chapter 43: special senses

How sensory systems work

Special sense organs have sensory receptors and supporting cell types

Eye, taste buds, ear

Sensory processing pathway

Stimulus (modality) to which receptor responds

ex: Sweet, light, sound, pain, etc

Receptor interacts with stimulus - Most sensitive to one modality

Interaction stimulates receptor

A conformational change occurs

Receptor physically stimulated - pressure

Nervous system transduction

initiation of electrical signal at effector

transmission: along afferent pathway to CNS

projection: Information arrives at a specific part of brain

Interpretation and perception [brain processes information]: Level of mental awareness can impact this

transmission: efferent pathways to effector

response

location of sensory receptors

On end of neuron (free nerve ending; unencapsulated) - pain receptors

On end of nerve but encapsulated - pain receptors

Separate (standalone) structure

Graded [receptor] potentials

amplitude of signal for stimulating receptor changes with

stimulus strength

Determines number of action potentials initiated by axon

Amplitude of action potential does not change

Pacinian corpuscle

vibrations and deep pressure, somatic mechanoreceptor

Exteroceptors

response to external stimuli

Interoceptors

response to internal stimuli

Visceral

associated with internal organs

Somatic

skin (touch, pressure, temperature), muscle

Proprioceptors

muscles, tendons and joints

Indicate body orientation and muscle/joint position

Mechanoreceptors

respond to mechanical change; touch, pressure, gravity, stretching, movement

Body position relative to gravity

Internal organs (fullness, blood pressure, lung inflation)

Chemoreceptors

chemical compounds, pH changes

Photoreceptors

light energy (photons)

Thermoreceptors

temperature (external and internal)

free nerve endings in skin and tongue

hypothalamus detects internal changes

pain receptors (extreme temps)

Electroreceptors

sense electrical potentials (currents)

earth's magnetic field

Locate prey or for orientation/navigation

Nociceptors

pain

strong tactile mechanical stimuli (pinching, cuts, hitting

Temperature extremes

Certain chemicals

emotional response to pain

Thalamus and brain stem send info to limbic system

interneurons

release endorphins and enkephalins

Bind opiate receptors

Inhibit release of substance P so pain signal stops

Frequency code

frequency of nerve impulses received by brain change

Population code

more receptors responding

Equals more impulses to CNS

sensory adaptation

occurs when receptor response rate decreases even if stimulus continues at

same intensity

Receptor sensitivity decreases

Smaller receptor potentials

Therefore, frequency of neuron action potentials decreases

Changes occur at synapse with receptor (amount of neurotransmitter released)

Result: way top ignore non-critical stimuli (some odors) but still be aware of critical stimuli

tonic receptors

slow adapting

Response continues as long as stimulus is there

response through the whole thing

phasic receptors

fast adapting

No response during center portion of stimulus

Phase: step - response at beginning and response at the end

Mechanoreceptors in skin

Free nerve endings [unencapsulated]

Detect touch, pressure, vibration, and pain

Meissner corpuscles [adapt quickly]

light touch and vibration

Ruffini endings [adapt slowly]

Heavy, continuous pressure,

skin stretching

High temperature

Pacinian corpuscles [discussed above]

Mechanoreceptors in muscle

Help maintain posture and monitor muscle activity

Continuously respond to tension and movement

Muscle spindles [muscle length]

Detect muscle movement

golgi tendon organs

force of contraction

Monitor tension in contracting muscles and tendons that attach muscle to bone

hair cells in invertebrates

Statocyst located in an infolding of epidermis

Receptors (hair cells) surround a centrally located statolith

body position to gravity receptors

Activity: gravity pulls on statoliths, Sensory hairs displace mechanically, Initiation of receptor potentials

hair cells in vertebrates

maintenance of body position, equilibrium, hearing, motion detection

structure: stereocilia (hair-like projections) extend into gelatinous cupula

vertebrate lateral line system

Structure: cupula with hair cells embedded located in canals along body surface, Detect movement/currents in water

how: Water movement displace cupula and hair cells bend, Bending towards shorter hair hyperpolarizes hair cell, decreased neurotransmitter release,

Bending towards longer hair depolarizes hair cell, increased neurotransmitter release

outer ear

composed of pinna (outer skin portion of ear), external auditory meatus (ear canal), and tympanic membrane (eardrum)

middle ear

includes ear bones (malleus, incus, stapes), auditory tube, muscles ad joints

middle ear function

regulating sounds amplification

Primarily amplify vibrations passing from tympanic membrane through oval window to perilymph in vestibular canal

Changes in muscle tension on bones

Auditory tube balances pressure and drains middle ear

inner ear functions

hearing and establishing equilibrium and balance

inner ear structures

cochlea, utricle and saccule, semicircular canal, vestibulocochlear nerve, membranous and bony labyrinth, oval and round windows

fluid in the inner ear

endolymph (semicircular canals and scala media) and perilymph (vestibular and tympanic canals)

inner ear: vestibular apparatus

Structure of utricle and saccule: hair cells covered by gelatinous cupula embedded with calcium

carbonate and protein stones (otoliths)

