The Human Brain

The human brain has three major subdivisions: brainstem, cerebellum, and the cerebrum. The central nervous system is first formed as a simple tube like 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 the cerebrum, as well as the spinal cord. As development continues, the brain is located within the cranium in the cranial cavity. See Figure 5-5 for illustrations of the adult brain.

Figure 5-5. Human brain: A side view, B bottom view.

a. The Brainstem. The term brainstem refers to that part of the brain that would remain after the removal of the cerebrum and the cerebellum. The brainstem is the basal portion (portion of the base) of the brain. The brainstem can be divided as follows:

FOREBRAINSTEM thalamus

MIDBRAINSTEM corpora quadrigemina

HINDBRAINSTEM pons medulla

(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 passing information up or down the neuraxis.

(3) The 12 pairs of cranial nerves connect at the sides of the brainstem.

b. Cerebellum. The cerebellum is the 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. The peduncles is a stemlike 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 cerebral cortex is the gray outer layer of each hemisphere. Deeper within the cerebral hemispheres the tissue is white. The "gray matter" represents cell bodies of neurons. The "white matter" represents the axons.

(2) 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 parental 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 third ventricle is located in the forebrainstem. The fourth ventricle is in the hindbrainstem. The fourth ventricle is continuous with the narrow central canal of the spinal cord.

5-12. THE HUMAN SPINAL CORD

a. Location and Extent. Referring to Figure 5-6, you can see that the typical vertebra has a large opening called the vertebral (or spinal) foramen. Together, these foramina form the vertebral (spinal) canal for the entire vertebral column. The spinal cord, located within the spinal canal, is continuous with the brainstem. The spinal cord travels the length from the foramen magnum at the base of the skull to the junction of the first and second lumbar vertebrae.

SUPERIOR VIEW

SUPERIOR VIEW

NEURAL ARCH

Figure 5-6. The spinal column.

NEURAL ARCH

Figure 5-6. The spinal column.

(1) Enlargements. The spinal cord has two enlargements. One is the cervical enlargement, associated with nerves for the upper members. The other is the lumbosacral enlargement, associated with nerves for the lower members.

(2) Spinal nerves. A nerve is a bundle of neuron branches that carry impulses to and from the CNS. Those nerves arising from the spinal cord are spinal nerves. There are 31 pairs of spinal nerves.

b. A Cross Section of the Spinal Cord (Figure 5-7). The spinal cord is a continuous structure that runs through the vertebral canal down to the lumbar region of the column. It is composed of a mass of a central gray matter (cell bodies of neurons) surrounded by peripheral white matter (myelinated branches of neurons). The gray and white matter are thus considered columns of material. However, in cross section, this effect of columns is lost.

Figure 5-7. A cross section of the spinal cord.

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