The human brain has three major subdivisions: brainstem, cerebellum, and cerebrum. The CNS is first formed as a simple tubelike structure in the embryo. The concentration of nervous tissues at one end of the human embryo to produce the brain and head is referred to as cephalization. When the embryo is about four weeks old, it is possible to identify the early forms of the brainstem, cerebellum, and cerebrum, as well as the spinal cord. As development continues, the brain is located within the cranium (para 4-13c(1)) in the cranial cavity. See figures 11-5A and 11-5B for illustrations of the adult brain.
Figure 11-5A. Human brain (side view).
Figure 11-5A. Human brain (side view).
a. The Brainstem. The term brainstem refers to that part of the brain that would remain after removal of the cerebrum and cerebellum. The brainstem is the basal portion (portion of the base) of the brain. The brainstem can be divided as follows:
(1) The brainstem is continuous with the spinal cord. Together, the brainstem and the spinal cord are sometimes known as the neuraxis.
(2) The brainstem provides major relays and controls for information passing up or down the neuraxis.
(3) The 12 pairs of cranial nerves connect at the sides of the brainstem.
b. Cerebellum. The cerebellum is a spherical mass of nervous tissue attached to and covering the hindbrainstem. It has a narrow central part called the vermis and right and left cerebellar hemispheres.
(1) Peduncles. A peduncle is a stem-like connecting part. The cerebellum is connected to the brainstem with three pairs of peduncles.
(2) General shape and construction. A cross section of the cerebellum reveals that the outer cortex is composed of gray matter (cell bodies of neurons) with many folds and sulci (shallow grooves). More centrally located is the white matter (myelinated processes of neurons).
(3) Function. The cerebellum is the primary coordinator/integrator of motor actions of the body.
c. Cerebrum. The cerebrum consists of two very much enlarged hemispheres connected to each other by a special structure called the corpus callosum. Each cerebral hemisphere is connected to the brainstem by a cerebral peduncle. The surface of each cerebral hemisphere is subdivided into areas known as lobes. Each lobe is named according to the cranial bone under which it lies: frontal, parietal, occipital, and temporal.
(1) The space separating the two cerebral hemispheres is called the longitudinal fissure. The shallow grooves in the surface of the cerebrum are called sulci (sulcus, singular). The ridges outlined by the sulci are known as gyri (gyrus, singular).
(2) The cerebral cortex is the gray outer layer of each hemisphere. The occurrence of sulci and gyri helps to increase the amount of this layer. Deeper within the cerebral hemispheres, the tissue is white. The "gray matter" represents cell bodies of the neurons. The "white matter" represents the axons.
(3) The areas of the cortex are associated with groups of related functions.
(a) For example, centers of speech and hearing are located along the lateral sulcus, at the side of each hemisphere.
(b) Vision is centered at the rear in the area known as the occipital lobe.
(c) Sensory and motor functions are located along the central sulcus, which separates the frontal and parietal lobes of each hemisphere. The motor areas are located along the front side of the central sulcus, in the frontal lobe. The sensory areas are located along the rear side of the central sulcus, in the parietal lobe.
d. Ventricles. Within the brain, there are interconnected hollow spaces filled with cerebrospinal fluid (CSF). These hollow spaces are known as ventricles. The right and left lateral ventricles are found in the cerebral hemispheres. The lateral ventricles are connected to the third ventricle via the interventricular foramen (of Monroe). The third ventricle is located in the forebrainstem. The fourth ventricle is in the hindbrainstem. The cerebral aqueduct (of Sylvius) is a short tube through the midbrainstem which connects the third and fourth ventricles. The fourth ventricle is continuous with the narrow central canal of the spinal cord.
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