Physiology of the Sense Organs. Ying Huang

Transcription

1 Section Ⅸ Physiology of the Sense Organs Ying Huang

2 Schedule for section Ⅸ Today------General principles, Vision Next Wednesday------Hearing & Equilibrium, Other senses

3 Clinical Investigation After a visit to a brothel in Arles, France, on December 23, 1888, Vincent Van Gogh, the nineteenth-century French painter, returned to his room, picked up a knife, and cut off his own ear. A local physician, Dr. Felix Ray, examined Van Gogh that night and wrote that the painter had been assailed by auditory hallucinations and in an effort to relieve them, mutilated himself by cutting off his ear. A few months later, Van Gogh committed himself to a lunatic asylum. By 1890, Van Gogh was dead by his own hand. Historians have assumed that Van Gogh suffered from epilepsy. But the painter s strange attacks of dizziness, nausea, and overwhelming tinnitus, which he recorded in desperate letters to his relatives, are more consistent with Ménière's disease, a condition that affects the inner ear.

4 GENERAL PRINCIPLES OF SENSORY PHYSIOLOGY

5 What is the Sensory System? Part of the nervous system consisting of sensory receptors that receive stimuli from internal and external environment and conduct this information to brain that processes this information.

6 What is it made of? Nose Eyes Ears Tongue Skin Brain Neurons Receptors

7 Traditional Classification of Sensory System General senses Touch Temperature Pain Special senses Vision Hearing Taste Smell Equilibrium

8 What Triggers These Senses Receptors Specialized endings Collect information about external and internal environment through a stimulus. Each receptor is specific to a certain type of stimulus An exception: a receptor can be activated by a nonspecific stimulus if its intensity is sufficiently high

9 Events occurring within a sensation 1.stimulation of the receptor 2.transduction (conversion) of stimulus into a graded potential vary in amplitude and are not propagated 3.generation of impulses when graded potential reaches threshold 4. Information is sent to CNS 5.integration of sensory input by the CNS

10 Receptors and Sense Organs Receptor is referred to the structure located on the body surface or within tissues to detect the changes in internal or external environments and to convert stimulus into electrical signals. The receptor is often associated with nonneural cells that surround it, forming a sense organ.

11 Receptors and Sense Organs Hair cells Ear

12 Receptors and Sense Organs Receptors detect a small range of energy levels Ear, 20~20,000 Hz vibration Taste buds, specific chemicals Eye, 380~750 nm electromagnetic wave

13 Complexity Range of Receptors

14 Types of Receptors-Location 1.Exteroceptors ( 外感受器 ) Teleceptors ( 距离感受器 ) Sight, hearing Contact receptor ( 接触感受器 ) Pressure, temp., touch, taste 2.Interoceptors ( 内感受器 ) Visceroceptors ( 内脏感受器 ) Blood pressure, pain Proprioceptors ( 本体感受器 ) Muscle spindle

15 Types of Receptors-Modalities 1.Mechanoreceptors respond to mechanical stimulus 2.Thermoreceptors changes of temperature can stimulate these receptors 3.Nociceptors brings information concerning pain 4.Electromagnetic receptors rod cells and cone cells of the eye which are stimulated by changes of intensity and wavelengths of the light 5.Chemoreceptors respond to chemical stimulus, such as taste, smell and blood oxygen

16 Properties of the Receptors 1.Adequate stimulus 2.Transducer function 3.Coding 4.Adaptation

17 1 Adequate Stimulus of Receptors Each type of receptor is highly sensitive to one type of stimulus The particular form of energy to which a receptor is most sensitive is called its adequate stimulus. E.g. The adequate stimulus for the rods and cones is light. The threshold for their nonspecific responses is much higher. Pressure on the eyeball will stimulate the rods and cones, but the threshold of these receptors to pressure is much higher than the threshold in the skin.

18 2 Transducer function The receptors translate the energy of the stimulus into an electrical change in specific sensory cells or afferent nerve endings. In the former case the electrical change is called receptor potential, and in the later case it is called generator potential. Converting one type of energy into another type (bioelectrical energy) is the process of transduction Our brain only deals with bioelectrical impulses so transduction must occur!

