BIOL 2305
Peripheral Nervous System - Afferent Division - Part II
Special Senses
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
1
Vision
Cross-Section of the Eye
Organization of the Retina
Anatomy Review of the Eye
Outer coat of the eyeball is called the fibrous tunic consisting of the cornea—the avascular, transparent
coat that covers the iris-- and the sclera-- the white coat of dense connective tissue covering the
remainder of the eye.
Lens – nonvascular structure made up of proteins called crystallins. It lies behind the iris and is held in
position by suspensory ligaments.
The lens divides the interior of the eyeball into an anterior cavity and a vitreous chamber.
Anterior cavity (segment) is filled with a watery fluid called the aqueous humor and further
divided into
anterior chamber - lies behind the cornea and in front of the iris
posterior chamber - lies behind iris and in front of suspensory ligaments
Posterior chamber (segment) is filled with a jelly-like substance called vitreous humor
Ciliary body – thick anterior portion consisting of ciliary processes which secrete aqueous humor and
ciliary muscles which alters the shape of the lens for accommodation
Iris – colored portion suspended between the cornea and lens and attached to the ciliary body, it consists
of circular and radial muscles, the hole in the center of the iris is the pupil, and it functions to regulate the
amount of light entering the eye.
Choroid – highly pigmented (black-brown appearance) epithelium, vascularized posterior portion, absorbs
scattered light that might distort image.
The third layer or inner coat is the retina (nervous tunic) which lines the posterior three fourths of the
eyeball
Slightly off center in the retina is the optic disc (blind spot) where the optic nerve exits the eyeball.
2
The Structure of the Eye
Modifies light before it is detected by the rods & cones. The light
level is altered by changing the size of the pupil and light waves are
focused by changing the shape of the lens
Electromagnetic Spectrum
Extends from high energy short wavelength gamma rays to low
energy long wavelength radio waves
Human visible light lies only within the range ~380-750 nm
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 photoreceptor cells
Reflected light is translated into mental image
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
Bright light: constrict to ~ 1.5 mm
Dark: dilate to ~ 8 mm
Pupillary reflex controlled by the autonomic nervous system (ANS)
3
Image Projection
The image projected onto the retina is inverted or upside down
Visual processing in the brain reverses the image
Convex structure of both cornea and lens produce convergence of diverging light rays that reach eye
Refraction of Light
The eye refracts the entering light to focus the image on the retina.
Refraction by the cornea and lens form an image on the retina by bending light rays due to the light
passing through media of different densities at an angle.
Constriction of the pupil prevents light from entering the eye through the periphery.
The curved corneal surface contributes most of the refractive ability of the eye
Imagine parallel light rays striking the surface of a transparent lens.
If the lens is flat and its surface is perpendicular to the rays, the light passes through without
bending.
If the surface is not perpendicular, however, the light rays bend.
Parallel light rays striking a concave lens are refracted into a wider beam.
Parallel rays striking a convex lens bend inward and focus to a point (convex lenses
converge light waves).
4
Optics
Mechanism of Accommodation
Accommodation – the process by which the eye adjusts the shape of the lens to keep objects in
focus; lens is regulated by the ciliary muscle
Zonule of Zinn – ring of fibrous suspensory ligaments that connect the ciliary body to the crystalline
lens of the eye
5
Mechanism of Accommodation
When the ciliary muscle is relaxed the lens is flattened, when it contracts the lens takes more of a
spherical shape. An increase in curvature is needed for near vision
Common Visual Defects
Myopia – nearsighted, far light source is focused in front of retina and appears blurry
Hyperopia – farsighted, near objects are focused past the retina, appearing blurry
Presbyopia – loss of elasticity-- the lens can no longer assume the spherical shape required to
accommodate for near vision. Normally occurs in middle aged people
6
Retina
Photoreceptors – rods & cones are first to detect light stimulus
Bipolar cells – generate 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)
Retinal Layers
Photoreceptor layer – Rod and cone cells specialized to transduce light
Rods – predominate in peripheral areas
Cones – are densely concentrated in the fovea-- the center of the visual field
Bipolar cell layer – The cells in this layer establish pathways for nerve impulses.
