The Nervous Impulse - Linn–Benton Community …cf.linnbenton.edu/mathsci/bio/waitea/upload/Lecture_02...probability of nerve impulse production. Figure 11.9b Hyperpolarizing stimulus

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Neural Signaling

Chapter 11

The Nervous System

The Nervous Impulse

• Dependent upon a resting potential across the

cell membrane

– Magnitude of potential is determined by

• Leakage channels for sodium and potassium

• Active transport carriers (Sodium/Potassium pump)

– The neuron is polarized

• Impulse results from depolarization

Factors that contribute to resting membrane potential

Check out the A&P Flix on Mastering “Resting Membrane Potential”

Figure 11.8

Finally, let’s add a pump to compensate

for leaking ions.

Na+-K+ ATPases (pumps) maintain the

concentration gradients, resulting in the

resting membrane potential.

Suppose a cell has only K+ channels...

K+ loss through abundant leakage

channels establishes a negative

membrane potential.

Now, let’s add some Na+ channels to our cell...

Na+ entry through leakage channels reduces

the negative membrane potential slightly.

The permeabilities of Na+ and K+ across the

membrane are different.

The concentrations of Na+ and K+ on each side of the membrane are different.

Na+

(140 mM )K+

(5 mM )

K+ leakage channels

Cell interior–90 mV



K+

Na+

Na+-K+ pump

K+

K+K+

K+

Na+

K+

K+K

Na+

K+K+Na+

K+K+

Outside cell

Inside cellNa+-K+ ATPases (pumps)

maintain the concentration

gradients of Na+ and K+

across the membrane.

The Na+ concentration

is higher outside the

cell.

The K+ concentration

is higher inside the

cell.

K+

(140 mM )Na+

(15 mM )

Figure 11.7

Voltmeter

Microelectrodeinside cell

Plasmamembrane

Ground electrodeoutside cell

Neuron

Axon

The Nervous Impulse

• Polarization

– Voltage across the plasma membrane

– Inside of the cell is more negative than the outside

• Resting potential

– Polarization leads to attraction between opposite

charges across the membrane

– When a neuron is at rest, average potential is -70mV

• Neurons use changes in membrane potential as

signals to receive, integrate, and send information

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The Nervous Impulse

• Types of changes

– Depolarization

• Decrease in membrane potential (interior becomes less

negative)

– Hyperpolarization

• Increase in membrane potential (inside becomes more

negative)

Figure 11.9a

Depolarizing stimulus

Time (ms)

Inside

positive

Inside

negative

Resting

potential

Depolarization

(a) Depolarization: The membrane potential

moves toward 0 mV, the inside becoming

less negative (more positive). This increases the

probability of nerve impulse production.

Figure 11.9b

Hyperpolarizing stimulus

Time (ms)

Resting

potential

Hyper-

polarization

(b) Hyperpolarization: The membrane

potential increases, the inside becoming

more negative. This decreases the probability

of nerve impulse production.

The Nervous Impulse

• Changes in polarization are produced by…

– Anything that changes ion concentration across the membrane

– Anything that changes membrane permeability to an ion (most important)

• Largely due to changes in the number of open ion channels

• Membrane channels

– Chemically gated (ligand gated)

– Voltage gated

Figure 11.6

(b) Voltage-gated ion channels open and close in response

to changes in membrane voltage.

Na+

Na+

Closed Open

Receptor

(a) Chemically (ligand) gated ion channels open when the

appropriate neurotransmitter binds to the receptor,

allowing (in this case) simultaneous movement of

Na+ and K+.

Na+

K+

K+

Na+

Neurotransmitter chemical

attached to receptor

Chemical

binds

Closed Open

Membrane

voltage

changes

The Nervous Impulse

• There also are mechanically gated membrane

channels

– Open in response to physical deformation (touch,

pressure, sound waves)

– Found in sensory receptors

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The Graded Potential

• Short lived, localized changes in membrane potential

– Due to incoming signals, usually energy or

neurotransmitters

• Channels open → ions flow

• Short distance signals

• May be depolarization or hyperpolarization events

Figure 11.10a

Depolarized region

Stimulus

Plasma

membrane

(a) Depolarization: A small patch of the

membrane (red area) has become depolarized.

Figure 11.10b

(b) Spread of depolarization: The local currents

(black arrows) that are created depolarize

adjacent membrane areas and allow the wave of depolarization to spread.

Figure 11.10c

Distance (a few mm)

–70

Resting potential

Active area

(site of initial

depolarization)

(c) Decay of membrane potential with distance: Because current

is lost through the “leaky” plasma membrane, the voltage declines

with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals.