19 Generator (Receptor) Potential Character: local excitation Not all-or-none electrotonic propagation temporal & spatial summation It can be summated to reach the threshold to produce AP in afferent fibers.

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21 3 Coding of Sensory Information The quality of sensation is dependent on sensory pathway including receptor, afferent fiber and cortex, which the stimulus eventually activates. The quantity (intensity) of sensation is associated with the frequency of APs in afferent fibers and the number of receptors activated by stimulus. The stronger the stimulus, the more APs are fired over a given time period

22 E.g. Stretch Receptors: Weak stretch causes low impulse frequency on neuron leaving receptor. Strong stretch causes high impulse frequency on neuron leaving receptor.

23 4 Adaptation of Receptors Definition: When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potentials in its sensory nerve declines over time 1.Rapidly adapting receptor (Phasic Receptors): The frequency of APs diminishes or stops if the stimulus is unchanging. 2.Slowly adapting receptor (Tonic Receptors): adapt slowly or not at all.

24 Adaptation of Receptors The height of the curve indicates the frequency of the discharge in afferent nerve fibers at various times after beginning sustained stimulation.

25 Significance of Adaptation The rapidly adapting receptors cease firing if strength of a continuous stimulus remains constant. Allow body to ignore constant unimportant information, e.g. Smell The slowly adapting receptors continue signal transmission for duration of stimulus. Monitoring of parameters that must be continually evaluated, e.g. baroreceptors.

26 Visual Sense Organ

27 Structure of the Eye Refractive system A lens system for focusing light on the receptors. Retina A light-sensitive tissue lining the inner surface of the eye. Blind spot

28 Function of Refractive System

29 Reduced Eye Refraction of Light in the Eye Vitreous humor

30 Accommodation of Refractive Function Gaze at a near object (less than 6m): 1. Accommodation of lens 2. Accommodation of Pupils Clear image on retina 3. Convergence of Eye Balls

31 Accommodation of Refractive Function Accommodation of Lens A more convex shape Lens ligaments

32 Accommodation of Refractive Function Accommodation of Pupils Sphincter pupillae contracts, the size of the pupil is decreased Pupillary light reflex Significance: regulate the amount of light entering the eye

33 Pupillary light reflex direct light reflex consensual light reflex Bright light is shone on the eye light sensitive cells in the retina, including rod and cone photoreceptors---the optic nerve fiber that carry the impulses initiating pupillary responses end in the pretectal region and the superior colliculi--- send signals to the oculomotor nerve, which terminates on the circular iris sphincter muscle. When this muscle contracts, it reduces the size of the pupil.

34 Pupillary light reflex Direct light reflex Consensual light reflex

35 Accommodation of Pupils Near reflex of the pupil: The pupil constricts when an individual looks at a near object

36 Near point of vision: The nearest point at which an object can be seen distinctly by the eye Far point of vision: The farthest point at which an object can be seen distinctly by the eye

37 Abnormalities of Refractive Function and Accommodation in the Eyes Emmetropia Ametropia

38 Myopia The anteroposterior diameter of the eyeball is too long. This defect can be corrected by the use of glasses with biconcave lenses so that parallel light rays are made to diverge slightly before they strike the eye.

39 Hyperopia The eyeball is shorter than normal and parallel rays of light are brought to a focus behind the retina. The use of glasses with convex lenses to aid the refractive power of the eye in shortening the focal distance corrects the defect.

40 Astigmatism The curvature of the cornea is not uniform. When the curvature in one meridian is different from that in others, light rays in that meridian are refracted to a different focus, so that part of the retinal image is blurred. Corrected with cylindric lenses placed in such a way that they equalize the refraction in all meridians

41 Presbyopia Glasses with convex lenses

42 The photoreceptor mechanism Retina: A light-sensitive tissue lining the inner surface of the eye. The optics of the eye: create an image of the visual world on the retina, which serves the same function as the film in a camera. Rod cells and cone cells Light activating rod cells and cone cells on the retina initiates chemical and electrical events that ultimately trigger nerve impulses. These are sent to visual centers of the brain through the fibers of the optic nerve.

