Cerebellum and brainstem

Upper and lower motor neurons

Below is a table showing the differences between UMN and LMN cells.

Cerebellum

The cerebellum helps maintain balance and posture, coordinates voluntary movements, and contributes to motor learning and cognitive functions such as language. It is part of the hindbrain and lies in the posterior cranial fossa.

It is divided into three lobes - anterior, posterior, and flocculonodular - along with a central vermis.

The cerebellar cortex has three layers:

• External molecular layer
• Middle Purkinje cell layer
• Internal granular layer

The molecular layer contains stellate and basket cells.

Climbing and mossy fibres are specific afferent fibre bundles in the cerebellum:

• Climbing fibres are terminations of the olivocerebellar tracts. They release the excitatory neurotransmitter aspartate to activate cerebellar Purkinje cells.
• Mossy fibres are the terminal branches of all other cerebellar afferents. They release the excitatory neurotransmitter glutamate to activate Purkinje cells.

Functional regions of the cerebellum include:

• The vermis, which is involved in movements of the trunk, neck, shoulder, thorax, abdomen, and hips
• The intermediate zone of the cerebellar hemispheres, which is involved in movements of the distal extremities
• The remaining lateral area of the cerebellar hemispheres, which is involved in planning sequential movements of the entire body and assessing movement errors

The deep cerebellar nuclei are fastigial, globose, emboliform, and dentate.

The cerebellum is supplied by the superior cerebellar artery, anterior inferior cerebellar artery (AICA), and posterior inferior cerebellar artery (PICA).

Midbrain

The midbrain, pons, and medulla collectively form the brainstem.

Key structures in the midbrain include:

• Pairs of superior and inferior colliculi
• Cerebral aqueduct
• Cerebral peduncles containing corticopontine, corticospinal, and corticobulbar tracts
• Substantia nigra, located in the peduncle

The trochlear and oculomotor nerves emerge from the midbrain. The interpeduncular fossa lies between the crus cerebri.

Functions and related regions:

• Superior colliculi: involved in conjugate gaze movements
• Inferior colliculi: involved in processing auditory information relayed from the cochlear nuclei via the lateral lemniscus
• Pretectal area: located near the superior colliculi and involved in the pupillary light reflex

The substantia nigra is rich in melanin, so it appears grey on gross examination. It uses dopamine and GABA as neurotransmitters.

Other important midbrain nuclei include:

• Red nucleus (involved with movement)
• Periaqueductal grey matter (plays a role in suppression of pain)
• Raphe nucleus (contains serotonergic neurons)

The ventral tegmental area contains dopaminergic neurons and is involved in reward mechanisms.

The medial longitudinal fasciculus lies anterior to the periaqueductal grey matter.

Pons

The pons lies inferior to the midbrain and connects to the cerebellum via the middle cerebellar peduncles.

Cranial nerve emergence:

• Cranial nerve V (trigeminal) emerges anteriorly and superiorly from the pons
• Cranial nerves VI, VII, and VIII emerge inferiorly in the groove between the pons and the medulla

The locus coeruleus is part of the reticular activating system and is located in the posterolateral pons. It produces norepinephrine as a neurotransmitter.

Pontine motor nuclei are located anteriorly.

Medulla oblongata

The medulla continues below the level of the foramen magnum as the spinal cord.

Cranial nerve emergence:

• Cranial nerve XII emerges anteriorly from the medulla
• Cranial nerves IX, X, and XI emerge posterior to the inferior olive

Anteriorly, two bulging masses called pyramids are visible. They contain fibres of the pyramidal (corticospinal) tract. The pyramidal decussation is also seen in the medulla and refers to the crossing of corticospinal fibres to the opposite side.

Lateral to the pyramids are the olivary bodies, which are part of the olivocerebellar system. The inferior cerebellar peduncles connect the medulla to the cerebellum.

Dorsally, the gracile and cuneate tubercles contain the gracile and cuneate nuclei, respectively. These nuclei are second-order neurons of the dorsal columns-medial lemniscus system:

• Gracile nuclei (medial): carry fibres from the lower extremities and trunk
• Cuneate nuclei (lateral): carry fibres from the upper body above T6, except for the face and ears

Medial longitudinal fasciculus

The medial longitudinal fasciculus is located in the brainstem ventral to the fourth ventricle and cerebral aqueduct. It connects the nuclei of cranial nerve VIII with nuclei controlling eye movements (cranial nerves III, IV, and VI). Its major role is execution of conjugate ocular movements.

Lateral lemniscus

The lateral lemniscus carries auditory input from the cochlear nuclei to the superior olivary complex, nuclei of the lateral lemniscus, inferior colliculi, and medial geniculate body. These then project to the auditory area in the temporal lobe.