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Sensory Perception

Since there are potentially many forms in which information can be gathered, several distinct types of receptors have evolved, each specialized to react to a particular stimulus in the environment. Five major types of receptors are recognized in humans.
  • Chemoreceptors: Chemoreceptors are sensitive to the quality and concentrations of chemical substances, and are the major receptors for the senses of taste and smell.
  • Photoreceptors: Sensitive to the quality and intensity of light, photoreceptors are responsible for our sense of vision.
  • Mechanoreceptors: Mechanoreceptors are sensitive to physical pressure, and are involved with several human senses, including hearing, touch, and our ability to determine the relative positions of different parts of our body (kinesthesis).
  • Thermoreceptors: Sensitive to changes in temperature, these are involved in our sense of touch.
  • Pain receptors: These receptors are sensitive to tissue damage, and are also involved in our sense of touch.
Other organisms, for example sharks, have electroreceptors that enable them to sense and react to an electric field. Still other organisms possess magnetoreceptors, which allow them to be aware of the earth’s magnetic field. Humans apparently evolved without the need for these other sensory abilities.
In humans, senses can be subdivided into two categories. The somatic senses receive information from receptors located on the skin (the sense of touch), as well as from receptors in muscles, joints, and various other internal locations (called proprioception). The special senses are those whose receptors are collected into large, complex sensory organs located in the head, and include taste, smell, hearing, equilibrium, and vision (sight). We will now list these senses, describing the types of receptors and the major structures and functions involved.
  • Touch: The sense of touch is actually mediated through a complex association of different types of receptors located on the skin. A combination of mechanoreceptors, thermoreceptors, and pain receptors collects information that we perceive as a single sensation containing all three types of information.
  • Proprioception: While not traditionally classified as a human sense, most of us realize that we obtain sensations giving us information about the interior of our bodies. For example, we can tell if many of our muscles are contracted or relaxed (including the perception of our own heartbeat and breathing), and we can certainly feel internal pain! As with touch, different types of receptors at various internal locations are responsible for this “sense”.
  • Hearing: The sense of hearing is made possible by the existence of the ear, a complex organ comprised of an external, middle, and inner section (see Figure 14.3). The stimulus for the sensation of hearing is the physical vibration of the molecules of the medium contacting the ear, usually air, in the form of sound waves. The external ear collects and transmits sound waves from the environment to the ear’s interior. The middle ear consists of the tympanic membrane (eardrum), which protects the interior of the ear and transmits vibrations of the air to the three ear bones, or ossicles, also located in this region. The names of the three ossicles are the hammer, the anvil, and the stirrup (or more technically the malleus, the incus, and the stapes). The vibrations are transmitted to a structure in the inner ear called the cochlea via the ossicles. The cochlea is a fluid filled compartment that contains the organ of Corti, a patch of tissue with many specialized mechanoreceptors. These hearing receptors are stimulated when the cochlear fluid vibrates, sending information to the brain via the auditory nerves. The human ear can distinguish frequencies in the range of 20 to 20,000 cycles/second. Other vertebrates can hear higher or lower pitched sounds.
  • Equilibrium: The sense of equilibrium is again not traditionally looked upon as a human sense, but we should recognize it as one. This sense provides information regarding the movement and orientation of the head in space. As with hearing, specialized mechanoreceptors located in the ear are responsible for the sensations, but the fluid that stimulates these receptors is located in the semicircular canals of the inner ear. Like proprioception, equilibrium may be a sense we take for granted!
Figure: The Ear
Vision (sight): The organ of vision is the eye, a complex structure with many component parts that functions in a very similar way to a conventional camera (see Figure 14.4). Light enters the eye through the transparent, protective cornea, and then through the lens, which focuses the light into an inverted image on the retina, located at the back of the eyeball. The amount of light entering the eye is controlled by the iris (the colored portion of the eye), a muscular structure that changes the size of the pupil, the opening in the iris. The retina contains the photoreceptors, which are classified into two categories based on their shape and function. Rods, responsible for “night vision”, contain the pigment rhodopsin, and are more light sensitive than the cones, which are responsible for color vision. Dim light does not stimulate the cones, but it does affect the rods; that is why we can distinguish shapes but not colors in dim light. Stimulated photoreceptors send information to the brain via the optic nerves. Most nocturnal vertebrates have retinas that contain rods only, in large quantities, so that while they can see well at night, they lack color vision almost entirely.
This survey of our senses reveals that instead of the traditional five, humans have at least seven different senses and possibly more (depending upon how they are classified).
Figure: The Eye

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