Me

mb

ran

e p

ote

nti

al

(mV

)

The Action Potential

�Long distance signal

� Initiated by sufficient depolarization at site of graded potential

� Must reach threshold – usually a change of ~100mV

�Opening of specific voltage gated channels

�Does not decrease in strength with distance

� “All or none”

�A.K.A. nerve impulse

Stimuli that Initiate Action Potentials

• Light

• Heat

• Chemicals

• Mechanical energy

• Chemical stimuli

from other

neurons

Sensory Neurons Motor & Association Neurons

Threshold stimulus always required

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Dendrites

(receptiveregions)

Cell body

(biosynthetic centerand receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon

(impulsegeneratingregion)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axon terminals(secretoryregion)

Dendriticspine

Neuron cell body

(a)

(b)

(impulseconductingregion)


�The players:

Sodium Channel

• Opens instantly

• Can’t sustain (self-inhibits)

Na+

Potassium

channel

Sodium

channel

Activation

gates

Inactivation gateK+

Potassium Channel

� Slow to open

� Slow to close

Na+

Na+

Potassium

channel

Sodium

channel

1 Resting state

2 Depolarization

3 Repolarization

4 Hyperpolarization

The events

Activation

gates

Inactivation gateK+

K+

Na+

K+

Na+

K+

Action

potential

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

The big picture

1 1

2

3

4

Time (ms)

ThresholdMem

brane p

ote

nti

al

(mV

)

Figure 11.11 (1 of 5)


• Resting potential is quickly restored

– Thousands of Na+/K+ pumps redistribute ions

– May seem like a huge task

– Only a small number of ions actually cross the

membrane

• Change in 0.012% of intracellular Na+ concentration


• Once initiated, AP is self-propagating

– Once Na+ channels in one region are inactivated, no

new AP is generated there

• Continues along axon in one direction at

constant velocity

• Factors affecting conduction velocity:

– Axon diameter – larger diameter = faster conduction

– Degree of myelination

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• Myelinated Neurons

– Nodes of Ranvier – only place where current can

pass through the membrane

– Saltatory conduction – 30X faster than continuous

conduction


• Refractory period

– Neuron cannot respond to a second stimulus

• During repolarization

– Limits number of impulses per second

Figure 11.14

Stimulus

Absolute refractory

period

Relative refractory

period

Time (ms)

Depolarization(Na+ enters)

Repolarization(K+ leaves)

After-hyperpolarization


• Coding for stimulus intensity

– All APs are independent of stimulus strength

– CNS must discern strong from weak signals to

initiate appropriate response

– Stimulus intensity is coded for by frequency of

action potentials

Dendrites

(receptiveregions)

Cell body


Nucleolus

Nucleus

Nissl bodies

Axon


Axon hillock


Node of Ranvier

Impulsedirection



Dendriticspine

Neuron cell body

(a)

(b)


?

The Synapse

• Nervous system operates through chains of neurons connected by synapses

• Syn = “to clasp or join”

• Junction between…

– Adjacent neurons

– Neuron and an effector cell

• Mediates information transfer

– Electrical

– Chemical

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The Synapse

• Presynaptic neuron

– Conducts impulses toward the synapse

• Postsynaptic neuron– Transmits impulses away from the synapse

• Most neurons are both

Dendrites

Cell body

Axon

Axodendriticsynapses

Axoaxonic synapses

Axosomaticsynapses

(a)

Axosomaticsynapses

Cell body (soma)of postsynaptic neuron

Axon of presynaptic

neuron

(b)

The Synapse

• Electrical Synapses

– Direct transmission of electrical signals from one cell to

another

– Less common than chemical synapses

• Neurons are electrically coupled (joined by gap junctions)

• Communication is very rapid

– May be unidirectional or bidirectional

• Important in

– Embryonic tissue

– Some brain regions

– Synchronizing groups of neurons (example: jerky eye movements)

Electrical Synapses The Synapse

• Chemical Synapses

– Indirect communication between cells

– Electrical signal of AP is changed to a chemical signal (neurotransmitter) in the presynaptic neuron

– Neurotransmitter is released into the synaptic space and diffuses toward the postsynaptic neuron

– Postsynaptic neuron changes chemical signal back to electrical signal for conduction along its own axon

– Unidirectional

– Here’s how it works…

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Chemical Synapses

1. Action potential

arrives at the

axon terminal in

the presynaptic

neuron

Chemical Synapses

2. Voltage-gated

Ca2+ channels

open, and Ca2+

enters the axon

terminal

Chemical Synapses

3. Ca2+ entry causes

synaptic vesicles

to release

neurotransmitter

by exocytosis

Chemical Synapses

4. Neurotransmitter

diffuses across the

synaptic cleft and

binds to specific

receptors on the

postsynaptic

neuron’s

membrane

Chemical Synapses

5-6. Binding of the

neurotransmitter

opens ion

channels and

creates graded

potentials in the

postsynaptic

neuron

Chemical Synapses

7. Graded

potentials

become nerve

impulses and are

conducted to the

next cell in the

chain

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The Synapse

• Postsynaptic membranes generally do not

generate action potentials

• When the neurotransmitter binds, one of two

types of graded potential occur

– Excitatory Postsynaptic Potentials (EPSP)

– Inhibitory Postsynaptic Potentials (IPSP)

EPSPs

• Excitatory Postsynaptic Potentials (EPSP)

– Depolarizes postsynaptic cell membrane

– Excitatory neurotransmitters

– Helps trigger AP at the axon hillock

Dendrites

(receptiveregions)

Cell body


Nucleolus

Nucleus

Nissl bodies

Axon


Axon hillock


Node of Ranvier

Impulsedirection



Dendriticspine

Neuron cell body

(a)

(b)


Figure 11.18a

An EPSP is a local

depolarization of the

postsynaptic membrane

that brings the neuron

closer to AP threshold.

