What is the mechanism that stops signals from sense organs reaching conscious regions of brain and from brain to effectors? Is there any kind of threshold variation that requires a stronger stimulus during sleep or some other mechanism?
There is another mechanism, at least, I can't talk to whether it is involved in the sleep dampening of senses, all the senses except smell have a portion of their input, routed through the thalamus. Evidence of Blind Sight, suggests that the input that runs through the brain stem, is not linked to the "Experience" of vision, that while it can be used for processing, and thus can influence our actions, the brain is not aware that it is receiving the signals.
Current wisdom is that the Nucleus Reticularis Thalami, acts as a inhibition based system somehow shutting off the sensory feeds to the brain, however this has recently come into some dispute, due to the fact that reverse tracing using a metabolic poison that moves retrograde to the signal, suggests that in fact the connections that were thought to link to the Nucleus Reticularis Thalami, belong to the thalamus instead. If this is the case, then pre-activation in the bottom-up attention system, is more complex than we thought previously involving feedback from the NRT into the thalamus, and gating in the thalamus.
Could it be that these connections change, depending on how much counsciousness/counscious attention you give to the corresponding sense? - When you say "he brain is not aware that it is receiving the signals", which particular part of the brain do you refer to?
Ah, I see that my "Awareness Model" is not shared. When I say the brain is not aware of receiving the signals I mean the subjects that have blind sight all report not being able to see, but often navigate as if they can tell the object is there, for instance they won't trip over a chair in the middle of a room.
This is because the sensory link that comes up through the brain stem, is not part of conscious awareness, and thus the brain is literally not reporting the fact that it has this information to that portion of the brain that "Experiences" vision. It is not that the brain does not receive the information but that it does not report receiving the information to the experience center, and therefore literally is not aware of it. The Unconscious Implicit reaction system however is using the information to navigate whether awareness exists or not.
There is no "Consciousness/Unconsciousness attention aspect, this is clearly part of the Unconscious processing centers. And is not available for conscious attention to "Illuminate". Think of it as a parallel system rather than an attention based effect. Because the brain stem mechanisms predated the ability of the brain to report "Experience" it doesn't connect to the newer "Experience" processing centers, and thus can't be "experienced".
Is the mechanism that inhibits signals from the sense organs during sleep any different than the mechanism that inhibits them at other times?
When concentrating on the task at hand we are not "conscious" of the signals from all our sense organs. Without this screening/inhibiting we could never get anything done. So isn't this the same mechanism just working harder when we are asleep?
Well that is the theory that I am working on. In essence, under concentration, and during meditation perhaps as well as during sleep, the senses that go through the thalamus, are shut down by gating either in the Nucleus Reticularis Thalami (NRT) or in the Thalamus itself. When this happens, the processing centers are more interested in the internal signals such as slow-wave signals that are thought to echo the inputs from the What and Where centers as detected during awake periods back into the hippocampal area possibly to allow a more widespread mapping than the episodal memory alone would be able to do. However, an interesting side effect of sleep shutdown is that it goes two ways, the inputs are gated, and so are the outputs, I have suggested that this might be made possible by the Nucleus Accumbens (NAc). Which has the ability to attenuate signals by changing their priorities via the NRT or thalamus perhaps.
GABAergic inhibition of the thalamus, most likely deriving from the NRT as Graeme suggests, is probably a part of the reason that sensory stimuli doesn't penetrate during sleep. This is consistent with the relatively strong response during NREM sleep to olfactory stimulation, which does not pass throught the thalamus. This is also consistent with the well defined role of the NRT in certain types absence seizures. I don't think this is the complete picture, however, since auditory and tactile stimuli can elicit cortical activity such as sleep spindles. This suggests that part of the disruption of normal sensory processing is due to the state of the cortex which probably involves local inhibition leading to the classically described cortical DOWN state and periodic, synchronized activation producing the corresponding UP states in pyramidal neurons which are the cellular correlates of slow wave activity. REM sleep is a bit of a different story, however.
If the Thalamus based inhibition is more of a lack of pre-activation, sleep spindling might still occur, when the patterns heard during sleep have a high salience of their own, and do not need pre-activation to link to.