Sensory Input Entering the Cortex - V4 Layer

[Gmanme]

hi,

Can someone tell me excatly where the sensory input enter the cortex? Mainly tell me at what cortical layer( I've heard V4)? I am talking about sensory information directly from outside the cortex ( through eyes/ear/etc..) not input from other lobes or areas in grey matter.

Any other info on the "flow' of synapitical fireings would be usefull.

Thanks all

 

[atyy]

http://www.frontiersin.org/Journal/10.3389/neuro.01.1.1.002.2007/full
"These thalamic relay cells project to the middle cortical layer 4 and to a lesser extent lower layer 3 and to layer 6 of specific cortical regions, especially primary sensory regions, in a topographically precise manner (Jones, 1998 )."

http://www.ncbi.nlm.nih.gov/pubmed/15711543 (link to free text at top right)
The thalamorecipient layers (4 and upper 6) were dominated by simple cells ...

http://jn.physiology.org/content/102/3/1483.long
"Classic (but necessarily somewhat oversimplified) views of “feedforward connectivity” in cortical circuits suggests that information propagates from middle layers mostly first up to supragranular layers and then down to infragranular ones (Binzegger et al. 2004; Thomson and Bannister 2003; Thomson and Lamy 2007)."

 

[Gmanme]

So essentially sensory info enters through the middle layers, (mainly layer 4, but also 3&5?) and moves both directions, up and down through the layers. At the same time? is that vaguely correct?

Q1- Also, does the flow of synaptic firings move through the layers then back to layer 4 and moved to other lobe areas? or does it more just dissipate?

Q2- Are the electrical firings from neurons a wave that could possibly cancel or or amplify each other?, or is it not possible for the electrical current to be on the same paths at the same time??

Sorry if I am vague, or don't make sense, I am a software engineer, not a neurologist.

 

[atyy]

So essentially sensory info enters through the middle layers, (mainly layer 4, but also 3&5?) and moves both directions, up and down through the layers. At the same time? is that vaguely correct?

It's an oversimplification, but a general pattern seems to be that sensory information enters from the thalamus into layer 4, then goes to the upper layers (2,3) which in turn send information to the lower layers (5,6). This flow of information is excitatory. There are inhibitory connections, but these are within each layer.

Q1- Also, does the flow of synaptic firings move through the layers then back to layer 4 and moved to other lobe areas? or does it more just dissipate?

Information from the primary sensory cortices moves to other cortical areas. The cortex (not necessarily primary sensory cortex) also sends information to places like the basal ganglia, which is involved in movement; or the amygdala, which is involved in emotion.

http://en.wikipedia.org/wiki/Visual_cortex
http://en.wikipedia.org/wiki/Basal_ganglia
http://en.wikipedia.org/wiki/Amygdala

http://elife.elifesciences.org/content/2/e01157 has amazing videos of a patient "awakened by a sleeping pill". How this works isn't known, but the authors talk about the intracortical connections, as well as connections from the cortex to the basal ganglia and back to other parts of the cortex that may explain such phenomena.

Q2- Are the electrical firings from neurons a wave that could possibly cancel or or amplify each other?, or is it not possible for the electrical current to be on the same paths at the same time??

The main mechanism is that one neuron spiking either excites or inhibits another neuron's spiking. If a wave description is sometimes helpful, one should still keep the underlying mechanism in mind.

 

[Pythagorean]

Q2- Are the electrical firings from neurons a wave that could possibly cancel or or amplify each other?, or is it not possible for the electrical current to be on the same paths at the same time??

Many neurons exhibit binary firing above threshold. They either fire or they don't. The underlying passive currents, though, that set excitability for the cell, do superimpose (cancel and amplify). In the dendrites, this seemingly happens a lot as incoming inputs mix with inputs that are reflecting off the soma (backpropagation in dendrites).

There are also contributions from axo-axonic synapses that can lead to what's called antidromic spike propagation (when spikes travel backwards). It's conceivable that a forward traveling and backward traveling spike meet and create some kind of nonlinear analog of superposition. However, the peak of the action potential is just about the reversal potential of sodium. Once it reaches that potential, the driving force becomes zero (sodium is "in equilibrium") and the potential can't go any higher, so it wouldn't simply be twice as high of a spike.

In the case of two spikes traveling in the same direction, it's mostly "not possible for the electrical current to be on the same paths at the same time" because as each piece of axon fires, the channels on it enter a refractory state where they can't be excited for a moment, so the action potential leaves wake of temporarily disabled channels behind it. (Note this isn't the case if they're traveling opposite directions until the moment they meet at which point one might predict that neither wave travels past that point as the channels on either side are in refraction).