Porth's Essentials of Pathophysiology, 4e

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Nervous System

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An important concept is that the more rostral or recently developed parts of the neural tube gain domi- nance or control over regions and functions at lower levels. They do not replace the earlier developed cir- cuitry but merely dominate it. After damage to the more vulnerable parts of the forebrain, as occurs with severe brain injury, a brain stem–controlled organism remains that is capable of breathing and may survive if the envi- ronmental temperature is regulated and nutrition and other aspects of care are provided. However, all aspects of intellectual function, experience, perception, and memory usually are permanently lost. Hindbrain During embryonic development, the hindbrain becomes subdivided into the cerebellum, the medulla oblongata— also referred to as the medulla —and the pons. The CSF-filled tube becomes the fourth ventricle, which is continuous with the cerebral aqueduct of the midbrain (see Fig. 34-14). The cerebellum, the “little brain,” is an important movement control center (see Chapter 36). It receives massive axonal inputs from the spinal cord and the pons. Neurons in the spinal cord provide information about the body’s position in space, and those in the pons relay information from the cerebral cortex specifying the goals of the intended movements. Brain Stem. The brain stem is the region of the brain that connects the cerebrum with the spinal cord. It consists of themidbrain, medulla oblongata, and the pons. In the brain stem, the reticular formation has been greatly expanded. Certain groups of neurons termed vital centers are located in the reticular formation of the pons and medulla. These centers include the respiratory, cardiovascular, and vaso- motor centers. Still other centers in the brain stem are con- cerned with swallowing, intestinal movements, control of micturition, and a variety of more specific functions such as salivation and pupillary diameter.

sends a signal via an efferent motor pathway, signaling an effector response. A signal is also sent to the brain, alerting it to the sensation and response. This happens so quickly that people may experience the sensation of pain almost simultaneously with the development of the reflex, even though the brain is aware of the situation. The withdrawal reflex is initiated by a painful (noci- ceptive) stimulus and quickly moves the body part away from the offending stimulus, usually by flexing a limb part. The withdrawal reflex is powerful, taking precedence over other reflexes associated with locomo- tion. All the joints of an extremity (e.g., finger, wrist, elbow, shoulder) typically are involved. This complex, polysynaptic reflex also shifts postural support to the opposite side of the body with a crossed extensor reflex and simultaneously alerts the forebrain to the offending stimulus event. The withdrawal reflex also can produce contraction of muscles other than the extremities. For example, irritation of the abdominal viscera may cause contraction of the abdominal muscles. The Brain The brain is divided into three regions, the hindbrain, the midbrain, and the forebrain (Fig. 34-14). The hind- brain includes the medulla oblongata, the pons, and its dorsal outgrowth, the cerebellum. Midbrain structures include two pairs of dorsal enlargements, the superior and inferior colliculi (not shown). The forebrain, which consists of two hemispheres covered by the cerebral cor- tex, contains central masses of gray matter, the basal ganglia (discussed in Chapter 36), and the rostral end of the neural tube, the diencephalon with its adult deriva- tives—the thalamus and hypothalamus. The olfactory and optic nerves are also considered part of the fore- brain. In contrast, the motor and sensory nuclei of the 3rd to 12th cranial nerves are located in the midbrain and hindbrain.

Corpus callosum

Forebrain

Third ventricle

Frontal lobe

Pineal body

Occipital lobe

Interventricular foramen

Cerebral aqueduct

Anterior commissure

Pituitary gland

Fourth ventricle

Midbrain

Pons

Cerebellum

Hindbrain

Medulla oblongata

Central canal

FIGURE 34-14. Midsagittal section showing structures of the hindbrain, midbrain, and forebrain.

Spinal cord