neuron that receives the neurotransmitter becomes the post-synaptic neuron. Furthermore,this is the point where electrical transmission within the neuron stops and chemical

neurotransmission across synapses begins once again.

Structure of a Neuron (next page)



Nature of an Electrical Charge

The flow of current through a conductor is caused by the movement of electrons,

which are negatively charged particles, from a point where they are in a high concentration to

a point where they are in low concentration. The difference between the number of electrons

at these two different points is what is known as voltage, measured in volts. The amount of

voltage difference between the two points plays an effect on current flow, making it more

rapid.

Neuronal transmission within the neuron is electrical in nature, and follows these

same principles in the transmission of impulses from one end of the nerve cell to the other.

Resting Potential

The inside of the neuron contains a large number of negative ions and the outside with

an equally large number of positive ions. These two kinds of ions are separated by the cell

membrane, which prevents the entry of positive ions into the neuron, as well as the exit of

negative ions from the neuron. Owing the polarity caused by the presence of oppositely

charged ions on either side of the cell membrane, this state of the nerve cell is termed as the

polarized condition. The difference between the positive outside of the membrane and the



negative inside of the cell is what is known as resting potential, and usually stands at a value

of -70mV.

Action Potential

When a nerve cell receives a synaptic message from a neighbouring cell, and electric

charge is sparked off in the axon, starting at the axon hillock. This is where the role of the

nerve fibres as electrical conductors comes in. the electrical impulse generated within the

neuron that travels along the length of the axon to finally trigger the next synaptic chemical

neurotransmission, is called the action potential. This action potential travels like a wave

along the nerve fibres – this means that the action potential generated at one point creates

another gradient of voltage between the active point and the point adjacent to it that is in

resting potential. Thus, the movement of the action potential along the axon is like a wave of

depolarization that moves through its length point by point.



How does the action potential work to depolarize the nerve cell? This is achievable

because of the afore-mentioned presence of positive and negative ions that exist on either side

of the cell membrane. A voltage difference exists, which makes the flow of current a potent

possibility. However, this flow of current cannot take place at other times owing to the

membrane that is blocking the flow of ions into and out of the nerve cell. However, this

barrier also contains a series ion channels at every point that can be stimulated to open and

close, thus letting ions in and out. When the post-synaptic nerve cell receives a chemically

neurotransmitter from the pre-synaptic nerve cell, it accordingly stimulates the opening of

these ion channels. The opening of the ion channels allows the positively charged ions to

flow into the axon while the negatively charged ions to move outwards, thus creating a

depolarized state.

It has already been stated that action potential works by creating a gradient in the

neighbouring point, thus allowing the electrical charge to move like a wave along the length

of the axon. However, how is this maintained if the very first point in the cell is depolarized

by the opening of the ion channels? This is answered by the mechanism of the two kinds if

ion channels that exist along the axonal membrane – the sodium ion channels and the

potassium ion channels.

Sodium ions (+Na) are positively charged ions that are found in the outside of the cell

membrane in greater concentration than inside it. Potassium ions (+K) are also positively

charged ions, but are usually found inside the axonal membrane, though in lower

concentration than that of the negative ions that exist inside it. This means that a large

number of sodium ions are found outside the cell membrane, which is the area that has a

positive charge. A small number of potassium ions are found inside the cell membrane, which

has a predominantly negative charge.

When action potential is generated, the first ion channels to open are the sodium

channels. The opening of the ion channels leads to a rushing in of sodium ions to the

negatively charged inside of the axonal membrane.1 This leads to rapid depolarization, as the

charges become balanced on the inside as well as the outside due to the entry of the positively

1 As mentioned in the section on ‘Nature of an Electrical Charge’, electrical charge works along the lines of the

movement of negatively charged ions, i.e. electrons, from an area of higher to lower concentration; however, inthe neuron since it is the positive ion, i.e. the sodium ion, that is initiating the current flow (by the opening of the

ion channel), current flow is indicated as positive to negative. In actuality there are ions that are moving in bothdirections.



charged sodium ions into the negatively charged axon. The rapid flow of current leads to a

sudden switch in voltage – what was -70mV before the flow of current (resting potential)

increases by about 100mV, leading to a current voltage of +30mV. The transmembrane

voltage is thus changed from negative to positive – and this, in turn, leads to the opening of

the next set of ion channels, i.e. the potassium ion channels.

Just as rapidly as the sodium channels flowed into the axonal membrane, the potassium channels flow out of it. This in turn causes the voltage within the membrane to

switch back to its original negative value, and pass on the gradient to the neighbouring point

in the axonal membrane.

Nature of Resting Potential

The mechanism of resting potential has been discussed in an earlier section; however,

an understanding of its nature and maintenance is important to gaining a clear understanding

of the concept of resting potential in a neuron.



