Ask a physiologist

Have you ever wondered why your heart rate increases when you get frightened or undertake exercise? Or why your stomach growls when you’re hungry? Physiology and physiologists are here to answer your questions! Take a look at the questions below or submit a new one to be answered.

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On one level, yes, more precisely than for any other organism. This is because, uniquely, you can ask a person for their date of birth, which is one of the first things that a doctor will ask, because it is vital, providing a context for interpreting other symptoms and signs that the patient may have. This is important because it has allowed medical science to correlate disease processes closely with age in humans.

On another level, no. As far as I’m aware there isn’t anything like tree rings in people, which would allow you to date them precisely to, say, within a year. However, there are things that change as the body ages: bones get thinner, muscle bulk lessens, teeth wear and fall out, injuries accumulate, as do skin blemishes. We all notice the effects of these things on people’s appearance and we are quite good at accurately eyeballing someone’s age to within a decade, or even five years or so.

Doctors tend to distinguish between chronological age – how old someone actually is – and biological age – how old they appear to be. This recognises the fact some people age more quickly than average (especially if they indulge in damaging activities such as smoking, substance misuse or excessive alcohol intake), while others with the right mix of genes age more slowly. This means that every so often we get our eyeball estimate wrong and have met older people delighted to be taken for younger than their real age!

Although there’s no general method for dating people who are alive, there may be special instances. For example, it wouldn’t surprise me if there’s a clever physical method of identifying the signature of the Chernobyl nuclear accident in people’s tissues, and perhaps even more accurately in those who were undergoing their pubertal growth spurt at the time.

Once people have died, we lose a lot of the clues that allow us to guess someone’s age. Carbon dating means that we can tell quite accurately how long ago a person died. Analysis of tooth wear can provide information, but requires assumptions about the diet. Archaeological or forensic estimates of adult age based on bones tend to be rather vague – young, middle aged and old being about the level of it. Anyone who can devise a method of accurately assessing a dead person’s chronological age from their bones will open up a whole new area of research.

This is one of the ‘big’ questions for which we do not yet have a complete answer. The human brain contains upwards of 100 billion neurones (nerve cells) which form a network that continuously sends and receives signals, then processes them to control every aspect of our behaviour – from simple tasks such as breathing, to the complex such as perceiving emotion, or recalling memories. Neurones “communicate” with each other at specialised sites called synapses where one neurone releases a chemical (a neurotransmitter) that binds to specific proteins (receptors) on a second neurone. A major neurotransmitter in the brain is glutamate and its receptors play a critical role in memory storage. When glutamate acts on its receptors, positive ions such as Na+, K+ and Ca2+ flow though a pore in the receptor, producing a tiny electrical signal in the neurone and this is the starting point for communication between brain cells.

Glutamate synapses can undergo a process called synaptic plasticity. What does this mean? Put simply, if two neurones communicate with each other a lot, then a remarkable thing happens – efficiency of the communication between them increases. This idea was first suggested by Donald Hebb in the 1940s, but demonstrating this effect took many more years. Experiments performed by two physiologists, Timothy Bliss and Terje Lømo, and published in The Journal of Physiology in 1973 reported the discovery of the phenomenon we now call long-term potentiation. This is the process now considered to be a mechanism by which memories are stored.

So where is memory stored? One answer to this is part of your question is: at synapses. However, you are probably asking where in the brain memories are stored. Well, it depends on the type of memory.

We have working memory, which you are using right now so you can make sense of what you are reading. We also use working memory to do mental arithmetic, or to remember a telephone number. Working memory is thought to be stored in the frontal and parietal lobes of the brain.

We also have long-term memory, which can be divided into several types. Semantic memory is our ability to recall facts e.g. giraffes come from Africa, and the basal ganglia and cerebellum are important for this. Next we have the ability of recognising situations that alter our emotions – we feel comfortable when surrounded by our family and friends but may become fearful in an unfamiliar situation. The amygdala plays a vital role in emotional memory.

