The Nervous SystemThe Nervous SystemAP BiologyAP Biology

Why do animals need a nervous Why do animals need a nervous system?system? • Because the world Because the world

is always coming is always coming at you!at you!

Remember…think aboutthe bunny… Poor bunny!

The Nervous SystemThe Nervous System

• Function: environment is Function: environment is constantly changing – nervous constantly changing – nervous system detects those changes system detects those changes and helps the organism and helps the organism respond/adaptrespond/adapt

• Irritability: ability to respond to a Irritability: ability to respond to a stimulusstimulus

The Nervous SystemThe Nervous System

• Nervous System detects Nervous System detects (sensory input), processes (sensory input), processes (integration), and responds (integration), and responds (motor output)(motor output)

• Peripheral Nervous System Peripheral Nervous System detects and respondsdetects and responds

• Central Nervous System Central Nervous System processes informationprocesses information

Nervous SystemNervous System

• Central nervous systemCentral nervous system– brain & spinal cordbrain & spinal cord

• Peripheral nervous systemPeripheral nervous system– nerves from sensesnerves from senses– nerves to musclesnerves to muscles

cerebrum

cerebellum

spinal cord cervicalnerves

thoracicnerves

lumbarnerves

femoral nerve

sciatic nerve

tibialnerve

The Neuron (Nerve Cell)The Neuron (Nerve Cell)• Three types of neurons:Three types of neurons:

–Sensory – carry impulses from Sensory – carry impulses from the sense organs (receptors) to the sense organs (receptors) to the CNSthe CNS

–Motor – carry impulses from the Motor – carry impulses from the CNS to the muscles or glands CNS to the muscles or glands (effectors)(effectors)

–Interneurons – connect and Interneurons – connect and carry impulses between sensory carry impulses between sensory and motor neuronsand motor neurons

Three components of NeuronsThree components of Neurons

1.1. Cell body – largest part; most Cell body – largest part; most metabolic activities take place metabolic activities take place here; contains nucleushere; contains nucleus

2.2. Dendrites – carry impulses from Dendrites – carry impulses from the environment or other the environment or other neurons toward the cell bodyneurons toward the cell body

Three components of NeuronsThree components of Neurons

3.3. Axon – long fiber that carries Axon – long fiber that carries impulses away from the cell impulses away from the cell bodybody

• Terminal branches – Terminal branches – branching of axonbranching of axon

• Synaptic knobs – ends of Synaptic knobs – ends of axon; contain vesicles with axon; contain vesicles with neurotransmittersneurotransmitters

Fun facts about neuronsFun facts about neurons• Most specialized cell in Most specialized cell in

animalsanimals

• Longest cellLongest cell

– blue whale neuronblue whale neuron

• 10-30 meters10-30 meters

– giraffe axongiraffe axon

• 5 meters5 meters

– human neuronhuman neuron

• 1-2 meters1-2 meters

Nervous system allows for 1 millisecond response timeNervous system allows for 1 millisecond response time

The Nerve ImpulseThe Nerve Impulse• Resting potential – a nerve cell has Resting potential – a nerve cell has

an electric potential because an electric potential because OPPOSITELY charged ions are on OPPOSITELY charged ions are on each side of the membraneeach side of the membrane

• Anions are mainly on the inside of Anions are mainly on the inside of the the cell; cations on the outsidethe the cell; cations on the outside

-----------------------------------------++++++++++++++++++++++++

The Nerve ImpulseThe Nerve Impulse• Ungated ion channels allow ions to Ungated ion channels allow ions to

diffuse across the plasma membranediffuse across the plasma membrane

–These channels are always openThese channels are always open

• This diffusion does not achieve an This diffusion does not achieve an equilibrium since sodium-potassium equilibrium since sodium-potassium pumps transport these ions against pumps transport these ions against their their gradientsgradients

HyperpolarizationHyperpolarization• Gated KGated K++ channels channels

open open K K++ diffuses diffuses out of the cell out of the cell the membrane the membrane potential becomes potential becomes more negativemore negative

