The nervous system is responsible for sensing the external and internal environments of an organism, and for inducing muscle movement. Human sensation is achieved through the stimulation of specialized neurons, organized into five different modalities—touch, balance, taste, smell, hearing, and vision. The touch modality includes pressure, vibration, temperature, pain, and itch. Some animals are also able to sense magnetism and electric fields. Modality, timing, intensity, and location of the stimulus are the four features that allow the brain to identify a unique sensation.
The neurons specialized to detect sensation are also called receptors because they are designed to receive information from the environment. Each receptor responds only to a stimulus that falls within a defined region, called its receptive field. The size of the stimulus can affect the number of receptors that respond, and the strength of the stimulus can affect how much they respond. For example, when a cat sits on your lap, a large population of receptors responds to the cat's weight, warmth, claws, and the vibrations from its purring.
Transduction refers to the transfer of environmental energy into a biological signal signifying that energy. Sensory receptors either transduce their respective stimuli directly, in the form of an action potential —the electrochemical communication between neurons—or they chemically communicate the transduction to a neuron . Neurons collect information at their dendrites , a long, branched process that grows out of the cell body. Information travels through the cell body, and then reaches the axon , another long process designed to transfer information by apposing, or synapsing on, another neuron's dendrites. In some receptors, the axon is branched, and is specialized for both initializing and transmitting action potentials.
Touch receptors are a type of mechanoreceptor because they are activated by mechanical perturbation of the cell membrane. The axon is located in either shallow or deep skin, and may be encapsulated by specialized membranes that amplify pressure. When the appropriate type of pressure is applied to the skin, these membranes pinch the axon, causing it to fire. The action potential travels from the point of origin to the neuron's cell body, which is located in the dorsal root ganglion. From there, it continues through another branch of the axon into the spinal cord, even as far as the brainstem.
A very similar system allows proprioceptors to convey information concerning the position of the limbs and body, and the degree of tension in muscles. The axon of the nerve cell is located either in the muscle, tendon, or joint, and firing is instigated by pinching, as with touch receptors.
Nociceptors convey information about pain and include temperature, mechanical, and polymodal receptor types. Temperature nociceptors are activated only by extremely high or low temperatures. Mechanical nociceptors are activated by extremely strong pressure against the skin. Polymodal nociceptors are activated by high temperature, pressure, or chemicals released from damaged cells. Most nociceptors are free nerve endings unassociated with specialized membranes.
Campaniform organs are insect proprioceptors that play a role in feedback control of body position, as well as normal walking motor programs that allow the coordinated movement of all six legs.
Vision receptors are called photoreceptors because the stimuli that activate them are photons of light. The two types of photoreceptors are called rods and cones. Rods only sense the intensity of light, while cones can sense both intensity and color. While cones function best in bright light, rods function better in dim light. Furthermore, rods are located diffusely over the retina at the back of the eye, but cones are located in the central line of vision in a region of the retina called the fovea. For this reason, dim objects in the darkness can be viewed better from peripheral vision than from direct focus. There are three kinds of cones in the vertebrate eye—one responsive to wavelengths of light corresponding to the color blue, one responsive to red wavelengths, and one responsive to green wavelengths. These three colors form the entire range of colors that humans can perceive.
Visual information is carried along the optic nerve and passes through several relay points before reaching the primary visual area of the cerebral cortex, located at the back of the brain. From there it splits into two routes. The dorsal pathway processes depth and motion while the ventral pathway processes color and form.
Insects have compound eyes organized in hundreds or thousands of practically identical units called ommatidia, each of which has its own lens system and its own tiny retina. This gives the eye a faceted appearance. The insect's retinula cell is a sensory neuron that contains photopigments similar to rods and cones. Overall, compound eyes resolve images about one sixtieth as well as vertebrate eyes. This is because the image is fractionated into so many ommatidia lenses and is difficult to reconstruct again in the brain. However, insects can have a greater range of vision than vertebrates, one that encompasses nearly the entire sphere of space around their bodies, due to the relatively large size of their eyes.
