Senses

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Every sense has a sensor, which is a mechanism for providing information back to the brain about our surroundings or about the internal state of the body.

We have many more than five senses, and I’m not talking about the 6th ESP (extra-sensory perception) that was once popular.  We have a multitude of senses as we will discuss below; at least 20 or more.

Primary senses of the head

These first few senses are the most familiar to us.  These senses have direct connections to the brain so that the information gets there quickly.

facial senses

Sight or vision (opthalmoception) – sensitivity to light through the eyes.  There are two separate receptors; rods and cones.  Rods are sensitive to light intensity, and cones are sensitive to colours and don’t work so well in dim light.  Humans are trichromatic, meaning that we have separate cone colour receptors for red, green and blue light.  In dim light we are slightly tetrachromatic, meaning that the rod receptors play a greater role.  Stereoscopic vision is an additional visual sense, which is a post-sensory function carried out by the brain (in the Occipetal Lobe).  Human eyes are aligned to the visible light spectrum (red through violet) of our sun.  We have very little sensitivity to infrared, but we can feel infra-red as heat using temperature sensors in the skin.  Our eyes do not see ultraviolet rays, but again our skin has some sensitivity and uses the UV rays for vitamin D production.  The receptor cells in the eye are sensitive to ultraviolet, but the lens of the eye blocks the UV rays so they don’t reach the cells at the back of the retina. Our ability to see at night is poor as we do not have a tapetum lucidum which some other animals do (like dogs and cats).

Hearing (audioception) – sensitivity to audio sound waves through the ears.  This is usually quoted to be from around 20 Hz to 20 kHz but most people have reduced capability.  We tend to feel low frequency sounds below 100 Hz (through the vibration sensors in our skin) more than hearing them, and as we age our high frequency range decreases (my upper limit is now around 12 kHz).  Some animals have much better hearing ability than humans, to above 100 kHz (birds, bats, etc).  Just as having two eyes provides us with stereoscopic vision which provides significantly better perception of distance, in the same way having two ears allows for binaural hearing and the ability to locate the source of a sound much more accurately.  Binaural hearing is an additional sense, and relies on brain processing (in the Auditory cortex in the Temporal Lobe).

Taste (gustaoception) – is a chemical sense which uses five types of receptors in the tongue: sweet, sour, salty, bitter and umami (or savoury).  There is debate whether metallic, calcium and fatty acids are also basic taste types.  Flavour is a post-sensory combination of taste, smell, texture and temperature.

Smell (olfaction) – is another chemical sense received through the nose.  Unlike the limited range of taste receptors, there are many different types of smell receptors that are sensitive to different types of molecules (probably more than 388).  A smell can be very unique and hard to describe, and due to the close linkage with the brain, specific smells can be linked to and trigger memories (and also allergic reactions) and they can act directly on our unconscious brain system.

The senses of sight, hearing, taste and touch (see below) are relayed from the brainstem to the Temporal, Parietal and Occipetal Lobes of the brain for further processing and interpretation.  The nose receptors (smell, and possibly also vomeronasal) go directly into the Limbic brain.  This is something we have inherited from our animal ancestors.  Being such a fundamental and primitive sense, it is also buried deep in our unconscious brain and we have little control over it.  For example, in some people certain odours can trigger allergic reactions, or bring up memories of traumatic experiences – and being buried at such deep levels in the brain it is hard to change our behaviour.

Skin senses

We know that our skin is responsive to touch (pressure), but there are other sensory receptors in the skin covering our entire body.  The density of the sensors does vary; for example, there are more nerve endings on the hands and feet than on the back or legs.

Skin senses

Touch (tactioception) – is the response to pressure.  There are sensors are in the skin, hair follicles, tongue and throat.  There are two separate pressure sensors:  fine touch and crude touch.  An example of crude touch is moving the palm of one hand quickly over the forearm on the other side of your body.  You are aware that the forearm has been touched, but not the precise location.  Injuries involving the temperature or pain receptors may deliver sense information along with crude touch – resulting in us knowing that we have been hurt or injured, but not precisely where.  Fine touch requires information from two closely located nerve sensors, and it enables quite precise identification of the part of our body that has come into contact with another object.  Fine identification of the area touched is important for the sense of proprioception, which will be discussed further below.  Fine and crude touch sensors work in parallel, but are carried by separate nerve pathways to the brain.

Vibration (mechanoreception) – there are four different skin receptors that are sensitive to mechanical vibration.  Merkel (5 – 15 Hz), Meissner’s (10 – 50 Hz), Pacinian (to 250 Hz, sudden stimuli only), Ruffini (slow reaction to skin stretch, eg feeling an object slipping away, and important for the Kinesthetic sense).  Sustained pressure is detected by Bulbous sensors.

