23A: Cortex Flashcards
The cerebral cortex develops from the embryonic _______
pallium
How does the cerebral cortex develop?
as outpocketings of the proscencephaon, the most anterior (rostral) part of the neural tube. Specifically, the cerebral cortex is a telencephalic structures, with the two hemispheres forming laterally on either side of the telencephalon
By ~___ days gestation, the cerebral hemipsheres have grown over most of the rest of the brain
- Note that from this age to about 6 months gestations, the cortical surface is smooth or lissencephalic, and by birth at 9 months the surface of the cortex has developed its gyri and sulci
By birth at 9 months, when the surface of the cortex has developed its gyri and sulci, it is said to be __________
gyrencephalic
What makes humans the smartest?
Humans have one the largest brain to body weight ratio (not a perfect correlation). However, note that there is not credible correlation between brain size and intelligence wthin humans or any other species
What are the lobes of the cerebral cortex?
frontal, parietal, occipital, and temporal
What are the three main types of cortex based on histology?
allocortex,
isocortex (6 layers, at least at some pt in development)
and mesocortex (less regular and may have 3-5 layers)
Describe Allocortex
(ex. hippocampal formation, olfactory cortex). Three layered
Three types of cortex have also been defined in terms of age of evolutionary origin, namely:
paleocortex (e.g. olfactory cortex)
archicortex (e.g. hippocampal formation)
Neocortex (synonymous with isocortex)
During development, archicortex is formed from what?
medial pallidum
During development, paleocortex is formed from what?
lateral pallidum
During development, neocortex is formed from what?
dorsal pallidum
Describe the layers of isocortex
From pia to deeper white matter:
Layer 1 (aka the molecular layer)- cell poor and all that is present in mature brain is GABAergic interneurons
Layers II and III- continuous and hard to tell apart from one another (aka the superficial pyramidal cell layer (after the most common cell type))
Layer IV contains many small cells (aka the granular layer)
Layer V- the deep pyramidal cell layer (the largest pyramidal cells are found here)
Layer VI- contains multiple cell types (aka the polymorphic layer)
Note about isocortex
Since layer IV is the granular layer, layers I-III are known as supragranular layers and layres V and VI are subgranular layers (or infragranular)
How are neurons classified?
pyramidal or nonpyramidal on the basis of soma shape plus the configuration of dendrites.
spiny or aspiny
Describe Pyramidal cells
they have a pear-shaped soma and a single dominant apical dendrite (as well as a basal rosette of dendrites). These cells send their axon deep to the deep white matter and are the projection neurons of the cortex. They project locally and to other cortical areas, as well as subcortical areas.
Pyramidal cells are excitatory, using ______ or ________ as their primary NTM
glutamate or aspartate
Describe nonpyramidal neurons
Mostly GABAergic interneurons (local circuit neurons that only project locally within a given area of cortex). GABAergic nonpyramidal neurons are typically multipolar (several similar sized dendrities from the soma) or bipolar (2 dendrities on opposite ends of the soma)
In layer IV of the primary sensory cortex, there is another type of nonpyramidal neuron that uses glutamate as its NTM, namely:
spiny stellate cells (these cells may be a subtype of pyramidal cell but they only project locally)
Describe spiny vs. aspiny neurons
What are some nonpyramidal cell types?
chandelier, basket, and neuroglia form cells
The dominant input to most cortical neurons comes from where?
other cortical neurons. In particular, the excitatory pyramidal neurons are highly interconnected
The main extrinsic input to most cortical neurons comes from where?
the thalamus
Another source of extrinsic input are the widely projecting brainstem nuclei (modulatory), which include:
the locus ceruleus (nor), raphe nuclei (5-HT), ventral tegmental area (dopamine), and basal forebrain nuclei (ACh)
There are two types of thalamic input to the cortex, specific and non-specific. Describe specific input.
