Download Neuroscience 14b – Organisation of the Cerebral Cortex

Neuroscience 14b - Organisation of the Cerebral Cortex
Anil Chopra
1. Explain the vertical and horizontal organisation of the cells of the neocortex
2. Explain briefly the function of association, commissural and projection fibres in
cerebral white matter
3. Compare the attributes of primary and association cortex
4. List the main functions of the association cortex in each cortical lobe and the type
of deficits caused by lesions there
5. Describe the interhemispheric differences in cortical function
6. Outline the anatomical basis of the olfactory system
7. List the advantages and disadvantages of modern imaging as a tool for studying
human cognitive function and compare the merits of PET and fMRI
Methods of Studying Cortical Function
Lesions
This is the oldest method of studying cortical function. The effects of cerebral lesions
were observed and the consequences produced gave us an indication as to the regular
physiology of the cortical area this was however very limited in a number ways.
- poor reproducability
- inter-subject variation
- lack of pre-morbid measures
- plasticity/ redundancy
- lesions are uncontrollable
Electroencephalogram – EEG
This records electrical activity in the brain using electrodes which pick up small
changes in membrane potential. It has a very good time resolution but a low spatial
resolution in comparison with MRI.
An evoked potential is the electrical response recorded on the EEG to a stimulus.
They are often very low in amplitude so in order to distinguish them from things such
as background noise, signal averaging is required. This is time locked for the
stimulus, and the average potentials are recorded in response to the repeated stimulus.
It will show which parts of the brain are associated with that stimulus. Motor function
can also be assessed.
Positron Emission Tomography
This is a relatively un-invasive technique used to measure oxygen, glucose
consumption and receptor function. A radioisotope is injected into the patient’s
circulation which reacts with electrons to emit gamma rays. More metabolically active
tissues will emit more electrons and hence more gamma rays. It has a poor time
resolution but a very good spatial resolution.
Functional Magnetic Resonance Imaging – MRI
This is an un-invasive procedure which like PET scans, has poor time resolution but
very good spatial resolutions. Its results do however depend on the oxygenation level
of the blood.
Grey Matter
The human cerebral cortex is around 2.4 mm thick and its surrounds the cerebral
hemispheres. It is split up into 6 distinct layers:
1. Molecular layer I, which contains few scattered neurons and consists mainly of
extensions of apical dendrites and horizontally-oriented axons.
2. Outer granular layer II, which contains small pyramidal neurons and numerous
stellate neurons making intracortical connections to tother parts of the cortex.
3. Outer pyramidal layer III, which contains medium-size pyramidal neurons, as
well as non-pyramidal neurons with vertically-oriented intracortical axons. It
receives efferents from layer III and sends afferents to layers I and III.
4. Inner granular layer IV, which contains different types of stellate and pyramidal
neurons, and receives inputs from the thalamus.
5. Inner pyramidal layer V, which contains large pyramidal neurons it is the
principal source of subcortical efferents i.e. it projects to the brainstem, spinal
cord and corpus striatum.
6. Fusiform layer VI, which contains few large pyramidal neurons and many small
spindle-like pyramidal and fusiform neurones; it sends efferent fibers to the
thalamus, establishing a very precise reciprocal interconnection between the
cortex and the thalamus.
All the layers receive inputs from the reticular activation system and brainstem
monoaminergic nuclei.
Brodmann’s Areas: Korbinian
Brodmann split up the cerebral cortex into a number of areas each defined by their
function.
White Matter
There is a large amount of white
matter contained within the
cerebral hemispheres. They
consists of different types of
fibres:
Association Fibres: these
connect one area of the cortex
with another within the same
hemisphere.
Commissural fibres: interconnect
cortical areas of two corresponding
hemispheres via the corpus
callosum. These ensure that the
memory traces in one hemisphere
are available to the other
hemisphere.
Projection Fibres: these project from the cortex to
the subcortical regions, and vice versa (incoming
projections mainly come from the thalamus).
o Thalamocortical, corticothalamic, corticopontine
fibres pass through both limbs
o Corticospinal, corticobulbar fibres pass through
posterior limb
Types of Cortex
The neocortex is the largest and most complex part of the cortex. The archicortex and
paleocortex are phylogenetically older and part of the limbic system. There are 2 main
types of cortical area:
- Primary Cortical Area
o Smaller
o Predictable function
o Topographically organised
o Unilateral representation
o Symmetrical
o Stimulation leads to simple moves or
sensory experiences.
- Association Cortex
o Although they are structurally organised, there is no topographic
arrangement.
o Bilateral representation
o Asymmetric
o Normally adjacent to the primary area
o Their stimulation does not lead to simple reproducible effects.
o Can be divided into polymodal and supramodal.
There has also been a third proposed type of cortical area – the higher order areas
which carry out further processing of information from primary modalities. They
supplement the primary motor areas and integrate information coming from the
different systems. e.g. V2-V2 in the visual cortex.
They can be divided up into polymodal and supramodal:
Polymodal (multimodal) association cortex – analyses information from more than
one modality eg posterior parietal cortex
Supramodal cortex – underpins complex behaviour which does not have obvious
sensory or motor function. eg. language areas, prefrontal cortex
‘Association cortex’ usually refers to secondary and tertiary.
Functions of the Association Cortex
Frontal Lobes
 Association area plans sequences of responses or changes in response to fit the
demands and controls of the emotional state
 The higher order area is in charge of motor planning especially of eye movements
and speech (Broca’s area)
 Associated with judgement, foresight, personality, appreciation of self in relation
to the world
 Damage can lead to
o Inability to organise and solve problems
o Personality and emotional changes. Patients become more impulsive,
aggressive – Disinhibition.
o Disorders of eye movement.
o Disorders with speech production.
o Disorders in working memory.
Temporal Lobes
 Association areas involved with learning.
 Higher order area involved with comprehension of language (Wernicke’s Area) in
left and object recognition in the right.
 Damage can lead to
o Disorders learning verbal information in left
o Disorders learning visiospatial information in right.
o Agnosia receptive anaphasia. (cannot recognise visual objects with sight.
Parietal Lobes
 Primary sensory area with somatosensory system.
 Higher order sensory areas.
 Association areas where many sensory modalities and motor inputs converge to
build a picture of where the body is in the environment.
 Posterior parietal association cortex creates a spatial map of the body and
surroundings from multi-modality information.
 Damage can lead
o Lack of sensation of one half of the body.
o Inability to make voluntary eye movements.
o Attentional deficits.
o Inability to organise movement is space – apraxia.
o Defects in route finding and spatial awareness.
o Disorders of language.
o Agnosia.
Occipital Lobes
 Visual association cortex analyses different features of visual images.
 Colour and form are analysed in the ventral pathways.
 Spatial relationships and movement are analysed along the dorsal pathway.
 Damage can lead to disorders of visual perception.
Interhemispheric differences
Left hemisphere is more concerned with language and sequential analysis.
Right hemisphere more concerned with shape, spatial relationships, music.