Cerebral Cortex Layers

The 6 layers of the cerebral cortex look different depending on which stain is used to observe them, and each stain provides us with different insights into their structure. The golgi stain stains a limited number of cells at random in their entirety. This is useful because otherwise, neurons are so densely packed that It would be difficult to observe their structure. The Nissl stain stains ribosomal RNA on the rough endoplasmic reticulum dark blue, and hence shows cell bodies and proximal dendrites. The Weigert stain shows myelinated fibers and hence axonal distribution.
Let’s describe some of the cell types are found in these 6 cortical layers. Pyramidal cells are some of the largest neurons in the brain, and have long apical dendrites that extend towards the cortex surface, as well as a set of basal dendrites. Betz cells are the largest pyramidal cells. Stellate cells are neurons with a star-like shape formed by their dendrites. Rosehip neurons are inhibitory neurons with a bushy appearance.
Now, let’s cover the layers. Layer one is closest to the outer surface, and layer 6 is just above the white matter. The layers above layer IV, the internal granular layer, are called the supragranular layers, while the layers below are called the infragranular layers.
Layer I is called the molecular, or plexiform, layer and has few neurons. Neurons that are found there include rosehip neurons, some spiny stellate cells and cajal-retzius cells. Other cells include subpial granular layer cells and glial cells. This layer is mostly made up of transverse fibers - extensions of the dendrites of pyramidal neurons. Layer II is called the external granular layer, and it contains lots of stellate neurons, as well as small pyramidal neurons. Layer III is called the external pyramidal layer, and it is predominantly occupied by small and medium-sized pyramidal neurons. There are also other neurons with vertically-oriented axons. Layer IV is called the internal granular layer. It is occupied by stellate and pyramidal cells. Layer V is called the internal pyramidal layer, and it features scattered Betz cells, which again, are large pyramidal cells. Layer VI is called the multiform, or polymorphic layer. This is the layer where cells of multiple forms merge with the white matter. The neurons in this layer differ in the shape and distribution of their axons and dendrites, as well as their physiological properties. Some of the cell types include pyramidal neurons and stellate neurons, as well as bipolar, or fusiform neurons.
Now that we’ve discussed the layers, let’s do a brief overview of the connections formed by each layer. Layer I gets substantial input from the thalamus, specifically from matrix, or M-type thalamus cells. Neurons in the first three layers are the principal targets of interhemispheric corticocortical afferent fibers. So they receive communications from cortical neurons in the other hemisphere. Layer III is also the principal source of corticocortical efferents, meaning that it signals to other cortical neurons. Layer IV receives inputs from core, or C-type, thalamus cells. The infragranular layers primarily connect the cortex with subcortical regions. Betz cells in layer V are important for voluntary motor control, and this layer is especially well developed in motor cortical areas. Betz cells are found in the primary motor cortex of the frontal lobe and their axons travel through the internal capsule, then through the brain stem and spinal cord, forming the corticospinal tract. They also project to the basal ganglia, reticular formation, cerebellum, and other areas. Finally, layer VI has reciprocal connections with the thalamus.
Now that we’ve discussed the connections formed by each layer, it is easier to understand what we observe with the Weigert stain. Exner’s plexus is the name for the fibers in the molecular layer. The band of bechterew is the name for the myelinated fibers in the superficial portion of layer III. The outer and inner bands of Baillarger are myelinated fibers coursing through layers IV and V respectively. Finally, there are the deep tangential fibers and vertical fibers. Note that not every region of the cortex has the same number of layers. For example, the primary olfactory cortex has only 3 histological layers.

[@vmmfilho]

The last sentence was the most important one. Not every part of the cortex has 6 layers, which help us differentiate them in their functions. Thanks man!

[@MichaelPayPlus]

This video explains the concepts very clearly and the graphics are very well made! Awesome job!

