Organization Of The Cerebral Cortex And Thalamus
The organization of the outer layer of the cerebral cortex (neocortex) can be considered in various ways. One way uses cytoarchitectural maps such as Brodmann’s areas – which equates to some extent with the functional organization of this structure into motor, sensory and associative areas, as evidenced by the laminar organization of the cortex. An area of cortex that is predominantly sensory in character has a prominent layer IV, while cortical motor areas have a prominent layer V.
An alternative approach is to view the cortex as being organized vertically. This vertical organization has become known as the columnar hypothesis and proposes that the ‘column’ of cortex is the basic unit of cortical processing.
Anatomical organization of the cerebral cortex
The neocortex is classically described as consisting of six layers, although in certain areas of the cerebral cortex further subdivisions are used, e.g. the primary visual cortex (see Chapter 26).
• The thalamic afferent fibres, relaying sensory information, project to layer IV often with a smaller input to layer VI. They terminate in discrete patches.
• This input then synapses onto interneurones within the cortex which in turn project vertically to neurones in layers II, III and V, and from there project to other cortical and subcortical sites, respectively.
This, the weight of synaptic relations within the cerebral cortex is in the vertical direction. This arrangement of synaptic connections is well seen in the somatosensory and visual cortices (see Chapters 26 and 31). In many cortical areas with a motor function, the motor output from that cortical area is such that it is directed back at the motor neurones controlling the muscles that move the sensory receptors which ultimately project to that same area of cortex – so-called input–output coupling (see Chapter 39).
Developmental organization of the cerebral cortex
In the mammalian CNS the entire population of cortical neurones is produced by a process of migration from the proliferative zones that are situated around the cavities of the cerebral ventricles. The radial glial fibres, which guide and may even give rise to the migrating neurones, span the fetal cerebral wall and direct the neurones to their correct cortical location in the developing cortical plate from the ventricular and subventricular zones (see Chapter 1). Thus, developmentally, the cortex forms in a vertical fashion.
Neurophysiological organization of the cerebral cortex
Neurophysiologically, if a recording electrode is passed at right angles through the cortex, it encounters cells with similar properties. However, if the electrode is passed tangentially then cells shift their response characteristics. This has been shown in many cortical areas (eg. Chapter 26).
This columnar organization of the cortex ensures that topography can be maintained and that the reorganization of the cortex in the event of a change in the peripheral input is relatively straight forward (see Chapter 49).
Functional organization of the cerebral cortex
Serial processing models
The original models proposed that information processing was performed in a serial fashion, such that the cortical cells form a series of hierarchal levels. Thus, one set of cells performs a relatively straightforward analysis, which then converges on another population of neurones that perform a more complex analysis (see, for example, Chapter 26). The ultimate prediction of these hierarchical models is that one neurone at the top of the hierarchy will register the percept – the ‘grandmother’ cell.
Parallel processing models
The discovery of the X, Y and W classes of ganglion cells in the retina (see Chapter 24) led to the development of a competing theory that proposed that information is analysed by a series of parallel pathways, with each pathway analysing one specific aspect of the sensory stimulus (e.g. colour or motion with visual stimuli; see Chapter 26). This theory does not exclude hierarchical process- ing but relegates it to the mode of analysis within separate parallel pathways. In practice, the cortex employs both modes of analysis. Distributed processing models It should be stressed that cortical columns are not to be viewed as a static mosaic structure, as one column may be a member of a number of different pathways of analysis. This organization has been termed the distributed system theory and describes the brain as a complex of widely and reciprocally interconnected systems, with the dynamic interplay of neural activity within and between these systems as the very essence of brain function. Consequently, one column may be a member of many distributed systems, because each distributed system is specific for one feature of a stimulus and one column may code for several features of the stimulus.
Anatomical and functional organization of the thalamus
The thalamus is made up of a number of discrete nuclei and is more than a simple relay station as it receives extensive connec-tions from the cortex and brainstem structures and is also critically involved in levels of arousal. The main nuclei of the thalamus are:
• The anterior nucleus – which is associated with the limbic system and prefrontal cortex (Chapters 34 and 45).
• The ventroanterior and ventrolateral nuclei – which are associated with motor systems (see Chapters 38–41).
• The ventroposterior nuclei – which are associated with somato-sensory systems (see Chapters 31 and 32).
• The pulvinar – which is associated with posterior parietal cortex (see Chapter 34).
• The medial geniculate nucleus – which is associated with the auditory pathways (see Chapter 28).
• The lateral geniculate nucleus – which is associated with the visual system (see Chapter 25).
• The intralaminar nuclei – which is associated with pain path- ways and basal ganglia (see Chapters 33 and 41).
• The reticular nucleus – which is associated with levels of arousal and some forms of epilepsy (see Chapters 43 and 44).