Tuesday, May 20, 2008

Biological Neuron Model and Artificial Neuron Model

Biological Neuron Model
It is claimed that the human central nervous system is comprised of about 1,3x1010 neurons and that about 1x1010 of them takes place in the brain. At any time, some of these neurons are firing and the power dissipation due this electrical activity is estimated to be in the order of 10 watts. Monitoring the activity in the brain has shown that, even when asleep, 5x107 nerveimpulses per second are being relayed back and forth between the brain and other parts of the body. This rate is increased significantly when awake. A neuron has a roughly spherical cell body called soma (Figure 1). The signals generated in soma are transmitted to other neurons through an extension on the cell body called axon or nerve fibres. Another kind of extensions around the cell body like bushy tree is the dendrites, which are responsible from receiving the incoming signals generated by other neurons.
Fig 1 Typical neuron
An axon (Figure 2), having a length varying from a fraction of a millimeter
to a meter in human body, prolongs from the cell body at the point called
axon hillock. At the other end, the axon is separated into several branches,
at the very end of which the axon enlarges and forms terminal buttons.
Terminal buttons are placed in special structures called the synapses which
are the junctions transmitting signals from one neuron to another (Figure 3).
A neuron typically drive 103 to 104 synaptic junctions

Fig2. Axon
The synaptic vesicles holding several thousands of molecules of chemical
transmitters, take place in terminal buttons. When a nerve impulse arrives
at the synapse, some of these chemical transmitters are discharged into
synaptic cleft, which is the narrow gap between the terminal button of the
neuron transmitting the signal and the membrane of the neuron receiving it.
In general the synapses take place between an axon branch of a neuron and
the dendrite of another one. Although it is not very common, synapses may
also take place betweentwo axons or two dendrites of different cells or between
an axon and a cell body.

Figure 3. The synapse
Neurons are covered with a semi-permeable membrane, with only 5 nanometer thickness. The membrane is able to selectively absorb and reject ions in the intracellular fluid. The membrane basically acts as an ion pump to maintain a different ion concentration between the intracellular fluid and extracellular fluid. While the sodium ions are continually removed from the intracellular fluid to extracellular fluid, the potassium ions are absorbed from the extracellular fluid in order to maintain an equilibrium condition. Due to the difference in the ion concentrations inside and outside, the cell membrane become polarized. In equilibrium the interior of the cell is observed to be 70 milivolts negative with respect to the outside of the cell. The mentioned potential is called the resting potential.
A neuron receives inputs from a large number of neurons via its synaptic connections. Nerve signals arriving at the presynaptic cell membrane cause chemical transmitters to be released in to the synaptic cleft. These chemical transmitters diffuse across the gap and join to the postsynaptic membrane of the receptor site. The membrane of the postsynaptic cell gathers the chemical transmitters. This causes either a decrease or an increase in the soma potatial, called graded potantial, depending on the type of the chemicals released in to the synaptic cleft. The kind of synapses encouraging depolarization is called excitatory and the others discouraging it are called inhibitory synapses. If the decrease in the polarization is adequate to exceed a threshold then the post-synaptic neuron fires.
The arrival of impulses to excitatory synapses adds to the depolarization of soma while inhibitory effect tends to cancel out the depolarizing effect of excitatory impulse. In general, although the depolarization due to a single synapse is not enough to fire the neuron, if some otherareas of the membrane are depolarized at the same time by the arrival of nerve impulses through other synapses, it may be adequate to exceed the threshold and fire.
At the axon hillock, the excitatory effects result in the interruption the regular ion transportation through the cell membrane, so that the ionic concentrations immediately begin to equalize as ions diffuse through the membrane. If the depolarization is large enough, the membrane potential eventually collapses, and for a short period of time the internal potential becomes positive. The action potential is the name of this brief reversal in the potential, which results in an electric current flowing from the region at action potential to an adjacent region on axon with a resting potential. This current causes the potential of the next resting region to change, so the effect propagates in this manner along the axon membrane.

Figure 4. The action potential on axon
Once an action potential has passed a given point, it is incapable of being reexcited for a while called refractory period. Because the depolarized parts of the neuron are in a state of recovery and can not immediately become active again, the pulse of electrical activity always propagates in only forward direction. The previously triggered region on the axon then rapidly recovers to the polarized resting state due to the action of the sodium potassium pumps. The refractory period is about 1 milliseconds, and this limits the nerve pulse transmission so that a neuron can typically fire and generate nerve pulses at a rate up to 1000 pulses per second. The number of impulses and the speed at which they arrive at the synaptic junctions to a particular neuron determine whether the total excitatory depolarization is sufficient to cause the neuron to fire and so to send a nerve impulse down its axon. The depolarization effect can propagate along the soma membrane but these effects can be dissipated before reaching the axon hillock.
However, once the nerve impulse reaches the axon hillock it will propagate until it reaches the synapses where the depolarization effect will cause the release of chemical transmitters into the synaptic cleft. The axons are generally enclosed by myelin sheath that is made of many layers of
Schwann cells promoting the growth of the axon. The speed of propagation down the axon depends on the thickness of the myelin sheath that provides for the insulation of the axon from the extracellular fluid and prevents the transmission of ions across the membrane. The myelin sheath is interrupted at regular intervals by narrow gaps called nodes of Ranvier where extracellular fluid makes contact with membrane and the transfer of ions occur. Since the axons themselves are poor conductors, the action potential is transmitted as depolarizations occur at the nodes of Ranvier. This happens in a sequential manner so that the depolarization of a node triggers the depolarization of the next one. The nerve impulse effectively jumps from a node to the next one along the axon each node acting rather like a regeneration amplifier to compensate for losses. Once an action potential is created at the axon hillock, it is transmitted through the axon to other neurons.
It is mostly tempted to conclude the signal transmission in the nervous system as having a digital nature in which a neuron is assumed to be either fully active or inactive.However this conclusion is not that correct, because the intensity of a neuron signal is coded in the frequency of pulses. A better conclusion would be to interpret the biological neural systems as if using a form of pulse frequency modulation to transmit information. The nerve pulses passing along the axon of a particular neuron are of approximately constant amplitude but the number generated pulses and their time spacing is controlled by the statistics associated with the arrival at the neuron's many synaptic junctions of sufficient excitatory inputs .
The representation of biophysical neuron output behavior is shown schematically inFigure 5 At time t=0 a neuron is excited; at time T, typically it may be of the order of 50 milliseconds, the neuron fires a train of impulses along its axon. Each of these impulses is practically of identical amplitude. Some time later, say around t=T+τ, the neuron may fire another train of impulses, as a result of the same excitation, though the second train of impulses will usually contain a smaller
number. Even when the neuron is not excited, it may send out impulses at random, though much less frequently than the case when it is excited.

Figure 5. Representation of biophysical neuron output signal after excitation at tine t=0

A considerable amount of research has been performed aiming to explain the electrochemical structure and operation of a neuron, however still remains several questions, which need to be answered in future.

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