What's the Brain?
Everybody knows that, one of the most important organs in the human body, is the brain. We also have the liver, the kidneys, the heart, the lungs, the stomach, the intestines, and so on. But, when we think or we do something, the principal commander is the brain. The feelings, the sensations, all the senses, are commanded by the brain. Even the organs are commanded by him, with an exception to the heart, that is commanded by nervous cells that give electric impulses to the heart. These cells are self-propelling, that means, without being controlled by the brain.
The brain is the principal organ of the CNS(Central Nervous System, view History of the Brain), adjoining to the spinal cord, the principal transmitter of the nervous impulses, that are conduced to the part of the body, giving special instructions (pain, scratching, etc...), where that action was made. We will see this later, with the neurotransmitters. Well, let's go to the brain structure.
The brain is made by four lobes and by specific fields, that, for e.g., made us see, ear, catch, and so on.... We can see an image of the brain, in the chapter Brain Functions, that will explain you more about it. The brain has two hemispheres, left and right, that control opposite parts, left-right; right-left. The brain is constituted, himself, by neurons. In the "ramification" of the brain the spinal cord, we have the nerves. We will begin from the neurons that represent the structural unit of the nervous system. We can see a draw in Figure 1.
The neuron has a nucleus, in the middle of the cell, and a cellular membrane. In the edge we have the dendrites, were occurs the synapse. And you will ask "What's the synapse?". First, we have to explain what are the principal neurotransmitters, that are noradrenaline, acetylcholine, dopamine, serotonin, GABA, and glycine. This transmission is unidirectional and sub-threshold activation of many synaptic terminals leads to summation. Neuronal links are known to give some measure of feedback control by processes of pre and post-synaptic inhibition.Chemical mediators transmit excitation by increasing permeability to sodium, or inhibition by increasing the permeability of the post-synaptic membrane to chloride, and so stabilize the resting potential. The same neuron may respond to different transmitters released on its surface whose influence may be excitatory or inhibitory. It's a process, in which neurotransmitters are released by synaptic vesicles, between two or more neurons, were it depolarizes or stabilizes the post-synaptic membrane. The transmitter is then removed, either by active re-uptake or destruction, specific enzymes being involved for different transmitters. The receptors, which are proteins bound to the synaptic membrane, are specific for each transmitter or its analogues, the number of receptors being much in excess of the normal quantity release of transmitter by an impulse. It seems likely that the amount of impulse traffic can modify the number of available receptors, so causing changes which may underlie processes such as "learning". Just as there are internuncials and collateral branches which give a negative feed-back control to motor neurons, there is a chemical feed-back process by which pre-synaptic receptors can inhibit the release of transmitters. Here is a figure, which shows the diagram of biochemical events at cholinergic and adrenergic endings.
ACH, Acetylcholine ; ACE, Acetylcholinesteras
e ; NA, Noradrenaline ; MAO, Monoamine oxidase ; COMT, Catechol-O-Methyl transferase ; X, receptor.The main transmitters may be inhibited or enhanced by certain drugs. The general principals are similar for each transmitter. Its action is enhanced if more of its precursors is provided, if its release is triggered by another chemical, if a receptor agonist is provided or, if the inactivating enzyme or re-uptake mechanism is blocked. Alternatively the effectiveness of transmission is reduced if its synthesis is reduced, its release blocked, or if the receptor is blocked. New transmitter roles for the peptide enkephalins are now emerging. After synthesis, noradrenaline is stored in presynaptic vesicles. Transmitter released by the effect of the nerve impulse, diffuses across the synaptic cleft to the receptor site from which, unlike acetyl chorine which is destroyed, noradrenaline is taken up again by the axonal membrane into the adrenergic terminal. Monoamine oxidase (MAO) and catechol-O-methyl transferase, destroy noradrenaline not bound by the synaptic vesicles. MAO inhibitor drugs, like phenelzine (Nardil) and tranylcypromine (Parnate) prevent the destruction of noradrenaline outside the synaptic vesicle and enhance excitement. MAO inhibitors also permit dangerous hypertension by tyramine released from foods and alpha adrenoceptor antagonists.
Now, we will demonstrate you the spinal cord. The spinal cord (Figure 4) is just like a "tail" of the brain. It's situated in the brainstem, measures 40-45 cm, and it goes from the atlas superior border until the first lumbar vertebra. In his terminal phase, inferior, it attenuates to much (terminal cone) and it continues until the holy region by the terminal thread (filum terminalis). His transversal diameter measures approximately 2 cm, and it's bigger than the antero-posterior diameter, that means, the spinal cord it's a little bit flattened and presents an section approximately oval. Back, in each side, enters in her the posterior roots of the nerves, which follow along the longitudinal lines; similarly, in antero-lateral lines, we see get out the anterior roots of the same nerves.
The spinal cord has no uniform caliber; it presents two fusiform dilatations, one in the cervical region, another in the lumbar region. The voluminous nerves, that innerve the superior and inferior members, are related precisely with the cervical and lumbar dilations. At the middle of the anterior part of the spinal cord we verify a longitudinal furrow - medium-anterior furrow - and in the bottom of the same, when the two borders move away, some fibers are seen to lay across, from a side to another, transversely.
Figure 4
The Spinal Cord and the Nervous System
Finally, there are three types of nerves: the sensitive nerves, that transport messages to the brain sended from the eyes, ears, skin and other organs; the motor nerves, that transport the brain signals to the muscles, making the movement of the body; and finally the mixed nerves.
In this chapter we turn aside a bit of the context, not only for a better compreession of this site, but for a more extended acknoldgement of the brain, Central Nervous System (C.N.S.) and Peripherical Nervous System (P.N.S.).
