HUMAN ANATOMY & PHYSIOLOGYCopyright Wiley, 2020548CHAPTER 16In the previous four chapters we described the organization of the nervous system. In this chapter, we explore the levels and...

HUMAN ANATOMY & PHYSIOLOGY Copyright W iley, 2020 548 CHAPTER 16 In the previous four chapters we described the organization of the nervous system. In this chapter, we explore the levels and components of sensation. We also examine the pathways that convey somatic sensory nerve impulses from the body to the brain and the pathways that carry impulses from the brain to skeletal muscles to produce movements. As sensory impulses reach the CNS, they become part of a large pool of sensory input. However, not every nerve impulse transmitted to the CNS elicits a response. Rather, each piece of incoming information is combined with other arriving and previously stored information in a process called integration. Integration occurs at many places along pathways in the CNS, such as the spinal cord, brainstem, cerebellum, basal nuclei, and cerebral cortex. You will also learn how the motor responses that govern muscle contraction are modified at several of these levels. To conclude this chapter, we introduce three complex integrative functions of the brain: (1) wakefulness and sleep, (2) learning and memory, and (3) language. Q Did you ever wonder how drugs such as aspirin and ibuprofen relieve pain? Sensory, Motor, and Integrative Systems Sensory, Motor, and Integrative Systems and Homeostasis The sensory and motor pathways of the body provide routes for input into the brain and spinal cord and for output to targeted organs for responses such as muscle contraction. c16SensoryMotorAndIntegrativeSystems.indd Page 548 10/14/16 2:33 PM f-512 /208/WB01989/9781119287759/ch16/text_s 520 Copyright W iley, 2020 16.1 Sensation 549 In this chapter we discuss the somatic senses and visceral pain. The special senses are the focus of Chapter 17. Visceral senses were discussed in Chapter 15 and will be further described in association with individual organs in later chapters. The Process of Sensation The process of sensation begins in a sensory receptor, which can be either a specialized cell or the dendrites of a sensory neuron. A given sensory receptor responds vigorously to one particular kind of stimu- lus, a change in the environment that can activate certain sensory receptors. A sensory receptor responds only weakly or not at all to other stimuli. This characteristic of sensory receptors is known as selectivity. For a sensation to arise, the following four events typically occur: 1. Stimulation of the sensory receptor. An appropriate stimulus must occur within the sensory receptor’s receptive field, that is, the body region where stimulation activates the receptor and produces a response. 2. Transduction of the stimulus. A sensory receptor converts the energy in the stimulus into a graded potential, a process known as transduction. Recall that graded potentials vary in amplitude (size), depending on the strength of the stimulus that causes them, and are not propagated. (See Section 12.3 to review the diff er- ences between action potentials and graded potentials.) Each type of sensory receptor exhibits selectivity: It can transduce (convert) only one kind of stimulus. For example, odorant molecules in the air stimulate olfactory (smell) receptors in the nose, which trans- duce the molecules’ chemical energy into electrical energy in the form of a graded potential. 3. Generation of nerve impulses. When a graded potential in a sen- sory neuron reaches threshold, it triggers one or more nerve im- pulses, which then propagate toward the CNS. Sensory neurons that conduct impulses from the PNS into the CNS are called first- order neurons (see Section 16.3). 4. Integration of sensory input. A particular region of the CNS receives and integrates (processes) the sensory nerve impulses. Conscious sensations or perceptions are integrated in the cerebral cortex. You seem to see with your eyes, hear with your ears, and feel pain in an injured part of your body because sensory impulses from each part of the body arrive in a specific region of the cerebral cor- tex, which interprets the sensation as coming from the stimulated sensory receptors. Sensory Receptors Types of Sensory Receptors Several structural and functional characteristics of sensory receptors can be used to group them into diff erent classes. These include (1) microscopic structure, (2) location of the receptors and the origin of stimuli that activate them, and (3) type of stimulus detected. MICROSCOPIC