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Chapter 11: 11.5: The Senses

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Humans do not have the acute sensory abilities of some other animals. For example, we cannot detect very low concentrations (for example, in ppm) of illegal drugs or explosive chemicals the way that some pigs can. Nor can we hear the high-frequency sounds that dogs can hear, or see the infrared portion of the electromagnetic spec- trum that rattlesnakes can "see." However, the combination of all of our senses enables us to observe and respond to our environment successfully.
Toothed whales, dolphins, many bats, some birds, and some insectivorous mam- mals have the ability to locate objects through echolocation. Echolocation is the transmission of a sound and the interpretation of the sound that is reflected as an echo (Figure 1). This specialized adaptation allows some animals to navigate in envi- ronments where visibility is limited.
Scientists have long assumed that humans are incapable of echolocation. Recently, however, neuropsychologists at the University of Western Ontario's Centre for Brain and Mind and Toronto's Rotman Research Institute have used functional magnetic resonance imaging (fMRI) to investigate the abilities of people who are blind and have learned to echolocate. Daniel Kish, a man in his 40s who lost his sight at 13 months of age, cannot recall a time when he did not use vocal clicks to navigate. His parents noted that he began to make clicking sounds at about 18 months of age. Based on fMRI images, scientists have learned that the part of the brain that processes visual input in sighted people (the visual cortex) becomes active when echolocation is used. In this section, you will explore the five senses of sight, hearing, taste, smell, and touch to understand how we acquire and use information from our internal and external environments.
In Section 11.4, you learned about the efferent system of the PNS. You will now learn about the afferent system of the PNS, whose receptors sense stimuli outside or on the surface of the body and send information to the CNS. There are five types of sensory receptors that receive stimuli and send information to the CNS:
• Mechanoreceptors detect mechanical energy, such as changes in pressure, body position, or acceleration. Auditory receptors in the ears are mechanoreceptors.
• Photoreceptors detect the energy of light. In vertebrates, photoreceptors are mainly located in the retina of the eye.
• Chemoreceptors detect specific molecules or chemical conditions, such as acidity. Taste buds on the tongue are examples of chemoreceptors.
• Thermoreceptors detect the flow of thermal energy. These receptors are located in the skin, where they detect changes in the temperature of the body surface.
• Nociceptors, or pain receptors, detect tissue damage or noxious chemicals. Their activity registers as pain. Nociceptors are located in the skin and in some internal organs.
Some animals also have receptors that detect electrical or magnetic fields. Songbirds, for example, can use the detection of Earth's magnetic field as one method of orientation during migration. Traditionally, humans are said to have five senses: vision, hearing, taste, smell, and touch. In reality, we can detect almost twice as many kinds of environmental stimuli. We can also detect external thermal energy; internal temperature; gravity; acceleration; internal pH; and the internal concentrations of substances such as oxygen, carbon dioxide, salts, and glucose. Some of the receptors that play a role in these senses are associated with the autonomic nervous system.
Sensory receptors are not evenly distributed throughout the body. Some parts of the body, such as the lips and fingers, have many more touch receptors per unit area and are therefore more sensitive than other parts of the body. Some sensory receptors, such as touch receptors, are positioned individually in body tissues, while others are part of complex sensory organs, such as the eyes or ears. Sensory organs, particularly those that receive external stimuli, such as the eyes, ears, and antennae, usually occur in pairs.
In many sensory systems, the effect of a stimulus is reduced if it continues at a con- stant level. This reduction is called sensory adaptation. Some receptors adapt quickly and broadly, but others adapt only slightly. For example, when you get into bed, you are initially aware of the touch and pressure of the covers on your skin. Within a few minutes, however, the sensations lessen or are lost, even though you are still under the covers.
In some sensory receptors, biochemical changes in the receptor cells contribute to adaptation. When you move from a dark movie theatre into the bright sunshine, the photoreceptors in your eyes adapt to the sudden bright light, partly through the breakdown of some of the pigments that absorb light. Sensory adaptation is crucial to survival. Adaptation of the photoreceptors in our eyes keeps us from being blinded indefinitely as we pass from the dark into bright sunlight. Sensory adaptation also increases the sensitivity of receptor systems to changes in environmental stimuli.
