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• General properties of sense organs • sense organs code several aspects of a stimulus • 1. Modality • all sense organs detect a specific form of energy of the stimulus • A. light (photoreceptors) • B. chemical (chemoreceptors) • taste (gustation) • smell (olfaction) • C. mechanical energy (mechanoreceptors) • touch and pressure • muscle stretch • auditory and balance • baroreceptors • D. thermal energy (thermoreceptors) • hot and cold • also electro-receptors & magneto-receptors present in certain animals but not humans 2. Sense organs have the property of transduction • the energy of stimulus is altered to electrical energy • energy of stimulus is usually increased (amplified) or reduced during transduction 3. 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) 4. Location -location of stimulus is detected by the receptive field of the sensory neuron -the area of the sensory surface of the body, eg, skin, that when stimulated increases the AP frequency of that neuron receptive field (RF) eg, in skin RF of SN1 RF of SN2 sensory neuron 1 sensory neuron 2 (SN1) (SN2) duration of stimulus Sensory receptor potential -40 mV 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 on stimulus off tonic receptors phasic receptors ||||||| ||| action potentials generated in sensory axon tonic receptor stimulus 200 Increase in AP freqn Is maintained AP frequency (Hz) 0 Time phasic receptor stimulus Increase in AP freqn is not maintained -only lasts a brief period AP frequency Time Somatic sensory organs (somatosensory) • sensory organs in skin, muscles and tendons (exteroceptors, detect external stimulus) and in internal organs (interoceptors, detect internal stimulus) • the sensory endings have proteins embedded in membrane which detect a specific type of sensory stimulus A) Touch & pressure receptors • class Aβ axons • myelinated. • large diameter (10 µm). • 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 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 nociceptors 1. Aδ (Adelta) fibres (fast pain system) -axons are medium diameter (2 µm), thinly myelinated, 15 m/s conduction velocity -sensitive to abnormally high mechanical , heat & chemical 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, <1µm diameter, 0.5m/s velocity -sensitive to high mechanical, heat & cold & chemical stimuli -non-adapting -continuous throbbing pain -not well localised pain -very heterogeneous -nociceptors release transmitters in CNS & also many neuropeptides peripherally which can induce vasodilation and have other actions -both peripheral and central terminals respond to a variety of 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 • • • 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 Golgi tendon organ Iii) Muscle spindle organs • detect length and movement of muscles • composed of 3-12 modified muscle fibres called the intrafusal muscle fibres • the two end regions of the intrafusal muscle fibres have contractile filaments • innervated by gamma (γ) motor (efferent) axons • (normal contractile fibres are called extrafusal fibres, and innervated by alpha (α) motor 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 and nuclear chain) Properties of sensory axons to muscle spindles type diameter condn vel. endings adaptation group I axons Aα 20µm 100m/s primary phasic group II axons Aβ 10µm 50m/s secondary 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 freqn 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 50 AP freqn 10 stretch time • Role of intrafusal muscle fibres • Contraction of intrafusal muscle fibres maintains sensitivity of the muscle spindles when a muscle contracts Vision -waves of photons -visible light is wavelength 400 (violet) to 700 nm (red) of electromagnetic spectrum The eye -light enters eye via pupil -size of pupil is controlled by the iris, which contains two types of muscle -pupil dilates or contracts to control amount of light entering eye In dim light -contraction of radial (dilator) muscle of iris via sympathetic stimulation -size of pupil increases In bright light -contraction of circular (constrictor) muscle of iris via parasympathetic stimulation -size of pupil decreases Focusing of light -light is focused on retina by cornea and lens -adjustment of focusing is by changing shape of lens Distant sight -ciliary muscles relax (sympathetic stimulation) -suspensory ligaments become taut -lens becomes thin, flattened Close sight -ciliary muscles contract (parasympathetic stimulation) -suspensory ligaments relax -lens thick, rounded (accomodated) • • • • • • • • • • • • • • • Retina -contains light sensitive cells, the photoreceptors called