DepolarizationDepolarization• Gated NaGated Na++

channels open channels open NaNa++ diffuses into diffuses into the cell the cell the the membrane membrane potential becomes potential becomes less negativeless negative

The Nerve ImpulseThe Nerve Impulse• Action Potential – a rush of NaAction Potential – a rush of Na++ flow flow

into the membrane causing an into the membrane causing an imbalance in the charge on each side imbalance in the charge on each side of the membraneof the membrane

• This causes the POLARITY to shift This causes the POLARITY to shift and a wave (impulse) moves down and a wave (impulse) moves down the length of the neuronthe length of the neuron

----------------------------++++++++++++++++++++++++-----------

+

Step 1: Resting State

Step 2: Threshold

Step 3: Depolarization

Step 4: Repolarizing

Step 5: Undershoot

Stimulus

Na+ gates open

Na+ gates close & K+ gates open

K+ gates close

• During hyperpolarization or During hyperpolarization or undershoot, Naundershoot, Na+ + channels are closedchannels are closed

–Neuron cannot depolarize in Neuron cannot depolarize in response to another stimulus: response to another stimulus: refractory periodrefractory period

–The refractory period assures The refractory period assures impulse conduction is impulse conduction is unidirectionalunidirectional

MyelinMyelin• Is composed of 80% lipid and 20% Is composed of 80% lipid and 20%

proteinprotein

• Used for insulation and to help Used for insulation and to help speed up the nerve impulsespeed up the nerve impulse

• Wraps around the axon of some Wraps around the axon of some neuronsneurons

MyelinMyelin

• Gaps in myelin sheath cells Gaps in myelin sheath cells called Nodes of Ranvier – allow called Nodes of Ranvier – allow impulses to move more quickly impulses to move more quickly down neuronsdown neurons

MyelinMyelin

• In Saltatory Conduction, only the In Saltatory Conduction, only the Nodes of Ranvier depolarize and Nodes of Ranvier depolarize and therefore conduct an impulse therefore conduct an impulse fasterfaster

The SynapseThe Synapse• SYNAPSE: the space between the SYNAPSE: the space between the

axon of one neuron and the dendrite axon of one neuron and the dendrite of anotherof another

• Axon terminals have vesicles Axon terminals have vesicles containing chemicals: containing chemicals: NEUROTRANSMITTERS NEUROTRANSMITTERS

• These chemicals are secreted from These chemicals are secreted from the axon of one neuron the axon of one neuron stimulates stimulates receptor sites on the effector or the receptor sites on the effector or the dendrite of the next neurondendrite of the next neuron

Neurotransmitter Action at SynapseNeurotransmitter Action at Synapse

1.1. Action potential arrives at axon Action potential arrives at axon terminal of presynaptic neuronterminal of presynaptic neuron

2.2. Synaptic vesicles rupture, releasing Synaptic vesicles rupture, releasing neurotransmitter into synapseneurotransmitter into synapse

3.3. Neurotransmitter diffuses across Neurotransmitter diffuses across synapse & binds to receptor protein synapse & binds to receptor protein on postsynaptic cellon postsynaptic cell

4.4. Postsynaptic cell is excited or Postsynaptic cell is excited or inhibitedinhibited

5.5. Neurotransmitter in synapse is Neurotransmitter in synapse is deactivateddeactivated

SynapseSynapse

synapsesynapse

Junction between nerve cellsJunction between nerve cells– 1st cell releases chemical to trigger 1st cell releases chemical to trigger

next cellnext cell– where drugs affect nervous systemwhere drugs affect nervous system

• NeurotransmittersNeurotransmitters are the chemicals are the chemicals which allow the transmission of signals which allow the transmission of signals from one neuron to the next across from one neuron to the next across synapses. synapses.

• They are also found at the axon endings They are also found at the axon endings of motor neurons, where they stimulate of motor neurons, where they stimulate the muscle fibers. the muscle fibers.

• They and their close relatives are They and their close relatives are produced by some glands such as the produced by some glands such as the pituitary and the adrenal glands.pituitary and the adrenal glands.