Neurotransmitter binding

opens chemically gated

ion channels, allowing

the simultaneous pas-

sage of Na+ and K+.

Time (ms)

(a) Excitatory postsynaptic potential (EPSP)

Threshold

Stimulus

Me

mb

ra

ne

po

ten

tia

l (m

V)

IPSPs

• Inhibitory Postsynaptic Potentials (IPSP)

– Hyperpolarizes postsynaptic cell membrane

– Increases membrane permeability to K+ or Cl-

– Inhibitory neurotransmitters

– Decreases chance of AP

Figure 11.18b

An IPSP is a local

hyperpolarization of the

postsynaptic membrane

and drives the neuron

away from AP threshold.

Neurotransmitter binding

opens K+ or Cl– channels.

Time (ms)

(b) Inhibitory postsynaptic potential (IPSP)

Threshold

Stimulus

Me

mb

ra

ne

po

ten

tia

l (m

V)

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Neurotransmitters

• Excitatory– Acetylcholine

• Receptors also activated by

nicotine, muscarine

– Norepinephrine• Most common NT used by

the sympathetic nervous

system

– Glutamate • Most abundant NT in

vertebrates

• Component of concern in MSG

Neurotransmitters

• Inhibitory

– Serotonin (brain, GI tract)

– GABA (brain and retina)

– Glycine (spinal cord, brain,

and retina)

Thought Question

Strychnine is a pesticide that is used against small

vertebrates (birds, rodents). This chemical is an

antagonist to glycine. What symptoms might an

animal or human experience if they ingest this

substance?

The Synapse

• Inactivation of NT’s

1. Diffusion

2. Reuptake

3. Degradation (enzymatic inactivation)

• Examples

– Cholinesterase & Ach

• Found in synapses

– Monoamine Oxidase & Norepinephrine (many others)

• Bound to mitochondrial membrane in most cells

Figure 11.17

Action potential

arrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Ion movement

Graded potential

Reuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Postsynapticneuron

1

2

3

4

5

6

The Synapse

• Pharmacology

– Anticholinesterase neurotoxins

• Causes excitotoxicity (overstimulation)

• Example: Organophosphates, Nerve Gas

– Local Anesthetics

• Block Na+ channels

• Inhibitory

• Example: Lidocaine

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The Synapse

• Cocaine

– Prevents dopamine reuptake

– In response, the brain stops making dopamine

– User unable to experience pleasure without the

drug

Neural Integration

• Summation

– A single EPSP cannot induce an action potential

• EPSP’s can summate to reach threshold

– IPSP’s can also summate with EPSP’s

• Cancel each other out

Summation

• Types

– Temporal summation

• One or more presynaptic neurons transmit impulses in

rapid-fire order

– Spatial summation

• Postsynaptic neuron is stimulated by more than one

terminal at the same time

Figure 11.19a, b

Threshold of axon ofpostsynaptic neuron

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

Resting potential

E1 E1 E1 E1

(a) No summation:

2 stimuli separated in time

cause EPSPs that do not

add together.

(b) Temporal summation:

2 excitatory stimuli close

in time cause EPSPs

that add together.

Time Time

E1 E1

Figure 11.19c, d

E1 + E2 I1 E1 + I1

(d) Spatial summation of

EPSPs and IPSPs:

Changes in membane

potential can cancel each

other out.

(c) Spatial summation:

2 simultaneous stimuli at

different locations cause

EPSPs that add together.

Time Time

E1

E2 I1

E1

Neuronal Integration

• Neurons function in groups, and each group

contributes to wider neuronal function

• There must be integration – the parts must

work together to form a more complex whole

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Neuronal Integration

• First level of neuronal integration: neuronal

pools

• Patterns of connections within a neuronal pool:

neuronal circuits

• Features

– Allow for a wide variety of neuronal interaction

– May consist of thousands of neurons

– Often include excitatory and inhibitory neurons

Figure 11.21

Presynaptic

(input) fiber

Facilitated zone Discharge zone Facilitated zone

Simple Neuronal Pool

Neuronal Circuits

• Diverging circuit

– One incoming fiber stimulates an ever-increasing

number of fibers

– May affect a single pathway or several

– Common in both sensory and motor systems

– Example: A single neuron in the brain can activate

100 or more motor neurons in the spinal cord and

thousands of skeletal muscle fibers

Figure 11.22a

Neuronal Circuits

• Converging circuit

– Opposite of diverging circuits

– Results in either strong stimulation or inhibition

– Also common in sensory and motor systems

– Example: Different sensory stimuli can elicit the

same memory

Figure 11.22c, d

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The Nervous Impulse - Linn–Benton Community …cf.linnbenton.edu/mathsci/bio/waitea/upload/Lecture_02...probability of nerve impulse production. Figure 11.9b Hyperpolarizing stimulus