1. Sensory System John Paul L. Oliveros, MD, DPPS

2. Section A General principles

3. General Principles Awarenesss of our external and internal world is brought about by neural mechanisms that process afferent information Stimulus energy  receptor potentials (graded potentials)  action potentials (Nerve fibers) Sensory system Part of the nervous system that consists of sensory receptors Neural pathways Processing areas of the brain Sensory information Information processed by a sensory system May or may not lead to conscious awareness of the stimulus Sensation Sensory information that reaches consciousness Perception A persron’s understanding of the sensation’s meaning

4. Receptors Sensory Receptors Initiates neural activity at the border betwee the nervoussystem and the outside world Change stimulus energy (pressure, temperature, light, soundwaves, etc) Can either be: Specialized ending s of afferent neurons Separate cellthat affect the ends of afferent neurons Stimulus Energy that impinges upon and activates a sensory receptor Stimulus transduction The process by which stimulus is transformed into an electrical response Adequate stimulus The type of energy to which a receptor responds in normal functioning Receptors respond best to only a very narrow range of stimulus energy (lowest threshold)

5. Receptor Potential Transduction process in all sensory receptors involve the opening and closing of ion channels that receive information about the outside world Receptor potential A change in the membrane potential on a specialized receptor membrane It is a Graded potential separate receptors: graded potential causes release of neurotransmitter Receptors on afferent neuons: A local current must flow to a part of an axon that can produce an action potential 1st node of Ranvier Part of myelinated afferent neurons capable of producing action potentials

6. Receptor Potential Graded potential magnitude determines action potential frequency but not action potential magnitude Factors controlling receptor potential magnitude Stimulus strength Rate of change of stimulus strength Temporal summation of successive receptor potentials Adaptation Decrease in receptor sensitivity Results in decrease in frequency of action potentials in an afferent neuron despite maintenance of the stimulus at a constant strength

7. Neural pathways in sensory system Sensory pathway A bundle of parallel 3-neuron chains Sensory units A single afferent neuron with all its receptor endings Receptive field Portion of the body that when stimulated leads to activity in a particular afferent neuron

8. Ascending pathways Central processes Part of afferent neurons that enter the brain or spinal cord and synapse with interneurons 2nd order neurons Interneurons that synapse with afferent neurons Synapse with 3rd, 4th, etc interneurons until the cerebral cortex is reached

9. Ascending pathways Specific ascending pathways Ascending pathways in the brain and spinal cord that carry information about single types of stimuli Somatosensory cortex Lies in the parietal lobe of the brain behind the junction of the parietal and frontal lobes Where information from somatic recepotrs are transmitted Information from skin, skeletal muscles, tendon and joints Visual cortex At the occipital lobe Where spefic pathways from the eyes transmit Auditory cortex Where specific pathways from the ears transmit Loacted at the temporal lobe

10. Ascending pathways Nonspecific ascending pathways Activated by sensory units of several different types Signal general information Polymodal neurons 2nd order neurons that respond to inputs from several afferent neurons, each activated by a different stimulus

11. Association Cortex and Perceptual Processing Cortical Association Areas Areas of the brain outside the primary cortical sensory areas but are adjacent to them Elaborates perception information from the primary sensory cortical areas Regions closests to the primary sensory cortical areas process information in fairly simple ways and serves basic sensory function Regions farther from the primary sensory cortical areas process information in more complicated ways Arousal Attention Memory Language Emotional and motivational significance (frontal lobe/ limbic system)

13. 1. there is heirarchical processing of afferent information along individual pathways

14. 2. information is processed by parallel pathways, each of which handles a limited aspect of neural signals generated by the sensory transducers

16. Primary sensory coding Stimulus intensity Distinguishing intensity Frequency of action potentials Inc. Stimulus strengthinc. Receptor potential  inc. Action potential frequency single receptor Other receptors of the same neuron Recruitment Calling in of receptors on additional afferent neurons

17. Primary sensory coding Stimulus location Factors: Main factor: Site of the stimulated receptor amount of convergence of neuronal input in ascending pathways: inversely related to acuity/precision Size of the receptive fieldcovered by a receptor Overlap of nearby receptive fields

