Portions of the brain stem function as "the spinal cord for the head.'' The facial, jaw, tongue, pharynx, larynx, and the neck muscles are innervated by efferent axons of brain stem somatic and "special visceral'' lower motoneurons (cranial motoneurons), just as the muscles of the arms or the legs are innervated by spinal somatic lower motoneurons. Axons of these cranial motoneurons emerge from the ventral or ventrolateral surface of the brain stem, leave the cranial cavity via specific holes ("foramina") in the skull, and continue on as part of the various cranial nerves. Similarly, on the sensory side, the skin of the face and head is innervated mostly by afferent/sensory axons of tri-geminal and upper cervical sensory neurons. The cell bodies of these afferent neurons are in ganglia outside the cranial cavity (i.e., the cavum trigeminale for the trigeminal nerve) and the central processes enter the brain and synapse with secondary sensory neurons in dorsal portions of the medulla and pons, just as spinal afferents synapse in the dorsal horn of the spinal cord.
The rostral end of the neuraxis also receives sensory input from special sensory organs located at the front end ofthe individual, the portion that first moves into a new environment. Olfactory receptors are stategically placed at the front end of the respiratory system. Eyes and ears are also positioned at the front end, as is the vestibular apparatus located within the inner ear. Taste receptors and their sensory nerves are at the front end of the digestive system. The central processes of sensory neurons carrying information from hearing, vestibular apparatus, and taste receptors enter the brain at the level of the medulla and pons and synapse with second-order sensory neurons located in dorsally situated sensory nuclei. Special sensory neurons also detect internal bodily states (blood pressure, blood oxygen, and carbon dioxide levels; stomach distention;
and blood sugar level) and the central processes of these cells also reach the brain stem via the lower cranial nerves.
There is no direct communication between different groups of cranial motoneurons, so during, for example, swallowing, coordinated movements of the lips and the tongue are controlled by inputs from integrating premotor neurons with axonal collaterals innervating particular subgroups of lower motoneurons and not by collaterals of the lower motoneurons. Many of the integrative premotor neurons are also located in the brain stem. They help coordinate the behavioral, respiratory, cardiovascular, visceral, gastrointestinal, eliminative, and reproductive events essential for survival of the individual and the species. In turn, the premotor neurons are controlled by interneurons that are controlled by higher order integrative interneurons in the brain stem and in other parts of the brain. These interneurons receive inputs from the afferent side of the nervous system so that their genetically determined patterned output can also reflect the influence of past and present environmental events. Different groups of brain stem premotor neurons and interneurons are described in Table I.
As already noted, in the brain stem not all particular subgroups of neurons are arranged in discrete "nuclei" clearly defined with conventional Nissl stains. Certain brain stem neurons (including lower motoneurons, premotor neurons, and interneurons) are arranged diffusely in regions of the medulla, pons, and mid-brain, scattered between ascending and descending fiber bundles. The net-like appearance of the regions containing these neurons led to the designation "reticular formation," a term that was originally used in a purely descriptive anatomical sense. However, in the 1950s and 1960s, an additional functional meaning was assigned to the reticular formation so that the name began to refer to a hypothetical anatomical-physiological neuronal entity responsible for regulation of the level of arousal and consciousness as well as the maintenance of bodily posture. Instead of being used in a descriptive anatomical way, the reticular formation was promoted to a functional concept, a brain stem system which, by virtue of its nonspecific connectivity, could act as a kind of volume control for the degree of conscious arousal, among other things. The rostral part of the brain stem reticular formation became the "ascending reticular activating system" and loss of consciousness with brain stem injury was attributed to damage to this system. The caudal part of the brain stem reticular formation was seen as a source of descending excitatory or inhibitory inputs to various brain stem centers and to the spinal cord.
