SENSORY MECHANISMS

Chapter 50

Sensory Mechanisms in Mammals

Chapter 50.1-50.4

The body NEVER stops collecting data about the world around it Sense brain

action React to the

environment we are in Link the immediate

stimulus to a memory

We may interpret more to a stimulus than is really there.

48.20

Picture of brain showing where senses are located

Sensations vs Perceptions

Action potential from a sense neuron

Brain’s interpretation of the sensation

Color, smell, sound, taste, texture

Created by the brain…do not really exist in the outside world

Vocab from the reading

Sensory reception Sensory receptors

Exteroreceptors Interoreceptors

Sensory transduction Receptor potential Amplification Transmission Integration Sensory adaptation

Sensory Transduction

Mechanoreceptors

Pressure, touch, stretch, motion, sound

Hair cells detect motion

Hair is really cilia or microvilli Vertebrate ears

Pain receptors

“naked” dendrites (branches of neurons) Called nociceptors

Vital to survival

Thermoreceptors

Register changes in external and internal temperatures

Posterior hypothalamus is the body’s “thermostat”

Chemoreceptors

Glucose, water, oxygen, carbon dioxide, amino acids, any necessary chemical substances

Gustatory: taste Olfactory: smell

Sweet, salty, sour, bitter, umami are the main categories

Electromagnetic receptors

Visible light, electricity, magnetic pull

Infrared receptor

photoreceptorWe think whales know migration patterns by sensing the earth’s magnetic field

Specialized sense organs “Ampullae of Lorenzini”

Detect electrical fields created by other fish Can find a ray buried in the sand

VISION

When light enters a vertebrate eye, it travels through the jelly and strikes the photoreceptors of the retina. But the neurons of the retina are actually pointed backward. It is as if we are gazing at our own brain. Light has to make its way through several layers of neurons and a web of capillaries before it finally gets to the nerve endings that can detect it.

Once light strikes the backward pointing photoreceptors of the retina, the photoreceptors then have to send their signals back up through the layers of the retina toward the front of the eye…

This achitecture is, as the evolutionary biologist George Williams has bluntly put it, “stupidly designed.”

Zimmer, C. Evolution: The Triumph of an Idea. 2001. Harper Collins Publishers. Pages 128-129

Vertebrates have Single Lens Eyes

Brain “sees”, NOT the eyes

FOVEA: spot with most cone cells

OPTIC DISC: blind spot

Ms. Bjelko’s Eye

Taken in 2004

Focus and Lens Shape The thicker the lens,

the more sharply the light is bent as it enters the eye

Ciliary muscles contract to accommodate vision of close objects, making lens thicker

Visual pigments

CONES Retinal (similar to vitamin A in structure)

bonded to opsin (membrane protein)

RODS Retinal bonded to a different opsin protein

form Rhodopsin

How does this work molecularly?

LIGHT

Rhodopsin

Retinal changes shape and separates from opsin

Bright light “bleaches” rhodopsin (the process above) and rods stop working temporarily. Causes cones to take over.

Cause of temporary blindness going from very dark to very light places.

Black and white vision at molecular level (highly simplified!!!) Light induced shape change in rhodopsin

starts a G protein linked chain reaction that Releases glutamate (neurotransmitter) to

bipolar cells in retina. What happens next depends on the type of

glutamate receptor on the bipolar cell.

Color vision (highly simplified)

3 types of cones and corresponding opsins in retina- TOGETHER CALLED PHOTOPSINS

red, green, blue The wavelength of light each specializes in

overlap so we can see shades and other colors.

APPLICATION: Color blindness is genetic- sex linked- absence/deficiency of 1+ photopsin

Rods and Cones

125mill in humans Monochrome More sensitive to light Allow vision at night

6mill in humans Colors Less sensitive to light More light needed to

stimulate

Which species will have more rod cells?

a. nocturnal animal

b. diurnal animal

Strange Fact

Color vision exists in ALL CLASSES of vertebrates, but NOT ALL SPECIES

Good Color Vision: most fish, birds, amphibians, reptiles, primates

Limited Color vision: cats, most mammals (b/c they are nocturnal)

Path of Light through the Eye

The eye is like a camera.

