Commissures, Association Fibers, and Projection Fibers

What is a commissure?

What is a commissure?

Connects cortical areas across hemispheres

What are the three parts of the corpus callosum?

• Genu

• Body

• Splenium

What is the main function of the corpus callosum?

Connect cortical areas across hemispheres

(Corpus Callosum is the largest white matter buncle with around 200 million heavily myelinated nerve fibers_

What is the function of the genu?

Connects prefrontal cortices

(executive function and speech planning_

What is the function of the body of the corpus callosum?

Connects premotor, primary somatosensory, and somatosensory association

What is the function of the splenium of the corpus callosum?

aka forceps major

Connects occipital lobes and connects temporal lobes

What is the function of the anterior commisure?

Responsible for acute, sharp pain sensation

• Connects L + R amygdala and ventromedial temporal cortices

What is the function of the posterior commisure?

Bilateral pupillary light reflex

• Crosses the midline on the dorsal aspect of rostral end of the cerebral aquaduct

What is the external capsule?

• White matter of external capsule contains fibers known as corticocortical association fibers

• Separates claustrum from lentiform nucleus

• These fibers are responsible for connecting the cerebral cortex to another cortical area

• Route for cholinergic fibers from the basal forebrain to the cerebral cortex

What does the external capsule separate?

Separates the claustrum from lentiform nucleus

• Fibers run between the most lateral segment of the lentiform nucleus and the claustrum and joint the internal capsule around the lentiform nucleus

What is the extreme capsule?

Bidirectional communication linking the inferior frontal gyrus (Broca’s), the middle part of the superior temporal gyrus (Wernicke’s) and inferior parietal lobule (angular gyrus),

What is the function of the extreme capsule?

Suggested that is plays a prominent role in language function

What does the extreme capsule connect?

Connects the claustrum, Broca’s Area (inf. frontal gyrus), Wernicke’s Area (middle of superior temporal gyrus), and Angular Gyrus

What is the cingulum?

• Gray matter!

• Projects bidirectionally through the limbic system around the posterior end of the corpus callosum and then rostrally forward deep to the parahippocampal gyrus

• Part of the papez circuit

What is the function of the cingulum?

Function is enigmatic / not well understood

What does the cingulum connect?

Connects the medial frontal, cingulate , parietal, and temporal cortices

What is an association fiber?

It connects areas of the same hemisphere

What is the sagittal striatum?

Visual pathway (optic radiations)

(Fibers run from thalamus (lateral geniculate nucleus - LGN) carrying axons of visual pathway caudally to primary visual cortex along calcarine sulcus)

What are projection fibers?

Connect cerebral cortex to brainstem and spinal cord

• Afferent: project to outer cortical layers (input)

• Efferent: project from pyramidal neurons (output)

What are two types of projection fibers?

• Corona radiata

• Internal capsule (anterior limb, genu, posterior limb, corticospinal tract)

What are the corona radiata?

• Lies at the level of the lateral ventricles

• Contains descending fibers projecting from the motor cortex to basal ganglia , midbrain motor nuclei (corticobulbar tract) and the spinal cord (corticospinal tract) AND ascending fibers from the thalamus to the cerebral cortex

What does the corona radiata connect?

Connects the motor cortex to the basal ganglia, midbrain motor nuclei (corticobulbar tract), and spinal cord (corticospinal tract)

ALSO connects thalamus to cerebral cortex

What is the internal capsule?

• V-shaped wedge in between key basal ganglia structures

• Carries fibers of many disparate functional modalities, including motor, sensory, cognitive, and emotional fiber pathways.

In what type of cross section can you see all components of the internal capsule?

Axial cross-sections

What does the anterior limb of the internal capsule separate?

Separates the head of the caudate nucleus from rostral aspect of the lentiform nucleus (putamen and globus pallidus)

What does the anterior limb of the internal capsule contain?

Frontopontine Tracts (all of which are primarily anteroposterioly oriented in contradistinction to the posterior limb)

What is the genu of the internal capsule?

The middle (bend) of the internal capsule

What does the genu of the internal capsule contain?

Contains the corticobulbar tract (which predominately terminates at the cranial motor nuclei)

What does the posterior limb of the internal capsule contain?

• Corticospinal motor tract to UE, trunk, LE (rostral); (FAL)

• Somatosensory pathways from UE, trunk, LE (caudal) (fal)

What does the posterior limb of the internal capsule separate?

Separates the lentiform nucleus from the thalamus

What is the corticospinal tract? What does it connect?

Major efferent projection fibers that connect motor cortex to the brain stem and spinal cord

What percentage of fibers of the corticospinal tract cross at the medullary pyramids?

90%