43 The photoreceptor mechanism Structure of retina

44 Photoreceptors in Retina 1) Scotopic vision system Rod cell: The photosensitive pigment in the rods is called Rhodopsin

45 Rod cell Not color sensitive Mainly more sensitive to light (scotopic vision) Peripheral area of retina Night blindness

46 Photoreceptors in Retina 2) Photopic vision system Cone cell:

47 Cone cell Color sensitive Less sensitive to light Mainly fovea in retina

48 Visual Transduction in Rods System Photochemistry of rhodopsin-retinal visual cycle in the rod Rhodopsin: opsin retinene

49 Mechanism for Rod Receptor Potential Generation of a hyperpolarization receptor potential

50 In dark In the presence of light Ionic currents in the photoreceptor cell ( The terminal releases less transmitters when hyperpolarized )

51 Hyperpolarization of photoreceptor in response to light Intensity-dependent Saturation

52 Color Vision (Cone, Photopic vision) Trichromatic theory

53 Color Blindness

54 Information Process in Retina

55 Other Phenomina Related with Vision Visual acuity: The degree to which the details and contours of objects are perceived Snellen visual acuity chart

56 Other Phenomina Related with Vision Dark adaptation: If a person spends a considerable period of time in brightly lighted surroundings and than moves to a dimly lighted enviroment, the retinas slowly become more sensitive to light. The decline in visual threshold during this process is known as dark adaptation. Light adaptation Mechanisms of adaptations: Changes in concentrations of rhodopsin or color photochemicals to resolve fine detail

57 Visual field: Not circular White > yellow > red > green Blind spotular White>yellow>red>green

58 Binocular vision and stereopsis The central parts of the visual fields of the 2 eyes coincide; therefore, anything in this portion of the field is viewed with binocular vision. Binocular vision is often assigned an important role in the perception of depth to produce stereopsis.

59 HEARING

60 Outline Belongs to special senses Receptors: Hair cells in the inner ear Sense organ: Ear Adequate stimulus: Sound wave Coding: Pitch and loudness

61 Structure and function of ear External ear-funnels sound waves from environment Middle ear-cavity between tympanic membrane and cochlea Inner ear-where sound is actually transmitted to nerves (It also receives the changes of the position of the head to balance centers in the brain.)

62 1. External Ear: Pinna (auricle): directs sound waves into the auditory canal External auditory Canal:conducts sound to the eardrum

63 2. Middle Ear Tympanic membrane (Eardrum): thin membrane that vibrates in response to sound, and transfers sound energy to bones of the middle ear Ossicles:three tiny bones amplify sound and transfer sound energy to the inner ear Tympanic cavity: small cavity surrounding ossicles Auditory tube: connecting the middle ear with the throat, equalizing pressure during yawning or swallowing

64 Role of middle ear in sound transmission Inward movement of the tympanic membrane by a sound pressure wave causes the chain of ossicles to push the footplate of the stapes into the oval window Sound Force Amplification by the Ossicles Greater pressure at oval window than tympanic membrane, moves fluid of cochlea The Attenuation Reflex Response where onset of loud sound causes tensor tympani and stapedius muscle contraction Function: Adapt ear to loud sounds

65 Mechanisms involved in transformer process 1.Size difference between Tympanic Membrane and Stapes Footplate 2.Lever action

66 Impact of size difference on Middle Ear Transformer Action Tympanic membrane 59.4 mm2 Stapes footplate 3.2 mm2 Pressure = force/area Pressure gain: 59.4/3.2 = 18.6 (times)

67 Impact of Lever Action on Middle Ear Transformer Action pressure gain: 1.3 times

68 Total Amount of Amplification Pressure Gain Contribution from 18.6 Size difference 1.3 Lever action 24.2 Total pressure gain (18.6 x 1.3)

69 The malleus takes the pressure from the inner surface of the tympanic membrane and passes it by means of the incus to the stapes in such a way that the pressure is amplified about 24 times as it moves.

