A.3.1. The Nerve Cell


Introduction:
In this and the next Basic Physiology chapter, we discuss in more details two types of cells in the body that demonstrate several basic physiological principles that are very useful to know and understand before studying specific organ systems. These are a) the nerve cells and b) the muscle cells. In this and the next pages, we will concentrate on the physiology of the nerve cells.

A. What is a nerve cell?
1.
A nerve cell, also called a neuron, is a cell that is specialized in passing on messages, which are called ‘action potentials’

2.
These action potentials are electrical potentials that are picked up at one end of the neuron and are then propagated to the other end. It is like an electrical wire!
3.
Our body contains billions of nerve cells, most of them located in the brain but there are also many nerve cells outside the brain, located throughout the body.


4.
A nerve cell, like any other living cells, has a body (called ‘soma’), which contains the usual cellular components such as a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and a plasma membrane. (see A.2.1. Structure of the cell).
5.
But, typical for a nerve cell, are the following three parts:
a. dendrites
b. axon
c. pre-synaptic terminals
6.
The dendrites are folds of the cell body that extend into the extracellular space, relatively short in length. Their function is to pick up a signal from another nerve cell.

a typical nerve cell
7.
The axon is a long extension of the cell body and its function is to transmit the electrical signal from the dendrites and the soma (=cell body), often over a long distance, to the next nerve cell.
8.
The pre-synaptic terminal is part of the system that connects this nerve cell to the next nerve cell. This connection system is called a ‘synapse’.


B. Typical nerve cells

top?

1.
Typical of a nerve cell is that it has many dendrites (10-100) and many terminals (100-1000) but only one (or a few) axon.




2.
In this diagram, you can see several pre-synaptic terminals (from other neurons) close to the membrane of a few dendrites. It is also possible for these pre-synaptic terminals to get close to the membrane of the soma of the nerve cell. These terminals are indicated in yellow.
a typical nerve cell
3.
Note that there is always a cleft (a small space) between the pre-synaptic terminal and the plasma membrane of the dendrite or the soma.
4.
This cleft is called the synaptic cleft (much more on this in A.3.7. The Chemical Synapse).


5.
In general, there are three different types of nerve cells in the body:
a. sensory neurons
b. motor neurons
c. inter neurons
6.
Sensory neurons are sensitive to stimuli such as touch, temperature, sound or light and send their signals to the brain.


7.
Motor neurons receive signals from the brain and stimulate muscles to contract.

8.
Inter neurons are nerve cells that connect to other nerve cells, as in the brain or in the spinal cord.

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A.3.1. The Nerve Cell

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