Neural Conduction and Synaptic Transmission (i.e., Electricity and Chemistry)

Neural Conduction and Synaptic Transmission(i.e., Electricity and Chemistry)

Neurons

Figure 2.5 A typical neuron and synapse Klein/Thorne: Biological Psychology© 2007 by Worth Publishers

Figure 2.6 The four major types of synapses Klein/Thorne: Biological Psychology© 2007 by Worth Publishers

Neural Conduction

An Electrical Process

Resting Membrane Potential

Ions and the resting membrane potential

• K+

– Potassium ions, positive charge


• Na+

– Sodium ions, positive charge


• Cl-

– Chloride ions, negative charge


• Inside the neuron– K+

– Protein-

• Outside the neuron– Na+

– Cl-

What the ions naturally want to do

• Force of diffusion– It’s getting crowded in here

• Electrostatic pressure– Opposites attract– Similarities repel

Figure 4.4 The influence of diffusion and electrostatic pressure on the movement of ions into and out of the neuronKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

What the neural membrane is making the ions do

• Differential permeability– Playing favorites

• K+ and Cl- pass through easily• Na+ -- not so easy to pass through the membrane• Proteins: not a chance!

– Ion channels: like doors

What the neural membrane is making the ions do

• Sodium-potassium pump

• Three Na+ out for every two K+ cells in

Figure 4.5 The sodium-potassium pumpKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Putting it all together…• Na+ ions

– Want to go inside neuron because• There are fewer of them inside (force of diffusion)• There is a negative charge inside (opposite to their positive

charge)

– But• Neuron’s membrane not very permeable to Na+ ions• Sodium-potassium pump keeps kicking them out

– Therefore, most Na+ ions stay outside neuron

Putting it all together…• K+ ions

– Want to go outside neuron because• There are fewer of them outside (force of diffusion)• Neuron’s membrane very permeable to K+ ions

– But• There is a positive charge outside (similar to their positive

charge), so they are repelled by the outside• Sodium-potassium pump keeps kicking them back into

neuron

– Therefore, most K+ ions stay inside neuron

Putting it all together…• Cl- ions: can’t make up their minds

– Want to go inside neuron because• There are fewer of them inside

• Neuron’s membrane very permeable to Cl- ions

– Also want to stay outside of neuron because• The charge outside is positive (and their own charge is

negative

– Therefore, Cl- ions keep going back and forth, distribution of Cl- ions is held at equilibrium.

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Postsynaptic Potentials

Getting the membrane potential to change from -70 mv

Figure 2.6 The four major types of synapses Klein/Thorne: Biological Psychology© 2007 by Worth Publishers

Figure 4.11 Overview of synaptic transmission Klein/Thorne: Biological Psychology© 2007 by Worth Publishers

• Like relay team passing a baton• Causes something called

“postsynaptic potentials” to happen

Postsynaptic potentials can do one of 2 things…

• Depolarize neuron

• Hyperpolarize neuron

Postsynaptic potentials

• Depolarize

• Decrease resting potential

• Become less negative

• E.g., from -70 mV to – 67 mv


• Increase likelihood that neuron will fire

• Excitatory postsynaptic potentials: EPSPs


• Hyperpolarize

• Increase the resting potential

• Become more negative

• E.g., from -70 mV to -72 mV


• Decrease likelihood that neuron will fire

• Inhibitory postsynaptic potentials: IPSPs

Characteristics of EPSPs and IPSPs

• Notes

There are a bunch of EPSPs and IPSPs There are a bunch of EPSPs and IPSPs happening in the same neuron at oncehappening in the same neuron at once

How EPSPs or IPSPs add up

• Spatial summation – A bunch of EPSPs/IPSPs combine together

Figure 4.14 Spatial summation and temporal summationKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

How EPSPs or IPSPs add up

• Temporal summation– When EPSPs/IPSPs are coming in real fast,

the next one happens before the previous one fades away

– They add together over time

Figure 4.14 Spatial summation and temporal summationKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Getting a neuron to fire

• EPSPs and IPSPs travel until they reach near the axon hillock


• Remember…– EPSP make membrane’s resting potential

less negative (e.g., from -70 mv to -68 mv)– IPSPs make membrane’s resting potential

more negative (e.g., from -70 mv to -75 mv)

• When combined they cancel each other out, and whichever is stronger wins

Getting a neuron to fire: Example 1

• EPSPs add up to change resting potential from -70 mv to -60 mv (change of +10 mv)

• IPSPs add up to change resting potential from -70 mv to -75 mv (change of -5 mv)


• Net difference of +5 mv, from -70 mv to -65 mv

• The resting membrane potential to become less negative

• The end result is the membrane is depolarized


• EPSPs add up to change resting potential from -70 mv to -68 mv (change of +2 mv)

• IPSPs add up to change resting potential from -70 mv to -75 mv (change of -5 mv)


