BIO3303 Sensory 1.pdf

BIO3303 – Sensory Physiology 1 Control systems I – Nervous system (Chpts. 5 and 8) Sensory physiology (Chpt. 7) Control systems II - Endocrinology (Chpt. 4) Muscles, locomotion and animal energetics (Chpts. 6, 12 and 14) Sensory Physiology Introduction to Sensory Physiology Chemoreception (olfaction...

BIO3303 – Sensory Physiology 1 Control systems I – Nervous system (Chpts. 5 and 8) Sensory physiology (Chpt. 7) Control systems II - Endocrinology (Chpt. 4) Muscles, locomotion and animal energetics (Chpts. 6, 12 and 14) Sensory Physiology Introduction to Sensory Physiology Chemoreception (olfaction and gustation) Mechanoreceptors Hearing Photoreception and vision Thermoreception, and Magnetoreception Fig. 6.17/7.19 Sensory Physiology (1 of 6) By the end of this lecture, you should know: BIO3303 What is sensory physiology? What is a sensory receptor? Classification of sensory receptors What is a stimulus? Steps in sensory reception Aspects of sensory stimulus encoding: modality, intensity, location and duration Introduction to Sensory Physiology What is sensory physiology? The study of how sensory stimuli are transduced by sensory receptors and processed by the nervous system. Fig. 7.1/8.2 Sensory System Organization Sensory system: sense organ + afferent sensory neuron + area of brain to which these neurons project e.g. Visual system: eyes, optic nerve, brain areas concerned with processing visual information Sense organ: receptor cells + accessory non-neural tissues e.g. Eye Fig. 13.10, Hill et. al. 2004 Fig. 13.19, Hill et. al. 2004 Sensory System Organization Sensory receptors: cell that is specialized to detect incoming sensory stimuli e.g. Rods and cones Receptor proteins: proteins on membranes of sensory receptors (cell) specialized to detect incoming sensory signals e.g. opsin Opsin Opsin Rod http://www.karger.com/gaz ette/64/fernald/fig_3.html Types of sensory receptors 1. Sensory neuron (sensory receptor neuron) Is an afferent neuron Stimulus leads to a type of graded potential called: “generator potential” and AP generated if threshold reached 2. Epithelial sensory receptor cell Graded potential called: “receptor potential” Releases neurotransmitter causing graded potentials and APs in an associated primary afferent neuron Sensory neuron Fig. 6.2/7.3 Sensory neurons have some unique features Technically unipolar, with one main branch extending from cell body the splits into two branches (bifurcates) -> “pseudounipolar” One branch has dendrites at the end; receives signal, generates graded potentials AP generated at trigger zone, instead of at axon hillock – behave the same, but location is different AP travels along length of both branches, bypassing the cell body Other branch has axon terminals for neurotransmitter release dendrites trigger zone AP axon terminals Sensory neuron Classification of sensory receptors Human-centered classification of senses that we consciously use (first developed by Aristotle >2000 years ago) – very simplistic Classification of sensory receptors Classification based on location of the stimulus: Exteroceptors: detect stimuli occurring on the outside of the body e.g. Pressure, temperature, light, taste, etc. Interoceptors: detect stimuli occurring inside the body e.g. Blood pressure, blood oxygen Teleceptors: detect stimuli occurring at some distance from the body e.g. Vision and hearing Proprioceptors: detect body position and orientation relative to the environment e.g. skin and muscle receptors, vestibular apparatus of inner ear These are not particularly useful classifications to physiologists – has not relation to how the receptor works Classification based on stimulus modality (type of stimulus it detects) For example: Types of stimulus Sensory receptor Chemical Mechanical Chemoreceptors mechanoreceptors Light photoreceptors Temperature Magnetic fields thermoreceptors magnetoreceptors Electrical fields electroreceptors Stimulus: form of external energy to which a receptor responds Sensory receptors Most receptors have a preferred stimulus modality → “adequate stimulus” e.g. rods and cones = light Enough stimulus energy can overcome specialization e.g. sufficient pressure on eye = light signal Some receptors sensitive to multiple modalities → polymodal receptors e.g. Ampullae of Lorenzini (shark nose): electricity, touch and temperature e.g. some Nociceptors (humans and other animals): detect strong stimuli of temperature, pressure, chemicals, among others →detection of pain Steps in sensory reception 1. Reception of the stimulus 2. Transduction of the stimulus: stimulus converted into changes in membrane potential (electrical signal) 3. Transmission of the signal to the integrating centre 4. Perception of the stimulus at the integrating centre Fig. 6.1/7.2 Stimulus encoding Problem of sensory encoding: Different types of stimulus of varying intensity and duration from different places, but only one signal in nervous system: the action potential How do APs encode for… A. Modality (type of stimulus) B. Intensity C. Location D. Duration Ch. 13, Hill et. al. 2004 A. Encoding Stimulus Modality Specificity - Most receptors are maximally sensitive to only 1 type of stimulus (adequate stimulus) Single sensory receptor type