Why “sleep”?

In a previous article, we talked a bit about narcolepsy as one of the very intriguing sleep disorders. It was perhaps easy to understand why people suffering from narcolepsy could have a pretty hard time performing several normal tasks; however, most of us would probably relate less to narcolepsy. But something which almost everyone can agree to have experienced regularly, in one way or another, is sleep. In comparison with disorders associated with it or derived from its impairments, sleep itself might not seem so interesting. We all do it and we can’t deny how much we enjoy it and long for it after it stops. Yet, there is much more to sleep than we think.

Sleep is very important for the normal functioning of any being. For animals as well as for humans, sleep helps in energy conservation, body restoration, predator avoidance and learning aid. Different animals have different sleep-wake cycles, from nocturnal animals (like rodents), which sleep during the day and are active at night, and animals which sleep with only half of the brain (like dolphins), all the way to diurnal animals, like humans. Although humans are advised to sleep approximately 8 hours per night, some people sleep very little (around 2-3 hours/night) and still function perfectly fine. An example of such a situation is presented in the textbook of Rosenzweig et al. (pg. 389).

But what triggers sleep and how is it regulated?

Most of us are certainly able to recall a dream the next morning and the memory of that dream is usually accompanied by feelings and emotions we sometimes do not even experience in real life. We are often under the impression that our dream has lasted the whole night. In fact, there are two stages of sleep, one of which is associated with the formation of dreams. These stages, known as non-REM sleep and REM sleep, succeed each other in cycles lasting approximately 90 minutes. Just to define the terms, REM means rapid eye movement and represents the part of sleep with the most increased brain activity. Interestingly, during REM the brain seems to consume more oxygen than during arousal!

Normally, when we fall asleep we slip¬†into the non-REM stage or the slow-wave sleep (SWL). This, in turn, is divided into four other stages: from light sleep to very deep sleep. During this phase, the brain is said to be truly resting and the body appears to repair its tissues. No dreams can be seen! The movement of the body is reduced, but not because the muscles are incapable of moving; it’s the brain which does not send signals to the body to move! One interesting feature of non-REM sleep is sleep-walking.¬†This peculiar behaviour some people show while asleep¬†usually takes place during the fourth (last) part of the non-REM sleep, when the person is the¬†deepest sleep. This is the reason why it is very difficult to wake a¬†sleepwalker up.

In turn, REM sleep (which starts after a 30-minute non-REM period) is the “active” part of our sleep. This time, the brain sends commands to the body, but the body seems to be in an almost complete¬†state of¬†atonia¬†(immobility). The heart rate and breathing become irregular and the brain is not resting. In fact, our dreams happen during this time and more importantly, our long-lasting memories are thought to be integrated and consolidated.

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When it comes to sleep regulation, many neuroendocrine systems and brain functions play a role. The circadian (or sleep-wake cycle), which is controlled primarily by the suprachiasmatic nuclei, in the hypothalamus, need special attention. For the purpose of this article, I won’t focus on¬†the circadian clock¬†now, but I will come back to this in a future article. The autonomic nervous system and parts of the brain such as the brainstem, the limbic system, especially the amygdala, and the forebrain modulate different aspects and stages of sleep. Amygdala, which I mentioned in a previous article about emotions and decision-making, is a brain region involved in the emotions such as fear. It also appears to be very active during¬†REM-sleep and may account for the awful nightmares we often experience.

Many cognitive functions, such as intelligence, performance and emotions are associated with disrupted non-REM as well as¬†REM sleep. To be more specific, REM-sleep loss appears to be associated with increased anxiety and stress and loss of emotional neutrality – this means that a person deprived of REM-sleep is more likely to react negatively to neutral emotional stimuli than in normal conditions. The explanations vary, but most of the studies agree that impaired REM sleep triggers increased release of noradrenaline, hyperactivity of amygdala and decreased function of prefrontal cortex (which tells¬†“stop!” to the amygdala when it goes crazy). At the same time, people deprived of non-REM sleep could experience¬†depression, due to deficiency in another neurotransmitter, this time an inhibitory one, called GABA (gamma-aminobutyric acid).¬†Other problems linked to sleep deprivation are attention deficits, working memory impairments and usually affected divergent thinking (creative, innovative thinking).

