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Sensory Receptors Laboratory Worksheet Exercise: Sensory Receptors Sense Organs - Sensory Receptors A sensory receptor is a specialized ending of a sensory neuron that detects a specific stimulus. Receptors can range from simple nerve endings of a sensory neuron (e.g., pain, touch), to a complex combination of nervous, epithelial, connective and muscular tissue (e.g., the eyes). Axon Synaptic info. Sensory end bulbs Receptors Figure 1. Diagram of a sensory neuron with sensory information being detected by sensory receptors located at the incoming end of the neuron. This information travels along the axon and delivers its signal to the central nervous system (CNS) via the synaptic end bulbs with the release of neurotransmitters. The function of a sensory receptor is to act as a transducer. Transducers convert one form of energy into another. In the human body, sensory receptors convert stimulus energy into electrical impulses called action potentials. The frequency and duration of action potential firing gives meaning to the information coming in from a specific receptor. The nervous system helps to maintain homeostasis in the body by monitoring the internal and external environments of the body using receptors to achieve this. Sensations are things in our environment that we detect with our 5 senses. The 5 basic senses are: Sight Hearing Touch Taste Smell An adequate stimulus is a particular form of energy to which a receptor is most responsive. For example, thermoreceptors are more sensitive to temperature than to pressure. The threshold of a receptor is the minimum stimulus required to activate that receptor. Information about Receptor Transmission Sensory receptors transmit four kinds of information - modality, location, intensity and duration. 1. Modality - describes the type of stimulus (or the sensation) it produces. A stimulated receptor typically elicits the same perception in the brain. 2. Location - the ability of the brain to identify the site of the stimulation. The precision with which the location of a stimulus is perceived is called acuity. 3. Intensity - refers to the strength of the signal that is detected by the receptors. If a stimulus intensity increases, it can be encoded three ways: 1) increasing action potential rate; 2) recruitment of more nerve fibers; 3) activating nerve fibers with higher thresholds (less sensitive fibers). 4. Duration – is the way a nerve fiber changes its rate of firing over time. Some receptors fire briefly when a stimulus begins, then become 'silent' and fire again briefly when the stimulus ends (e.g., corpuscles of touch). Other receptors fire continually, but all receptors exhibit some adaptation. 2 Adaptation occurs after a long period of stimulation. The receptors responds by decreasing action potential firing. The way you adapt to the hot water of a shower is an example of sensory receptor adaptation. In terms of adaptation, there are phasic receptors and tonic receptors. Phasic receptors generate a burst of action potentials when first stimulated; then they quickly adapt and stop transmitting impulses even if the stimulus continues. In other words, they are fast to adapt to the stimulus. Examples of phasic receptors are touch and smell receptors. Tonic receptors are slow to adapt and generate nerve impulses continually. Examples of tonic receptors are proprioceptors (for balance) and baroreceptors (for blood pressure). Sensory Receptors Classification Table 1. Shows a summary of sensory receptor classifications and naming, using modalities for the general receptors they stimulate and their specific names. Classification Sensory Modality General Type of Specific Name of Receptor Receptor Cutaneous Heat Thermoreceptor Ruffini's End Organ Senses Cold Thermoreceptor Krause's End Bulbs Touch (vibration) Mechanoreceptor Meissner's Corpuscles Pressure Mechanoreceptor Pacinian Corpuscles Pain Nociceptor Free Nerve Endings (Chemoreceptor) Tickle and Itch Mechanoreceptor Free Nerve Endings Proprioceptors Muscle stretch Mechanoreceptor Muscle Spindles Muscle contraction Mechanoreceptor Tendon Organs Limb position Proprioceptor Joint Receptors Special Senses Low intensity of Light Photoreceptor Rods (Black and White) High intensity of Light Photoreceptor Cones (Color) Sound Mechanoreceptor Inner Ear (Hair Cells) Equilibrium Chemoreceptor Inner Ear (Vestibular) Smell (olfaction) Chemoreceptor Nasal Mucosa (Vomeronasal Organ) Taste (gustation) Chemoreceptor Taste Buds (Papillae of Tongue) Classification Sensory Modality General Receptor Specific Receptor Region Visceral Senses Blood Pressure Mechanoreceptor Aortic Arch, Carotid Sinus Blood Chemistry Chemoreceptor Aortic and Carotid Bodies Plasma Oncotic Pressure Chemoreceptor Osmoreceptors Plasma Temperature Thermoreceptor