Brainstem

The brainstem connects the cerebrum/diencephalon to the spinal cord, comprising three parts (rostral to caudal): midbrain, pons, medulla oblongata: housing the majority of cranial nerve nuclei (III-XII), major ascending/descending tract relay and crossing points, and the reticular formation.

Cerebral peduncles:

• The paired, ventral bundles of descending fibres (corticospinal, corticobulbar, and corticopontine tracts) forming the midbrain's anterior surface, together with the substantia nigra, they form the crus cerebri; the term "cerebral peduncle" is sometimes used more broadly to encompass the crus cerebri, substantia nigra, and tegmentum collectively, though the narrower usage (crus cerebri alone) is common in clinical teaching.

Tectum:

• The midbrain's posterior (dorsal) surface, comprising the superior and inferior colliculi.

Superior colliculus:

• Involved in visual reflexes and the coordination of eye movements toward visual stimuli (saccadic eye movement control), receiving input from the retina and visual cortex.

Inferior colliculus:

• A key auditory relay station, receiving ascending auditory pathway input and relaying to the medial geniculate nucleus (thalamus) en route to the primary auditory cortex; also involved in auditory reflexes (e.g., the startle response to loud sound).

Substantia nigra:

• Located between the crus cerebri (anteriorly) and the tegmentum (posteriorly), divided into pars compacta (dopaminergic, nigrostriatal pathway) and pars reticulata (basal ganglia output).

Red nucleus:

• A prominent, reddish (iron-rich) nucleus in the midbrain tegmentum, giving rise to the rubrospinal tract, and receiving major input from the cerebellum (via the superior cerebellar peduncle) and the motor cortex, functionally significant in some mammals for limb flexor tone control, though its role in humans is considered comparatively less dominant than the corticospinal system.

Cerebral aqueduct:

• Runs through the midbrain's centre, connecting the third and fourth ventricles, surrounded by the periaqueductal grey matter (involved in descending pain modulation and autonomic/defensive behavioural responses).

Cranial nerves at midbrain level:

• CN III (oculomotor): Nucleus in the midbrain (at the level of the superior colliculus), emerging from the interpeduncular fossa.
• CN IV (trochlear): Nucleus in the midbrain (at the level of the inferior colliculus), the only cranial nerve to emerge from the brainstem's dorsal surface and the only one to fully decussate.

Basis pontis (ventral pons):

• The pons's bulky anterior portion, containing transverse pontocerebellar fibres (connecting to the cerebellum via the middle cerebellar peduncle), longitudinally coursing corticospinal/corticobulbar fibres passing through, and the pontine nuclei.

Pontine nuclei:

• Scattered grey matter nuclei within the basis pontis, receiving corticopontine fibres from the cerebral cortex and relaying (via pontocerebellar fibres, crossing the midline) to the contralateral cerebellar hemisphere, forming the cortico-ponto-cerebellar pathway, the major route by which the cerebral cortex informs the cerebellum of intended movements for coordination purposes.

Middle cerebellar peduncle:

• The largest of the three cerebellar peduncles, carrying the pontocerebellar fibres from the pontine nuclei to the cerebellum, a purely afferent (to the cerebellum) peduncle, unlike the other two peduncles which carry mixed afferent/efferent traffic.

Fourth ventricle relations:

• The pons (with the upper medulla) forms the floor of the fourth ventricle, while the cerebellum forms its roof, several cranial nerve nuclei create visible surface elevations on this floor (e.g., the facial colliculus, formed by the abducens nucleus with the looping facial nerve fibres around it).

Cranial nerves at pontine level:

• CN V (trigeminal): Main sensory and motor nuclei in the pons (with the spinal trigeminal nucleus extending down into the medulla/upper cervical cord, and the mesencephalic nucleus extending up into the midbrain).
• CN VI (abducens): Nucleus at the pontomedullary junction, its fibres looping around by the facial nerve's genu to form the facial colliculus on the fourth ventricle floor, a classic anatomical relationship explaining why a single small dorsal pontine lesion can affect both nerves together.
• CN VII (facial): Motor nucleus in the pons, with its fibres taking an unusual looping course (first passing posteromedially around the abducens nucleus, forming the internal genu of the facial nerve, before coursing anterolaterally to exit), this looping course explains the facial colliculus landmark.
• CN VIII (vestibulocochlear): Cochlear and vestibular nuclei at the pontomedullary junction.

Pyramids:

• Paired, ventral longitudinal ridges formed by the corticospinal tract fibres descending toward the spinal cord, the site of the pyramidal decussation at the medulla's lower end, where the majority (~85-90%) of corticospinal fibres cross to the contralateral side to form the lateral corticospinal tract.

Olives:

• Paired, oval swellings lateral to the pyramids, overlying the inferior olivary nucleus (a large, folded grey matter structure providing a major climbing fibre input to the contralateral cerebellum, important for motor learning and error correction).

Gracile tubercle:

• A small elevation on the posterior medulla surface, overlying the gracile nucleus (the first-order-to-second-order synapse point for the dorsal column-medial lemniscus pathway's lower limb/lower trunk fibres, the fasciculus gracilis).

Cuneate tubercle:

• A small elevation lateral to the gracile tubercle, overlying the cuneate nucleus (the equivalent synapse point for the upper limb/upper trunk fibres, the fasciculus cuneatus).

Inferior cerebellar peduncle:

• One of the three cerebellar peduncles, carrying predominantly afferent (to the cerebellum) fibres, including the posterior spinocerebellar tract, olivocerebellar fibres (from the inferior olivary nucleus), and vestibulocerebellar fibres.

Cranial nerves at medullary level:

• CN IX (glossopharyngeal), CN X (vagus), CN XI (accessory: cranial root component), CN XII (hypoglossal): All have their nuclei within the medulla.

• The brainstem carries essentially all the major ascending (sensory) and descending (motor) long tracts connecting the cerebrum to the spinal cord, running through it in a broadly consistent anatomical arrangement at each level (though the precise position shifts somewhat rostrocaudally): corticospinal/corticobulbar fibres run ventrally (cerebral peduncles/basis pontis/pyramids); the medial lemniscus (carrying dorsal column information after decussating in the lower medulla) runs in a paramedian position; the spinothalamic tract runs more laterally; and cranial nerve nuclei are positioned according to their embryological column of origin (motor nuclei more medial/ventral, sensory nuclei more lateral/dorsal), this consistent, level-specific arrangement is the anatomical basis for the classic "crossed" brainstem syndromes (ipsilateral cranial nerve deficit combined with contralateral long tract signs), a hallmark localising pattern distinguishing a brainstem lesion from a lesion at any other level of the nervous system.