Module 12: Clock Outputs

what are the two ways that clock outputs can be characterized?

• message that the clock is sending to the rest of the brain

• what is the medium by which this message is delivered

what are the ways that neurons can deliver messages?

• classical synaptic transmission, mediated by axons making directed synapses with post-synaptic neurons

• releasing signaling molecules that diffuse to their targets through extracellular fluid, or via the circulatory system

how far can non-synaptic diffusion travel in the brain?

across millimeters, and potentially through cerebral ventricles to the neuroaxis

what analogy describes paracrine/endocrine signaling?

radio broadcasting - one sender, many possible receivers

what happens to sleep patterns in SCN-ablated rats during the baseline day?

they become arrhythmic but get the same total sleep as intact rats

what does the similar total sleep in SCN-ablated rats suggest?

SCN does not control total daily sleep duration

what role does the SCN likely play in sleep-wake regulation?

it likely promotes wake during the wake phase and sleep during the sleep phase

why is it unlikely that the SCN only stimulates wake or sleep?

because if it did only one, it would affect total sleep duration, which it doesn’t

sleep rebound

after deprivation, sleeping more than usual for that time of day

how does sleep in intact rats compare to SCN-ablated rats during the wake phase?

intact rats get much less sleep, even with comparable sleep deprivation

what is suggested about the SCN’s role during the wake phase in the ablated study?

SCN actively opposes sleep during the wake phase

what does the ablated rat study and its phenomenon relate to humans?

if humans stay up all night, they usually can’t sleep much the next morning, showing a similar SCN effect

why were intact rats able to sleep more than the ablated rats during the usual sleep phase?

the SCN actively promotes sleep during the normal sleep phase, without it, you cannot achieve the high levels of sleep

what was observed about activity in temporal chimera hamsters?

they were active only when both clocks (24h and 20h) signaled the active phase. If either clock indicated rest, no activity occurred.

what does the temporal chimera hamsters suggest about the SCN’s role during the rest phase?

the SCN emits a dominant inhibitory signal during its rest phase that suppresses activity, even if another SCN promotes activity?

what are the two main ways the SCN can transmit its circadian “message”?

• axonal outputs making directed synapses with brain targets (classical transmission)

• secretion of signaling molecules that diffuse to nearby or distant targets

what did the polymer capsule study conclude about SCN restoring behavioral circadian rhythms?

it can do so without making axonal connections with the brain

TGF-alpha

molecule that binds to EGF receptors in the region just above the SCN

What happened when TGF-alpha was infused above the SCN?

• running stopped, suggesting that it blocked activity but did not shift the clock