The Central Nervous System
(page 2 of 3)

The diencephalon.

    The diencephalon is the part of the brain that surrounds the third ventricle. There are three main areas of gray matter that make up the diencephalon, and these are the thalamus, the hypothalamus and the epithalamus.

    The thalamus relays the sensory information to the cortex. It contains several nucleus that receive specific information and retransmit them to different cortical regions. For example, the ventral posterolateral nucleus gets information from somatic sensory receptors (sensing touch, pressure, pain, etc..) and distributes this information to different areas of the cortex. The lateral geniculate nucleus serves as a relay for visual information, while the medial geniculate body relays auditory information.

    The thalamus also serves as a relay for the integration of certain motor information. The ventral lateral nucleus relays information from the cerebellum and the ventral anterior nucleus receives from the basal ganglia. The anterior nuclei transmit information from the hypothalamus and participate to the regulation of emotions and several visceral functions (gut feelings). Finally, the reticular thalamic nucleus, acting on neighbor thalamic nuclei, influence the focus and attention we pay to the different sensory modalities.

    The hypothalamus, suggested by its name, is located just under the thalamus. It contains several nuclei that control important functions for physiological homeostasis (maintenance of equilibrium of body functions). Several of those nuclei are directly or indirectly involved in the control of the autonomic nervous system: control of blood pressure, frequency and intensity of cardiac contractions, respiration, as well as gastrointestinal functions.

     The hypothalamus is also involved in regulating food intake, thirst and water balance. It is at the center of many emotions and behaviors. And, some other nuclei are associated with fear, anger and even sexual pleasure.

    Another important function of the hypothalamus concerns its participation to the endocrine system, the control of few hormones. The paraventricular and supraoptic nuclei respectively produce the antidiuretic hormone (ADH, vasopressin) and the oxytocin. These hormones are then released into the blood stream via the posterior pituitary (the neurohypophysis). Other nuclei control the release of several factors via the anterior pituitary (the adenohypophysis). In turn, those factors drive other endocrine glands in our body and regulate other hormonal functions.

    Finally, the last part of the diencephalon is the epithalamus, located above the thalamus. This is the smallest part of the diencephalon, but its importance remains vital. It is composed mainly of the pineal gland which secretes melatonin, a hormone that participates actively in the regulation of sleep-wake cycles and mood. This hormone also has important antioxidant properties.

The brainstem.

    The brainstem, located between the diencephalon and the spinal cord, includes the midbrain, the pons and the medulla. These structures, in addition of serving as the passage way for the fibers that travel between the higher centers and the spinal cord, contain multiple nuclei of gray matter involved mainly in the control of autonomic functions (automatic and necessary functions for the survival: like breathing and blood circulation).

     The midbrain includes the brainstem which contains the main descending bundle of motor fibers toward the spinal cord; it's the pyramidal tracts. There is also the cerebellar peduncles, a network of fibers connecting the cerebellum. And, there is also several areas of gray matter including the central gray matter surrounding the ventricle of the midbrain. The latter region plays a role in the suppression of pain and has been link, with other regions, to fear, fight or flight reactions. The midbrain also contains nuclei that give rise to two pairs of cranial nerves: the oculomotor nerve (III) and the trochlear nerve (IV).

     There are four large protuberances visible on the dorsal of the midbrain: the superior colliculus which coordinate the movements of the eyes and the head, and the inferior colliculus which serve as relays between the ear and the auditory cortex. Ventrally, there is a large nucleus called the substantia nigra which is a major dopamine center that projects to the globus pallidus which is in turn involved in the coordination of certain movements. The degeneration of the substantia nigra is involved in the development of Parkinson's disease which is characterized with motor tremor. Finally, there are several small nuclei in the reticular formation, like the red nucleus which play a role in motor control.

     The next section of the brainstem is the pons. It constitutes an important relay between superior centers and the spinal cord, and between the motor cortex and the cerebellum. In addition, of its nuclei are involved in the control of breathing and blood pressure. There are three pairs of cranial nerves exiting at the level of the pons: the trigeminal nerve (V), the abducens nerve (VI) and the facial nerve (VII).

     The following section of the brainstem is the medulla. Like for the midbrain and the pons, the medulla is a passage for fibers from the motor cortex, through the pyramidal tract, and to the spinal cord. Located at the end of this medulla oblongata, just before entering the spinal cord, 90% of these fibers decussate (go to the other side). Hence, the left motor cortex controls the muscles on the right side of the body and vice-versa. The medulla also contains fibers connecting the cerebellar peduncles, carrying proprioception information's, to the cerebellum.