Horizontal and Amacrine cells – modulate bipolar cells
Bipolar cells send graded potentials to ganglion cells which may depolarize and initiate action
potentials
Ganglion cell layer – neurons whose axons merge to form the optic nerve which passes through the
optic disc and optic tract to thalamus which passes the visual information to the occipital lobe
7
Photoreceptors
Rods
Light-sensitive
Don’t distinguish colors-- 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 – respond to selectively various wavelengths of light
Photoreceptors contain photopigments which absorb various wavelengths of light. They are made up
of two components:
Opsin – a protein that is an integral part of the disc membrane
Retinene – derivative of Vitamin A that is bound within the interior of the opsin molecule
Photoreceptors
8
Rods v. Cones
30 times more rods than cones (100 million to 3 million)
Cones used for color perception, have low sensitivity to light, being turned on only by bright light but
they have high visual acuity-- there is very little convergence of cones in retinal pathway
Rods used for shades of gray, have low acuity but high sensitivity (high convergence) so they respond
to dim light
Dark adaptation – phenomena involving switching between rods and cones
Dark & Light Adaptation
Sensitivity to light based on the amount of photopigment present in rods and cones. Breakdown of
photopigments during exposure to bright light tremendously decreases photoreceptor sensitivity
Dark adaptation: In dark, the photopigments broken down during light exposure are gradually
regenerated. Sensitivity gradually increases so that you begin to see in the darkened surroundings –
only highly sensitive rejuvenated rods are turned on by the dim light
Light adaptation: when transitioning from dark to high levels of light, the increased amount
photopigment produced during dark causes extreme sensitivity and images appear “bleached out.”
After photopigments broken down, normal contrast reappears
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.
9
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.
10
Phototransduction
Phototransduction
In darkness: Rods and cones release the inhibitory 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
11
Summary
1. In the absence of light the Na+ channels stay open due to the binding of second messenger cGMP
causing depolarization, keeping Ca+ channels open at the synaptic terminals which continually produce
NT glutamate which inhibits the bipolar cells.
2. When light hits retinene it changes shape and activates the photopigment which activates a G protein
called transducin which activates an enzyme that degrades cGMP which permits the chemically gated
Na+ to close which hyperpolarizes the receptor potential lead to closure of the Ca+ gates and reduction
of NT.
3. Photoreceptors hyperpolarize on light absorption! Opposite of normal receptor mechanism
4. Removal of inhibition has the same effect as direct excitation of the bipolar cell
5. Greater the illumination, the greater the removal of inhibition of the bipolar cell, the greater excitation of
these cells
Convergence and Ganglion Cell Function
The Retina & Visual Acuity
12
Visual Integration/Pathway
Afferent pathway:
Rods or cones  bipolar cells  ganglion cell axons  fibers from each medial retina chiasm  fibers from
both retinas  optic tracts terminate in midbrain and thalamus  optic radiation carries information from
thalamus to primary visual cortex
Vision Integration / Pathway
Optic nerve
Optic chiasm
Optic tract
Thalamus
Visual cortex
13
Sound and Hearing
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.
14
Middle Ear: Structures & Functions
Middle ear houses the ossicles (malleus, incus, and stapes) which transfer the vibratory movements
of the tympanic membrane to the fluid of the inner ear
The stapes is attached to the oval window, the entrance to the cochlea – the “hearing” portion of the
inner ear
As the oval window is pushed in and out, the resultant pressure produces wavelike movements in the
inner ear fluid at the same frequency as the original sound waves
Cochlear Anatomy
The cochlea is a coiled tubular system lying deep within the temporal bone. It is divided lengthwise into
three fluid filled compartments.
Cochlear duct (scala media) – the middle compartment filled with endolymph fluid
Scala vestibuli – upper compartment filled with perilymph fluid
Scala tympani – lower compartment also filled with perilymph fluid
The round window seals the scala tympani from the middle ear.
