Taste, smell, touch, sight and hearing: these are the five ‘traditional’ senses that obviously allow us to comprehend our surroundings. But there are at least five other less celebrated senses that contribute to our ability to function.
Proprioception – sense of body position
Knowing what your hands, feet and other limbs are doing without having to physically look and think about them is an important part of normal bodily function. This sense of self, known as proprioception, is sometimes referred to as a ‘sixth sense’. It works through input supplied by the sensory nerves, known as proprioceptors, in the muscles, tendons, joints and vestibular system that respond to position and movement.
We know how important this system is by studying those who suffer from sensory disorders. Even the movement of a finger requires a concentrated effort as the person can’t feel what the finger is doing without visual verification.
Equilibrioception – sense of balance/acceleration
The ability to stand up and walk on two legs is made possible by a good sense of balance, known as equilibrioception, achieved through the complex co-operation of the ears, eyes and our sense of proprioception.
Deep inside our ears is a highly precise network of canals and tubes known as the vestibular system, which contains a special fluid known as endolymph. The movement of this fluid within the system – caused by, for instance, the motion of walking – triggers a sensory response that leads to muscular movement as well as the stabilisation of the eyes.
Rotational movement is detected by semicircular canals; linear acceleration is sensed by specialised structures known as otoliths. Both components work together to ensure any sort of motion can be counteracted to keep you upright.
Sense of pain
Sensing pain can mean the difference between life and death. It is pivotal for ensuring we know we are in danger.
From initial stimulus to final response, a complex sequence of signal pathways along the central nervous system, which includes the brain and spinal cord, culminate in the brain issuing a muscular response or drawing attention to the initial triggering of the pain.
Pain itself can be sensed almost anywhere throughout the body through special pain receptors, known as nociceptors.
Skin nociceptors cover our body surfaces. These can be classified as: high-threshold mechanonociceptors that respond to intense mechanical triggers such as pinching, cutting or the stretching of skin; chemical nociceptors triggered by chemical substances such as strong acids; and polymodal nociceptors, which are more ‘generalist’ painsensors, responding to high-intensity mechanical, thermal and chemical stimuli.
Sleep nociceptors are another type of pain receptor found in the skin and deep within tissue. They are normally unresponsive to stimuli, hence their name, but can ‘wake up’ in response to inflammation or after tissue is injured. It is not fully understood what causes these pain receptors to suddenly kick into action. One possibility is that constant stimulation from damaged tissue (such as after getting a bruise) causes the nociceptors to respond.
Thermoception – sense of temperature
Temperature is sensed by specialised thermoceptors. They are thought to be part of the transient receptor potential channel (TRP) family, which consist of about 28 different channels that transmit signals between cells in response to a wide variety of stimuli.
Like other types of sensing nerves and receptors, TRP channels work by means of an action potential: a build-up of electric charge that triggers a signal.
Changes in temperature cause ions – such as potassium, sodium and calcium – to move across the membrane of the thermoceptor. When enough ions accumulate, the thermoceptor sends a pulse of electric charge to the brain.
Thermoceptors send signals down different channels to pass on different information. The TRPV1 channel is a hot receptor, for example: it is activated in response to temperatures above 40 degrees Celsius. On the other hand, TRPM8 is a cold receptor: it responds to temperatures below 20 degrees Celsius.
Thermoceptors don’t always act alone; when coupled with nociceptors they can trigger both a temperature and pain signal, such as when you spill boiling water on your hand.
Sense of time
Unlike the senses covered so far, our sense of time is not thought to be due to a specific receptor. Rather it may be due to a cooperative effort within the brain, where different parts help us to tell the passing of different lengths of time.
Past evidence has pointed to an area of the brain known as the striatum, which is involved in voluntary movements, as being a central part of our internal clock. The striatum uses surrounding parts of the brain to combine temporal information, like how things around us move and are positioned in relation to us, to determine the passage of time.
We can see how important the striatum is to time perception from studies of patients with Parkinson’s disease. Researched published in the Journal of Cognitive Neuroscience found they have a reduced ability to tell the time, likely due to input to their stratium being disrupted.