Cellular organisation and

biophysical properties of the

cerebral cortex

•! Cellular components of the cerebral cortex

•! How cells are organised in three dimensions

•! How cells are interconnected to form distinct circuits

•! Patterns of electrical activity

Different types of cortex

Neocortex (or isocortex):

•! Formed by 6 cellular layers

•! Marked expansion during mammalian evolution

•! Thicker and differentiated in many areas

“Older” cortices:

•! Less than 6 layers

•! Relatively conserved during evolution

Archicortex: hippoccampus

Paleocortex: pyriform cortex of the medial temporal lobe

Neocortical areas

Lateral view of the left hemisphere of the owl

monkey brain, showing the location of some

areas

“Unfolded” cortex, showing most of the

sensory and motor areas

5 mm

•! Each of the main

sensory modalities is represented by

several areas in the cortex.

•! Red= vision

•! Blue=hearing

•! Green= somatic

sensory

•! There are also

several motor areas

(purple).

•! The “blank” regions

are more remotely linked to sensory

processing or motor

control. They form the association cortex

Cortical cytoarchitecture

1

6

6

1

•! Although the 6-

layered scheme is present throughout

the neocortex, the lamination is slightly

different for each

area.

Preparation of monkey visual cortex,

stained by the method of Nissl. This

stain reveals the locations of cell

bodies (dark spots)

Magnified view

neurone

glia

Cortical neurones: pyramidal cells

Preparations stained by the Golgi method. Only a

few of the neurones in this field are revealed.

Apical dendrites

Cell body

Axon

Basal

dendrites

•! PYRAMIDAL CELLS are the main type of cortical excitatory neurone.

•! They are the only cells that project long axons to other brain areas. However,

most pyramidal cells project locally, forming intrinsic connections.

•! They have a long apical dendrite with multiple branches, which projects towards

the pia mater, as well as a complex basal dendritic tree.

•! The axons emerge towards the white matter.

Cortical neurones: stellate cells •! STELLATE CELLS lack apical dendrites. They

are further subdivided into:

•! SMOOTH STELLATE CELLS: They are

the cortical inhibitory interneurones.

•! Use GABA (!- aminobutyric acid) as

neurotransmitter.

•! Act by modulating the activity of

pyramidal cells.

•! Found in all cortical layers.

•! SPINY STELLATE CELLS: A class of

small cortical excitatory interneurones.

•! Found primarily in layer 4 of the

cortex. Also known as granular cells

(hence layer 4= granular layer)

•! Receive the bulk of the thalamic

afferents, and relay these inputs to pyramidal cells.

Interneurones= cells that only synapse with

other cells within the same area

A GABAergic cortical interneurone

Cortical neurones in layers

Spiny stellate cells: receive

the area’s main excitatory

inputs

Large pyramidal cells: long

extrinsic projections

Small pyramidal cells:

intrinsic projections and

short extrinsic projections

Smooth stellate cells (not

shown) are found spread

across all cortical layers.

“Vertical”

excitatory

interactions 1. Main excitatory inputs arrive

in the granular layer (layer 4).

2. This information is processed

and integrated in the supragranular layers (2 and 3).

3. The processed data are

relayed to other cortical areas.

4. These data are also relayed

to the infragranular layers (5

and 6), which then send feedback projections to “earlier”

areas or to the thalamus.

5. The feedback also modulates

the processing in the granular

and supragranular layers

Most excitatory interactions in the cortex

occur “vertically”, i.e., across the layers.

Preparation of monkey visual cortex (Gallyas method).

This stain reveals the locations of myelinated axons

(darker= more myelin). Note that the myelinated axons

tend to run across the layers

V1

V2 •! These interactions give

rise to functional

columns, running from

layer 1 to layer 6.

•! Every cell in a column shares some functional

properties.

Example:

•! All cells in a right eye

dominance column respond

more strongly to stimulation of

the right eye than to the left

eye.

•! However, cells in different

layers also have slightly

different characteristics (e.g.

monocularity in layer 4).

Summary

•! The cellular circuits that form the cortex are somewhat

stereotyped across areas.

•! The principal inputs that define an area’s function arrive in

layer 4, usually to small excitatory interneurones (granule

cells). This information is then processed by cascades of

neurones, including other layers.

•!The circuits feed back to “earlier” levels of processing at

all stages.

•! Many of the excitatory interactions are columnar.

Optical imaging

•! Cortex that is more active has different optical properties. It reflects light in a different way than

non-active cortex.

•! In optical imaging, the light reflected by the cortex is monitored while an animal sees different

visual patterns.

•! The spatial distribution of columns of cells that react to different stimulus parameters can then be

mapped with a video camera.

