Boundaries and landmarks
Externally, the pons appears as a prominence. Its lateral boundary is conventionally considered to be a line passing through the exit points of the roots of the trigeminal (nervus trigeminus, cranial nerve V) and facial (nervus facialis, cranial nerve VII) nerves.
Lateral to this imaginary line, the pons transitions into the middle cerebellar peduncle (pedunculus cerebellaris medius), which connects it to the cerebellum. Through these peduncles, pontocerebellar fibers (fibrae pontocerebellares) run from the pontine nuclei to the cerebellar cortex.
Internal structure: pathways
The tracts of the pons are divided into longitudinal and transverse.
- Transverse fibers (fibrae pontis transversae) form the bulk of the anterior part of the pons.
- Longitudinal fibers (fibrae pontis longitudinales) belong to the pyramidal system. They include the corticospinal, corticonuclear, and corticopontine tracts.
Among the descending extrapyramidal tracts passing through the pons, the rubrospinal tract (tractus rubrospinalis) stands out. It originates in the midbrain, descends through the pons, and is responsible for muscle tone. The pontoreticulospinal tract (tractus pontoreticulospinalis) originates from the pons's own reticular formation.
Trapezoid body and lemnisci
At the border between the anterior and posterior parts of the pons lies the trapezoid body (corpus trapezoideum). It consists of transverse fibers originating from the anterior cochlear nucleus. These fibers partially decussate, terminate in the trapezoid body's own nuclei, or continue into the lateral lemniscus (lemniscus lateralis).
Cranial nerve nuclei and reticular formation
The posterior part of the pons contains the nuclei of four pairs of cranial nerves:
- Trigeminal nerve (cranial nerve V)
- Abducens nerve (cranial nerve VI)
- Facial nerve (cranial nerve VII)
- Vestibulocochlear nerve (cranial nerve VIII, partially)
In addition, the tegmentum contains the reticular formation (formatio reticularis), which gives rise to important descending tracts regulating muscle tone and autonomic functions.
Briefly: answers to topical questions
Which specific nuclei of the trigeminal nerve (cranial nerve V) are located in the pons?
The pons contains the motor nucleus and two sensory nuclei of the trigeminal nerve.
- Motor nucleus (nucleus motorius n. trigemini) — located in the middle part of the pons, medial and anterior to the sensory nuclei.
- Pontine (main sensory) nucleus (nucleus pontinus n. trigemini / nucleus principalis n. trigemini) — lies in the posterior part of the pons, responsible for tactile sensation (touch).
- Spinal nucleus (nucleus spinalis n. trigemini) — extends from the pons into the medulla oblongata and upper cervical segments, conducting pain and temperature sensation.
Which nuclei of the facial nerve (cranial nerve VII) lie in the tegmentum of the pons?
The motor nucleus of the facial nerve is located in the tegmentum of the pons.
- Motor nucleus (nucleus nervi facialis) — lies in the ventrolateral part of the pontine tegmentum (within the reticular formation).
Other nuclei of the cranial nerve VII complex are also located in the posterior part of the pons (corresponding to the tegmentum):
- Superior salivatory nucleus (nucleus salivatorius superior) — a parasympathetic (secretory) nucleus, including the lacrimal nucleus (nucleus lacrimalis).
- Nucleus of the solitary tract (nucleus tractus solitarii) — a sensory nucleus responsible for taste sensation.
Which descending extrapyramidal tracts pass through the pons?
The following descending extrapyramidal pathways pass through the pons or are associated with its structures according to literature:
- Rubrospinal tract (tractus rubrospinalis) — originates from the red nucleus of the midbrain, and after decussating in the midbrain, descends through the pons and medulla oblongata.
- Rubropontine tract (tractus rubropontinus) — connects the red nucleus with the motor nuclei of the pons.
- Pontoreticulospinal tract (tractus pontoreticulospinalis) — originates from the cells of the pontine reticular formation, does not decussate, and passes through the medulla oblongata into the anterior funiculus of the spinal cord.
Class and exam questions
Where is the lateral border of the pons located?
It is defined by a line drawn through the exit points of the roots of the V (trigeminal) and VII (facial) cranial nerves.
Which fibers form the trapezoid body?
The trapezoid body is formed by transverse fibers from the anterior cochlear nucleus (nucleus cochlearis ventralis).
What is the difference between longitudinal and transverse fibers of the pons?
The transverse fibers form the bulk of the anterior part, while the longitudinal fibers belong to the pyramidal system (including the corticospinal and corticonuclear tracts).
What else to study on this topic
- Corticopontine tracts and their origin from various cortical lobes
- Connections of the vestibular nuclei with the cerebellum and spinal cord
- Pyramidal and extrapyramidal systems (general overview)
- Topography of the trigeminal and facial nerve nuclei in the brainstem
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