70 3. Inner Ear Includes sense organs for hearing and balance Filled with perilymph

71 Inner Ear A maze of bony chambers Cochlea: snail shaped fluid-filled structure Vestibule Oval window: thin membrane, transfers vibrations from stapes to fluid of cochlea Round window: absorbs energy and equalizes pressure in the cochlea

72 Cochlea Snail-shaped organ with a series of fluid-filled tunnels. Converts mechanical energy into electrical energy Three chambers: scala vestibuli, scala tympani, scala media (cochlear duct) Basiliar and vestibular membranes

73 Cochlea At the end of the cochlea, the helicotrema joins the scala vestibuli and the scala tympani.

74 Fluids in the cochlea Perilymph: fills the scala vestibuli and scala tympani. similar in composition to extracellular fluid (High in Na+and low in K+). Endolymph: fills the scala media. Similar to intracellular fluid (High in K+and low in Na+).

75 Cochlea - Organ of Corti a structure contains approx. 16,000 cochlear hair cells located on basilar membrane Stereocillia, kinocilium : at apex of hair cells, embedded in tectorial membrane Gel-like tectorial membrane is capable of bending stereocillia, konocilium and activating receptors Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex

76 Function of Corti Relative shearing motion of basilar membrane and tectorial membrane makes the hair cells bend and depolarize, changes transmitters release

77 Properties of Sound Sound travels in waves as light does Pitch: determined by frequency, the number of cycles per second of a sound wave, measured in hertz (Hz) Loudness:determined by amplitude (height) of the sound wave, measured in decibels(db) Timbre: determined by complexity and shape of the sound wave, gives each sound its unique quality

78 Pitch of sound

79 Loudness of Sound 0 db = hearing threshold 50 db = normal conversation 90 db = danger zone 120 db = Rock concert 130 db = Pain threshold

80 Sound Transmission and Transduction Sound waves Tympanic membrane vibrations Ossicles transmit & amplify vibration Via oval window to perilymph then endolymph basal membrane resonance oscillation hair cells translate the vibration into generator potentials auditory nerves transmits nerve impulses to auditory cortex

81 Conduction of Sound 1. The air conduction

82 Air Conduction of Sound

83 Conduction of Sound 2. The bone conduction

84 Sound Transmission and Transduction Sound waves Tympanic membrane vibrations Ossicles transmit & amplify vibration Via oval window to perilymph then endolymph basilar membrane resonance oscillation hair cells translate the vibration into generator potentials auditory nerves transmits nerve impulses to auditory cortex

85 Electrical Potentials in Cochlea 1, Endocochlear Potential (EP) Putting the electrode in the scala media and a +80 mv potential with respect to a neutral point of perilymph in scala tympani can be discovered

86 Intracellular Potential (IP) or hair cell resting potential: -80 mv

87 Electrical Potentials in Cochlea Difference between extracellular and intracellular potential: Top: mV endolymph in scala media Base: 80mV perilymph in scala tympani

88 Electrical Potentials in Cochlea Mechanoreceptor in hair cell When the stereocilia of a hair cell move toward the tallest cilium, the hair cell is depolarized; when the stereocilia bend in the opposite direction, the hair cell is hyperpolarized.

89 Sound Transduction Vibrations from sound waves move tectorial membrane Hair cells (stereocillia) are bent by the membrane Generator potential is induced in hair cell An action potential starts in the cochlear nerve

90 Electrical Potentials in Cochlea 2,Cochlear Microphonic Potential (CM) When cochlear is activated by sound, the membrane potential recorded in cochlear or near cochlear which is generated from hair cells. It reproduces frequency of a sound wave perfectly. 3, Action Potential (AP) Electrical activity from the auditory nerve Can be measured from anywhere in the cochlea or in the auditory nerve

91 Coding of sound Coding Information About Sound Intensity Firing rates of neurons Number of active neurons Coding Information About Sound Frequency Location of Basilar membrane activated Frequency: Highest at base, lowest at cochlea apex

92 How to discriminate the frequency of the sound? Traveling Wave Theory

93 The Traveling Wave Theory Sound wave entering at the oval window is to cause the basilar membrane to vibrate different frequencies cause vibrations at different locations (places) along basilar membrane higher frequencies at base, lower frequencies at top

94 The Traveling Wave Theory

95 Hearing impairments Deafness Conduction deafness Sensorineural deafness Tinnitus Ménière's syndrome attacks of dizziness, nausea, caused by excess endolymph in the media canal

