University:
Kentucky State University
Course:
BIO 303 | Human Physiology
Academic year:

2020
Views:

472

Pages:

37
Author:

nmarie0886sck

Sensory Receptors; Neuronal Circuits For Processing Information 75 Objectives ➢ Define receptors (Transducers) and classify them ➢ Describe the generator (receptor) potential and its importance in sensory coding ➢ List the types of somatic receptors in the skin ➢ Explain the mechanism of sensory coding ➢ Interpret the mechanism of receptor adaptation and classify the types of receptors accordingly (Phasic and Tonic receptors) ➢ Describe sensory neuronal processing and its functional importance 76 Types of Sensory Receptors: Classification by Modality (Stimulus they transduce) ➢ Mechanoreceptors ➢ detect deformation, Touch and Prssure ➢ Thermoreceptors ➢ detect change in temperature ➢ Nociceptors ➢ detect tissue damage (pain receptors) ➢ Electromagnetic (Photoreceptors) ➢ detect light (Rods and Cones) ➢ Chemoreceptors ➢ taste, smell, CO2, O2, etc. 77 Classification by Location ➢ Exteroceptors – sensitive to stimuli arising from outside the body ➢ Located at or near body surfaces ➢ Include receptors for touch, pressure, pain, and temperature ➢ Interoceptors – (visceroceptors) receive stimuli from internal viscera ➢ Monitor a variety of stimuli (distension of viscera, pain) ➢ Proprioceptors – sense of position- monitor degree of stretch ➢ Located in musculoskeletal organs (muscle, tendons and skin around joints) 78 Types of Sensory Receptors 79 80 81 Merckel’s disc for mechanical sensation (Touch in hairy skin) Iggo dome receptors 82 Tactile Receptors    Free nerve endings (Ad and C fibers)  detect touch and pressure  found everywhere in the skin and other tissues Meissner’s corpuscles (Ab)  rapidly adapting (within a fraction of a second) and detect movement of light objects over skin  found on nonhairy skin (glabrous skin), fingertips and lips Merkel’s discs (Ab)  respond rapidly at first and then slowly adapt, detect the “steady state”  found on hairy as well a glabrous (non hairy) skin 83 Tactile Receptors  Hair end organ   Ruffini’s end organ   adapts rapidly and detects movement over the body slowly adapting and respond to continual deformation of the skin and joint rotation Pacinian corpuscle   very rapidly adapting and is stimulated only by rapid movement detects vibration and other rapid changes in the skin 84 Tactile Sense Transmission    Meissner’s corpuscles, hair receptors, Pacinian corpuscles and Ruffini’s end organs transmit signals in type Ab nerve fibers at 30-70 m/sec. Free nerve endings transmit signals in type Ad nerve fibers at 5-30 m/sec, some by type C unmyelinated fibers at 0.5-2 m/sec. The more critical the information the faster the rate of transmission. 85 Sensory Receptors: General structure Receptor area is None-excitable region so as it can discriminate different intensities, otherwise it will not be able to differentiate strengths of stimuli 86 Conversion of Receptor and Generator Potentials into Action Potentials Receptor Potential Generator Potential Law of Specific Nerve Energies   Sensation characteristic of each sensory neuron is that produced by its normal or adequate stimulus. Adequate stimulus: ◼  Requires least amount of energy to activate a receptor. Regardless of how a sensory neuron is stimulated, only one sensory modality will be perceived (specificity of receptors) ◼ Allows brain to perceive the stimulus accurately under normal conditions. 88 Sensation ➢ Each of the principle types sensation; touch, pain, sight, sound, is called a modality of sensation. ➢ Each receptor is responsive to one type of stimulus energy. Specificity is a key property of a receptor, it underlines the most important coding mechanism, the labeled line principle ➢ How the sensation is perceived is determined by the characteristics of the receptor and the central connections of the axon connected to the receptor. 89 Receptor Excitation ➢ mechanical deformation which stretches the membrane and opens ion channels ➢ application of chemicals which also opens ion channels ➢ change in temperature which alters the permeability of the membrane through changing the metabolic rate ➢ electromagnetic radiation that changes the membrane characteristics 90 General Structure of Receptors Conducting Region Receptor Region Spike Generating Region 91 Receptor Excitation 92 Receptor Potential ➢ The membrane potential of the receptor ➢ Excitation of the receptor results from a change in this potential. ➢ When the receptor potential rises above the threshold, action potentials appear and the receptor is active. ➢ The greater the intensity of the stimulus, the greater the receptor potential, and the greater the rate of action potential generation. 