Temperature (thermoception) – we have separate sensors in the skin for hot and cold.  These temperature sensors in the skin are separate from the sensors responsible for the internal temperature regulation within the body.

Pain (nociception) – we have separate sensors for chemical, mechanical and thermal stimuli, and they are there to tell us about external substances that could result in damage to our physical body.  These nerves to do not respond until a certain threshold has been reached.  At that point they begin sending signals to the brain, which can in some cases result in an immediate reflex reaction (usually triggered in the brainstem) without requiring the conscious brain to receive the information and make a decision about what to do.  When I was 3 years old I put my hand on the hot plate on the stove, resulting in an immediate reflex reaction!  Pain receptors are located on the skin and also in some joints and internal body organs.  There are no pain receptors in the brain.

Ultraviolet – the skin has melanocyte receptors for ultraviolet light.  This is responsible for the skin turning brown (melanin production) for protection, and also used for manufacturing vitamin D which is important for body health.  The receptors are sensitive to ultraviolet UVA (400 – 315 nm) and UVB (315 – 280 nm) wavelengths, and whilst they do their best to darken the skin as a shield from radiation, long exposure to the suns rays can be very damaging to the skin.

uv spectrum

Before leaving the skin sensors there are some other interesting ones that may or may not be classed as separate senses.  Itching is related to nociception (pain), but uses different receptors and nerve pathways.  Pain results in a withdrawal reflex, whereas itch results in a scratching reflex.  Itch could be considered as another sense.  Tickling is another interesting response, and is different from both itch and pain.  There are two types of tickling – knismesis and gargalesis.  Knismesis is light tickling, it is more of an annoyance like an itch (eg a small insect crawling over your skin) and does not result in laughter.  Gargalesis is stronger tickling and often results in an involuntary laughter response.  It doesn’t seem to be unique to humans, but only a few other types of primates exhibit this behaviour.  Knismesis seems to use the itch nerves, whereas gargalesis involves both the pain and touch receptors.  If you try to tickle yourself, you can invoke the knismesis response but not gargalesis.

There are other skin reactions such as goose-bumps, flushing and blushing which are involuntary reactions to certain sensory or emotional states and are not usually categorised as separate senses.  The trigger mechanism seems to be via the parasympathetic nerve pathways from the brain in response to other sensory stimuli.

Senses requiring brain processing

Some basic senses also require some post-sensory processing in the inner brain.  Stereoscopic vision and binaural hearing have already been mentioned above.  Here are some more:

Balance (equilibrioception) – whilst we may not think of it as a separate sense, the sense of balance is extremely important.  It relies on input from sensors near the ears, and also information from the eyes and proprioception (see below).  It is an involuntary sense, and dizziness (or vertigo) can result if it is disrupted.  Ear infections and drinking alcohol can upset the sensors in the ears – the end result being that you fall over!

Proprioception (kinesthesia) – is an important and often-overlooked sense.  It is the knowledge of where the body is and its surroundings.  Information from the stretch receptors in joints, tendons and muscles, are used by the brain (Parietal lobe) to build a picture of where all the body parts are and your immediate surroundings.  The somatosensory cortex builds a map of where our body is and our immediate surroundings.  Proprioception is what enables to you touch your nose with your finger when your eyes are closed (unless you are drunk; alcohol demonstrates how the abilities of the body are quickly affected, and a good reason why you shouldn’t drink and drive).  There are both conscious and unconscious forms of proprioception.  Unconscious proprioception is responsible for some reflex behaviours and is handled by the cerebellum.  It is the conscious proprioception that is of greatest interest to us.

Proprioception is a fascinating area to explore and develop if you are spiritually inclined.  Martial arts, Tai Chi, Qi Gong, Hatha Yoga and other physically demanding activities like ballet, dance, gymnastics or circus acrobats all serve to develop the body’s proprioception skills which assists in the development of the prefrontal cortex of the brain (more on this later).  All spiritual schools begin with giving devotees physical exercises that are intended to awaken these brain areas.  During long periods of motionless meditation the brain can lose touch with where its body parts are, leading to the reported experiences of growing larger or growing smaller – areas in the sensory cortex of the brain have become temporarily confused.  The physical body has not actually grown bigger or smaller, or started levitating – the proprioceptive senses have become temporarily confused.  It is proprioception that is responsible for itchy or painful phantom limbs after amputation – again, a fault in the brain’s proprioception processing centres results in it believing that the amputated limb is still there.  I recommend spending some time researching proprioception to fully understand this important sense that we don’t consider very often.

