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Lecture 13

BIOC32- Lecture notes (Lecture 13-19)

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University of Toronto Scarborough
Biological Sciences
Joanne Nash

1 Lecture 13: Sensory Physiology •Special senses: Always conscious, and get processed -Vision, Hearing, taste, smell, equilibrium (balance) •Somatic senses: Pain, itch, touch, temperature, body position -Touch: can be conscious and unconsciously processed -Body position: or Proprioception, awareness in space -pH and blood pressure: Subconscious reflexes •Hypothalamus: Detect changes in pH and blood pressure -Integrate most of the sensory information (e.g. vision, taste, blood pressure) -Exception: Smell information goes directly to olfactory bulb, does not process in hypothalamus ➣ General properties of sensory system 1) Activated by stimulus as in physical energy (e.g. lightwaves, soundwaves) 2) Stimulus activates sensory receptor -Signal transduction: Convert physical stimulus into electrical intracellular signal (graded potential) -Generator /Receptor potential: Changes in membrane potential (Vm), similar to graded potential 3) Conduct AP to CNS when threshold has reached 4) Integration of signal in CNS: Somatosensory cortex: consciously perceived Brain stem: unconsciously perceived/processed ➣ Type of sensory receptor: •Sensory receptors are classified by structures: 1) Olfactory / Somatosensory receptor: Have free nerve endings, not always myelinated 2) Somatosensory receptor: Found in skins, have enclosed nerve endings Myelinated = faster conduction 3) Auditory, visual and gustatory receptors: -Have specialized receptor cells, with synapses which conducts AP (e.g. hair cell) -Usually myelinated •Can be classified by type of stimulus that most sensitive to: -Chemoreceptors: Respond to chemical stimuli that binds to receptors (e.g. taste, smell, O2, glucose, pH) -Mechanoreceptor: Senses mechanical stress or strain, such as pressure (baroreceptors) -Senses vibration of skin, gravity, cell stretch (Osmoreceptor) -Senses acceleration (Proprioceptor), sound in ear -Thermoreceptors: Sense temperature -Photoreceptor: senses light, also can be activated by physical pressure (e.g. punch) -Nociceptors: Respond to painful stimuli 2 •Adequate stimulus: Form of energy which the receptor is most responsive (e.g. photoreceptor to light) -Sensory receptors can be activated by different types of energy when in high intensity ➣ Receptive field (For touch and visual simulus) • Receptive field: Able to determine location of somatic (touch) and vision stimuli -Stimuli are activated by stimuli that falls within this physical area (receptor field) •Receptive field may associated with only 1 secondary sensory neurons •Convergence: Overlap of receptive fields into one sensing information -having several primary neurons converge onto one secondary neurons -Sensitivity / ability to locate stimulus will be decreased and not that accurately -Two point discrimination: Stimulation of 2 primary neurons, and converge onto 1 secondary neuron -Have larger receptive field, and less sensitive -Usually happen in area of arms, legs, back •Small receptive field: Stimulation of 2 primary neurons lead to 2 activated secondary neurons -Have smaller receptive field, which is more sensitive (2 different signals, not convergent into one) -Have 2 separate stimuli to be decoded as 2 separate sensations -Usually in area of finger tips ➣ Coding of auditory information •Do not require receptive field to locate stimulus = By time differences -Uses timing of stimulation to compute location -Uses time difference of sound stimuli to reach 2 sides of auditory cortex, to locate sound source -e.g. Sound source at the left = Left side of ear receives sound faster than right = On the left •Ears sensitive to different frequencies of sound waves -Higher frequency when it’s closer to that side of ear •Lateral inhibition: Increase contrast between activated receptive fields and inactive neighbors -Middle neurons with strongest response is decoded -Another neurons beside will be also activated Presynpatic