Download 1 - U-System

1. Cerebral Cortex
- six-layered neocortex in all areas except hippocampus and olfactory areas near uncus
Cell types
Pyramidal cells- numerous; large, cone-shaped, apex toward cortical surface with long apical
dendrite; basal dendrites from base of pyramid and extends horizontally in cortex; principal
output neurons of cortex; axon towards white matter
Nonpyramidal cells- small, multipolar, short axons; also called stellate cells; principal
interneurons of cortex (some with GABAergic inhibitory output)
Layering
Molecular (Plexiform) Layer – Layer 1; few neurons, synaptic interactions occur here (certain
incoming fibers end on apical dendrites)
- Beneath plexiform layer are four alternating layers with different proportions of cell types:
- Layers 3 and 5 are pyramidal; pyramidal cells in layer 3 send axons to other cortical areas;
those in layer 5 send axons to striatum and spinal cord
- Layer 4 is nonpyramidal; incoming fibers from thalamic relay end in layer 4
- Layer 2 is a mix of nonpyramidal and small pyramidal cells
- Layer 6 is a mix of pyramidal cells and some spindle-shaped modified pyramidal cells; send
axons to thalamus
- Areas processing incoming info are thin, have lots of small nonpyramidal and pyramidal cells,
and few big pyramidal cells; this is granular cortex (primary sensory areas)
- Motor cortex is thick and has lots of big pyramidal cells overwhelming the small cells
(agranular cortex)
2. Columnar organization
- apical and basal dendrites of pyramidal cells are a reflection of two perpendicular directions of
cortical organization
- columns run perpendicular to cortical surface
- columns in visual cortex, somatosensory cortex, prefrontal cortex
- Cortical areas project to other areas in same hemisphere (ipsilateral); to neighboring areas via
short U-fibers that dip under one or two sulci; to faraway areas through longer association
bundles (arcuate fasciculus is one that arcs above insula and interconnects anterior and
posterior parts of a hemisphere including Broca’s and Wernicke’s area)
3. – Corpus callosum is the largest commissure in the CNS; it interconnects most areas of each
hemisphere’s cortex with mirror-image and other sites in the contralateral hemisphere
- the temporal lobe sends many of its interconnecting fibers through the anterior commissure
(contains a few crossing olfactory fibers)
Cortical Maps
- Each elemental function, like somatic sensation, vision or voluntary movement, has a primary
cortical area associated with it
- Each function also has a nearby association area that works on more complicated aspects of the
same function; these unimodal association areas have higher THs, larger/bilateral receptive
fields, and more complex properties
- destruction of primary somatosensory cortex causes a somatosensory deficit, but not a total
loss; this is true because there is parallel processing occurring (thalamic info goes to both
primary and association areas, which can function by themselves)
- there are also more complex (multimodal) association areas that receive multiple types of info,
and cortical areas involved in limbic function
4. Primary areas
- Primary somatosensory cortex occupies postcentral gyrus and is made up of 3 strips (3 1 2
from anterior to posterior)
- Primary auditory cortex is hidden in lateral sulcus, in transverse temporal gyri on top of
the temporal lobe
- Primary visual cortex is in the walls of the calcarine sulcus
- Primary motor cortex (large corticospinal neurons) occupies precentral gyrus
5. Unimodal (simple) association areas
- Primary visual cortex surrounded by two concentric belts of visual association cortex filling
up rest of occipital lobe (additional visual areas exist in temporal lobe)
- Auditory association cortex is lateral to primary auditory cortex and occupies most of
superior temporal gyrus
- Somatosensory association cortex is in superior parietal lobule and its continuation onto
medial surface of parietal lobe; 2nd mapping of body surface in parietal lobe, looking like a
distorted mirror image of the one in primary somatosensory cortex (head at bottom of
postcentral gyrus, body onto insula – second somatosensory area)
- Premotor cortex is a anterior to primary motor cortex; association area for motor system;
part of Premotor cortex anterior to where head is represented contains frontal eye field,
responsible for triggering voluntary rapid eye movements (saccades) to contralateral side;
motor association cortex has supplementary motor area on medial side of hemisphere
6. Complex (multimodal) association areas
1. Parietal-occipital-temporal junction receives and integrates multiple kinds of information; it
is made up of angular and supramarginal gyri
2. Large area of frontal lobe in front of motor and premotor cortex, on both the lateral and
medial surfaces of the hemisphere; this is called prefrontal cortex
- Language functions contained within these multimodal association areas
- Broca’s area occupies the opercular and triangular parts of the left inferior frontal gyrus;
damage here causes problems producing language
- Wernicke’s area is the posterior part of the left auditory association cortex; damage here
causes problems comprehending language
7. Limbic areas
- The cingulate and parahippocampal gyri, the orbital cortex and insula, and the anterior end
of the temporal lobe associated with hippocampus and amygdala  limbic cortex