Time (chronoception) – we all have an inner clock operating in our subconscious which keeps track of passing time, and in healthy people it is remarkably accurate.  If I need to wake up at 5 am in the morning to catch an early plane flight I almost always do so without needing to use an alarm clock.  My internal clock is constantly keeping track of time and my subconscious is able to wake me through an activation link through the thalamus at close to the required time.  Similarly, when I awake in the night and make a quick estimate of the time in the darkness before switching on the light and reading the clock, I am usually accurate to within a minute or two.  We can all access our internal clock to some degree.  The brain area responsible seems to be the Suprachiasmatic Nucleus (SCN) in the Hypothalamus.  Alcohol and drugs can alter our perception of time, as can long periods of meditation.  Our perception of time is known to slow down during life-threatening emergencies.  The mechanism for this is not fully understood, but it seems that the brain diverts activity from the Default Mode Network or DMN (described later in the section on the Mental Body) to the Central Executive Network for optimum decision-making during the crises.  Normally the DMN fully occupies our conscious thinking experience, and a temporary continuous activation of the CEN for a few seconds is an unusual and profound experience for us to look back on (assuming the crisis did not result in us being killed!).  Some people undergo a spiritual awakening, realising the greater potential of our conscious brain beyond our usual DMN experience.  The ability for us to consciously control the perception of time was explored in Star Trek Insurrection 1998 movie, A Perfect Moment, when Anij demonstrates to Jean-Luc Picard this ability as they watch a hummingbird beat its wings in slow motion (which is in the region of 10 – 80 beats per second).

Internal organs (interoception)

There are sensors within our internal organs and joints, mostly under control of the parasympathetic nervous system and Limbic brain, to ensure that our body is kept in a state of balance (homoeostasis).  It is hard to get a definitive list, but some of the most common ones are listed below.

Stretch receptors – in the gastrointestinal tract, these are used to detect the build-up of gas.  There are stretch receptors in blood vessels, and the dilation of certain blood vessels may be one cause of headaches.  There are stretch receptors in joints, ligaments and muscles that provide information for proprioception and body movements.  The knee jerk is a well known involuntary reflex response used to test the proper operation of these motor nerve pathways. Alteration in atmospheric pressure can be detected as small changes in the sensors in joints, thus giving credibility to claims by some people that they feel pain when the weather is about to change (especially if they are arthritic).

Tension – as well as stretch sensors the muscles and tendons also have tension sensors, such as the Golgi tendon organ.  The stretch and tensions sensors work together as part of the autonomic reflexes in movement.

Fullness sensors in the bladder and rectum indicate when we are ready to urinate or defecate.  There is some degree of voluntary control here, but if you try and hold on long enough involuntary reflexes will take over.

Respiration rate – through pulmonary stretch receptors in the lungs.  This is under involuntary control for many people, and when we are asleep our respiration rate is typically about 12 breaths per minute for adults (it is faster for children and sick people).  Breath control is one of the first techniques taught in meditation as it is one means to bring rest and some degree of control over the parasympathetic nervous system through the vagus nerve and the Insular cortex.  Some sources suggest that 6 breaths per minute may be the optimum respiration rate for us at rest, as it would align with the heart rate variability of about 0.1 Hz (ie once every 10 seconds or 6 times per minute) also known as Mayer waves.  Over a period of 10 seconds the heart rate tends to speed up slightly, and decrease slightly.  When you breath in the heart rate tends to speed up slightly, and when you exhale it slows down slightly.  There may be health benefits by having Mayer Wave alignment with the respiration rate.  Many sources expound the benefits of breathing deeper and slower suggesting that such people are healthier.  Slow breathing techniques have been taught as a method for helping asthmatics.  The Insular cortex and Anterior Cingulate areas of the brain are linked to the respiration rate and homeostasis, and also also though the be where our sense of self-awareness and ego reside.  Also see Cheyne-Stokes breathing.

Vomiting – is an involuntary reflex.  There is a chemoreceptor trigger zone in the medulla oblongata of the brain which monitors the blood and cerebrospinal fluid for toxins, and can trigger the expulsion of the stomach contents out through the mouth.

Gag reflex triggered by sensors in the pharynx, used to prevent large foreign objects from being swallowed.  Some people are more sensitive than others and may have issues swallowing large medicinal pills.

Carbon Dioxide and Oxygen – chemical receptors in the brain monitor carbon dioxide levels, and also low oxygen.  The carbon dioxide level is used to keep the pH of the blood between 7.43 and 7.45.

Glucose – monitored by sensors in the pancreas.

Thirst and salt levels – receptors in the medulla around the area postrema monitor  blood volume and osmolite concentration.

Hunger – levels of certain hormones (leptin, ghrelin) in the blood can lead to hunger contractions being triggered in the stomach.  The hypothalamus is also involved.  Ignoring the hunger pangs for a day or two results in them going away and once you don’t feel hungry you can fast for quite a long period of time (or go on a hunger strike).

