The Central Nervous System
(The Brain)
(page 1 of 3)

Generalities.

    Brain is a general term to describe the content in our skull, but there may be more appropriate terms. We say the central nervous system (CNS) to designate the encephalon (the portion which is in our head) and the spinal cord (in the middle of our vertebral column). The brain (or the encephalon) could be divided in distinct portions: the telencephalon, the diencephalon, the mesencephalon, the metencephalon, the myelencephalon and the rhombencephalon. The telencephalon is the largest part of the brain, it is the most advanced (evolutionary speaking) portion, and this portion occupies the major part of our skull. The forebrain includes the telencephalon and the diecephalon; the midbrain is the mesencephalon; whereas the hindbrain, which is also named the rhombencephalon, includes the metencephalon and the myelencephalon. Thus, the brain per see includes several portions of the central nervous system, and its functioning is even more complicated compared to this nomenclature.

    The brain is like a super computer for our body. It is responsible for integrating sensory information and generate appropriate responses that will allow our body to react to its environment, to survive and to reproduce. It also has a role in hormone regulation, memory, consciousness, and the sleep and wake cycles.

    The basic cell of the nervous system is the neuron. This cell type is responsible for transporting the information in the form of electrical impulses. The nervous system includes nearly a hundred billion (10¹¹) neurons, and each neuron can have up to ten thousand (10⁴) synapses (connections) with other neurons. One can hardly imagine the number of combinations and permutations involved in the process of all kinds of information and commands. But, not all these permutations are viable. The brains of all humans are similar in most ways: they have the same specific regions responsible for controlling your breathing, your heart or your hormonal system, the same specific regions for moving the tip of your big toe, perceiving light, colors or movement, and the same specific regions to control your language skills. And, surprisingly our brain also have many similarities to that of other mammals: that is that similar regions that are responsible for the same functions, just slightly less evolved.

    As if things were not complicated enough, the CNS does not only contains neurons. For the proper functioning of the neurons, our brain needs the support from other cell types, the glial cells: astrocytes help, among other tasks, to clean-up metabolites that could otherwise intoxicate our neurons, and they also help to form a barrier between our blood and our neurons; neurolemmocytes (oligodendrocytes) wrap around neuronal axons and act as insulation sheath; and microgliocytes provide immunity in the CNS. For more information about the glial cells, which by the way are much more numerous than neurons, I present this short video about the role of gliocytes.

Development of the central nervous system (CNS).

    For this section I only present a series of figures illustrating the embryonic development of the central nervous system (CNS). You will learn about the development of the telencephalon, the diencephalon, the midbrain, the metencephalon, the myelencephalon, the spinal cord and the cranial nerves. I summarize the function of these regions in the following sections.

The ventricles.

    Inside the brain, there is a system of ventricles (holes). I will further discuss about these ventricles in a following section, in an another page. For now, I will just mention that these ventricles form a contiguous cavity inside the brain. This cavity is filled with a fluid, the cerebrospinal fluid (CSF), which serves, among others thing, to absorb shocks that our head, and consequently our brains may suffer. In this section, I will only present here the anatomical development of the ventricular system.

    The cerebrospinal fluid (CSF) is produced by choroid plexus in different areas of the brain ventricles. It flows down the ventricular system and escapes through a few holes, located in the medulla and the spinal cord. Then the CSF flows outside of the brain, between the CNS and the meninges. In the subarachnoid space, at the level of the sagittal sinus, it is absorbed back into the venous system. Hence, there is a flow of CSF from the center of the brain to the periphery. The brain contains about 1.5 L of CSF which is renewed every eight hours.

The cerebral hemispheres.

    The largest portion of the brain (83%) consists of the cerebral hemispheres. The surface of the hemisphere is the cortex. This is the region of our brain which is folded, wrinkled. This folding provides more surface area, thus increased number of neurons. A fold is named a gyrus, while a grooves is called a sulcus. The deeper valleys that separate different portions of the cortex, are called fissures. Gyrus and sulcus constitute important anatomical landmarks and are used to define various regions of the cortex.

    Evolution of mankind is mainly due to the increased complexity of the cerebral cortex. Indeed, the numerous sulcus and gyrus (folds and pleats) have significantly increased the cortical surface. If we could unfold and flatten the cerebral cortex, its thickness would not exceed 2 to 4 mm, but its surface area would be around 1m². The cortex contains roughly a billion cells arranged in six layers, and it account for nearly 40% of the mass of the brain. These six layers are composed of neuronal cell bodies, and are referred to as gray matter, while the regions composed primarily of neuronal axons are referred to as white matter.

    Different regions of the cortex are responsible for different functions, and these regions are interconnected to integrate several sources of information and produce appropriate responses. For visual information, for example, a region could perceive contrasts, another one would perceive contours, another one will detect movement and one more region might determine depths, sizes and distances, and finally another one will integrate all these informations to determine the meaning of what is seen and trigger an appropriate response if needed. Despite this enormous complexity, using electrophysiological techniques, our knowledge of brain function have evolved considerably in the last century. We now know, for example, what are the neurons primarily responsible for somatosensory perception (the sense of touch) and those primarily responsible for the activation of our muscles (the primary motor cortex). In addition, in the motor and sensory cortices for example, we know there is an organization in the arrangement of neurons that represent our body. The neurons near the center of our brain (sagittal line) receive and send information from and to the lower parts of the body, whereas those located more laterally send and receive information from our face and mouth. This somatotopic organization, which is called the homunculus, is not representing evenly all the parts of our body. There are many more neurons involved in the perception or control of our face or our hand, compared to the number of neurons innervating our elbow. This suggests that the perception and the motor control of the thumb or the face, for example, are much more refined for the thumb and the face than it is for the elbow or the trunk. These homunculi (somatic representations) for the motor and somatosensory cortex, are illustrated in the figure bellow. Please, note that this somatotopic organization is not limited to the sensorimotor functions, but to most brain functions even though it is not always well known or well defined.

Sensory and motor homunculi.

    Our brain is divided into two hemispheres, left and right. In general, the left hemisphere receives and controls the right side, while the reverse is true for the right hemisphere. But this is not an absolute rule, as some functions do not work in pairs, or do not show bilateral symmetry. Speech and spatial orientation are good examples: we have only one mouth and it is inconceivable that the left half of your mouth wants to say something different than the right half, and it would be inconceivable the left half of our body would be moving in a different direction than the right half. For most people, the left hemisphere contains our speech centers while spatial orientation centers are located in our right hemisphere. But even this is not absolute; for a left-handed person, it is often the opposite. There are 10 to 20% of people who have a reversed laterality, or do not really have lateralization of brain functions. In addition, there are several connections between the two hemispheres of the brain (e.g.: the corpus callosum) so, for several functions the brain really works as a whole.

    Nevertheless, for several functions, crossing between the left and right hemisphere is clear. Among others, this is the case of the motor function and somatosensory perception. They process decussation of their fibers (crossing or changing sides) at the level of the pyramids. When an injury occurs in the right hemisphere, the left side of the body becomes paralyzed and insensitive, as this is often the case during stroke (AVC), and vice versa.

    Under the cortex, in the middle of the brain, in the heart of the white matter, we find the basal ganglia, which consist primarily of the caudate nucleus, putamen and globus pallidus. Although their functions are not always well defined, they are known to receive information from sensory areas and project their fibers to the frontal and premotor cortices. They are indirectly involved in the initiation, termination and the intensity of movement. For example, they could be involved in arm swing when walking or other stereotyped movements that are produced without thinking, without paying attention. Malfunction of these nuclei could produce some tics or other involuntary movements.