Nervous System(Ch. 48)

• Every time you move a muscle & every time you think a thought, your nerve cells are hard at work. They are processing information: receiving signals, deciding what to do with them, & dispatching new messages off to their neighbors. Some nerve cells communicate directly with muscle cells, sending them the signal to contract. Other nerve cells are involved solely in the bureaucracy of information, spending their lives communicating only with other nerve cells. But unlike our human bureaucracies, this processing of information must be fast in order to keep up with the ever-changing demands of life.

Why do animals need a nervous system?

• What characteristics do animals need in a nervous system?

– fast – accurate– reset quickly

Remember…think aboutthe bunny… Poor bunny!

Overview of information processing by nervous systems

Sensor

Effector

Motor output

Integration

Sensory input

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Nervous system cells

dendrites

cell body

axon

synaptic terminal

Neuron a nerve cell

Structure fits function many entry points for

signal one path out transmits signal

signal direction

signaldirection

dendrite cell body axon synapse

myelin sheath

Fun facts about neurons• Most specialized cell in

animals• Longest cell

– blue whale neuron• 10-30 meters

– giraffe axon• 5 meters

– human neuron• 1-2 meters

Nervous system allows for 1 millisecond response time

• Think dominoes! – start the signal

• knock down line of dominoes by tipping 1st one trigger the signal

– propagate the signal• do dominoes move down the line?

no, just a wave through them! – re-set the system

• before you can do it again, have to set up dominoes again reset the axon

Transmission of a signal

Transmission of a nerve signal• Neuron has similar system

– protein channels are set up – once first one is opened, the rest open in

succession• all or nothing response

– a “wave” action travels along neuron – have to re-set channels so neuron can react

again

Cells: surrounded by charged ions

• Cells live in a sea of charged ions– anions (negative)

• more concentrated within the cell• Cl-, charged amino acids (aa-)

– cations (positive)• more concentrated in the extracellular fluid• Na+

Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+

Cl-

K+ Cl- Cl- Cl-K+

aa-K+ Cl- Cl-

aa- aa-aa-

aa- aa-K+

K+channel leaks K+ +

–

Cells have voltage!• Opposite charges on opposite sides of cell membrane• This is an imbalanced condition.• The positively + charged ions repel each other as do the negatively -

charged ions. They “want” to flow down their electrical gradient and mix together evenly.

• This means that there is energy stored here, like a dammed up river.• Voltage is a measurement of stored electrical energy. Like “Danger High

Voltage” = lots of energy (lethal).– membrane is polarized

• negative inside; positive outside• charge gradient• stored energy (like a battery)

+ + + + + + + ++ + + + + + +

+ + + + + + + ++ + + + + + +

– – – – – – – ––– – – – –

– – – – – – – ––– – – – –

Measuring cell voltageVoltage = measures the difference in concentration of charges.The positives are the “hole” you leave behind when you move an electron.Original experiments on giant squid neurons!

unstimulated neuron = resting potential of -70mV

How does a nerve impulse travel?• Stimulus: nerve is stimulated

– reaches threshold potential • open Na+ channels in cell membrane• Na+ ions diffuse into cell

– charges reverse at that point on neuron• positive inside; negative outside • cell becomes depolarized

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

The 1stdomino

goesdown!

Gate

+ –

+

+

channel closed

channel open

How does a nerve impulse travel?• Wave: nerve impulse travels down neuron

– change in charge opens next Na+ gates down the line • “voltage-gated” channels

– Na+ ions continue to diffuse into cell– “wave” moves down neuron = action potential

– – + + + + + +– + + + + + +

– – + + + + + +– + + + + + +

+ + – – – – – –+ – – – – – –

+ + – – – – – –+ – – – – – –Na+

wave

The restof the

dominoes fall!

How does a nerve impulse travel?• Re-set: 2nd wave travels down neuron

– K+ channels open• K+ channels open up more slowly than Na+ channels

– K+ ions diffuse out of cell– charges reverse back at that point

• negative inside; positive outside

+ – – + + + + +– – + + + + +

+ – – + + + + +– – + + + + +

– + + – – – – –+ + – – – – –

– + + – – – – –+ + – – – – –Na+

K+

wave

Setdominoesback upquickly!

How does a nerve impulse travel?• Combined waves travel down neuron

– wave of opening ion channels moves down neuron– signal moves in one direction

• flow of K+ out of cell stops activation of Na+ channels in wrong direction

+ + – – + + + ++ – – + + + +

+ + – – + + + ++ – – + + + +

– – + + – – – –– + + – – – –

– – + + – – – –– + + – – – –Na+

wave

K+Readyfor

next time!

How does a nerve impulse travel?• Action potential propagates

– wave = nerve impulse, or action potential– brain finger tips in milliseconds!