Input from the thalamus that projects to a single cortical area and typically concerns a specific modality (ex. VL to motor cortex,, VPL for somatosensory cortex, lateral geniculate for visual cortex or medial geniculate to aduitory cortex)
There are two types of thalamic input to the cortex, specific and non-specific. Describe non-specific input.
Comes from thalamic nuclei that integrate info from many sources and this input is thought importnt for general brain states and arousal (e.g. intraaminar and midline thalamic nuclei)
The input from specific thalamic nuclei is centered in what isocortex layer?
layer IV (in primary sensory areas this synapse is on the spines of spiny stellate cells)
and some II, III, and V (all excitatory)
The input from non-specific thalamic nuclei is centered in what isocortex layer?
layer I (local interneurons and apical tuft of pyramidal cell apical dendrites)
Note that all of the extrinsic inputs enter the cortex from the deep white matter and travel vertically
Pyramidal cells are the principal projection neurons of the cortex. In general, pyramidal cells in what layers are the main source of cortico-cortical connections?
II and III. These include association fibers that project ipsilaterally (local and long distance) and callosal/commissural projections (cross to equivalent area of contralateral cortex via the corpus callosum)
Layer II/III pyramidal cells also send some axons to the subcortical telencephalon (especially the basal ganglia/striatum)
Layer __ pyramidal neurons are the main putput of cortex in general
V. These cells project to various subcortical regions, including the spinal cord (eg. corticospinal tract), pons (corticopontine), tectum (corticotectal), and basal ganglia (corticostriatal)
Pyramidal cells in the more superficial part of layer 5 (5A) tend to be thinner, with a less robust apical dendrite and project to contralateral cortex and to subcortical telencephalic targets like the striatum
Pyramidal cells in the deeper parts of layer 5 (5B) are more robust in form and tend to project beyond the telencephalon (e.g. spinal cord, tectum, pons, brain stem)
Layer 6 pyramidal cells primarily project where?
the thalamus- thus, thalamus proejct to the cortex and cortex projects back to the same areas of thalamus. This feed back loop is the basis of thalamocortical rhythms observed in EEGs (thes rhythms are important in regulation of sleep-wake cycle, consciousness, and absence epilepsy
Chandelier cell function
selectively contact the basal dendrite of pyramidal cells (and especially the axon initial segment). This latter connection allows the GABAergic (inhibitory) chandelier cells to powerfully inhibit the output of the layer V pyramidal cells (and thus the output of the local circuit)
Basket cell function
Axon terminals of basket cells terminate as baskets that surround the soma of pyramidal cells, again allowing inhibitory control at the final summing pount for synaptic inout from the whole dendritic tree at the soma. Thus basket and chandelier cells have powerful inhibitory function on cortical output
Double banquet (bipolar) cell function
also GABAergic and inhibitory but its primary axonal termination is on more distal branhces of the apical and basal dendrities. This makes these cells more influential on LOCAL signal processing in the dendrites (where most excitatory input to pyramidal cells occurs on spines)
What are cortical columns?
The cortex is thought to be composed of repeated modules called cortical columns, that describes the vertical arrangement of inputs and outputs
Cortical column functioning:
The thought is that specific (excitatory) thalamic input is to a small group of layer IV spiny stellate cells. These excitatory layer IV cells in turn project to layers II and III pyramidal cells. The layer II/III pyramidal cells (excitatory) project to layer V/VI pyramidal cells (excitatory). These cells are the excitatory output of the column
Axons of layer V pyramidal cells project to other columns locally, other areas of cortex, and subcortically
The anatomy of pyramidal cell axons, together with lateral inhibition by interneurons, set the dimensions of the column
How do cortical columns develop?
The basic idea is that all of the pyramidal cell members of a developmental mini column are ancestors of a single precursor cell in the ventricular zone of the embryonic cerebral vesicles. The precursors give rise to pyramidal cells and radial glia.
