Cerebral Cortex

last authored: Sept 2009, David LaPierre

 

The cerebral cortex has a surface area of about 2200 cm2 and contains an estimated 1.5-2 x 1010 neurons. There are an estimated 3 x 1014 synapses (Boron and Boulpaep, 2006). Much gray matter is located on the brain surface, but some is inside. The basal ganglia are gray matter deep in the brain.

 

The cortex is highly folded, with each ridge called a gyrus and each groove called a sulcus. Brains differ considerably, though there are some fairly constant major features. It is divided into 5 lobes, as well as other smaller structures:

  • frontal
  • prefrontal
  • parietal
  • temporal
  • occipital
  • limbic
  • insula

Frontal Cortex

 

structures

  • lies in front of the central sulcus (of Rolando) and above the lateral sulcus; separated from the limbic lobe by the cingulate sulcus

 

functions

  • the precentral gyrus contains the primary motor cortex and the premotor and supplementary motor areas
  • Broca's area, usually the left inferior frontal gyrus, is involved in production of written and spoken language
  • prefrontal cortex involved in personality, insight, foresight

 

Prefrontal Cortex

 

The prefrontal cortex is associated with mood and executive decisions involving social behaviour.

dorsolateral - establishes and maintians social goals

medial - empathy and regulates negative emotion

orbitofrontal - assigns emotional value to a situation following decision-making projections from amygdala and thalamus

 

 

connections

The PFC has connections with:

Parietal Cortex

structures

  • extends from central sulcus to an imaginary line separating it from the occipital lobe
  • medially, bounded by calcarine sulci
  • postcentral gyrus contains primary somatosensory cortex

 

functions

  • associated with somatosensation, processing of tactile and proprioreceptive information, and integration with experience and emotion
  • inferior lobe, usually left, associated with language comprehension
  • complex aspects of spatial orientation and perception

 

Temporal Cortex

 

strutures

  • Located below the lateral fissure

 

functions

  • primary auditory cortex located on superior temporal lobe; processes auditory information
  • Wernicke's area, usually on the left temporal lobe, important in language comprehension
  • much of temporal lobe, especially inferior surface, associated with higher order processing of visual information
  • medial aspects associated with learning and memory

 

Occipital Cortex

 

strutures

  • No clear demarcation laterally, though separated from the parietal lobes by the parietoccipital fissure.
  • primary visual cortex contained in walls of calcarine sulcus, separating parietal and temporal lobes

 

 

functions

 

Limbic Cortex

 

structures

  • encircles the cortex-diencephalon junction
  • contains cingulate and parahippocampal gyri, the latter which forms a medial bump called the uncus.
  • hippocampus is folded into temporal lobe

 

functions

 

Insula

 

structure

lies buried within lateral sulcus

 

 

functions

  • gustatory and autonomic functions
  • may be involved in visceral function and nociception, though it is unclear.

 

Cortical Output

Output axons from the cortex come primarily from the pyramidal cells of layer V, which synapse in the brainstem and spinal cord, and from layer VI, which projects back to the thalamus.

The internal capsule connects the cortex with the rest of the CNS, particularly the thalamus and the brainstem. Internal capsule is continuous with the cerebral peduncles in the midbrain.

The corpus collosum is a massive collection of white matter connecting the two hemispheres.

 

 

Motor and Sensory Maps

The brain organizes motor output and sensory input using spatial maps. These are present to increase efficiency and enforce togetherness of related inputs and outputs. Maps also help during development as neuronal connections are formed.

It is likely a single point of input will activate a large population of neurons, meaning maps are fuzzy. A given sensory input will tend to activate an even larger group of neurons as the signal is disseminated during encoding and processing, with signal intensity will vary according to location.

 

Sensory Maps

Almost all sensory receptors, be they somatic sensory or visual, are laid out in planar sheets. Freqency maps and chemical maps are also generated by sensory signals.

 

Each sensory signal can be mapped many times within the brain, forming different, unique maps.

 

Homunculus

The homunculus is arranged, laterally to medially, with face, hand, arm, head, trunk, legs, then genitals. Relative size is determined by sensory fibre density.

 

 

Motor Maps

The primary motor map is arranged similarly to the sensory map, and there are many interconnections between the primary motor and somatosensory areas.

 

In some areas, motor and sensory functions can overlap each other precisely, such as occurs in the superior colliculus. The SC receives direct information from the retina and the visual cortex and can then cause head and eye movements to being the image into the centre of the fovea. The superior colliculus also has auditory and somatosensory maps, and together these inputs help orient to prominent stimuli.

 

 

Map Plasticity

Maps can change over time, with substantial reorganization of sensory and motor areas during development, behaviour, training, or damage.