bouton of middle secondary neurons -Laterally inhibit another neurons -Only the tertiary neurons with strongest response at the primary neuron will be activated -Able to increase sensitivities of the stimulation, and identify the location of stimulus ➣ Coding and processing of sensory stimuli 3 •CNS distinguishes 4 properties of stimuli: 1) Modality: Type of sensory stimulation (All modalities are similar as in neural signaling) 2) Location of stimulus 3) Intensity of stimulus 4) Duration of stimulation •Sensory modality: Indicated by type of sensory neuron activated (e.g. photoreceptor/thermal) -Sensation is determined by where neurons terminate in brain -Location of cortex activated determine what we perceive, no matter types of stimulus (e.g. Photoreceptor activated by pressure → End up in visual cortex → Perceive as light) •Labelled line coding: -Specific brain region associate signal from specific groups of receptors with special modality -e.g. Cold receptor always activate specific neurons that terminates brain region that decode sense of cold •Intensity and Duration of stimulus: -Receptor potential strength and duration varies with size of stimulus -Receptor potential is integrated at the trigger zone -Stimulus intensity: Proportional to frequency of action potentials -Stronger the stimulus, have higher frequency of A.P = More neurotransmitter release -Stimulus duration: Proportional to stimulus duration -Neurotransmitter release depends on patterns of A.P reaching to terminal ➣ Adaptation to stimulus: Tonic and Phasic •Adaptation: when sensory neurons stop responding, consists 2 types of adaptation •Tonic adaptation: Fire rapidly initially, then slow down generally -Will remain firing as long as stimulus is continue to present -Involved in parameters that need to be continuously measured by body (e.g. pressure receptor, some of the touch receptor, able to continue senses weight) •Phasic adaptation: Rapidly adapting, rapidly fire at first and then stop quickly (e.g. smell) -IF stimulus remains, constant firing will ceases -Attuned to changes in stimulus, allow stimulus to be ignored if not threat to well being ➣ Decoding somatic sensory pathways •Involved in sense of touch, proprioception, temperature, nociception •Receptors found in skin and viscera, and trigger AP in primary sensory cortex -Primary and secondary neurons synapses are located in spinal cord and medulla •Thalamus: Consists relay nucleus •Nociceptors (pain), touch and temperature sensation: -Have synapses located early in spinal cord = able to act and response more quickly -Will be primary relayed in spinal cord Primary sensory neuron on Spinal cord → Thalamus → Somatosensory cortex •Vibration, fine touch, proprioception sensation: 4 -Have synapses located in medulla oblongata Primary sensory neuron on Medulla → Thalamus → Somatosensory cortex •Different types of sensation will rely differently ➣ Somatosensory cortex and body •Somatosensory cortex is topographically organized -Each part of body is mapped onto specific part of cortex -Topographical regions can be expanded more when used frequently •Larger region of cortex devoted to body part = More sensitive of that body part •Receptors for given body part are located near each other -e.g. cold receptors of arm are adjacent to pressure receptors for the arm ➣ Touch receptors in skin •Have free ending nerves that are umyelinated •Pacinian corpuscle: Response to vibration •Ruffini corpuscle: where stretch receptor located, deep within skin •Merkel receptor: sensitive to texture Melssner’s corpuscle: sensitive to stroke ➣ Temperature (thermal) receptor •Thermal receptors terminate in subcutaneous layers of skin •Cold receptors: Sensitive to temperatures lower than body temperature (e.g. menthol) •Heat receptors: Sensitive to temperatures higher than body temperature -Adapt to temperature between 20 – 40 degree with phasic adaptation •Pain receptors are activated when it’s higher than 45 degree ➣ Nociceptors (Pain receptor) •Have free nerve endings connected to ion channels ( Ionotropic receptor = Faster) •Respond to strong noxious, harmful