• They are chemicals that communicate They are chemicals that communicate information throughout our brain and information throughout our brain and body. body. 

• The brain uses neurotransmitters to tell The brain uses neurotransmitters to tell your heart to beat, your lungs to your heart to beat, your lungs to breathe, and your stomach to digest.  breathe, and your stomach to digest. 

• They can also affect mood, sleep, They can also affect mood, sleep, concentration, weight, and can cause concentration, weight, and can cause adverse symptoms when they are out of adverse symptoms when they are out of balance. balance.

• Neurotransmitter levels can be depleted Neurotransmitter levels can be depleted many ways.many ways.

• Stress, poor diet, neurotoxins, genetic Stress, poor diet, neurotoxins, genetic predisposition, drug (prescription and predisposition, drug (prescription and recreational), alcohol and caffeine recreational), alcohol and caffeine usage can cause these levels to be out usage can cause these levels to be out of optimal range.of optimal range.

Types of NeurotransmittersTypes of Neurotransmitters• Two kinds of neurotransmitters – Two kinds of neurotransmitters –

INHIBITORYINHIBITORY and and EXCITATORYEXCITATORY.  .  • Excitatory neurotransmitters are not Excitatory neurotransmitters are not

necessarily exciting necessarily exciting – stimulate the brain.  stimulate the brain. 

• Inhibitory - calm the brain and help Inhibitory - calm the brain and help create balance are called .  create balance are called .  – balance mood and are easily balance mood and are easily

depleted when the excitatory depleted when the excitatory neurotransmitters are overactive.  neurotransmitters are overactive. 

Small molecule neurotransmittersSmall molecule neurotransmitters

TypeType NeurotransmitterNeurotransmitter Postsynaptic effectPostsynaptic effect

AcetylcholineAcetylcholine ExcitatoryExcitatory

Amino acidsAmino acids Gamma aminobutyric Gamma aminobutyric acidGABAacidGABA

InhibitoryInhibitory

GlycineGlycine InhibitoryInhibitory

GlutamateGlutamate ExcitatoryExcitatory

AspartateAspartate ExcitatoryExcitatory

Biogenic aminesBiogenic amines DopamineDopamine InhibitoryInhibitory

Nor adrenalineNor adrenaline ExcitatoryExcitatory

SerotoninSerotonin InhibitoryInhibitory

HistamineHistamine ExcitatoryExcitatory

ACETYLCHOLINEACETYLCHOLINE• Acetylcholine was the first Acetylcholine was the first

neurotransmitter to be discovered.neurotransmitter to be discovered.• It is responsible for much of the It is responsible for much of the

stimulation of muscles, including the stimulation of muscles, including the muscles of the gastro-intestinal muscles of the gastro-intestinal system.  system. 

• It is also found in sensory neurons and It is also found in sensory neurons and in the autonomic nervous system, and in the autonomic nervous system, and has a part in scheduling REM (dream) has a part in scheduling REM (dream) sleep. sleep.

• There is a link between acetylcholine There is a link between acetylcholine and Alzheimer's disease:  There is and Alzheimer's disease:  There is something on the order of a 90% loss of something on the order of a 90% loss of acetylcholine in the brains of people acetylcholine in the brains of people suffering from Alzheimer's, which is a suffering from Alzheimer's, which is a major cause of senility. major cause of senility.

• Outside the brain, acetylcholine is the Outside the brain, acetylcholine is the main neurotransmitter in the main neurotransmitter in the parasympathetic nervous system – the parasympathetic nervous system – the system that controls functions such as system that controls functions such as heart rate, digestion, secretion of saliva heart rate, digestion, secretion of saliva and bladder function.and bladder function.

• The plant poisons curare cause The plant poisons curare cause paralysis by blocking the acetylcholine paralysis by blocking the acetylcholine receptor sites of muscle cells.  receptor sites of muscle cells. 

• The well-known poison botulin works by The well-known poison botulin works by preventing the vesicles in the axon preventing the vesicles in the axon ending from releasing acetylcholine, ending from releasing acetylcholine, causing paralysis.  causing paralysis. 