96 Deafness Conduction deafness Transmission of sound waves through middle ear is impaired. Impairs all sound frequencies. May be caused by damage to ossicles by an infection of the middle ear or immobilization of the stapes in otosclerosis. Treatment: Hearing aids- amplify sounds Sensorineural/perceptive deafness Transmission of nerve impulses anywhere from the cochlea to the auditory cortex is impaired. Often impairs ability to hear some pitches more than others. May be caused by many pathological processes or exposure to extremely loud sounds. Treatment: Cochlear implants- electrically stimulate nerve fibers in response to sound

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98 Outline Belongs to special senses Receptors: Hair cells in the inner ear Sense organ: Vestibular organ in the inner ear Adequate stimulus: Rotational and linear acceleration

99 Structure of vestibular organ Ability to detect head position and movement (or acceleration) to maintain a steady balance when it is still or moving Semicircular canals Utricle Saccule

100 Semicircular canals Three semicircular canals on one side on three directions, helps sense all possible head-rotation angles Receptors in the semicircular canals Crista ampullaris, located in the expanded end (ampulla) of the canal Consists of hair cells Cupula (gelatinous cap) covers the hair cells

101 Utricle and saccule Receptors in the Utricle and saccule: Otolithic Organ (Maculae) Hair cells are embedded in the otolithic membrane Otoliths (tiny stones) float in a gel around the hair cells

102 Function of Semicircular canals Detects rotational acceleration To maintain balance while turning

103 Function of Utricle Detects changes relative to gravity (linear acceleration) from vertical movement

104 Function of Saccule Detects backward-frontward (horizontal) movement (linear acceleration) from acceleration

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106 Vestibular reaction Vestibular autonomic reaction Motion sickness: mismatch between visually perceived movement and the vestibular system's sense of movement Dizziness, fatigue and nausea Treatment: Antimotion drugs (e.g. Dramamine), depression of vestibular inputs

107 Vestibular reaction Nystagmus (The Vestibulo-Ocular Reflex, VOR) When spinning is suddenly stopped, eyes continue to move in the direction opposite of the spin, then jerk rapidly back to the midline. When a person begins spinning, the cupula bends in the opposite direction,and eyes slowly drift in opposite direction, then jerk rapidly back to the midline. If the spining suddenly stops, inertia of endolymph causes it to continue moving in the direction of spin,and eyes movement is opposite to those at the begining. This is a normal phenomenon that helps maintain balance during spinning.

108 Otoscopy is used to have a visual examination of the ear Audiometer measures various frequencies to test hearing A tuning fork compares the conduction of sound in one ear or between the two ears.

109 Summary Each type of receptor is excited most effectively by only one modality of stimulus known as the adequate stimulus. The stimulus is converted into an electrical potential. The intensity of the stimulus is associated with frequency of APs and the number of receptors activated by stimulus. Ear is the sense organ for both hearing and equilibrium, both use hair cells in the inner ear as receptors.

110 Ear care Day March 3rd

111 Olfaction-the sense of smell The sense of smell is activated by neurons called olfactory receptors which are covered with cilia. Olfactory receptors are yellowish-brown masses along the top of the nasal cavity. Responds to molecules dissolved in mucus or lipids

112 Gustation-the sense of taste Humans perceive five types of taste: bitter, sour, salty, sweet, and umami(meaty flavor). Papillae are structures on the surface of the tongue that contain the taste buds. Taste Buds are organs that sense the taste of food, contain receptor cells called taste cells responsive to each of the taste categories.

113 Somatic senses 1.Pain Fast pain: The nervous system quickly responds to a pain initiating event to the central processing unit Slow pain: persistent, indistinct source Referred pain is a phenomenon of pain perceived at a site adjacent to or at a distance from the site of an injury's origin. (during ischemia brought by a myocardial infarction where pain is often felt in the neck, shoulders, and back rather than in the chest.)

114 2. Warmth and cold Changes in temperature in dermis, skeletal muscles, liver and hypothalamus Free nerve endings Cold receptors are much more than warm receptors

115 3.Touch-pressure Unencapsulated receptors: free nerve endings Merkelsdics-fine touch Encapsulated receptors: Meissners corpuscles -fine touch Pacinian corpuscles -deep pressure

116 4. Proprioception monitors of muscle stretch Muscle spinder Tendon organ