Orientation, response, function

utrice

horizontal acceleration/deceleration

Left to right tilt

saccule

vertical acceleration/deceleration

front to back tilt

inner ear: semicircular canals

orientation: each in different planes (X, Y, Z)

structure includes an enlarged end (ampulla) that houses the

sensory structure with the receptors (crista)

function: detecting angular acceleration

CN 8

inner ear: cochlea

structure: spiral tube that connects to middle ear via oval window and ends at round window

the canals in the cochlea

Scala vestibuli: entery point - where you come in

Scala media: cochlear duct - contains endolymph

Scala tympani: bottom

organ of corti

[mechanoreceptor] in scala media

uses hair cells to detect sound (pressure) waves

structure: hair cells for receptors, basilar membrane and tectorial membrane

how organ of corti detects sound

stapes presses on oval window - bulges inward

wave motion set up in vestibular fluid

initiates waves in tympani

basilar membrane and organ of corti move up and down

receptors (hairs) contact tectorial membrane and flex

receptor potential and possibly action potential initiated

distinguishing pitch

Basilar membrane has different thickness and stiffness its length

pitch

depends on wave frequency (hertz or Hz)

loudness

depends on wave amplitude

hair cells more intensely stimulated

cochlear nerve transmits more impulses per second

tone

depends on harmonics produced

Chemoreceptors: gustation - taste

Modalities [recognized taste]

(sweet, sour, salt, bitter, umami [glutamate])

Receptor location on tongue

Often localized to specific region of tongue

Also surface of soft palate and upper esophagus

cranial nerves involved in taste

7, 9, 10

olfaction (smell)

occurs in olfactory epithelium

structure: Bipolar cells: one axon extends down to epithelial surface (bind odorant)

Cribriform plate of ethmoid bone: base ethmoid bone, axon pass through (cranial nerve 1) connect olfactory bulb

Olfactory bulb: anterior end of olfactory tract

Olfactory tract: transmits to olfactory cortex

smell: receptor activation

Process: odorant binds a receptor on a cilium of olfactory receptor cell

G protein is activated

cAMP synthesized

ligand-gated Na channels open

Convergence: same receptors converge in olfactory bulb

photoreceptors

Function: photopigments (e.g., opsins) absorb light energy

phylum: most metazoans

eye spots

light sensitive structures - often non-image forming

Cnidarians, platyhelminthes, some arthropods, some mollusks, echinoderms

light sensitive structure - image forming

Lens: concentrates light and focus image on photoreceptors

Brain integration: interprets image coming in along optic tract

structures with lenses

Compound eye: insects

Camera eye

Direct Camera eye: receptors face incoming light (cephalopods)

Indirect Camera eye: receptors face away from incoming light (humans)

ommatidium

Biconcave lens and crystalline cone: focus light onto photoreceptors

Retinular cells: have light sensitive membrane with rhodopsin

Sensitivity: good for movement, but not determining shape

Optic nerve: formed by nerves from receptor cells

Final image: mosaic

major tissue layers in the eye

Sclera: outer layer, protects protection and rigidity

Choroid: middle layer, pigmented

Retina: inner layer, Nervous layer, Pigmented layer

cornea

thinner, transparent sclera, initial focusing

iris

smooth muscle, regulate pupil size and light entry

pupil

center "hole"

lens

transparent, elastic, focus image retina

as it gets older, lens gets stiffer

ciliary body

ciliary processes secrete fluid

ciliary muscle changes lens shape

aqueous humor

anterior cavity between cornea and lens

vitreous humor

posterior cavity between lens and retina

fovea

concentration of cones (colored vision), keenest vision (best)

more rods than cones

blind spot

no receptors, optic nerve exists (CN 2)

accommodation

change in lens shape to focus the image on retina

structures involved in accommodation

ciliary muscles - circular and attached to suspensory ligaments

suspensory ligaments - attached to lens

process of accommodation

Ciliary muscle contraction (sympathetic): Releases tension on ligaments - lens thickens

Ciliary muscle relaxation (parasympathetic): Put tension on ligaments - lens thins

emmetropia

normal vision

myopia

near sighted, image behind retina

Hyperopia

far sighted, image behind the retina

astigmatism

irregularities in cornea or lens

presbyopia

loss of near vision as accommodation decreased - old age

nervous control of pupil

Sympathetic fibers effect on pupil dilation

Parasympathetic fibers effect on pupil dilation

retina

Receptor portion of retina actually extension of CNS

During development receptors back out of CNS so are facing backward

contains rods and cones

rods

Location: toward margin of retina

Shape: tall, thin

Activity: dim light

Photopigment: rhodopsin

Image: grey and black and white, detect movement

Number: 20 times more than cones (100 million/eye)

cones

Location: concentrated in fovea

Activity: color vision, response to higher intensity of light (daylight vs midnight), perceive fine detail

Photopigments: opsin (photopigment) and retinene (derived from Vitamin. A)

Cones: different opsin in each type of cones

cone receptor types

sensitive to different wavelets (colors) of light

color blindness

Deficiency of one or more types of cones

Usually an inherited X-linked condition

Phototransduction in light

hyperpolarize , reduce neurotransmitter release

1. isomerization of retinal activates enzyme that breaks down cGMP

2. cGMP gates Na channels close

3. hyperpolarizing receptor potential

4. glutamate release turned off, which excites bipolar cell

phototransduction in dark

1. opsin binds to retinal in the cis form

2. cyclic GMP opens nonspecific channels that permit passage of Na and other cation into rod cell

3. depolarization with more neurotransmitter release compared to light exposed

photoreceptor adaptation

Adaptation involves rods, cones, and other cells of the retina

light adaptation

initial receptor sensitivity is to dark

Moving from dark to light

"Over stimulation" of rods and cones

Adaptation occurs as rod function is inhibited and cone function favored

Occurs in minutes

dark adaptation

initial receptors sensitivity is to light

Moving from light to dark

Cones cease functioning in low light

Rods initially not functional - rhodopsin has been bleached

Rhodopsin regenerates in dark (can take up to an hour or loner till fully functional)

binocular vision

humans; information enters both eyes at the same time, important in judging distance and depth perception

monocular vision

rabbits and horses; eyes farther apart; wider visual field

optic nerve

bundle of nerve fibers from each eye

optic chiasm

crossing over

optic tract

combined visual pathways from both eyes