Brainstem

Anatomical position of the brainstem

The brain (encephalon) is functionally divided into four levels. While the first level (cerebral cortex) is responsible for logical thinking and memory, the second (basal nuclei) for muscle tone regulation and involuntary movements, and the third (limbic system) for emotions and endocrine regulation, the brainstem occupies the basic, fourth level.

It is here that the centers regulating autonomic functions are located, and major ascending and descending pathways pass through. In addition, the brainstem is anatomically and functionally connected to the cranial nerves (nervi craniales). Before exiting the cranial cavity through specialized foramina, these nerves are accompanied by meninges, and their nuclei are localized directly within the brainstem structures.

Topography of cranial nerve nuclei

The brainstem is traditionally divided into three parts. Important topographical landmarks in this region also include adjacent structures: the thalamus (thalamus), optic tract, subcortical visual centers (superior colliculi), and auditory centers (inferior colliculi and medial geniculate body).

Midbrain (Mesencephalon) This region contains structures associated with the oculomotor apparatus and proprioception:

• CN III (oculomotor nerve): has a motor nucleus, emerges on the medial surface of the cerebral peduncle.
• CN IV (trochlear nerve): motor nucleus lies at the level of the inferior colliculi. This is the only nerve that emerges on the dorsal surface of the brainstem (wrapping around the cerebral peduncle).
• CN V (trigeminal nerve): houses its mesencephalic nucleus (proprioceptive sensitivity).

Pons (Pons) The motor and sensory roots of the trigeminal nerve emerge from the lateral surface of the pons.

• CN V: includes the motor (masticatory) and main sensory nuclei (touch).
• CN VI (abducens nerve): motor nucleus is hidden deep within the facial colliculus.
• CN VII (facial nerve): contains motor, lacrimal, and superior salivatory (parasympathetic) nuclei. The fibers of this nerve form the genu of the facial nerve (genu n. facialis), looping around the nucleus of CN VI.
• CN VIII (vestibulocochlear nerve): vestibular and cochlear nuclei project into the lateral angles of the rhomboid fossa.

Medulla oblongata (Medulla oblongata)

• CN IX (glossopharyngeal nerve): includes the inferior salivatory nucleus.
• CN X (vagus nerve): contains the dorsal motor nucleus (parasympathetic).
• CN XI (accessory nerve): has its own motor nucleus with a spinal component.
• CN XII (hypoglossal nerve): motor nucleus is located medially, and the nerve itself emerges between the pyramid and the olive.

Additionally, the spinal trigeminal tract and nucleus of CN V, responsible for pain and temperature sensation of the face, run through the entire medulla oblongata.

Brainstem pathways

The brainstem is permeated by tracts connecting various parts of the nervous system.

Descending (motor) pathways Transmit impulses from higher centers to the spinal cord:

• Tractus rubrospinalis (Monakow's tract): originates from the red nucleus, undergoes the ventral tegmental decussation (Forel's decussation) in the midbrain. Belongs to the extrapyramidal system, controls muscle tone and automated movements.
• Tractus vestibulospinalis: runs from the vestibular nuclei (Deiters' nucleus), maintains balance.
• Tractus reticulospinalis: pathways from the reticular formation of the pons and medulla oblongata for regulating reflexes and autonomic functions.
• Fibrae corticopontinae: descending fibers from the cortex to the pontine nuclei.

Ascending (sensory) pathways

• Fasciculi gracilis and cuneatus (columns of Goll and Burdach): convey conscious proprioceptive and tactile sensitivity, terminating in the nuclei of the same name in the medulla oblongata.
• Tractus spinalis n. trigemini: carries pain and temperature information.
• Tractus spinoolivaris: transmits information from the spinal cord to the olivary nuclei.

Connections with the cerebellum The cerebellum actively exchanges signals with the brainstem. It receives the anterior spinocerebellar tract (Gowers' tract, via the superior cerebellar peduncles) and posterior spinocerebellar tract (Flechsig's tract, via the inferior cerebellar peduncles), pontocerebellar fibers, as well as pathways from the olivary nuclei (tractus olivocerebellaris), vestibular nuclei, and gracile and cuneate nuclei (via external arcuate fibers). Efferent signals return via the dentatothalamic tract (tractus dentatothalamicus) to correct voluntary movements, as well as to the red nucleus (tractus cerebellorubralis).

Class and exam questions

What else to study on this topic

• Detailed anatomy of cerebellar connections (afferent and efferent pathways)
• Topography of the rhomboid fossa and projection of vestibular nuclei
• Autonomic (parasympathetic) nuclei of cranial nerves
• Subcortical auditory and visual centers (colliculi and geniculate bodies)
• Reticular formation: pontine and medullary pathways

Other topics in the "CNS" section

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