SEROTONINSEROTONIN • SEROTONINSEROTONIN is an inhibitory is an inhibitory

neurotransmitter – which means that it neurotransmitter – which means that it does not stimulate the brain. does not stimulate the brain. 

• Adequate amounts of serotonin are Adequate amounts of serotonin are necessary for a stable mood and to necessary for a stable mood and to balance any excessive excitatory balance any excessive excitatory (stimulating) neurotransmitter firing in (stimulating) neurotransmitter firing in the brain.  the brain. 

• If you use stimulant medications or If you use stimulant medications or caffeine in your daily regimen – it can caffeine in your daily regimen – it can cause a depletion of serotonin over time.  cause a depletion of serotonin over time. 

• Low serotonin levels leads to an Low serotonin levels leads to an increased appetite for carbohydrates increased appetite for carbohydrates (starchy foods) and trouble sleeping, (starchy foods) and trouble sleeping, which are also associated with which are also associated with depression and other emotional depression and other emotional disorders.  It has also been tied to disorders.  It has also been tied to migraines, irritable bowel syndrome, migraines, irritable bowel syndrome, and fibromyalgia. and fibromyalgia.

• Low serotonin levels are also Low serotonin levels are also associated with decreased immune associated with decreased immune system function.system function.

• In addition to mood control, serotonin In addition to mood control, serotonin has been linked with a wide variety of has been linked with a wide variety of functions, including the regulation of functions, including the regulation of sleep, pain perception, body sleep, pain perception, body temperature, blood pressure and temperature, blood pressure and hormonal activityhormonal activity

• Largest amount of serotonin is found in Largest amount of serotonin is found in the intestinal mucosa.the intestinal mucosa.

• Although the CNS contains less than Although the CNS contains less than 2% of the total serotonin in the body, 2% of the total serotonin in the body, serotonin plays a very important role in serotonin plays a very important role in a range of brain functions. It is a range of brain functions. It is synthesized from the synthesized from the amino acid amino acid tryptophantryptophan..

• Gamma amino butyric acidGamma amino butyric acid((GABA)GABA) is the is the major inhibitory neurotransmitter that is major inhibitory neurotransmitter that is often referred to as “nature’s VALIUM-often referred to as “nature’s VALIUM-like substance”.  When GABA is out of like substance”.  When GABA is out of range (high or low excretion values), it range (high or low excretion values), it is likely that an excitatory is likely that an excitatory neurotransmitter is firing too often in neurotransmitter is firing too often in the brain.  GABA will be sent out to the brain.  GABA will be sent out to attempt to balance this stimulating over-attempt to balance this stimulating over-firing.firing.

• People with too little GABA tend to People with too little GABA tend to suffer from anxiety disorders, and drugs suffer from anxiety disorders, and drugs like Valium work by enhancing the like Valium work by enhancing the effects of GABA.  Lots of other drugs effects of GABA.  Lots of other drugs influence GABA receptors, including influence GABA receptors, including alcohol and barbiturates.  If GABA is alcohol and barbiturates.  If GABA is lacking in certain parts of the brain, lacking in certain parts of the brain, epilepsy results.epilepsy results.

HISTAMINEHISTAMINE• Amino acid HistidineAmino acid Histidine is the precursor of is the precursor of

an important neurotransmitter an important neurotransmitter histamine.histamine.

• Histamine is present in venom and Histamine is present in venom and other stinging secretions.other stinging secretions.

• HistamineHistamine is a biogenic amine involved is a biogenic amine involved in local in local immune responsesimmune responses

• Regulate Regulate physiological function in the physiological function in the gut gut

• Act as a Act as a neurotransmitterneurotransmitter. . • Triggers the Triggers the inflammatory responseinflammatory response. .

