02 The Neuron
The neurons are the most complex cells that the Evolution has devised up to now in these parts of our Galaxy.
To think that the Neuron, which is extraordinary small, a thousandth of millimetre, can have up to 20.000 inputs, and according to their state, keep on firing with well regulated different frequencies, potential discharges of fixed values, overcomes our technological imagination.
However already similar functional elements are studied at the atomic level of proteins! (exploiting the strangeness of Quantum World)
To think that we have in hour head approximately 100 billion of neurons and that the nature has found the way to make them work together, fills us with wonder for the Creation.
But to think that We, fruit of said Creation, cannot do better and much more quickly than 15 billion years, it means that we prefer to keep our eyes closed.
THE NEURON
The neurons transmit the functional signals from one point of the body to the other.
Many types of neurons are known, different in operation and structural details depending on the specific roles which they carry out, but the greater part of them shares the common followings characteristic:
Description of a typical Neuron
A typical neuron, and we can distinguish at least 50 types, possesses a cellular body said Soma, relatively voluminous, which contains all the elements common to a cell: the nucleus and the various endoplasmatic organelle.
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| Fig. 2-1a Typical structure of vertebrates neurons | |
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The characteristic of a neuron is however the presence of numerous endoplasmatic extroflections (thread-like prolongations of the external membrane of the cell) the dendrites, and of a similar, but much longer prolongation, the Axon that starts from a cone extension, said Emergent Cone . |
| Fig. 2. 2 Morphological variants among neurons depending of their functions |
In the examples the Soma and the dendrites are in black and the axons in red.
The axons can be very long, as for instance the Sciatic nerve that starts from the Bone Marrow and ends in the back limbs, They are branched. Their ramifications connects to the dendrites of the other neurons trough specialized terminations defined synaptic terminations. An axon can give origin to hundreds or even thousands of these terminations.
The dendrites of a neuron are even more numerous and they can reach the number of ten thousand.
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Every neuron, especially those of the central nervous system, represent, with the respective synaptic joints, the last station of thousand of neural terminations, some excite (in green) others inhibit (in red) In every instant an Action Potential can originate in correspondence of the emergent cone, provided that the combined effect of the ionic flows induced by the synapses, both excitatory and inhibitory, succeeds in depolarising the Soma Cone bringing it to the threshold value. |
| Fig. 2-3a The integration of multiple synaptic prominences |
The zone of separation between the ramification of the axon and the dendrites is said Synapse, and it is there that communication among neurons happens.
The communication is modulated by the neural transmitters and neural modulators of the already quoted structure of the Value system.
The membrane, is the border of separation between the external environment and the inside. Inside, the Rest negative potential is maintained by a double diffusion of ions in contrary directions, (Na+, K+, Cl -) due to the respective external and internal concentrations.
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(a) The intracellular and extra cellular solutions have different ionic compositions. In this image of a mammal cell the concentrations are in millionth of litre. Potassium ions (K *) have the tendency to spread to the outside of the cell according to their gradient of concentration. Consequently inside the cell a negative charge builds up. (b) When the membrane is characterized by a Rest potential, it similarly occurs a static diffusion of potassium ions toward the outside, and a static diffusion of sodium ions toward the inside. |
| Fig 2-4 Electrochemical bases of the potential of plasma lemma at rest. |
In the neurons the membrane has some specialized channels that allow the cell to modify its own polarization according to a signal coming from the external environment. In the case of a Receptor cell, it can be excited by photoreceptors for Vision, mechanic receptors for Hearing, or associative receptors for Inter neurons.
The effect can be that of increasing the relative potential of the neuron, hyperpolaristion, or to decrease it, depolarisation
The changes depend on the intensity of the stimulus, a signal more intense opens a greater number of channels.
The transmission of the signals along the ramifications of the neurons, doesn't happen with electrical current (flow of electrons) but with the propagation of an impulse of potential. A difference of potential exists in correspondence of the membrane between its inside part, the cytoplasm, and its external part.
Considering 0 V. the external potential at rest, the inside has a potential around 70 mvs (from -50 to 100 mvs).
When is reached a threshold, (-55 / -50 mvs) the depolarisation starts a signal said Action potential, of the type: everything or nothing (on – off), that is independent from the entity of the depolarising stimulus, provided that the threshold has been reached.
If the signal is of hyperpolarisation, the effect is to make a possible polarization more difficult, that is a stronger stimuli to overcome the threshold will be required. The Action potential remains for a short time of few milliseconds, however sufficient to propagate itself downstream .
To this follows a refractory period in which an eventual subsequent depolarisation, doesn't have effect.
