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Sensory perception MD1009pgSensoryPercep1onlynchma2013_14 PG2000pgSensoryPercep1onlynchma2013_14 Peripheral Nervous System Nerve fibers that carry information between CNS & other parts of the body Afferent division – Sends information from internal and external environment to CNS Visceral afferent Incoming pathway for information from internal viscera (organs in body cavities) Efferent division – Sends information from the CNS Sensory afferent Somatic Sensation from body surface & proprioception Special senses Vision, hearing Smell, taste Perception Conscious interpretation of external world derived from sensory input Why sensory input does not give true reality perception – Humans have receptors that detect only a limited number of existing energy forms – Information channels in our brains are not high-fidelity recorders – Cerebral cortex further manipulates the data Stimulus A change detectable by the body Exists in different modalities (eg heat, light, sound ……) Perceived by receptors (afferent neurons have sensory receptors) Receptors convert heat/light/sound etc into electrical signals Sequence Stimulus Receptor Receptor potential Sensory transduction Receptors Structures at peripheral endings of afferent neurons Detect stimuli (change detectable by the body) Convert forms of energy into electrical signals (action potentials) – Process is called transduction Action potential (in afferent fibre) Sensory receptors Sense Stimulus Chemoreceptors Taste/Smell O2/CO2/pH Photoreceptors Rods/Cones Mechanoreceptors Touch/Pressure Balance, Motion Hearing Muscle fibre length Joint position Hair cell movement Blood pressure Thermoreceptors Skin Deep (in CNS) Nociceptors Nerve endings Pain Osmoreceptors OTHERS Electroreceptors Blood osmolarity Magnetoreceptors –in animals (not humans) Sense organs have the property of transduction The energy of stimulus is altered to electrical energy The energy of stimulus is usually increased (amplified) or reduced during transduction Receptors • May be – Specialized ending of an afferent neuron – Separate cell closely associated with peripheral ending of a neuron • Stimulus alters receptor’s permeability which leads to graded receptor potential • Usually causes nonselective opening of all small ion channels • This change in membrane permeability can lead to the influx of sodium ions. This produces receptor (generator) potentials. • The magnitude of the receptor potential represents the intensity of the stimulus. • A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS. Intensity Amplitude of stimulus is coded by the frequency of action potentials (APs) (Weber –Fechner law) 500 AP frequency (Hz) 0 stimulus intensity (log scale) Generator Potential Local depolarization = Na+ influx Generator potential = local depolarization of the membrane of a sensory neuron. Graded response correlates with the strength of a stimulus applied to the associated receptor organ If the generator potential is large enough (ie threshold is reached), an action potential is triggered. Generator Potential in a Pacinian corpuscle Conversion of Receptor Potentials into an AP Receptor Potential Ini1a1on site of AP is different in efferent neurons or interneuron compared with afferent neurons duration of stimulus -40 mV Sensory receptor potential sensory potential (mV) -70 mV Time Generation of sensory potential • the sensory stimulus generates a sensory receptor potential in the sensory cell • usually a depolarization • amplitude is proportional to stimulus • generated by Na+ influx via channels of specific sensory receptor protein channels • sensory potential may decline if stimulus is prolonged (called sensory adaptation) Sensory adaptation • sensory receptors vary in degree of adaptation • tonic receptors are non or slowly adapting • phasic receptors are rapidly adapting stimulus on off tonic receptors | | | | | | | phasic receptors | | | action potentials generated in sensory axon Tonic receptor Phasic receptor AP frequency (Hz) 200 s1mulus Increase in AP frequency is maintained 0 Time s1mulus Increase in AP frequency is not maintained -only lasts a brief period How is sensory information coded (perceived)? IMPORTANT FACTORS Type of stimulus Location of stimulus Intensity of stimulus Type of stimulus (Modality) Is coded by receptor and pathway eg light, photoreceptors, visual cortex Specific receptor type Activates specific pathway Transmits to specific area of cerebral cortex How is sensory information coded (percieved)? IMPORTANT FACTORS: Type, Location & Intensity of stimulus Location of stimulus Is coded by receptive field eg skin Location of receptor determines… Specific pathway and…… Transmits information to somatosensory cortex How is sensory information