Olfactory nerve

Internal structure of the brainstem

The brainstem is a crucial transit and integrative hub of the central nervous system. A vast number of pathways pass through it:

• Ascending and descending pathways: they act as communication cables connecting the brain to peripheral regions.
• Fiber decussations: a significant portion of neural pathways crosses to the opposite side.
• Synaptic relays: some pathways do not simply pass through, but synapse onto the cell bodies of subsequent neurons.

The substance of the brainstem contains numerous accumulations of gray matter known as nuclei. These include cranial nerve nuclei (pairs III–XII) as well as structures forming motor feedback regulatory loops. The latter include the red nucleus and substantia nigra (located in the midbrain), the pontine nuclei, and the olivary nuclei (in the medulla oblongata). The superior and inferior colliculi of the tectum of the midbrain serve as subcortical visual and auditory centers acting as intermediate relay stations for impulses.

Reticular formation

A special place in the anatomy and physiology of the brainstem is occupied by the reticular formation. This is a diffuse network of neurons that permeates virtually the entire brainstem and performs three fundamental functions:

1. Autonomic regulation: centers ensuring vital processes—cardiac activity, blood circulation, and respiration—are concentrated in this zone.
2. Ascending activating influence: the formation continuously affects the cerebral cortex, ensuring and maintaining the state of wakefulness.
3. Descending influence: impulses descending from the reticular formation strictly regulate the activity of spinal cord motor neurons.

Clinical brainstem syndromes

Due to the colossal density of nuclei and pathways within a very limited volume of the brainstem, even a minimal focal injury provokes the simultaneous development of several neurological syndromes (e.g., vascular).

A most characteristic sign of brainstem pathology is alternating syndromes.

• Classical picture: cranial nerve dysfunction occurs on the side of the lesion (ipsilaterally), while limb muscle paralysis occurs on the opposite side (contralaterally).
• Special case: according to Russian medical literature, the term alternating hemiplegia is used specifically. It denotes a lesion of the corticospinal tract in the area of the pyramidal decussation, which manifests as armparesis on the side of the lesion and leg paresis on the opposite side. In other situations, the general term "alternating syndrome" is used.

Levels of cranial nerve lesions

Clinical manifestations of any cranial nerve dysfunction depend on the precise localization of the lesion. It is customary to distinguish four main levels of damage:

1. Supranuclear: descending pathways running from higher CNS centers (cerebral cortex) to the nerve nuclei in the brainstem are damaged.
2. Nuclear: the cranial nerve nuclei themselves are destroyed.
3. Fascicular: the fibers of the cranial nerve roots that are still inside the brainstem (prior to their exit from the brain tissue) are affected.
4. Peripheral: damage to the nerve trunk occurs after it has already left the brainstem.

Class and exam questions

What else to study on this topic

• Motor feedback regulatory loops of the brainstem
• Subcortical visual and auditory centers (tectum / corpora quadrigemina)
• Autonomic centers of the reticular formation
• Vascular brainstem syndromes
• Topography and functions of cranial nerve nuclei (pairs III–XII)

Other topics in the section "DeMyer's The Neurologic Examination" / "Duus' Topical Diagnosis in Neurology"

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