Temperature – internal body temperature is regulated through sensors in the hypothalamus in the brain, and the normal state is about 36.8 degrees Celcius.  The specific heat of water (which comprises about 60% of the human body) helps considerably in keeping out body temperature regulated.  Water has an unusual specific heat curve with the minimum falling at around 40 degrees, not too far from the typical human body temperature.  This is one of many aspects showing how our bodies are optimally aligned with the physical properties of nature as a result of millions of years of evolution.
waterspecificheat
Tibetan Tummo meditators can increase their body temperature by up to 8 degrees through special breathing techniques.  This allows them to meditate on blocks of ice and with ice cold sheets draped over their naked body.  This ability is real, but whilst it demonstrates an unusual ability and mind control, it doesn’t necessarily mean spiritual enlightenment.

Humans do not have the ability to hibernate for long periods of time like bears, but the Mammalian Diving Reflex is an innate ability triggered by cold water on the face.  Some people who are awake when this happens report experiencing an altered state of consciousness – this is not a spiritual experience but is profound enough to demonstrate to them that alternate states of conscious reality do exist.I have come across texts that state that alternate nostril breathing is responsible for temperature regulation (internal temperature sensors are used to bring about homeostasis).  I don’t believe that this is an involuntarily temperature regulation mechanism, but it may be that deliberate alternate nostril breathing can change the temperature of the body.

May or may not be senses

Here are a couple more senses that are weak and may or may not be present at all in human beings.  Whether all these are indeed senses or not is still disputed.

Vomeronasal (pheronomal, VNO) – an additional olfactory sense, which may or may not be transmitted through cranial nerve zero or XIII.  Many animals have a vomeronasal organ (VNO).  In some animals it is highly developed, but not so much in primates and development in humans seems to cease during early embryo development.  However, there does seem to be a nerve connection from the nasal cavity directly to the amygdala for this purpose.  It is known that the menstrual cycle of women living together (such as in a Nunnery) tends to synchronise, and that this is probably due to chemical transmission.  The VNO has been hypothesised as the sensory apparatus (rather than sense of smell through the nose).  It has not been conclusively proven that this is the case.

Magnetoreception – some animals possess this sense for navigation (such as homing pigeons), and some mammals demonstrate the ability to orient themselves with magnetic fields.  It is disputed whether humans possess this ability and no sensor has been identified.  Magnetotactic bacteria living within the human body could be responsible. The claimed abilities for Dowsing (water divining) could be explained by the body’s ability to detect and flinch at small changes in electromagnetic fields, but this has never been conclusively proven.

Eidetic imagery – the so-called photographic memory.  Some people demonstrate a remarkable ability to vividly store and recall information and it has been suggested that this is a separate and independent sense, not a function of the normal memory circuits.  As a child, Mozart was able to listen to and perfectly memorise music, and write it down later (as he did with Allegri’s Miserere).  Other examples of well studied humans are listed here.  In ancient times, certain people were chosen for committing to memory the scriptures such as the Hindu Mahabharata and Ramayana.  They had to do this and be able to recall the text without changing it in any way – this sounds very much like eidetic memory.  More discussion on this on memory.

Were you surprised at the length of this list?  We do indeed have far more than five senses; the true number being well in excess of 20 if you total up hot and cold as separate senses, and the various types of mechoreceptors as separate senses, etc.

Our sensory receptors connect “us” to our physical environment.

Synesthesia

Before leaving the topic of the senses, I want to mention synesthesia.  This is a phenomena where stimulation of one sense leads to it being experienced in another way.  A common example is to perceive letters and numbers as having colours, or hearing some everyday sounds as having different colours.  The condition is fairly rare, but given that all sensory inputs to the brain are processed by neural networks it isn’t surprising that there can be “cross-talk” or interference.  We can also learn to process some sensory information differently, as blind people do in order to compensate for the loss of vision.  I am highlighting the phenomenon because some people report hearing sounds or seeing colours (or scents, or tastes) during deep meditation, and many teachings explain that certain sounds and colours are associated with certain chakra centres.

Chakra sounds

There is a plausible mechanism for this experience when the attention is focused internally.  We don’t have a normal sensory mechanism for describing activity within the different body organs and glands (and the corresponding chakra centres), so when such input from the autonomic nervous system reaches the sensory interpretation area of the brain, sounds and colours are the result.  Not everyone experiences this during meditation, and vivid experiences are more likely to be an indication of trouble (ie high activity or stress in the areas of these chakra centres, often an over-excited sexual centre) than enlightenment.  The Om sound is what is heard when there isn’t stress or over-activity in nerve pathways elsewhere in the body.