The action potential represents an episode of the type all or nothing whose amplitude is not influenced by the entity of the stimulus. However the strongest signals, produce an action potential of greater frequency than the weak ones. If the stimulus is intense then the neuron shoots repeatedly at the maximum frequency that is allowed by the refractory period. (10/100 Hz)
Propagation of Action potential
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The action potential is propagated along the dendrite branch toward the Soma and from the Soma toward the extremities of the axon. The strong temporary polarization of one section influences the neighbouring section, depolarising it in turn and therefore opening the voltage dependent channels. This effect is not produced by a displacement of the Action potential but is produced ex novo, in rapid succession along the membrane of the neuron. |
| Fig 2-5a Jumping Conduction |
In an axon covered by Myelin the depolarisation caused by an Action potential in correspondence of a Knot of Ranvier diffuses in the inside portion of the axon jumping from knot to knot.
Propagation speed depends on the diameter of the dendrite or axon and varies from a few centimetres per second, in the case of thin appendages to 100 meters per second in the case of the giant axons of certain invertebrates as the giant squids and the lobsters.
In the vertebrates, man inclusive, to accelerate the speed of transmission we have a different device. The channels for the voltage dependent ions, are assembled in the interruptions of the sleeve of Myelin that protects the nervous fibre, that are said knots of Ranvier. In this way the action potential is not uniformly propagated along the fibre but jumps from one knot to the other.
The nervous transmission among neuron
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We are speaking of the Synapse. With this term is indicated the junction between the terminal membrane of one of the ramifications of the axon, with a dendrite of another neuron. At the arrival of the Action potential the membrane termination frees molecules of neurotransmitter that migrate in the dendrite through the receptors cells, allowing the passage of ions that depolarise it. According to the receptors the polarization can be excitatory or hyperpolarizing, in the last case the results is inhibitory. Immediately the neurotransmitter is decomposed by particular enzymes and returns to the point of departure where it is recycled. Neurotransmitters are of many kind, among the more common Aceticolin, then Amines: Serotonine, Dopamine and the Neuropeptides: Endorphins and in general others that keep being discovered together with their receptors and their specializations. |
| Fig. 2-9 Synapse operation |
All these transfers of potential happen because of transmigrations of ions favoured by the activation of channels formed by specialized proteins, and they are known in their particular features, but it is not the case to go in details. It will be enough to keep in mind the phenomenon of the passage of the signal from a neuron to the others.
NEURON OPERATION
In this chapter we examine the neuron as a functional element of the Brain, pointing out the elaboration and the transfer of the information.
The information in the neuron enters from the dendrites that past learning has reinforced, they can be even a few thousand. The information is elaborated in the Soma and parts along the Axon that in turn reaches, with its branched terminations, the dendrites of other neurons.
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The operation of the neurons has been copied in a simplified way, by the scientists and simulated in computers to overcome the impasse of their operation with sequential software, with which they didn't succeed in resolving the problems of the Perceptive Categorizzation that the world present. In other words for example, to distinguish in a screen that this is “An automobile, in motion, of such brand!” |
| Fig 2-6 Electronic microscope photo of a neuron and his synapses |
The trick to which the nature has resorted, or if you prefer, the road that the evolution has discovered, is to put together some apparatuses that are modified by experience, and that know how to learn by the semi-permanent change of an enormous number of synapses which represents the information.
SYNAPSES
The Synapses are the interstices between the filament of a ramification of a neuron axon, with a dendrites of another neuron. The transfer of the Action Potential is modulated in strength by the presence of neuromodulators, which in turn are released by the Value System that has been described.
Through this system the synapses learn to change their ability to transmit the signals. In dependence of the state of the synapses the upcoming signals can be amplified or attenuated to cause excitatory or inhibiting effects.
DENDRITES
They are those bulges of the external membrane that transport the signal to the Soma, the centre of the neuron. A neuron can have a number of dendrites, from very little as 10, to as much as 10.000/20.000
Receptors, present in the inside of the dendrites terminations, allow the unidirectional acquisition of the signal and are also able to intensify them when they receive signals in short succession before the refractory phase occur. Synapses exert a very important role in the learning by strengthening themselves or growing weak or even disappear, as function, for example, of the frequency of solicitations in time.
SOMA
In the Soma happens the aggregation of Action potentials coming from the Dendrites. Aggregation happens taking in account the operation frequency, the strength of the received signals modulated by neurotransmitters, and the proximity of the dendrites to the emergent cone.
EMERGENT CONE
The action potential starts from the emergent cone along the axon only when its potential overcomes a threshold of around -50 mVs, that is, more positive or less negative than is required, generally of 15/20 mVs
ACTION POTENTIAL
When it overcomes his threshold, the potential of action departs. That therefore always represents an event of “everything or nothing” of the same power and of the duration of few milliseconds. To it follows a refractory period of negative hyperpolarisation in which firing cannot be repeated, and that prevents the action potential to return back along the axon.
The stimuli of greater intensity produce some impulses of greater frequency, this allows to distinguish them from weaker impulses.
It is believed that different frequencies are associated to different areas and different neural functions.
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| Fig 2-4a Effects on graduated potentials on Action Potential of the Neuron |