Nervous System OrganizationNervous System Organization• Cnidaria - nerve netCnidaria - nerve net

– loose organization of loose organization of bi-directional neurons bi-directional neurons

–no centralizationno centralization• Flatworms - ladderFlatworms - ladder

–2 anterior ganglia (rudimentary 2 anterior ganglia (rudimentary brain) with paired, brain) with paired, longitudinal longitudinal nerve cordsnerve cords

–paired sensory organs paired sensory organs (eyespots)(eyespots)

Nervous System OrganizationNervous System Organization• Segmented worms - advanced ladderSegmented worms - advanced ladder

–prominent brainprominent brain

–solid, fused, ventral solid, fused, ventral nerve nerve cordcord

–segmentally arranged segmentally arranged ganglia ganglia

Nervous System OrganizationNervous System Organization• ArthropodsArthropods

–prominent brainprominent brain

–solid, fused, ventral solid, fused, ventral nerve nerve cordcord

–extensive fusion of ganglia extensive fusion of ganglia

–well-developed sensory organswell-developed sensory organs

–exhibit complex behaviorsexhibit complex behaviors

Organization of the NSOrganization of the NS• The human nervous system is The human nervous system is

divided into 2 major divisions: divided into 2 major divisions:

–Central Nervous System (CNS)Central Nervous System (CNS)

• Control center of body, brain and Control center of body, brain and spinal cordspinal cord

–Peripheral Nervous System (PNS)Peripheral Nervous System (PNS)

• Nerves (bundles of axons)Nerves (bundles of axons)

CNS: Parts of the BrainCNS: Parts of the Brain

• ForebrainForebrain

• MidbrainMidbrain

• HindbrainHindbrain

HindbrainHindbrain• CerebellumCerebellum

–coordinates muscular movementscoordinates muscular movements• MedullaMedulla oblongataoblongata

–regulates heart rate, blood regulates heart rate, blood pressure and breathingpressure and breathing

–contains reflex centers for contains reflex centers for vomiting, swallowing, sneezing, vomiting, swallowing, sneezing, hiccupping, and coughinghiccupping, and coughing

• PonsPons –helps regulate respirationhelps regulate respiration

ForebrainForebrain• ThalamusThalamus – switching station for – switching station for

sensory input for all senses but sensory input for all senses but smell; relays sensory info to smell; relays sensory info to cerebrum and motor info from the cerebrum and motor info from the cerebrumcerebrum

• HypothalamusHypothalamus – control hunger, – control hunger, thirst, fatigue, anger, and body temp; thirst, fatigue, anger, and body temp; regulates pituitary glandregulates pituitary gland

ForebrainForebrain• CerebrumCerebrum – divided – divided into into

left and right left and right hemispheres hemispheres

• CorpusCorpus callosumcallosum – – major connection major connection between hemispheresbetween hemispheres

–Left hemisphere primarily Left hemisphere primarily responsible for right side of body; responsible for right side of body; right hemisphere primarily right hemisphere primarily responsible for left sideresponsible for left side

ForebrainForebrain• CerebralCerebral cortexcortex – outer covering of – outer covering of

gray mattergray matter

–The more convoluted the surface, The more convoluted the surface, the more surface area, the more the more surface area, the more neuronsneurons

ForebrainForebrain• CerebrumCerebrum – divided into frontal, – divided into frontal,

temporal, parietal, and occipital temporal, parietal, and occipital lobeslobes

• Frontal lobeFrontal lobe–Contains the Contains the

primary motor primary motor cortex cortex (controls (controls

actions of actions of skeletal muscles) and olfactory skeletal muscles) and olfactory cortex (smell)cortex (smell)

ForebrainForebrain• Parietal lobeParietal lobe

–Contains the primary Contains the primary somatosensory cortex and somatosensory cortex and gustatory cortex (taste)gustatory cortex (taste)

• Temporal lobeTemporal lobe–Contains Contains auditory auditory

cortex cortex (sound) (sound)• Occipital lobeOccipital lobe

–Contains visual Contains visual cortex cortex (sight)(sight)

PNSPNS

• Sensory – transmits impulses Sensory – transmits impulses from the sense organs (such as from the sense organs (such as the ears and taste buds) to the the ears and taste buds) to the CNS CNS