coded (perceived)? IMPORTANT FACTORS: Type, Location & Intensity of stimulus Intensity of stimulus 2 factors involved in coding Frequency of action potentials (frequency coding) No of receptors activated (population coding) Frequency of action potential in neuron Number of receptor activated Increased frequency of action potential Leads to an increased receptor potential And higher stimulus strength The somatic sensory/somatosensory system The system enables reaction to stimuli that are relayed from -Thermoreceptors -Nociceptors -Mechanoreceptors -Chemoreceptors Information passes through sensory nerves to tracts in the spinal cord to the brain The primary brain area involved in processing is the primary somatosensory area in the parietal lobe of the cerebral cortex MD1009pgSoma1csensorySystemlynchma2013_14 PG2000pgSoma1csensorySystemlynchma2013_14 Somatic sensory organs (somatosensory) Sensory organs in skin, muscles and tendons are exteroceptors (detect external stimulus) Sensory organs in internal organs are interoceptors (detect internal stimulus) Sensory endings have proteins embedded in membrane which detect a specific type of sensory stimulus A) Touch & pressure receptors • class Aβ axons (A are myelinated and biggest, and α>β>γ>δ) • myelinated. • large diameter (10 mm). • conduction velocity - 50 m/s • include sensory axons with free nerve endings and those with a terminal accessory organ or corpuscle i) rapidly adapting • detect light touch on smooth skin and low frequency vibrations ii) slowly adapting -detect prolonged touch and pressure Meisner's corpuscles - rapid adaptation - not as but not as fast as Pacinian - most common receptors of fingers, palms and soles - smaller receptive field than Pacinian Pacinian corpuscle -rapid adaptation -very sensitive - large receptive field Ruffini -slow -large receptive field -sensitive to stretching (deep skin, ligaments and tendon) Merkel's disks -slow -small receptive field -for light touch (eg finger tips lips and genitals Acuity • Refers to discriminative ability • Influenced by receptive field size and lateral inhibition The receptive field determines whether 2 distinct points can be perceived Figs 9.15 and 10.11 Lateral Inhibition: Enhances contrast between areas of strong and weak stimulation Fig. 10.10 Pain Is primarily protective Triggers awareness that tissue damage is occurring Memory of pain triggers learning and future avoidance Pain is accompanied by motivated behavioral responses and emotional reactions Perception -influenced by other experiences Nociceptors 1. Mechanical nociceptors - Respond to mechanical damage eg cut, crush or pinch 2. Thermal nociceptors - Respond to temperature extremes 3. Polymodal nociceptors - Respond equally to all kinds of damaging stimuli Properties Non adapting Prostaglandins greatly enhances receptor response to noxious stimuli Nociceptors ……….. release transmitters in CNS & also neuropeptides peripherally These can induce vasodilation and have other actions Peripheral and central terminals respond to chemicals & pH which regulate their sensitivity Hyperalgesia (sensitization) to pain -caused by prolonged severe tissue damage (burns, arthritis) -an increased sensitivity (lowering of the threshold) of C fibre nociceptors, either at site of injury (primary) or at adjacent sites (secondary). Itch receptors -specialised free nerve ending Temperature receptors -further specialised free nerve endings for hot and cold B) Nociception (pain perception) -nociceptors are specialized receptors sensitive to noxious stimuli (unpleasant, aversive, potentially tissue damaging) -pain is the conscious experience of noxious stimuli -the receptors are free nerve endings devoid of myelin sheath -the ending have specific proteins sensitive to noxious stimuli Two major types of fibres that respond to nociception 1. Aδ (Adelta) fibres (fast pain system) -axons are medium diameter (2 mm), thinly myelinated, 15 m/s conduction velocity -sensitive to abnormally high mechanical stimuli -rapidly adapting -well localised pain -sensation of sharp pain, eg, pinprick, and initial response to noxious heat 2. C fibres (slow pain system) -axons are non-myelinated, <1mm diameter, 0.5m/s velocity -sensitive to high mechanical, heat & cold & chemical stimuli -non-adapting -continuous throbbing pain -not well localised pain -very heterogeneous SUMMARY TABLE How pain is perceived Somatosensory pathway 1st order sensory neuron Afferent neuron with its peripheral receptor that first detects stimulus 2nd order sensory neuron Either in spinal cord or medulla. Synapses with 3rd -order neuron 3rd order sensory neuron Located in thalamus Pain 2 best known pain neurotransmitters – Substance P • Activates ascending pathways that transmit nociceptive signals to