rods and cones -photoreceptors contain a pigment(s) which absorbs light -photoreceptors are stimulated by intensity of light (luminance) Rods contain photopigment rhodopsin sensitive to all wavelengths high sensitivity to light (even as low as a single photon) night sight found over all retina except fovea 100 million in total Cones contain photopigments cone opsins (several different types) different absorption spectra for red, green or blue low sensitivity to light (>100 photons) daylight sight located mainly in central retina called the macula, and especially the central area of the macula, the fovea 6 million in total Myopia Nearsightedness distant objects cannot be focused eyeball grows too long correction - concave lens Visual disorders Hyperopia -farsightedness - near objects cannot be focused -lens cannot accommodate -cells of lens die with age, & lens loses elasticity (presbyopia) -correction - convex lens normalised with convex lense • • • • • Cataract causes blindness loss of transparency of lens (cells become opaque with ageing) diagnosed as a grey patch in pupil correction - lens removed & replaced with prosthetic (artificial) lens • • • • • • Glaucoma increase in pressure in anterior chamber of eye (aqueous humor) due to narrowing of a small canal in cornea near edge of iris initially blurring of peripheral vision later full blindness as optic nerve is damaged correction - relief of pressure by opening canal • • • • • Macular degeneration blurred/distorted central vision degeneration of retinal cells in macula dry (atrophic) (90% of cases) - block of blood flow to macula wet (exudative) (10%) - new weak blood vessels grow in retina which leak fluid Floaters • deposits in the vitreous humour, which is normally transparent. • visible because of the shadows they cast on the retina, appearing as spots, or fragments of cobwebs, which float slowly before the observer's eyes • due to degenerative changes, mainly in elderly • shrinkage of the vitreous humour, causing collagen in the humour to break down into fibrils, or detachment of the vitreous humour from surrounding tissue, which may sometimes cause retinal detachment • Mechanism of hearing • • • Hearing of sound amplitude sound waves oscillate the eardrum pressure waves transmitted through ear bones to a fluid filled chamber, the cochlea duct (scala media) • • • sensory cells are called hair cells hair cells generate sensory potentials in response to sound depolarization evokes release of vesicles containing neurotransmitter (L-glutamate) • • neurotransmitter evokes APs in afferent sensory axon of the auditory nerve NB, the hair cells are also innervated by an efferent nerve Hearing -propogated sound is oscillations of air pressure -the ear detect changes in the air pressure amplitude of air pressure (intensity) increase decrease time -amplitude is expressed in db (log scale) -0 db is the auditory threshold -30-40db whisper,50-60db conversation,100-120db rock concert (ear damage!) -frequency (pitch) is measured in Hz (cycles / sec) -humans detect 20-20,000 Hz -auditory threshold varies with frequency -most sensitive at 500-4000Hz (speech frequency) • Mechanism of hearing • • • Hearing of sound amplitude sound waves oscillate the eardrum pressure waves transmitted through ear bones to a fluid filled chamber, the cochlea duct (scala media) • sensory cells are called hair cells, located in the organ of Corti (the organ in the inner ear that contains the auditory sensory cells) • • hair cells generate sensory potentials in response to sound depolarization evokes release of vesicles containing neurotransmitter (L-glutamate) • • neurotransmitter evokes APs in afferent sensory axon of the auditory nerve NB, the hair cells are also innervated by an efferent nerve kinocilium vesicles containing transmitter L-glutamate • • • • • • Hair cells these have ~100 hairs (stereocilia) projecting upwards and one kinocilium with no sound, cilia of hair cells project directly upwards -auditory (cochlear) axons have a “resting” frequency of APs at ~20 Hz sound waves cause oscillation of basilar membrane which moves up and down, and cilia deflected from side to side upward movement of basilar membrane, stereo cilia deflected one way, causes a depolarizing sensory receptor potential and an increase in AP frequency downward movement of basilar membrane, stereocilia deflected other way, causes a hyperpolarizing sensory receptor potential and a decrease in AP frequency • Frequency discrimination • • • individual frequencies are detected by different part of cochlea end nearest oval window is narrow and stiff vibrates and detects high freqn • • • end nearest tip (helicotrema) is wide and