18. Primary sensory coding

19. Primary sensory coding Lateral inhibition More important in localization than the different sensitivites of receptors throughout the receptor field Information from afferent neurons whose receptors are at the edge of the stimulus is inhibitted compared to information from the stimulus’ center Increases contrast between relevant and irrelevant information May occur at any levels of the pathway but mostly on the early stages

20. Primary sensory coding Stimulus duration Receptors differ in the way they respond to a constantly maintained stimulus adaptation Rapidly adapting receptors: Important in signaling rapid change On response On-off response Slowly adapting receptors: Maintain response at or near the initial level of firing regardless of the stimulus duration For prolonged events (posture)

21. Central control of afferent information Reticular formation and cortex: main control

22. Section B Specific Sensory Systems

23. Somatic sensation Somatic sensation: Skin Muscles Bones Tendons joints Activation gives rise to sensations touch, Pressure Warmth Cold pain awareness of the position of the parts and their movement Each sensation has a specific receptor type Information enters both specific and non-specific pathways Specific pathways cross to the opposite side of the brain (somatosensory cortex

24. Somatic sensation

25. Somatic sensation Somatosensory cortex Endings of axons of the specific pathways are grouped according to the location of the receptors giving rise to the pathways

26. Touch-pressure Skin mechanoreceptors Rapidly adapting receptors Touch Movement vibration Slowly adapting receptors pressure

27. Sense of posture and movement Receptors for postures and movement Muscle-spindle stretch receptors Vision and vestibular organs Mechanoreceptors in joints, tendons, ligaments, skin Kinesthesia Sense of movement at a joint

28. Temperature Thermoreceptors Warmth receptors Respond to temp between 30c-43c Increase discharge rate upon warming Cold receptors Stimulated by small decrease in temperature

29. Pain Nociceptors Detect stimulus that causes tissue damage Respond to intense mechanical deformation, excessive heat, and many chemicals (several secreted by damaged cells) Hyperalgesia Increased sensitivity to painful stimuli Last for hours after the stimulus is over Referred pain Sensation of pain is experienced at a site other than the injured/diseased part Due to activation of intrneurons by incoming nociceptive afferents Visceral and somatic afferents often converge in the same interneurons in the pain pathway Analgesia Selective suppresion of pain without effects on consciousness or other sensation Stimulation-produced analgesia Electrical stimulation Transcutaneous electric nerve stimulation Electrodes are placed on the surface of the skin above the painful site or nerves leading from it Stimulation of non-pain, low threshold fibers leads to inhibition of neurons in the painful pathway Acupuncture Needles are introduced into specific parts of the body to stimulate afferent fibers which causes analgesia It ivolves endogenous opiod neurotransmitters

30. Vision Light Receptors of the eyes are only sensitive to visible light Wavelength Distance between 2 successive wave peaks of the electromagnetic radiation Frequency Measured in hertz (cycles per second) Varies inversely with wavelength Visible spectrum Between 400-700nm Light of different wavelength is percieved as colors

31. Vision Optics of vision Retina Focuses the image being viewed Thin layer of neural tissue lining the back of the eyeball Contains Rods Cones ‘neurons Lens and cornea Optical system that focus the impinging light rays into an image upon the retina Surface are curved to bend light rays coming from different directions and focus them into a single point at the retina Fovea centralis Area in the retina with the greatest visual clarithy Area where light rays from the cornea/lens are focused Image is upside down and reversed right to left

32. Vision Optics of vision

33. Vision Optics of vision Accomodation Process of focusing and adjusting image on the retina Cornea Greater part of focusing image on the retina Lens Adjustments for distance made by changing its shape Cilliary muscles Controls the shape of the lens Stimulated by parasymphatetic Sphincter like and draws lens towards it as it contracts Accomodation for viewing near objects Zonular fibers Attaches the ciliary muscles to the lens Pulls lens to flatten it to focus distant objects Relaxes to make lens more spherical to focus near objects

34. Vision Accomodation Include the following mechanisms To view near objects Moving of lens slightly towards the back of the eye Turn the eyes inward and towards the nose (convergence) Constrict the pupil To view far objects Opposite of the above mechanisms

