Brainstem

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1. Cerebellar flocculus

The flocculus is labelled here as the ‘cerebellar flocculus’ which is useful because it informs us the flocculus belongs to the cerebellum. Moreover, the flocculus works together with another cerebellar component, the nodulus, as part of the flocculonodular lobe: this lobe has important input to the vestibular system (as part of the vestibulocerebellum) and also influences eye movement.

Show list

1.
Cerebellar flocculus
2.
Cerebellar hemisphere
3.
Cerebral peduncle
4.
Basal pons (basis pontis)
5.
Pyramid of rostral medulla
6.
Inferior olive (inferior olivary nucleus)
7.
Caudal medulla

In this view we see:

An anterior view of the brainstem, partially obscuring the much broader posteriorly located cerebellum.
All three divisions of the brainstem, from superior to inferior: the midbrain, pons and medulla. The midbrain, pons and medulla share a number of features including longitudinal fibre tracts, a tegmentum, and clusters of neurons called nuclei. While some longitudinal fibre tracts begin and terminate in the brainstem, others pass through it on their way between the cerebrum above and spinal cord below. These longitudinal tracts are present at all levels of the midbrain, pons and medulla.

Location

Parts of the midbrain are visible in Images 8, 9 and 10, with Image 2 demonstrating the relationship of the midbrain to the rest of the brain. The midbrain comprises the superior part of the brainstem and is located between the diencephalon above, and the pons below. The posteriorly situated colliculi (seen in Image 9) provide a rough guide to the rostrocaudal extent of the midbrain.

Structure

In early formation, the brain is sometimes divided into forebrain, midbrain and hindbrain. This is appropriate in the embryo because at four weeks of development an embryo's forebrain, midbrain and hindbrain are all similar sized bulges of the neural tube. However, at three months, the forebrain has grown to such an extent that it completely overshadows the midbrain. In the adult the midbrain is just a small section at the top of the brainstem.

Despite its small size in an adult, the midbrain contains a number of important structures, including the cerebral peduncles, cerebral aqueduct, tectum and substantia nigra, the colliculi and two cranial nerve nuclei (CN III and CN IV). Many of these structures can be seen in Image 10.

Function

Conduit fibres passing through the midbrain – can be divided into descending and ascending fibres, including:

bilateral descending fibres such as the corticospinal, corticobulbar, tectospinal and reticulospinal tracts all have a role in movement.
bilateral ascending fibres such as the spinothalamic tracts carry sensory information about pain and temperature to the thalamus and cerebrum.

Cranial nerve nuclei – both the oculomotor nerve (CN III) and trochlear nerve (CN IV) are primarily involved in movement of the eyeball. The Edinger-Westphal nucleus within the oculomotor complex also controls sphincter pupillae and ciliary muscles.

Reticular formation – the reticular activating system of the reticular formation has a role in keeping the cortex alert and enhancing its excitability.

Damage to the midbrain – can be caused by herniation of the brain through the foramen magnum due to increased intracranial pressure caused by a bleed (haemorrhage). This can be fatal.