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 Rod Neurons Pigmented epithelium Bipolar cell Amacrine cell Horizontal cell Optic nerve fibers Ganglion cell

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 Outer segment Disks Rod Inside of disk Cell body Synaptic terminal Rhodopsin Cytosol Retinal Opsin trans isomer LightEnzymes cis isomer

Phototransduction Retinal Changes Shape Opsin inactivated Retinal restored Rods contain the pigment rhodopsin, which changes shape when absorbing light

Photo-transduction cGMP levels high Transducin (G protein) Pigment epithelium cell Inactive rhodopsin (opsin and retinal) (a)In darkness, rhodopsin is inactive, cGMP is high, and ion channels are open. Disk Na + K+K+ Membrane potential in dark = -40mV Tonic release of neurotransmitter onto bipolar neurons Neurotransmitter decreases in proportion to amount of light. Membrane hyperpolarizes to -70 mV. Light Activated retinal Decreased cGMP Opsin (bleached pigment) Cascade (c)In the recovery phase, retinal recombines with opsin. Retinal converted to inactive form Retinal recombines with opsin to form rhodopsin. (b)Light bleaches rhodopsin. Opsin decreases cGMP, closes Na + channels, and hyperpolarizes the cell. Na + Na + channel closes K+K+ Activates transducin 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 Transducin Inactive rhodopsin Disk membrane Active rhodopsin Plasma membrane cGMP Na + GMP Na + Membrane potential (mV) EXTRACELLULAR FLUID Light Hyper- polarization Time –70 Dark 0 –40 Photo-transduction

Light Responses Rhodopsin active Na + channels closed Rod hyperpolarized Bipolar cell depolarized No glutamate released Dark Responses Rhodopsin inactive Na + channels open Rod depolarized Bipolar cell hyperpolarized Glutamate released 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 Convergence and Ganglion Cell Function

The Retina & Visual Acuity Light adapted eye has greatest visual acuity at the fovea - Photopic vision (cones) Dark adapted eye has least visual acuity at the fovea but has greater acuity in the parafoveal region Scotopic vision (rods) Fovea

Visual Integration / Pathway 2x binocular vision plus accessory structures Optic disk - blood supply optic nerve Retina 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 and flexible) Frequency producing maximum vibration Base (narrow and stiff) 16 kHz (high pitch) 8 kHz 4 kHz 2 kHz 1 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 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 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.