Chapter 49

Sensory and Motor Mechanisms

(pp 1045-1062)

IB-202-16-06

Sensory input, motor output and behavior!

The detection and processing of sensory information and the generation of motor output is the physiological basis for all animal behavior. Behavior is not a linear sequence of sensing,

brain analysis and action, but rather a continuing process. As animals move they are probing the environment through that movement, sensing

changes and using the information to generate the next action. It is a continuous cycle.

An example of sensing and acting• Bats use sonar to detect their prey• Moths, a common prey for bats can detect the bat’s

sonar with sensory hairs in the abdomen and attempt to escape by diving in a spiral pattern towards the ground.

• Both of these organisms have complex sensory systems that facilitate their survival.

Figure 49.1

• The sensory and effector structures that make up these systems have been transformed by evolution into diverse mechanisms that sense various stimuli and generate the appropriate physical movement

• The first step is converting the stimulus into another form. Sensory receptors transduce stimulus energy to electrical signals. The electrical signals are transformed into action potentials and travel to the the brain via sensory neurons

And the brain interprets them as a perception of the stimuli and generates an appropriate response. (Cross talk—some people see colors when they hear music!—What is going on?) Action potentials going from ear to visual center????

• Sensations begin with the detection of stimuli by sensory receptors

• There are many kinds of receptors: heat, cold, pain, pressure, light, hearing, osmotic, oxygen etc. Some are located in the surface tissues of the body and others within the brain, circulatory system and visceral organs.

• Locations:• Exteroreceptors

– Detect stimuli coming from the outside of the body such as pressure waves, light and heat/cold.

• Interoreceptors– Detect internal stimuli chemoreceptors, osmoreceptors, pressure

etc.

Functions Performed by Sensory Receptors

• All stimuli represent forms of energy

• Sensation involves converting this energy into a change in the membrane potential of sensory receptors

• Sensory receptors perform four functions in this process– Sensory transduction, amplification,

transmission, and integration– The stretch receptor and hair receptor

represent these processes.

• Two types of sensory receptors exhibit these functions– A stretch receptor in a crayfish

Figure 49.2a

(a)Crayfish stretch receptors have dendrites embedded in abdominal muscles. When the abdomen bends, muscles and dendrites

stretch, producing a receptor potential in the stretch receptor. The receptor potential triggers action potentials in the axon of the stretch

receptor. A stronger stretch producesa larger receptor potential and higherrequency of action potentials.

Muscle

Dendrites

Stretchreceptor

Axon

Mem

bran

epo

tent

ial (

mV

)

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Action potentials

Receptor potential

Weakmuscle stretch

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Strongmuscle stretch

Action potential has much more energy than a decrease in receptor potential! An example of an amplification!

– A hair cell found in vertebrates

of action potentials in the sensory neuron. Bending in the other direction has the opposite effects. Thus, hair cells respond to the direction of motion as well as to its strength and speed.s

(b) Vertebrate hair cells have specialized cilia or microvilli (“hairs”) that bend when sur-rounding fluid moves. Each hair cell releases an excitatory neurotransmitter at a synapse

with a sensory neuron, which conducts action potentials to the CNS. Bending in one direction depolarizes the hair cell, causing it to release more neurotransmitter and increasing frequency

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Action potentials

No fluidmovement

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Receptor potential

Fluid moving inone direction

–50

–70

0

–70

0 1 2 3 4 5 6 7Time (sec)

Fluid moving in other direction

Mem

bran

epo

tent

ial (

mV

)

Mem

bran

epo

tent

ial (

mV

)

Mem

bran

epo

tent

ial (

mV

)

“Hairs” ofhair cell

Neuro-trans-mitter at synapse

Axon

Lessneuro-trans-mitter

Moreneuro-trans-mitter

Figure 49.2b

Depolarization of hair cell!

Hyperpolarization of hair cell. Less likely to generate an action potential!

Use of a neurotransmitter step and amplification step!

Sensory Transduction

• Sensory transduction is the conversion of stimulus energy into a change in the membrane potential of a sensory receptor

• This change in the membrane potential is known as a receptor potential (resting potential changes from -70 to -60)

• Many sensory receptors are extremely sensitive– With the ability to detect the smallest

physical unit of stimulus possible

Transmission

• After energy in a stimulus has been transduced into a receptor potential– Some sensory cells generate action

potentials, which are transmitted to the CNS

• Sensory cells without axons – Release neurotransmitters at synapses with

sensory neurons

Integration

• The integration of sensory information– Begins as soon as the information is

received– Occurs at all levels of the nervous system

• The integration of sensory information begins as soon as the information is received. It occurs at all levels of the nervous system

• Some receptor potentials are amplified through summation

• Some receptor potentials are decreased (attenuated) with repeated stimulation. This is called sensory adaptation.

