The Central Nervous System

The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.

The Cerebrum

The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum with two distinct halves, a right and left cerebral hemisphere (Figure 15.11). Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises a continuous, wrinkled, and thin layer of gray matter that wraps around both hemispheres, the cerebral cortex, and several deep nuclei. A (plural = gyri) is the ridge of one of those wrinkles, and a (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex (Figure 15.12).

 

Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy (cytoarchitecture) of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann's areas, which is still used today to describe the anatomical distinctions within the cortex. The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. For example, Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.

Beneath the cerebral cortex are sets of nuclei known as basal nuclei that augment cortical processes (Figure 15.13). Some of the basal nuclei in the forebrain, for example, serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the cholinergic basal forebrain nuclei. Some other basal nuclei control the initiation of movement. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep an urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)

 

 

The Diencephalon

The word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the peripheral nervous system all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum.

The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus (Figure 15.14). There are other structures, such as the epithalamus, which contains the pineal gland, and the subthalamus, which includes the subthalamic nucleus, one of the basal nuclei.

 

Thalamus

The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The cerebrum also sends information down to the thalamus, which usually communicates motor commands.

Hypothalamus

Inferior and slightly anterior to the thalamus is the hypothalamus, the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system.

The Brain Stem

The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 15.15). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems.

The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem.

 

Midbrain

One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal.

The midbrain includes four bumps known as the colliculi (singular = colliculus), which means “little hill” in Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps, and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit but do not.

Pons

The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem.

Medulla

The gray matter of the midbrain and pons continues into the medulla, also known as medulla oblongata. This diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, general brain activity, and attention. The medulla contains autonomic nuclei with motor neurons that control the rate and force of heart contraction, the diameter of blood vessels, and the rate and depth of breathing, among other essential physiological processes.

The Cerebellum

The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum and looks like a miniature version of that part of the brain (Figure 15.16). The cerebellum integrates motor commands from the cerebral cortex with sensory feedback from the periphery, allowing for the coordination and precise execution of motor activities, such as walking, cycling, writing, or playing a musical instrument.