In contrast, receptors that detect painful stimuli show little or no adaptation. Pain detectors are essential for survival. They signal a potential danger to some part of the body, and they maintain the signal until a response by the animal compensates for the stimulus causing the pain.
The auditory organ for humans is the ear. The pinna (outer ear) focuses sound waves. Inside the auditory canal, sound waves strike a thin sheet of tissue, called the tym- panic membrane or eardrum, causing it to vibrate. These vibrations generate vibra- tions in the auditory ossicles, the chain of tiny bones located in the air-filled middle ear. Mammal ears have three auditory ossicles: the malleus (hammer), the incus (anvil), and the stapes (stirrup) (Figure 4, next page). The stapes meets the inner ear at the oval window. The oval window is a thin, elastic membrane, where vibrations in bone are converted to vibrations in fluid in the cochlea.
The inner ear contains several fluid-filled compartments: the semicircular canals (which enable us to keep our balance and be aware of our body position) and a spiral tube called the cochlea. The vibrations in the fluid cause vibrations in the basilar membrane, which forms part of the floor of the cochlea and anchors the
sensory hair cells. Vibrations of the basilar membrane cause the hair cells to bend. This stimu- lates them to release a neurotransmitter that triggers action potentials in afferent neurons leading from the inner ear. Each frequency of sound waves causes hair cells in a different segment of the basilar membrane to initiate action potentials. There are more than 15 000 hair cells, which are connected to afferent neurons. The axons of the afferent neurons are bundled together in the auditory nerve, leading to the thalamus. Signals are routed from the thalamus to the auditory centre of the temporal lobe, where they are interpreted as sounds.
A duct called the Eustachian tube leads from the air-filled middle ear to the throat. The Eustachian tube protects the eardrum from damage caused by changes in envi- ronmental atmospheric pressure. As you swallow or yawn, the Eustachian tube opens, allowing air to flow into or out of the middle ear. This equalizes the pressure on both sides of the eardrum.
Chemoreceptors are receptors that are involved in taste and smell. They also sense levels of oxygen, carbon dioxide, and hydrogen ions in the body. All chemorecep- tors probably work through membrane receptor proteins that bind with specific molecules in the environment. Taste receptors form part of a structure called a taste bud, which is a small pear-shaped capsule with a pore that opens to the exterior at the top (Figure 5). The sensory hairs of taste receptors pass through the exterior pore of a taste bud and are exposed to the inside of the mouth. The opposite ends of the receptor cells synapse with the dendrites of an afferent neuron.
Humans have about 10 000 taste buds, each 30 to 40 μm in diameter. The taste buds are scattered over the tongue, the roof of the mouth, and the throat. The taste buds on the tongue are embedded in outgrowths called papillae (from papula meaning "pimple"), which give the tongue its rough texture. Taste receptors on the human tongue respond to five basic tastes: sweet, sour, salty, bitter, and umami (savoury).
Signals from taste receptors are relayed to the thalamus. From there, some signals travel to gustatory or taste centres in the cerebral cortex, which interpret them as dif- ferent tastes. Some signals travel to the brain stem, which links tastes to involuntary visceral and emotional responses. A pleasant taste may lead to salivation, the secre- tion of digestive juices, and sensations of pleasure. An unpleasant taste may produce nausea and vomiting.
Each olfactory receptor cell has 10 to 20 sensory hairs, which project into a layer of mucus that covers the olfactory area in the nose. To be detected, airborne molecules must dissolve in the watery mucus. Olfactory receptors are distinct among sensory receptors because they make direct connections with brain interneurons, rather than making connections via afferent neurons. Nerves from olfactory bulbs conduct signals to the olfactory centres of the cerebral cortex, where they are interpreted as tantalizing or unpleasant odours. As with taste, some olfactory bulbs also connect to the brain stem, where nerve signals elicit emotional and visceral responses similar to those caused by different tastes.
As already mentioned, there are more than five senses. Most animals have ther- moreceptors, which can detect changes in the temperature of their surroundings. Some snakes, such as pit vipers, use a pair of pit organs (thermoreceptors) located in depressions just below each eye to detect infrared radiation (thermal energy). This enables them to detect the body thermal energy of endothermic prey, such as mice and other small mammals (Figure 7).
