Brainstem
Truncus encephali
- Latin synonym: Truncus encephalicus
- Related terms: Brain stem
Definition
The brainstem is a critical, stalk-like structure situated at the base of the cerebral hemispheres. It serves as the primary conduit for communication between the cerebrum and the rest of the body. Connecting the cerebrum to the spinal cord and cerebellum, the brainstem regulates essential functions such as breathing, consciousness, blood pressure, heart rate, and sleep. Think of the brainstem as the brain's power supply: if the brain hemispheres were a computer monitor, the brainstem would be the power cable—any damage, however minor, can lead to significant neurological impairments due to its indispensable role.
Parts of the brainstem:
The brainstem is anatomically divided into three main sections:
Situated at the superior end where it interfaces with the thalamus and hypothalamus at the midbrain-diencephalic junction. | |
Located below the midbrain, connecting with it at the ponto-mesencephalic junction. | |
Positioned below the pons, connected at the ponto-medullary junction, and extends to the cervico-medullary junction at the level of the foramen magnum. On the dorsal surface of the pons and upper medulla oblongata lies the cerebellum. |
Cranial Nerves:
Of the 12 pairs of cranial nerves, 10 emerge from the brainstem. The trochlear nerve (CN IV) is unique as it originates from the dorsal midbrain, while the others emerge from the brainstem’s ventral aspect. Here’s an overview of their origins from various parts of the brainstem and their functions:
Oculomotor (CN III) and Trochlear (CN IV): Crucial for eye movement. | |
Trigeminal (CN V): Facilitates facial sensation and chewing. | |
Junction of pons and medulla | Abducent (CN VI), Facial (CN VII), and Vestibulocochlear (CN VIII): CN VII controls facial expressions; CN VIII manages hearing and balance. |
Glossopharyngeal (CN IX), Vagus (CN X), and Accessory (CN XI): These nerves support functions like laryngeal, pharyngeal, and gastrointestinal activity, with the accessory nerve also serving muscles such as the trapezius and sternocleidomastoid. Hypoglossal (CN XII): Manages tongue movement. |
Functional Groups of the Brainstem:
The brainstem’s critical components are grouped into four functional areas:
Comprise motor and sensory nuclei and associated structures. | |
Long Tracts | Include corticospinal, corticobulbar, and other descending pathways, essential for somatomotor and autonomic functions. |
Cerebellar Circuitry | Encompasses cerebellar peduncles and additional nuclei such as the pontine and red nuclei. |
Features widespread projection systems (cholinergic, noradrenergic, serotonergic, and dopaminergic) that influence consciousness and functions like sleep regulation, and key nuclei like the periaqueductal gray and rostral ventral medulla that aid in pain modulation. |
Ventral Aspect Features:
The brainstem’s ventral surface showcases several important features:
Features cerebral peduncles and the interpeduncular fossa with perforations for small arteries. | |
Distinguished by transverse pontocerebellar fibers and a groove accommodating the basilar artery. | |
Notable for the anterior median fissure, lateral pyramids, and olivary nuclei. |
Dorsal Aspect Features:
The brainstem’s dorsal surface includes:
Four colliculi (two superior and two inferior) critical for visual and auditory processing. | |
Visible structures include the medial eminence, facial colliculus, and sulcus limitans, with adjoining cerebellar peduncles. | |
Contains the posterior median sulcus, gracile and cuneate tubercles, and sensory pathways like the fasciculus gracilis and fasciculus cuneatus. |
Please note that this was an overview of the brainstem anatomy and functions. The details for individual structures will be described in more detail on their respective pages.
References
Basinger H, Hogg JP. Neuroanatomy, Brainstem. [Updated 2023 Jul 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK544297/
Blumenfeld, H. (2010). ‘Chapter 12: Brainstem I: surface anatomy and cranial nerves’, in Neuroanatomy through clinical cases. (2nd ed.) Sunderland, Mass.: Sinauer Associates, pp. 459–464.
Blumenfeld, H. (2010). ‘Chapter 14: Brainstem III: Internal structures and vascular supply’, in Neuroanatomy through clinical cases. (2nd ed.) Sunderland, Mass.: Sinauer Associates, pp. 575–578.
