The Fab Four
The two special senses of hearing and balance use mechanical energy to perceive sensation, while the two special senses of smell and taste detect chemical signals.
Hearing
Sound waves (mechanical energy) move through the air and are funneled into the auditory tube via the external ear (pinna). This canal and the eardrum (tympanic membrane) at the deep end make up the outer ear. As waves impact the eardrum, the membrane vibrates, and energy that is transferred through a series of small bones in the middle ear, the chamber on the opposite side of the eardrum.
The small bones in the middle ear are the malleus, incus, and stapes (also called the hammer, anvil, and stirrup, respectively). They transfer the mechanical energy of the sound waves from the eardrum to the cochlea, the organ of the inner ear that houses the auditory receptors. The stapes is attached to an oval window on the body of the cochlea. Waves are transferred into the fluid-filled tunnel of the coiled cochlea.
The cochlea resembles a snail shell. The long fluid-filled tube, through which the sound waves pass, is packed inside. The tube is divided into an upper and lower chamber by the sensory organ of Corti. The very end of the tube remains open and allows the sound waves in the upper chamber to run the full length of the tube. In doing so, the waves coursing through the fluid compress a membrane in the organ of Corti, which then compresses down upon receptor cilia to cause an electrical signal.
These receptor cilia, like the organ of Corti, run the full length of the tube. The area in the cochlea where the receptors are stimulated corresponds to the wavelength of the sound and distinguish low from high tones. Think of depressing the key on a piano. Depending on where on the keyboard you push, a different sound will emerge.
Frequency range in humans
Humans can’t perceive all sound wavelengths. Individuals with normal hearing can detect sound in the frequency range of 20 Hz (hertz) to 20,000 Hz. (A hertz is 1 cycle per second, with a cycle being how often an alternating current changes direction, or frequency.) Many animals can hear sounds that humans cannot hear, such as ultrasound (higher than 20,000 Hz) and infrasonic (lower than 20 Hz). Dogs can detect ultrasound, which is why they can hear the mail truck coming long before you can.
Balance
Balance, which was added in recent years to the list of special senses, also has receptors located in the inner ear and is associated with the cochlea. While sound is detected in the body of the cochlea, balance is monitored in three semicircular canals attached to the body of the cochlea. Oriented in three planes, which detect movement in three dimensions, these canals respond to acceleration and deceleration and use the law of inertia to function.
The law of inertia
Also called Newton’s first law of motion, the law of inertia simply says that an object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted on by an opposing force.
Fluid called endolymph fills each semicircular canal, and much like any object, moves relative to the body and follows the law of inertia. Receptor cells project cilia upward and into this fluid. At rest, the cilia stand straight and no signal is sent to the body. However, when the body starts to move, the hair cells (and cilia) accelerate at the same rate as the body. But the endolymph remains stationary for a brief moment—just long enough to bend the cilia of the receptors and transduce an electrical signal that motion has been initiated. Likewise, when the body stops, the fluid remains in motion and bends the cilia in the other direction. The brain perceives this as halting.
Smell
The sense of smell, or olfaction, begins in the lining of the nasal cavity. Secretory cells coat the surface of the nasal epithelium with a watery, protein-rich fluid that functions to trap chemicals as they are brought into the nasal cavity during an inhalation.
Once they are trapped in the watery surface zone, these chemicals are detected by the olfactory receptor cells, which resemble neurons more than epithelium. On their surface and projecting into the watery, chemical-filled fluid are dendrites, often called cilia. The cilia increase the surface area where proteins can bind to the chemicals and begin a signal transduction pathway that leads to an electrical signal that is sent to the brain. At the base of these cells, axons transmit the electrical signals on to mitral cells located in what is called the olfactory bulb, which is connected to the olfactory centers of the brain via the olfactory tract.
The olfactory tract is often incorrectly called the olfactory nerve. However, the axons of the olfactory receptors collectively form the olfactory nerve proper (filia olfactoria).
Taste
Taste (gustation) is the second of the chemically mediated specialized senses and is initiated on the dorsal surface of the tongue via barrel-shaped structures called taste buds. Located on the lateral portions of specialized papillae (projections), the taste buds are a collection of supportive and receptor cells used for taste. Hairlike microvilli from the receptor cells project into this opening to collect the chemicals dissolved in the saliva on the surface of the tongue. These receptor cells form synapses with sensory neurons at the base of the taste bud and transmit information away from the tongue and on to the taste center of the brain.
Taste buds are capable of detecting several chemical stimuli, but are thought to specialize in one or two. They are distributed across the surface of the tongue and are restricted only by the locations of the papillae on which they are located. Receptors meditate five basic tastes that require distinct signal transduction mechanisms to convey the stimulus and transduce it into electrical energy. Sweet, salty, sour, and bitter are four of the basic tastes that have been identified for decades. While most are familiar with these, the most newly identified of the basic tastes is umami, which is a savory flavor and common in Asian cuisines.