• K+ gates open more slowly than Na+ gates

+ + + + – – + ++ + + – – + +

+ + + + – – + ++ + + – – + +

– – – – + + – –– – – + + – –

– – – – + + – –– – – + + – –Na+

K+

wave

In theblink ofan eye!

Voltage-gated channels• Ion channels open & close in response to changes in charge across

membrane – Na+ channels open quickly in response to depolarization & close

slowly– K+ channels open slowly in response to depolarization & close

slowly• Na+ channel closed when nerve isn’t doing anything.

+ + + + + – + ++ + + + – – +

+ + + + + – + ++ + + + – – +

– – – – – + – –– – – – + + –

– – – – – + – –– – – – + + –Na+

K+

wave

Structure& function!

How does the nerve re-set itself?• After firing a neuron has to re-set itself

– Na+ needs to move back out– K+ needs to move back in– both are moving against concentration gradients

• need a pump!!

+ + + + + – – ++ + + + + – –

+ + + + + – – ++ + + + + – –

– – – – – + + –– – – – – + +

– – – – – + + –– – – – – + +Na+

Na+Na+

Na+ Na+Na+

K+K+K+K+ Na+ Na+

Na+Na+Na+

Na+Na+

Na+Na+

Na+

Na+

K+K+K+K+

K+K+

K+ K+

wave

K+

Na+

A lot ofwork todo here!

How does the nerve re-set itself?• Sodium-Potassium pump

– active transport protein in membrane• requires ATP

– 3 Na+ pumped out– 2 K+ pumped in– re-sets charge

across membrane

ATP

That’s a lot of ATP !

Feed me somesugar quick!

• Dominoes set back up again.• Na/K pumps are one of the main drains on ATP

production in your body. Your brain is a very expensive organ to run!

Neuron is ready to fire again

Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+

K+

K+ K+ K+ K+

K+

aa-K+ K+ K+

aa- aa-aa-

aa- aa-

+ + + + + + + ++ + + + + + +

+ + + + + + + ++ + + + + + +

– – – – – – – –– – – – – – –

– – – – – – – –– – – – – – –

resting potential

1. Resting potential2. Stimulus reaches threshold

potential3. Depolarization

Na+ channels open; K+ channels closed

4. Na+ channels close; K+ channels open

5. Repolarizationreset charge gradient

6. UndershootK+ channels close slowly

Action potential graph

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mV DepolarizationNa+ flows in

20 mV

30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

2

3

4

5

6

Mem

bra

ne

po

ten

tial

Myelin sheath

signaldirection

Axon coated with Schwann cells insulates axon speeds signal

signal hops from node to node saltatory conduction

150 m/sec vs. 5 m/sec(330 mph vs. 11 mph)

myelin sheath

myelin

axon

Na+

Na+

++ + + + –

–

action potential

saltatoryconduction

Multiple Sclerosis immune system (T cells)

attack myelin sheath loss of signal

Synapse

Impulse has to jump the synapse!– junction between neurons– has to jump quickly from one

cell to next

What happens at the end of the axon?

How does the wave

jump the gap?

axon terminal

synaptic vesicles

muscle cell (fiber)

neurotransmitteracetylcholine (ACh)receptor protein

Ca++

synapse

action potential

Chemical synapse Events at synapse

action potential depolarizes membrane

opens Ca++ channels neurotransmitter vesicles

fuse with membrane release neurotransmitter to

synapse diffusion neurotransmitter binds with

protein receptor ion-gated channels open

neurotransmitter degraded or reabsorbed

We switched…from an electrical signal

to a chemical signal

• Calcium is a very important ion throughout your body. It will come up again and again involved in many processes.

Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM)

Postsynapticneuron

Synapticterminal

of presynapticneurons

5 µ

m

Nerve impulse in next neuron • Post-synaptic neuron

– triggers nerve impulse in next nerve cell• chemical signal opens ion-gated channels • Na+ diffuses into cell• K+ diffuses out of cell

– switch back to voltage-gated channel

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

K+

K+K+

Na+ Na+

Na+

ion channel

binding site ACh

Here wego again!

