The diencephalon, or interbrain, is located between the cerebral hemispheres and the brain stem. One can see it by cutting into the central section of the brain. The diencephalon includes the thalamus and the hypothalamus. (Fig. 6-10.) The two parts of the thalamus form the lateral walls of the third ventricle (see Figs. 6-1 and 6-5). Nearly all sensory impulses travel through the masses of gray matter that form the thalamus. The role of the thalamus is to sort out the impulses and direct them to particular areas of the cerebral cortex. The hypothalamus is located in the midline area inferior to the thalamus and forms the floor of the third ventricle. It helps to maintain homeostasis by controlling body temperature, water balance, sleep, appetite, and some emotions, such as fear and pleasure. Both the sympathetic and parasympathetic divisions of the autonomic nervous system are under the control of the hypothalamus, as is the pituitary gland. The hypothalamus thus influences the heartbeat, the contraction and relaxation of blood vessels, hormone secretion, and other vital body functions.
The Limbic System
Along the border between the cerebrum and the diencephalon is a region known as the limbic system. This system is involved in emotional states and behavior. It includes the hippocampus (shaped like a sea horse), located under the lateral ventricles, which functions in learning and the formation of long-term memory. It also includes regions that stimulate the reticular formation, a network that extends along the brain stem and governs wakefulness and sleep. The limbic system thus links the conscious functions of the cerebral cortex and the automatic functions of the brain stem.
The Brain Stem
The brain stem is composed of the midbrain, the pons, and the medulla oblongata (see Fig. 6-1). These structures connect the cerebrum and diencephalon with the spinal cord.
Figure 6-10 Regions of the diencephalon. The figure shows the relationship among the thalamus, hypothalamus, and pituitary gland (hypophysis).
The midbrain, inferior to the center of the cerebrum, forms the superior part of the brain stem. Four rounded masses of gray matter that are hidden by the cerebral hemispheres form the superior part of the midbrain. These four bodies act as centers for certain reflexes involving the eye and the ear, for example, moving the eyes in order to track an image or to read. The white matter at the anterior of the midbrain conducts impulses between the higher centers of the cerebrum and the lower centers of the pons, medulla, cerebellum, and spinal cord. Cranial nerves III and IV originate from the midbrain.
The pons lies between the midbrain and the medulla, anterior to the cerebellum (see Fig. 6-1). It is composed largely of myelinated nerve fibers, which connect the two halves of the cerebellum with the brain stem as well as with the cerebrum above and the spinal cord below. (Its name means “bridge.”) The pons is an important connecting link between the cerebellum and the rest of the nervous system, and it contains nerve fibers that carry impulses to and from the centers located above and below it. Certain reflex (involuntary) actions, such as some of those regulating respiration, are integrated in the pons. Cranial nerves V through VIII originate from the pons.
The Medulla Oblongata
The medulla oblongata of the brain stem is located between the pons and the spinal cord (see Fig. 6-1). It appears white externally because, like the pons, it contains many myelinated nerve fibers. Internally, it contains collections of cell bodies (gray matter) called nuclei, or centers. Among these are vital centers, such as the following:
* The respiratory center controls the muscles of respiration in response to chemical and other stimuli.
* The cardiac center helps regulate the rate and force of the heartbeat.
* The vasomotor center regulates the contraction of smooth muscle in the blood vessel walls and thus controls blood flow and blood pressure. The ascending sensory fibers that carry messages through the spinal cord up to the brain travel through the medulla, as do descending motor fibers. These groups of fibers form tracts (bundles) and are grouped together according to function.
The motor fibers from the motor cortex of the cerebral hemispheres extend down through the medulla, and most of them cross from one side to the other (decussate) while going through this part of the brain. The crossing of motor fibers in the medulla results in contralateral control-the right cerebral hemisphere controls muscles in the left side of the body and the left cerebral hemisphere controls muscles in the right side of the body, a characteristic termed contralateral (opposite side) control. The medulla is an important reflex center; here, certain neurons end, and impulses are relayed to other neurons.
The last four pairs of cranial nerves (IX through XII) are connected with the medulla.
The cerebellum is made up of three parts: the middle portion (vermis) and two lateral hemispheres, the left and right (Fig. 6-11). Like the cerebral hemispheres, the cerebellum has an outer area of gray matter and an inner portion that is largely white matter. However, the white matter is distributed in a treelike pattern. The functions of the cerebellum are as follows:
* Help coordinate voluntary muscles to ensure smooth, orderly function. Disease of the cerebellum causes muscular jerkiness and tremors.
* Help maintain balance in standing, walking, and sitting as well as during more strenuous activities. Messages from the internal ear and from sensory receptors in tendons and muscles aid the cerebellum.
Figure 6-11 The cerebellum. (A) Posterior view showing the two hemispheres. (B) Midsagittal section showing the distribution of gray and white matter. The three parts of the brain stem (midbrain, pons, and medulla oblongata) are also labeled.
* Help maintain muscle tone so that all muscle fibers are slightly tensed and ready to produce changes in position as quickly as necessary.
These techniques include:
* CT (computed tomography) scan, which provides photographs of the bone, soft tissue, and cavities of the brain (Fig. 6-12 A). Anatomic lesions, such as tumors or scar tissue accumulations, are readily seen.
* MRI (magnetic resonance imaging), which gives more
views of the brain than CT and may reveal tumors, scar
tissue, and hemorrhaging not shown by CT (see Fig. 6- 12 B).
* PET (positron emission tomography), which visualizes
the brain in action (see Fig. 6-12 C).
The interactions of the brain’s billions of nerve cells give
rise to measurable electric currents. These may be recorded using an instrument called the electroencephalograph. Electrodes placed on the head pick up the electrical signals produced as the brain functions. These signals are then amplified and recorded to produce the tracings, or brain waves, of an electroencephalogram (EEG).
The electroencephalograph is used to study sleep patterns, to diagnose disease, such as epilepsy, to locate tumors, to study the effects of drugs, and to determine brain death. Figure 6-13 shows some typical normal and abnormal tracings.
Figure 6-12 Imaging the brain. (A) CT scan of a normal adult brain at the level of the fourth ventricle. (B) MRI of the brain showing a point of injury (arrows). (C) PET scan.
Figure 6-13 Electroencephalography. (A) Normal brain waves. (B) Abnormal brain waves.