Download Neocortex Cell Types

Neocortex Cell Types


There are 2 primary neuron types in the neocortex; the stellate (granule) cells
and the pyramidal cells.
Other cell types include the horizontal cells (of Cajal), fusiform cells, and cells
of Martinotti.
Stellate (Granule) Cells




These come in a wide assortment of shapes.
They are typically small (< 10 micrometres) multipolar neurons.
Their short axons do not leave the cortex.
Stellate cells are the principal interneurons of the neocortex.
Pyramidal Cells








Pyramidal cells range in size from 10 micrometres in diameter to 70-100
micrometres of the giant pyramidal cells (Betz cells) of the motor cortex.
A long apical dendrite leaves the top of each pyramidal cell and ascends vertically
to the cortical surface.
A series of basal dendrites emerges from nearer the base of the cell and spreads
out horizontally.
The apical dendrites of pyramidal cells are studded with dendritic spines.
These are numerous small projections that are the preferential site of synaptic
contact.
Dendritic spines may be the sites of synapses that are selectively modified as a
result of learning.
Most or all pyramidal cells have long axons that leave the cortex to reach either
other cortical areas or to various subcortical sites.
Therefore, pyramidal cells are the principal output neurons.
The cells may be broadly divided into two categories: those which are presynaptic to
excitatory (Type I, asymmetrical) synapses, and those which are presynaptic to inhibitory
(Type II, symmetrical) synapses. The excitatory cells include the pyramidal cells and the
spiny stellate cells. These two types account for up to 85% of the cells in the cortex, and
around 75% of all synapses in the cortex are between cells in this category. Pyramidal
cells are more predominant than spiny stellate cells. Pyramidal cells typically receive
inhibitory synapses on their cell bodies, and excitatory synapses at their dendritic spines.
The spiny stellate cells are part of a continuum with pyramids. However, the spiny
stellate cells are smaller, and generally lack a long main projection axon. They are
considered to be primarily a type of local circuit neuron.
The inhibitory cells include Martinotti cells, among others. Martinotti cells have very
compact dendritic arborizations, and have an ascending main axon which typically
reaches layer I of the cortex.
Cortical Layers
Number











Name
I
Molecular layer
II
External granular layer
III
External pyramidal layer
IV
Internal granular layer
V
Internal pyramidal layer
VI
Multiform (polymorphic layer)
The cells of the neocortex are arranged as a 6-layered structure.
The most superficial is the cell-poor molecular layer and the deepest is the
multiform (polymorphic) layer, which is populated largely by fusiform cells.
Between these two layers are 4 layers that are alternatively mostly populated by
stellate or pyramidal cells.
The six neocortical layers are not equally prominent everywhere.
In the motor cortex, layers II-V are dominated by large pyramidal cells to the
extent that individual layers are no longer obvious.
Because of the apparent lack of stellate cells, this type of cortex is known as
agranular.
In contrast, the primary sensory cortex projects mainly to cortical regions and
does not give rise to many long axons.
Layers II-V are dominated by small stellate cells and pyramidal cells.
This type of cortex is thus known as granular cortex.
The agranular and granular types of cortex are collectively called heterotypical.
There is a continuum of structure types ranging between the thick (4.5 mm)
agranular cortex to the thin (1.5 mm) granular cortex.
The intermediate types of cortex in which the 6-layered structure can be seen is
called homotypical cortex.
Local cortical circuits.
There may be a tendency for cells in the infragranular layers to send local axonal
collaterals to the supragranular layers, and conversely; but some collaterals also occur in
the same layers as the cell bodies of their origin. These collaterals generally ramify in the
immediate horizontal vicinity of the cell body, but they sometimes travel long distances
horizontally before ramifying.
Deep pyramidal cells (layers V-VI) have apical dendrites ramifying in layer IV and
above, while cells of layers II and III ramify in layers I, II and III only. Within a layer,
sub-populations of pyramidal cells may sometimes be identified by the layers of
ramification of their apical dendrites, and also by the targets of their axons Spiny stellate
and local plexus (local circuit) neurons occur primarily in layer IV. Inhibitory
interneurons are found in all layers of the cortex.
The function of inhibitory interneurons is evidently to balance the predominance of
excitatory interactions in the pyramidal system, to prevent runaway activation due to
positive feedback. Activation levels of inhibitory neurons in a cortical region may
tend to be highly correlated with the activation levels of excitatory neurons in that
same region.
References
Braitenberg, V. (1977). On the texture of brains. New York: Springer-Verlag.
Braitenberg, V., & Schuz, A. (1991). Anatomy of the cortex. Berlin: Springer-Verlag.
Deacon, T. W. (1990). Rethinking mammalian brain evolution. American Zoologist, 30,
629-705.
Felleman, D. J., & Van Essen, D. C. (1991). Distributed hierarchical processing in
primate cerebral cortex. Cerebral Cortex, 1, 1-47.
Hinton, G., Dayan, P., Frey, B., & Neal, R. (1995). The "wake-sleep" algorithm for
unsupervised neural networks. Science, 268, 1158-1161.
Hopfield, J. J. (1995). Pattern recognition computation using action potential timing for
stimulus representation. Nature, 376, 33-36.
Ingber, L. (1995). Statistical mechanics of multiple scales of neocortical interactions.
Karten, H. J., & Shimuzu, T. (1989). The origins of neocortex: connections and
lamination as distinct events in evolution. Journal of Cognitive Neuroscience, 1, 291-301.
Kolb, B., & Whishaw, I. Q. (1990). Fundamentals of human neuropsychology. New
York: W. H. Freeman.
Mumford, D. (1991). On the computational architecture of the neocortex: I. The role of
the thalamo-cortical loop. Biological Cybernetics, 65, 135-145.
Mumford, D. (1992). On the computational architecture of the neocortex: II. The role of
cortico-cortical loops. Biological Cybernetics, 66, 241-251.
Mumford, D. (1994). Neuronal architectures for pattern-theoretic problems. In C.Koch &
J.Davis (eds.),
Pandya, D. N., Seltzer, B., & Barbas, H. (1988). Input-output organization of the primate
cerebral cortex. In Comparative Primate Biology, Vol. IV : Neurosciences (pp. 39-80).
New York: Alan R. Liss."Large-Scale Neuronal Theories of the Brain". Cambridge: MIT
Press, pp.125-152.
Toyama, K., Matsunami, K., Ohno, T., & Tokashiki, S. (1974). An intracellular study of
neuronal organization in the visual cortex. Experimental Brain Research, 21, 45-66.
White, E. L. (1989). Cortical circuits. Boston: Birkhauser.