Viii

Dorsal and ventral cochlear nuclei; medial, lateral, superior, and spinal vestibular nuclei

General cutaneous from posterior one-third of head and rest of body

Dorsal root ganglia

Spinal dorsal roots

Gracile and cuneate nuclei

gustation

Geniculate VII, petrosal IX, nodose X

VII, IX, X

nTS, spinal nucleus ofV, and paratrigem-inal islands

Baro- and chemoreceptors, receptors in heart, lung, and abdominal organs

Petrosal IX, nodose X

IX, X

nTS, spinal nucleus of V, and paratrigeminal islands

(continues)

(continues)

Table I (continued)

Interneurons, including premotor neurons

Type of neuron

Brain stem nucleus

Interneurons with no primary afferent input and no direct projections to motoneurons

Premotor cells for cranial somatic motoneurons

Premotor cells for cervical (phrenic) and thoracic spinal respiratory neurons

Presympathetic motoneurons

Preparasympathetic motoneurons (cranial outflow)

Preparasympathetic motoneurons (sacral spinal outflow)

Premotor cells for hypothalamic magnocellular neurons

Inferior olive, lateral reticular nucleus, cerebellar nuclei, vestibular nuclei, nuclei pontis, arcuate nuclei, superior olive, nucleus intercalatus, prepositus hypoglossi, locus coeruleus and subcoeruleus, dorsal tegmental nuclei, A7 catecholamine cells, parabrachial, Kolliker-Fuse, pedunculopontine and cuneiform nuclei, PAG

Ventral pontine nuclei (relay nuclei between cerebral cortex and cerebellum), near various regions of the pons and medulla, many still undefined; neurons loosely scattered between fibers without formation of defined nuclei are referred to as the "reticular formation''

Some Kolliker-Fuse and parabrachial neurons, Botzinger complex neurons, rostral inspiratory and more caudal expiratory neurons in the ventrolateral medulla, some nucleus tractus solitarius; raphe magnus, parapyramidal and more caudal raphe neurons

Paraventricular nucleus of hypothalamus, A5 catecholamine cells, C1 catecholamine cells and other intermingled noncatecholamine cells, raphe magnus, parapyramidal and more caudal raphe neurons

See details in text

Raphe and parapyramidal nuclei, rostral ventrolateral medulla, A5 region, Barrington's nucleus, and paraventricular and preoptic nuclei of the hypothalamus

A1 and A2 catecholamine-synthesizing neurons, and possibly some midbrain raphe neurons

"General somatic efferents. ^Special visceral efferents.

The medial longitudinal faciculi are paired paramid-line tracts containing axons which interconnect cranial nerve nuclei, especially those concerned with horizontal eye movements.

III. BLOOD SUPPLY AND CEREBROSPINAL FLUID CIRCULATION: BLOOD-BRAIN BARRIER, AREA POSTREMA, AND VENTRAL SURFACE OF THE MEDULLA

The brain stem, up to the level of the rostral midbrain, is supplied by the single midline basilar artery, formed by union of the paired vertebral arteries. At its rostral extent, the basilar artery bifurcates into paired posterior cerebral arteries which continue on to supply the medial temporal lobe and the occipital poles of the cerebral hemispheres. This knowledge is very important in clinical practice because patients with brain stem ischemic stroke (vertebrobasilar ischemia) present with a clinical picture (see Section V) quite different from that displayed by patients with ischemia in forebrain territory supplied by the paired internal carotid arteries. The posterior communicating arteries connect the vertebrobasilar arterial supply with the internal carotid supply, forming part of the circle of Willis.

Cerebrospinal fluid (CSF) is filtered from the blood by the choroid plexus in the lateral, third, and fourth ventricles of the brain. Fluid secreted in the forebrain descends through the midbrain aqueduct and enters the large dorsal brain stem pool bounded anteriorly by the dorsal surface of the medulla oblongata and the pons (known as the floor of the fourth ventricle) and posteriorly by the cerebellum and by special folds of the meninges. These folds have openings (formamina of Luschka and Magendie) through which CSF can pass en route to the subarachnoid space surrounding the brain and to the dorsally positioned arachnoid villae which absorb the CSF back into the bloodstream. From the fourth ventricle, the CSF also passes caudally down the central canal of the spinal cord. Occlusion of the aqueduct or of the foramina of Luschka and Magendi obstructs the flow of CSF, causing increased pressure in rostral portions of the system so that the forebrain ventricles enlarge. This condition is known as hydrocephalus and it may be associated with drowsiness or coma.

The "inside" of the brain (the brain parenchyma and the cerebrospinal fluid) is chemically isolated from the "outside" of the brain (the cerebral blood vessels). A particular configuration of the basement membranes in the small cerebal blood vessels results in a blood-brain

Table II

Cranial Nerves

Nerve No. Nerve name" Site for CNS motoneurons CNS site for termination of primary sensory axons

I Olfactory (S) Forebrain

II Optic (S) Forebrain (thalamus)

III Oculomotor (M) Dorsomedial midbrain —

IV Trochlear (M) Dorsomedial midbrain —

V Trigeminal (S, M) Pons Pons, medulla, and upper cervical spinal cord

VI Abducent (M) Dorsomedial pons —

VII Facial (S, M) Ventrolateral pons Nucleus tractus solitarius

VIII Vestibulocochlear (S) Vestibular nucleus in dorsolateral medulla

IX Glossopharyngeal (S, M) Ventrolateral pons and nucleus ambiguus Nucleus tractus solitarius

X Vagus (S, M) Dorsal motor nucleus of the vagus and Nucleus tractus solitarius nucleus ambiguus

XI Accessory (M) Nucleus ambiguus and upper spinal cord —

XII Hypoglossal (M) Hypoglossal nucleus in dorsomedial —

medulla aM, motor; S, sensory.

barrier that protects the brain from circulating agents which might interfere with normal neurotransmitter action. One particular group of cells on the dorsal aspect of the medulla oblongata forms the area postrema, a specialized circumventricular organ which samples the blood side of the blood-brain barrier and sends action potential messages to the brain, with axons of the area postrema neurons projecting widely within the brain stem, particularly to the parabrachial nuclei.

Cells lining certain portions of the ventral surface of the medulla oblongata, either glial cells or neurons, may be sensitive to different properties of the CSF, especially its hydrogen ion concentration (pH). Some theories of the neuropathological mechanism underlying sudden infant death syndrome (cot death) postulate that the normal sensitivity of these cells is somehow reduced so that when the airway becomes obstructed, affected infants fail to arouse, to turn their head, and to increase their breathing in the normal manner.

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