Our nervous system can be separated into two gross divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS). The central nervous system is composed of the brain and spinal cord, and is also referred to as the neuraxis. The nerves of the peripheral nervous system exit and re-enter the spinal cord. When they exit the spinal cord, they innervate muscles, organs, and tissues of the body.

CNS

The brain is housed in the part of the skull known as the cranium. A synonym for brain is encephalon. The largest mass of brain tissue is called the cerebrum, which consists of the cerebral hemispheres, basal gangl ia, and rhinencephalon (which is that part of the brain that has to do with olfaction or smelling).

The cerebral hemispheres look identical, although the functions of the various parts may differ on the right and left sides. Each hemisphere can be divided into four primary lobes: Frontal, Parietal, Temporal, and Occipital. These lobes ca n be identified on the brain’s surface by certain landmarks, the gyri and sulci. A gyrus is an elevation on the surface of the brain caused by the folding in of the cortex. A sulcus is a groovelike depression on the brain surface that sepe rates the gyri. Another name for sulcus is fissure.

The frontal lobe is bounded inferiorly by the lateral sulcus or Sylivan fissure, and posteriorly by the central sulcus or Rolandic fissure. In the frontal lobe there is a long gyrus just anterior to the central s ulcus known as the precentral gyrus. This makes up the majority of what is known as the primary motor cortex or primary motor strip. Another important area of the frontal lobe is solely in the left hemisphere lobe. Part of the inferior (t hird) frontal gyrus is known as Broca’s area. This area of the brain plays a significant part in the production of fluent, well-articulated speech. Damaging the same area in the right hemisphere does not have a significant impact up on speech production.

The parietal lobe is bounded anteriorly by the central sulcus, inferiorly by the posterior end of the lateral sulcus, and posteriorly by an imaginary border line with the occipital lobe. The first gyrus posterior to the central sulcus is th e postcentral gyrus, which is also called the primary sensory cortex. This is the part of the brain that recieves most sensory input from other parts of the body.

The temporal lobe is where auditory processing in the brain takes place. It is bounded superiorly by the lateral fissure and posteriorly by an imaginary line with the occipital lobe. A region known as the primary auditory cortex is situated in the inferior wall of the lateral fissure. You can see this area if you peel back the lateral fissure to reveal the inner surface of the sulcus that is formed by folding in of the superior temporal gyrus. The posterior part of the superior temporal gyrus is known as Wernicke’s Area, and is very important for language processing.

The occiptial lobe, which occupies the small area behind the parietal lobe, is marked by imaginary lines rather than prominent sulci. It is involved in vision.

Besides dividing the brain into lobes, the brain can also be divided into three major divisions based on evolutionary development. These three divisions are the Hindbrain, Midbrain, and Forebrain. The hindbrain is the oldest part of the brain. It can be subdivided into the Myelencephalon and Metencephalon. The myelencephalon is the area where the spinal cord merges into the brain. This area is called the Medulla Oblongata. The fourth ventricle is also loca ted in the Medulla, which is an enlargement of the central fluid filled canal of the brain. The upper part of the hindbrain is the metencephalon. The posterior or dorsal part of the metencephalon is the cerebellum, while the anterior or ve ntral part is the pons.

The midbrian is also called the mesencephalon. It is located immediately above the pons and is the narrowest part of the brainstem. The midbrain contains the tectum or roof of the brainstem. On the tectum are four swellings called colliculi. The inferior colliculi are relay points in the central auditory nervous system and the superior colliculi are relay points in the visual nervous system. The tegmentum is also part of the midbrain, and this is where the substa ntia nigra and red nucleus, which are involved in motor control, are located. There is a fluid filled cavity in the mesencephalon known as the cerebral aqueduct. It connects the fourth ventricle to the third ventricle in the forebrain.

The newest part of the brain is the forebrain. It can be subdivided into the diencephalon and telencephalon. The structures that make up the diencephalon are located lateral to the third ventricle. The two main structures of the diencephalon are the thalamus, which is a primary sensory and motor relay station of the brain, and the hypothalamus which controls aspects of emotional behavior, aids in the regulation of body temperature, food & water intake, sex ual and sleep behavior, and control over the pituitary gland which releases hormones. The telencephalon is composed of the two hemispheres of the cerebrum, including all of the aforementioned lobes. Also considered part of the telencephalon is the basal ganglia, which is a collection of neuclei laying deep to the cerebral cortex.