• Both of these responses can be viewed as integration at the receptor level.

Types of Sensory Receptors• Based on the energy they transduce,

sensory receptors fall into five categories– Mechanoreceptors– Chemoreceptors– Photoreceptors– Thermoreceptors– Pain receptors– Electromagnetic receptors includes (photo,

electrical and magnetism)

Mechanoreceptors

• Mechanoreceptors sense physical deformation– Caused by stimuli such as pressure, stretch,

motion, and sound

• The mammalian sense of touch relies on mechanoreceptors that are the dendrites of sensory neurons. These are naked nerves and depolarization of the endings leads to an action potential.

Figure 49.3

Heat

Light touch Pain

Cold

Hair

Nerve Connective tissue Hair movement Strong pressure

Dermis

Epidermis

Pain Receptors

• In humans, pain receptors, also called nociceptors– Are a class of naked dendrites in the

epidermis– Respond to excess heat, pressure, or

specific classes of chemicals released from damaged or inflamed tissues

Chemoreceptors

• Chemoreceptors include– General receptors that transmit information about

the total solute concentration of a solution– Specific receptors that respond to individual kinds of

molecules. Best example is that of a male moth’s antennae sensing pheromone (bombykol) put out by female moth a mile upwind. Male responds when only 40 receptors bind compound / sec out of 20,000 receptors.

• The most sensitive and specific chemoreceptors known is present in the antennae of the male silkworm moth

Figure 49.4 0.1

mm

Electromagnetic Receptors

• Electromagnetic receptors detect various forms of electromagnetic energy– Such as visible light, electricity, and

magnetism

• Many mammals appear to use the Earth’s magnetic field lines to orient themselves as they migrate. There is also good evidence that birds use magnetic field lines during long migrations.

Figure 49.5b

(b) Some migrating animals, such as these beluga whales, apparentlysense Earth’s magnetic field and use the information, along with other cues, for orientation.

Thermoreceptors

• Thermoreceptors, which respond to heat or cold– Help regulate body temperature by signaling

both surface and body core temperature.

– Infrared reception in pit vipers (rattlesnakes).

Pit Vipers (rattlesnakes) have infrared receptors.

• Some snakes have very sensitive infrared receptors– That detect body heat of prey against a colder background

Figure 49.5a

(a) This rattlesnake and other pit vipers have a pair of infrared receptors,one between each eye and nostril. The organs are sensitive enoughto detect the infrared radiation emitted by a warm mouse a meter away. The snake moves its head from side to side until the radiation is detected equally by the two receptors, indicating that the mouse is straight ahead.

Pits below eyes.

Snake can detect .002C temp change within the pit. Can sense a rat 40 cm away if its body temp 10C above the environmental. Receptor just branched ending of the sensory axon.

Can also sense direction because of depth of pit!

• Concept 49.2: The mechanoreceptors involved with hearing and equilibrium detect settling particles or moving fluid

• Hearing and the perception of body equilibrium– Are related in most animals

Sensing Gravity and Sound in Invertebrates

• Most invertebrates have sensory organs called statocysts– That contain mechanoreceptors and function in their sense of

equilibrium

Figure 49.6

Ciliatedreceptor cells

CiliaStatolith

Sensory nerve fibers

Statolith is a secretion of protein and calcium carbonate! In fish they increase in size as the fish grows and can be used to age fish by counting the annual rings!

• Many arthropods sense sounds with body hairs that vibrate or with localized “ears” consisting of a tympanic membrane and receptor cells. Cockroach escape response!

Figure 49.7

1 mm

Tympanicmembrane

Sensory Perception in Aquatic Vertebrates• The lateral line system of fishes and

tadpoles contains mechanoreceptors– With hair cells that respond to water

movement

Figure 49.12 Nerve fiber

Supporting cell

Cupula

Sensoryhairs

Hair cell

Segmental muscles of body wall Lateral nerve

ScaleEpidermis

Lateral line canal

Neuromast

Opening of lateralline canal

Lateralline

Neuromast includes the gelatinous cupula, sensory hairs and hair cells!

Water flows through the channel and deforms the cupula. Also pressure waves in the water deform it!

Hearing and Equilibrium in Vertebrates.

• In most terrestrial vertebrates– The sensory organs for hearing and

equilibrium are closely associated in the ear

Deformation of hair cells basis for hearing in mammals.

