Although superficially continuous with the hippocampal gyrus, the uncus forms
morphologically a part of the
rhinencephalon.
An important landmark that crosses the inferior surface of the uncus is the
band of Giacomini.[2]
The term comes from the Latin word
uncus, meaning hook, and it was coined by
Félix Vicq-d'Azyr (1748–1794).[3]
Clinical significance
The part of the
olfactory cortex that is on the temporal lobe covers the area of the uncus, which leads into the two significant clinical aspects of the uncus: uncinate fits and
uncal herniations.
Seizures, often preceded by hallucinations of disagreeable odors, often originate in the uncus.
In situations of
tumor,
hemorrhage, or
edema, increased pressure within the cranial cavity, especially if the mass is in the middle fossa, can push the uncus over the tentorial notch against the
brainstem and its corresponding
cranial nerves and can result in a
brain herniation. If the uncus becomes herniated the structure lying just medial to it,
cranial nerve III, can become compressed. This causes problems associated with a non-functional or problematic CN III - the pupil on the
ipsilateral side fails to constrict to light and absence of medial/superior movement of the orbit, resulting in a fixed, dilated pupil and an eye with a characteristic "down and out" position due to dominance of the abducens and trochlear nerves. Further pressure on the midbrain results in progressive lethargy, coma and death due to compression of the mesencephalic
reticular activating system. Brainstem damage is typically
ipsilateral to the herniation, although the contralateral
cerebral peduncle may be pushed against the tentorial notch, resulting in a characteristic indentation known as
Kernohan's notch and ipsilateral
hemiparesis, since fibers running in the cerebral peduncle
decussate (cross over) in the lower medulla to control muscle groups on the opposite side of the body.
The landmark that helps you find the amygdala on a coronal section of the brain.
Function
A sparse amount of literature exists to propose a comprehensive overview of the functionality of the uncus. A study has indicated that psychotic-like experiences were associated with reduced expansion within the uncus between the ages of 14 and 19 in cannabis-using individuals.[4]
Additional images
Position of uncus (red)
Basal view of a human brain
Scheme of
rhinencephalon. (Uncus labeled at bottom right.)