BMS1052_L4a_Sensation_Intro.pptx

BMS1052 – Lesson 4a Introduction to sensory systems  Sensation and Perception are different Sensation is the process of encoding of events and stimuli by the nervous system Sensation depends on low level physical, biochemical and neural events e.g. here is a set of 12 lines with a ran...

BMS1052 – Lesson 4a Introduction to sensory systems  Sensation and Perception are different Sensation is the process of encoding of events and stimuli by the nervous system Sensation depends on low level physical, biochemical and neural events e.g. here is a set of 12 lines with a range of orientations, and which intersect in certain locations Perception is the process by which the brain interprets sensory information based on its prior experience of the world e.g. Here is a cube pointing down/left, or a cube pointing up/right  BMS1052 – Lesson 4a Introduction to sensory systems Learning objectives Sensory systems have several common anatomical and functional properties. At the end of this lecture, you should be able to: Describe factors affecting sensory transmission and transduction in a range of sensory systems Describe how stimulus information is encoded in action potentials Describe the functional organisation of sensory neurons into topographic maps Reading Chapters 9-12 of Bear, Connors and Paradiso. These chapters are covered across the 2 Sensation lessons. For more detailed information, see: - Ch 22 of Fundamental Neuroscience, Squire, Berg, Bloom, du Lac, Ghosh & Spitzer. (search Fundamental Neuroscience at books.google.com)  Transduction: conversion of one form of energy into another Physical structures (e.g. the eyeball, outer ear; skin) influence the transmission of stimulus energy to the receptor cells. In all sensory receptor cells, the stimulus causes a change in membrane potential (a receptor potential) Hearing - sound pressure waves / Touch – displacement / Taste and smell - molecules and ions / Vision - light energy  Transduction: conversion of one form of energy into another Physical structures (e.g. the eyeball, outer ear; skin) influence the transmission of stimulus energy to the receptor cells. In all sensory receptor cells, the stimulus causes a change in membrane potential (a receptor potential) Hearing - sound pressure waves / Touch – displacement / Taste and smell - molecules and ions / Vision - light energy Light activates a G- protein coupled receptor Stretch-sensitive Na+ channels. Opening them leads to membrane depolarisation e.g. Na+ channels for salt; G- and action potentials. protein coupled receptors for sweet / bitter flavours  Sensory coding: sensory neurons encode information in the rate and timing of action potentials Now that we understand how action potentials are generated and propagate, let’s simplify our representation of the activity in a single neuron How could we encode two different stimuli? Different rates vs for different stimuli Different vs timings for Raste different stimuli rs Time Action potentials = neuronal spiking = neuronal firing. So we often talk about spiking rates or firing rates (action potentials per second)  Sensory coding: in a rate code, different stimuli evoke different firing rates Stimulus Evoked This neuron encodes the orientation of response a visual stimulus in the rate of APs Notice how unpredictable (stochastic) the timing of individual action potentials is! The visual stimulus is just a stationary bar of light. Sensory coding: a neuron’s responses to repetitions of an identical stimulus are highly Stimulus variable Trial 1 Trial 2 Raste Trial 3 rs Trial 4 Time Average across trials Mean Response Peristimulus time (spikes/s histogram ) (PSTH) Time To accommodate variability (and therefore unreliability) of responses, every stimulus is encoded by a population of neurons  Sensory coding: neurons can encode, or are tuned for, multiple stimulus features e.g. APs in a neuron in visual cortex can be affected by luminance, contrast, orientation, motion direction, colour, … APs in a neuron in auditory cortex can be affected by sound intensity, frequency, location, … APs in a neuron Neuralin somatosensory responses cortex can be affected byNeural pressure, vibration responses frequency, location, … Action potential Action potential rate rate 0 0 Stimulus intensity Stimulus property (e.g. visual contrast; sound level; touch (e.g. visual orientation; sound pressure) frequency; touch location) The curves above show how responses of 4 hypothetical neurons might respond to different types of stimuli.  Sensory coding – temporal codes Rate coding theories suggest that the precise timing of action potentials doesn’t matter – only the number of spikes within a certain time window is important. Temporal coding theories suggest that the precise timing of action potentials is informative. Neuron 1 Stimulus A: neuron 1 spikes after neuron 2 Neuron 2 Stimulus B: neuron 2 spikes after neuron 1 Different neuronal systems might use either a rate or a temporal code (or both). We will focus on rate codes, except in the auditory system, where  What is a map? Spatial representation of physical information or relationships Þ Systematic and orderly way of representing structure. Þ Adjacent physical features are adjacent in the map.  Somatosensory cortex contains a somatotopic map of the skin surface. Auditory cortex contains a map of sound frequency (pitch) Visual cortex contains map of visual space Adjacent groups of neurons in V1 encode adjacent Adjacent groups of neurons Adjacent groups of neurons spatial locations in S1 encode adjacent in A1 encode adjacent regions of the skin surface pitches  BMS1052 – Lesson 4a Introduction to sensory systems Learning objectives Sensory systems have several common anatomical and functional properties. At the end of this lecture, you should be able to: Describe factors affecting sensory transmission and transduction in a range of sensory systems Describe how stimulus information is encoded in action potentials Describe the functional organisation of sensory neurons into topographic maps Reading Chapters 9-12 of Bear, Connors and Paradiso. These chapters are covered across the 2 Sensation lessons. For more detailed information, see: - Ch 22 of Fundamental Neuroscience, Squire, Berg, Bloom, du Lac, Ghosh & Spitzer. (search Fundamental Neuroscience at books.google.com)