Figure 49.8

Pinna

Auditory canal

Eustachian tube

Tympanicmembrane

Stapes

Incus

Malleus

Skullbones

Semicircularcanals

Auditory nerve,to brain

Cochlea

Tympanicmembrane

Ovalwindow

Eustachian tube

Roundwindow

Vestibular canal

Tympanic canal

Auditory nerve

BoneCochlear duct

Hair cells Tectorialmembrane

Basilarmembrane

To auditorynerve

Axons of sensory neurons

1 Overview of ear structure 2 The middle ear and inner ear

4 The organ of Corti 3 The cochleaOrgan of Corti

Outer earMiddle

ear Inner ear

3 chambers!

Hearing• Vibrating objects create percussion waves

in the air– That cause the tympanic membrane (ear

drum) to vibrate

• The three bones of the middle ear– Transmit the vibrations to the oval window on

the cochlea to the fluid of the inner ear.

• These vibrations create pressure waves in the fluid in the cochlea– That travel through the vestibular canal and into the

tympanic canal. They ultimately strike the round window where they are dissipated.

Figure 49.9

Cochlea

Stapes

Oval window

Apex

Axons ofsensoryneurons

Roundwindow Basilar

membrane

Tympaniccanal

Base

Vestibularcanal Perilymph

• The pressure waves in the vestibular canal – Cause the basilar membrane to vibrate up and

down causing its hair cells to bend

• The bending of the hair cells depolarizes their membranes– Sending action potentials that travel via the

auditory nerve to the brain

• The cochlea can distinguish pitch– Because the basilar membrane is not uniform along

its length (thinner at one end), it vibrates more vigorously at a certain frequency! Louder greater amplitude deforms hair more.

Cochlea(uncoiled)

Basilarmembrane

Apex(wide and flexible)

Base(narrow and stiff)

500 Hz(low pitch)1 kHz

2 kHz

4 kHz

8 kHz

16 kHz(high pitch)

Frequency producing maximum vibration

Figure 49.10

Receptor potential causes influx of Ca+ which in turn causes release of transmitter and action potential!

Equilibrium

• Several of the organs of the inner ear– Detect body position and balance

Equilibrium

• The utricle, saccule, and semicircular canals in the inner ear function in balance and equilibrium

Figure 49.11

The semicircular canals, arranged in three spatial planes, detect angular movements of the head.

Body movement

Nervefibers

Each canal has at its base a swelling called an ampulla, containing a cluster of hair cells.

When the head changes its rateof rotation, inertia prevents endolymph in the semicircular canals from moving with the head, so the endolymph presses against the cupula, bending the hairs.

The utricle and saccule tell the brain which way is up and inform it of the body’s position or linear acceleration.

The hairs of the hair cells project into a gelatinous cap called the cupula.

Bending of the hairs increases the frequency of action potentials in sensory neurons in direct proportion to the amount of rotational acceleration.

Vestibule

Utricle

Saccule

Vestibular nerve

Flowof endolymph

Flowof endolymph

CupulaHairs

Haircell

Hearing and Equilibrium in Other Vertebrates

• Like other vertebrates, fishes and amphibians– Also have inner ears located near the brain

• Concept 49.3: The senses of taste and smell are closely related in most animals

• The perceptions of gustation (taste) and olfaction (smell)– Are both dependent on chemoreceptors that

detect specific chemicals in the environment

• The taste receptors of insects are located within sensory hairs called sensilla– Which are located on the feet and in

mouthparts

Figure 49.13

EXPERIMENT Insects taste using gustatory sensilla (hairs) on their feet and mouthparts. Each sensillum contains four chemoreceptors with dendrites that extend to a pore at the tip of the sensillum. To study the sensitivity of each chemoreceptor, researchers immobilized a blowfly (Phormia regina) by attaching it to a rod with wax. They then inserted the tip of a microelectrode into one sensillum to record action potentials in the chemoreceptors, while they used a pipette to touch the pore with various test substances.

Num

ber

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pot

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tials

in f

irst

seco

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of r

esp

ons

e

CONCLUSION Any natural food probably stimulates multiple chemoreceptors. By integrating sensations, the insect’s brain can apparently distinguish a very large number of tastes.

To brain

Chemo-receptors

Pore at tip

Pipette containingtest substance

To voltagerecorder

Sensillum

Microelectrode

50

30

10

00.5 MNaCl

Meat 0.5 MSucrose

Honey

Stimulus

Chemoreceptors

RESULTS Each chemoreceptor is especially sensitive to a particular class of substance, but this specificity is relative; each cell can respond to some extent to a broad range of different chemical stimuli.