The radial glia migrate to layer I and neurons migrate along the radial glia to their mature laminar location, where they fully differentiate
T or F. Interneurons are not generated in the ventricular zone of the cerebral vesicles
T. Rather they derive from cells from the medial ganglionic eminence (and cauda ganglionic eminence) and then migrate to the neocortex to give rise to a majority of cortical GABAergic interneurons
Currently the belief is that cell types and local circuits are similart throughout the neocortex. How then does the cortex deal with all of its many functions?
What are some ways that cortical areas can be defined?
- by histology (Brodmann’s areas)
- connections (e.g. with thalamic nuclei)
- function
What is an example of heterotypic cortex?
primary motor complex (precentral gyrus)- in motor cortex, layer IV (granular layer) is almost absent (thus, aka agranular cortex). In contrast, layer V is very large in motor cortex (large pyramidal cells project to spinal cord and subcortical motor centers)
The primary sensory cortex are also heterotypic. However, in these areas, layer IV (the thalamic input zone) is large and layer V is small (thus, these are aka granular cortex, or koniocortex)
What are Brodmann areas?
52 distinct areas of human cortex based upon density and types of cells found in each area.
Where is the primary motor cortex located?
in the precentral gyrus (BA 4)
Where is the primary somatosensory cortex located?
BA 3,1,2
Where is the primary visual cortex located?
banks of calcarine fissure in BA 17
Where is the primary auditory cortex located?
in Heschl’s gyrus (transverse gyri: areas 41,42)
What is BA 6?
premotor cortex (ex of unimodal association cortex)
What are BA 5 and 7?
superior parietal love- unimodal association areas for somatesthesias
Homuncoli of the primary motor and sensory cortex
Describe the somatosensory cortex
this cortex receives input from the thalamus to areas 3,1,2. These areas are arrayed in the postcentral gyrus from anterior to posterior (as below). Note how area 3 is further subdivided into an anterior 3a and a posterior 3b (seperate homunculi exist for each sub area)
Areas 3a and 2 receive input from where?
afferents in muscle and deep tissues
Areas 3b and 1 receive input from where?
cutaneous inputs (there is further subdivision by the particular type of cutaneous inputs (pain, flutter, vibration, etc.)
Where do BA 3,1,2 project to?
BA 5 and 7 (unimodal sensory association cortex). In these areas, info about different sub modalites is combined. In addition, dynamic attributes (e.g. direction and speed of movement of objects on skin, texture) are also analyzed here.
This unimodal association area in turn projects to heteromodal areas in parietal, frontal, and temporal cortex to further manipulate it.
What is the binding problem?
What is the main function of parietal association cortex?
attention to the physical world
What is the main function of temporal association cortex?
important for naming things (what is it?)
T or F. There is considerable overlap in the functions of association cortex from the lobes
T. Functional MRI suggest that connections between cortical areas may be more important for particular functions, rather than strict localization of function to particular grat matter
Thalamic input can also define particular areas of cortex. Parietal and occipital association cortex receive its specific input from where?
the pulvinar nucleus
Thalamic input can also define particular areas of cortex. Temporal association cortex receive its specific input from where?
lateral posterior nucleus of the thalamus
Thalamic input can also define particular areas of cortex. Prefrontal association cortex receive its specific input from where?
medial dorsal nucleus (MD)
What happens if the parietal cortex is damaged in the dominant hemisphere (left side for most people)?
pts suffer from language disorder (aphasias)
What happens if the parietal cortex is damaged in the NON-dominant hemisphere (left side for most people)?
a phenomena called sensory neglect, in which these pts. ignore sensory experience on the half of the body contralateral to the injury
What is the largest component of the cortex?
the prefrontal association cortex (nearly 1/3rd of cortical volume)- it is also the latest part of the brain to develop postnatally
What are the parts of the PFC?
the orbitofrontal and dorsolateral components- collectively, these are responsible for determining personality and our sense of ‘self’
What is the main function of the orbitofrontal prefrontal cortex?
considered part of the limbic system and is associated with aggression and emotions
What is the main function of the dorsolateral prefrontal cortex?
for working memory (e.g. keeping numbers in mind to make a phone call) and planning behaviors
