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Ch. 7 – Sensory Physiology Objectives: 1. Steps to Sensory Perception 2. Categories of Sensory Receptors and Stimuli They Respond To.

3. Somatic Senses versus Special Senses

Part 1: Steps to Sensory Perception 1. Stimulus – acts on a sensory receptor. 2. Transduction – stimulus causes an action potential in sensory neuron. 3. Conduction – info. about stimulus ascends by sensory neurons through afferent path to brain. 4. Perception – “awareness” and evaluation of the stimuli by the brain.

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1. The Stimulus CNS Distinguishes 4 Stimulus Properties: A. TYPE of stimulus (which receptor activated) see more about receptor types later B. Stimulus Location – where in body C. Stimulus Intensity – how strong stimulus is (by number of receptors activated & frequency of APs coming from receptor) D. Stimulus Duration – how long (duration of

APs in sensory neurons) Somatosensory cortex

B. Stimulus Location Ex: 2-point touch discrimination = test of mechanoreceptors for touch in the skin Receptor with more dendrites = bigger response

Lateral inhibition if outer edge of receptor stimulated

If stimulus applied to location having high density of sensory receptors, more APs form, brain has better sensory perception. Stimulus needs to hit “center” of receptor. If hits only outer (lateral) edge, 4 stimulation of receptor inhibited (called “lateral inhibition”)

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C. Stimulus Intensity

Ex. IF small Stimulus Weak stimulus causes only few APs in receptor

Sensory receptor (= transducer) Afferent sensory neurons CNS Integration, perception

Minimal perception

C. Stimulus Intensity

IF BIG Stimulus

Sensory receptor (= transducer) Afferent sensory neurons

stronger stimulus causes more frequent APs in the receptor.

CNS Integration, perception

Greater perception

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D. Stimulus Duration how long (duration of APs

in sensory neurons)

2 responses to sustained stimulation : Tonic receptors  Slow or no adaptation  Continuous APs form for duration of stimulus For functions that must be continuously monitored (Ex. Baroreceptors for blood pressure) Phasic receptors  Rapid adaptation  APs stop if stimulus remains constant  Allow body to ignore constant unimportant info. Ex. Smell, mild temperatures.

2. Stimulus Transduction Stimulus received by receptor & “transduced” (converted) into a graded AP in sensory neuron.

Types of Sensory Receptors: A) sensory neuron – the

AP

receptor IS an afferent neuron.

B) special receptor cell – intermediate cell senses stimulus, releases neurotransmitter, which stimulates sensory neuron.

AP

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Stimulus Transduction = Graded AP Response in Sensory Receptor Stimulus too small

Large stimulus

Sensory Neuron

AP

Stimulus

Moderate stimulus

No AP in receptor

Some APs in receptor

X

!

No neurotransmitter released

Neurotrans. released

Many APs in receptor

!!!!! LOTS of neurotrans. released

3. Conduction of Stimulus info. about stimulus ascends by sensory neurons through afferent path to brain thalamus

 Primary sensory neurons - signal from receptor goes to spinal cord or medulla

Left

Right

 Secondary sensory neurons signal crosses over (in spinal cord or medulla) to thalamus  Tertiary sensory neurons  somatosensory cortex in parietal lobe (post central gyrus) 

4. CNS Perception of Stimulus

medulla

Spinal cord

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Review • Steps to sensory perception (stim., transduction, conduction, perception) • Transduction & perception of stimuli based on: – Type of signal (based on type of receptor stimulated) – Signal location (are more than one receptor involved) – Signal strength (stronger stim = more APs in sensory neuron) – Signal duration (some sensory neurons “adapt”, others don’t)

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Part 2: Categories of Sensory Receptors & Stimuli They Respond To 1. Chemoreceptors – chemicals (taste, smell, pH, and O2, CO2)

2. Thermoreceptors – heat (> 37C) and cold (< 37C) 3. Mechanoreceptors – touch, pressure, vibration. – equilibrium and hearing in inner ear. 4. Photoreceptors – for light (in retina of eye) 5. Proprioceptors – for feedback on movement or stretch (in muscles, joints, tendons, & ligaments) 6. Cutaneous receptors – in skin for touch, pressure, heat, cold, pain.

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Part 3: Somatic Senses versus Special Senses • Somatic senses = proprioceptors & cutaneous receptors all over body 1. Proprioceptors = Stimuli from skin, muscles, joints, ligaments, and tendons. Sense stretch, movement, body posture. 2. Cutaneous receptors – sense touch, pressure, heat, cold, & pain (mechanoreceptors, thermoreceptors, nocireceptors).