STRUCTURE On a microscopic level, sensory recep- tors may be one of the following: (1) free nerve endings of first-order 16.1 Sensation OBJECTIVES • Define sensation, and discuss the components of sensation. • Describe the diff erent ways to classify sensory receptors. In its broadest definition, sensation is the conscious or subconscious awareness of changes in the external or internal environment. The nature of the sensation and the type of reaction generated vary according to the ultimate destination of nerve impulses (action potentials) that convey sensory information to the CNS. Sensory impulses that reach the spinal cord may serve as input for spinal reflexes, such as the stretch reflex you learned about in Chapter 13. Sensory impulses that reach the lower brainstem elicit more complex reflexes, such as changes in heart rate or breathing rate. When sen- sory impulses reach the cerebral cortex, we become consciously aware of the sensory stimuli and can precisely locate and identify spe- cific sensations such as touch, pain, hearing, or taste. As you learned in Chapter 14, perception is the conscious interpretation of sensa- tions and is primarily a function of the cerebral cortex. We have no perception of some sensory information because it never reaches the cerebral cortex. For example, certain sensory receptors constantly monitor the pressure of blood in blood vessels. Because the nerve impulses conveying blood pressure information propagate to the car- diovascular center in the medulla oblongata rather than to the cere- bral cortex, blood pressure is not consciously perceived. Sensory Modalities Each unique type of sensation—such as touch, pain, vision, or hear- ing—is called a sensory modality (mō-DAL-i-tē). A given sensory neu- ron carries information for only one sensory modality. Neurons relaying impulses for touch to the somatosensory area of the cerebral cortex do not transmit impulses for pain. Likewise, nerve impulses from the eyes are perceived as sight, and those from the ears are per- ceived as sounds. The diff erent sensory modalities can be grouped into two classes: general senses and special senses. 1. The general senses refer to both somatic senses and visceral senses. Somatic senses (somat- = of the body) include tactile sen- sations (touch, pressure, vibration, itch, and tickle), thermal sensa- tions (warm and cold), pain sensations, and proprioceptive sensations. Proprioceptive sensations allow perception of both the static (nonmoving) positions of limbs and body parts (joint and muscle position sense) and movements of the limbs and head. Visceral senses provide information about conditions within internal organs, for example, pressure, stretch, chemicals, nausea, hunger, and temperature. 2. The special senses include the sensory modalities of smell, taste, vision, hearing, and equilibrium or balance. c16SensoryMotorAndIntegrativeSystems.indd Page 549 10/14/16 2:33 PM f-512 /208/WB01989/9781119287759/ch16/text_s 521 johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight johnup Highlight Copyright W iley, 2020 550 CHAPTER 16 Sensory, Motor, and Integrative Systems in the inner ear, gustatory receptors in taste buds (Figure 16.1c) and photoreceptors in the retina of the eye for vision. The olfactory recep- tors for the sense of smell are not separate cells; instead, they are located in olfactory cilia, which are hair like structures that project from the dendrite of an olfactory receptor cell (a type of neuron). You will learn more about the receptors for the special senses in Chapter 17. A sensory receptor responds to a stimulus by generating a graded potential known as a receptor potential (Figure 16.1a–c). In sensory receptors that are free nerve endings or encapsulated nerve endings, if the receptor potential is large enough to reach threshold, it triggers one or more nerve impulses in the axon of the sensory neuron (Figure 16.1a, b). The nerve impulses then propagate along the axon into the CNS. In sensory receptors that are separate cells, the receptor potential triggers release of neurotransmitter through exocytosis of sensory neurons, (2) encapsulated nerve endings of first-order sen- sory neurons, or (3) separate cells that synapse with first-order sen- sory neurons. Free nerve endings are bare (not encapsulated) dendrites; they lack any structural specializations that can be seen under a light microscope (Figure 16.1a). Receptors for pain, tempera- ture, tickle, itch, and some touch sensations are free nerve endings. Receptors for other somatic and visceral sensations, such as