flexible vibrates and detects low frequency ~20,000 hair cells located along cochlea, each innervated by a sensory axon in cochlea nerve each hair cell & axon responds to a different frequency of sound • • NB, two types of hair cells are present i) inner – carry out the signalling to CNS as described ii) outer – do not signal. Sound alters their membrane potential, which in turn causes a length change of the hair cell (shorten or lengthen – called electromotility) -the change in length of the hair cell amplifies basilar membrane movement • Deafness • most common physical disability (#10% popn) Conductive deafness • sound waves inadequately conducted through external and middle ear • several causes • 1. deposition of calcium salts at joints between ear bones and therefore loss of mobility • occurs gradually with age • corrected with a hearing aid, which amplifies sound • 2. blocking of ear canal with earwax (glue ear) • 3. rupture of eardrum • 4. middle ear infections • • Sensory-neural deafness -defect in hair cells / auditory nerve eg, neural presbycusis, age related neurodegeneration of hair cells, in elderly especially affects detecting high frequency sound. -can also occur in young with hereditary defects Correction cochlear implant. Electrodes surgically implanted which transduce sound to electrical signals & stimulate auditory nerve directly • Further Disorder of Hearing • Tinnitus (meaning “ringing") is the perception of sound within the human ear in the absence of external sound. -range of causes, eg, hearing impairment caused by loud noise (especially) & during ageing, ear infections, foreign objects or wax in the ear, nose allergies that prevent fluid drain, wax build-up, withdrawal from a certain drugs, eg, benzodiazapine addiction very common - one in five people between 55 and 65 years old report tinnitus symptoms • • • • Sense of equilibrium, balance detected by the vestibular organs semicircular canals and otolith organs - detect changes in position and motion of the head 1. Semicircular canals • detect rotation of head • three fluid filled canals (bones) at right angles to each other • bulbous expansion at base called ampulla • ampullae contains hair cells with cilia projecting into a gelatinous mass, the cupula which protrudes into the fluid (endolymph) in the ampulla (cilia) • • • • • • hair cells have high resting discharge of APs of ~100 / sec hair cells have 1 large kinocilium & 30 stereocilium any rotation of head causes fluid movement in at least one canal inertia of fluid (endolymph) causes cilia to be deflected to side bending of cilia in one direction causes depolarization & increase in AP frequency of vestibular axons from resting 100 Hz to a higher frequency bending of cilia in other direction – hyperpolarization & decrease in AP frequency from 100 / sec to a lower frequency the three canals give information about rotation in all directions • output via vestibular nerve to brain • • • • • • 2. Otolith organs detect position of head in space, relative to gravity, ie, head tilt two organs, the utricle and saccule both are sac-like structures in a bony chamber between semicircular canals and cochlea contain hair cells protruding into a gelatinous mass containing otoliths (small crystals of calcium carbonate) • • • • • • • • • • Detection of head tilt: a) standing (head upright) cilia project upwards in utricle head tilt from vertical causes cilia to bend in directection of tilt (stereocilia towards kinocilium) by gravitational force depolarization & increase in freqn of APs head tilt back - cilia are bent in opposite direction – hyperpolarization & decrease in AP freqn b) lying down cilia project upwards in saccule head tilt from horizontal causes cilia to bend in direction of tilt increase in freqn of APs Disorders of Balance • Vertigo - loss of balance & sometimes nausea -often caused by problems of the inner ear. i) labyrinthitis (inflammation within the inner ear) & vestibular neuritis (inflammation of the vestibular nerve) -characterized by the sudden onset of vertigo and may be associated with hearing loss -viral or bacterial infection ii)Benign paroxysmal positional vertigo (BPPV) - the most common -characterized by the sensation of motion initiated by head movements -the crystals in the otolith organs become dislodged and move into the semicircular canals, which become sensitive to head tilt movements (abnormal) iii) Meniere’s disease -dysfunction of the semicircular canals -an increased pressure of the endolymph fluid in the canals, due to increased secretion, or blockage of drainage, and dilated membranous sacs -symptoms include severe vertigo, tinnitis (ringing in the ears) and hearing loss.