Signal conducted to the spinal cord first, crosses over in spinal cord or thalamus, then to somatosensory part of brain.

• Special Senses – Stimuli of smell, taste, sight, and sound. (olfactory receptors, photoreceptors, mechanoreceptors) – Have specialized sensory receptors in head – Signal conducted directly to the brain

Somatic Senses and Their Receptors: 1. Proprioceptors = Stimuli from skin, muscles, joints, ligaments, and tendons. Sense stretch, movement, body posture.

2. Cutaneous receptors – sense touch, pressure, heat, cold, & pain. • Touch & pressure (mechanoreceptors) – Merkel's disks around hair follicles (slow to adapt) – Meissner's corpuscles and Pacinian corpuscles (quick to adapt stimulus applied long time, stop responding)

pacinian

meissner

merkel

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Somatic Senses and Their Receptors: 1. Proprioceptors = Stimuli from skin, muscles, joints, ligaments, and tendons. Sense stretch, movement, body posture.

2. Cutaneous receptors – sense touch, pressure, heat, cold, & pain. • Touch & pressure (mechanoreceptors)

• Heat & Cold (thermoreceptors) for > 37C and < 37C - adapt only between 20 and 40C - nociceptors activated if temp > 45C

Somatic Senses and Their Receptors: 1. Proprioceptors = Stimuli from skin, muscles, joints, ligaments, and tendons. Sense stretch, movement, body posture.

2. Cutaneous receptors – sense touch, pressure, heat, cold, & pain. • Touch & pressure (mechanoreceptors) • Heat & Cold (thermoreceptors)

• Pain (nociceptors) - Activated by strong, noxious stimulu - 3 categories of pain stimuli: mechanical, thermal, chemical - Might activate 2 pathways: A) Reflex movement (signal to spinal cord then to motor neuron) Body part retracts from stimulus (before you realize it hurt) B) Brain perception - signal goes up to somatosensory cortex (OUCH! you realize it hurt) - signal goes to limbic system & hypothalamus (Emotional Distress)

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Referred Pain Pain in organs is poorly localized May be displaced if Multiple 1° sensory neurons converge on single ascending tract in spinal cord

Review • Types of receptors – Chemoreceptors – Mechanoreceptors – Thermoreceptors – Photoreceptors – Proprioceptors – Cutaneous receptors

For “somatic senses”

• Somatic senses versus special senses – Cutaneous receptors • Touch & pressure mechanoreceptors • Heat & cold thermoreceptors • Nociceptors 18

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Special Senses and Their Receptors:

• • • • •

Taste (gustation) Smell (olfaction) Vision Hearing (auditory) Equilibrium (vestibular)

Special Senses and Their Receptors:

Taste (gustation) - receptors found on taste buds. - Food / liquid molecules activate different receptors Sweet (G-protein receptor) Sour (ion-gated receptor) – based on H+ ions Salty (ion-gated receptor) – based on Na+ Bitter (G-protein receptor) – can be toxins Umami (G-protein receptor) - based on glutamate - pronounced “oo-MAH-mee”

Fig 7.5

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Special Senses and Their Receptors:

Smell (olfaction)

• Olfactory receptor neurons on epithelium of upper nasal cavity. • Odor molecules bind to receptor • Axons of olfactory neurons form olfactory nerve (Cranial nerve 1) • Signal to olfactory cortex (temporal lobe)

Fig 7.6

Special Senses and Their Receptors:

Vision

• Light enters pupil •Light bends passing through lens •Light focuses on retina (sensory area with photoreceptors: rods & cones)

• photoreceptors depolarize (form AP) & release glutamate to “bipolar cells”

•At retina, image is upside-down & reversed (visual cortex fixes this later)

• Retina merges w/optic disk & optic nerve

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Fig. 7-24

Vision

Ciliary muscle

• If image on retina not focuses, lens changes shape using ciliary muscle (called “accomodation”).

lens

• Relaxation of ciliary muscle = lens gets thinner, to view something far.

• Contraction of ciliary muscle = lens gets thicker, to view something close.