Hearing receptors, or hair cells, are mechanoreceptors located within a bony spiral structure called the cochlea. Sounds are interpreted by the brain from patterns of air pressure caused by the vibration of objects. Sounds can also travel through water or solid objects. In mammals, the pressure in the air is transformed into mechanical pressure by three ear bones called the malleus , incus , and stapes , located in the middle ear.
Pressure waves that strike the tympanum, a thin membrane separating the middle from the outer ear, force it to push inward. The malleus is attached to the incus and the incus to the stapes, so that the mechanical activity of the tympanum is transferred to a coiled structure of the inner ear called the cochlea. Because this is a fluid-filled structure, the pushing and pulling of the stapes generates waves in the fluid. A semi-flexible membrane called the basilar membrane is located within the fluid, and also conducts the waves of pressure. The wave-like motion of the basilar membrane causes a series of hearing receptors grounded in the basilar membrane to be pushed up against another membrane just above it, the tectorial membrane.
Hair-like extensions, stereocilia, at the apex of the hair cell push and bend against a tectorial membrane, when the basilar membrane reaches the peak of its wave phase. This instigates a change in the electrochemical properties of the cell. The basilar membrane is formed so that only a particular region of hair cells is pushed up to the peak of the wave form for any one frequency, or tone, of sound. The frequency that a particular hair cell responds to is its receptive field. Hair cells are closely coupled to the auditory nerve, and transmit their auditory information to neurons from this nerve, which then travels up through the brain.
Insect hearing receptors are typically located on the legs, and sometimes on the body, of the insect. In crickets, the ear is located on the tibia of the foreleg. The eardrum of crickets is called a tympanum, and consists of an oval membrane on one side of the leg, and a smaller round tympanum on the other surface. Inside the tympanum is a large air sac formed by the leg trachea, which is the insect respiratory organ. Transduction mechanisms in the cricket ear are not well understood, but the vibration of the tympanum is known to stimulate the activity of auditory neurons behind the membrane.
Vertebrate balance receptors are located in a specialized organ in the inner ear called the vestibular organ. This structure is located directly adjacent to the cochlea, and is composed of a triplet of semicircular canals, each of which is oriented in a different plane—the X, Y, or Z axis. Movement of liquid in these tubes caused by rotation of the head or body are measured by vestibular hair cells. The stereocilia of these cells are embedded in a gelatinous material called the otolithic membrane.
Gravity and body movements cause the otolithic membrane to slide, which cause the stereocilia to bend in a particular direction. This leads to electrochemical changes in the hair cell, causes an action potential in the associated nerve ending. Information from the vestibular system allows eye and head movements to fix on a particular target, and to stabilize a moving image. It also allows organisms to balance—for example, when a cat walks atop a fence.
Smell receptors, or olfactory sensory neurons, are chemoreceptors , meaning that the binding of molecules causes these neurons to fire. Olfactory neurons extend a single dendrite to the surface of the skin in the nose, where it expands—along with dendrites from other neurons—to form a large knob. Thin hair-like projections extend from this knob into the thin layer of mucus within the nose. These projections contain a diverse array of receptors for odorants, so that all olfactory neurons are able to respond to a particular scent. The number that actually do respond is relative to the concentration of the scent molecules in the air.
Taste-detecting, or gustatory, organs are also chemoreceptors and are located in functional groupings called taste buds on the tongue, palate, pharynx, epiglottis, and the upper third of the esophagus. Taste cells have a very short life span, which is why each unit contains a population of stem cells that continuously divides, producing progeny cells to replace the dying taste cells. The remainder of the cell types in the taste bud has hair-like projections called microvilli that extend into a pore at the top of the taste bud. When taste molecules bind or interact with the microvilli, the taste cell undergoes an electrochemical change that is conveyed to an associated neuron; however, taste cells are not neurons. Four basic taste sensations can be distinguished by humans: bitter, salty, sour, and sweet.
see also Growth and Differentiation of the Nervous System; Nervous System.
