Animal Sensory Systems

sensory receptor cells

cells in sensory system that transduce external stimuli into changes in membrane potentials

two types of sensory receptor cells

- specialized neurons (the receptor cell is also a neuron)

- specialized sensory cells which synapse with a neuron (the receptor cell secretes neurotransmitters to stimulate changes in membrane potential in the synapsed neuron)

do sensory receptor cells depolarize or hyperpolarize?

both

five types of specialized sensory receptors

mechanoreceptors, chemoreceptors, photoreceptors, nociceptors, thermoreceptors

Mechanoreceptors:

respond to physical deformation of the cell membrane from mechanical energy or pressure, including touch, stretch, motion, or sound

Chemoreceptors:

respond to specific molecules, often dissolved in a specific medium (such as saliva or mucus), or airborne molecules

Photorecetpors:

respond to radiant energy (visible light in most vertebrates; visible as well as UV light in many insects)

Nociceptors:

respond to "noxious" stimuli, or essentially anything that causes tissue damage

Thermoreceptors:

respond to heat or cold

five special senses

olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing

somatosensation

respond to stimuli like temperature, pain, pressure, and vibration.

The intensity or degree of a stimulus is often encoded in three different ways:

- rate/frequency of action potentials produced by sensory receptor

- number of receptors activated

- specific receptors are activated

how is sense of touch detected?

mechanoreceptors in skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system

does touch involve one or multiple stimuli and receptors?

multiple

mechanoreceptors on surface of skin

sensitive to lighter toucher and can precisely localize gentle touch

mechanoreceptors deeper in skin

activated by stronger pressure and not as highly sensitive

how is firmer touch detected?

mechanoreceptors deeper in skin and activation of more mechanoreceptors which induce more frequent action potentials

auditory stimuli

sound waves, which are mechanical pressure waves that move through a medium, such as air or water

four main characteristics of a sound wave

frequency, wavelength, period, and amplitude

what characteristics of sound waves affect hearing?

frequency, wavelength, amplitude

frequency

number of waves per unit of time, which is heard as pitch

relationship between frequency, pitch, and wavelength

high frequency = high pitch = shorter wavelength

amplitude

dimension of a wave from peak to trough, in sound is heard as volume

how does amplitude affect sound?

louder sound = greater amplitude

outer ear

- Sound waves are collected by the external, cartilaginous part of the ear

- Sound waves then travel through the auditory canal and cause vibration of the ear drum (tympanic membrane)

middle ear

The eardrum transmits sound to the middle ear by vibrating the ossicles

ossicles

three small bones of the middle ear which collect force and amplify sounds

what animals have ossicles?

mammals

inner ear

- The ossicles transmit the vibrations to a thin membrane called the oval window, which is the outermost structure of the inner ear.

- vibrations in oval window create pressure waves in fluid inside the cochlea

- basilar membrane in cochlea tranduces sound waves into action potentials using mechanoreceptors called hair cells

- basilar membrane vibrates and press hair cells against tectorial membrane which bends hair cells and initiates action potentials in afferent neurons that communicate sounds stimuli to the brain

cochlea

cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal

basilar membrane

flexible membrane that runs the length of the cochlea

tectorial membrane

A membrane located above the basilar membrane; contains hair cells that bend in response to sound

how does the basilar membrane flexibility change?

thicker, stiffer, and narrower at one end of cochlea and thinner, floppier, and broader at the other end

how does frequency affect basilar membrane vibration?

higher frequency vibrates stiffer region and lower frequency vibrates flexible region

how is pitch detected?

region of the basilar membrane vibrates in response to a sound

where is the site of transduction from sound waves to action potentials?

organ of corti

organ of corti process

hair cells are held in place above the basilar membrane with their hair-like stereocilia embedded in the tectorial membrane above them

steps of sound wave flexes in basilar membrane

1. hair cells on basilar membrane flexed against tectorial membrane

2. stereocilia bends which causes potassium ion channels to open

3. hair cell is bathed in a lot of potassium to depolarize it

4. synaptic vesicles in hair cell fuse to plasma membrane because of depolarization

5. neurotransmitters into synaptic cleft between hair cell and synapsed afferent neuron

6. is depolarization is sufficient action potential transmitted to chochlear nerve

7. intensity determined by how many hair cells and pitch determined by where hair cells are on the basilar membrane

vestibular system

linear acceleration (gravity) and angular acceleration and deceleration detected by inertia on receptive cells

how is gravity detected in vertebrates?

head position

how is angular acceleration and deceleration detected?

turning and tilting of head

how does the vestibular membrane work?

it uses hair cells in the vestibular labyrinth

how do hair cells in the vestibular labyrinth detect signals?

- Some hair cells lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals, like tiny rocks, that move in response to gravity. Any time the head is tilted at an angle or is subject to acceleration or deceleration, these crystals cause the gelatin to shift, bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance.

