Corpus: Brain stem
from Latin: truncus - trunk; cerebrum - brain
Synonym: truncus cerebri, brainstem
1. Definition
The brain stem is the part of the brain that lies below the diencephalon, excluding the cerebellum. It includes:
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain) with the pons (bridge) and medulla oblongata (myelencephalon)
2. Embryology
The brain stem consists of all parts of the brain that develop from the 2nd and 3rd cerebral vesicles. This definition includes the cerebellum, though it is not typically considered part of the brain stem. The 2nd cerebral vesicle becomes the mesencephalon, and the 3rd cerebral vesicle becomes the rhombencephalon, which forms the parietal flexure in the 6th week and divides into the metencephalon and myelencephalon.
3. Function
The brain stem contains pathways for both protopathic and epicritic sensitivity, cerebellar lateral cord pathways, and pathways of the pyramidal and extrapyramidal systems. It regulates, coordinates, and modulates essential vital functions, such as controlling heart rate and respiration, and is necessary for brain stem reflexes (e.g., eyelid closure reflex, cough reflex).
4. Anatomy
The brain stem can be divided into three longitudinally oriented levels from ventral to dorsal, particularly in the mesencephalon area:
4.1. Base of cerebral peduncles
The base of the cerebral peduncles and the tegmentum are collectively known as the cerebral peduncle. The base of the cerebral peduncles consists of white matter in the form of descending motor pathways that collect in the cerebral peduncles of the mesencephalon, the anterior part of the pons, and the pyramid of the medulla oblongata. The substantia nigra is situated at the center of the mesencephalon.
4.2. Tegmentum
The tegmentum contains pathways that interconnect within the brain stem and connect to ascending sensory pathways (e.g., medial lemniscus). Between the pathways is grey matter in the form of nuclei. The nuclei of the oculomotor nerve (III), trochlear nerve (IV), parts of the nuclei of the trigeminal nerve (V), and the red nucleus are located in the mesencephalon. The remaining cranial nerve nuclei (V to XII) are located in the pons and medulla oblongata. Functionally, the origin and end nuclei of cranial nerves III-XII are assigned to the peripheral nervous system. Some core areas are parasympathetic nuclei whose fibers run with the corresponding cranial nerves (e.g., accessory nucleus of the oculomotor nerve, superior salivatory nucleus, inferior salivatory nucleus, dorsal nucleus of the vagus nerve).
The cranial nerve nuclei are arranged in a columnar and topical-functional manner, corresponding to the distribution pattern of cell migration after neurulation, starting from the base and wing plate of the neural tube. Another grey matter area in the medulla oblongata is the inferior olivary complex. The reticular formation is the reticular grey matter in the entire brain stem, responsible for the control of attention and alertness (ascending reticular activating system, ARAS), circulation, respiration, and as a vomiting center.
4.3. Tectum
The tectum includes structures such as the quadrigeminal plate and serves as the visual and auditory reflex center.
5. Histology
6. Histochemistry
6.1. Material deposits
The brain stem contains various nuclear areas, each with different chemical substances and neurotransmitters. Iron can be detected in different sections, particularly in the substantia nigra and pallidum using the Berlin blue reaction. Iron is stored as small particles in neurons and glial cells, with high iron content being characteristic of nuclear groups involved in the extrapyramidal motor system.
6.2. Neurotransmitters
The distribution of neurons and their neurotransmitters varies regionally within the brain stem. Catecholaminergic and serotoninergic neurons are prevalent in the tegmentum, while cholinergic nuclei are found mainly around the motor cranial nerve nuclei. Additionally, the pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus contain nitrergic neurons, which produce nitric oxide. Noradrenaline content is high in the tegmentum and lower in the tectum, while dopamine is primarily found in the substantia nigra.
6.3. Enzyme pattern
Enzyme distribution in the brain stem follows a specific pattern, with higher activity of oxidative enzymes in the grey matter than in the white matter. Enzyme activity is particularly high in the cranial nerve nuclei, lower olive, and pons nuclei.
7. Source
- 3D model: Dr Claudia Krebs (Faculty Lead) University of British Columbia