The rhombencephalon refers to the caudal area of the neural tube, a structure from which the brain and spinal cord originate during early stages of human embryonic development. Also called the hindbrain, the rhombencephalon gives rise to the myelencephalon and metencephalon regions, forming the fourth ventricle and a large portion of the brainstem primarily responsible for vegetative functioning such as blood pressure and heart rate. Damage to the hindbrain produces autonomic nervous system failure, uncoordinated and delayed motor responses, as well as cognition difficulties.
Myelencephalon structures encompass the medulla oblongata, the bottom part of the fourth ventricle and the ninth through twelfth cranial nerves. Located at the lower-most end of the hindbrain sits the medulla oblongata, a pyramid-shaped organ chiefly responsible for regulation and control of cardiovascular and respiratory system activities, including heartbeat, breathing, and blood pressure. Cranial nerves nine and ten, the glossopharyngeal and vagus, respectively, help control blood pressure within the carotid arteries and the aorta, the largest artery of the body through which oxygenated blood moves into systemic circulation. While the accessory nerve is needed for movement of the head, such as in turning and nodding, the twelfth cranial nerve, the hypoglossal, allows use of the tongue.
Two organs develop from the metencephalon: the cerebellum and pons. Resembling the larger cerebrum, the cerebellum provides over half of the brain's nerve cells and is necessary for posture and movement coordination. Situated beneath the midbrain lie bundles of neuronal tracts referred to as the pons, creating a bridge connecting the cerebellum and medulla to the midbrain. This section of the rhombencephalon includes part of the fourth ventricle, belonging to a group of interconnected cavities containing cerebrospinal fluid (CSF), serving as the central nervous system's protection against harmful substances, trauma, and excessive pressure. The pons also plays a role in sleeping and patterns of arousal through the reticular formation, a network of nuclei.
A rare brain anomaly affecting certain newborns, referred to as rhombencephalosynapsis, is a condition resulting in a fused cerebellum in which the vermis, a rhombencephalon structure separating the cerebellar hemispheres, is absent at birth. Characterized by muscular weakness, rapid involuntary eye movement, head tremors, and an unsteady gait, this disorder causes developmental delays in motor skills such as standing and walking. Cognitive and behavior problems are also common, but depend on the extent of involvement. Children with rhombencephalosynapsis are sometimes born with physical defects of the hands in which fingers are fused together or are absent. Life expectancy is typically much shorter, as those suffering from the disease die during childhood.