Aging people seem to sleep less and this deprivation is also associated with conditions like Alzheimer’s. Moreover, sleep deprivation can kill you! Sustained sleep loss can cause low immune system and drop in body’s temperature, which can make bacterial infections fatal. Another consequence of sleep loss is increased metabolic rate, which leads to weight loss and eventually death. Don’t think this could be a good idea for a diet! More like for “die”!!!¬†Having said that, most people should try their best to get enough hours of sleep.

I hope this article convinced you of the importance of sleep and as usual, any questions or comments are welcome ūüôā

Further information:

Article 1 Рabout REM-sleep and emotional discrimination 

Article 2 Рabout non-REM sleep and GABA 

Article 3 Рabout how sleep loss affects behaviour and emotions

Article 4 Рa review on many articles about the link between sleep deprivation and emotional reactivity and perception

Bear et al., 2006. Neuroscience ‚Äď Exploring the Brain. s.l.:Lippincott Williams & Wilknins¬†

Rosenzweig et al., 2010. Biological Psychology РAn Introduction to Behavioural, Cognitive and Clinical Neuroscience. 6th edition. Sinauer Associates Inc.,U.S., pg. 380-401

Both images by Gabriel Velichkova

Smells, learning and memory

After seeing this article’s title, you might have thought: “That sounds rather boring. I mean, what is so interesting about the nose?” Perhaps the “memory” part aroused your curiosity, though. If that’s so, you might find the following reading worth having a look at, as you could discover some surprising things about the “nose”.

I’d like to begin by emphasising something important: we don’t actually smell with our noses; it’s the brain that identifies different odours through the central olfactory pathways, but we’ll get to that soon. What does happen in our nasal cavity is the activation of the olfactory receptors (a type of neurone) of the primary olfactory system, by chemical stimuli¬†called¬†odourants. The binding of odourants to the olfactory receptors’ cilia triggers the transduction process, which involves G-protein stimulation, formation of the cyclic AMP (cAMP) and membrane depolarisation, by the opening of ion channels (calcium, sodium and chloride). This complex signalling cascade results in a receptor potential which is then coded as an action potential (provided the receptor potential reaches a certain threshold) and then transmitted further along the receptor’s axons (remember, they are actually neurones!) The axons form the olfactory nerve, but they also group in small clusters and converge onto the two olfactory bulbs, in spherical structures known as glomeruli. Here, the axons synapse upon second-order neurones which form the olfactory tracts and finally project to the olfactory cortex (involved in the perception of smell) and to some structures in the temporal lobes, the medial dorsal nucleus (in the thalamus) and the orbitofrontal cortex. The last two are thought to play an important role in the conscious perception of smells. A pretty intricate process, isn’t it? But it is¬†a lot more to olfaction than this!

Running parallel to the primary olfactory system is the accessory olfactory system. This has been shown to detect our favourite smelling chemicals, the pheromones. As I am sure most of you are aware of, pheromones are involved in reproductive behaviours, identifying individuals, aggression and submission recognition. Not only the type of chemical stimuli, but also the structures in the accessory olfactory system are different: the vomeronasal organ in the nasal cavity, the accessory olfactory bulb and last but not least the hypothalamus and amygdala (and hippocampus) as the final axonal targets. The amygdala and hippocampus are known for their implications in emotions and long-term memories (check out article about memory). Thus, olfaction also plays an important role in the integration of different odours in emotion processes, as well as explicit memory and associative meanings to odours.

Interestingly, each receptor cell is defined by only one receptor protein, which is encoded by a single receptor gene. These genes form the largest family of mammalian genes: 1000 in rodents, 350 in humans. The receptor cells have unique structures and are divided into different types according to their sensitivity to odours: each receptor type is activated by a single odour; nevertheless, one odour can activate many receptor types and the combination as well as the frequency, rhythmicity and temporal pattern of receptor stimulations encode for odour information.

