Neocortex

Question 1

What are the two main advantages of the columnar organization of the neocortex?

Answer 1

The advantages of the columnar organisation of the neocortex are:

1 - If one column fails, neighbouring columns are able to overtake the function of the failed column.

2 - The neocortex can evolve quickly, because the same wiring structure can be expanded by increasing the number / size of columns in the circuit (rather than evolving a new wiring circuit).

Question 2

List the layers of the neocortex.

Answer 2

Layers of the neocortex:

1 - Molecular.

2 - External granular.

3 - External pyramidal.

4 - Internal granular.

5 - Internal pyramidal.

6 - Multiform.

Question 3

Summarise the connectivity of all 6 layers of the neocortex.

Answer 3

1 - Layer 4 neurones receive input from the thalamus.

2 - This information is sent up to layers 2/3.

3 - Layer 2/3 neurones combine input from layer 4, forming subnetworks.

4 - Information from these layer 2/3 subnetworks is sent down to layer 5.

5 - Layer 5 neurones integrate this information, forming subnetworks based on the type of information encoded, and where the information is to be sent.

6 - Layer 5 neurones send this integrated information to layer 6.

7 - Layer 6 uses this information to provide feedback to the thalamus, forming a feedback loop.

*Remember layer 1 serves as a means of communication between layers 2-5.

Question 4

List the characteristics of the molecular layer (1) of the neocortex.

Answer 4

Molecular layer (1):

• Low cell density.
• Contains horizontally-set fusiform cells.

Connectivity:

• Most of the layer consists primarily of the dendritic processes of cells of lower layers 2-5.
• Layer 1 fusiform cells form synapses with these dendrites, connecting dendrites of the lower layers. This serves as a means of communication between other layers.
• Axons of fusiform cells only make local connections.

Question 5

List the characteristics of the external granular layer (2) and external pyramidal layer (3) of the neocortex.

*The reason for why these layers are grouped is explained in the last card.

Answer 5

• The external granular layer (2) contains small granular cells.
• The external pyramidal layer (3) contains pyramidal cells of variable size.
• Both layers have a high cell density.

Connectivity:

• Receives and combines input from the internal granular layer (4), and creates subnetworks from this input.
• These subnetworks send axons down into the internal pyramidal layer (5) for integration.
• Layers 2/3 also send axons horizontally to other regions of the neocortex.

Question 6

List the characteristics of the internal granular layer (layer 4) of the neocortex.

Answer 6

Internal granular (4):

• Many different cell types.

Connectivity:

• Receives most of the neocortical input from the thalamus.
• Layer 4 neurones send axons up into layers 2/3 for integration and sorting of thalamic information.
• Layer 4 neurones also send some axons into the multiform layer (6), which receives the rest of the thalamic input.

Question 7

List the characteristics of the internal pyramidal layer (layer 5) of the neocortex.

Answer 7

Internal pyramidal (5):

• Medium to large sized pyramidal cells.
• Contains two types of excitatory neurones: corticocortical and corticosubcortical neurones

Connectivity:

• Receives input from layers 2/3.
• Both corticocortical and corticosubcortical neurones integrate information from layers 2/3 based on the type of information encoded, and where the information is to be sent.
• Both of these neurones send axons down into the multiform layer (6).
• Corticocortical neurones also send integrated information horizontally to other regions of the neocortex.
• Corticosubcortical neurones also send integrated information to subcortical neurones.
• Both of these neurones also send axons to the basal ganglia.

Question 8

List the characteristics of the multiform layer (layer 6) of the neocortex.

Answer 8

Multiform (6):

• Contains fusiform and pyramidal cells.
• Contains two types of excitatory neurones: corticocortical and corticothalamic neurones.

Connectivity:

• Corticocortical neurones receive integrated input from the internal pyramidal layer (5).
• Corticocortical neurones of layer 6 ‘form another large group of pyramidal cells that send long horizontal axons which form connections across cortical columns and cortical areas’.
• Corticocortical neurones of layer 6 also relay this information to corticothalamic neurones of layer 6.
• Corticothalamic neurones of layer 6 send this information into the thalamus, creating a feedback loop between the thalamus and the cortex.
• Corticothalamic neurones also receive some of the input from the thalamus, and from layer 4.

Question 9

Why are layers 2 (external granular) and 3 (external pyramidal) of the neocortex often considered a single layer?

Answer 9

• Layers 2 (external granular) and 3 (external pyramidal) are often considered together as layer 2/3 because:

1 - They are difficult to distinguish experimentally.

2 - They have similar patterns of connectivity.