Refractive Disorders: Myopia (Nearsightedness) • Distant object brought into focus in front of the retina – Elongated eyeball – Abnormally high convexedness to cornea or lens – Too much refractive power to see distant objects • Corrected w/ concave lenses Fig 7.24

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Refractive Disorders: Hyperopia (Farsightedness) • Close object brought into focus behind of the retina – Shortened eyeball – Abnormally low convexedness to cornea or lens – Too little refractive power to see near objects • Corrected w/ convex lenses

Fig 7.24

Refractive Disorders: Astigmatism • Oblong shape to cornea or lens (not perfectly symmetrical curve) – Different parts refract light to different degrees • Corrected with uneven lens that compensates for the asymmetry

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Neural Pathways for Vision

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Retinal Cells • Photoreceptors –rods and cones

• Bipolar cells –Neurons that receive neurotransmitter from photoreceptors –Synapse with ganglion cells

• Ganglion cells –Neurons that receive neurotransmitter from the bipolar cells –Conduct APs to CNS (visual cortex of occipital lobe) by the optic nerve.

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Photoreceptors

• Rods

– More numerous than cones – Cannot distinguish different colors • Highly sensitive to light (low light levels detected) • Low visual acuity (image not as sharp)

• Cones – Found mainly in fovea centralis – Can distinguish among colors • Low light sensitivity (need more light to see with cones) • High acuity (image is sharper)

Photopigments • Rhodopsin (rod cell photopigment) –Consists of retinal (vitamin A derivative) and opsin (protein) –Retina undergoes a photochemical change when struck by light •Ultimately leads to generation of action potentials in ganglion cells • Cone cell photopigment –Each cone has one of 3 different opsins –Respond to different wavelengths of light •Blue (S), Green (M), or Red (L)

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Color vision • We perceive colors based on wavelengths of light that pigments in the objects of our visual world reflect, absorb, or transmit. • Ex. an object appears red because it absorbs shorter wavelengths (blueish-greenish), while it reflects the longer wavelengths, (red). to excite the photopigment of the retina most sensitive to red.

• Light perceived as white is a mixture of all wavelengths, and black is the absence of all light. • Color vision = activation of the photopigments in cone cells. Human retinas have three kinds of cones—one responding optimally at long wavelengths (“L” cones), one at medium wavelengths (“M” cones), and the other stimulated best at short wavelengths (“S” cones). 31

Color vision

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Color vision • Although each type of cone is excited most effectively by light of one particular wavelength, there is actually a range of wavelengths within which a response will occur. • Our ability to discriminate color also depends on the intensity of light striking the retina. In brightly lit conditions, the differential response of the cones allows for good color vision. In dim light, however, only the highly sensitive rods are able to respond. • Though rods are activated over a range of wavelengths that overlap with those that activate the cones, there is no mechanism for distinguishing between frequencies. Thus, objects that appear vividly colored in bright daylight are perceived in shades of gray at night. 33

“Afterimage” or bleaching of photopigments Black and white afterimage: After viewing black & white image any length of time (~30 seconds), then replace with a white field, see afterimage of the negative of the image that is seen earlier.

Color afterimage: After viewing color image ~30 sec, then view white field, image replaced by the complementary color.

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Did it look like this?

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Did it look yellow-ish?

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Color Blindness • Defects in color vision from sex-linked recessive mutations in the cone pigments. The most common form of color blindness. Red-green color blindness, is sex-liked (mostly in men), affecting 1 out of 12. • Men with red-green color blindness either lack the red or the green cone pigments entirely or have them in an abnormal form. Because of this, the discrimination between shades of these colors is poor.

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Macular Degeneration • The macula lutea = specialized region of the retina for visual acuity.

• Macular degeneration = defect in this region causing loss of vision in the center of the visual field. • The most common form of this disease increases with age, occurring in approximately 30% of individuals over the age of 75, and referred to as age-related macular degeneration (AMD). 42

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Special Senses and Their Receptors:

Hearing (auditory)

Review Ear anatomy – Outer Ear > Pinna or auricle > External auditory canal >  Tympanic membrane (vibrates when struck by sound waves)

Tympanic membrane

Fig 7.13

Hearing (auditory)  Middle Ear has auditory ossicles: > malleus – near tympanic membrane > incus > stapes – connect to oval window These ossicles conduct vibration from tympanum to oval window.