Studies in Drosophila have shown another very important function of the olfactory processes: the associative learning. Gustatory unconditional and odour conditional signals both converge on the antennal lobe and mushroom body of the Drosophila, establishing learning efficacy of appetitive and aversive memories in classical conditioning. The release of certain catecholaminergic neurotransmitters such as dopamine and octopamine (the insect analogue of noradrenaline) are involved in the aversive and appetitive behaviours, respectively. In an incredibly revealing study, Lee Chi-Yu and his colleagues developed this topic in much more detail. I strongly recommend you have a look at it here.

As you might have guessed, given the fact that olfaction has a wide range of implications, its impairments are present in different mental diseases. Olfaction deficits or absence (anosmia) have been identified in Alzheimer’s disease and dementia, whereas olfactory hallucinations and weird smells are one of the main symptoms of schizophrenia.

Hopefully, you didn’t find this article too long or confusing. Did you find out new information about olfaction? If you have any questions or comments, please feel free to upload your¬†posts. I’m looking forward to them as always.

For further information:

Bear et al., “Neuroscience”, third edition, Lippincott Williams & Wilkins

My friend’s article

Another very relevant article

Photo by Isuru Priyaranga 

Spatial memory, grid cells and…honeycomb?!

Remember when we talked about memory? One very important aspect you should never forget is that there are many types of memory (stored in different areas of the brain). Today I’d like to discuss a particular and very important type of memory,¬†known as¬†spatial memory.¬†As a matter of fact, I’m going to be even more specific: this article deals with one of the several types of cells involved in spatial memory, which were recently discovered. These were called¬†grid cells¬†(the name¬†is very suggestive, as you’ll see in a few moments).

You may wonder why I chose this topic. Obviously, it is of great interest for many scientists, but to be honest, my own subjectivity played a role in here as well. For those who haven’t met me, you should know this: there is no one in the world less capable of spatial self-orientation than myself! Therefore, when I heard that a young Norwegian couple (the Mosers) won the Noble Prize in 2014 for discovering some neurones specialised in location memory, it came as no surprise that I became extremely curious about their research.

As I mentioned before, grid cells are not the only neurons involved in spatial memory. In the 1970, John O’Keefe discovered navigation in the brain along with the cells that generate this process, known as¬†place cells.¬†These neurons are located in the hippocampus (a structure of the temporal lobe associated with¬†long-term and spatial memory) and appear to code for location, by creating a spatial map of the environment. They are dependent on the motion’s speed and direction.

Grid cells represent a type of place cells, although unlike place cells, they are predominantly found in the medial entorhinal cortex. Some grid cells were also found in the prefrontal cortex, which is involved in forming episodic memory. A quite peculiar characteristic about these neurons is that they fire whenever the subject is at certain locations and these signalling locations form a hexagonal pattern, very similar to a honeycomb (which explains the rather odd image I chose for this article).

This honeycomb-like pattern is not only mathematically precise, but it is also very efficient as it uses a minimum number of grid cells to achieve the highest-possible resolution, thus saving energy.¬† It was also discovered that the firing patters remain constant during the subject‚Äôs movement, regardless its speed or direction. Moreover, unlike the response of place cells, which is based on visual stimuli, the grid cells‚Äô firing is maintained even in the absence of sensory stimuli, thus showing an algorithm based on self-motion.¬†In an interview for Deutsche Welle Magazine, Edvard Moser says:¬†“It is thought that these [grid cells] are part of an internal map that is based on our own movement, so that these cells signal the distances we move and the directions we take”.

The discovery raises scientists’ hopes up in treating neurodegenerative diseases linked to memory, such as Alzheimer’s disease, as it has been demonstrated that the first symptoms of dementia appear in the entorhinal cortex. At the same time, for other people, like me, grid cells may offer an explanation as to why they still can’t find their best friend’s house, after having been there several times.

Articles related to the subject:

 Nature

 New Scientist 

 Deutsche Welle 

Image created by Isuru Priyaranga