ANATOMY OF THE NERVES

Anatomy of  the nerves

Cells of the nervous system:

There are basically two types of cells in the nervous system, named as Glial cells and the neurons or nerve cells.

The main function of the Glial cells is as follows:

1.      To nourish neurons.

2.      To provide a structural support to the neurons

3.      They help in the removal of waste products from the neurons.

4.      Insulating neurons are another function of the Glial cells.

Neurons, the other cells play a significant role in the communication along the nervous system and hence can be regarded as the communicators of the Nervous system. Neuron, the messenger in the body receives information, then process it according to the nature of the information and finally pass it along. They not only communicate with one another, but also with the sensory organs and the muscles in the body. (1)

Physical features of the nerve cells or neurons:

Soma or cell body:

Dendrites:

Dendrites are the extensions at the beginning of the neuron which are just like a tree. These projections help to increase the surface area of the cell body. Dendrites have a function of receiving information from other neurons or the sense organs in the body.

Soma:

It is the most important part of the neuron as it contains the most vital organ which is the nucleus. Besides nucleus it also contains other cellular organelles like mitochondria, Golgi apparatus etc. The soma is a place where the signals from the dendrites are received and further passed on. The cell body and the nucleus within have the main function of maintaining the functional role of the neuron. At the end of the soma there is a structure termed as axon hillock which controls the firing of the neuron.

Axon:

It is the single long body which extends from the cell body or soma and performs a function of sending information to the muscles, other neurons ans sense organs in the body. The surface are of the axon plays an important role in the transmission of the neural signals, as larger the axons, faster is the rate of transmission of the signals in the body.

There is a coating fatty substance around the axons by the name of Mylein sheath, but not all the axons have myelin coating. This mylein coating is produced by the Glial, thus protecting the nerves. The presence of this myelin sheath enables the axon to transmit the nerve impulse at a faster rate as compared to the situation when the myelin sheath is damaged or absent.

There are terminal buttons at the end of the axon which are responsible for the release of the neurotransmitters. These neurotransmitters are chemical compounds which travel across the neighboring neurons and cause their activation for a particular response. A junction existing between the axon of one neuron and the dendrites of the neighboring neuron is called a synapse. (2)

Communication between neurons:

As mentioned earlier, the neurons having the larger axons can send nerve impulses faster as compared to the ones with shorter neurons. The neurons with large axons are termed as the giant squids. The communication between neurons for the purpose of electrochemical conduction is as follows:

The resting potential:

There is a presence of fluid both inside and outside the neurons as it serves as a medium to conduct electrochemical signal across the body. There is the presence of the positively or negatively charged  molecules or atoms in this fluid, called as ions. There is a continuing in and out of the of these ions in and out of the neurons across the cell membranes. These ions can be positive like sodium and potassium ions and negative like chloride ions. A nucleus at its resting potential is inactive, in this state there is a slightly higher concentration of the negatively charged neurons inside the neuron. This situation creates a negative charge inside the neuron, thus arising the condition of resting potential which is about -70 MV.

The action potential:

On some stimulation to the neuron, the voltage gated channels in the cell membrane open up, allowing the inward movement of the sodium ions, so causing an increased amount of the positively charged ion in the neuron. This results in an action potential, which causes a short lived change in the electric charge inside the neuron. As a result the channels get closed, thus inhibiting the initiation of any nerve impulse. This short period of time results in a dormant span of about 1-2 milliseconds and is called as absolute refractory period. The neural impulses always follow the All or None Law, which means neurons only fire an impulse when a stimulation reaches a certain threshold. Otherwise, weak stimuli don’t result in the firing of the nerve impulse.

There are certain drugs and poisons that alter this axon conduction like levetiracetam  which is an adjunct therapy in the treatment of epilepsy. The exact mechanism of the levetiracetam  is unknown, however it is believed that it inhibits the voltage gated channels. It is believed that it impedes impulse conduction across the synapses.

Similarly, valproic acid which is another anti-convulsant drug is used to enhance the transmission of the GABA by inhibiting the GABA transaminase which is responsible for the breakdown of GABA. Besides, it also blocks the voltage gated sodium channels.

In the same way there are toxins that can affect the cell at any step of the neural transmission. As we know that the flow of sodium ions in the neuron is a vital step in the conduction of the nerve impulse along the axons. So any disturbance in this mechanism can severely lead to the damage in the conduction of the nerve impulse in the body. A toxin tetrodotoxin is reported to be specific in blocking the sodium ion channels in the cell membrane of the neurons. Its mechanism of action involves the blocking the conduction of the nerve impulse along the nerve fibers and the axons. It happens as tetrodotoxin competes with the sodium cation, thus entering the sodium ion channels and results in binding.  As a result, the patients dies of the respiratory paralysis.(3)

 One of the most famous toxin alcohol also inhibits the axonal transmission by inhibiting the excitatory channels on the post synaptic neuron. Besides, it also lowers the rate of action potential coming from the presynaptic neuron. In the same way caffeine, nicotine, heroin and cocaine can also lead to the inhibition of the transmission of the nerve impulse.

References:

1.       http://sparkcharts.sparknotes.com/health/generalanatomy/section12.php

2.       Anand I Rughani, MD (Feb 15, 2013). Brain Anatomy . [ONLINE] Available at: http://emedicine.medscape.com/article/1898830-overview#a1

3.      Steven Gilbert (2012). Tetrodotoxin. [ONLINE] Available at: http://toxipedia.org/display/toxipedia/Tetrodotoxin