stimuli -Can be respond to heat and capsaicin (chemical that makes chili hot) •Initiates withdrawal reflex responses as protective and adaptive mechanism -Initiated unconsciously, also activates somatosensory cortex causing sensation of pain -e.g. hand pulling away from heat 5 •Two types of pain: Fast pain or slow pain 1) Fast pain: Sharp and more localized, usually with cold and mechanical stimuli -Activation of small myelinated Aδ fiber 2) Slow pain: Dull and throbbing pain, more diffuse and less localized -Activation of small unmyelinated C fibers, located further inside of epidermis -Stimulated by heat, cold. Mechanical stimuli Inhibit ascending pathway = no pain •Pain can be suppressed in dorsal horn: -Caused by tonically active interneurons, which lower pain perception ➣ Gate control theory of pain modulation •When no pain presented: -Absence of input from C fibers Inhibits GABA release -Tonically active inhibitory neuron suppresses pain pathway •When in strong pain: -C fibre stops inhibition of pathway, painful signal is able to be set to the brain -Activates sensory cortex, hypothalamus, limbic system -May cause sweating, emotion responses •Strong pain decreased by mechanical stimuli: -Pain can be modulated by simultaneous somatosensory input -Lower pain response by yelling, with more touch somatosensory response •C-fibre: Synapse onto inhibitory neurons -Activated C-fibre → Simultaneous activation of ascending pathway + Inhibition of interneuron inhibition •Visceral pain: Known as referred pain -When pain caused in viscera is experience elsewhere in the body (Viscera= inside body) -Due to multiple sensory neurons converge on single ascending tract in spinal cord -Will not able to localized the pain source precisely Lecture 14: Sensory Physiology  Olfactory system: Sense of smell •Do not integrated in thalamus, can be response and integrate quicker •Olfactory neurons: Have free nerve ending (simple sensory neurons) •Olfactory cells: Primary sensory neuron, located in olfactory epithelium high upper in nasal cavity -Stem cells in epithelial layer replace broken down olfactory cells every 2 months •Olfactory bulb: Located underneath the forebrain 6 -Receives input from primary olfactory neurons (Olfactory receptor cells) -Primary free nerve ending at nose forms cranial nerve I, or known as olfactory nerve -Synapses in olfactory bulb onto secondary olfactory neurons (Converge) -Process an integrates incoming information from odorant receptors -Then will rely the most dominant smells to olfactory cortex Information flow in olfactory system: Primary sensory neuron → Olfactory bulb → Secondary sensory neuron → Olfactory cortex (Olfactory epithelium) •Conduction of AP along from: Olfactory primary sensory neuron → Olfactory bulb •Secondary neurons terminate in olfactory neurons: -Also send outputs to amygdala and hippocampus, trigger emotion and memory ➣ Signal transduction in olfactory system •Olfactory receptors are concentrated in olfactory epithelium: -Dendrites on surface extend to olfactory bulb -Terminals of olfactory receptors line the epithelium, each terminal is embedded in mucus lining •Odorant molecules dissolve and penetrate mucus → Bind to odorant receptor -Each odorant receptor is sensitive to several odorant molecules •Olfactory receptor: G-protein coupled receptor -Liked to G protein of G (G ), causes increases in cAMP level olfactory olf -Causes opening of Na channels → Influx of Na+ → Membrane depolarization  Gustatory system: Taste •Closely linked to olfaction, with 5 main taste sensation based on 5 receptor types: Salty: Activated by Na+ Sour: Activated by H+ Bitter: Activated by toxins Sweet: Activated by amino acid Umami: Acitvated bt glutamine and monosodium glutamate •Taste receptor: Located on taste buds clustered on tongue -Each bud contains 50-150 taste cells and their support cells •Taste cells: Non-neural epithelial cells -Have tip protrudes into oral cavity (taste pore) -Tip contains microvilli to increase surface area in contact with mouth environment •Tastant: Taste substance needs to be dissolved in mucus and saliva in mouth -Taste ligand will interact with the taste receptor → Generate receptor potential -Cause conduction of AP along primary gustatory sensory neurons ➣ Mechanism of taste transduction •Have 2 types of taste cells based on structures: 7 1) Taste cells for salty and sour 2) Taste cells for sweet, bitter, umami •Taste cells for salty and sour: Activate ionic channels -Synapses with primary gustatory sensory neurons Primary -Release serotonin from presynaptic terminal gustatory -Sour receptors also located in spinal cord: To regulate H+ in CSF •Taste cells for sweet, bitter, umami: Have receptor that are G-protein coupled neurons -Do not form traditional synapses -Receptor cell releases ATP to activate primary sensory (Not via synapses) ➣ Taste transduction 1) Ligands activate taste cell: -Salt and sour transduced by activation of ion channels -Bitter, umami and sweet receptors are GPCR (G gusducin) 2) Various intracellular pathways are activated 3) Ca2+ signal in cytoplasm triggers exocytosis of serotonin / ATP formation 4) Serotonin neurotransmitter or ATP is released 5) Primary sensory neuron fires and AP are sent to the brain ➣ Ear and perception of sound •Pinna: Direct soundwave into the ear cannal •Cochlear: where hearing receptor are located -Where transduction of soundwaves into action potential •External ear: Consists of pinna and ear cannal -Pinna and ear canal: Refer as accessory structures -Sealed at internal end by tympanic membrane, which separate external and middle ear •Middle ear: Area which filled with air -Consists of 3 bones to conduct sound: Mallelus, Incus, Stapes -Middle bones able to protect from damage = Detached if soundwave is too high (vibrate too rapid) -Responsible in amplify soundwave, and protect ear from damage •Inner ear: area which filled with liquid -Consists of vestibular apparatus, semicircular cannals, cochlear - Vestibular apparatus + semicircular cannal: sense equilibrium -Cochlear: Involved in hearing -Round and oval windows: Separates cochlear from middle ear, sound dissipated in round window ➣ Signal transduction in cochlear • Cochlear contains 3 fluid-filled compartments: 1) Vestibular duct 2) Tympanic duct 3) Cochlea duct 8 •Tympanic duct: Connected through helicotrema, contains perilymph -Have fluid that have similar components with extracellular fluid (High Na ) •Cochlea duct: Flexible membrane containing hair cells + + -Contains endolymph, which high in K and low Na (Similar to intracellular fluid) •Hair cells: Located in cochlear duct, contain hearing receptor -will bend when pushed by sound waves in cochlear duct through fluid •Cochlear: Nerve transmit AP from hair cells to auditory cortex •Organ of corti: Located within cochlear duct -Contain hearing receptors, with hair cells, beginning of primary sensory neurons and support cells -Lies between basilar and tectorial membrane -Fluid in vestibular duct will hit tectorial membrane -Moves in response to waves passing through vestibular duct -Create up and downwards oscillations → Bend hair cells ➣ Conversion of soundwave into hearing 1) First transduction: Soundwaves strike tympanic membrane → Become vibration 2) Soundwave energy is transferred to 3 bones in middle ear and vibrate 3) Second transduction: Vibration in bone → water movement -Stapes is attached to membrane of oval window -Vibration of oval window create fluid waves in cochlear 4)Third transduction: Water movement → Membrane vibration -Fluid waves push on flexible membranes of cochlear (Tectorial membrane → Basilar membrane) -Hair cells bend and release neurotransmitter 5) Fourth transduction: Neurotransmitter release onto primary sensory neurons -AP then travels through cochlear nerve to the brain 6) Energy from waves transfer across cochlear duct → Tympanic duct → Middle ear at round window -then energy is dissipated back into middle ear at round window -Excess soundwave = Travel through tympanic duct and exit at round window -Cochlear duct: contains sensory receptor which converts soundwaves to hearning ➣ Signal transduction in hair cells of Organ of corti •Hair cells: Non-neural receptor cells -Contain 50-100 