The Human Brain : From Neurone to Nervous System

THE CEREBRAL CORTEX


  1. The cerebral cortex covers the surface of the two cerebral hemispheres, and is folded - the folded surfaces are called gyri, whereas the groves between the folds are called sulci. The central sulcus is an important marker, which separates the frontal and parietal lobes of the brain.

  2. Beneath the cortex is the white matter, consisting of large bundles of axons connecting different areas of cortex with each other and with other structures within the brain. The largest band of axons connects the two hemipheres and is called the corpus callosum.

  3. In the centre of each hemisphere is a lateral ventricle containing cerebrospinal fluid; these are continuous with the ventricular system of lower levels of the CNS.

  4. The hemispheres are divided into lobes. The cortex can be subdivided into areas, with different functions and cellular composition, which are variations on a basic structure consisting of six layers.

  5. Some are concerned with receiving or processing sensory information from the body, the eye or the ear, and others are concerned with motor functions.

  6. The cortex is divided into areas with different functions and cellular architecture; the structures are often referred to as Brodmann's areas, and their functions have been realised because of changes in function following lesions or electrical stimulation, or by recording the properties of neurones during specific behaviours.

  7. As a result we recognise a primary motor area, the pre-central gyrus, that connects the cortex with alpha motoneurones; a primary somatosensory area in the post-central gyrus, concerned with the reception of sensory information from the skin, muscles and viscera; and primary visual and auditory areas that receive signals from the eyes or ears - these are in the occipital cortex and the superior temporal gyrus respectively.

  8. Between these are areas of association cortex concerned with processing and integrating sensory and motor information, and other higher functions of the CNS, such as attention, decision-making, memory and learning.

  9. Key Words: Motor cortex, Somatosensory cortex, Visual cortex, Auditory cortex, Association cortex







Layered Structure of the Cerebral Cortex


Image source: Kandel 200 via http://theanalogicalinstinct.blogspot.co.uk/, and (right)pixshark.com

The neurons of the cerebral cortex are arranged in six distinctive layers. The appearance of the cortex depends on what is used to stain it. The Golgi stain reveals neuronal cell bodies and dendritic trees. The Nissl method shows cell bodies and proximal dendrites. A Weigert stain for myelinated fibers reveals the pattern of axonal distribution. (Kandel 2000)

Layers of the Cerebral Cortex

In humans the cortex is 2-4 mm thick and has six layers, numbered I to VI, starting from the pial surface:

  • Layer I, the molecular layer, receives substantial inputs from the thalamus.
  • Layer II, the external granular layer contains densely packed stellate cells.
  • Layer III, the external pyramidal layer, receives inputs from other areas of the cortex, including the contra-lateral hemisphere.
  • Layer IV, the internal granular layer, contains stellate neurones and is the main target of thalamocortical pathways as well as fibres crossing over in the corpus callosum.
  • Layer V, the internal pyramidal layer, contains large pyramidal neurones whose axons project through the internal capsule to subcortical structures including the striatum, brainstem and spinal cord. The largest of these are known as Betz cells.
  • Layer VI, the polymorphic or multiform layer has precise reciprocal connections with the thalamus.

More Details of the Cellular Structure of the Cerebral Cortex and it Connections.


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