The axonal membrane contains an interesting mechanism of ion pumps that

contribute towards the maintenance of resting potential in the absence of stimulation. These

ion pumps include sodium pumps as well as potassium pumps. As has already been

discussed, it is clear that the concentration of positively charged sodium ions is much higher

outside the axonal membrane. The sodium pump is actively at work in order to expel any

excessive sodium ions during resting potential. Similarly, the potassium pump is also actively

at work in order to contain a certain number of potassium ions during resting potential. It is

important to note that only a certain number of potassium ions are worked to be contained

inside the membrane, i.e. a small enough number that their collective charge does not upset

the voltage difference cause by the higher positive charge outside the membrane. It is also

these pumps that are work to bring the neuron back to its initial state of depolarization, when

the exchange of sodium and potassium ions takes place during transmission of nerve impulse.

In actuality, a very few number of sodium and potassium ions make the exchange during this

process, and this number is not enough to significantly modify the positive and negative

charges of the outside and inside of the membrane respectively. However, the ion pumps are

still at work in expelling sodium ions and re-containing potassium ions, in order to prevent an

accumulation of either kind in the wrong space which would cause a consequent

depolarization of the neuron.

Another mechanism adopted by the neuron in order to maintain the voltage

difference, and thus the resting potential, is by the size of the ions that it contains. A number



of negatively charged ions contained within the membrane are of too large a size to be able to

pass through the pores of the membrane. Thus, this restricts the flow of electrons from an

area of higher to lower concentration, i.e. from inside the membrane to the outside of it,

helping maintain the polarized state of the neuron.

Nature of the Action Potential

The mechanism of the action potential has already been discussed; however, action

potential further involves a few more characteristics that become important in understanding

its functioning.

Impulses can travel at different speeds along an axon – however, this is NOT

regulated by the intensity of electrical impulse that is generated. The action potential follows

an ‘all-or-none’ law. This means that if, and only if, a certain threshold of stimulus intensity

is achieved, will and electrical current be initiated. Stronger stimuli do not result in larger or

faster nerve impulses.

How this works is, when a certain threshold of intensity is achieved, then the nerve

cell itself generates the current, i.e. the action potential, in the manner discussed in the earlier

section. Since the generation of current is internal to the neuron, the generation of the currentin terms of the speed of the impulse is independent of the stimulus. If the critical threshold is

not reached, then no action potential is generated. If the critical threshold is reached, then

action potential is generated. If intensity is higher than the critical threshold, action potential

is, of course, generated, but the speed or nature of the impulse does not differ at all from an

impulse generated by exactly meeting the critical threshold.

Thus, action potential is based on an ‘all-or-none’ law – hitting threshold results in

complete generation of current; not meeting it evokes no response, not even a weak one. The

speed of impulse is dictated by the diameter and myelination of the neuron that is being

stimulated, and not on the nature or intensity of the stimulus.





potential. This technically concludes that conduction of impulse can happen multi-

directionally within the axon, and this is called antidromic conduction. However, chemical

neurotransmission, i.e. synaptic transmission, only happens in one direction. Therefore, when

the antidromic depolarized waves meet the site of the first synapse, i.e. the direction of the

dendrites of the neuron in which the action potential was generated itself, these waves wane

out. This results in the commonly understood pattern of transmission – from receptors to

synaptic junctions to synaptic terminals, and from there to the next neuron. This type of

conduction is termed as orthodromic conduction.

During the generation of action potential, it has been discussed how sodium ions are

let into the membrane via sodium channels. This is also due to the increased permeability of

the membrane towards the entry of sodium ions. Once the action potential has passed on, the

permeability of the membrane rapidly declines. This is followed by a temporary increase in

the permeability of the membrane towards potassium ions (those that were contained in the

membrane during resting potential but travel outwards during action potential). This becomes

the reason for a temporary period of hyperpolarization that follows the depolarization.

Furthermore, the passing of an action potential is followed by an absolute refractory

period. Due to the increased permeability towards sodium ions, the neuron will not respondto new activation temporarily. Furthermore, for several milliseconds after the end of the

absolute refractory period, it becomes difficult for a new action potential to be generated in

the neuron. This is, as mentioned, due to the hyperpolarization that follows.



Finally, the role of sodium and potassium pumps has already been mentioned in an

earlier section. As discussed, during the exchange of sodium and potassium ions that takes

place during generation of action potentials, very few ions are actually involved in a single

instance, thus no significant difference is brought about to the overall charge of the inside and

outside of the membrane. However, prolonged and repeated trains in generation of action

potential can result in measurable changes in ionic concentration. Thus, the sodium and

potassium pumps work to compensate for the leakages that take place through the ionic

channels. The ratio is 3:2 – for every three sodium ions that are expelled, two potassium ions

are taken in.

References:

Foxit Software. (2014, June 26). Brain Campaign. Retrieved from Brain Campaign Web site:

http://psych.hanover.edu/Krantz/neurotut.html

Leukel, F. (1976). Introduction to Physiological Psychology. C.V. Mosby Company.

Oja, S. S., & Saransaari, P. (n.d.). Neurons, Action Potentials, and Synapses. Encyclopedia of

Life Support Systems.

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Neuronal Transmission Within the Neuron