7 neurons

Neurons&

The NervousSystem

Neural Signaling

Response to stimulus involves:

•detection of stimulus•conduction of signal•processing•response

receives information

nucleus & organellesintegrates

info

carries signalto other cells

neuroglialcell

synapticterminal

p887

Speed of impulse – 30-90 m/sec

p887

Neuroglia• structural & functional support of neurons (surround axons)• two important neuroglial cells:

-Schwann cells (PNS)-oligodendrocytes (CNS)

•Cells wrap around axons several times to form myelin sheath (Figure).

Function?•Nodes of Ranvier (1-2 mm apart)

Louis Antoine Ranvier 1878discovered myelin and nodes.

CNS-brain-spinal cord

PNS(peripheralnervoussystem)-sensory neurons-motor neurons

Interneuronslink CNS & PNS

p886

p894

Ganglion

Cell bodies

Myelin sheath

Artery Vein Axon

100 µm

Nerve-consists ofhundreds (thousands)of axons wrapped together in connectivetissue.

bullfrog

-A mass of nervecell bodies.

Initiation of action potential

all or nothingmust reach a threshold

-Membrane potential isthe voltage differenceacross a cell’s membrane(cytoplasm is morenegative than outside the cell – resting potential is -70mV)

-Nerve impulses aredetected as a wave ofelectrical activity.(electrochemical change)

-With an all or nothingresponse, how is intensitydetected?

RestingPotential

p889

Resting Membrane Potential-Polarized

K+ leak Na+/K+ ATP channel pump

K+ leak channels maintain negative voltage inside the cell. There are few Na+ leak channels.

p893

p893

& Potassium Channel1. 2.

3. 4.

channelinactivated

Propagation of nerve impulse

polarized depolarized repolarized

Voltage gated Na+ channel

How do voltage gated channels work?

Science4/3/10

K+ voltage gated

channels

Area of depolarization Potassiumchannel

Sodiumchannel

Area of repolarization

Area of depolarization

Action potential

Action potential

Resting state Depolarization

Repolarization Return to resting state

Extracellularfluid

Sodiumchannel

Potassiumchannel

Cytoplasm

1 2

3 4

2

1

3

4

Na+/K+ ATP Pump

-Average neuron contains 1,000,000 pumps.

-Speed – 200 Na+ ions & 135 K+ ions per second.

Fig. 44.11 p878

p894

Neural circuits

Convergence

Divergence

Synapticvesicles

Neurotransmittermolecules

Receptor

Plasma membrane of postsynaptic

neuron

Presynapticterminal

Synapticcleft

Na+

0.25 µm

presynapticneuron

postsynapticneuron

synapticterminal

What happens to theneural transmitter?

20 nM(10-9 M)

p896

Excitatory Neurotransmitter

Promotes Depolarizationof Postsynaptic Neuron

Inhibitory Neurotransmitter

Hyperpolarizes the cell

p898

Green = excitatoryRust = inhibitory

Events at the Synapse

p897

Cocaine-Binds the dopamine transporters and prevents reuptake ofthe neurotransmitter.

-Dopamine continues to stimulate the postsynaptic cell.

Effect of Alcohol on the NS

•Increases absorbance of K+

-neuron cannot repolarize

-no repolarization - prevents propagation of action potential.

-no action potential, no influx of Ca+2, hence no release of neural transmitter.

•Leads to slurred speech, slow reflexes, blurred vision & loss of inhibition.

Sensory Receptors•Mechanoreceptors - pressure•Energy Detecting Receptors

heatcoldlight - photoreceptors

•Chemoreceptorsconcentration of a particular compound

How do they work? mechanoreceptors – stretch receptors -fire when stimulated

- no adaptation sensory receptors – fire at a continuous basal level - undergo adaption

Ex. – chemoreceptors & photoreceptors

thermal

Ruffinicorpuscle(pressure)

Der

mis

500 µm

Paciniancorpuscle(deep pressure,touch)Hair follicle

receptor(hair displacement)

Merkel disc(touch, pressure)

Meissnercorpuscle(touch, pressure)

HairFree nerveendings(pain)

Ep

ider

mis

Su

bcu

tan

eou

sti

ssu

e

Pressure

Sodiumchannelopens

Sodiumchannelclosed

papilla – locationof taste buds.

“wall-like”

“leaf-like”

“mushroom-like”

Taste buds – a collectionof chemosensitive epithelial cells associated with a sensory neuron.

Circumvallatepapilla

Each taste bud is “onion”shaped structures of between 50-100 taste cells.

Molecules in the fooddissolve in saliva and contact the taste receptors through the taste pores.

Smith &MargolskeeScientific AmericanMarch 2001

SaltsNa+ ions enter Na+ channels& depolarize the cell. The cell repolarizes by openingK+ gates.


Sour - Acids

1. H+ directly enter channels.

2. H+ bind to Na+ channels causing them to open.

3. H+ bind to K+ channels andclose these channels (no K+

leaves).

Taste Cell


Sugar molecules bind to a receptor. This activates a G-protein & and the secondarymessenger cAMP causingK+ leak channels to close. The Na+ leak channels allow Na+ in a & the neuron depolarizes.

G protein

GTP

K+ channelopen

Adenylylcyclase

Proteinkinase A

Sugar molecule

Receptor

K+ channelcloses

activates

1 2 3

4

5

6Gustducin

cAMP closes K+ leakchannels, but Na+ leakchannels stay open.

The Tongue Taste Map


1. Although each neuron responds more strongly to one type of tastant, it canalso generate a stimulus to otherdissimilar molecules.

2. Specific tastes might be distinguishedby the brain due to a pattern of activityacross neural networks.

3. Smith & Margolskee claim that tastediscrimination depends on the relativeactivity of different neuron types, eachof which must contribute to the overallpattern of activity in order to distinguishbetween different stimuli.

Technology

7 neurons