PNS – Afferent Division Sensory Physiology

Part 2

Special Senses – External Stimuli

Figure 10-4: Sensory pathways

• Vision• Hearing• Taste• Smell• Equilibrium

Cross-Section of the Eye

Figure 17.6b, c

Organization of the Retina

• Light enters the eye through the pupil, diameter of pupil modulates light

• Shape of lens focuses the light on the retina• Retinal rods and cones are photoreceptors• Reflected light translated into mental image

Vision

Figure 10-36: Photoreceptors in the fovea

Pupils• Bright light they constrict to ~ 1.5 mm• Dark they dilate to ~ 8 mm. • Controlled by the autonomic nervous system,

pupillary reflex

Image Projection•The image projected onto the retina is inverted or upside down. Visual processing in the brain reverses the image

Image Projection• Convex structures of eye produce

convergence of diverging light rays that reach eye

Figure 10-30a

Refraction of Light

Figure 10-31a

Optics

Figure 10-31b

Optics

Figure 10-32a

Mechanism of Accommodation• Accommodation is the process by which the eye adjusts the shape

of the lens to keep objects in focus

Mechanism of Accommodation

Figure 10-32b

Figure 10-33a

Common Visual Defects

Retina• Photoreceptors - rods and cones detect light stimulus• Bipolar - generate APs• Amacrine & Horizontal cells – local integration of APs• Ganglion cells converge form optic nerve

Cone

Photoreceptors

Retina

RodNeurons

Pigmentedepithelium

Bipolarcell

Amacrinecell Horizontal

cellOpticnervefibers

Ganglioncell

Photoreceptors• Rods - light-sensitive but don’t distinguish colors; monochromatic, night vision• Cones - Three types; red, green, & blue, distinguish colors but are not as sensitive,

high acuity day vision

Photo-transduction• Each rod or cone contains visual pigments consisting of a light-

absorbing molecule called retinal bonded to a protein called opsin

Outersegment

Disks

Rod

Insideof disk

Cell body

Synapticterminal

Rhodopsin

Cytosol

Retinal

Opsin trans isomer

Light Enzymes

cis isomer

Phototransduction

Retinal Changes Shape

Opsin inactivated

Retinal restored

• Rods contain the pigment rhodopsin, which changes shape when absorbing light

Photo-transduction

cGMPlevels high

Transducin(G protein)

Pigment epithelium cell

Inactiverhodopsin

(opsin and retinal)

(a) In darkness, rhodopsin isinactive, cGMP is high, and ion channels are open.

Disk

Na+

K+

Membrane potential in dark = -40mV

Tonic release of neurotransmitteronto bipolar neurons

Neurotransmitter decreases in proportionto amount of light.

Membranehyperpolarizes

to -70 mV.

Light

Activatedretinal

DecreasedcGMP

Opsin (bleachedpigment)

Cascade

(c) In the recovery phase, retinal recombines with opsin.

Retinal converted to inactive form

Retinal recombineswith opsin to

form rhodopsin.

(b) Light bleaches rhodopsin. Opsin decreases cGMP, closes Na+

channels, and hyperpolarizes the cell.

Na+

Na+ channelcloses

K+

Activatestransducin

• Photons "bleach" opsin, retinal changes shape and released, transduction cascade, decreased cGMP, Na+ channel closes, K+ opens , hyperpolarization reduces NT release

Light INSIDE OF DISK

CYTOSOL

PDE

TransducinInactiverhodopsin

Diskmembrane

Activerhodopsin

Plasmamembrane

cGMP

Na+

GMP

Na+

Membranepotential (mV)

EXTRACELLULARFLUID

Light

Hyper-polarization

Time

–70

Dark0

–40

Photo-transduction

Light Responses

Rhodopsin active

Na+ channels closed

Rod hyperpolarized

Bipolar celldepolarized

No glutamatereleased

Dark Responses

Rhodopsin inactive

Na+ channels open

Rod depolarized

Bipolar cell hyperpolarized

Glutamatereleased

• In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons called bipolar cells

• Bipolar cells are hyperpolarized

• In the light, rods and cones hyperpolarize, shutting off release of glutamate

• The bipolar cells are then depolarized

Photo-transduction

Figure 17.18

Convergence and Ganglion Cell Function

The Retina & Visual Acuity

Light adapted eye has greatestvisual acuity at the fovea -Photopic vision (cones)

Dark adapted eye has leastvisual acuity at the fovea buthas greater acuity inthe parafoveal regionScotopic vision (rods)

Fovea

Visual Integration / Pathway

2x binocular visionplus accessory

structuresOptic disk - blood supply

optic nerveRetina

Retinal cells

• Optic nerve

• Optic chiasm

• Optic tract

• Thalamus

• Visual cortex

Vision Integration / Pathway

Figure 10-29b, c: Neural pathways for vision and the papillary reflex

The Ear / Auditory Physiology

External Ear Structures & Functions• Pinna—Collects sound waves and channels them into the external

auditory canal.

• External Auditory Canal—Directs the sound waves toward the tympanic membrane.

• Tympanic membrane—Receives the sound waves and transmits the vibration to the ossicles of the middle ear.

