Senses

Human Senses

Everyone knows that we have 5 senses, right? Actually, human beings have quite a few more. Science formally acknowledges that humans have at least 11 senses, and some sources list 19, 20 or more. There is no formal definition of what a sense is. I think of a sense as an input receptor that provides information to the brain, in the same way that an input device is attached to a computer to measure temperature, pressure, etc. Twelve pairs of cranial nerves (CN) emanate from the brain, and these provide an indication of the importance of certain senses. These nerves are (see also diagram at bottom of page):

CN	Name	Function
I	Olfactory	Smell
II	Optic	Vision
III	Oculomotor	Eyelid and eyeball movement
IV	Trochlear	Eye movement downward and sideways
V	Trigeminal	Chewing; face and mouth touch and pain
VI	Abducens	Eye movement laterally
VII	Facial	Taste; facial expressions; tears; saliva
VIII	Vestibulocochlear	Hearing; equilibrioception
IX	Glossopharyngeal	Taste
X	Vagus	Major nerve to vital organs of body including heart, digestion; taste
XI	Spinal	Swallowing; shoulder and head movement
XII	Hypoglossal	Tongue movements

We are highly dependent on 6 senses (sight, hearing, touch, taste, smell and balance), all of which have direct connections to the brain. However, there are other senses, some of which utilise information from the basic 6 along with additional brain processing. Other sense ability exists weakly, or in a few individuals, or may be accidental senses resulting from receptors responding under unusual circumstances.

The table below summarises the senses, along with phenomena which human beings seem to be able to detect (in which case it demonstrates sensitivity, though not necessarily a "sense"). The senses are in approximate descending order beginning with the most familiar scientifically accepted, through to those that are poorly understood or disputed. This list is not necessarily exhaustive.