     The medulla is also the place of emergence to several cranial nerves. This is the case of the hypoglossal nerve (XII: chewing, swallowing and speech), the glossopharyngeal nerve (IX: taste and swallowing), the vagus nerves (X: vegetative junctions) and vestibulocochlear nerve (VIII: auditory relay and equilibrium). The medulla also contains several nuclei important for the control of vegetative functions: the control of cardiovascular and respiratory functions, as well as for other functions such as the regulation of body temperature, salivation, swallowing, coughing, sneezing and vomiting, for example.

The cerebellum.

     After the cerebral hemispheres, the cerebellum is the second largest part of the brain. It's a little more than 10% of the brain volume. The cerebellum serves as a site of integration of several modalities of sensory-motor information and adjusts muscle contractions to produce coordinated movements. For example, a ball comes in your field of vision, your eyes perceive it, your head turns, your arm stretches and reach out to catch the ball. Furthermore, all this was performed in a split second, without thinking and without ever losing balance, since your body had to rotat and your weight had to be transfered. Thus, for coordinating motor activity, the cerebellum act like a coprocessor of the brain.

     The cerebellum is so folded that it looks like a cauliflower. But in fact, it resembles the cortex, just a little more wrinkled. It has gray matter at the surface, fibers underneath constituting the white matter, and some ganglia deeper in its structure. And, like the motor and sensory cortices, neurons are arranged somatotopically, representing different parts of the body (see figure on the left side). However, by contrast to the cerebral cortex, the cerebellum does not really have decussations (fibers crossing between the left and right sides).

     The cerebellum is divided into three main lobes (anterior, posterior and flocculonodular or median), and a central stem called the vermis. The anterior and posterior lobes coordinate movements. The medial part of these lobes is associated with the motor activities of the center of the body (pelvis, trunk and head) while the intermediate portions of the lobes are associated with the motor activities of the extremities (arms, hands, legs and feet). The sides of these lobes are involved in the motion planning and the execution thereof. Finally, the flocculonodular lobe receives information from the vestibular system of the inner ear (the semicircular canals) and helps to coordinate movements in order to maintain balance (extension of the right arm for example will require adjustments in other parts of the body).

     At the same time that the motor cortex sends a command to the muscles, it sends this information to the cerebellum. While the movement is being executed, the cerebellum receives proprioceptive information which is the information about the state of contraction of different muscles and the stretch state of the tendons and ligaments. At the same time, it also receives information from the semi-circular canals and from the vision system. All this providing information about the balance and the position body in space. Based on all these parameters, the cerebellum calculates if corrections are needed for the final movement to match what was initiated and intended by the motor cortex. It make sure that the resulting movement is executed at the perfection, without overshoot and without loss of balance. Otherwise, without these fine tuned adjustments, one could trip because of a small crack in the sidewalk, or loss balance when his foot adjusts to extend the step in order to avoid that crack.

     Indeed, damage to the cerebellum causes a lack of muscle tone and poor coordination of movements (ataxia). The effects of alcohol on the cerebellum partly explain the loss of balance and motor coordination.

The limbic system.

     There are several systems, several neural networks, which, although located in different parts of the central nervous system (CNS), participate to the same function. In this section, I present to you, the limbic system and the reticular formation.

     The limbic system consists of several nuclei involved in the control of emotions and mood. It includes, among others, the amyloid bodies that serve to assess the danger and trigger fear reactions, and the cingulate gyrus which is involved in emotional responses, in non-verbal language of our emotions as well as in frustrations.

     In the figure on the left, we can note that the olfactory bulb, responsible for our sensitivity to odors is connected to the limbic system. This explains the importance of odors in our reactions and emotional memories. That is maybe why we sometimes say that we can not small this or that person.

     The limbic system also receives a lot of information from different regions of cortex allowing us to react emotionally to environmental stimuli. This is where the fight occurs between our logic (the facts) and our feelings (the perception). Is it the feelings that will prevail over the logic or is it the logic that will allow us to better control our feelings? This will depend on the individual, the importance of the experience and the will to deal with these sometimes conflicting information's. In addition, several projections of the limbic system pass through the hypothalamus where the emotional tension can induce visceral disorders, such as heartburn, digestive disorders and even hypertension.

     In a world of science and reason, we must teach our limbic system to control our nerves! But often it is our animal side, our instinct that wins. A matter of survival, I guess.

The reticular formation.

     The reticular formation constitute an important link between different sensory modalities from all parts of the brain. These channels are particularly important to adjust the idle state, the alertness of the brain. Thus, the more senses that are activated, the more vigilant or alert we usually become. And even during sleep, sensory stimulation would manage to awake us. This somewhat primitive system is quite important for survival.

     The reticular formation can adapt to the relevance of the sensory information such that we do not pay attention to this information if they seem normal or if they become normal. For example, asleep in front of the television, music or dialogues do not wake us. But if someone turn off that television set, this change in sensory excitement may wake us up more easily.

     Because the reticular formation is important for the control of our wake state, it is the inhibition of reticular centers that reduces our vigilance and allows our brain to enter asleep.