Midbrain

The midbrain, also known as the mesencephalon, is the smallest, most cranial part of the brainstem. It is located between the diencephalon (which includes the thalamus and hypothalamus) and the pons.

On a transverse section, midbrain can be divided into three main regions: the crus cerebrum, the tegmentum (the ventral part) and the tectum (the dorsal part).

To demarcate the boundaries between these regions, you need to know two important structures:

• The Substantia Nigra: A key pigment-stained structure involved in movement control through the release of dopamine, affecting the basal ganglia. It has two parts: pars reticulata (with GABAergic neurons) and pars compacta (with dopaminergic neurons).

• The cerebral aqueduct which allows cerebrospinal fluid (CSF) to flow from the third ventricle to the fourth ventricle.

The three sub-regions of the midbrain are:

• Crus Cerebri: (in front of the substantia nigra).

• Midbrain tegmentum: (between substantia nigra and cerebral aqueduct), and

• Tectum: (behind the cerebral aqueduct).

The part of the ventricular system which is visible in the midbrain is called the cerebral aqueduct which contains CSF and interconnects the third and fourth ventricles. A Periaqueductal Gray (PAG) region surrounds the cerebral aqueduct, which is crucial for pain modulation and autonomic function.

Various important nuclei in the midbrain include:

• The superior and inferior colliculi, involved in visual processing and reflexes, while the inferior colliculi are involved in auditory processing.

• Substantia nigra,

• Cranial nerve nuclei (including trochlear, oculomotor, and Edingar Westphal nuclei),

• the red nucleus (Involved in motor coordination and movement regulation.),

• Mesencephalic nuclei of trigeminal nerve,

The motor tracts include:

• the corticospinal and corticonuclear tracts,

• frontopontine and temporopontine fibers,

• the medial longitudinal fasciculus, and

• the decussating fibers of the red nucleus.

The important sensory tracts include:

• the medial lemniscus,

• spinal lemniscus,

• lateral lemniscus, and

• trigeminal lemniscus.

• And the decussating fibers of the superior cerebellar peduncles.

Lastly, a network of neurons is found in the tegmentum region called the reticular formation, implicated in arousal, sleep, pain modulation, and visceral (including cardiovascular) control. It includes structures like the serotoninergic raphe nuclei and reticular nuclei.