Internal Structure and Nuclei
The core of the midbrain contains specific collections of gray matter—nuclei—that facilitate both cranial nerve innervation and global movement coordination.
A key structure for the masticatory apparatus is the mesencephalic nucleus of the trigeminal nerve (nucleus mesencephalicus nervi trigemini). It lies within the periaqueductal gray matter, lateral to the cerebral aqueduct. The uniqueness of this nucleus lies in its composition of pseudounipolar neurons, which provide proprioceptive innervation to the muscles of mastication. This allows the body to precisely control bite force.
Additionally, elements of the extrapyramidal system are located within the brainstem structures, including the midbrain. The most significant of these is the red nucleus (nucleus ruber). It serves as an essential relay center regulating involuntary components of movement and muscle tone.
Extrapyramidal Pathways
The extrapyramidal system is a complex network formed by axons of basal nuclei and brainstem nuclei. The midbrain actively participates in its function.
Two major descending (efferent) tracts originate from the gray matter of the midbrain:
- Rubrospinal tract (tractus rubrospinalis, or Monakow's pathway). Originates in the red nucleus and descends into the spinal cord, regulating skeletal muscle tone.
- Tectospinal tract (tractus tectospinalis). Originates from the midbrain roof (tectum mesencephali) and mediates involuntary reflex reactions in response to external stimuli.
For coordinated function, the red nucleus receives input from other regions of the central nervous system: corticorubral fibers (fibrae corticorubrales) from the cerebral cortex, and the cerebellorubral tract (tractus cerebellorubralis) from the cerebellum. Additionally, there is feedback to the thalamus via fibrae rubrothalamicae.
Transit Fibers of the Cerebral Peduncles
The massive white matter bundles on the ventral surface are the cerebral peduncles (pedunculi cerebri). Powerful descending tracts of the pyramidal system, responsible for conscious, voluntary movements, pass through them.
Specifically, the pyramidal tract (tractus pyramidalis) descends from the cortex through the corona radiata and internal capsule, after which fibers (fibrae corticospinales) run in a dense bundle through the cerebral peduncles. They then continue through the pons and into the medulla oblongata, forming distinct bulges known as the pyramids before entering the spinal cord.
Furthermore, corticonuclear fibers (fibrae corticonucleares) descend from the cortex through the brainstem. They transmit impulses to the motor cranial nerve nuclei located within the brainstem structures.
Afferent Pathways
The midbrain also serves as a destination for sensory (afferent) pathways. Ascending tracts are formed by axons of dorsal root ganglion cells or sensory nuclei of the spinal cord.
These pathways, carrying information from extero-, proprio-, and interoreceptors, ascend through the spinal cord. Some fibers cross the midline (decussate), while others remain ipsilateral. As impulses ascend through the brainstem (including the medulla, pons, and rhomboid fossa structures), they are distributed between the cerebellum, thalamus, and midbrain tectum. The third-order neurons of sensory pathways reside in the thalamus, from which information projects via thalamic radiations (radiatio thalami) to the respective cortical sensory areas.