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17 General Senses 17 General Senses UNIT OUTLINE General Sensory Receptors Activity 1: Identifying General Sensory Receptors Activity 2: Examining the Microscopic Structure of General Sensory Receptors Activity 3: Performing a Two-Point Discrimination Test n this unit we investigate sensory receptors, structures that respond to changes, or stimuli, in the external environment and within the body. Most sensory re- Iceptors—those that respond to touch, pressure, cold, heat, pain, stretch, vibra- tion, and changes in body position—are widely distributed throughout the body and are classified as general sensory receptors. Somatic sensory receptors are found in the skin and in musculoskeletal organs, whereas visceral sensory recep- tors are located in visceral organs. Sensory receptors for the special senses, by con- trast, are housed in specialized sense organs in the head. The special senses, which include gustation (taste), olfaction (smell), vision, hearing, and equilibrium, will be our focus in Unit 18. THINK ABOUT IT Which brain structure serves as a relay station for most sensory impulses? __________________________________________________________________ Sensory impulses originating from general sensory receptors in the skin are relayed to which functional area of the cerebral cortex? _______________________________ What is the difference between sensation and perception and where does each occur? __________________________________________________________________ _________________________________________________________________ ▪ 355 PRE-LAB ASSIGNMENTS Pre-lab quizzes are also assignable in To maximize learning, BEFORE your lab period carefully read this entire lab unit and complete these pre-lab assignments using your textbook, lecture notes, and prior knowledge. PRE-LAB Activity 1: Identifying General Sensory Receptors 1. Use the list of terms provided to label the accompanying illustration of cutaneous receptors. Check off each term as you label it. □ lamellated corpuscle a □ tactile corpuscle b □ free nerve endings □ Ruffini ending c d 2. Fill in the blanks with the appropriate term(s): ________________________ a. light touch receptor located in the papillary layer of the dermis ________________________ b. deep pressure receptor ________________________ c. light touch receptor located in the epidermis ________________________ d. receptors that detect pain and temperature PRE-LAB Activity 2: Examining the Microscopic Structure of General Sensory Receptors 1. Where specifically in the skin are tactile (Meissner) corpuscles located? _________________________ __________________________________________________________________________________ 2. Where specifically in the skin are lamellated (Pacinian) corpuscles located? ______________________ __________________________________________________________________________________ PRE-LAB Activity 3: Performing a Two-Point Discrimination Test 1. Which instrument will be used to determine the ability of a subject to discriminate between two points? __________________________________________________________________________________ 2. What is the relationship between two-point receptor density and tactile sensitivity? ________________ __________________________________________________________________________________ General Sensory Receptors Because action potentials, once generated, are identical, the body distinguishes among different stimuli entirely by the Sensory receptors respond to various stimuli, both external region of the cerebral cortex that receives the sensory input and internal, and they are often classified according to the originating from the receptor. source of the stimulus. These receptors act as transducers— Sensory receptors can be classified in various ways. One that is, they convert stimuli into afferent nerve impulses. classification method groups sensory receptors according to 356 UNIT 17 | General Senses 357 the type of stimulus to which they respond. Based on this musculoskeletal organs and their associated connective tis- criterion, sensory receptors can be divided into five groups: sue coverings. Two types of proprioceptors—muscle spindles mechanoreceptors, which respond to touch or pressure; and Golgi tendon organs—respond to changes in muscle chemoreceptors, which respond to chemicals; photorecep- length and in joint position and movement, respectively. tors, which respond to light; nociceptors, which respond to Receptors in the skin, or cutaneous receptors (Figure 17-1), intense thermal, chemical, or mechanical stimuli that result in are of two general types: free nerve endings and encapsulated pain; and thermoreceptors, which respond to temperature. receptors. Free nerve endings, which are the dendrites of sen- Sensory receptors can also be