The vestibular membrane forms the ceiling of the cochlear duct and separates it from the scala
vestibule, the basilar membrane forms the floor of the cochlear duct – holds the organ of Corti
Sound and Hearing
Sound waves vibrate tympanic membrane
Auditory ossicles conduct and amplify the vibration; stapes (last ossicle) 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 of the Organ of Corti are pushed against the tectorial membrane
15
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  greater
bending of stereocilia  greater the amount of NT
released  increased frequency of APs produced
Summary
1. Sound waves enter the external auditory canal, strike the tympanic membrane vibrating the auditory
ossicles.
2. The ossicles mechanically amplify the vibrations and then strike the oval window
3. The oval window acts like a piston by creating fluid pressure waves in the perilymph of the cochlea.
4. The fluid pressure of perilymph in the scala vestibuli is transmitted to the scala tympani and eventually
to the round window.
5. The pressure waves push against the vestibular membrane causing the fluid pressure of the
endolymph inside the cochlear duct to increase and decrease.
6. The pressure fluctuations of the endolymph vibrates the basilar membrane which moves the hair cells
of the spiral organ against the tectorial membrane The bending of the microvilli produces receptor
potentials that lead to the generation of nerve impulses to vestibulocochlear nerve which passes to the
thalamus and then to the temporal lobes.
7. The mechanical deformation of the hairs alternately opens and closes mechanically gated ion channels
in the hair cells resulting in alternating depolarization and hyperpolarization changes in potential.
8. The inner hair cells synapse on afferent neurons of the auditory nerve.
9. Bending of the hair cells in one direction causes depolarization which increases the rate on NT release,
which increases the rate of firing of afferent neurons
10. Hyperpolarization causes less NT to be released when the hair cells are displaced in the opposite
direction
16
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
17
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
Detects position and motion of the head
Consists of the semicircular canals and the otolith organs
18
Vestibular Apparatus
Cristae are receptors within ampullae that detect rotational acceleration
Maculae are receptors within utricle and saccule (the otolith organs) that detect linear acceleration
and gravity
Vestibular Apparatus: Semicircular Canals
Semicircular canals – provide information about rotational acceleration
Project in 3 different planes
19
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
Endolymph provides inertia so that the sensory processes will bend in direction opposite to the angular
acceleration
Rotational Forces in the Cristae
Semicircular Canals Summary
1. Semicircular canals detect rotational or angular acceleration- change of movement. It consists of three
circular canals arranged in planes that lie at right angles to each other, they are filled with endolymph
2. The receptive hair cells are located in the ampulla. The hair cell holds 20-50 microvilli (stereocilia) and
one cilium – the kinocilium which are embedded in cupula –a caplike gelatinous layer
3. When the head is set in motion the cupula moves through the fluid which does not initially move due to
inertia, causing the cupula to sway or bend.
4. At constant speed, the fluid eventually catches up with the ampulla and the ampulla returns to resting
position.
5. When motion of the head stops, the ampulla stops, but the fluid lags behind causing the ampulla to be
bent in the opposite direction (similar to the motion of seaweed during the tide)
6. Each hair cell is oriented so that it depolarizes when the stereocilia are bent toward the kinocilium,
bending in the opposite direction hyperpolarizes. The hair cells synapse on the vestibular nerve
20
Vestibular Apparatus: Otolith Organs
Maculae are receptors within utricle and saccule (the otolith organs) that detect linear acceleration,
head position, and gravity
Vestibular Apparatus: 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
21
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
Summary of Otolith Organs
1. Provides information on the position of the head relative to gravity and linear acceleration
2. The otolith organs, utricle and saccule are situated between the semicircular canals and the cochlea.
3. The hairs of the receptors in these organs also protrude into an overlying gelatinous sheet whose
movement displaces the hairs and results in changes in hair cell potential
4. Tiny crystals of calcium carbonate (otoliths) are suspended in the layer making it heavier and giving it
more inertia than the surrounding fluid
5. Utricle hairs are oriented vertically and the saccule hairs are lined up horizontally
6. Utricle hairs - when the head is tilted in any direction other than vertical, the hairs are bent in the
direction of the tilt because of the gravitational force pulling on the top heavy gelatinous layer
a. The utricle hairs are also displaced by any change in linear motion because the top heavy layer
labs behind the endolymph and hair cells causing the hairs to bend in the opposite direction of
head movement.