higher levels for further processing – Glutamate • Major excitatory neurotransmitter AN IMPORTANT NOTE: The brain has built in analgesic system (a) Suppresses transmission in pain pathways as they enter spinal cord (b) Depends on presence of opiate receptors • Endogenous opiates – endorphins, enkephalins, dynorphin Proprioceptors • • detect position of body in space, particularly position of body limbs gives rise to the kinaesthetic or "6th sense" (an automatic non-conscious awareness of body and limb position) • two main proprioceptive sense organs: i) Golgi organs -located on tendons -free nerve endings -detect length and movement of joints ii) Muscle spindle organs • • • • • • detect length and movement of muscles composed of 3-12 modified muscle fibres called the intrafusal muscle fibres the 2 end regions of the intrafusal muscle fibres have contractile filaments innervated by gamma motor (efferent) axons the central region of each intrafusal muscle fibre does not contain contractile filaments, but has a swollen sensory region innervated by sensory (afferent) axons sensory endings on the muscle spindles are activated by stretch 2 types of muscle spindles nuclear bag nuclear chain NOTE: Normal contractile fibres are called extrafusal fibres, and innervated by alpha motor axons) Role of intrafusal muscle fibres Contraction maintains sensitivity of the muscle spindles when a muscle contracts Properties of sensory axons to muscle spindles Group 1 axons Group 2 axons Type Aα Aβ Diameter 20µm 10µm Conduction velocity 100m/sec 50m/s Endings Primary Secondary Adaptation Phasic Tonic Primary endings (group 1 axons) • • • • • resting discharge rate of firing of AP in the sensory axons is ~10 / sec increase in AP discharge when muscle spindle is stretched, ie, when muscle is stretched increase in AP frequency is proportional to rate of stretch - detect velocity of muscle stretch AP frequency rises as high as 500/sec during very rapid stretch are phasic receptors, ie, rapidly adapt in response to stretch of muscle and muscle spindle 500 Rapid adaptation AP frequency 10 stretch time Secondary endings (group II axons) • • • • increase in AP discharge when muscle is stretched - from resting frequency of 10 / sec increase in AP frequency is proportional to increased length of muscle spindle AP frequency rises only to a maximum of ~ 50/sec during stretch are tonic receptors, ie, only slow adaption in response to stretch of muscle and muscle spindle detect absolute of muscle length AP frequency • 50 10 stretch time Chemical Senses Taste and smell Receptors are chemoreceptors Influence digestion & appetite (digestive juices) Stimulation of receptors induces pleasurable or objectionable sensations and signals presence of something to seek or to avoid Taste (Gustation) • Chemoreceptors in oral cavity and throat • Taste receptors life span =10 days • • • • Taste bud consists of (a) Taste pore Fluids in mouth contact receptor cells • • • • • • (b) Taste receptor cells Modified epithelial cells with microvilli Microvilli contain receptor sites that bind chemical molecules Taste Tastant = taste-provoking chemical Tastant binds receptor Ion channels open Depolarizing receptor potential Action potentials set up in terminals of afferent nerve fibers with which receptor cell synapses Signals conveyed to brain stem, thalamus to cortical gustatory area 5 Primary Tastes Salty - Stimulated by chemical salts, especially NaCl Sour - Caused by acids which contain a free hydrogen ion, H+ Sweet - Evoked by configuration of glucose Bitter - Chemically diverse tastants eg alkaloids, toxic plant derivatives, poisonous substances Umani - Meaty or savory taste Influenced by • Information from other receptors, especially odor • Temperature and texture of food • Psychological experiences associated with past experiences Smell (Olfaction) • Olfactory receptors in nose are specialized endings of renewable afferent neurons • Olfactory mucosa - 3cm2 of mucosa in ceiling of nasal cavity Olfactory mucosa contains 3 cell types Olfactory receptor cell is the afferent neuron. Its receptor is in the olfactory mucosa & signals to the brain. Axons of olfactory receptor cells form the olfactory nerve Supporting cells secrete mucus Basal cells are precursor cells. Olfactory receptor cells are replaced about every 2 months. • Odorants – Molecules that can be smelled • To be smelled, substance must be – Sufficiently volatile that some of its molecules can enter nose in inspired air – Sufficiently water soluble that it can dissolve in mucus coating the olfactory mucosa 1000 different types of olfactory receptors Odorants act through 2nd messenger systems to trigger action potentials Afferent signals - sorted according to scent component by glomeruli within olfactory bulb