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Sensory Pathways 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education contents Principles of Biology 131 Sensory Receptors and the CNS Sensory Pathways All animals gain information about the external and internal environment through sensory pathways that involve four basic steps: reception, transduction, transmission, and perception. Sensory reception is a process in which specialized structures called sensory receptors detect a stimulus. Some sensory receptors sense external stimuli, like pressure, temperature, chemicals, or light levels, while others detect internal stimuli, like blood pressure and oxygen levels. Ion channels in the plasma membrane respond to the stimulus by opening or closing, which changes the relative internal and external ion concentrations. As a result, the membrane potential changes through a process called sensory transduction. If the change in membrane potential is sufficiently large, an action potential is generated. Neurons carry the action potential to the central nervous system (CNS) in a process called transmission. Perception, the awareness of a stimulus, occurs at the brain. Sensory receptors are present on neurons, or on cells associated with neurons. Sensory neurons have specialized dendrites that contain sensory receptors. For example, sensory receptors in skin have specialized structures called lamellae that deform in response to pressure, resulting in the sense of touch (Figure 1a). Other sensory receptors, such as those responsible for the sense of taste, are found in specialized epithelial cells that form synapses with neurons (Figure 1b). Regardless of cell type, a stimulus causes ion channels to open or close, which in turn causes the membrane to depolarize or hyperpolarize. If the membrane depolarizes enough, an action potential is typically generated. If the membrane becomes hyperpolarized, generation of an action potential is typically inhibited. Many sensory receptors generate action potentials even in the absence of a stimulus. In this case, a stimulus changes the rate at which action potentials occur. Neurons containing sensory receptors directly transmit the action potential to the CNS. Epithelial cells containing sensory receptors release synaptic vesicles in response to a stimulus. Neurotransmitter travels across the synaptic cleft and binds receptors on the sensory neuron, which generates an action potential that is transmitted to the CNS. http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 1/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Figure 1: Sensory receptors may be present on neurons or epithelial cells associated with neurons. (a) Lamellae are modified dendrites of neurons present in skin that are able to detect pressure. (b) Sensory cells in taste buds form synapses with neurons. © 2014 Nature Education All rights reserved. Test Yourself Explain how sensory pathways are regulated through membrane ion permeability. Submit Many sensory receptors can detect very weak stimuli. For example, the human eye is capable of detecting a few photons of light. During transduction, the signal may be strengthened through a process called amplification. Amplification often involves signal transduction pathways. The signal transduction pathway that occurs when sugar binds a taste sensory receptor is shown in Figure 2. In this pathway, binding of sugar molecules to G protein­coupled receptors causes a signal transduction cascade that causes potassium channels to close and calcium channels to open. The membrane depolarizes, and an action potential is generated. As a result, synaptic vesicles fuse with the plasma membrane. Figure 2: Signal transduction pathways result in amplification. Binding of a sugar molecule to a taste sensory receptor, which is located in a taste bud, results in a signal transduction cascade that amplifies the http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 2/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education signal. © 2014 Nature Education All rights reserved. If a stimulus continues without changing, the sensitivity of sensory receptors eventually changes through a reversible process called sensory adaptation. Sensory adaptation enables an animal to adjust to changing environmental conditions. For example, sensory adaptation allows the human eye to adjust to different light levels. When light intensity decreases, the pupil increases in size and light sensory receptors, called rods and cones, become more sensitive to light. The reverse process occurs when light intensity increases. Sensory adaptation also allows an animal to ignore stimuli that might otherwise be distracting. For example, sensory adaptation allows a person to tune out the hum of a refrigerator so they can focus on a conversation. Test Yourself Compare the process of amplification to the process of sensory adaptation. What is the advantage of each process? Submit Perception of sensory information. Through a process called integration, a number of separate weak stimuli can be aggregated so that they are perceived as a single strong stimulus by the CNS. Integration begins in the sensory cell when a number of separate stimuli are added together to depolarize the membrane sufficiently to generate an action potential. Integration continues during transmission, when many action potentials generated by a single sensory receptor are added into a single signal. Integration also occurs in the CNS, where signals from multiple sensory receptors are integrated into a single signal. As a result of integration, many small stimuli are perceived as a single strong stimulus. For example, if many auditory sensory cells generate many action potentials in rapid succession, the resulting integrated signal is perceived as a single, loud sound. If only a few auditory sensory cells generate a few action potentials, the integrated signal is perceived as a quiet sound. Sensory neurons all transmit action potentials to the CNS, yet various types of sensory information are perceived differently. This is because neurons associated with a particular sense transmit information to a specific area of the brain. For example, sensory information from the olfactory neurons in the nose travels to the olfactory bulb in the brain, which integrates the information and transmits the perception of smell. Sensory information from the ears travels to the auditory cortex, which integrates the information and transmits the perception of sound. Once the sense is identified, the information is transmitted to other parts of the brain so that it can be interpreted. For example, information about a perceived odor is transmitted to the amygdala, a region of the brain that stores emotional memories. If the odor was perceived during a prior emotional event, it can evoke both a memory of the event in which the odor was previously perceived and the emotion associated with the memory. IN THIS MODULE Sensory Pathways Types of Sensory Receptors Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 3/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education Stem Cells Stem cells are powerful tools in biology and medicine. What can scientists do with these cells and their incredible potential? PRIMARY LITERATURE Brain preplay anticipates the future Preplay of future place cell sequences by hippocampal cellular assemblies. View | Download An artificial self­assembling retina from stem cells Self­organizing optic­cup morphogenesis in three­dimensional culture. View | Download Innovation in Cannabis medicine Cannabinoid potentiation of glycine receptors contributes to cannabis­induced analgesia. View | Download SCIENCE ON THE WEB A Career Using Your Nose What is it like to be a senior fragrance evaluator? Out Of Balance Learn about medical conditions that involve dysfunction in the vestibular system page 662 of 989 3 pages left in this module http://www.nature.com/principles/ebooks/principles­of­biology­104015/29145683/1 4/4 4/7/2015 Sensory Receptors and the CNS | Principles of Biology from Nature Education contents Principles of Biology 131 Sensory Receptors and the CNS Types of Sensory Receptors Receptors are classified into five different groups based on the type of stimulus to which they respond: mechanoreceptors, chemoreceptors, electromagnetic receptors, thermoreceptors, and nociceptors (pain receptors). Mechanoreceptors. Mechanoreceptors detect forms of mechanical energy, including pressure and sound. Deformation of mechanoreceptors causes ion channels to open or close. Some mechanoreceptors in the skin, called lamellae, are modifications that occur at the tip of dendrites. Lamellae consist of many membrane layers that deform in response to pressure, which opens or closes ion channels and results in the sense of touch. The frequency of action potentials generated depends on the level of distortion of the membrane layers. Another type of mechanoreceptors found in dendrites that wrap around the base of hair follicles is able to detect movement of the hair. Figure 3: Skin sensory receptors. Skin contains various types of receptors, including mechanoreceptors, temperature receptors, and pain receptors. Mechanoreceptors close to the skin surface are able to detect light pressure. Receptors deeper in the skin are only activated in response to strong pressure. © 2014 Nature Education All rights
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