35. Vision Abnormalities Presbyopia Normal part of aging process Increasing stiffness of the lens making accomodation difficult Cataract Opacity of the lens Usually due to changing color due to age Nearsightedness / myopia Unable to see distant objects clearly Eyeball is too long Far images focus at a point in front of the retina Farsightedness / hyperopia Eye is too short Near objects are focused behind the retina Near vision is poor Astigmatism Lens and cornea doesn’t have a smoothly spherical surface Glaucoma Aqueous humor is formed faster than it is removed Increase intraocular pressure Leading cause of irreversible blindness Axons of the otic nerve die

36. Vision Iris Controls the amount of light entering the eye Ringlike pigmented muscular tissue Color is not significant Sympathetic innervation Radial muscles contract Pupils enlarge Parasymphatetic innervation: Sphincter muscles contract Pupils contract Pupils Whole in the center of the iris Where light enters the eye

37. Vision Photoreceptor cells

38. vision Photoreceptor cells Rods Extremely sensitive to very low levels of illumination Cones Less sensitive and respond to bright light Choroid Pigmented layer behind the retina Absorbs light and prevents reflection back to the rods and cones Photopigments Inside photoreceptors Absorb light 4 types: Rods (1): rhodopsin) Cones (3) Each contains: Opsin Group of integral proteins that surround and binds a chromophore molecule Differs in each of the 4 photopigments Light filters differently in each photopigments and thus absorbs light most effectivly at different spectrum Chromophore Light sensitive part of the photopigment Same in all 4 photopigments A derivative of vit. A (retinal)

39. Vision Photoreceptors

40. Vision Light retinal changes shape  photoreceptor hyperpolarization decrease release of neurotransmitter (glutamate)  hyperpolarization of bipolar cell In the dark: Retinal has resting shape Photoreceptor cell partially depolarized More neurotransmitter is transmitted Dark adaptation Temporary blindness when one steps into a darkend room from bright sunlight At brighlight: rhodopsin completely activated At dark: at least 10mins needed to restore rhodopsin to resting state Neural Pathways of vision

41. Vision Color vision The colors we perceived are related to the wavelengths of light that are reflected, absorbed, or transmitted by the pigments in the objects of our visual world White light Mixture of all colors Blacklight Absence of all light Begins with the activation of the photopigments of the cone receptor cells(red, green, blue)

42. Vision Color vision Ganglion cells general brightness: receives input from all 3 colors Opponent color cells: Code for specific color Excitatory input from one type of receptor and inhibitory from another Color blindness AKA color deficiency Lack red or green pigments entirely or have them in abnormal form Trouble perceiving red vs green

43. Vision Eye movement Controlled by 6 skeletal muscles 2 basic movements Fast movements AKA saccades small jerking movements Rapidly bring eye from one fixation point to another Allow search for visual field Prevent adaptation Move during certain periods of sleep (watching visual imagery of dreams) Slow movements Involve in tracking visual objects moving throught the visual field Compensation during movements of the head

44. Hearing Sound Sound energy: Medium: gaseous, liquid, or solid medium Vibration of the mediums’ molecules Vibrating objects can serve as a sound source Sound wave: Zones: Zones of compression Molecules close together Pressure is increased Zones of rarefaction Molecules are far apart Pressure is less Sound wave (cont) Consists of rapidly alternating pressures Amplitude Determined bydifference between the 2 zones Related to the loudness of the sound Frequency Number of zones in a given time Determines the pitch we hear Keenly audible frquency: 1000-4000hz Audble frquency: 20-40,000hz

45. Hearing

46. Hearing Sound transmission in the ear External auditory canal Help amplify and direct sound Tympanic membrane Vibrate at the same frequency of the sound waves Middle ear cavity Air filled cavity in the temporal bone Auditory/eustachian tube Connects the middle ear to the pharynx Exposes the middle ear to atmospheric pressure Normally close but opens during yawning, sneezing, swallowing to equal middle ear pressure to atmospheric pressure Pain during sudden change of altitude because of pressure difference between middle ear and atmosphere

47. Hearing Middle ear Sound waves amplified by chain of bones that act as pistons and couple the motions of the tympanic membrane to the oval window Malleus Incus stapes Force of sound waves transferred from tympanic membrane to oval window