In mammals, distinct thermoreceptors respond to hot and cold. Three kinds of Ca2+ channels act as thermoreceptors. One responds when the temperature reaches 33 °C, and another responds when the temperature goes above 43 °C and starts to be painful. Both receptors probably play a role in thermoregulation. A third receptor responds at temperatures of 52 °C and above, producing only a pain response. This receptor is not believed to be involved in thermoregulation. Two cold receptors are also found in mammals. One responds between 8 and 28 °C and probably plays a role in thermoregulation. The second responds to temperatures below 8 °C and appears to be associated with pain rather than thermoregulation. The mechanisms that control the opening and closing of hot and cold receptor channels are not currently known. Thermoreception in humans is performed by several specialized receptors. Some neurons in the hypothalamus of mammals function as thermoreceptors, sensing changes in brain temperature and receiving afferent thermal information. They are sensitive to shifts from normal body temperature and trigger involuntary responses, such as sweating, panting, or shivering, which restore normal body temperature. Maintaining the human body at normal body temperature (about 37 °C) is important for numerous processes. Temperatures that are too high or too low can cause serious complications, including impeded nerve transmission, irregularities in heart rate,
internal bleeding, organ damage, and cell death.
Free nerve endings (branched endings of sensory neurons) are the simplest type of thermoreceptors. They are either unmyelinated or thinly myelinated, and they adapt slowly to stimulation by thermal energy. Ruffini endings, which are located deep within the skin, are another type of slowly adapting receptor. Ruffini endings respond to temperatures above 45 °C, and also to touch and pressure. Closer to the surface of the skin, another type of thermoreceptor is activated at temperatures of 20 °C or lower. When this type of receptor senses a temperature below 10 °C, it alerts the body to cold through a pain response. Anyone who has experienced frostbite can attest to the pain response caused by extreme cold.
In Figure 8, you can see the flushed face that results as body temperature is elevated by exercise. The change in body temperature is detected by thermorecep- tors and then transmitted and interpreted by the hypothalamus, which sends out a signal to dilate the blood vessels in the skin. This increases the blood flow near the skin, increasing the transfer of thermal energy to the air, which cools down the body. Local nerves also stimulate vasodilation of the blood vessels in the skin by the release of a neurotransmitter known as substance P (a calcitonin gene-related peptide neu- rotransmitter). Capsaicin, the characteristic ingredient in hot peppers, triggers this same response, causing the local sensation of heat and vasodilation in the skin.
Pain receptors, called nociceptors, detect damaging stimuli, which are interpreted by the brain as pain. Pain is a protective mechanism that prompts animals to do something to remove or decrease the damaging stimulus immediately. Often pain elicits a reflex response, such as withdrawing the hand from a hot stove, which occurs before we are consciously aware of the sensation. Whereas many sensory receptors aid in normal homeostasis, pain receptors respond to more serious and potentially damaging internal or external conditions. Pain receptors are the most numerous type of sensory receptor. For each square centimetre of skin, there are around 200 pain receptors, 15 pressure receptors, 6 cold receptors, and 1 heat receptor.
Mechanical damage, such as a cut, pinprick, or blow to the body, can cause pain. Some nociceptors are specific for a particular type of damaging stimulus, whereas others
respond to more than one type of stimulus. Axons that transmit pain signals are part of the somatic system of the PNS. They synapse with the interneurons in the grey matter of the spinal cord and activate neural pathways to the CNS by releasing the neurotransmit- ters glutamate and/or substance P. Glutamate-releasing axons produce sharp, prickling sensations that can be localized to a specific body part, such as the pain of stepping on a tack. Substance P-releasing neurons produce dull, burning, or aching sensations that are not easily localized, such as the more widespread pain when you stub your toe.
As part of their protective function, pain receptors adapt very little, if at all. Some pain receptors increase the rate at which they produce action potentials if the stimulus continues at a constant level. The CNS also has a pain-suppressing system. In response to stimuli such as exercise, hypnosis, and stress, the brain releases endor- phins. Endorphins are natural painkillers that bind to membrane receptors on sub- stance P neurons, reducing the amount of neurotransmitter released.