The hindbrain, or RHOMBENCEPHALON, consists of the PONS, CEREBELLUM and MEDULLA OBLONGATA. This term is often encountered in the context of the DEVELOPMENT of the CENTRAL NERVOUS SYSTEM. In contrast, the BRAINSTEM also includes the MESENCEPHALON and DIENCEPHALON. The hindbrain develops from the third of the three primary vesicles of the early embryonic brain. As the brain develops to the five-vesicle stage, the rhombencephalon differentiates into the METENCEPHALON rostrally and MYELENCEPHALON caudally. In the adult brain, the metencephalon includes the cerebellum and pons while the myelencephalon becomes the medulla oblongata.
During development of the hindbrain, the central canal of the embryonic VENTRICULAR SYSTEM enlarges to form the fourth ventricle. In the course of this development, the fourth ventricle opens and expands laterally with the consequence that sensory nuclei of the CRANIAL NERVES of the hindbrain move to a lateral rather than the dorsal position seen in the SPINAL CORD. Similarly, the motor nuclei assume a medial rather than ventral position. This developmental process is a major feature critical to an understanding of the topography and organization of cranial nerve nuclei in the hindbrain. During this process, the roof of the fourth ventricle becomes very thin as it is formed by a layer of ependyma and PIA MATER. The cerebellum eventually expands to lie over and cover the roof of the ventricle in the adult.
Three large fibre tracts, the INFERIOR, MIDDLE and SUPERIOR CEREBELLAR PEDUNCLES attach the cerebellum to the pons. The inferior cerebellar peduncle carries afferent axons from the spinal cord and the INFERIOR OLIVE, the middle cerebellar peduncle carries afferent axons from the pontine nuclei of the pons, and the superior cerebellar peduncle carries efferent axons from the cerebellum to the RED NUCLEUS of the mesencephalon and to the THALAMUS.
The fibres of the middle cerebellar peduncle gather on the ventral (anterior) and lateral surface of the PONS, forming a striking white, bridge-like elevation (pons is Latin for ‘bridge’). The PONTINE NUCLEI, a large collection of neurons giving rise to projections to the contralateral cerebellum, lie deep to the ventral fibres of the middle cerebellar peduncle. The pontine nuclei and their efferent axons occupy roughly half of the pons, while the gray matter dorsal to them forms the PONTINE TEGMENTUM which contains the major cranial nerve nuclei of the pons. The darkly-pigmented ADRENERGIC cells of the LOCUS COERULEUS are found in the dorsal pontine tegmentum. The locus coeruleus has extensive connections with higher levels of the nervous system and has been implicated in many physiological functions related to ATTENTION and EMOTION as well as in the pathophysiology of ANXIETY and DEPRESSION. The COCHLEAR NUCLEUS and VESTIBULAR NUCLEUS are found dorsolaterally while the NUCLEUS ABDUCENS, a motor nucleus, is found medially. The motor TRIGEMINAL NUCLEUS and FACIAL NUCLEUS are located ventrolaterally, consistent with their embryological origin as special visceral efferent neurons which migrate to a ventrolateral position similar to that of the more caudally located special visceral efferent neurons of the NUCLEUS AMBIGUUS in the medulla oblongata. The white matter of the pons is composed of the cerebellar peduncles and longitudinally oriented ascending (for instance, the MEDIAL LEMNISCUS, SPINAL LEMNISCUS, MEDIAL LONGITUDINAL FASCICULUS) and descending (for instance, the CORTICOSPINAL TRACT, medial longitudinal fasciculus) pathways connecting other levels of the NEURAXIS. The medulla oblongata has similarities to the pons in that it contains a large ventrally located precerebellar relay nucleus, the inferior olive, that is closely related both in development and connections to the pontine nuclei. The major medullary somatosensory nuclei are the dorsal column nuclei, which relay information about proprioception and fine touch from the body to the contralateral ventral posterolateral thalamic nucleus, and the spinal trigeminal nucleus, which mediates PAIN and TEMPERATURE sensations from the face and relay it to the contralateral ventral posteromedial nucleus of the thalamus. The NUCLEUS OF THE SOLITARY TRACT relays visceral information to higher levels of the neuraxis. The core of the hindbrain contains the pontine and medullary reticular formations which give rise to widespread ascending and descending projections. Ascending projections, particularly from the pontine reticular formation, are considered to represent part of the anatomical substrate for the ASCENDING RETICULAR ACTIVATING SYSTEM.
DAVID A.HOPKINS
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