synapses with only one afferent neuron. “Labeled-line Theory” (1850’s Johannes Müller) Modality based on receptor type and afferent neuron Requires high degree of receptor specificity Fig. 13.1, Hill et. al. 2004 A. Encoding Stimulus Modality Polymodal receptors How can they encode more than one modality? Labelled Line Sensory cells have only one type of receptor -> only 1 sensory neuron Cross-fiber Coding Single receptor cell, many receptor proteins, responds to different tastants Afferent neurons synapse with many receptor cells Chandrashekar et. al. 2006 Nature 444, 288-294 A. Encoding Stimulus Modality Polymodal receptors How can they encode more than one modality? Likely through temporal patterns of APs – sensory neuron Morse code 1. Different receptors in the same sensory receptor cell may lead to graded potentials that generate APs at different temporal rates 2. Different sensory receptors may have different sensitivities to a given stimulus; sensory neuron can compare relative intensity coming from each receptor (stronger vs weaker graded potential), leading to temporal pattern in APs Across-fiber Coding 1. Single receptor cell, many receptor proteins, responds to different stimuli 2. Afferent neurons synapse with many types of receptor cells Chandrashekar et. al. 2006 Nature 444, 288-294 B. Encoding Stimulus Intensity APs are all-or-none electrical events of same magnitude Graded (receptor/generator) potentials vary in magnitude and affect rate of APs stimulus intensity coded through changes in frequency of APs ‘low’ ‘higher’ strength strength stimuli Graded (generator) potentials Impulse (AP) frequency Fig. 7.4, Randall et. al. 2002 B. Encoding Stimulus Intensity Stimulus can activate one or many receptor cells Large stimulus activates larger number of receptor cells e.g. strong mechanical force, or high amplitude (loud) sound = large deformation Stimulus-Response Relationships Dynamic range: range of intensities over which stimuli are encoded by receptor cells Stimulus too weak = no AP response Stimulus very strong = no increase in AP response Fig. 6.4/7.5 Stimulus-Response Relationships Maxing out the dynamic range receptor proteins saturated all ion channels have opened or closed Membrane potential change maxes out (i.e. reaches equilibrium potential of ion) Maximum rate of release of neurotransmitters Maximum frequency of APs in afferent neuron (set by refractory periods) Stimulus-Response Relationships Different sensory receptors with varying dynamic range Sensory receptor A detects broad range of stimulus intensities, but has low power to discriminate among different intensities Sensory receptor B detects narrow range; but provides fine discrimination within that range Problem: Jet engine is 1.4 million times as loud as the faintest sound that a human can hear. Fig. 6.4/7.5 Stimulus-Response Relationships Range fractionation: Group of sensory receptors, each sensitive to a different range, work together to provide finer discrimination across a wider range of intensities Population coding: intensity coded through the behavior of the population (multiple) of sensory receptors Usually there is much more overlap than shown Responses integrated in upstream neurons and processing centers Range fractionation extends the dynamic range and increases sensory discrimination Fig. 6.4/7.5 Stimulus-Response Relationships Logarithmic encoding Some receptors encode stimulus intensity logarithmically: Discrimination of stimulus: fine at low intensities coarse at high intensities e.g. Candle light in dark room vs bright room Fig. 6.4/7.5 C. Encoding Stimulus Location Receptive Field: region of sensory surface that, when stimulated, generates a response in the primary afferent neuron two-point discrimination test Neurons with small Small Receptive Fields Large Receptive Field receptive fields provide more precise localization of stimulus (greater acuity) Populations of neurons with overlapping receptive fields improves ability to locate stimuli Population coding: information about stimulus encoded in pattern of firing of multiple neurons Perceived as two points Perceived as single point C. Encoding Stimulus Location Lateral inhibition increases contrast between signals from neurons at the center of the stimulus and neurons on the edge, allowing further discrimination Strong stimulus at center (B) causes skin to bend in receptive fields of adjacent neurons (A and C) Neuron B release lots of Nt which stimulates lateral interneurons Lateral interneurons release inhibitory Nt, preventing release of Nt in A and C. Fig. 6.3/7.4 D. Encoding Stimulus Duration Tonic receptors: fire APs as long as the stimulus continues → convey information about how long the stimulus lasts Receptor adaptation: AP frequency declines even when the stimulus intensity is maintained at a constant level Phasic receptors : fire APs only when the stimulus begins (adapt quickly) → convey information about changes in stimulus but not duration Fig. 6.5/7.6 Sensory Physiology (1 of 6) Summary of key points What is sensory physiology? What is a sensory receptor? Classification of sensory receptors What is a stimulus? Steps in sensory reception Aspects of sensory stimulus encoding: modality, intensity, location and duration