Principles of Sensory Physiology PDF

© Barry Mason 2014. Not for use or reproduction without permission. Principles of Sensory Physiology copyright © Dennis Kunkel Dr. Frank Huynh...

© Barry Mason 2014. Not for use or reproduction without permission. Principles of Sensory Physiology copyright © Dennis Kunkel Dr. Frank Huynh Department of Cellular and Physiological Sciences University of British Columbia © Barry Mason 2014. Not for use or reproduction without permission. Objectives By the end of this lecture you should be able to: 1) define a sensory receptor 2) identify some sensory receptors and what they detect 3) understand the general mechanisms of how sensory receptors relay signals to the CNS, including how sensory intensities are coded 4) describe two different types of receptor adaptation PRINCIPLES OF SENSORY PHYSIOLOGY SPECIAL AND GENERAL SENSES The special senses are modalities carried by cranial nerves They are: – Olfaction – Cr. I – Vision – Cr. II – Taste – Cr. VII and Cr. IX – Hearing and balance (equilibrium) – Cr. VIII The general or somatic senses (somatosensory) – Detected from all parts of the body (and head) and transmitted to CNS via: – Cr. V (trigeminal) – All spinal nerves except C.1  DEFINITION AND CLASSIFICATION OF SENSORY RECEPTORS Sensory receptors are transducers – devices that convert one form of energy into another form. – They detect various stimuli and convert them into action potentials. They include, with reference to the modality being detected: Photoreceptors – light: rods and cones of the retina. Thermoreceptors – changes in temperature, central (hypothalamus) and peripheral (skin) Nociceptors – pain Mechanoceptors – mechanical stimuli – can be subdivided into: – exteroceptors – respond to stimuli from outside the body eg. touch receptors – proprioceptors – give information about position of the body, or its parts eg. muscle spindles.  Classes of Sensory Receptors Mechanoreceptors: tactile Light touch, vibrations, surrounded by connective tissue, rapidly adapt Touch and deep pressure, rough Most sensitive to surfaces, vibrations and touch, vibrations, rapid slow to adapt adaptation, Stretch (deformation), torque surrounded by (rotational force), detect connective tissue deformation in joints, slow to adapt  Objectives By the end of this lecture you should be able to: 1) define a sensory receptor 2) identify some sensory receptors and what they detect 3) understand the general mechanisms of how sensory receptors relay signals to the CNS, including how sensory intensities are coded 4) describe two different types of receptor adaptation  THE SENSORY RECEPTOR RECEPTOR AND GENERATOR POTENTIALS The membrane must be depolarized to a threshold level to generate an action potential (AP) – Remember – AP needs open voltage-gated Na+ channels. The generator potential (GP) is a depolarization of the peripheral, receptive portion of the sensory axon. – caused by sensory stimulus – exception: in rods and cones, the GP is a hyperpolarization If the GP is big enough to reach threshold, action potentials will be produced. – APs propagate to the CNS. In myelinated sensory axons, the action potential is initiated at the 1st node of Ranvier.  (First node of Ranvier in myelinated axons) (axon hillock) (axon hillock)  THE SENSORY RECEPTOR The GP is similar to the EPSP in that it: Compare to action potentials which:  THE SENSORY RECEPTOR The GP is similar to the EPSP in that it: – can be graded in amplitude, i.e. the bigger the stimulus, the bigger the GP – does NOT cause the membrane to be refractory – is NOT actively propagated. Compare to action potentials which:  THE SENSORY RECEPTOR The GP is similar to the EPSP in that it: – can be graded in amplitude, i.e. the bigger the stimulus, the bigger the GP – does NOT cause the membrane to be refractory – is NOT actively propagated. Compare to action potentials which: – are all or none. – cause the membrane to be come refractory. – are actively propagated by regenerating themselves all along the axonal membrane.  THE SENSORY RECEPTOR The mechanism responsible for the GP depends on the type of receptor: – In all cases is due to the opening or closing of ion channels – Result is depolarization (except in photoreceptors) The GP of somatosensory mechanoreceptors – direct effect of mechanical stimuli on stretch-sensitive channels. Non-selective and allow both Na+ and K+ to pass. Net result is depolarization due to greater driving force for Na+. The GP of nociceptors, photoreceptors, and chemoreceptors – Separate cells or G-protein-coupled mechanisms that influence channels indirectly.  Receptor Potential in Specialized Afferent Ending Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Specialized Afferent Ending Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Specialized Afferent Ending Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2b Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2a Sherwood, 5th Cdn Ed.  Receptor Potential in Separate Cell Ending Ex. auditory, photoreceptors Fig. 5-2a Sherwood, 5th Cdn Ed.  HOW IS STIMULUS INTENSITY CODED? How are different intensities of stimuli translated into neural activity? Two coding strategies: 1. Frequency coding – Greater the intensity – greater the frequency of action potentials in individual axons. – NOT A LINEAR FUNCTION 2. Population coding – With increased intensity, more individual receptors are recruited.  Stimulus strength Increased stimulus strength leads to increased graded potential Fig. 6-3, Sherwood 9th Ed.  Stimulus strength Increased stimulus strength leads to increased action potential frequency along afferent neuron Fig. 6-3, Sherwood 9th Ed.  Stimulus strength Increased stimulus strength leads to increased action potential frequency along afferent neuron and greater quantity of neurotransmitter release from afferent Fig. 6-3, terminals Sherwood 9th Ed.  Stimulus strength Is this frequency coding or population coding? Fig. 6-3, Sherwood 9th Ed.  Stimulus strength Is this frequency coding or population coding? How do you get population coding? Fig. 6-3, Sherwood 9th Ed.  Objectives By the end of this lecture you should be able to: 1) define a sensory receptor 2) identify some sensory receptors and what they detect 3) understand the general mechanisms of how sensory receptors relay signals to the CNS, including how sensory intensities are coded 4) describe two different types of receptor adaptation  Receptor Adaptation Most sensory receptors have the property of adapation. Decreased depolarization despite sustained stimulus strength Tonic receptors – adapt slowly or not at all – Continue to generate AP and relay info to CNS Ex. Muscle stretch receptors (which monitor muscle length) or joint proprioceptors (which monitor degree of joint flexion) CNS must get this continuous info to maintain posture and balance.  Receptor Adaptation Phasic receptors – adapt rapidly – No longer responds to maintained stimulus Ex. Touch receptors on skin that signal change in pressure Think of putting clothes or accessories on. You quickly forget that you are wearing them. When you take them off though, you are aware of it (ie. the “off response”) Speed of Adaptation Off response is depolarization seen when stimulus is removed Fig. 6-4  Objectives By the end of this lecture you should be able to: 1) define a sensory receptor 2) identify some sensory receptors and what they detect 3) understand the general mechanisms of how sensory receptors relay signals to the CNS, including how sensory intensities are coded 4) describe two different types of receptor adaptation