7. The saccule functions similarly except that it responds to the tilting of the head away from a horizontal
position (getting out of bed)
22
Taste (Gustation)
Taste
Taste Receptors – clustered in taste buds (~10,000)
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
Each taste bud has a taste pore from which
fluid in the mouth can contact the receptor
cells
The receptor cells are modified epithelium
with many surface folds (mircovilli) to increase
surface area. Epithelial cells differentiate into
supporting cells and then into receptor cells.
Taste receptors have a life span of about ten
days
Taste cells are not neurons, but depolarize upon stimulation and if reach threshold, release NT that
stimulate sensory neurons
23
Taste (Gustation)
1. Each taste bud contains taste cells responsive to each of the different taste categories.
2. A given sensory neuron may be stimulated by more than 1 taste cell in a # of different taste buds
3. One sensory fiber may not transmit information specific for only 1 category of taste
4. Brain interprets the pattern of stimulation with the sense of smell; so that we perceive complex
tastes
5. The microvilli plasma membranes contain receptor sites that bind selective chemical molecules
(tastants). Binding of a tastant with the receptor site produces a depolarizing receptor potential
6. Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of
the mouth.
7. The nerves relay to the brain stem and thalamus and on the cortical gustatory area
Taste Receptor Distribution
Salty:
Direct entry of Na+ is responsible for depolarization
Na+ enters through Na+ channel and depolarizes the taste cell, resulting in exocytosis of the
neurotransmitter serotonin which excites the primary gustatory neuron
Sour:
Transduction mechanisms remain uncertain
Ultimately, H+ -mediated depolarization of the taste cell resulting in serotonin release
Sweet, Bitter, and Umami:
All release ATP as NT which acts both on sensory neurons
Sweet (sugars) – glucose and others bind with a taste receptor that activates a G protein
Gustducin which turns on cAMP second messenger pathway which ultimately blocks K+
channels, leading to depolarization and triggering release of ATP
Bitter – alkaloids, poisons also activate a G protein Gustducin which sets off a second
messenger pathway, triggering release of ATP
Umami – triggered by the amino acid glutamate that activates a G protein Gustducin, ultimately
triggering release of ATP
24
Taste Transduction
Summary of Taste Transduction
25
Smell (Olfaction)
Smell
Olfactory epithelium with olfactory receptors, supporting cells, basal cells
Basal cells are precursors to new olfactory receptors which are replaced every two months
Olfactory receptors are modified neurons
The axons of the olfactory receptor cell collectively form the olfactory nerve
Surfaces are coated with secretions from olfactory glands
Olfactory reception involves detecting dissolved chemicals as they interact with odorant binding
proteins
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
26
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
Olfaction Overview
1. The receptor portion of an olfactory receptor cell consists of an enlarged knob bearing several long
cilia that extend like a tassle to the surface of the mucosa.
2. The cilia contain binding sites for the attachment of odorants – molecules that can be smelled, they
must sufficiently volatile (vaporized) and water soluble to dissolve in the mucosa
3. Human nose contains 5 million receptors of about 1000 different types. Each receptor responds to
only one discrete component of an odor- the odor is dissected. A given receptor can respond to
particular odor component shared in common by different scents
4. Binding of an appropriate scent signal to an olfactory receptor activates a G protein triggering cAMP
cascade leading to Na + gates to open bringing about a depolarizing receptor potential that can
generate and action potential in the afferent fiber.
5. The afferent fiber synapse in the olfactory bulb, each bulb is lined by neural junction called
glomeruli (little balls)
6. The glomeruli serve as the first relay station in the brain for processing olfactory information and
play a key role in organizing scent perception
7. Within each glomerulus, the terminals of receptors cells carrying information about a specific scent
component synapse with the next cells in the olfactory pathway, the mitral cells which refine the
smell signals and relay them to the brain – limbic system and the thalamic-cortical route
8. Although the olfactory system is sensitive and highly discriminative, it is also quickly adaptive
27