The Organization of Neocortical Lamina

0:00 – 0:30  [Basic Definition]  In about 90% of the cerebral cortex you would find 6 layers of cells and these layers are called a lamina.  The cell density in each of the six lamina varies across the brain, and this is the basis for dividing the cerebral cortex into Brodmann’s areas.  Lamina 1 contains axons but few cell bodies.  Lamina 2 and 3 contain neurons that project to other cortical areas.  Lamina 4 contains neurons that receive most incoming signals from other parts of the nervous system.  And Lamina 5 and 6 contain neurons that project to areas outside of the cortex such as the thalamus and spinal cord.

0:30-2:30  

If we slice into motor cortex and do a Nissel stain and look at the cell bodies, we see something like this. It’s interesting to compare what we see here to what we saw in primary visual cortex. There’s a number of differences that we can see immediately. First though, ignore the fact that it sort of looks like there’s fewer cells here – that’s not real, we probably just have a weaker stain here in motor cortex.

First of all, you’ll notice that there’s like a number missing here. It goes like, one, two, three, five. But what about four? It turns out that you don’t really see a distinct layer four like we do in sensory cortices. Recall that layer 4 is the layer that gets sensory input from the thalamus. That doesn’t happen in motor cortex, because it’s not a sensory cortex. So we don’t need sensory input from the thalamus. So there is essentially no layer four.

In place of layer 4, we see an expanded layer five. Why might we have an expanded layer 5? Layer 5 is the main subcortical output layer. Layer 6 is also an output layer, sending output to thalamus. But layer 5 sends output everywhere, including the basal ganglia, output to the brainstem, output to the spinal cord.This is where our corticospinal neurons live. Because motor cortex needs to send so much output to subcortical brain structures, layer five is big.

If we zoom in on layer five, we see these ginormous cells here. This is like a regular cell. And this seemingly like eight times bigger cell is called a Betz cell. As far as I know, these cells have only been observed in primates. They’re smaller in numbers – I think about 5% of your corticospinal neurons are Betz cells. These cells are pretty interesting. They’re huge.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about the neocortical lamina.  But Neuroscience is an interdisciplinary field, so there are many words for this term.  The neocortical lamina are often referred to simply as layers – so for example, you may see the neocortex defined as the six-layered region of the cerebral cortex.  When comparing and contrasting the lamina in two different regions of the neocortex, you may hear the terms cytoarchitecture which is the study of cellular organization, and histology which is the general study of microscopic biological structures.

3:00-4:00  [Here’s a real world example]  Detailed knowledge of the neocortical lamina has played and continues to play an important role in the study of neuro-anatomy. For example, did you know that many years before regions of the cerebral cortex were named according to function, they were named according to structure of the neocortical lamina. In the early 1900s, Korbinina Brodmann looked at areas of the brain under a microscope and found 6 layers of cells in the cerebral cortex.  The thickness of each lamina varied from region to region, and these differences were used to define the 52 Brodmann’s areas that are still used to this day when labeling parts of the brain.  Recall that the connectivity patterns of the cells in each lamina are distinct.  For example, lamina 4 contains the cells that receive signals from the thalamus, so this lamina is particularly dense in the primary sensory areas.  Importantly, while Brodmann’s areas were defined based on cytoarchitecture, which is the organization of cells, we now know that these regions are not only structurally different, but functionally different as well.  For example, Brodmann area 17, which is defined according to the density of cells in each lamina, is also known as the primary visual cortex, when defined according to function.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapter 10: “The Central Visual System”

2) Neuroscience (Purves)

• Chapter 12: “Central Visual Pathways”