Neuron structure of the brain

Nerve cells, or neurons, are the structural constituents of the brain. Typically, neurons are five to six orders of magnitude slower than silicon logic gates. Neural events happen in the millisecond range as compared to events in silicon chip which happen in the nanosecond range. However, the brain makes up for the relatively slow rate of operation of a neuron by having a truly staggering number of neurons ( 10 billion) with massive interconnections ( 60 trillion) between them. The brain is a highly complex, non-linear, and parallel computer. It has the capability of performing certain computations many times faster than the fastest digital computer in existence today.

 

At birth, a brain has great structure and the ability to build up its own rules through experience. Experience is built up over the years, with most dramatic development of the human brain taking place in the first two years from birth; but development continues well beyond that stage. During this early stage of development, billions of interconnections between the neurons are formed.

 

The neuron nerve cells, called neurons, are the fundamental elements of the central nervous system. The central nervous system is made up of about 10 billion neurons.

 

Neurons have five specialist functions: 

  1. They receive signals coming from neighbouring neurons,

  2. They integrate these signals,

  3. They give rise to nerve pulses,

  4. They conduct these pulses,

  5. They transmit them to other neurons which are capable of receiving them.

A neuron is built up of three parts :

  1. the cell body,

  2. the dendrites,

  3. the axon. 

Fig. 2.1 Main parts of the Neuron

 

The body of the cell contains the nucleus of the neuron. Each neuron has  a hair-like structure of dendrites around it. They branch out into a tree-like form around the cell body. The dendrites are the principal receptors of the neuron and serve to connect its incoming signals.

 

The axon or nerve fibre is the outgoing connection for signals emitted by the neuron. An axon is a long cylindrical connection that carries impulses from the neuron. The connection between two neurons takes place at synapse, where they are separated by a synaptic gap of the order of one-hundredth of a micron. The signals reaching a synapse and received by dendrites are electrical impulses. It is assumed that a synapse is a simple connection that can impose excitation or inhibition, but not both on the receptive neuron.

 

The neuron is able to respond to the total of its inputs aggregated within a short-time interval called the period of latent summation. The response of the neuron is generated if the total potential of its membrane reaches a certain level. i.e. the neuron generates a pulse response and sends it to its axon only if the conditions necessary for firing are fulfilled.

 

Incoming impulses can be excitatory if they cause the firing, or inhibitory if they hinder the firing of the response. A more precise condition for firing is that the excitation should exceed the inhibition by the amount called the threshold of the neuron, typically a value of about 40 mV. The incoming impulses to a neuron can only be generated by neighbouring neurons and by the neuron itself. Usually, a certain number of incoming impulses are required to make a target cell fire.

 

After carrying a pulse, an axon fibre is in a state of complete non-excitability for a certain time called the refractory period. For this time interval the nerve does not conduct any signals, regardless of the intensity of excitation. Thus, we may divide the time scale into consecutive intervals, each equal to the length of the refractory period. This will enable a discrete-time description of the neurons' performance in terms of their states at discrete time instances.

 

 

 

 

TOM SCARFF
1 Martello Court
Portmarnock
Dublin
Ireland.


Email: tscarff@eircom.net