Brain Stem

The brain stem is the most caudal portion of the brain, and structurally it serves to connect the cerebrum with the spinal cord as well as to anchor the cerebellum behind it. Its three major segments are the midbrain, which lies just below the diencephalon and is continuous with the thalamus, the pons, which is below the midbrain and anterior to the fourth ventricle, and the medulla, which is continuous with the spinal cord below. In addition to harboring many cranial nerve nuclei and tracts, the brain stem contains long ascending and descending tracts to and from higher structures, as well as an important integrative structure called the reticular formation.

As reviewed previously, cranial nerves III-XII all have their central termini in the brain stem. Thus, the brain stem serves as a general relay station conveying sensory, motor, and autonomic information between the CNS and the tissues and organs of the face and body. Damage to the brain stem can thus have a wide range of effects on cranial nerve function that can often be detected clinically, as is well-described by the observations of classic neurology.

The long tracts that are found within the brain stem are all continuations of tracts that begin above or below. Four major tracts deserve mention. First is the corticospinal tract, which begins in the precentral gyrus of the frontal lobe and descends to the spinal cord, where it provides supraspinal input to motor neurons that directly innervate voluntary muscles. In the brain stem, this tract occupies the ventral portion of the midbrain, pons, and medulla, and near the bottom of the medulla it crosses (decussates) so that most corticospinal fibers travel to the opposite side of the spinal cord. Thus, as in the visual system, there is a crossing of neural systems that renders one side of the brain responsible for nervous activity on the other side of the body. The second major tract is the corticobul-bar tract, which performs a role similar to that of the corticospinal tract, but terminates in various brain stem motor nuclei. The other two long tracts of the brain stem are sensory. The first is the medial lemniscus, which is a continuation of the dorsal column system in the spinal cord that conveys information about vibration and position sensation to the ventral posterior lateral (VPL) nucleus of the thalamus and thence to the somatosensory cortex of the parietal lobe. The other is the spinothalamic tract, a similar fiber system that transmits pain and temperature sensation from the spinal cord again to the VPL nucleus of the thalamus and ultimately the parietal cortex.

The integrative function of the brain stem is carried out by a collection of diffusely organized nuclei and tracts in the brain stem core called the reticular formation. This area is defined more by its physiological characteristics than by discrete anatomic boundaries, and it includes some important neuro-transmitters, among them norepinephrine from the locus ceruleus and serotonin from the dorsal raphe nuclei. The reticular formation participates in sensation, movement, and autonomic function, but perhaps its most important role is in the maintenance of consciousness. A portion of the reticular formation in the upper pons and midbrain, called the ascending reticular activating system (ARAS), sends projections to the intralaminar nuclei of the thalamus, which in turn project to the entire cerebrum (Fig. 8). By virtue of the general cerebral activation enabled by this system, the ARAS has a crucial role in wakefulness and is largely responsible for the sleep and wakefulness cycle of normal humans. Damage to the ARAS, as from a brain stem stroke or traumatic brain injury, may produce a loss of arousal and result in the clinical state of coma. Thus, the ARAS is necessary for the level of consciousness; conversely, the content of consciousness, as will be seen, is elaborated by more rostral structures in the brain.

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