[@pedrammahmoudi4052]

Thank you very much! This seems like perfect timing as I was looking for a video like this on CNS Histology just as my finals are about to come up!
Keep up the good work

[@davidschmid6963]

beautiful drawings! love the splitting of the layers :)

[@Arunava_Gupta]

This was a very good video with great pics, explanation and presentation style. 👍

[@tobiaszb]

TBT by Hawkins.
L2 Object coded.
L3 orientation of sensor,
L4 Feature @ location,
L5 sensation @ orientation,
L6 location (allocation to be precise)

[@KarlMagnusBlindheim]

Brilliant video! Thank you so much for your effort and excellent communication skills. You have a new fan!

[@ericchang9568]

does the signal travel from layer 1 to 6 or 6 to 1?

[@gengpan]

in ephys, it is hard to tell layers some time to me. So far I mainly determine layer based on neuron density but which I feel not precise.

[@MatMabee]

This video was phenomenal. Thank you very much.

[@abdualaziz8046]

Thank you, very informative in a simple way

[@afnannaaz3004]

please do cover all the topics from anatomy its really helpful in fun manner

[@omerkeremtastepe2271]

This video is legendary man thank you so much!

[@jvamaral]

Do you recommend any book or article for that topic?

[@ae3846]

very good job thanks

[@jtkw]

Thanks for the video. It is so well done! One suggestion for future video: Optogenetics. There are loads of video on this topic. However, NONE (and I have watched a few dozens of them looking for this answer) discussed HOW scientists identify specific neurons so that, for example, channel-rhodopsin genes are ONLY placed within specific clusters of several hundred neurons and not its neighbouring neurons? Thank you in advance!

[@Fadel-wj9wk]

Thank you

[@doricenguemadje5761]

Thanks you for your explanation. My question is which layer is most prominent in the sensory areas and in the motor areas I still don't understand that. Thank.

[@Dubbah08]

Excellent video, thank you so much!

[@chaimaa4303]

thank you

[@ibraheemalma]

thank you.

[@robinhodson9890]

Do you have any information on how cortical neurons activate, or how information flows through them, as compared with artificial neural networks? The two are extremely different, eg no separate training mode in biology, and no back-propagation.

[@jaykuchera5058]

very well done. Thanks

[@Joeythegoats]

you could also explain Von Economo division

[@shine9796]

Thank you so much, it really helps me!

[@InquilineKea]

if layer 1 through thalamus, then does thalamus send neurons THROUGH other layers (an inefficient scheme) to reach layer 1?

[@ahmadbuz2010]

Do you know the answer now?

[@LovinBxDJxB]

very nice video!!! really helped a lot

[@ErenNeseli]

Thanks for the video but our book says there are no pyramid cells in the 4th layer. 🤷🏻‍♂️

[@ellyza2096]

There are in fact pyramid cells in the 4th layer, just not as prominently as in the 5th

[@salisaabdirisaq7122]

Brodmann’s area please main areas and their function

[@Dunedain432]

Seeing as the substantia alba is underneath these layers, I'm guessing that must mean that these 6 layers essentially depict the substantia nigra, correct?

[@ecaterinasavenco6680]

No 🙂 Substantia nigra is a subcomponent of the basal ganglia, which is a subcortical region, while this video covers the 6 layers of the Neocortex, which is the most outer part of the cortex.

[@blueover6580]

@@ecaterinasavenco6680 lol im too dumb to understand all i needed was the info at 2:32

[@homognitor3378]

@@ecaterinasavenco6680 ого, все таки англоязычные более информативные, не так ли ?

[@ecaterinasavenco6680]

@@blueover6580 oh no! Don’t say that! This stuff can be confusing 🧠😵‍💫

[@bublisoniyak6227]

Awesome!

[@shalinikalai8915]

Pls upload subdivisions of anatomy

[@citizensofheaven2987]

Wow could you refer me to a good book like this and the human anatomy of the human brain or maybe a book with the human body names of the entire anatomy layout? The brain. The heart. The lungs and how they all do what they do. Sorry for the long ? I’m autistic but want to know more to I was wondering maybe there’s a book human anatomy u could refer me to?

[@alialdarrat5324]

Nice

[@Fadel-wj9wk]

Thank you