Cerebral White Matter

Association Fibers

Association fibers are axons of neuronsThe functional cells of the nervous system that transmit signals. that connect different gyri of the brain.  These are neurons that are completely contained in the brain.  Also, they are all myelinated for fast transmission. 

There are short one that connect adjacent gyri.  These are lightly myelinated.  If we need to connect gyri that are not adjacent, the neuron will be more heavily myelinated.  So, let’s say that the premotor cortex is trying to consolidate motor output for you to cross a street.

 The premotor cortex needs information about your position in relation to a curb. The visual association area has this info.  The premotor cortex may also need some information about the beep from the crosswalk sign. It indicates when it’s OK to start crossing.  These pieces of info from the association areasBrain regions that integrate sensory input and coordinate responses. will use an association fiber to reach the premotor cortex.

Projection Fibers

Projection fibers are much more profuse in the brain.  They connect lower, like physically inferiorBelow or toward the lower part of the body. areas of the brain to areas more superiorAbove or toward the upper part of the body. in the brain. For example, the premotor cortex is connected to the cerebellumThe brain region responsible for balance, coordination, and fine motor control. by heavily myelinated neurons that are projection fibers.

 There are two large-scale structures that projection fibers make.  The inter capsule consists of a bunch of projection fibers. These fibers connect the thalamusThe brain’s relay center, sending sensory information to the cerebral cortex. and basal nucleiClusters of neurons in the brain that help regulate movement and coordination.. These structures are actually really close to each other.  But, these areas control precise movements so, they need quick communication.  There is also a collection of projection fibers called the corona radiataThe layer of follicular cells that surrounds a developing oocyte.. 

The name is a reflection of how these fibers fan out in the cerebrumThe largest part of the brain, responsible for thought, memory, and voluntary movements..  Coronal means crown and radiata basically means radiating.  The corona radiata is just an anatomical name describing its shape.  What does it do?  It is part of that reticular activating system that maintains your consciousness.  Remember?  The reticular activating system involves these neurons deciding on your study environment. They determine if you can study in the cafeteria or in a sensory deprivation tank.

Commissural Fibers

We have seen this term before: commissural.  There were commissures in the spinal cordThe central nervous system structure that relays signals between the brain and body. that connected the left to the right side of the spinal cord.  Same applies here.  The anteriorThe front of the body or toward the front when standing in the anatomical position. and posterior commissures are myelinated neurons that cross from the left hemisphere to the right hemisphere. 

In the physical lab, when we dissect brains, I first ask students to separate the two hemispheres. This opens up that longitudinal fissureA deep groove separating the left and right hemispheres of the brain. a bit and you can see these commissures. 

Corpus Callosum

The corpus callosumThe large bundle of nerve fibers connecting the two cerebral hemispheres. is just a commissural fiber.  It’s a really big one and is the one used most often by the brain.