• Net difference of -3 mv, from -70 mv to -73 mv

• The resting membrane potential to become more negative

• The end result is the membrane is hyperpolarized


• The end result that matters is how the EPSPs and IPSPs cancel each other out near the axon hillock

Getting a neuron to fire• If it so happens that, near the axon hillock

– The net combination of EPSPs/IPSPs– Depolarizes the membrane (makes it less negative)– To a point called threshold potential

Action potential

• Membrane becomes depolarized to about 40 mV

• All-or-nothing

Voltage-activated ion channels

• When a neuron’s membrane reaches the threshold of excitation, ion channels open

• Na+ ions (previously could not permeate the membrane) can now rush into the neuron

• As a result, membrane potential goes to about 40mv

Voltage-activated ion channels

• K+ ions (they start out being inside the neuron) now rush out of the neuron– Force of diffusion– When membrane potential is now positive,

also driven out by electrostatic pressure

Refractory period

• Absolute refractory period– Lasts 1 to 2 milliseconds– Impossible for another action potential to

happen


Refractory period

• Relative refractory period– Possible for another action potential to

happen– But need extra-strength stimulation


Action potential travels down axon

Figure 4.9 Propagation of the action potential along an unmyelinated axonKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Action potential travels down axon

• Action potentials are nondecremental – do not become weaker as they travel

• Travel very slowly

• Action potential only causes those ion channels in one small spot of the membrane to open

• To travel down the axon, needs to nudge the adjacent ion channels

Conduction of Action Potential in Myelinated Axons

Figure 4.10 Propagation of the action potential along an myelinated axonKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Conduction of Action Potential in Myelinated Axons

• This time,– Action potential travels rapidly– Action potential simply hops from one node of

Ranvier to another• Saltatory conduction (saltare = dance)

– Action potential grows weaker as it travels• But, still strong enough to initiate another action

potential at the next node of Ranvier

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Synaptic Transmission of Signals

A Chemical Process

Chemical signals

• In your nervous system there are chemicals called “neurotransmitters.”

• Neurons produce neurotransmitters.

Neurotransmitters

• Neurotranmsitters are packed into synaptic vesicles.

• Synaptic vesicles are found at the terminal buttons.

Release of neurotransmitters• Action potential travels down axon and reaches synapse• This causes Ca2+ (calcium) ion channels to open

Figure 4.11 Overview of synaptic transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Release of neurotransmitters

• Ca2+ ions cause synaptic vesicles to join to presynaptic membrane

• Vesicles release neurotransmitters into synaptic cleft

• Neurotransmitters get passed on to the next neuron

Figure 4.11 Overview of synaptic transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

What happens next?

It’s like playing pinball

Back to where we started…

• Neurotransmitter binds with receptor, causes ion channels to open

• If Na+ channels open, then Na+ ions enter neuron, depolarizes membrane EPSP

Back to where we started…

• If chloride channels open, the Cl- ions enter neuron, hyperpolarizes membrane IPSP

• If potassium channels open, the K+ ions leave the neuron, hyperpolarizes membrane IPSP

What happens to the leftover neurotransmitters?

• Reuptake– Neurotransmitters return to presynaptic

buttons

Figure 4.18 Termination of neural transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

What happens to the leftover neurotransmitters?

• Degradation– Neurotransmitters broken apart in the

synapse by enzymes

Figure 4.18 Termination of neural transmissionKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Neurotransmitters

Chemicals in the Nervous System

Neurotransmitters

• Acetylcholine (Ach)– Muscles– Memory: Alzheimer’s

• Gamma-aminobutyric acid (GABA)– Seizures– Huntington’s disease

Neurotransmitters

• Epinephrine (aka adrenaline)

• Norepinephrine

– Activation of cardiovascular system

Neurotransmitters

• Dopamine– Schizophrenia– Parkinson’s

• Serotonin– Depression– Aggression

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Agonists and Antagonists

Agonists

• Synthesis of neurotransmitter

• Helps with release

• Obstructs autoreceptor

• Pretends to be neurotransmitter

• Prevents reuptake

Figure 4.16 AutoreceptorsKlein/Thorne: Biological Psychology© 2007 by Worth Publishers

Antagonists

• Obstacle to synthesis

• Obstacle to neurotransmitter release

• Fools autoreceptor

• Blocks receptor

How some drugs work

• Cocaine– Agonist of norepinephrine and dopamine– Prevents reuptake of leftover norepinephrine

and dopamine– Therefore, effects of these neurotransmitters

are increased

How some drugs work

• Botulinium toxin– Antagonist of acetylcholine– Prevents acetylcholine from being released– Therefore, effects of these neurotransmitters

are decreased– Small amounts used to paralyze certain

muscles

Neural Conduction and Synaptic Transmission (i.e., Electricity and Chemistry)

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