Optical imaging

Complete pattern of orientation columns in area V1 of the tree shrew. The

location of columns of cells sensitive to different orientations is coded by the

different colours.

Columnar organisation of V1 in primates •! Multiple stimulus

parameters are mapped

within V1, including ocular

dominance, boundary

orientation and colour.

•! Note that these columnar

systems are NOT mutually

exclusive. For example, a

cell which is in a column that

codes for vertical orientation

is ALSO in a given ocular

dominance column.

•! All the information needed

to analyse what is

happening in a point of the

visual field is contained

within a block of cortex less

than 1 mm wide (detached).

“blobs”(regions rich in

colour-sensitive cells)

Ocular

dominance

columns

Orientation

columns

Columns aggregate in spatially precise

patterns to form topographic maps

Stimulus on the computer

screen. The black and white

“checkers” indicate that the stimulus was blinking all the

time, in order to cause a more

effective activation of visual

cortex

Pattern of activation on the surface of V1. Because only

one eye was stimulated, the cortical “image” of the

stimulus has a striped appearance, due to the ocular dominance columns (darker= columns activated by the

stimulated eye).

One

“hypercolumn”

Summary

•! Columns of cells sensitive to a given parameter form

regular functional maps which repeat themselves in a

modular fashion.

•! “Global” topographic or cognitive maps are formed by the

aggregation of many modules, in ways that reflect the

area’s function.

Lateral connections between columns

•! In addition to the columnar interconnections, cortical cells form horizontal,

or intrinsic connections.

•! These connections are important to integrate information across different

parts of the same area (for example, in visual cortex, to integrate what is

happening in different parts of the visual scene)

Result of an experiment in which a tracer substance was injected into a single cortical layer (black

oval). In addition to columnar interconnections, the tracer revealed axons that project horizontally

within the same layer, and also form collateral branches in other layers (less extensively).

1

2 & 3a

3b

3c

4"

4#

5

6

250 !m

4-5 mm !

1-2 mm !

Lateral connections between columns

•! The synaptic sites of horizontal axons are highly specific. They may travel

several millimetres, ignoring several columns, and then form tightly clustered

terminal fields. This clustering is particularly clear in layer 3.

Schematic diagram of the

pattern of horizontal

connections of pyramidal cells in layer 3 from one cortical

column (darker blue) to other

columns (lighter blue).

Specificity of horizontal connections

•! In V1, long-range horizontal connections connect cells that are selective for

the same orientation, representing different parts of the visual field.

•! One possible function for this would be signalling the continuity of a

contour.

Results of two experiments

in which a tracer was

injected in a column sensitive to a particular

orientation (black and white

inserts). The terminal

clusters concentrate on

columns that are selective to the same or nearby

orientations.

Dynamic ensembles in the cortex

•! The extensive integration allowed by horizontal connections allows groups

of pyramidal cells to fire as synchronised ensembles.

•! In the visual system, this synchronised firing may be the neural signal to

indicate continuity of a contour or an object, and separate those from other

objects.

“Static” optical imaging map

of the columns sensitive to

vertical orientation, obtained by averaging many seconds

of stimulus presentation.

This square represents a

region of cortex about 2mm

by 2 mm. LIGHTER regions are the ones activated by the

vertical stimulus.

This movie was generated using the technique of spike-triggered optical imaging, in which the data

collection was synchronised to the occurrence of action potentials in a cell selective for vertical

orientation. It represents the activity from 200 msec prior to the action potential, to 200 msec after the action potential.

At the time of spike occurrence, the activity spreads to adjacent columns of cells selective to the

same orientation whether the spike was elicited by visual stimuli, or occurred spontaneously. Thus,

“ensembles” of concurrently active neurones are continuously being created as the scene

changes.

Synchronised firing and contour integration

•! Two neurones with overlapping receptive fields (1 and 2) were studied

simultaneously in the middle temporal area (a region of visual cortex which analyses direction of motion). The two cells were selective for different directions of motion

(arrows).

•! Synchronised firing was found only when a single object (black bar) was moved

across both receptive fields, in a direction intermediate between the “optimal” for each cell (A).

•! Stimulation with two bars, each “optimal” for one cell, disconnected the

“ensemble”- each cell was now part of a separate network, each representing one

object (B).

Time 0 Time 0

Summary

•! Cortical pyramidal cells, especially in the supragranular

layers, are integrated via horizontal axons.

•! The pattern of connections afforded by horizontal axons is

very precise.

•! Cells form dynamic ensembles in which they fire action

potentials synchronously.

•! In sensory areas, this synchronised firing may be the

neural correlate of the perception of an object as distinct

from other objects. In association areas- representation of

a thought, memory, emotion?