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BIOL 2305
Peripheral Nervous System - Afferent Division - Part II
Peripheral Nervous System – Afferent – Special Senses
Peripheral Nervous System (PNS) – all neural structures outside the brain and spinal cord
Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings
PNS provides links to and from the external environment
Special Senses – External Stimuli
Vision
Hearing
Taste
Smell
Equilibrium
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Cross-Section of the Eye
Organization of the Retina
Vision Overview
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 is translated into mental image
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Fovea centralis
Fovea centralis in macula lutea contains high density of cones (responsible for sharp central vision)
Retina is a neural layer composed of:
Ganglion cells
Bipolar cells
Photoreceptor cells
Pigment epithelium contains pigment cells that absorb
excess light
Pupils
In bright light, they constrict to ~ 1.5 mm
In the dark, they dilate to ~ 8 mm
Controlled by the autonomic nervous system, pupillary reflex
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Image Projection
The image projected onto the retina is inverted or upside down.
Visual processing in the brain reverses the image
Convex structures of eye produce convergence of diverging light rays that reach eye
Refraction of Light
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Optics
Mechanism of Accommodation
Accommodation – the process by which the eye adjusts the shape of the lens to keep objects in focus
Zonule of Zinn - ring of fibrous suspensory ligaments that connect the ciliary body to the crystalline
lens of the eye
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Mechanism of Accommodation
Common Visual Defects
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Retina
Photoreceptors – rods & cones are first to detect light stimulus
Bipolar – generates action potentials
Amacrine & Horizontal cells – integrate and regulate input from multiple photoreceptor cells
Ganglion cells’ axons converge to form the optic nerve (cranial nerve II) and exit the eye, creating the
optic disc (blind spot)
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Photoreceptors
Rods
Light-sensitive
Don’t distinguish colors- are monochromatic, night vision
High sensitivity (low light, night vision)
Low Acuity
Cones
Three types:
red cones
green cones
blue cones
Distinguish colors
Lower sensitivity (daylight, day vision)
High acuity
Photoreceptors
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Phototransduction
Rods contain the pigment rhodopsin, which changes shape when absorbing light
Rods: Rhodopsin = opsin + retinal
Cones contain a close analog called iodopsin
Cones: Iodopsin = photopsin + retinal
Phototransduction
Each rod contains visual pigments consisting of a light-absorbing molecule called retinal bonded to a
protein called opsin, forming rhodopsin.
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Phototransduction
Absence of light: retinal is bound to opsin
Presence of light: retinal alters conformation from cis (bent) to trans (straight) isomer
Trans retinal isomer no longer binds to opsin and is released from the pigment molecule in the process
known as bleaching.
Bleaching of opsin activates a G protein called Transducin, which activates Phosphodiesterase (PDE),
which breaks down cGMP (cyclic GMP) into GMP (guanosine monophosphate), inactivating it.
With less cGMP, Na+ channels begin to close, causing the photoreceptor cell to hyperpolarize.
Hyperpolarization of the photoreceptor (rod or cone) causes a decrease in the release of the inhibitory
neurotransmitter glutamate.
Bipolar cells are now allowed to depolarize, sending action potentials to ganglion cells whose axons
converge as the optic nerve and exit the posterior of the eye at the optic disc.
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Phototransduction
Phototransduction
In the dark, rods and cones release the neurotransmitter glutamate into synapses with neurons called
bipolar cells
Bipolar cells become hyperpolarized and are unable to “fire”
In light, rods and cones hyperpolarize, shutting off release of the inhibitory neurotransmitter glutamate.
The bipolar cells are allowed to self-depolarized
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Convergence and Ganglion Cell Function
The Retina & Visual Acuity
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Visual Integration/Pathway
Vision Integration / Pathway
Optic nerve
Optic chiasm
Optic tract
Thalamus
Visual cortex
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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.
Sound and Hearing
Sound waves vibrate tympanic membrane
Auditory ossicles conduct and amplify the vibration, stapes moves oval window
Movement at the oval window applies pressure on the perilymph of the vestibular duct
Pressure waves move through vestibular membrane through endolymph to distort basilar membrane
Hair cells (stereocilia) of the Organ of Corti are pushed against the tectorial membrane
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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
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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
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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
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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.
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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.
Rotational Forces in the Cristae
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Vestibular Apparatus
Cristae are receptors within ampullae that detect rotational acceleration
Maculae are receptors within utricle and saccule that detect linear acceleration and gravity
Otolith Organs: Maculae
The otolith organs sense linear acceleration and head position
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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.
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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 barrelshaped 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 (Gustation)
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
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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 Ca2+ channels
Sweet and Bitter:
Mediated by receptors coupled to G-protein (gustducin).
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Summary of Taste Transduction
Smell (Olfaction)
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
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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
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