#	Sense	Type	C/U	Loss of	Location	Description
1	Sight (vision)	P	C	Blindness	Retina back of eye, rods and cones. The Pineal gland may also be light sensitive.	Conveys visible light information to the brain, visible light being red through violet (380 - 750 nm wavelength). Separate senses for colour and brightness and possibly depth (stereopsis). Cell types are rods (black/white vision in low light) and cones (colour vision). Recently ipRGC (Intrinsically Photosensitive Retinal Ganglion Cells) have been identified in the eye which provide information for the pineal gland (melatonin) and circadian rhythm synchronisation. The ipRGC are weakly senstive to blue light.
2	Hearing (audition)	M	C	Deafness	Ear canals	Convey information on loudness and pitch/frequency for sound waves (20 Hz - 20 kHz). The ear is also sensitive to phase which explains why sound above the audible frequency range can sound different to the ear.
3	Touch (somatic)	M	C	Anesthesia	Nerve endings all over skin surface, muscle and internal organs. Tongue	Sense of pressure on skin. Separate sensors for pain (see Nocicpetion below). The touch receptors are mapped by the homunculus in the brain, and this is essential for other senses such as Proprioception (see below). The tongue detects sensations such as dryness, metallicness, coolness, prickliness, numbness, kokumi (heartiness).
4	Taste (gustation)	C	C	Ageusia	Tongue	At least five different receptors: sweet, salty, sour, bitter and umami (savoury). May also be taste receptors for fatty acids and calcium.
5	Smell (olefaction)	C	C	Anosmia	Nasal cavity	Hundreds of different receptors responding to many different odours. See also pheronomic sense below.
6	Equilibrioception (balance, vestibular)	M	C	Vertigo, motion sickness, disequilibrium	Eyes, ears, brain	Separate senses for angular momentum and linear acceleration. Works in conjunction with eyes and Proprioception. Brain processing is required for proper operation.
7	Proprioception (kinesthetic)	M	C	Phantom limb syndrome Pinnocchio illusion	Ear and stretch receptors on muscles	Sense of relative position of the different parts of the body. This is the sense that means you can touch your finger to your nose when your eyes are closed. Brain processing is required for operation.
8	Nociception	M, T, C	C	Congenital analgesia	Skin, joints, bones and internal organs	Uses separate sense receptors from touch. Chemo pain receptor signals take a slow path to the brain, whereas thermal or mechanical events result in signals taking a fast path to the brain. It may be hard to determine the exact location of the source of pain from some internal organs, eg sciatica pain is felt down the leg whereas the problem is compression in lumbar spine.
9	Thermoception	T	C	-	Skin Hypothalamus	Separate receptors for heat (above body temperature) and cold (below body temperature). Internal homeostatic thermoceptors regulating internal body temperature.
10	Magnetoception	Mg	U	-	Ethmoid bone of nose	Whilst known to be well developed in some birds, bees and turtles, humans seem to have very weak magnetoception capability and this seems to vary between individuals (depending on degree of magnetite deposits in brain). Animals use the sense for navigation. This sense may explain the ability of some people to dowse.
11	Time	P	U	-	ipRGC cells	Using input from light sensitive cells which help establish the circadian rhythm, the sense of time is established through brain processing functions and be highly accurate. For example, some people on retiring to bed are able to set the time at which they will wake up. Drugs affecting brain neurotransmitters such as dopamine and adrenaline can interfere with the perception of time.
12	Eidetic imagery	?	C/U	-	??	This is the photographic memory capability. A few individuals exhibit exceptional ability, for example being able to recite pi to 100,000 decimal places. The mechanism is not understood, but it is possible that this is a true "sense" rather than simply enhanced memory.
13	Infrared (IR)	P	C	-	Eye	Humans have very limited visual ability in the infrared region (wavelengths above 750 nm). Some animals have a tapetum lucidum which enhances night vision. There is some variation between individuals, and whilst poor some IR vision has been demonstrated.
14	Ultraviolet (UV)	P	C	-	Eye Skin	As with infrared, the human eyes have some weak Ultraviolet capability (UVA range - 315 to 400 nm). UVB (280 - 315 nm) exposure on the skin is necessary for the production of Vitamin D, essential for good body health. When the skin receives too much UV radiation the result is sunburn.
15	Ionic	C	U	-	?	Ability to detect airborne ionic charge, such as from negative air ionizers. There has been much research and debate, and no firm conclusions have been drawn (good summary here). Possible link through respiration intake affecting seratonin production (ie Chemo).
16	Pheromonic (vomeronasal)	C	U	-	Nose	The vomeronasal organ (VNO) is highly developed in many mammals and is found at the base of the nasal cavity. In humans it is regresses after birth and there is no agreement as to whether the organ functions or not. Women appear to be more sensitive to men, possibly unconsciously using pheromones to select mates based on histocompatibility complex (MHC). When many women cohabitate together their menstrual cycles tend to synchronize, and pheromones have been suggested as the unconscious mechanism.
17	Proximal	?	C/U	-	?	Feeling of physical closeness to another object. Whilst we seem to have the ability to do this even when we cannot see/hear/smell/touch another person or object, this sense ability may be the combined unconscious (ie levels so weak they are below conscious awareness) input from several senses combined with brain processing.
18	Electrical	E	C/U	-	Nervous system	The human nervous system is electrical, and the human body radiates a weak electromagnetic field (this is the aura detected by Kirlian photography). Direct contact with an electric current of more than a few milliAmps is felt as pain in the body and can cause muscle spasms. There has been concern about the affect of electromagnetic fields on the body, such as radiation from cellphones, microwave ovens, and high voltage 50 Hz power lines. There is no firm scientific agreement, but some research indicates that strong electromagnetic fields may cause disease (such as leukemia or cancer) in some individuals.
19	Barometric	M	U	-	Joints?	Sensitivity to atmospheric pressure. Some people report aching joints or migraines when the weather is changing. Science has yet to provide a verifiable explanation, but lab tests suggest that humans do have some sensitivity to small changes in atmospheric pressure. Pressure receptors in the joints are a possibility.
20	Geogravimetric	?	C	-	?	Ability to sense differences in mass. This is demonstrated by the ability to avoid large objects in total darkness. Could be a sense formed by brain processing of several other senses.

Type: C = Chemoreception, E = Electroreception, M = Mechanreception, Mg = Magnetoreception, O = Osmoreception, P = Photoreception, T = Thermoreception

C/U (third column): C = Conscious, U = Unconscious

Diagram of the human nervous system (from Wikipedia)

Human nervous system