Oval window

 

Cochlear branch of Vestibulocochlear nerve

Vestibular duct (perilymph) Cochlear duct (endolymph)

Tympanic membrane

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Hearing (auditory) Inner Ear (fluid-filled bony labryinth) has:

1. Cochlea (Organ of Corti) – for hearing 2. Otolith organs & Semicircular Canals - for balance (cover later)

Cochlea Cochlear branch of Vestibulocochlear nerve

Oval window

Vestibular duct (perilymph) Cochlear duct (endolymph)

 

Tympanic membrane

Hearing (auditory) 1. Cochlea (Organ of Corti) for hearing  oval window vibrations travel through “perilymph” & vibrate basilar membrane  moves “endolymph” (fluid) in cochlea  bends stereocilia on hair cells of cochlear basilar membrane, release Basilar Cochlear branch neurotransmitter membrane

Oval window

of Vestibulocochlear nerve

Vestibular duct (perilymph)



Cochlear duct (endolymph)

 

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Basilar membrane Oval window

Cochlear HAIR CELL

AP

Vestibulocochlear nerve

 bends stereocilia on hair cells of cochlear basilar membrane, releases neurotransmitter (glutamate)

Hearing (auditory) 1. has Cochlea (Organ of Corti) for hearing  oval window vibrations travel through “perilymph”  & vibrate basilar membrane “endolymph” (fluid) in cochlea (Organ of Corti)  bends hair cells on basilar membrane, release neurotransmitter  which cause AP in sensory neurons Basilar Cochlear branch (cochlear branch of membrane of Vestibulovestibulocochlear nerve )  cochlear nerve > Signal goes to auditory cortex (temporal lobe) > Brain perceives sound

Oval window

Vestibular duct (perilymph) Cochlear duct (endolymph)

Louder sounds cause more vibrations

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Hearing (auditory) 2 (3) types of Hearing Loss 1. Conduction deafness (read Clinical App Pg 183 & ONLINE)  External or middle ear problem  Can be helped by hearing aids, which amplify sound waves Many possible causes: • Cerumen = • Otitis media =

• otosclerosis = hereditary disorder causing progressive deafness due to overgrowth of bone in the inner ear.

2. Sensory deafness  Most common: gradual loss of receptor (hair) cells  Or interuption in sound wave conduction between cochlea to auditory cortex Need cochlear implant

3. Central  Not common - damage to ascending neural pathways

Balance (vestibular apparatus) Inner Ear (bony labryinth) 1. Cochlea (Organ of Corti)

2. Otolith organs & Semicircular Canals

Semicircular canals

Fig 7.13

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Balance (vestibular apparatus) Otolith organs (utricle & saccule) have sensory receptors (stereocilia on maculae) that bend with linear movement and head position. 

Semicircular canals have ampulla with sensory receptors (stereocilia) for rotational acceleration 



Equilibrium also interpreted with input from vision & stretch receptors in muscle

Balance (vestibular apparatus) Otolith organs (utricle & saccule) have sensory receptors (stereocilia on maculae) that bend with linear movement and head position. 

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Balance (vestibular apparatus) Fluid-filled Semicircular canals have ampullae with sensory receptors (hair cells with stereocilia) that bend with head rotation. 

when rotate head, stereocilia of hair cells bend an AP forms, release neurotransmitter, which stimulates vestibular branch of vestibulocochlear nerve.

Balance (vestibular apparatus)

IF rotate head LEFT, hair cells bend RIGHT IF rotate head RIGHT, hair cells bend LEFT

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Balance (vestibular apparatus) • Vestibular information is integrated with info. from proprioceptors. • Vestibular information is used to: – control eye movement – maintain posture and balance – provide awareness of body position and acceleration. • Vestibular nerve fibers transmit information through the brainstem to cerebellum (fine motor control) and thalamus to motor cortex of frontal lobe.

Balance (vestibular apparatus) Cerebellar ataxia – read Clinical App Pg 138 = lack of coordination due to cerebellar damage

-

Info from vestibular apparatus ascends to cerebellum Cerebrum needed for fine motor control Damage leads to errors in speed, force, & direction of movement Person might move as if drunk (they’re not) Might reach to grab an object & miss Might show “intension tremors” (when trying to perform a task, limb trembles near completion of movement) https://www.youtube.com/watch?v=5eBwn22Bnio

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Balance (vestibular apparatus) “Vestibular Nystagmus” (vestibulo-ocular reflex) = as your head rotates in a body spin, then stop suddenly, your eyes slowly drift in opposite direction of spin, then eyes dart back to center. This due to fluid in semicircular canals still moving. Continues until fluid stops moving.

https://youtu.be/JaCpG0n6Jas

Review • • • •

Taste perception Smell perception Vision Hearing - causes of deafness • Balance – Vestibular Nystagmus (vestibulo-ocular reflex) – Cerebellar ataxia

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