93 Generator Potentials   In response to stimulus, sensory nerve endings produce a local graded change in membrane potential. Potential changes are called receptor or generator potential. ◼ Analogous to EPSPs. 94 Relationship between Receptor Potential and Action Potentials 95 The effect of stimulus strength on RP amplitude Amplitude of Receptor potential Greater sensitivity region Lesser sensitivity region Slop of the curve Stimulus Strength 96 The effect of the amplitude of RP on the frequency of impulses generated 97 Adaptation of Receptors ➢ When a continuous stimulus is applied, receptors respond rapidly at first, but response declines until all receptors stop firing. 98 Adaptation ➢ Rate of adaptation varies with type of receptor. ➢ Therefore, receptors respond when a change is taking place (i.e., think of the feel of clothing on your skin.) 99 Adaptation of Sensory Receptors    Receptors responding to pressure, touch, and smell adapt quickly Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles, and interoceptors that respond to chemical levels in the blood Pain receptors and proprioceptors do not exhibit adaptation 100 Slowly Adapting (Tonic) Receptors ➢ continue to transmit impulses to the brain for long periods of time while the stimulus is present ➢ keep brain apprised of the status of the body with respect to its surroundings ➢ will adapt to extinction as long as the stimulus is present, however, this may take hours or days ➢ these receptors include: muscle spindle, golgi tendon apparatus, Ruffini’s endings, Merkels discs, Macula, chemo- and baroreceptors 101 Sensory Adaptation   Tonic receptors: ◼ Produce constant rate of firing as long as stimulus is applied.  Pain. Phasic receptors: ◼ Burst of activity but quickly reduce firing rate (adapt) if stimulus maintained. ◼ Sensory adaptation:  Cease to pay attention to constant stimuli. 102 Rapidly Adapting (Phasic) Receptors ➢ respond only when change is taking place ➢ Rate and Strength of the response is related to the Rate and Intensity of the stimulus ➢ important for predicting the future position or condition of the body ➢ very important for balance and movement ➢ types of rapidly adapting receptors: pacinian corpuscle, semicircular canals in the inner ear 103 Importance of Signal Intensity ➢ Signal intensity is critical for interpretation of the signal by the brain (i.e., pain). ➢ Gradations in signal intensity can be achieved by: ➢ 1) increasing the number of fibers stimulated, spatial summation ➢ 2) increasing the rate of firing in a limited number of fibers, temporal summation. 104 Signal Intensity An example of spatial summation Coding in the sensory system  Intensity is coded for by: ◼ ◼   Frequency of action potential The No. of neurons stimulated Location is coded for by the labeled line principles Type of stimulus is coded for by the kind of receptor stimulated (Adequate stimulus) and specificity of the receptors. 106 Mapping of the Primary Somatosensory Area ➢ Mapping of the postcentral gyrus. ➢ Size of the cortical region representing a body part depends on density of receptors on that part and the sensory impulses received from that part. Receptive Fields  Area of skin whose stimulation results in changes in the firing rate of the neuron. ◼  Back and legs have few sensory endings. ◼  Area of each receptor field varies inversely with the density of receptors in the region. Receptive field is large. Fingertips have large # of cutaneous receptors. ◼ Receptive field is small. 109 Signal Intensity An example of spatial summation Two-Point Touch Threshold  Minimum distance at which 2 points of touch can be perceived as separate. ◼  Measures of distance between receptive fields. Indication of tactile acuity. ◼ If distance between 2 points is less than minimum distance, only 1 point will be felt. 111

Sensory Receptors Neuronal Circuits for Processing Information

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