N4: White Matter Organization

what is a neuron responsible for

responsible for receiving + sending electrical impulses

what is a nerve responsible for

responsible for carrying nerve impulses to / from the spinal cord

what is a tract

a bundle of axons

• that travels together within the CNS (brain/spinal cord)

location of association fibers

found within each cerebral hemisphere

function of association fibers

allows communication to occur between areas of the same hemisphere

• ex. linking motor + sensory regions on one side

location of commissural fibers

found in the corpus callosum

function of commissural fibers

allows communication + coordination between BOTH hemispheres

• now they can work together 

location of afferent fibes

found in sensory pathways

• enters the CNS through the spinal cord/cranial nerves

function of afferent fibers

transmits sensor information → brain (for processing)

location of efferent fibers

found in motor pathways

• leaves the CNS

• through the spinal cord/motor cranial nerves 

function of efferent fibers

sends motor commands from the brain + spinal cord → muscles + glands

location of short association fibers

found just beneath the gray matter (cerebral cortex) 

function of short association fibers

allows quick communication between nearby cortical regions

• connects neighbouring gyri (folds) within the same lobe

location of long association fibers

runs deeper within the white matter

• extends between distant cortical regions (lobe → lobe)

function of long association fiber

allows communication between distant parts of the same hemisphere

• allows integration of information from multiple lobes 

what is a cingulate gyrus

a gyrus (fold) on the medial surface of the brain

• on top of the corpus callosum 

what is a cingulum

bundle of white matter fibers beneath the cingulate gyrus

• association tract that connects the frontal, parietal + temporal lobes 

description of the corpus callosum

the largest bundle of white matter fibers

location of the corpus callosum

lies deep within the brain

• below the cerebral cortex

• above the lateral ventricles

function of the corpus callosum

• connects the two hemispheres

  • allows them to share information

• transmits motor, sensory + cognitive signals between the R + L sides

description of the genu

the front (anterior) cured part of the corpus callosum 

location of the genu

lies just above the frontal lobes

• bending forward before turning downward

function of the genu

connects the R + L frontal lobes of the brain

• which is important for coordination of movement, decision-making, emotional control

description of the body of the corpus callosum

the middle, largest portion of the corpus callosum

location of the body of the corpus callosum

it runs horizontally

• between the genu / splenium

• over the lateral ventricles 

function of the body of the corpus callosum

it connects the..

• parietal lobes + parts of the frontal lobes of both hemispheres 

• allows integration of sensory information + coordination of complex functions (spatial awareness, language) 

description of the splenium

the posterior, rounded par of the corpus callosum 

location of the splenium

lies above + in front of the cerebellum

• over the occipital lobes

function of the splenium

it connects the occipital lobes of the two hemispheres

description/location of the forceps minor

fibers that extend laterally (forward)

• from the genu 

• into the frontal lobes

function of forceps minor

they connect the frontal lobes

• allows coordination of the frontal lobe functions (ex. planning, reasoning, emotion) 

description/location of the forceps major

fibers that extend laterally (backward)

• from the splenium

• into the occipital lobes

function of the forceps major

they connect the occipital lobes

• allows visual information to be shared between hemispheres

description of projection fibers

• bundles of white matter axons

• contains afferent (sensory) and efferent (motor)

location of projection fibers

found deep within the brain

• runs vertically between the cerebral cortex, thalamus + brainstem

what do projection fibers include

includes the..

• corona radiata

• internal capsule

• pyramids

function of projection fibers

carries signals between the higher / lower parts of the nervous system

• afferent - sensory → cortex

• efferent - motor → body

description of the corona radiata

fan-shaped collection of projection fibers

location of the corona radiata

lies just beneath the cerebral cortex

• spreads upwards + outward → frontal, parietal, occipital lobes

• superior to..

  • gray nuclei

  • internal capsule 

function of the corona radiata

it connects the cortex with deeper brain structures

• forms the upper part of the projection fiber system

description of the internal capsule

thick band of white matter fibers

• contains both sensory + motor tracts

location of the internal capsule

found deep in each hemisphere

• between the caudate nucleus + thalamus (medially)

• lentiform nucleus (laterally) 

function of the internal capsule

• serves as the main route for communication 

  • between the cerebral cortex + lower centres (brainstem + spinal cord)

• carries motor commands down 

• carries sensory signals up 

description of pyramids

two large ridges of white matter

• contains descending motor pathways 

location of pyramids

on the ventral (front) side of the medulla

• which is part of the brainstem

function of pyramids

carries voluntary motor signals from the motor cortex → spinal cord to control movement

• where pyramidal decussation occurs (when they cross over to the opposite sides)

  • the reason the left side of the brain controls the right side of the body

description of cerebral peduncles

pair of large bundles of white matter

location of cerebral peduncles

located on the ventral (front) surface of the midbrain 

• just below the internal capsule 

• lies between the optic tracts + pons 

function of cerebral peduncles

either..

• carries motor information from the cerebral cortex → brainstem/spinal cord

• carries sensory information upward

definition of homunculus

a map of the body

what is the homunculus represented on

through the motor / sensory cortexes

what does the homunculus do

it shows how different parts of the body are controlled/sensed by specific areas of the cortex 

• shows how the brain allocates space based on function/sensitivity (NOT PHYSICAL SIZE)

what does somatotopic representation mean

when body information is organized in an orderly, body-based fashion

where is the motor homunculus located

on the pre-central gyrus

• of the frontal lobe (primary motor cortex)

where is the sensory homunculus located

on the post-central gyrus

• of the parietal lobe (primary somatosensory cortex)

how is the body represented on the cortex

1. foot/leg - midline (medially)

2. torso - superior

3. arm/hand - super-lateral

4. face - lateral (just above the lateral fissure)

why are the hands + face larger than the feet on the homunculus

because they..

• require finer motor control

• have more receptors

• use more cortical space