Cerebral Cortex
last authored: Nov 2013, David LaPierre
last reviewed:
Introduction
The cerebral cortex is the highly folded, predominant structure of the brain, composed of ridges called gyri (sing: gyrus) and grooves called sulci (sing: sulcus). Brains differ considerably, though there are some fairly constant major features.
The 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). Gray matter, which is composed of the neuronal cell bodies, is predominantly located on the brain surface, while the axon-containing white matter is deeper.
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. Maps can change over time, with substantial reorganization of sensory and motor areas during development, behaviour, training, or damage.
A single point of input will activate a large population of neurons. As the signal is disseminated during encoding and processing, the number of involved neurons grows, with signal intensity will vary according to location.
Cortical output axons from the cortex originate 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 central nervous system, particularly the thalamus and the brainstem. The internal capsule is continuous with the cerebral peduncles in the midbrain.
Major Lobes
The five lobes of the brain are as follows:
- frontal
- prefrontal
- parietal
- temporal
- occipital
Frontal Cortex
The frontal cortex lies in front of the central sulcus (of Rolando) and above the lateral sulcus; separated from the limbic lobe by the cingulate sulcus.
Functions of the frontal cortex include:
- executive processing, emotions, motor function
- primary motor, premotor, and supplementary motor areas (precentral gyrus)
- Broca's area - production of written and spoken language (usually left inferior frontal gyrus)
Prefrontal Cortex
The prefrontal cortex is located below the frontal cortex.
The prefrontal cortex is associated with mood and executive decisions involving social behaviour, including personality, insight, foresight. Specific functions of the prefrontal cortex include:
- establishment and maintenance of social goals (dorsolateral)
- empathy and regulation of negative emotion (medial)
- assignment of emotional value following decision-making (orbitofrontal, with connections to amygdala and thalamus)
Parietal Cortex
The parietal cortex extends from central sulcus to an imaginary line separating it from the occipital lobe. Medially, bounded by calcarine sulci. The postcentral gyrus contains primary somatosensory cortex.
The parietal cortex performs the following functions:
- sensation
- processing of tactile and proprioreceptive information and integration with experience and emotion
- language comprehension (inferior lobe, usually left)
- complex aspects of spatial orientation and perception
Temporal Cortex
The temporal cortex is below the lateral fissure.
The temporal cortex performs the following functions:
- processing auditory information (primary auditory cortex; superior temporal lobe)
- language comprehension (Wernike's area, usually left temporal lobe)
- higher order visual processing (especially inferior surface)
- learning and memory (medial)
Occipital Cortex
The occipital cortex is located posteriorly. It has no clear demarcation laterally, though separated is from the parietal lobes by the parietoccipital fissure. The primary visual cortex is contained in walls of calcarine sulcus, separating the parietal and temporal lobes.
The occipital cortex is almost exclusively involved with processing visual information.
Other Structures
The corpus collosum is a massive collection of white matter connecting the two hemispheres.
The Limbic Cortex
The limbic system encircles the cortex-diencephalon junction. It contains cingulate and parahippocampal gyri, the latter which forms a medial bump called the uncus. The hippocampus is folded into temporal lobe.
The limbic system is important in emotions, drive-mediated behaviour, and memory.
Insula
The insula lies buried within the lateral sulcus.
It has gustatory and autonomic functions. Although less clear, it may also be involved in visceral function and nociception.
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.
Resources and References
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