classified based on the sory neurons, are the least specialized receptors; they respond source of the stimulus. Exteroceptors, which respond to primarily to temperature and pain. Some free nerve endings stimuli from the external environment, are usually located are associated with epidermal cells and are called Merkel cell near the body surface. Examples are the sensory receptors fibers (or tactile discs); others are wrapped around a hair fol- found in the skin and the specialized sensory receptors for vi- licle (found only in thin skin) and are called hair follicle sion and hearing located in the eye and ear, respectively. In- receptors (or root hair plexuses, not shown). Both Merkel cell teroceptors (also called visceroceptors) respond to stimuli fibers and hair follicle receptors respond to light touch. Encap- within the body. Examples include chemoreceptors (such as sulated receptors are nerve endings that are encapsulated by those located in the medulla oblongata that respond to blood connective tissue cells. Tactile corpuscles (also called Meiss- carbon dioxide levels), stretch receptors in the walls of hollow ner corpuscles) are located in the dermal papillae and function organs such as the urinary bladder, and baroreceptors in arte- as light touch receptors. Lamellated corpuscles (also called rial walls that respond to changes in blood pressure. Pacinian corpuscles) are located deeper in the reticular layer A third type of receptor based on stimulus type is pro- of the dermis and respond to deep pressure. Ruffini endings prioceptors. Like interoceptors, proprioceptors respond (also called bulbous corpuscles) lie in the dermis, hypodermis, to internal stimuli, but these receptors are located only in and joint capsule and respond to deep and continuous pressure. Merkel cell Free nerve fiber endings Epidermis LM (1000×) Tactile corpuscle Dermis Axons of Ruffini ending sensory neurons (bulbous corpuscle) LM (200×) Lamellated corpuscle Figure 17-1 Cutaneous receptors. 358 UNIT 17 | General Senses Materials Needed ACTIVITY 1 □ Skin model and anatomical charts Identifying General Sensory Receptors □ Muscle model or anatomical charts □ Colored pencils Learning Outcomes 1. Identify cutaneous receptors—free nerve ending, lamellated Instructions corpuscle, hair follicle receptor, Ruffini ending, tactile corpuscle, and Merkel cell fiber—on a skin model or anatomical chart, and 1. CHART Complete the following chart as you identify then describe the location and function of each. each of the types of sensory receptor on an anatomical 2. Identify proprioceptors (Golgi tendon organ and muscle spindle) model (or on an anatomical chart). Information for the on an anatomical model or chart, and then describe the location first structure has been filled in for you. and function of each. Sensory Receptor Location Stimulus Type Received In What Way(s) Can This Receptor Be Classified? Free nerve Skin (epidermis and dermis) Temperature; Exteroceptor: responds to external stimulus ending pain; Thermoreceptor: responds to temperature light touch Nociceptor: responds to pain Mechanoreceptor: responds to touch Golgi tendon organ Lamellated corpuscle Muscle spindle Hair follicle receptor Ruffini ending Tactile corpuscle Merkel cell fiber UNIT 17 | General Senses 359 2. Sketch a longitudinal section of the skin. Instructions a. Label and then color the following structures: 1. Observe a longitudinal section (or a photomicrograph) epidermis, blue; dermis, green; free nerve endings, of the skin showing a tactile corpuscle under low power. yellow; tactile corpuscle, brown; lamellated Sketch and label the following structures: epidermis, corpuscle, purple; Ruffini ending, red; hair follicle dermal papilla, dermis, tactile corpuscle. receptor, orange. b. Draw a black box around the labels for the receptors classified as free nerve endings. c. Draw a black circle around the labels for the recep- tors classified as encapsulated receptors. Total magnification: _____ × Describe the location of the tactile corpuscle. Switch to the high-power objective (or view a more- magnified photomicrograph), observe the cells of the tactile corpuscle, and describe their appearance. 2. Observe a histological section (or a photomicrograph) of the skin showing a lamellated corpuscle under low power. Sketch and label the following structures: dermis, lamellated corpuscle. ACTIVITY 2 Examining the Microscopic Structure of General Sensory Receptors Learning Outcomes 1. Distinguish between a tactile corpuscle and a lamellated corpuscle in a longitudinal section of the skin. 2. Compare and contrast a tactile corpuscle and a lamellated corpuscle with respect to structure and function. Total magnification: _____ × Materials Needed □ Anatomical models Describe the location of
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