sense organs

sense organs Our sensory experiences, conscious and unconscious, derive from sensory receptors distributed throughout the body. But the five main senses — sight, hearing, taste (gustation), smell (olfaction), and touch — are associated with specialized structures known as sense organs (the skin being the ‘organ’ for the classical sense of touch). In addition, a sense organ in the inner ear known as the vestibular apparatus provides our sense of balance and equilibrium. It detects movement, and gravity (strictly speaking, angular and linear acceleration of the head).

The variety of specialized sensory experiences is very much greater than the classical five. The skin, as well as being sensitive to tactile contact, can also detect and distinguish warmth and cold, vibrations at various frequencies, and a whole range of different kinds of pain, itch, and tickle. Each of these definable ‘sub-modalities’ depends on the activation of a particular type of specialized nerve ending in the skin (or of more than one type of nerve ending). Visual and auditory senses also have a range of ‘sub-modalities’, each associated with a distinctly different sensory experience — for instance colour, movement, and brightness for vision; loudness, pitch, and position in space for hearing. These different aspects of sensory experience depend not only on receptor cells with different receptive characteristics in the eye and ear, but also on the way in which information from the sense organs is processed in the brain. Indeed, much of our sensory experience is determined not by the basic characteristics of our sense organs, but by the analysis of sensory messages in the brain, especially in the sensory areas of the cerebral cortex. The perception of the distances of objects in space through stereoscopic vision is a good example. This depends on the detection of tiny differences in the two retinal images resulting from the fact that the two eyes are separated in the head and therefore view the world from slightly different angles. Obviously this striking aspect of our visual experience is not represented in the signals from either eye alone, and can be derived only in the brain, where the signals from both come together.

At the heart of all sense organs are sensory receptors — specialized nerve cells or the endings of nerve fibres, which detect particular physical or chemical events outside the cell membrane. Much of the variety of sensory experience depends on the physical characteristics of the tissues that make up the rest of the sense organs. For instance, receptors sensitive to mechanical stimulation (mechanoreceptors) have been put to an extraordinary range of tasks in the body. The hair cells in the cochlear of the inner ear are mechanoreceptors: they respond selectively to sound because the rest of the ear delivers sound energy directly to them and protects them from any other form of mechanical stimulation. The hair cells of the vestibular apparatus are very similar to those of the cochlea, but they respond to tilt or rotation of the head, not sound, because those are the physical forces to which they are exposed. All the receptors in the skin that respond to touch and vibrations, those in muscles, tendons, and joints that detect stretch and tension, and even various receptive endings in the heart, lungs, and blood vessels that signal changes in blood pressure and inflation of the chest, are basically mechanoreceptors. Their particular sensitivities are determined by where they are placed in the body.

The non-neural components of sense organs serve, then, to protect the sensory cells and also to deliver particular forms of stimulation to them. The nasal passages support the delicate olfactory epithelium and direct the airflow across it. Chewing, tongue movement, and swallowing force a flow of macerated food, dispersed in saliva, over the taste buds on the tongue. The cornea and lens of the eye ensure that the light rays are focused on the rods and cones of the retina. The eye is unique among sense organs in that it not only houses the receptor cells, forms an image on them, and directs them towards objects of interest (by means of eye movements), but it also contains a large number of other interconnected nerve cells, in the retina. These process and analyse signals from the rods and cones and then transmit processed messages to the brain, along the optic nerve.

Sense organs are our windows on the world. But, like windows, they restrict our perception to what passes through them. We blissfully imagine that, if our senses are normal, we know everything that there is to know of the world around us. But our environment is full of physical energy and chemicals that our sense organs cannot detect. Many other animals have sense organs that can detect stimuli beyond the confines of the human senses. Our wonderful vision is restricted to the narrow band of wavelength within the electromagnetic spectrum that our photopigments can catch, and our eyes are blind to the ultraviolet and infra-red wavelengths that lie just beyond this visible band of the spectrum. But the eyes of many invertebrates, fish, and birds can detect ultraviolet light — male and female blue tits distinguish each other by brilliant ultraviolet feathers whose ‘colour’ is invisible to our eyes. The family of snakes called pit vipers, which includes rattlesnakes, have a second set of ‘eyes’, in the form of pinhole cameras set in the cheeks, each consisting of a cavity lined with thousands of heat receptors. Like a thermal imaging camera, these ‘pit organs’ sense infra-red radiation, enabling the snake to detect the positions of warmblooded prey in its vicinity. The ears of even a young child can detect frequencies only between about 50 and 20 000 cycles per second (Hz); but natural events, from thunderstorms to snapping twigs, generate much higher and lower frequencies. Whales and pigeons can hear frequencies of sound far below the capacity of the human ear. And many animals can detect sound frequencies up to 100 000 Hz: the vocal production and detection of such ultrasound is the basis of the radar-like echo location of bats.

Even more impressive (and more humbling to human beings) are sensory capacities found in many animals that differ from ours not just in degree but in kind. For instance, bees and many other insects can detect the plane of polarization of light (the axis of vibration of the light photons). This enables them to recognize the position of the sun, even when below the horizon or partially obscured by cloud, and hence it helps them to navigate. Pigeons (and probably many other animals) have magnetic sensory receptors through which they can detect the direction of the earth's magnetic field. A homing pigeon with a small bar magnet attached to the back of its head takes much longer to fly home! And certain fish have electroreceptive organs that are sensitive to weak electric fields in the water around them. Some emit electric pulses and use them to communicate. Others, such as sharks, use their electroreceptors when they attack other fish, sensing the minute electrical fields emitted by the gills of their prey.

Frances M. Ashcroft, and Colin Blakemore

See also eyes; hearing; proprioception; sensory receptors; somatic sensation; taste and smell; vestibular system; vision.