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Unformatted text preview: Sensory Receptors
Chapter 10 Various Types of Sensation (various type of receptors) Special Senses – vision, olfaction, gustation, audition, equilibrium Somatic Senses – touch, temperature, pain Proprioception – body movement and position Table 10-1 Types of Receptors Free nerve endings - dendrites of a neuron Encapsulated nerve endings Special receptor cell Figure 10-1 - Overview 4 Major Groups of Receptors (Table 10-2) Chemoreceptors Chemical stimuli Mechanical stimuli Temperature stimuli Light energy stimuli (photons) Mechanoreceptors Thermoreceptors Photoreceptors Table 10-2 Receptors have the ability to change the stimulus into an electrical event (transduction) Stimulus opens/closes ion channels in membrane of receptor – usually a net influx of Na+ (or other cation) to depolarize membrane Resulting depolarization = graded potential Receptor Potential Alters neurotransmitter release from receptor cell – this causes an electrical effect in the sensory neuron associated with the receptor cell The electrical event may initiate an action potential in the sensory neuron Sensation Properties Modality Each receptor is most sensitive to one type of stimulus Example: Mechanoreceptors in carotid artery are sensitive to changes in stretch of the wall of the artery. It is not sensitive to the pH or oxygen content of the blood within the artery. Projection to stimulus location The brain knows where the stimulus occurred based on: - which receptors were activated - where the sensory neurons terminate (specific area in the cerebrum) Figure 10-4 - Overview Intensity Number of receptors activated – as stimulus strength increases, more receptors will be activated; not all receptors have the same threshold Frequency of action potentials (remember there are no large or small AP’s – they’re all the same) Duration A longer stimulus will result in more action potentials. Stimulus duration affects adaptation of receptor. Receptor Adaptation Tonic receptors (respond to stimulus that requires constant monitoring) Slowly adapting – will continue to produce electrical signals as long as stimulus persists. Example: Mechanoreceptors that monitor blood pressure Phasic receptors (respond to stimulus changes) Rapidly adapting – will stop producing electrical signals if stimulus remains constant Example: Olfactory receptors (smell) Somatic Senses Touch Temperature Nociception (pain, itch) Proprioception Receptors located in the skin and viscera Touch Receptors Various kinds located in the skin Merkel discs Free nerve endings Meissner’s corpuscles Pacinian corpuscles Ruffini corpuscles Figure 10-11 Temperature Receptors Free nerve endings Cold and warm (in relation to body temp.) Adaptation between 20 - 40° C (68 - 104° F) No adaptation outside this range – causes tissue damage! Nociceptors Respond to STRONG stimuli Free nerve endings (chemical, temperature and mechanical stimuli) Chemicals released due to tissue damage will lower the threshold of these receptors Depending on the chemical we will “itch” (histamine) instead of “hurt” Pain is subjective (individual perceptions) Fast or Slow pain – effects of myelination Referred pain Many sensory neurons from various locations will converge on a pathway to the brain. Integrator interprets pain as coming from somatic receptors because it is the more frequent source of sensory input Figure 10-13 - Overview Special Senses Olfaction (chemoreceptors) Gustation (chemoreceptors) Audition (mechanoreceptors) Equilibrium (mechanoreceptors) Vision (photoreceptors) Audition (Hearing) Sound waves produce mechanical vibrations that produce fluid waves to stimulate hearing receptors (mechanoreceptors) where electrical activity is initiated and sent to the brain Sound waves:
Frequency (hertz, perceived as pitch) and amplitude (decibels, perceived as loudness) Humans: 20-20,000 Hz (1000-3000 Hz) >80 dB can cause damage Figure 10-18b Figure 10-17 Transmission of Sound P = F/A Tympanic membrane is 20 x larger than oval window, so wave Figure 15.31 is amplified Muscles of Middle Ear Figure 15.26 Figure 10-20 (4 of 8) Figure 10-20 (6 of 8) Figure 10-20 (8 of 8) Figure 10-19 - Overview Figure 10-20 (7 of 8) Figure 10-21 - Overview Sound Transduction Pinna collects sound waves and directs them into the external auditory canal (ear canal) Tympanic membrane vibrates Auditory ossicles vibrate: malleus → incus → stapes amplification occurs (oval window is smaller in area than the tympanic membrane) protection offered by muscles: stapedius and tensor tympani Oval window vibrates → creates fluid (perilymph) waves in the cochlea
Vestibular duct (scala vestibuli) → Helicotrema → Tympanic duct (scala tympani) → Roud Window (dissipates here) Cochlear duct (scala media) fluid (endolymph) waves cause vibration of basilar membrane (location of Organ of Corti) Bends stereocilia + kinocilium of hair cells against tectorial membrane Opens membrane ion channels – cation influx Depolarizes hair cells – results in opening of calcium channels and exocytosis of NT Increase in NT from hair cells results in depolarization (to AP?) in associated sensory neuron (Cochlear Nerve → Vestibulocochlear Nerve → Auditory Cortex of Brain) Bending back reverses the ion channel activity, etc. Pitch Perception Basilar membrane At base (near oval/round windows): Narrow, stiff