The problem was that the very nonspecific and all-inclusive nature of the reticular formation, conceived as a kind of functioning neuronal system, made it difficult to generate specific research hypotheses. This extended use of the term meant that the reticular formation became elevated to a magical entity, an easy shortcut explanation of complex and little understood physiological processes. This was not a helpful strategy. The term reticular formation should be restored to its original neuroanatomical status and emphasis should be placed on specifying the particular group of brain stem lower motoneurons, premotor neurons, and interneurons responsible for the various physiological functions. The cerebral hemispheres and the cerebellum can be viewed as an additional vast collection of interneurons whose coordinated discharge controls less complex sets of interneurons and premotor neurons in the brain stem reticular formation, thereby producing a finely coordinated discharge of brain stem motoneurons. Some axons descending from control neurons in the cerebral cortex synapse directly onto lower motoneurons in the brain stem or in the spinal cord, but a substantial proportion synapse
Figure 2 A series of photomicrographs of transverse sections through the human brain stem, stained for myelin by the Weil procedure. The insert in each figure gives the approximate level from which the section is taken Aq, aqueduct; Arc, arcuate nucleus; Coch, cochlear nucleus; Cu, cuneate nucleus; Cuext, external cuneate nucleus; Cun, cuneiform nucleus; dmnX, dorsal motor nucleus of the vagus; DR, dorsal raphe nucleus; Gr, gracile nucleus; IC, inferior colliculus; icp, inferior cerebellar peduncle; Int, nuclues intercalatus; IV, trochlear nucleus; IX fibers, intramedullary fibers of the glossopharyngeal nerve; lat lem, lateral lemniscus; LC, locus coeruleus; LRN, lateral reticular nucleus; LVe, lateral vestibular nucleus; mcp, middle cerebellar peduncle; ml, medial lemniscus; mlf, medial longitudinal fasciculus; MR, median raphe nucleus; MVe, medial vestibular nucleus; nA, nucleus ambiguus; nTS, nucleus ofthe tractus solitarius; PAG, periaqueductal gray; PB, parabrachial nucleus; PN, pontine nuclei; PPT, pedunculopontine tegmental nucleus; PrPH, nucleus prepositus hypoglossi; py, pyramidal tract; RM, nucleus raphe magnus; scp, superior cerebellar peduncle; sct, spinocerebellar tract; SN, substantia nigra; SpVe, spinal vestibular nucleus; ts, tractus solitarius; Vfibers, intramedullary fibers of the trigeminal nerve; VIfibers, intramedullary fibers of the abducent nerve; VIIfibers, intramedullary fibers of the facial nerve; VIIgenu, genu of the facial nerve; Vmes, mesenteric nucleus of the trigeminal nerve; Vsp, spinal nucleus of the trigeminal nerve; Vspt, spinal tract of the trigeminal nerve; XII, hypoglossal nucleus (modified with permission from Blessing, 1997).
with relevant brain stem interneurons and premotor neurons.
Much is already known concerning the lower motoneurons and the secondary sensory neurons in the different brain stem regions. Traditionally, these two groups are subclassified into particular categories, as summarized in Table I. Secondary sensory neurons are classified according to whether their peripheral processes terminate in somatic structures (skin and joints), in special sensory structures (hearing, vestibular function, and taste), or from internal receptors in the large arteries (baroreceptors and chemoreceptors) or in the viscera (heart, lungs, stomach, etc). Brain stem lower motoneurons are classed as somatic if their axons innervate striated muscle (e.g., the muscles of the face, jaw, and tongue) and as parasympathetic if their axons
innervate peripheral ganglionic neurons which, in turn, project to viscera in the head, neck, thorax, or abdomen (e.g., the blood vessels in the head, the salivary glands, the heart, lungs, and the abdominal viscera). Somatic efferents are subdivided (because of embryological considerations) into medially situated general somatic efferent nuclei (innervating extraocular muscles and tongue) and more ventrolaterally situated special visceral efferents, innervating (striated) muscles of chewing, facial expression, and swallowing.
Traditionally, the peripheral parasympathetic cell bodies are designated as "postganglionic" even though they are located within ganglia. It is the axons which are postganglionic. It is best to refer to the parasympathetic cell bodies as final parasympathetic moto-neurons. The motoneurons in the brain stem can be referred to as parasympathetic preganglionic moto-neurons or, in the appropriate context, simply as brain stem parasympathetic motoneurons. The "preganglio-nic'' terminology is reasonable, but it derives from an earlier period when attention was focused much more on the periphery than on the brain.
Brain stem and forebrain neurons with inputs to parasympathetic motoneurons can be referred to as preparasympathetic motoneurons. The sacral region of the spinal cord also contains parasympathetic preganglionic motoneurons (sacral parasympathetic motoneurons) so that some of the brain stem prepar-asympathetic motoneurons have long axons descending to the sacral cord from different brain stem groups (e.g., Barrington's nucleus for control of micturition).
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