Our perceptions of the world—its textures, colors, and sounds; its warmth, smells, and tastes—are created by the brain from electrochemical nerve impulses delivered to it from sensory receptors. These receptors transduce (change) different forms of energy in the “real world” into the energy of nerve impulses that are conducted into the central nervous system by sensory neurons. Different modalities (forms) of sensation—sound, light, pressure, and so forth—result from differences in neural pathways and synaptic connections. The brain thus interprets impulses arriving from the auditory nerve as sound and from the optic nerve as sight, even though the impulses themselves are identical in the two nerves. We know, through the use of scientific instruments, that our senses act as energy filters that allow us to perceive only a narrow range of energy. Our vision, for example, is limited to light in a small range of electromagnetic wavelengths known as the visible spectrum. Ultraviolet and infrared light, x-rays and radio waves, which are the same type of energy as visible light, cannot normally excite the photoreceptors in our eyes. The perception of cold is entirely a product of the nervous system—there is no such thing as cold in the physical world, only varying degrees of heat. The perception of cold, however, has obvious survival value. Although filtered and distorted by the limitations of sensory function, our perceptions of the world allow us to interact effectively with the environment.
Categories of Sensory Receptors
Sensory receptors can be categorized on the basis of structure or various functional criteria. Structurally, the sensory receptors may be the dendritic endings of sensory neurons. These dendritic endings may be free, such as those that respond to pain and temperature, or encapsulated within non-neural structures, such as those that respond to pressure. The photoreceptors in the retina of the eyes (rods and cones) are highly specialized neurons that synapse with other neurons in the retina. In the case of taste buds and of hair cells in the inner ears, modified epithelial cells respond to an environmental stimulus and activate sensory neurons.
Sensory receptors can be grouped according to the type of stimulus energy they transduce. These categories include (1) chemoreceptors, which sense chemical stimuli in the environment or the blood (e.g., the taste buds, olfactory epithelium, and the aortic and carotid bodies); (2) photoreceptors—the rods and cones in the retina of the eye; (3) thermoreceptors, which respond to heat and cold; and (4) mechanoreceptors, which are stimulated by mechanical deformation of the receptor cell membrane (e.g., touch and pressure receptors in the skin and hair cells within the inner ear).
Nociceptors are pain receptors that depolarize in response to stimuli that accompany tissue damage; these stimuli include high heat or pressure and a variety of chemicals. Depolarization can stimulate the production of action potentials in sensory neurons, which enter the spinal cord in the dorsal roots of spinal nerves and then relay information to the brain. However, the actual perception of the pain is enhanced or reduced by a person’s emotions, concepts, and expectations. This involves various brain regions that activate descending pathways in the spinal cord. Analgesia (pain reduction) depends to a large degree on the endogenous opioid neurotransmitters (including β-endorphin), but a non-opioid mechanism also functions to reduce the perception of pain. Receptors also can be grouped according to the type of sensory information they deliver to the brain.
Proprioceptors include the muscle spindles, Golgi tendon organs, and joint receptors. These provide a sense of body position and allow fine control of skeletal movements.
Cutaneous (skin) receptors include (1) touch and pressure receptors, (2) heat and cold receptors, and (3) pain receptors. The receptors that mediate sight, hearing, equilibrium, taste, and smell are grouped together as the special senses. In addition, receptors can be grouped into exteroceptors, which respond to stimuli from outside of the body (such as those involved in touch, vision, and hearing), and interoceptors, which respond to internal stimuli. Interoceptors are found in many organs, and include mechanoreceptors and chemoreceptors. An example of mechanoreceptors are those in blood vessels that respond to stretch induced by changes in blood pressure, and chemoreceptors include those that monitor blood pH or oxygen concentration in the regulation of breathing.
A major role of sensory receptors is to help us learn about the environment around us, or about the state of our internal environment. Stimuli from varying sources, and of different types, are received and changed into the electrochemical signals of the nervous system. This occurs when a stimulus changes the cell membrane potential of a sensory neuron. The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus. The central integration may then lead to a motor response.
Describing sensory function with the term sensation or perception is a deliberate distinction. Sensation is the activation of sensory receptor cells at the level of the stimulus. Perception is the central processing of sensory stimuli into a meaningful pattern. Perception is dependent on sensation, but not all sensations are perceived. Receptors are the cells or structures that detect sensations. A receptor cell is changed directly by a stimulus. A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. Transmembrane receptors are activated by chemicals called ligands. For example, a molecule in food can serve as a ligand for taste receptors. Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.