Vibrates in response to higher frequency waves (higher pitched sounds) At apex (helicotrema): Wider, floppy
Vibrates in response to lower frequency waves (lower pitched sounds) *Early processing of sound waves* Figure 10-22 - Overview Figure 15.32 Some fibers cross over to other side of brain so auditory cortex receives input from both ears. Eustachian tube (Pharyngotympanic tube) Connects the middle ear with the pharynx (back of the throat) Equalizes pressure inside the middle ear with atmospheric pressure – important that pressures on both sides of the tympanic membrane are equal Vision Photoreceptors convert light energy (photons) into electrical energy Light pathway: Cornea → Aqueous humor → Pupil → Lens → Vitreous humor → Retina Electrical pathway: Photoreceptors (rods + cones) → Bipolar cells → Ganglion cells → Optic nerve → Optic tract (at optic chiasm – crossover so both eyes send signals to both sides of brain) → Visual cortex (occipital lobe) Figure 10-28a Pupil Regulates amount of light entering eye Contraction of iris muscles changes diameter Bright light/near vision:
Circular fibers contract (parasympathetic) Constriction (↓ pupil size) Dim light/far vision:
Radial fibers contract (sympathetic) Dilation (↑ pupil size) Pupil Dilation and Constriction Figure 15.9 Lens Accommodation regulates degree of refraction (bending) of light rays Most refraction occurs as light passes through the cornea [Refraction: light rays bend when they pass through different media (cornea → aqueous humor → lens → vitreous humor → neural layer of retina to photoreceptor cells)] Accommodation of the Lens Normal eye : Objects need to be 20 feet away in order for focal point to be on retina Vary lens curvature to focus on objects closer and farther away Controlled by ciliary muscles connected to lens by suspensory ligaments (ciliary zonules) Figure 10-32 - Overview Close vision:
Greater curvature (more convex) to bend rays more Ciliary muscles contract (parasympathetic NS) Releases tension of suspensory ligament Lens shortens, thickens, bulges Greater refraction (Presbyopia – lens loses elasticity → reading glasses around 40) Far vision: Ciliary muscle relaxes Tension in suspensory ligament increases Lens is flatter Less refraction Photoreception of Rods + Cones Photoreceptor cells contain visual pigments One type (rhodopsin) in rods and 3 types in cones (respond to different wavelengths of light – color!) Packaged in discs within outer segments Have a lot of cation (Na+ and K+) channels in their cell membranes Rhodopsin: 11- cis retinal (from Vitamin A – stored in liver, then in pigmented epithelium) + opsin (protein) Figure 10-35d Figure 10-37 - Overview (1 of 6) In the Dark 11- cis retinal + opsin cGMP levels are high inside cell and cGMP is bound to Na+ channels in membrane
Result: Na+ channel is open Steady depolarization (-40 mV) due to net Na+ influx Steady release of NT (glutamate) from photoreceptor cells Figure 10-39 - Overview In the Light Pigment absorbs light and retinal changes shape 11- cis to all-trans isomer Retinal-opsin breakdown/separation (later regenerated in the dark) Chemistry of Visual Pigments Figure 15.20 Opsin activates the G protein transducin Transducin activates the enzyme PDE (phosphodiesterase) PDE converts cGMP → GMP cGMP was responsible for opening Na+ channels in membrane allowing Na+ influx (depolarization) Na+ channels close and K+ channels remain open → Hyperpolarization Decrease in NT release from cells Figure 10-39 - Overview Phototransduction Figure 15.22 ↓ NT release Excites (depolarizes, graded potential) bipolar neurons Bipolar excitation causes depolarization (action potentials) of ganglion neurons Ganglion neuron axons unit to form Optic Nerve (exits retina at Optic Disc – blind spot) Optic nerve → Optic tract (optic chiasm crossover) → Visual cortex Figure 10-35e Figure 10-35b Figure 10-29 - Overview Rods Dim light Black + white - Night vision Shades Peripheral vision (located in peripheral zone of retina) 20 rods: 1 cone Cones Bright light Color vision Dense in fovea (area of sharpest vision, best color perception) Can see better at night if not looking directly at object; can’t see color at night Different Wiring for Rods + Cones Rods: Many rods (100) feed into 1 ganglion neuron – Convergence! Result: Summation, but unclear about source of activation; vision is not as clear Cones: 1 cone: 1 ganglion neuron Result: Higher visual acuity of a small area of visual field Horizontal and Amacrine Cells Lateral pathways that modify message being transmitted Horizontal cells Between photoreceptor cells + bipolar cells Enhance contrast Between bipolar + ganglion cells Signal change in illumination level Amacrine cells Figure 10-35d ...
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This note was uploaded on 04/29/2009 for the course BIOL 425 taught by Professor Tondi during the Fall '08 term at George Mason.
- Fall '08