stereocilia arranged in ascending height -Stereocilia are attached by tip links (Protein bridge) -Kinocilium: The longest stereocilia, connected to tectorial membrane -Kinocilium moves when membrane moves, causing other stereocilia to move -Always be the first stereocilia that bent first, then other shorter stereocilia •Movement of stereocilia generates signal transduction: Change in Vm → Flux of Ca2+ -Signals to the primary sensory neurons 9 Rest: 10% ion channels open Have tonic signal sent by sensory neuron (Tonical active = Keep firing although it’s quiet) Excitation: Hair cells bend in one direction -Cell depolarizes -AP frequency increased in sensory neurons -Inhibition: Hair cell bend in opposite direction, ion channels close -Cell is hyperpolarized, and sensory neuron signaling decreases (No AP generated) ➣ Frequency of sound waves •Pitch: encode by frequency of soundwaves -Measure in Hz, frequency is refer to the number of peaks per second -High frequency = High pitch, low frequency = low pitch •Loudness of sound: encode by amplitude of soundwaves -Measure in dB, larger the amplitude = larger the sound •Lower the pitch = soundwaves hit more basilar membrane •Higher the pitch = Soundwaves hit more tectorial membrane (Closer to Oval window) ➣ Central processing of hearing •Vestibular cochlear nerve: Runs through cranial nerve VIII •Primary auditory sensory neurons: projects to cochlear nuclei in medulla oblongata •Secondary sensory neurons: May cross to other hemisphere = Both hemisphere get information from one ear -Information will be crossed after medulla oblongata •Information then related through pons and thalamus → Secondary neuron projects to auditory cortex Primary sensory neuron → Cochlear nuclei(Medulla) → Pons → Midbrain → Thalamus → Auditory cortex Lecture 15: Sensory physiology •Equilibrium: State of body in relation to self and environment -Requires sight, proprioceptor in muscle and joints -Requires semicircular canals, vestibular apparatus in inner ear •Two components for detection of equilibrium in inner ear: -Dynamic: Related to motion of body -Static: Position of body with respect to gravity (e.g. in elevator) •Meniere’s disease: Increase fluid pressure in vestibular apparatus due to infection in ear -Endolymph production exceeds endolynph drainage -Organ of corti is affected by fluid pressure within vesibular apparatus 10 ➣ Anatomy of vestibular apparatus and semi-circular canals •Vestibular apparatus: Interconnected fluid-filled chambers -Contain 2 sac-like otilith organs: Sacule and utricle -Senses linear acceleration (walking) and head position (in gravity) •Semi-circular canals: 3 semi-circular canals connect to utricle at bases -Consists Horizontal + Posterior + Superior circular cannals -Sense rotational acceleration -Like cochlear duct , filled with endolymph -Action of endolymph = Head rotated -Do not affected to gravity, senses dramatic changes in environment •Semicircular canals: Sense rotational acceleration in different directions -Monitor acceleration at right angles to each other: -Horizontal: monitors head turning (e.g. shaking head no) -Posterior: monitors left to right movement (e.g. cartwheel) -Superior: Sensitive to forward and backward (e.g. nod yes) -Ampulla: At the end of each canal -Contains sensory structures = Cristae -Cristae: Consists hair cells structure for equilibrium system (have cell receptors) -Contains gelatinous mass and cupola -Cupola: Stretches from floor of ampulla closing it off, hair cells are inside the cupola -Will be pushed over when canal moves, and stimulate hair cells -Have lag between body movement and haircell movement -Since haircells are moved in fluid = haircell will move backward when body stops ➣ Otilith organs in vestibular apparatus: •Sense speed of linear acceleration and head position (in space/gravity) •Have macculae as sensory structure (Cristae in semicircular canals) •Macculae: Contains haircells, gelatinous membrane contains CaCO3 -Hair cells embedded in otilith membrane •Otilith: Protein particle, or refer as ear stones -Binds to matrix proteins on membrane surface -Otilith