Rebecca M. Steinberg
Finger, Thomas E., D. Restrepo, Wayne L. Silver, et al. The Neurobiology of Taste and Smell. New York and London: Wiley-Interscience, 2000.
Hubel, David H. Eye, Brain and Vision. New York: Scientific American Library, 1995.
Kruger, Lawrence, ed. Pain and Touch. San Diego: Academic Press, 1996.
Singh, Naresh R., Nicholas James Strausfeld, et al. Neurobiology of Sensory Systems. New York: Plenum Press, 1989.
Whitlow, W. L., Arthur N. Popper, and Richard R. Fay. Hearing by Whales and Dolphins. New York: Springer, 2000.
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.
A sense organ is any collection of cells in an organism that responds to information about certain changes in the organism's internal or external environment. Each sense organ reacts to a particular type of stimulus. The sense organ converts the stimulus into a nerve impulse that is sent to the organism's brain to be processed and identified. Human sense organs are the eyes, ears, nose, mouth, and skin—each having its own particular type of receptors.
Sense organs are essential if an animal is to obtain any information about its surroundings. Without them, the individual organism would probably not survive long. Therefore, sense organs are essential to survival since they are the key link to the external, or outside, world. The type of sense organs an animal has greatly determines how it will perceive its environment. Humans see a world rich in colors. Animals who hunt at night see things only as shades of gray. Dogs recognize others mainly by their smell. Insects see an ultraviolet world all their own.
Although there is great variety in how different animals experience the world through their senses, they all have sensing systems that operate by using sensory receptors. Thus, all sense organs, no matter what they detect, have and need sensory receptors. A receptor is a group, or cluster, of nerve cells that react to a particular stimulus and receive information.
Sensory receptors make up the most familiar sense organs, such as the ears and eyes. These receptors can be classified according to the type of energy or stimulus to which they respond. Chemoreceptors (for taste and smell) respond to certain chemical compounds. Mechanoreceptors (for touch) respond to mechanical energy. Auditory receptors (for hearing) respond to sound wave vibrations, although some consider them to be a form of mechanoreceptor since the pressure waves of sound are a real, physical force. Photoreceptors (for sight) respond to light energy.
These four types of receptors correspond to the five human senses (taste and smell both use chemoreceptors) and, therefore, to our five major sense organs. However, there are other types of receptors that are used by organisms that are not necessarily linked to a particular sense organ. For example, certain insects such as ticks, and animals such as snakes, possess thermoreceptors with which they respond to temperature. A tick is aware of your presence and jumps on you by sensing your body heat. Certain fish also have electroreceptors in their skin that allows them to detect electrical energy given off by other objects. Another important type of receptor is known as the proprioceptors. These are sensory receptors located inside an animal's muscles and joints. Proprioceptors allow the animal to be aware of its entire body position, as well as to keep its balance. It is because of this sense that a person is able to dress in complete darkness.
The human body has two senses that employ chemoreceptors—taste (gustation) and smell (olfaction). The first uses taste buds (mainly in the tongue) as the receptors for dissolved chemicals. The second uses olfactory epithelium ("smelling skin") in the nasal cavities to detect airborne chemicals. The sense of touch is located in the skin. Here, mechanoreceptors are activated when they change shape by being pushed or pulled. Auditory, or hearing, receptors are located in the cochlea, deep within the ear. They detect or respond to pressure waves (since sound is actually a vibration of the air). Finally, the rods and cones in the retina of the eyes are the photoreceptors that enable people to see.
Humans would have none of their five senses if they did not also have a vast, branching network of nerves. These nerves take all of the coded messages to the brain, which translates or interprets them. The brain tells a person exactly what he or she is sensing. As a result, people do not really see with the sense organs called the eyes, for it is the brain that interprets the signals the eyes are sending. The brain tells a person that he or she is seeing a beautiful rainbow and not a dangerous fire. The role of the sense organs then is only to gather the information. Understanding that information is left to the brain.