Figure 17.28a

Sound and Hearing• Sound waves travel toward tympanic membrane, which vibrates

• Auditory ossicles conduct the vibration into the inner ear

• Movement at the oval window applies pressure to the perilymph of the cochlear duct

• Pressure waves move through vestibular membrane through endolymph to distort basilar membrane

• Hair cells of the Organ of Corti are pushed against the tectorial membrane

Cochlea and Organ of Corti

Organ of Corti• Ion channels open, depolarizing the hair cells, releasing

glutamate that stimulates a sensory neuron.• Greater displacement of basilar membrane, bending of

stereocilia; the greater the amount of NT released.• Increases frequency of APs produced.

Figure 10-21a

Signal Transduction in Hair Cells

• The apical hair cell is modified into stereocilia

Pitch Discrimination• Different frequencies of vibrations (compression

waves) in cochlea stimulate different areas of Organ of Corti

• Displacement of basilar membrane results in pitch discrimination.

Cochlea(uncoiled) Basilar

membrane Apex(wide andflexible)

Frequencyproducingmaximum vibrationBase

(narrow and stiff)

16 kHz(high pitch)

8 kHz4 kHz

2 kHz1 kHz

500 Hz (low pitch)

Sensory Coding for Pitch• Waves in basilar membrane reach a peak at different regions

depending upon pitch of sound.• Sounds of higher frequency cause maximum vibrations of

basilar membrane.

Vestibular Apparatus

Figure 10-23a, b: ANATOMY SUMMARY: Vestibular Apparatus

Vestibular apparatus provides information about movement and position in space

Vestibular Apparatus• Cristae are receptors within ampullae that detect rotational acceleration• Maculae are receptors within utricle and saccule that detect linear

acceleration and gravity

Vestibular Apparatus: Semicircular Canals

• Provide information about rotational acceleration.– Project in 3 different planes.

Figure 10-23b

Semicircular Canals• At the base of the semicircular duct is the crista ampullaris, where

sensory hair cells are located.– Hair cell processes are embedded in the cupula.

Semicircular Canals• Endolymph provides inertia so that the sensory processes will

bend in direction opposite to the angular acceleration.

Figure 10-24

Rotational Forces in the Cristae

Vestibular Apparatus• Cristae are receptors within ampullae that detect rotational acceleration• Maculae are receptors within utricle and saccule that detect linear

acceleration and gravity

Figure 10-25a

Otolith Organs: Maculae• The otolith organs sense linear acceleration and head position

Figure 10-25a

Otolith Organs

Stereocilia and Kinocilium

• When stereocilia bend toward kinocilium; membrane depolarizes, and releases NT

• When bends away from kinocilium hyperpolarization occurs

• Frequency of APs carries information about movement

Maculae of the Utricle and Saccule

• Utricle:– More sensitive to horizontal acceleration.

• During forward acceleration, otolithic membrane lags behind hair cells, so hairs pushed backward.

• Saccule:– More sensitive to vertical acceleration.

• Hairs pushed upward when person descends.

Taste (Gustation)• Taste Receptors - Clustered in taste buds• Associated with lingual papillae• Taste buds

– Contain basal cells which appear to be stem cells– Gustatory cells extend taste hairs through a narrow taste pore

Taste (Gustation)• Epithelial cell receptors

clustered in barrel-shaped taste buds

• Each taste bud consists of 50-100 specialized epithelial cells.

• Taste cells are not neurons, but depolarize upon stimulation and if reach threshold, release NT that stimulate sensory neurons.

Taste (continued)

• Each taste bud contains taste cells responsive to each of the different taste categories.

• A given sensory neuron may be stimulated by more than 1 taste cell in # of different taste buds

• One sensory fiber may not transmit information specific for only 1 category of taste

• Brain interprets the pattern of stimulation with the sense of smell; so that we perceive complex tastes

Taste Receptor Distribution• Salty:

– Na+ passes through channels, activates specific receptor cells, depolarizing the cells, and releasing NT.

• Sour:– Presence of H+ passes

through the channel, opens Ca+ channels

Taste Receptor Distribution (continued)

• Sweet and bitter:– Mediated by

receptors coupled to G-protein (gustducin).

Summary of Taste Transduction

Figure 10-16

• Olfactory epithelium with olfactory receptors, supporting cells, basal cells

• Olfactory receptors are modified neurons• Surfaces are coated with secretions from olfactory glands• Olfactory reception involves detecting dissolved chemicals as

they interact with odorant binding proteins

Smell (Olfaction)

Olfactory Receptors• Bipolar sensory neurons located within olfactory epithelium

– Dendrite projects into nasal cavity, terminates in cilia– Axon projects directly up into olfactory bulb of cerebrum– Olfactory bulb projects to olfactory cortex, hippocampus, and

amygdaloid nuclei

Olfaction• Neuronal glomerulus receives input from 1 type of olfactory receptor

• Odorant molecules bind to receptors and act through G-proteins to increase cAMP.

– Open membrane channels, and cause generator potential; which stimulate the production of APs.

– Up to 50 G-proteins may be associated with a single receptor protein. – G-proteins activate many G- subunits - amplifies response.