• The receptor cells for taste in humans are modified epithelial cells organized into taste buds

• Five taste perceptions involve several signal transduction mechanisms– Sweet, sour, salty, bitter, and umami (elicited

by glutamate)

• Transduction in taste receptors– Occurs by several mechanisms– Na and H+ (sour) diffuse through channels on

the taste receptor depolarizing it. Glutmate binds to Na channel opening it. Quinine (bitter) binds to K channels and closes them (depolarizing). All generate action potential. Sweetness next slide!

Taste pore Sugar molecule

Sensoryreceptorcells

Sensoryneuron

Taste bud

Tongue

G protein Adenylyl cyclase

—Ca2+

ATP

cAMP

Proteinkinase A

Sugar

Sugarreceptor

SENSORYRECEPTORCELL Synaptic

vesicle

K+

Neurotransmitter

Sensory neuron

• Sensing sweetness

Figure 49.14

4 The decrease in the membrane’s permeability to K+ depolarizes the membrane.

5 Depolarization opens voltage-gated calcium ion (Ca2+) channels, and Ca2+ diffuses into the receptor cell.

6 The increased Ca2+ concentration causes synaptic vesicles to release neurotransmitter.

3 Activated protein kinase A closes K+ channels in the membrane.

2 Binding initiates a signal transduction pathway involving cyclic AMP and protein kinase A.

1 A sugar molecule binds to a receptor protein on the sensory receptor cell.

Cell depolarizes because K builds up in the cell from the Na/K ATPase pump.

Smell in Humans

• Olfactory receptor cells– Are neurons that line the upper portion of the

nasal cavity

Olfaction• When odorant molecules bind to specific

receptors– A signal transduction pathway is triggered,

sending action potentials to the brain

Brain

Nasal cavity

Odorant

Odorantreceptors

Plasmamembrane

Odorant

Cilia

Chemoreceptor

Epithelial cell

Bone

Olfactory bulb

Action potentials

MucusFigure 49.15

1000 ordorant receptors in humans. Represents 3% of human genes! Probably don’t use them all anymore.

• Concept 49.4: Similar mechanisms underlie vision throughout the animal kingdom

• Many types of light detectors– Have evolved in the animal kingdom and

may be homologous

Vision in Invertebrates

• Most invertebrates– Have some sort of light-detecting organ.

Flatworms, some jelly fish, scallops (molluscs), crustaceans and insects.

Light

Light shining from the front is detected

Photoreceptor

Visual pigment

Ocellus

Nerve to brain

Screening pigment

Light shining from behind is blockedby the screening pigment

• One of the simplest is the eye cup of planarians– Which provides information about light intensity and

direction but does not form images

Figure 49.16

Eyes positioned so that light coming from one side does not illuminate eye on opposite side.

• Two major types of image-forming eyes have evolved in invertebrates– The compound eye and the single-lens eye

Compound Eyes• Compound eyes are found in insects and

crustaceans and consist of up to several thousand light detectors called ommatidia

Figure 49.17a–b

Cornea

Crystallinecone

Rhabdom

PhotoreceptorAxons

Ommatidium

Lens

2 m

m

(a) The faceted eyes on the head of a fly,photographed with a stereomicroscope.

(b) The cornea and crystalline cone of each ommatidium function as a lens that focuses light on the rhabdom, a stack of pigmented plates inside a circle of photoreceptors. The rhabdom traps light and guides it to photoreceptors. The image formed by a compound eye is a mosaic of dots produced by different intensities of light entering the many ommatidia from different angles.

• Single-lens eyes– Are found in some jellies, polychaetes,

spiders, and many molluscs– Work on a camera-like principle

The Vertebrate Visual System

• The eyes of vertebrates are camera-like– But they evolved independently and differ

from the single-lens eyes of invertebrates

Structure of the Eye

• The main parts of the vertebrate eye are– The sclera, which includes the cornea– The choroid, a pigmented layer– The conjunctiva, that covers the outer

surface of the sclera

– The iris, which regulates the pupil– The retina, which contains photoreceptors– The lens, which focuses light on the retina

• The structure of the vertebrate eye

Figure 49.18

Ciliary body

Iris

Suspensoryligament

Cornea

Pupil

Aqueoushumor

Lens

Vitreous humor

Optic disk(blind spot)

Central artery andvein of the retina

Opticnerve

Fovea (centerof visual field)

Retina

ChoroidSclera

• Humans and other mammals– Focus light by changing the shape of the lens

Figure 49.19a–b

Lens (flatter)

Lens (rounder)

