Structural characteristics

1. Stretching of roof plate at this plane of medulla oblongata in embryonic life causes outward deviation (abduction) of alar plate. This results widening of central canal to form cavity of fourth ventricle. Stretched dorsal surface of medulla oblongata forms floor of fourth ventricle.
2. Central gray matter presenting the cranial nerve nuclei pushed more dorsally to lie just beneath the dorsal surface of medulla oblongata.
3. Fibers from medulla oblongata which will connect cerebellum will form compact bundle of inferior cerebellar peduncle seen to be present in posterolateral part.
4. Bulge of olive containing inferior olivary nucleus is related to anterolateral and posterolateral sulci on it medial and lateral sides respectively.

Structural detail

1. On either side of ventral median fissure, beneath the bulge of pyramid, fibers of pyramidal (corticospinal) tract from compact bundle.
2. Ventromedial surface of pyramid presents a narrow semilunar strip of gray matter, called arcute nucleus. It is the detached part of pontine nuclei.
3. Lateral to pyramid, bulge of olive contains inferior olivary nucleus. It is irregular walled sac-like mass of gray matter. Open mouth of the sac faces medially and backwards. Adjacent to inferior olivary nucleus, medial and dorsal components of assessory olivary nucleus are seen.
4. Dorsolateral aspect of medulla oblongata at this level presents bulge of inferior cerebellar peduncle. It is a compact bundle of white matter connecting medulla oblongata with cerebellum in both directions.
5. Different fiber tracts in central core
   a) Medial: From before backwards close to the midline:
   
   1. Medial lemniscus: Formed by internal arcuate fibers, situated behind pyramid. This ascending tract passes upwards to reach thalamus.
   2. Tectospinal tract: Descending tract from tectum of midbrain to spinal cord.
   3. Medial longitudinal bundle: It is the fiber bundle connecting vestibular nucleus with motor nuclei of IIIrd, IVth, VIth and XIth cranial nerves.
   b) Lateral: Important ascending tracts passing from below upward:
   
   1. Spinal tract of trigeminal nerve.
   2. Ventral and lateral spinothalamic tracts.
   3. Ventral and dorsal spinocerebellar tracts.
6. Medullary part of brainstem reticular formation: Scattered nerve nuclei with reticulum (network) of fibers.
7. Cranial nerve nuclei (in central gray matter): Beneath the dorsal surface of medulla oblongata
   
   1. Hypoglossal nerve nucleus: Situated on either side of midline, just beneath the dorsal surface (on the floor of 4th ventricle) and behind medial longitudinal bundle.
   2. Dorsal nucleus of vagus: This nucleus is situated lateral to hypoglossal nerve nucleus. It is a mixed nucleus having general visceral efferent as well as afferent components which supply motor, secretomotor and sensory fibers to thoracic and abdominal viscera (upto midgut).
   3. Nucleus tractus solitarius: This is a composite nucleus of special visceral afferent column. It is ventrolateral to dorsal nucleus of vagus and receives taste sensation through sensory fibers of VIIth, IXth and Xth cranial nerves from anterior two-third and posterior one-third of tongue and also vallecula and epiglottis.
   4. Nucleus ambiguous: This nucleus is placed more ventrally. This is also a composite nucleus of special visceral efferent group which gives out motor fibers through IXth to XIth cranial nerves to supply muscles developed from mesoderm of IIIrd, IVth and VIth branchial arch respectively.
   5. Vestibular nucleus of VIIIth cranial nerve: It is proprioceptive type of special somatic afferent nucleus of vestibulocochlear nerve. It is present in lateral angle of dorsal surface of pontomedullary junction. Vestibular nucleus is made up of four parts–superior, inferior, lateral and medial.
   6. Cochlear nucleus of VIIIth cranial nerve: It is exteroceptive type of special somatic afferent nucleus of vestibulocochlear nerve. It is composed of dorsal and ventral components in close relation to inferior cerebellar peduncle.
   7. vii. Spinal nucleus (and spinal tract) of trigeminal nerve: It is situated medial to inferior cerebellar peduncle. Spinal tract is made up of bundles of those sensory fibers of trigeminal nerve which carry pain and temperature sensation from the skin of face. The fibers of spinal tract relay in cells of spinal nucleus of trigeminal nerve.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented) (2014)