Images appear upside down AND backwards on the retina.

Brain fixes it.

How your brain interprets the messages from your eyes.

Emmetropia- normal vision

Upside down image lands on retina

Myopia

Image forms in front of retina

“nearsighted”

Can see clearly at short distances

Corrected with diverging lenses

HyperopiaImage forms behind the retina

“farsighted”- can see clearly at long distances

Corrected with converging lenses

Taste and Smell

Chemical conversations

Much of the animal kingdom communicates via chemicals instead of sound.

Pheromones Scent marking territories

Defining Terms

Taste (olfaction)- detection of chemicals in solutions

Smell (gustation)- detection of chemicals in air

Animals in aquatic environments- no functional difference between taste and smell

Taste: Basic Mechanism

Molecule from food dissolves in liquid on tongue

Molecule reaches specific proteins in receptor cell membranes (modified epithelial cells in groups called taste buds)

Triggers depolarization of membrane Neurotransmitters released

Visual on next page

Taste: Basic Mechanism

All Taste Buds LOOK the same

4 different taste perceptions

1 more being studied: umami Meat or cheese flavor depending on the

source you read.

Smell: Basic Mechanism

Chemicals detected by cilia on olfactory receptor cells (in layer of mucus in upper nasal cavity.

Molecule binds to specific receptor molecules on plasma membrane

Triggers G protein signaling pathway involving adenylyl cyclase and 2nd messenger cAMP

CAMP opens Na+ channels, depolarizing membrane

Smell: Basic Mechanism

Olfactory bulb

Smell and Taste are Linked

Most of the thousands of smells we identify are based on a few primary odors that match up to the 4/5 taste perceptions

Lack of smell causes lack of sense of taste

Hearing and Equilibrium

Parts of the Ear

Fill in the labels on the drawing you were provided as we go through the functions of each part

Hearing is a mechanical system that works like a Rube Goldberg machine.

18 steps for a machine to drop coins in a bank

Many little steps to get a sound wave from your environment to your inner ear where it can be interpreted by your brain.

HOW do you hear? 1

Sound waves in the air travel down the outer ear canal

Hit the ear drum/ tympanic membrane Vibrations on ear drum, passed to the

malleus, incus, and stapes in that order Stapes transfers the vibrations to oval

window (membrane on cochlea) Vibrations produce pressure waves in the

fluid of the cochlea.

HOW do you hear? 2

Pressue wave in cochlea eventually strikes the round window

Pressure waves in vestibular canal push down on cochlear duct and basilar membrane

Basilar membrane vibrates up and down and hair cells brush against tectorial membrane at the frequency of the vibrations

Ion channels open in hair cells as they touch letting K+ in

Membrane depolarizes, neurotransmitter is released

HOW do you hear? 3

Neuron carries sensations to brain via auditory nerve

Sound detected by frequency of impulses in nerve

Volume determined by amplitude of sound wave (higher the amplitude, more frequent impulses to nerve)

Pitch determined by frequency of sound wave (short high frequency waves= high pitches, long, low freqency waves = low pitches

How well do we hear compared to other mammals? Human: 20-20,000hz Dog: up to 40,000 hz Bats: higher than 40,000hz

The Ear and Equilibrium

Utricle and Saccule Chambers behind oval window Utricle opens to semicircular canals- used for

equilibrium

Tell brain UP vs DOWN Help your brain determine your position in

space

49.19 a c

HOW??

Gravity always pulling down on the hairs Angle of hairs triggers chain reaction resulting

in neurotransmitters to nerve. Action Potentials in brain interpreted as your

angle to earth.