Summation of postsynaptic potentials

E1 E1 E1 E1E1E1 + E2 E1 + II

ActionpotentialAction

potentialRestingpotential

Threshold of axon ofpostsynaptic neuron

(a) Subthreshold, nosummation

(b) Temporal summation (c) Spatial summation (d) Spatial summationof EPSP and IPSP

Terminal branch of presynaptic neuron

Postsynaptic neuron E1

E1E1

E2

E1

IAxonhillock

0

–70

Mem

bra

ne p

oten

tial (

mV

)

Neurotransmitters• Acetylcholine

– transmit signal to skeletal muscle• Epinephrine (adrenaline) & norepinephrine

– fight-or-flight response • Dopamine

– widespread in brain– affects sleep, mood, attention & learning– lack of dopamine in brain associated with Parkinson’s

disease– excessive dopamine linked to schizophrenia

• Serotonin– widespread in brain– affects sleep, mood, attention & learning

• Nerves communicate with one another and with muscle cells by using neurotransmitters. These are small molecules that are released from the nerve cell and rapidly diffuse to neighboring cells, stimulating a response once they arrive. Many different neurotransmitters are used for different jobs: – glutamate excites nerves into action; – GABA inhibits the passing of information; – dopamine and serotonin are involved in the subtle messages of thought and

cognition. – The main job of the neurotransmitter acetylcholine is to carry the signal from

nerve cells to muscle cells. When a motor nerve cell gets the proper signal from the nervous system, it releases acetylcholine into its synapses with muscle cells. There, acetylcholine opens receptors on the muscle cells, triggering the process of contraction. Of course, once the message is passed, the neurotransmitter must be destroyed, otherwise later signals would get mixed up in a jumble of obsolete neurotransmitter molecules. The cleanup of old acetylcholine is the job of the enzyme acetylcholinesterase.

Neurotransmitters • Weak point of nervous system

– any substance that affects neurotransmitters or mimics them affects nerve function

• gases: nitrous oxide, carbon monoxide• mood altering drugs:

– stimulants» amphetamines, caffeine, nicotine

– depressants» quaaludes, barbiturates

• hallucinogenic drugs: LSD, peyote• SSRIs: Prozac, Zoloft, Paxil• Poisons

• Selective serotonin reuptake inhibitor

Pity the Test Mice

snake toxin blockingacetylcholinesterase active site

Acetylcholinesterase

acetylcholinesterase

neurotoxin in green

• Enzyme which breaks downacetylcholine neurotransmitter – acetylcholinesterase inhibitors =

neurotoxins• snake venom, sarin, insecticides

active site in red

Since acetylcholinesterase has an essential function, it is a potential weak point in our nervous system. Poisons and toxins that attack the enzyme cause acetylcholine to accumulate in the nerve synapse, paralyzing the muscle. Over the years, acetylcholinesterase has been attacked in many ways by natural enemies. For instance, some snake toxins attack acetylcholinesterase.Acetylcholinesterase is found in the synapse between nerve cells and muscle cells. It waits patiently and springs into action soon after a signal is passed, breaking down the acetylcholine into its two component parts, acetic acid and choline. This effectively stops the signal, allowing the pieces to be recycled and rebuilt into new neurotransmitters for the next message. Acetylcholinesterase has one of the fastest reaction rates of any of our enzymes, breaking up each molecule in about 80 microseconds.

Is the acetylcholinesterase toxin a competitive or non-competitive inhibitor?

Questions to ponder…

• Why are axons so long?– Transmit signal quickly. The synapse is the choke point. Reduce the

number of synapses & reduce the time for transmission

• Why have synapses at all?Decision points (intersections of multiple neurons) & control

points

• How do “mind altering drugs” work?– caffeine, alcohol, nicotine, marijuana…– Affect neurotransmitter release, uptake & breakdown. React with or

block receptors & also serve as neurotransmitter mimics

• Why are axons so long?– Transmit signal quickly. The synapse is the choke point.

Reduce the number of synapses & reduce the time for transmission

• Do plants have — or need — nervous systems?– They react to stimuli — is that a nervous system?

Depends on how you define nervous system.– But if you can’t move quickly, there is very little adaptive

advantage of a nervous system running at the speed of electrical transmission.

Organization of some nervous systems

Nerve netNervering

Radialnerve

Eyespot

BrainNerve cord

Transversenerve

Brain

Segmentalganglion

Ventral nervecord

Brain

Ventral nervecord

Segmentalganglia

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

Sensoryganglion

Spinalcord

(dorsalnervecord)

Brain

(d) Leech (annelid)(c) Planarian (flatworm)(b) Sea star (echinoderm)(a) Hydra (cnidarian)

(e) Insect (arthropod) (f) Chiton (mollusc) (g) Squid (mollusc) (h) Salamander (chordate)

The vertebrate nervous systemCentral nervous

system (CNS)Peripheral nervous

system (PNS)

Brain

Spinal cord

Cranialnerves

Gangliaoutside

CNS

Spinalnerves

Functional hierarchy of the vertebrate peripheral nervous system

Peripheralnervous system

Somaticnervoussystem

Autonomicnervoussystem

Sympatheticdivision

Parasympatheticdivision

Entericdivision

The parasympathetic and sympathetic divisions of the autonomic nervous system

Parasympathetic division Sympathetic division

Action on target organs: Action on target organs:

Location ofpreganglionic neurons:

brainstem and sacralsegments of spinal cord

Neurotransmitterreleased by

preganglionic neurons:acetylcholine

Location ofpostganglionic neurons:

in ganglia close to orwithin target organs

Neurotransmitterreleased by

postganglionic neurons:acetylcholine

Constricts pupilof eye

Stimulates salivarygland secretion

Constrictsbronchi in lungs

Slows heart

Stimulates activityof stomach and

intestines

Stimulates activityof pancreas

Stimulatesgallbladder

Promotes emptyingof bladder

Promotes erectionof genitalia

Cervical

Thoracic

Lumbar

Synapse

Sympatheticganglia

Dilates pupilof eye

Inhibits salivary gland secretion

Relaxes bronchiin lungs

Accelerates heart

Inhibits activity of stomach and intestines

Inhibits activityof pancreas

Stimulates glucoserelease from liver;inhibits gallbladder

Stimulatesadrenal medulla

Inhibits emptyingof bladder

Promotes ejaculation and vaginal contractionsSacral

Location ofpreganglionic neurons:

thoracic and lumbarsegments of spinal cord

Neurotransmitterreleased by

preganglionic neurons:acetylcholine

Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near

spinal cord

Neurotransmitterreleased by

postganglionic neurons:norepinephrine

Development of the human brainEmbryonic brain regions Brain structures present in adult

Forebrain

Telencephalon

Midbrain

Hindbrain

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

Midbrain Hindbrain

Forebrain

(a) Embryo at one month (b) Embryo at five weeks (c) Adult

MesencephalonMetencephalon

Myelencephalon

Spinal cord

Diencephalon

Telencephalon

Cerebral hemisphereDiencephalon:

Hypothalamus

ThalamusPineal gland

(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Cerebellum

Central canal

Spinal cord

Pituitarygland

Ventricles, gray matter, and white matter

Gray matter

Whitematter

Ventricles

Medulla, Pons and Midbrain

The Cerebellum

The Diencephalon

The Cerebrum

The human cerebrum viewed from the rear

Left cerebralhemisphere

Corpuscallosum

Neocortex

Right cerebralhemisphere

Basalnuclei

The human cerebral cortexFrontal lobe

Temporal lobe Occipital lobe

Parietal lobe

Frontalassociation

area

Speech

Smell

Hearing

Auditoryassociation

areaVision

Visualassociation

area

Somatosensoryassociation

area

Reading

Speech

Taste

Som

atos

enso

ry c

orte

x

Mot

or c

orte

x

Body representations in the primary motor and primary somatosensory cortices

Tongue

JawLips

Face

Eye

Brow

Neck

Thumb

Fingers

HandW

ristForearmE

lbow

ShoulderT

runk

Hip

Knee

Primarymotor cortex Abdominal

organs

Pharynx

Tongue

TeethGumsJaw

Lips

Face

Nose

Eye

FingersHand

ForearmE

lbowU

pper arm

Trunk H

ip

Leg

Thumb

Neck

Head

Genitalia

Primarysomatosensory cortex

Toes

Parietal lobeFrontal lobe

Mapping language areas in the cerebral cortex

Hearingwords

Seeingwords

Speakingwords

Generatingwords

Max

Min

Mechanism of long-term potentiation in the vertebrate brain

PRESYNAPTIC NEURON

NO

Glutamate

NMDAreceptor

Signal transduction pathways

NO

Ca2+

AMPA receptor

POSTSYNAPTIC NEURON

Ca2+ initiates the phos-phorylation of AMPA receptors,

making them more responsive. Ca2+ also causes more AMPA

receptors to appear in the postsynaptic membrane.

5

Ca2+ stimulates thepostsynaptic neuron to

produce nitric oxide (NO).

6

The presynapticneuron releases glutamate.1

Glutamate binds to AMPAreceptors, opening the AMPA-

receptor channel and depolarizingthe postsynaptic membrane.

2

Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneously

depolarized, the NMDA-receptorchannel opens.

3

Ca2+ diffuses into thepostsynaptic neuron.

4

NO diffuses into thepresynaptic neuron, causing it to release more glutamate.

7

P

Microscopic signs of Alzheimer’s diseaseSenile plaque Neurofibrillary tangle

20 m

Ponder this…Any Questions??

Make sure you can do the following:1. Compare and contrast the regulatory structures and

functions of the nervous and endocrine systems2. Diagram the processes by which nervous signals are

transmitted by and between neurons.3. Label all parts of a neuron4. Explain the relationships between the major divisions

of the mammalian nervous system.5. Explain the relationships between the major divisions

of the human brain6. Explain the causes of nervous system disruptions and

how disruptions of the nervous system can lead to disruptions of homeostasis.