pressure, vibration, and some touch sensations, are encapsulated nerve end- ings. Their dendrites are enclosed in a connective tissue capsule that has a distinctive microscopic structure—for example, lamellated cor- puscles (Figure 16.1b). The diff erent types of capsules enhance the sensitivity or specificity of the receptor. Sensory receptors for some special senses are specialized, separate cells that synapse with sensory neurons. These include hair cells for hearing and equilibrium FIGURE 16.1 Types of sensory receptors and their relationship to first-order sensory neurons. (a) Free nerve endings: in this case, a cold-sensitive receptor. These endings are bare dendrites of first- order neurons with no apparent structural specialization. (b) An encapsulated nerve ending: in this case, a vibration-sensitive receptor. Encapsulated nerve endings are dendrites of first-order neurons. (c) A separate receptor cell—here, a gustatory (taste) receptor—and its synapse with a first-order neuron. Sensory receptors respond to stimuli by generating receptor potentials. (a) First-order sensory neuron with free nerve endings (b) First-order sensory neuron with encapsulated nerve endings (c) Sensory receptor synapses with first-order sensory neuron Free nerve endings (dendrites) Triggers Triggers Triggers Cold stimulus Axon Axon Axon Propagate into CNS Propagate into CNS Receptor potential Vibration stimulus Encapsulated nerve ending Nerve impulses Nerve impulses Receptor potential Sugar molecule Gustatory (taste) receptor Synaptic vesicle Dendrite Release of neurotransmitter from sensory receptor Neurotransmitter Dendrite Q Which senses are served by receptors that are separate cells? c16SensoryMotorAndIntegrativeSystems.indd Page 550 10/14/16 2:33 PM f-512 /208/WB01989/9781119287759/ch16/text_s 522 johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Underline johnup Highlight johnup Highlight johnup Highlight johnup Highlight Copyright W iley, 2020 16.1 Sensation 551 you may already have guessed, this causes the frequency of nerve impulses in the sensory neuron to decrease. Because of adaptation, the perception of a sensation may fade or disappear even though the stimulus persists. For example, when you first step into a hot shower, the water may feel very hot, but soon the sensation decreases to one of comfortable warmth even though the stimulus (the high temperature of the water) does not change. Receptors vary in how quickly they adapt. Rapidly adapting re- ceptors adapt very quickly. They are specialized for signaling changes in a stimulus. Receptors associated with vibration, touch, and smell are rapidly adapting. Slowly adapting receptors, by contrast, adapt synaptic vesicles (Figure 16.1c). The neurotransmitter molecules lib- erated from the synaptic vesicles diff use across the synaptic cleft and produce a postsynaptic potential (PSP), a type of graded potential, in the sensory neuron. If threshold is reached, the PSP will trigger one or more nerve impulses, which propagate along the axon into the CNS. The amplitude of a receptor potential varies with the intensity of the stimulus, with an intense stimulus producing a large potential and a weak stimulus eliciting a small one. Similarly, large receptor poten- tials trigger nerve impulses at high frequencies in the first-order neuron, in contrast to small receptor potentials, which trigger nerve impulses at lower frequencies. LOCATION OF RECEPTORS AND ORIGIN OF ACTIVATING STIMULI Another way to group sensory receptors is based on the location of the receptors and the origin of the stimuli that activate them. • Exteroceptors (EKS-ter-ō-sep′-tors) are located at or near the external surface of the body; they are sensitive to stimuli origi- nating outside the body and provide information about the external environment. The sensations of hearing, vision, smell, taste, touch, pressure, vibration, temperature, and pain are conveyed by exteroceptors. • Interoceptors (IN-ter-ō-sep′-tors) or visceroceptors are located in blood vessels, visceral organs, muscles, and the nervous system and monitor conditions in the internal environment. The nerve impulses produced by interoceptors usually are not consciously perceived; occasionally, however, activation of interoceptors by strong stimuli may be felt as pain or pressure. • Proprioceptors (PRŌ-prē-ō-sep′-tors) are located in muscles, ten- dons, joints, and the inner ear. They provide information about body position, muscle length and tension, and the position and move- ment of your joints. TYPE OF STIMULUS DETECTED A third way to group sensory recep-