Animal Sensory Systems

sensory receptor cells

transduce external stimuli into changes in membrane potentials- may either depolarize or hyperpolarize in response to the stimulus

• if the changes in membrane potential are sufficient to induce action potentials → transduced to the nervous system (efferent division of PNS to CNS)

• sensory receptor cells can be either:

  • specialized neurons (receptor cell is a neuron)

  • specialized sensory cells which synapse with neurons (receptor secretes neurotransmitters to a neuron)

• different types:

  • mechanoreceptors

  • photoreceptors

  • chemoreceptors

  • nociceptors

  • thermoreceptors

stimuli are integrated in the ___ and responses are sent to appropriate body systems via the ___

stimuli are integrated in the CNS and responses are sent to appropriate body systems via the PNS

3 stages of sensory system

sensory input → integration → motor output

(afferent neurons → interneurons → efferent neurons)

processing sensory information

1. transduction within the PNS

   • receptor cells may depolarize or hyperpolarize

2. transmission to the CNS via the PNS

   • signals from different sensory systems transmit to different parts of the brain

3. integration in the CNS

   • sensory adaptation

   • incoming signals integrated at axon hillock via summation of potentials from many receptors

4. response via the PNS (usu motor output)

mechanoreceptors

detect pressure

• touch, sound, balance

• some types of mechanoreceptors are located near the upper layers of the skin, and thus are more sensitive to lighter touch and are able to precisely localize gentle touch

• mechanoreceptors located deeper in the skin are only activated by stronger pressure and are not as highly sensitive to identify the precise location of the touch

photoreceptors

respond to light

chemoreceptors

respond to taste/smell

• oldest of all sensory receptors

nociceptors

detect tissue damage- which our brains interpret as pain

thermoreceptors

respond to heat or cold

the intensity/degree of a stimulus is encoded in three different ways:

• rate/frequency of action potentials produced by the sensory receptor

• number of receptors activated

• which specific receptors are activated

frequency (sound)

number of waves per unit of time

• pitch

• high-frequency sounds are higher-pitched and shorter wavelength than low-frequency, long-wavelength sounds

amplitude (sound)

dimension of a wave from peak to trough

• volume

• sound waves of louder sounds have a greater amplitude than those of softer sounds

structure of the human ear

outer ear collects and amplifies signal which hits the tympanic membrane → ossicles vibrate to transmit signal to fluid-filled cochlea → soundwaves transduced into action potentials by hair cells between basilar and tectorial membranes within the cochlea

cochlea

fluid-filled whorled structure that contains the auditory mechanoreceptors that allow us to perceive pressure waves in the air as sound

• contains the basilar membrane

basilar membrane

flexible membrane that runs the length of the cochlea and contains the mechanoreceptors called hair cells- transduce sound waves into action potentials

stereocilia: tiny hair-like protrusions on hair cells- pressed against the tectorial membrane when basilar vibrates- stereocilia bend → intitiates action potentials

mechanoreceptors bend and open ion channels in response to pressure

we perceive volume based on:

how many hair cells are activated

we perceive pitch based on:

which hair cells are activated (which region of the basilar membrane vibrates)

stiffer region of the basilar membrane (narrow part)

vibrates in response to high frequency (higher-pitched) sounds

flexible region of the basilar membrane (wider part)

vibrates in response to low frequency (lower-pitched sounds)

vestibular system

detects positions and movement of our head in space

stimuli:

• linear acceleration (gravity)

• angular acceleration and deceleration

vestibular labyrinth

contain vestibular hair cells

• adjacent to cochlea

hair cells in the vestibular labyrinth detect stimuli in two ways:

• detect head position and movement through gelatin shifting and stereocilia bending caused by movement of calcium carbonate crystals (ear stones) in a gelatinous layer in response to the head tilting or accelerating/decelerating- bending signals to the brain for balance

• some hair cells project into a gelatinous cap called the cupula. When the head turns, the fluid in the canals shift → bending stereocilia and sending signals to the brain; when movement stops → movement of the fluid within the canals slows or stops.

to detect pressure changes, bony fish use:

lateral line systems comprised of hair cells in a cupula

statocysts

organs with dense statoliths adjacent to hair cells

• how invertebrates detect balance

wavelength (light)

detected as hue/color

• varies inversely with frequency

• light at red end of the visible spectrum has longer wavelengths; light at violet end has shorter wavelengths

amplitude (light)

perceived as brightness

eye cups

• in flatworms

• dimple-shaped; detect direction of a light source

compound eyes

• arthropods

• contain multiple lenses and detect shapes, patterns, and movements

pinhole eyes

• in the nautilus

• contain no lens and form simple, low-resolution images

simple eyes

• cephalopods and vertebrates

• contain a single lens and form high-resolution images

all photoreceptors contain:

a combination of a protein and a pigment molecule

the vertebrate eye contains:

• 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: curved structure that focuses light on retina (by bending) in conjunction with cornea

• retina: thin layer of photoreceptor cells and neurons

  • photoreceptor cells: light-detecting sensory cells

  • fovea: site of retina w/ only cones- area of highest visual resolution

• optic nerve: axons of the ganglion cells

cephalopod vs. vertebrate eye

• cephalopod eyes move the lens to focus rather than changing shape

• vertebrate eyes have an inverted retina, where blood vessels and nerves are in front of the photoreceptor cells instead of behind

  • results in vertebrates having blind spot as well as age-related macular degeneration, and increased chance of a detached retina

retinal

pigment contained in opsin (protein)

• reversibly changes shape when it is hit by a photon of light

opsin

protein that holds the retinal pigment and changes shape/activity when the retinal changes shape in response to absorption of light

• responsible for ability to perceive differences in color or hue

• each different version is capable of absorbing a different wavelength of light:

  • s opsin (short-wavelength opsin); m opsin (medium-wavelength opsin); l opsin (long-wavelength opsin)

rhodopsin

complex made up of retinal and opsin- allows us to detect light and color

• inactive in the dark

• activated by light

light stimulus results in:

hyperpolarization

cones

contain a single type of color-sensitive opsin (so 3 types of opsin = 3 types of cones)

• require high levels of light to work

• use for color vision

• good for detail

• heavily concentrated at the fovea

rods

contain a fourth type of opsin called rod opsin- activated by an intermediate wavelengths of light

• active in low light

• not color-sensitive

• good for detecting movement in field of vision

• heavily concentrated at the periphery (outer edges) of retina

rods and cones in the dark vs in the light

• in the dark: rods and cones are depolarized → releasing neurotransmitters to their synapsed bipolar cells

• in the light: rods and cones hyperpolarize → stop releasing neurotransmitter