Ciliarymuscle

Suspensoryligaments

Choroid

Retina

Front view of lensand ciliary muscleCiliary muscles contract, pulling

border of choroid toward lens

Suspensory ligaments relax

Lens becomes thicker and rounder, focusing on near objects

(a) Near vision (accommodation)

(b) Distance vision

Ciliary muscles relax, and border of choroid moves away from lens

Suspensory ligaments pull against lens

Lens becomes flatter, focusing on distant objects

• The human retina contains two types of photoreceptors– Rods are sensitive to light but do not

distinguish colors– Cones distinguish colors but are not as

sensitive

Sensory Transduction in the Eye

• Each rod or cone in the vertebrate retina– Contains visual pigments that consist of a

light-absorbing molecule called retinal bonded to a protein called opsin

• Rods contain the pigment rhodopsin– Which changes shape when it absorbs light

Figure 49.20a, b

Rod

Outersegment

Cell body

Synapticterminal

Disks

Insideof disk

(a) Rods contain the visual pigment rhodopsin, which is embedded in a stack of membranous disks in the rod’s outer segment. Rhodopsin consists of the light-absorbing molecule retinal bonded to opsin, a protein. Opsin has seven helices that span the disk membrane.

(b) Retinal exists as two isomers. Absorption of light converts the cis isomer to the trans isomer, which causes opsin to change its conformation (shape). After a few minutes, retinal detaches from opsin. In the dark, enzymes convert retinal back to its cis form, which recombines with opsin to form rhodopsin.

Retinal

OpsinRhodopsin

Cytosol

HC

CH2C

CH2C C

HCH3

CH3

H

CC

CH3 H CH3

CC

CC

CC

C

H

H

H

H

OH

H3C

HC

CH2C

CH2C C

HCH3

CH3

H

CC

CH3 H CH3

CC

CC

HH

CH3

H

CC

CH

O

CH3

trans isomer

cis isomer

EnzymesLight

Processing Visual Information

• The processing of visual information begins in the retina itself. This is accomplished by the interconnections with three types of cells before an action potential is transmitted to the brain via the optic nerve. Some of these connections are inhibitory while others are stimulatory.

• Absorption of light by retinal– Triggers a signal transduction pathway

Figure 49.21

EXTRACELLULARFLUID

Membranepotential (mV)

0

– 40

– 70

Dark Light

– Hyper- polarization

Time

Na+

Na+

cGMP

CYTOSOL

GMP

Plasmamembrane

INSIDE OF DISK

PDEActive rhodopsin

Light

Inactive rhodopsin Transducin Disk membrane

2 Active rhodopsin in turn activates a G protein called transducin.

3 Transducinactivates theenzyme phos-phodiesterae(PDE).

4 Activated PDE detaches cyclic guanosine monophosphate (cGMP) from Na+ channels in the plasma membrane by hydrolyzing cGMP to GMP.

5 The Na+ channels close when cGMP detaches. The membrane’s permeability to Na+ decreases, and the rod hyperpolarizes.

1 Light isomerizes retinal, which activates rhodopsin.

• In the dark, both rods and cones– Release the neurotransmitter glutamate into

the synapses with neurons called bipolar cells, which are either hyperpolarized or depolarized

• In the light, rods and cones hyperpolarize– Shutting off their release of glutamate

• The bipolar cells– Are then either depolarized or hyperpolarized

Figure 49.22

Dark Responses

Rhodopsin inactive

Na+ channels open

Rod depolarized

Glutamatereleased

Bipolar cell eitherdepolarized orhyperpolarized,depending onglutamate receptors

Light Responses

Rhodopsin active

Na+ channels closed

Rod hyperpolarized

No glutamatereleased

Bipolar cell eitherhyperpolarized ordepolarized, depending on glutamate receptors

• Three other types of neurons contribute to information processing in the retina

– Ganglion cells, horizontal cells, and amacrine cells

Figure 49.23Opticnervefibers

Ganglioncell

Bipolarcell

Horizontalcell

Amacrinecell

Pigmentedepithelium

NeuronsCone Rod

Photoreceptors

Retina

Retina

Optic nerve

Tobrain

• Signals from rods and cones– Travel from bipolar cells to ganglion cells

• The axons of ganglion cells are part of the optic nerve– That transmit information to the brain

Figure 49.24

Leftvisualfield

Rightvisualfield

Lefteye

Righteye

Optic nerve

Optic chiasm

Lateralgeniculatenucleus

Primaryvisual cortex

• Most ganglion cell axons lead to the lateral geniculate nuclei of the thalamus– Which relays information to the primary visual

cortex

• Several integrating centers in the cerebral cortex– Are active in creating visual perceptions