- Some hair cells project into a gelatinous cap called the cupula. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. When movement stops, the movement of the fluid within the canals slows or stops.

light

composed of electromagnetic waves and needs no medium

photon

packet of electromagnetic radiation

what two variables are important about light waves?

wavelength (color) and amplitude (brightness)

what detects light?

photoreceptors; cells that contain pigment-absorbing molecules that absorb light

eye cups

in flatworms, which are dimple-shaped structures that detect the direction of a light source

compound eyes

of arthropods, which contain multiple lenses and detect shapes, patterns, and movements

pinhole eyes

in the nautilus, which contain no lens and forms simple, low-resolution images

simple eyes

cephalopods and vertebrates, which contain a single lens and form high-resolution images

sclera

tough outer tissue (white of eye)

cornea

transparent sheet of connective tissue, functions with the lens to focus light on the retina

iris

pigmented ring of muscle that controls amount of light entering eye

pupil

hole in center of iris

lens

crystalline, curved structure that focuses light on the cornea (by bending, not by moving) in conjunction with the cornea

retina

thin layer of photoreceptor cells and neurons

what is in the retina?

photoreceptor cells, bipolar cells, ganglion cells, fovea

photoreceptor cells

light-detecting sensory cells

bipolar cells

intermediate connecting neurons

ganglion cells

neurons whose axons project to the brain via the optic nerve

fovea

site of retina with only cones, area of highest visual resolution

optic nerve

axons of the ganglion cells

how do cephalopod eyes focus?

they move the lens like a camera

how do vertebrate eyes focus?

changing in lens shape which causes age-related loss of resolution

vertebrate retina

inverted; blood vessels and nerves are in front of the photoreceptor cells; blind spot and age-related macular degeneration

cephalopod retina

nerves are behind the photoreceptor cells

retinal

light-absorbing pigment molecules embedded in the photoreceptor cells

opsin

protein that contains retinal; changes shape/activity when retinal changes shape in response to absorption of light

rhodopsin

retinal and opsin

what happens when light hits retinal?

it changes shape

three color-sensitive opsins

S opsin (short-wavelength opsin), M opsin (medium-wavelength opsin), and L opsin (long-wavelength opsin)

what are the two photoreceptor cells?

rods and cones

cones

each contain a single type of color-sensitive opsin, making each cone most sensitive to a particular hue or color of light

how many types of cones are there?

3 (red, green, blue)

Where are cones concentrated?

fovea

how can we detect a variation of colors?

combination of cones at the same time

rods

activated by an intermediate wavelength of light and is capable of working in low-levels of light

where are rods located?

periphery of the retina

what happens when a photon activates rhodosin?

a rod or cone cell hyperpolarizes when its rhodopsins are activated by light and stops releasing neurotransmitters, and it depolarizes when its rhodopsins are in the dark

what two senses are interconnected?

taste and smell

gustatory receptors

Detecting a taste relies on activation of specific chemical receptors in taste receptor cells

how does taste work?

When the specific chemical (tastant) binds the receptor, the receptor cell becomes depolarized and releases neurotransmitter on its synapsed afferent neuron

depolarization salty tastant

provides the sodium ions (Na+) that enter the taste receptor cells and excite them directly

sour tastant

acids cause an increase hydrogen ion (H+) concentrations in the taste receptor cells, thus depolarizing them

sweet, bitter, and umami tastants

cause activation of an enzyme that causes opening of an ion channel, thus depolarizing the taste receptor cells

how is spiciness detected?

activation of pain receptors

taste bud

cluster of gustatory receptors (taste receptor cells) that are located within the bumps on the tongue called papillae (singular: papilla)

why isn't taste strong when the mouth is dry?

tastants must be dissolved in saliva to bind with and stimulate the receptors on the microvilli

odors

molecules in the air we breathe

how does smell work?

1. odorants enter nose and dissolve in the olfactory epithelium

2. olfactory receptor responds when it binds to certain molecules inhaled from the environment by repulses

3. single dendrite in olfactory neuron trap odorant molecules in cilia

4. specific odorant binds to specific protein

olfactory epithelium

collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm2 in humans

what does the olfactory epithelium require?

mucus

olfactory receptor

dendrite of a specialized sensory neuron

what makes cilia in the nose sensitive to odorants?

variation in protein sequences

how do taste and olfactory receptors work together?

taste receptors are responsible for sense of taste while olfactory receptors are responsible for flavor

retronasal olfaction

flow of air from the back of the throat up to the olfactory epithelium via the back of the nose which provides taste

nociception

neural processing in response to tissue damage

two types of causes of pain

true sources of injury and harmless stimuli that mimic damaging stimuli

capsaicins

compounds that cause peppers to taste hot