There are endless things that surround us, but how can we detect their presence? Some could be seen like objects, colors, lights, etc.; others could be heard such as sounds, voices, music, etc. Foods and drinks may be salty, sweet, or sour; we can also tell which type of food is cooked without seeing it, using the sense of smell. We can feel surfaces, whether they are smooth or rough, cold or hot; we can also recognize objects in the dark by touching them.
All this knowledge about our environment is known as “special senses”, which are the receptors that perceive environmental stimuli of different forms.
These environmental stimuli may be visual; the organ that perceives them is the eye. The eye has a special tissue known as the retina, containing cells called rods and cones, which perceive images. Visual images are then transferred in the form of electrical impulses through the optic nerve to an area in the back of the brain called the occipital cortex, where the electrical impulses are perceived as an image; then, the understanding of the image occurs, such as perceiving a figure as a dog for example. This process is a special sense that is Vision.
Another form of environmental stimuli is auditory; the ear is the organ stimulated by them. At first, sound waves move the tympanic membrane of the ear; they are then transmitted through small bones in the middle ear to the inner ear. A special tissue called the Organ of Corti gets stimulated and transfers the sound waves into electrical impulses carried out through the auditory nerve to the temporal lobe in the cortex of the brain. At that point, sound waves are perceived and understood as being a sound of a cat for example, or a voice of somebody we know. This special sense is called Hearing.
The special sense that is responsible for smelling odors and perfumes is called Olfaction; the nose is the receptor organ for that sense. The tissue lining the nose is called the nasal mucosa; its upper part―olfactory mucosa―carries special receptors for different chemicals responsible for different odors. The olfactory mucosa transfers these chemical stimuli into electrical impulses carried through the olfactory nerve to the temporal lobe of the brain, where impulses are perceived and understood as a special odor as the odor of a perfume or coffee.
A fourth special sense is the Taste; the organ responsible for gustatory or taste sensation is the tongue. The tongue carries special receptors called taste buds; the chemical materials found in our food stimulate the taste buds, which send electrical impulses through many cranial nerves to the brain to recognize which type of food we are eating, even if we cannot see it.
The fifth and last special sense is tactile sensation or Touch; it is the ability to feel objects through the skin. There are many skin receptors for different forms of tactile stimuli such as pain, temperature, pressure, light touch, vibration senses, etc. All these sensations are carried through sensory nerves to the spinal cord, then to the sensory cortex in the parietal lobe of the brain to perceive touch and understand what you have touched.
Without any of these special senses, one will miss a lot of joy, and will be at risk. By these five special senses we are actually blessed.
Top image: Close up eye scanning. Credit: Freepik.
This article was first published in print in SCIplanet, Spring 2013 issue.