48. Hearing

49. Hearing Inner Ear/Cochlea Fluid filled, spiral shaped passage in the temporal bone Where the receptors cells are located Cochlear duct Fluid filled membranous tube Follows the cochlear spiral Divides the cochlea lengthwise Scala vestibuli On side of cochlear duct and ends on the oval window Scala tympani Below the cochlear duct and ends on the round window Oval window Separates inner ear from middle ear Basilar membrane Forms one side of the cochlear duct Organ of corti Sits on the basilar membrane Contains receptor cells Hair cells Mechanoreceptors with hairlike stereocilia Transform pressure waves in the cochlea into receptor potentials Movements of basilar membrane stimulate hair cells Tectorial membrane Move in relation to haircells Bend sterocilia to open ion channels Efferent nerve fibers From brainstem Dampen response for protectionr Afferent neurons Forms cochlear portion of cranial nerve VIII

50. Hearing Neural pathways of hearing Cochlear N.  brainstem interneurons  multineuron pathway  thalamus  auditory cortex Hearing aids Amplify incoming sounds Cochlear implants Used when there is extensive damage Restore functional hearing Directly stimulate the cochlear nerve with tiny electric currents Bypass the cochlea

51. Vestibular system Vestibular apparatus Series of fluid filled membranous tubes that connect with each other and with the cochlear duct Contains hair cells that detect changes in the motion and position of the head Consists of: 3 semicircular canals Utricle saccule Labyrinth Bony canals of the inner ear that contains the vestibular apparatus and cochlea

52. Vestibular system Semicircular canals Detect angular acceleration during rotation of the head along 3 perpendicular axes Nodding head up and down (yes) Turning head from side to side (no) Tipping the head so ear touches shoulder Receptor cells Also contains hairlike stereocilia Cupula Gelatinous mass that ensheaths the stereocilia Ampulla Slight bulge in the wall of each duct

53. Vestibular system Utricle and saccule Receptors: Mechanoreceptors with stereocilia Utricle: horizontalposition Saccule: vertical position Otoliths: Tiny calcium carbonate stones Embedded in gelatinous substance together with stereocilia Makes gelationous substance heavier than surrounding fluid Moves according to the force of gravity Provide information on: Linear acceleration Up and down Back and forth Changes in head position in relation to gravity

54. Vestibular system Vestibular information and dysfunction Hair cell  vestibular branch of cranial N. VIII  brainstem  multineuronal pathway  vestibular centers of parietal lobe 3 uses of vestibular information: 1. Control eye muscles to fix eye in the same point in spite of head movement nystagmus: large jerky back and forth movement of the eye in response to unusual vesdtibular input 2. To maintain upright posture 3. To provide conscious awareness of the position and acceleration of the body Vertigo: Illusion of movement (usually spinning) Accompanied by feelings of nausea and lightheadedness Occurs when there is a mismatch in the information from the various sensory systems e.g. Looking down from the building Motion sickness Unfamiliar patterns of linear and rotational acceleration are experienced and adaptation to them has not occured Meniere’s disease Involves the vestibular system Episodes of abrupt and severe dizziness, ringing of ears, bouts of hearing loss Due to increase fluid pressure in the membranous duct sytem of the inner ear

55. Chemical senses Receptors: chemoreceptors Taste Tastebuds Specialized organs for taste 10,000 + present 4 basic groups Sweet Sour Salty Bitter Pathways end up in mouth region of the somatosensory cortex

56. Chemical Senses Smell 80% of flavor of food is contributed by smell Odor of a substance is related to its chemical structure Olfactory receptor cells Lie in the olfactory epithelium in upper part of the nasal cavity Specialized afferent neurons With single enlarged dendrite that extends to the surface of the epithelium Cilia: Processes of dendrites Non-motile Bath in mucus Containreceptor proteins for olfactory stimuli Odorant 1000 or so different plasma membrane odorant receptor types Axons (cranial N. I) olfactory bulbs  olfactory cortex (limbic system) Limbic system: Emotional behavior Fodd getting behavior Sexual behavior Olfactory discrimination Increased in hunger More keen in women Smokig decrease sensitivity Decreases with age Decreases with nasal congestion

57. Chemical senses

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