Biology 103 - Main points/Questions

1. What does a neuron look like?

2. Why do membranes have charges?

3. How can these charges change?

Functions of the Nervous System • Process and coordinate:

– sensory input:• from inside and outside body

– motor commands:• control activities of peripheral organs (e.g.,

skeletal muscles)

– Integration – occurs in the central nervous system

– higher functions of brain:• intelligence, memory, learning, emotion

Coordinating all the different body systems and interacting with the external world are the job of the body’s control systems – the nervous system and the endocrine (hormone) system.

Aplysia (sea slug) neurons

• Neurons are nerve cells that transfer information within the body

• Neurons use two types of signals to communicate: – electrical signals (long-distance) and

– chemical signals (one cell to the next - short)

• Nervous systems process information in three stages: sensory input, integration, and motor output

Sensor

Sensory input

Integration

Effector

Motor output

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Whitematter

Spinal cord

Sensory information

Sensory neuron

Motor neuronInterneuron

Integration

Response

Three types of neurons

• These stages use three basic types of neurons – – sensory– association and– motor

Neuron Structure and Function• Most of a neuron’s organelles are in the

cell body• Most neurons have dendrites, that receive

signals from other neurons• The axon is typically a longer extension

that transmits signals to other cells • Many axons are wrapped by other cells

(glial cells) to speed signaling

A typical neuron & formation of the myelin sheath

Glial Cells

Big idea: Neuron membranes have a charge.

• Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential

• Messages are transmitted as changes in membrane potential

• The resting potential is the membrane potential of a neuron not sending signals

The Resting Potential• Why do neurons have a resting potential?

• Lets look at one ion - potassium (K+) – that is found in your neurons

• Cells have large amounts of potassium inside them and small amounts outside.

• Neurons have channels that let potassium cross the membrane – what does this do?

Electrochemical Gradients

Figure 12–9c, d

Electrochemical Gradients

Figure 12–9a, b

Innerchamber

Outerchamber

–90 mV

140 mM 5 mM

KCIKCI

K+

Cl–

Potassiumchannel

(a) Membrane selectively permeable to K+

• Potassium stops moving when charge is -90mV – Why?

The Resting Potential• Of course there are more charged ions and

molecules inside a neuron

• Sodium (Na+) is a key player in neuron signaling.

• There is lots of sodium outside the cell!

OUTSIDECELL

[K+]5 mM

[Na+]150 mM

INSIDECELL

[K+]140 mM

[Na+]15 mM

[A–]100 mM

(a)

• Two key ions for neurons

• Other molecules and ions add negative charge to the inside of a neuron.

The Resting Potential

• In your neuron the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside

• How do your neurons maintain this difference?

• Neurons are constantly working to maintain “resting” conditions

• This is because the membrane leaks ions• A neuron at rest contains many open K+

channels and few open Na+ channels; so lots of K+ diffuses out of the cell

Active resting in neurons

• Active transport allows cells to maintain concentration gradients that differ from their surroundings

• The sodium-potassium pump is one type of active transport system

EXTRACELLULAR

FLUID [Na+] high [K+] low

Na+

Na+

Na+ [Na+] low[K+] high CYTOPLASM

Cytoplasmic Na+ binds tothe sodium-potassium pump. 1

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

ATP P

ADP

2

Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3

K+ binds on theextracellular side andtriggers release of thephosphate group.

P P

K+

K+

4

Loss of the phosphaterestores the protein’s originalshape.

K+

K+

5

K+ is released, and thecycle repeats.

K+

K+

6

• K+ constantly leaks out of the neuron

• The flow of K+ ions out of the cell helps to maintain the resting potential

• A neuron at rest has a potential about -70 mV

Big idea: Action potentials are the signals conducted by axons

• Signals are passed down an axon as spikes in membrane potential

• These spikes, that briefly reverse membrane polarity, are called action potentials

• These action potentials are the basic form of communication for neurons

(a) Gentle touch

fires slowly1

silent2

2 1

• Neurons contain gated ion channels that open or close in response to stimuli

• Membrane potential changes in response to opening or closing of these channels

• What would happen if K+ permeability increased?

Changing membrane potential

3 Conditions of Gated Channels

1. Closed, but capable of opening

2. Open (activated)

3. Closed, not capable of opening (inactivated)

Stimuli

+50M

emb

ran

e p

ote

nti

al (

mV

)

–50 Threshold

Restingpotential

Hyperpolarizations–100

0 2 3 4

Time (msec)

(a) hyperpolarizations

0

1 5

• When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative

• This is called hyperpolarization

• What if Na+ gates open?

• If gated Na+ channels open and Na+ diffuses into the cell

• This causes a depolarization, a reduction in the membrane potential

Stimuli

+50M

emb

ran

e p

ote

nti

al (

mV

)

–50 Threshold

Restingpotential

Depolarizations

–1000 2 3 4

Time (msec)

(b) depolarizations

1 5

0

• If enough open the membrane in this region reaches threshold

• At this point a large number of Na+ channels open and sodium pours in

• What would this do to membrane potential?

Stimuli

+50M

emb

ran

e p

ote

nti

al (

mV

)

–50 Threshold

Restingpotential

Depolarizations

–1000 2 3 4

Time (msec)

(b) depolarizations

1 5

0

Strong depolarizing stimulus

+50M

emb

ran

e p

ote

nti

al (

mV

)

–50 Threshold

Restingpotential

–1000 2 3 4

Time (msec)

(c) Action potential

1 5

0

Actionpotential

6

• Membrane polarity flips!

• Then these channels shut & K+ open

• Potential drops back as K+ ions flow out

• This spike in charge is an action potential!

• This flipping and returning of the membrane potential is passed along a neuron down it’s axon

• The action potential flows down the axon as depolarization is pushed ahead of the action potential (propagation)

Big idea: Action potentials• starts with a slight of

membrane (closer to 0mv)– often no action potential is fired if

• at ~ -50mv channels open – allowing to pour (in/out)

depolarization

Threshold isn’t hit

Na+

gated

Big idea: Action potentials• at ~ -50mv gated channels open – allowing

Na+ to pour (out!)– this causes – They after a very short time

(~1msec.)

• channels also respond to voltage – but they are

– pours (in/out) – reversing the charge again

– They shut after driving charge

membrane potential to flipslam shut

K+

much slowerK+

below resting

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

ActionpotentialK+

K+

Na+

• Because the sodium gates lock shut an action potential cannot move “backwards”

• During the refractory period after an action potential, a second action potential cannot be initiated

• The refractory period is a result of a temporary inactivation of the Na+ channels

Figure 34.5 How an action potential is

generated

Generation of Action Potentials

What happens at the end of the axon?

• Axons end at a synapse • This is a small gap between one neuron

and another (or sometimes another cell)• Chemicals called neurotransmitters carry

information across the gap

Dendrites

Stimulus

Nucleus

Cellbody

Axonhillock

Presynapticcell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitter

A synapse between two neurons

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

• The presynaptic neuron synthesizes and packages neurotransmitter in synaptic vesicles located in the synaptic terminal

• The action potential causes the release of the neurotransmitter

• The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell

Postsynapticneuron

Synapticterminalsof pre-synapticneurons

5 µ

m