The term brainstem refers to a large group of structures that extend from the spinal cord posteriorly to the hypothalamus and thalamus anteriorly. This is the part of the brain that is oldest phylogenetically—fish have many of the same structures in their brainstems! Although this part of the brain is dwarfed by the enormous cerebral cortex in the human brain, it contains groups of neurons that are absolutely vital for the proper functioning of the human body. A series of 5 papers was published in the Journal of Comparative Neurology in 1980 and 1981 showing the neurogenetic timetables throughout the brainstem. The timetables are shown as bar graphs, with the height of the bar indicating the proportion of neurons that are generated on a particular day of development; the combined height of all the bars for a particular structure always nearly equals 100%. The position of the bars shifts from left (early developing) to right (later developing) from one structure to another showing that some nuclei are generated early, others a little later, and still others even later. The timetables have been grouped according to function, and neurogenetic gradients often exist when comparing related structures; for example, note the shift in the bars for the lateral ventral tegmental area (VTl, early developing) and the medial ventral tegmental area (VTm, later developing) in the leftmost panel, bottom group in the midbrain figure just below. Exploring all of the links in the files below will give you a full accounting of the neurogenetic gradients that exist in the brainstem. No doubt, the precise timing of neuronal production is a necessary prerequisite for proper development of a normally functioning adult brain.
The midbrain has a thick floor, the tegmentum, and a roof, the tectum. Here we present the neurogenetic timetables of the various neuronal groups (nuclei) in the tegmentum. There are motor neurons associated with eye movements (III and IV). Other nuclei are concerned with visual functions (nucleus of Darkschewitsch, Edinger-Westphal nucleus, and the parabigeminal nucleus). The red nucleus relays information between the cerebellum and the motor thalamus—to influence output from the motor cortex. The interpeduncular nucleus is closely associated with the habenula. The ventral tegmental area and both parts of the substantia nigra are closely associated with the basal ganglia. The raphe nuclei in the midbrain tegmentum are the most rostral extensions of raphe neurons in the medulla and pons. The central gray surrounds the narrow cerebral aqueduct and is associated with visceral functions.
The pons lies between the midbrain tegmentum and the anterior medulla. It is noted for a prominent bend (the pontine flexure) toward its anterior end. The pontine gray nuclei are prominent accumulations of neurons attached to the exterior part of that flexure. The development of the pontine gray is presented as part of the precerebellar system (see the cerebellar development page). Here, we deal with the development of other nuclei in the pons, mainly the trigeminal nuclei (associated with cranial nerve V), nuclei of the lateral lemniscus (associated with cranial nerve VIII), raphe nuclei, dorsal and ventral tegmental nuclei, the locus coeruleus, the parabrachial nucleus, and the pontine reticular formation.
The medulla is a posterior continuation of the pons. This part of the brain contains motor output nuclei of cranial nerves VI, VII, IX, X, XI, and XII. It contains sensory processing nuclei for cranial nerves V and VIII. The inferior olive, lateral reticular, and external cuneate nuclei are discussed in the cerebellar development page. Besides that, it contains a large raphe system and a prominent reticular formation that are concerned with the maintenance of vital bodily functions.