Speaker: Randy Bruno
Department: Department of neuroscience
Subject: the many layers of the neocortex
Location: Rotterdam, Erasmus MC
The neocortex is a part of the brain, made up of six layers. The neocortex has a severe influence on cognition. It is involved in functions like conscious thought, recognition, problem solving, motor commands and sensory reception.
Randy Bruno focuses on sensory reception. Sensory information first travels centrally to the thalamus. The thalamus relays the information to other structures in the brain, including the neocortex. The axons from the thalamus innervate the neocortex and especially layer four (L4). Many people believe that the information follows the pathway according to part A of the image. The information first enters L4, than continues to L 2/3 that dispatches it to L5. There was a small part of the information that directly entered L5, but this was always believed to be an unimportant shortcut. After this there is sensory transformation.
Randy Bruno has done research to disprove this theory. To do this he used the whisker system of rodents. This is a system a bit more sensitive than human skin. The axons from the thalamus are projected into every layer of the cortex. The axons are bimodal, with a peak in L5/6 and L4, but L4 is most densely innervated. Axons spike depolarization in each layer. You can look at the time of depolarization. L4 has an early onset, while L2/3 has a late onset. If the other system was true, L5/6 should have a later onset than L2/3, but this is not correct. Half of the cells in L5/6 have an onset as early as L4. So the shortcut may give much information.
Bruno performed an experiment in which a drug (lidocain) was pipetted into L4. This drugs blocks sodium channels, so prevents a response. You can look at the response evoked in L5 with sensory stimulants. According to the old theory there should be no response. But there was almost no difference with a functioning L4. The amplitude of the peak and the time of depolarization were very similar in both cases.
These experiments would disprove the old theory. They give prove for the theory of the system portrayed in part B of the image. Almost all input of L5 comes from the primary pathway and not via L2/3. This is sensory information.
There are axons passing from L2/3 to L5, but they do not give sensory information. Researchers are now trying to determine what kind of information is relayed. The other layers are probably not redundant. They have different biological structures and relay information to different places. Researches do not know what information is dispatched in these layers.
http://www.neuwritewest.org/blog/4167 11-10-15 (Deep cortical layers are activated directly by thalamus by C.M. Constantinople and R.M. Bruno in Science 28-06-2013)
The lecturer spoke clearly, but he still used very technical terms, especially on the slides. This sometimes made the lecture difficult to follow without a purely biological background. The lack of biological information made it easier to hear that the old theory was incorrect. Other people had difficulty with this.
I believe this is a very important topic. The neocortex so heavily influences our behavior and without it we would not be human. So I believe it is very important to know which kind of information is relayed in the neocortex.