The adult brain weighs approximately 3 pounds, or 2% of total body weight. Brain tissue is the most energy-consuming tissue in the body, receiving approximately 20% of the cardiac output and using approximately 20% of the body’s oxygen supply. Structurally the brain consists of external gray matter and internal white matter. The brain has three distinct parts: cerebrum, brainstem, and cerebellum. The cerebrum, divided into two hemispheres, represents almost 90% of the brain’s weight. Bands of connective tissue, called the corpus callosum, relay information between the two hemispheres. Each hemisphere consists of lobes named for the adjacent portion of the skull (i.e., frontal, temporal, parietal, and occipital). The brainstem, consisting of the midbrain, pons, and medulla, is continuous with the spinal cord and serves as an important relay and reflex center for the CNS. Nuclei for the cranial nerves are found in the brainstem. The brainstem also controls respiration, the cardiovascular system, gastrointestinal functions, equilibrium, and eye movement. ² The cerebellum controls activities below the level of consciousness (e.g., posture, equilibrium). Within the brain, there is a series of interconnected cavities called ventricles. Most cerebrospinal fluid (CSF) is produced within the ventricles by the choroid plexus and the ependymal cells.

Twelve pairs of cranial nerves arise directly from the brainstem. Each nerve is identified with a Roman numeral and name. Cranial nerves may be sensory, motor, or both. Cranial nerve functions are not consciously controlled; therefore assessment of cranial nerves provides an accurate picture of brainstem activity and neurologic function. Table 33-1 lists cranial nerves and their functions.

Two pairs of arteries connect to form the circle of Willis, the brain’s major blood supply. The terminal branches of each internal carotid artery branch into the posterior communicating artery and the middle and anterior cerebral arteries. This is commonly called the anterior circulation. The internal carotid arteries supply most of the cerebral hemispheres, basal ganglia, and the upper two thirds of the diencephalon (a division of the cerebrum). Two vertebral arteries unite to form a single basilar artery, which then divides into two posterior cerebral arteries. This artery complex supplies the cerebellum, brainstem, spinal cord, the occipital lobes, portions of the temporal lobes, and the posterior diencephalon. The anterior circulation supplies 80% of the blood to the brain, and the posterior circulation carries 20%. Figure 33-1 illustrates arterial blood supply to the brain. Venous blood drains from the brain through sinuses in the dura mater into the internal jugular veins.

The brain occupies 80% of the cranium. Vascular volume and CSF account for the remaining 20%. Cranial rigidity limits the brain’s ability to tolerate volume expansion. If one component increases, the other components must decrease to prevent pressure on the brain. Cerebral blood flow is altered by changes in cerebral perfusion pressure (CPP). CPP is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) (MAP − ICP = CPP). The normal CPP is 60 mm Hg. Normal ICP is 10 to 15 mm Hg and is measured with invasive monitoring. Responses in fluid alterations can result in CSF reabsorption, shunting to the spinal subarachnoid space, or a decrease in cerebral blood volume into the sinus cavity. The body’s compensatory mechanisms allow for minimal changes in cranial volumes with minimal increases in ICP. ¹

CSF is produced in the ventricles by the choroid plexus at a rate of 7 to 10 mL/hr. CSF protects the brain and spinal cord by forming a shock-absorbing cushion, providing nutrition via glucose transport, and removing metabolic waste products. CSF also compensates for changes in pressure and volume within the cranium. Table 33-2 summarizes normal CSF characteristics.

The spinal cord lies in the spinal canal of the vertebral bodies and is covered by meningeal layers. The adult spinal cord is approximately 16 to 18 inches long and extends from the brainstem to the intervertebral disk between L-1 and L-2. Sensory and motor neurons in the spinal cord conduct impulses to and from the brain. Unlike the brain, the spinal cord has white matter on the exterior and gray matter on the interior. Reflex arcs into the spinal cord operate without voluntary or conscious control. Table 33-3 lists these reflexes.