Finally, we need to consider what we refer to as ‘episodic memory’, which we use to recall our own experiences. The hippocampus is important for this type of memory. Use your episodic memory now to recall what you had for dinner last night. Most of you will be able to do that quite easily. Now what did you have for dinner three weeks’ past on Tuesday? Unless that date was a special event, such as your birthday, you are unlikely to recall what you ate. This is a feature of episodic memory – we find it easier to recall events if they are associated with other significant events. So for those of us who have no special association with that date, remembering that meal will need us to recall some other event that happened!

Gases emitted from burping and farting are different in origin. Burping, or eructation, to give it its medical name, is simply the escape of air from the stomach. When we eat or drink, we swallow air as well as food or liquid and an excess of air may escape as a burp. This happens more often if you eat too quickly or drink gassy liquids. Some of the air we take in from eating and drinking exits the stomach and passes into the intestines.

A fart, more formally known as flatulence, is the escape of gas mixtures from the large intestine, via the rectum. The gas mixture originates from two sources: some is from swallowed air and some is produced by microorganisms, such as yeast and bacteria, that live in our gut. I’m sure you have realised by now that eating certain foods produces more flatulence than others, such as beans, often result in flatulence. This is because these foods arrive in the bowel only partly digested and the gut microorganisms finish off the job, producing flatus gases in the process.

After many decades of research there is still much debate about the function of sleep. Everyone would acknowledge that a good night’s sleep makes you feel more alert, energetic and better able to function. Lack of sleep has clear consequences; it not only makes you grumpy in the morning, but also impairs concentration and performance, can disturb vision and slow reaction time. Chronic sleep disturbance can reduce immune function and increase the risk of obesity, diabetes and cardiovascular disease.

It seems likely that no single theory will explain why we sleep, though several have been proposed. One of the earliest ideas – the adaptive or evolutionary theory – suggested that sleep evolved as a survival strategy to keep an animal away from harm at vulnerable times, for example in darkness or when predators were active. However, it seems logical to suppose that remaining conscious – if inactive– might prove a safer policy. A related idea proposes that sleep is a mechanism to allow an individual to reduce energy expenditure, perhaps at times when the search for food is least likely to be successful.

Other theories have been proposed that suggest that sleep allows the body and brain to repair and replenish themselves. Activities associated with restoration such as growth hormone secretion, and protein synthesis occur during sleep, and babies (if parents are lucky!) sleep lots. Sleep plays a critical role in brain plasticity not only in infants but also in adults, and research indicates that sleep quantity and quality can have an important influence on learning and memory.

A hiccup occurs as a result of a sudden brief, often violent, involuntary contraction of the diaphragm, and the other muscles used when breathing in. Physiologists have speculated that there could be a “hiccup-generating centre” leading to stimulation of the phrenic nerve (the nerve that originates in the neck and passes down between the lung and heart to reach the diaphragm). But neither the centre nor the precise physiological trigger for the hiccup has been identified so far. The ‘hic’ noise comes when the breath is cut off by the closure of the vocal cords snapping the windpipe shut.

There are many theories as to why we hiccup although no one is really sure. One idea is that hiccups evolved when we walked on four legs, and that they helped us to swallow food stuck in our throats. Now that we walk upright, swallowing is aided by gravity. The sharp breath in creates a vacuum behind the food to help suck the lump down. This might explain why dogs eating food quickly are prone to hiccups. Other suggestions are that hiccups result from our evolution as sea creatures, when gills were used to breath. Finally, they could be linked to how we learn to suck as babies.

There are literally hundreds of recommended ‘cures’ for hiccups: drinking out of the back of a cup, a cold key down the shirt and being frightened are commonly used. Usually the best cure is holding your breath to gain control of your breathing muscles. Hiccups have been around for millions of years and the exact cause remains a mystery.

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