1.1 Introduction
All living beings are made of cells. Groups of cells with same or similar functions constitute a "tissue". In the brain, there is: i) a primary tissue whose cells (called "neurons") characterize its function; and ii) a secondary tissue whose cells (called "astrocytes and oligodendrocytes") work in support of neurons. These cells constitute the "glia". The same kind of cells constitutes the spinal cord.
Brain and spinal cord form the Central Nervous System (CNS), whereas nerves are classified as the Peripheral Nervous System (PNS).
The CNS is the "center" of our existence as living beings. It controls our personality and governs our thinking, memory, intelligence, speaking and writing, emotions, relationships and sex. Our five senses are functions of the CNS, which also supervises metabolism, breathing, cardiac activity and arterial pressure. All this enables us to establish appropriate relationships with other living beings and/or the environment.
To do that, both the CNS and the PNS must be healthy.
For instance, speaking depends on a number of requirements: the areas of the brain where the speaking functions are located must be intact; we must have a motivation to speak; talking implies and conveys emotions; our memory must work properly to recall facts, words and so on; and the nerves permitting to move our facial muscles, tongue and vocal cords must function.
It is important to understand that the integrity of the brain as "mind" and "reason" underlies the concept of life that we generally have. Tumors of the brain may alter this equilibrium, as well as the visual image that we have of ourselves, of our body and of the interaction between our body and the surrounding environment.
It is worth recalling that motor and sensory pathways have a diagonal criss-cross pattern: the left half of the brain controls the right part of our body, while the right half controls the left part. Only the pathways coming from the cerebellum do not cross one another: the site of the lesion is the same as the one of the symptom.
Along their way, motor and sensory paths are connected with a lot of other centers, which coordinate action and not mere gross movements, such as those evoked by directly stimulating the motor cortex. The stimulus reaches the 2nd motoneuron in the spinal cord and then travels along the peripheral nerve to the muscle in order to elicit action.
The sensory paths are ascending along the spinal cord, coming from the peripheral nerves. They cross and reach the thalami and then the parietal cortex.
Smell, vision, hearing and taste have different and specific pathways. Vision has a large and fairly definite representation in the occipital cortex. The other senses have less definite locations.
Metabolism and vegetative life are governed by the hypothalamus, automatically when they are at rest. When we have emotion, stress, sexual desire, fear or do physical efforts, etc., the hypothalamus alters metabolism to adapt the body to the new situation. Sitting at a dining table may seem an easy action; it is not. We must feel hungry, have a motivation to feed ourselves, be materially able to activate all the muscles that are required for the action and with the right coordination. At the same time, our brain may receive a pleasant message: we feel hungry, the food tastes well.