organs contains 2 strucures: Utricles and saccules Ultricles: Senses forward acceleration due to gravity -Hair cells orientated horizontally -When head tips back → otiliths slide back due to gravity → Gelatinous Otilith membrane slides -Causes hair cilia to bent setting off electrical signals -Gravity displaces otiliths and hair cells are activated -Hair cell movement is caused by weight of Otilith -Saccules: Senses vertical acceleration due to gravity -Hair cells orientated vertically 11 -Sensitive to vertical forces (e.g. dropping downwards in an elevator) ➣ Equilibrium pathways project to cerebellum and brain stem •Hair cells of vestibular apparatus → Stimulate primary neuron in vestibular nerve •Innervate 2 brain regions: 1) Vestibular nuclei of medulla 2) Cerebellum •Afferents from medulla terminate in cortex via thalamus Lecture 16: Sensory physiology ➣ Internal anatomy of eye •Eye: “Hollow” sphere divided into 2 compartments separated by lens -Compartments contain aqueous humor and vitreous humor •Len and cornea: Focus light on the retina -Bending of lens is controlled by zonules and cilary muscles •Retina: Contains blood vessels, photoreceptors, optic disk, fovea ➣ Light •Light enters eye through the cornea •Pupil: modulate amount of light -Increase amount of light = Decrease diameter of pupil opening -can change from 1.5mm- 8mm in diameter = 28 fold difference in pupil area •Lens: Focuses light waves on the retina -have muscles that control lens to focus light cause the lens to change shape (Zonules and ciliary muscles) •Photoreceptors of retina: Transduce light energy → electrical signal -Electrical signal processed through neural pathways ➣ Ciliary muscle and zonules (ligament): Control lens shape •Light converges on retina at focal point •Focal length: Distance from centre of lens to focal point -Lens change shape in order to change focal length -Lens rounded: Ciliary muscle contracted, ligaments slacken, light is bent -Lens flattened: ciliary muscle relaxed, ligaments pulled tight, light is not bent Focus more farther away 12 •Presbypoia: Loss of ability of lens to change in shape, occurs with aging •Accommodation: Process which eyes adjust lens shape to focus image on retina -When object >20 feet away, light is parallel when entering into eyes -When object is closer = Lens will be more convex to bend light rays to converge on retina -Focal length will be shorten when object is closer ➣ Common visual defect •Hyperopia: Known as far sightedness, corrected with convex lens -Occurs when focal point falls behind retina •Myopia: Known as near sightedness, corrected with concave lens -Occurs when focal point falls in front of retina, prevent the light from bending too much •Astigmatism: Images are distorted, when cornea is not perfectly dome shaped ➣ Phototransduction occurs at retina •Photoreceptors are at the posterior of retinal epithelium Pigment epithelium → Rod cell → Bipolar cell → Ganglion cell → Optic nerve •Information transfer is convergent: At bipolar cell, ganglion cells •Portion of photoreceptors: 15-45 Bipolar cells: 3 Ganglion cell: 1 •Convergence DO NOT occur at fovea -Has maximum acuity at fovea, one photoreceptor → 1 Bipolar → 1 Ganglion → Optic nerve •Optic disk: no photoreceptor at optic nerve, known as blind spot ➣ Two types of photoreceptor: Rods and Cones •Photoreceptors are at posterior of eye •Light travels through transparent layer •Outer segment: -Tips which touch epithelium, contain light sensitive visual pigment •Inner segment: Contains cell nucleus, organelles for ATP and protein synthesis •Outer fibre: or Basal segment -Contain synaptic terminals that release glutamate onto bipolar cells ➣ Rod cells •More numerous than cone cells (20:1 ratio) •Have visual pigment Rhodopsin: Function well in low light, affect by all types of wavelength •Responsible for sensing black and white colour ➣ Cones •Fovea has only cone cells, does not have rod cells -Have high acuity or sharp daylight vision, able to see in colour •Three types of cones: Blue, green, red -Each type responses to
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