The uncinate fasciculus is a
white matter
White matter refers to areas of the central nervous system (CNS) that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution ...
association tract
Association fibers are axons that connect cerebral cortex, cortical areas within the same cerebral hemisphere.
In human neuroanatomy, axons (nerve fibers) within the brain, can be categorized on the basis of their course and connections as ass ...
in the
human brain
The human brain is the central organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the activities of the ...
that connects parts of the
limbic system
The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.Schacter, Daniel L. 2012. ''Ps ...
such as the
temporal pole, anterior
parahippocampus, and
amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
in the
temporal lobe
The temporal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe ...
with inferior portions of the frontal lobe such as the
orbitofrontal cortex. Its function is unknown though it is affected in several
psychiatric conditions
A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitti ...
. It is one of the last white matter
tracts to mature in the human brain.
Anatomy
The uncinate fasciculus is a hook-shaped bundle of axons that links anterior portions of the temporal lobe with the
inferior frontal gyrus and the lower surfaces of the
frontal lobe. It arises in the anterior temporal lobe and
amygdala
The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex verteb ...
, in the
temporal lobe
The temporal lobe is one of the four Lobes of the brain, major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
The temporal lobe ...
curving in an upward pathway behind the
external capsule
The external capsule is a series of white matter fiber tracts in the brain. These fibers run between the most lateral (toward the side of the head) segment of the lentiform nucleus (more specifically the putamen) and the claustrum.
The white matt ...
inward of the
insular cortex and continuing up into the posterior part of the
orbital gyrus. It does not appear to have cell bodies in the hippocampus proper.
The average length of the uncinate fasciculus is 45 mm with a range 40–49 mm. Its volume in adults is 1425.9±138.6 mm
3, being slightly larger in men, at 1504.3±150.4, than women 1378.5±107.4.
It has three parts: a ventral or frontal extension, an intermediary segment called the isthmus or insular segment and a temporal or dorsal segment.
Function
The uncinate fasciculus is a bi-directional pathway between the temporal lobe and frontal lobe; it is traditionally considered to be part of the
limbic system
The limbic system, also known as the paleomammalian cortex, is a set of brain structures located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.Schacter, Daniel L. 2012. ''Ps ...
.
It has been proposed that the uncinate fasciculus allows mnemonic representations stored in the temporal lobe to interact with and guide decision making in the frontal lobe.
Diffusion tensor imaging
Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It ...
, a reconstruction model available from a diffusion
MRI scan, shows a greater
fractional anisotropy Fractional anisotropy (FA) is a scalar value between zero and one that describes the degree of anisotropy of a diffusion process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A ...
on the left side than on the right. The difference in this measure of anisotropy has been linked to the left hemispheric specialization for language. However, the use of
electrical brain stimulation upon it fails to disrupt language, suggesting it might not be involved in language, though it is possible that this disruption failed to happen because it was functionally compensated by alternative pathways.
The uncinate fasciculus appears to have a role in some, but not all, types of learning and memory. Reversal learning, in which a stimulus-reward association is learned over many trials, then reversed such that the initial stimulus is no longer is associated with a reward, is associated with individual variation in the uncinate fasciculus.
In addition, the ability to learn associations through trial and error, such as the pairing of a name with a face, correlates with uncinate fasciculus microstructure.
This relates to early work showing that surgical damage to the uncinate fasciculus is reliably associated with deficits in proper name retrieval.
Development
The uncinate fasciculus has the longest period of development in terms of fractional anisotropy as it alone amongst the major white fibre tracks continues to develop until the age of 30. Developmental disorders with core problems relating to memory retrieval, reward and valuation computation, and impulsive decision making may be linked to aberrations in uncinate fasciculus microstructure, e.g.
temporal lobe epilepsy and
conduct disorder
Conduct disorder (CD) is a mental disorder diagnosed in childhood or adolescence that presents itself through a repetitive and persistent pattern of behavior that includes theft, lies, physical violence that may lead to destruction, and reckles ...
.
Moreover it seems to be developmentally vulnerable. In 12-year-old males that were
preterm
Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is between 2 ...
, abnormalities measured by fractional anisotropy in the left anterior uncinate correlated with
verbal IQ, full-scale IQ, and
Peabody Picture Vocabulary Test-Revised scores. In 10-year-old children who have suffered socioemotional deprivation, the left uncinate fasciculus shows reduced fractional anisotropy compared to that in other children, and this might underlie their cognitive, socioemotional, and behavioral difficulties.
Clinical significance
Abnormalities within the fiber bundles of the uncinate fasciculus have been linked to a host of neuropsychiatric disorders, some consistently, some inconsistently. For instance, studies of individuals with
social anxiety
Social anxiety is the anxiety and fear specifically linked to being in social settings (i.e., interacting with others). Some categories of disorders associated with social anxiety include anxiety disorders, mood disorders, autism spectrum disord ...
and
schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social withdra ...
have reported changed in diffusion metrics of the uncinate. Unfortunately these findings frequently fail to replicate, the directionality of effects is mixed, and there is no specificity between findings in the uncinate and these particular diseases. An in-depth review of the clinical literature noted that "the uncinate fasciculus per se plays either a small role, or no role, in anxiety disorders " and that "the results of DTI studies measuring changes in uncinate fractional anisotropy values in schizophrenics are contradictory."
Future researchers should focus on using superior diffusion imaging methods as well correlating specific symptoms, rather than presence or absence of disease state, with changes in white matter.
In contrast, microstructural changes to the uncinate fasciculus have been consistently been linked to conduct disorder and psychopathy.
[
*] This is consistent with studies of gray matter volume which reported that neural regions linked by the uncinate fasciculus—the
orbitofrontal cortex and temporal polar cortex/
Brodmann area 38
Brodmann area 38, also BA38 or temporopolar area 38 (H), is part of the temporal cortex in the human brain. BA 38 is at the anterior end of the temporal lobe, known as the temporal pole.
BA38 is a subdivision of the cytoarchitecturally defined ...
(also known as the temporal pole/temporal polar cortex/anterior temporal lobe)— have decreased volume in individuals with antisocial personality disorder /psychopathy relative to control. Interestingly, Phineas Gage, a railroad worker who had an iron bar go through his left frontal lobe
also sustained damage to the uncinate fasciculus. After the accident, his personality was radically transformed, becoming impulsive, making poor decisions, and failing to follow social norms and conventions.
Last, consistent abnormalities in the uncinate fasciculus are found in
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in short-term me ...
,
semantic dementia, and
temporal lobe epilepsy.
Additional images
File:H. Mayo -Series of engravings-, 1827; brain Wellcome L0015852 - Uncinate fasciculus.png, Human brain with operculum removed. A part of uncinate fasciculus is visible (shown in yellow)
File:Gray751 - Uncinate fasciculus.png, Diagram showing principal systems of association fiber
Association fibers are axons that connect cortical areas within the same cerebral hemisphere.
In human neuroanatomy, axons (nerve fibers) within the brain, can be categorized on the basis of their course and connections as association fibers, ...
s in the cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. In ...
. (Uncinate fasc. visible at lower left, in red.)
File:Uncinate Fasciculus.jpg, Tractography showing uncinate fasciculus.
File:Tractography - Uncinate fasciculus - animation 2.gif, Animation of tractography data. Blue is the left, green is the right.
References
External links
* - "Dissection of the Left Hemisphere"
{{Authority control
Cerebral white matter