K-complexes are patterns of activity in which the voltage across the scalp rises then drops. K-complexes seem to kick off a period of reduced firing between neurons, perhaps because of the massive changes in the neuron’s polarity.
To explain why those large large fluctuations lead to a period of quiet, we need to dig into the way that signals propagate through the brain. EEGs measure the combined effect of action potentials in the brain. These action potentials are the electrical signals that travel from the cell nucleus, down the axons. However, the electrical potential is not what is transmitted between neurons. When an axon terminal reaches a sufficient voltage (positive or negative with respect to its surroundings) this opens ‘gates’ in those axons. What is released are neurotransmitters: chemical signals that bind with the receptor sites on other neurons. Those neurotransmitters cause a voltage change in the receiving neuron. If enough neurotransmitters from enough axons reach the dendrites of a given cell, gates open in the neuron, allowing either positively or negatively charged ions to enter the cell. If enough channels open to let in enough positive (or negative) ions then an action potential builds up again. For a K-complex to occur, there must be a large, coordinated flow of neurotransmitters that allow positive ions into the neurons, followed by a large, coordinated flow of neurotransmitters that allow negative ions to accumulate in the neurons. After that, the reserves of neurotransmitters are pretty much exhausted, which is why the rates of neuronal firing fall throughout the brain just after the K-complex.
It would seem from this that K-complexes are designed to keep us asleep, to exhaust the neurotransmitters rapidly and leave our brains in what is called a ‘down state.’(Cash et al. 2009) However, recent research suggests that there may be more to it than that. K-complexes may serve as a kind of sentry, telling us whether to stay asleep, or to wake up.
Combined EEG and fMRI recordings show that when K-complexes are picked up at the scalp, the brainstem, thalamus, and sensory areas of the brain are all doing…something. Thirty-seven incredibly resilient volunteers managed to fall asleep inside a banking, clanking fMRI tube, and their activation levels during K-complexes were recorded and analysed by Professor Kolja Jahnke and colleagues at Goethe University in Frankfurt. For some time it had been thought that K-complexes were largely spontaneous, but Prof Jahnke’s team discovered that sensory processing areas of the brain are active during K-complexes. They particularly noticed that auditory processing areas of the cortex were active. As they pointed out: ‘acoustical scanner noise is unavoidable during [fMRI].’ In other words, between the pumps driving helium around the coils, to the rapid vibrations of the coils themselves, the inside of an fMRI machine is loud. It seems as though, during the up-state of K-complexes, we’re open to processing sensory information, although the parts of the brain involved in conscious awareness (the frontal-parietal area) stays fast asleep.
That would explain why K-complexes can be useful ‘wake points’ if we hear a worrying noise in the night. But how do they keep us asleep? Activity along the midline of the brain in particular seemed to preserve sleep by triggering the downstate mentioned above.
K-complexes are then usually followed by sleep spindles. Sleep spindles are patterns of activity in the brain that are believed to help us go to sleep and stay there, although this might not be entirely true in very young children: babies whose brains produce more sleep spindles are no more difficult to wake up than those babies who produce fewer. (Horne et al. 2003) But in older children and adults, these spindles seem to be a necessary part of stage two NREM sleep. They might be part of the mechanism by which sleep helps us build memories and retain skills. People are more likely to remember more words, shapes and faces the next morning if they experienced more sleep spindles during the night. (Schabus et al. 2004; Clemens et al. 2005) Sleep can improve our physical skills too. People were asked to type the sequence 4-1-3-2-4 as quickly and accurately as possible with their non dominant hand. Those who napped between the practice and test phases did better than those who spend the intervening time awake. (Nishida and Walker 2007)
Babies and young children show a distinct pattern in their development of sleep spindles. Babies in their first year have rapid sleep spindles, which occur roughly once every 20 seconds, and they last for about 1-1.5 seconds. Children between the ages of four and 16 have slightly faster frequencies and similar durations with sleep spindles lasting around 1.5 seconds and occurring every five to six seconds. But between the ages of one and three, something unusual happens. Despite the density of learning that toddlers are undergoing, sleep spindles get further apart – once every 40 seconds at one year of age, to 110 seconds at 19 months, to every 83 seconds at two years and four months. The durations of spindles drop too, roughly halving from the length that babies and older children experience. (Scholle et al. 2007) No one is quite sure what is happening during those toddler years, or what other mechanisms might take over to aid learning, but we can be certain that there’s something that is just different about sleep in the toddler years. The development of K-complexes, too, is rapid for the first two years, variable until five, but doesn’t reach mature rates until at least early adolescence. (Metcalf et al. 1971) Perhaps some of this wakefulness is not so much disordered sleep as developmental stages.