The Structure and Function of Neurons

Typical neuron

Figure 1. Simplified diagram of a typical neuron
Figure 1. Simplified diagram of a typical neuron

Neurons are the highly specialised cells of the nervous system. Although they share the essential machinery common to all cells they are highly adapted to their role. Their primary function is the transfer of information both within and between cells, but neurons also work with other neurons to process information. There are various types of neuron throughout the nervous system but virtually all share common structural components; the cell body, a single axon and usually multiple dendrites. [1] (Figure 1).


The cell body, like other cells, contains the nucleus and cytoplasm containing the cell organelles, all of which are essential for the general cellular processes. [1]

A single axon extends from the cell body at a junction termed the axon hillock. The axon is a thin tube like structure (approx. 1µm across) that is a continuation of the cytoplasm. Axons may be very short, as in the brain, or much longer (over a metre) elsewhere, forming collective bundles which are what we term nerves. At the ends of the axon branches are the axon terminals which contact numerous other neurons via synapses. [1]

Also originating from the cell body are the dendrites. Although similar in structure to axons there are often many dendrites extending from the cell body and these tend to be shorter, thicker and more highly branched. [1]

When a neuron transmits information it is said to ‘fire’. Adjacent neurons typically contact each other with the axon terminal of one passing the signal (output) to the dendrite of another (input) across the synaptic gap or cleft. Synapses may be either electrical or chemical. [1] In electrical synapses the cell membranes are very close allowing ions to pass through pores from one cell to the other. This happens instantaneously with no synaptic delay. Electrical synapses are also capable of signalling in both directions, this is relevant to their function in forming networks in regions such as the cortex of the brain. [3]

Chemical synapses function through the release of neurotransmitters across the synaptic gap. Unlike electrical synapses a delay of around 1 ms occurs as the molecules are released and diffuse across the gap. Neurotransmitters bind with receptors on the surface of the postsynaptic cell initiating a new action potential and this activity continues until the neurotransmitters are. Unlike electrical synapses, the information only travels in one direction. [3]

The signal between cells, the synaptic potential, can be either excitatory or inhibitory. At the axon hillock of the receiving neuron all the inputs arriving via its dendrites are summated and if they are over a required threshold the neuron will fire. If this is the case a pulse of electrical energy, the action potential, is generated and propagates rapidly along the axon to the synapses. Generation of the action potential are facilitated by the rapid flow of ions across the membrane of the neuron changing the balance of electrical charge, causing what is termed depolarization. It is specialised voltage-gated ion channels in the axonal membrane that allow the propagation of this pulse. [2]

The action potential is an all-or-nothing response, it is the discrete unit of information in the nervous system and does not vary. However, the rate of discharge of action potentials does vary reaching a maximum rate of around 1,000 per second (1000Hz). The frequency of action potentials is the ‘language’ of the nervous system, adding qualitative information to the signal. [1] This rate is effectively limited by the time the axonal membrane takes to recover from the previous action potential, the refractory period. [2]


The speed that an action potential propagates along varies by several orders of magnitude depending on the neuron type. Two main factors determine the conduction velocity of different neurons, one of which is axon diameter. A greater diameter facilitates faster transmission as the greater ability of Na+ ions to spread sideways allows the next action potential to be generated further away. [2]

The other main factor is myelination of the axon. This makes conduction faster as its insulating properties prevent attenuation of the action potential allowing greater distance between its generation. In effect the action potential jumps between the un-insulated regions of the axon (the nodes of Ranvier) where the membrane is exposed. As these nodes can be up to 2mm apart this form of saltatory conduction is very rapid. It is also more efficient in terms of metabolic cost as there are less ion channels in the membrane. [2]


References

  1. Block 2 ‘The Sensory Nervous System’, Signals and Perception: The science of the Senses, The Open University, 2005. Section 2.1.1
  2. Block 2 ‘The Sensory Nervous System’, Signals and Perception: The science of the Senses, The Open University, 2005. Section 3.3.1

This essay was part of my 3rd level degree courses with the Open University. It scored 23 out of 25.

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Comments 5 comments

wanzulfikri profile image

wanzulfikri 4 years ago from Malaysia

A good read for science student like me. Good luck with your studies.


fordie profile image

fordie 4 years ago from China

Interesting. I've been reading up on artificial intelligence 'neurons'. Might be an interesting comparison for you - tho' the maths isn't friendly at first


surfgatinho profile image

surfgatinho 4 years ago from Cornwall UK Author

Try to avoid reading anything I don't have to at the moment! I did cover some stuff about computer models of neural nets - seems to be about building interconnections. Get the impression we have an awful long way to go before we really know how the brain works though.


fordie profile image

fordie 4 years ago from China

The whole brain, yes. The clever thing is that even simple mathematical calculations can act as neurons, which can then 'learn' to do quite 'intelligent' things. Favours a belief in evolution


surfgatinho profile image

surfgatinho 4 years ago from Cornwall UK Author

We have around 100 billion neurons - not sure how many of these are in the brain. Each of these in the brain is capable of making many, many interconnections with other neurons. In total we are looking at many trillion interconnections. On top of this, unlike computers, neurons are not simple binary on/off switches - they have firing rate too.

The mind boggles, literally!

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