Hypothalamus Pain Nociceptor Free Nerve Endings (Chemoreceptor) 3 Other ways to Classify Sensory Receptors A) By Modality (type of stimulus to which each is most sensitive): Thermoreceptors - stimulated by changes in temperature (heat and cold). Mechanoreceptors - stimulated by the physical deformation of the plasma membrane caused by touch, pressure, stretch, tension or vibration. Can also be called stretch receptors. Nociceptors - pain receptors, stimulated by physical or chemical damage to tissue. Damage can result from trauma, ischemia (reduced blood flow) or excessive heat and chemicals. Photoreceptors - stimulated by light or changes in light intensity (in retina of the eyes). Chemoreceptors - stimulated by chemicals, including food, odors, & molecules in body fluids. B) By Origin of their stimuli: Exteroceptors - detect changes external to the body, including surface receptors (e.g., for touch, temperature and cutaneous pain). They also include receptors for vision, hearing, taste and smell. Interoceptors or Visceroceptors - these detect stimuli that originate in the internal organs (viscera); they are responsible for feeling visceral pain, nausea, stretch and pressure. Proprioceptors - detect changes in the position of the body and movement of the body or its parts. They are located in muscles, tendons, joint capsules and the inner ear. Surface pain can be accurately located whereas Visceral pain is referred pain and typically poorly localized; e.g., heart pain felt in the shoulder and arm, kidney pain can be felt in the lower back. C) By General or Special categories: General Senses are somatic or somatosensory senses and have receptors that are widely distributed in the body rather than limited to specific locations. These receptors occur in the skin, muscles, tendons, joint capsules and viscera (internal organs). They detect touch, pressure, stretch, heat, cold and pain, as well as other stimuli that we are not consciously aware of, such as blood pressure and chemistry of body fluids. Special Senses have receptors which are very complex and are limited to the head and innervated by cranial nerves. There are 5 special senses: 1. Vision 2. Hearing 3. Equilibrium (Balance) 4. Gustation (Taste) 5. Olfaction (Smell) 4 Table 2. From the information provided throughout this worksheet about receptors, take a look at the types of “stimulus” below that we are going to study and fill in the table for the modality, general type of receptor, and the specific name (if applicable) for each stimulus. Stimulus Modality General Receptor Specific Receptor (Sensation Produced) Activated (Name) 1) (stepping on a tack) 2) (flame) 3) (an ice cube) 4) (coins on an arm) 5) (a molecule to taste) 6) High intensity Low intensity (Light) 7) (measuring sensitivity) 5 Sensory Lab – Data and Q’s from Activities Part 1: In the Lab We examined how sensitive the skin on different areas of the body is to touch using calipers (see drawing to the right) to assess ‘Two-Point Sensitivity’ on the palm, the fingertip, the back of the neck, and the inside of the forearm. Data was collected in pervious physiology labs and the averages are shown in Table 3. In the diagram to the right, we see that some areas of the body have different receptive fields than others. By measuring the distance that 2 points can be perceived as separate (and not one point) we can determine the variations in sensitivity in different regions of the body. Q1. In the diagram of receptive fields above, which area is more sensitive, the one with the larger receptive field, or the one with the smaller receptive fields? ________________________________. Q2. Why? Table 3. Here are the results each area of the body, showing the average smallest distance (mm) at which two points can be identified as separate. Palm Fingertip Back of Neck Inside Forearm Class Average: 9.1 2.3 18.5 21.2 Q3. From the class average data in Table 3: a) Which of these areas is the least sensitive? b) Which of these areas is the most sensitive? c) For the most sensitive area, why do you suppose it was so much more sensitive than the others? Part 2: Sensory Modality Read the “Sense Organs” section above (p.1-2) and define the terms and answer the question using these concepts to explain your perceptions. Q4. The function of a sensory receptor is to act as a ______________________; it converts one form of ________________ into another. Q5. Define adequate stimulus of a receptor: Q6. Define the threshold of a receptor: 6 Q7. Consult Table 1 (p 2) in the lab manual for information about these receptors. a) Nociceptors detect ____________, their receptors are ____________ _____________ endings. b) The general type of receptors the detect heat and cold in the skin are? ______________________. c) The specific type
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