• Motor – transmits impulses from Motor – transmits impulses from the CNS to the muscles or the CNS to the muscles or glands (somatic or autonomic)glands (somatic or autonomic)

• Somatic – conscious movement Somatic – conscious movement of the bodyof the body

• Autonomic – regulates activities Autonomic – regulates activities that are automatic or involuntarythat are automatic or involuntary

–Sympathetic (stress, high Sympathetic (stress, high energy) and Parasympathic energy) and Parasympathic (leisure, rest) are antagonistic (leisure, rest) are antagonistic systems that turn an autonomic systems that turn an autonomic response on or offresponse on or off

Sympathetic effects:Sympathetic effects:• dilates pupildilates pupil• accelerates heartbeat & respirationaccelerates heartbeat & respiration• inhibits stomach & intestine activityinhibits stomach & intestine activity• relaxes urinary bladderrelaxes urinary bladderParasympathetic effects:Parasympathetic effects:• constricts pupilconstricts pupil• slows heartbeat & respirationslows heartbeat & respiration• stimulates stomach & intestine activitystimulates stomach & intestine activity• contracts urinary bladdercontracts urinary bladder

Reflex ArcReflex Arc• Some actions don’t/can’t wait for Some actions don’t/can’t wait for

your brain to interpret the signalyour brain to interpret the signal

• Reflexes are involuntary actions; Reflexes are involuntary actions; they travel from ____ to ____:they travel from ____ to ____:

–Receptors (nerve “endings”)Receptors (nerve “endings”)

–Sensory neuronsSensory neurons

–InterneuronsInterneurons

–Motor neuronsMotor neurons

–Effectors (muscles or glands)Effectors (muscles or glands)

Types of Sensory ReceptorsTypes of Sensory Receptors• Thermoreceptors – detect heat and Thermoreceptors – detect heat and

coldcold

• Pain receptors (nocioceptors) – Pain receptors (nocioceptors) – detect chemicals released from detect chemicals released from injured cellsinjured cells

• Mechanoreceptors – Mechanoreceptors – detect mechanical detect mechanical energy (touch, energy (touch, pressure, vibration)pressure, vibration)

Types of Sensory ReceptorsTypes of Sensory Receptors• Chemoreceptors – detect chemicalsChemoreceptors – detect chemicals

• Photoreceptors – detect light energyPhotoreceptors – detect light energy

• Electroreceptors – detect electrical Electroreceptors – detect electrical fieldsfields

How are sounds sensed?How are sounds sensed?

• The ear captures, transmits, and The ear captures, transmits, and converts sound into electrical converts sound into electrical signalssignals

• Ear has three basic parts:Ear has three basic parts:

1.1. Outer earOuter ear

2.2. Middle earMiddle ear

3.3. Inner earInner ear

How are sounds sensed?How are sounds sensed?

• Outer ear: external ear (pinna) and Outer ear: external ear (pinna) and auditory canal auditory canal

–Funnels soundFunnels sound

–Sound waves vibrate the Sound waves vibrate the tympanic membranetympanic membrane

How are sounds sensed?How are sounds sensed?

• Middle earMiddle ear

–Tympanic membrane (ear drum)Tympanic membrane (ear drum)

–Three tiny bones: malleus Three tiny bones: malleus (hammer), incus (anvil), stapes (hammer), incus (anvil), stapes (stirrup); transfer vibrations to (stirrup); transfer vibrations to the oval window on the cochleathe oval window on the cochlea

–Eustachian tube – equalize Eustachian tube – equalize pressurepressure; connects middle ear to ; connects middle ear to pharynxpharynx

How are sounds sensed?How are sounds sensed?

• Inner ear: cochleaInner ear: cochlea–converts vibrations into converts vibrations into

electrical signalselectrical signals–As the oval window vibrates, it As the oval window vibrates, it

sets the cochlear fluid in motionsets the cochlear fluid in motion–Moving fluid brushes over hairsMoving fluid brushes over hairs–Bending of hairs is sensed by Bending of hairs is sensed by

mechanorecptors and sends the mechanorecptors and sends the signal to the brain (auditory signal to the brain (auditory nerve)nerve)

EquilibriumEquilibrium• Equilibrium is maintained by the Equilibrium is maintained by the

semicircular canalssemicircular canals

EquilibriumEquilibrium• The semicircular canals are The semicircular canals are

arranged in the X, Y, and Z planesarranged in the X, Y, and Z planes

–Therefore, any movement in any Therefore, any movement in any direction will be perceiveddirection will be perceived

• Fluid in the canals brushes over Fluid in the canals brushes over hairshairs

• Movement of hairs is sensed and Movement of hairs is sensed and the signal is sent to the brainthe signal is sent to the brain

EquilibriumEquilibrium• Dizziness can be due to the Dizziness can be due to the

momentum of the fluid in the momentum of the fluid in the canalscanals

–You’ve stopped moving, but the You’ve stopped moving, but the movement of the fluid in the movement of the fluid in the semicircular canals makes you semicircular canals makes you think you’re still movingthink you’re still moving

Fish “Hearing” – Lateral Fish “Hearing” – Lateral LinesLines

• Contains mechanoreceptors that Contains mechanoreceptors that function similarly function similarly to mammalian to mammalian inner earinner ear

• Gives info about Gives info about direction and direction and velocity of water velocity of water flowing over fish’s flowing over fish’s bodybody

How is light sensed?How is light sensed?• Sclera – tough, white layerSclera – tough, white layer

• Conjunctiva – external cover of Conjunctiva – external cover of sclera; keeps eye moist; conjuctivitis sclera; keeps eye moist; conjuctivitis (pink (pink eye)eye)

How is light sensed?How is light sensed?• Cornea – transparent covering in Cornea – transparent covering in

front of eyefront of eye

• Choroid – thin, pigmented layer Choroid – thin, pigmented layer lining interior surface of the sclera; lining interior surface of the sclera; prevents light rays from scattering prevents light rays from scattering and distorting the imageand distorting the image

• Iris regulates size of pupil/amount of Iris regulates size of pupil/amount of light into eyelight into eye

How is light sensed?How is light sensed?• Lens focuses light on retinaLens focuses light on retina

• Retina – Contains photoreceptors Retina – Contains photoreceptors (Except at the optic disk where the (Except at the optic disk where the optic nerve attaches)optic nerve attaches)

–Rods: Black and WhiteRods: Black and White

–Cones: ColorCones: Color

• Optic nerve takes electric signals Optic nerve takes electric signals from eye to brainfrom eye to brain

Rods and ConesRods and Cones• ~ 125 million rod cells~ 125 million rod cells

–Rod cells are light sensitive but do Rod cells are light sensitive but do not distinguish colorsnot distinguish colors

• ~ 6 million cone cells~ 6 million cone cells

–Not as light sensitive as rods but Not as light sensitive as rods but provide color visionprovide color vision

–Most highly concentrated on the Most highly concentrated on the fovea – area of retina lacking rodsfovea – area of retina lacking rods

How are scents sensed?How are scents sensed?

• Insects smell through their legs Insects smell through their legs and antennaeand antennae

Male silkworm moth Bombyx mori

Sensory hairs on antennae detect pheromones released by female

How are scents sensed?How are scents sensed?

• Olfactory nerves are stimulated Olfactory nerves are stimulated when chemicals touch themwhen chemicals touch them

• Different chemicals create Different chemicals create different responses in the different responses in the olfactory nerves; hence we detect olfactory nerves; hence we detect different smellsdifferent smells

How are tastes sensed?How are tastes sensed?• Taste buds on tongue act just like the Taste buds on tongue act just like the

olfactory nervesolfactory nerves

–Different chemicals stimulate the Different chemicals stimulate the nerves in the taste buds differently; nerves in the taste buds differently; hence we detect different tasteshence we detect different tastes

• Four “primary” tastes Four “primary” tastes are bitter, sour, salty, are bitter, sour, salty, and sweetand sweet