13.2-Electrochemical Impulse.pdf

LEARNING OUTCOMES:

1) Explain the formation and transmission of an action potential -including all-or-none response and

-intensity of response;

-the transmission of a signal across a synapse; and -the main chemicals and transmitters involved,

-i.e., norepinephrine, acetylcholine and cholinesterase

(p.415-425)

Discovery of the Action Potential

Background Knowledge:

Voltage: Difference in charge between 2 points

Diffusion: Movement of particles down a concentration gradient

Voltage Gated Ion Channels: protein channel in the cell membrane that opens and closes at specific voltage levels, allowing the diffusion of specific ions

Which side is more positive? More negative? + + + + + + + +

Action Potential = a nerve impulse

-Stages that occur before, during, & after an Action Potential: 1) Resting Potential (-70mV)

2) Depolarization (+40mV) 3) Repolarization

4) Hyperpolarization (below -70mV) 5) Refractory Period (returns to -70mV)

1) Resting Potential

• the nerve is at rest

• -70mV accross the cell membrane

• High conc. of Na+ _{outside cell}

• High conc. of K+ _{inside cell}

DRAW:

2) Depolarization

• initiated by a stimulus

• Voltage-gated Na+ _{channels open allowing}

Na+ _{to flow IN}

> causes the Voltage to change to +40mV

• NOTE: Na+ _{channels open at -50mV}

> the stimulus must cause enough Na+ _to

flow in change the membrane potential to -50mV (threshold) or the Action Potential will not occur

DRAW:

3) Repolarization

• initiated by the +40mV membrane potential

• restores the original polarity and causes "hyperpolarization" (-75mV)

• voltage-gated K+ _{channels open at +40mV}

allowing K+ _{to flow OUT}

• voltage-gated Na+ _{channels close at +40mV}

4) Hyperpolarization

• membrane potential becomes more negative than the original -70mV

• caused by K+ _{gates staying open after}

the membrane potential is restored

DRAW:

5) Refractory Period

• returning the membrane to resting potential (-70mV)

• lasts 1-10ms

• neuron cannot have another action potential until it's over

• Na+/K+ pump turns on

Movement of the Action Potential

• Domino affect

> a wave of Action Potentials occur down the neuron

• 1 direction due to Refractory Period

• (+) ions move toward (-) area causing the voltage to change (-70mV => -50mV)

http://www.learnalberta.ca/content/sebap/html/index.html

-myelenated vs unmylenated -ms damage vs normal

-action potential -voltage

Myelinated vs Unmyelinated Axons

• Voltage-gated channels are concentrated at the Nodes of Ranvier between Schwann cells.

All-or-None Response

• either the Neuron fires completely or it does not fire at all > Action Potential either occurs or it doesn't

• Threshold Level must be met (-50mV) before voltage-gated Na+

channels will open

• neurons do not touch each other

• Synapse: small spaces between neurons, or between neurons and effectors

• Neurotransmitters: chemicals released from the presynaptic neuron, diffuse across the synaptic space, and cause the postsynaptic neuron to depolarize

• Synapses slow nerve transmission

> the more synapses involved the slower the transmission – Reflex Arc is FAST because it involves a few synapses

Figure 4 Label with the following terms

presynaptic membrane postsynaptic membrane synaptic cleft synaptic

vesicle_{molecules in} transmitter

synaptic vesicle

receptor

dendrites

end plates axons

• a chemical messenger that alters the membrane potential of the postsynaptic neuron

> Examples:

– Acetylcholine - Increases Na+ _permeability

– Norepinephrine

• Acetylcholine: most common neurotransmitter

> excitatory effect (opens Na+ _{channels to depolarize neuron}

and cause Action Potential)

Question:

What would happen if acetylcholine remains in

the receptor site?

Problem:

The postsynaptic cell would be in a constant state

of depolarization = constant muscle spasms

Solution:

Cholinesterase

- an enzyme released into

the synapse to break down acetylcholine.

Puffer Fish Neurotoxin

• blocks the Na+ channels

Nerve Gas

• inactivates cholinesterase

> amt. of acetylcholine in synaptic cleft increases every impulse > Action Potentials repeatedly

– stimulation of muscle repeatedly

Number these events in the correct order.

(a) ____ An action potential is stimulated at the postsynaptic membrane, and an impulse travels down the dendrite.

(b) ____ An enzyme destroys the neurotransmitter substance and clears out the synaptic cleft.

(c) ____ The impulse reaches the synapse from the axon. Calcium channels open and calcium ions diffuse into pre-synaptic neuron.

(d) ____ Calcium influx cause stimulates synaptic vesicles to move to the presynaptic membrane.

(e) ____ The neurotransmitter substance diffuses across the cleft.

(f) ____ The neurotransmitter substance fits into receptor sites on the postsynaptic membrane.

(g) _____Synaptic vesicles dump neurotransmitter substance into the synaptic cleft.

Events of a Synaptic Transmission

Pu ll Pu ll 2 Pu ll Pu ll 4 Pu ll Pu ll 5 Pu ll Pu ll 3 Pu ll Pu ll 6 Pu ll Pu ll 1 Pu ll Pu ll 7

• The acetylcholine from one axon terminal is usually not enough to cause depolarization of the post-synaptic neuron.

• Usually, neurotransmitters from a few different pre-synaptic knobs are needed to induce an action potential

Other Neurotransmitters

Neurotransmitter

Function

Too Much

Too Little

Dopamine • body movements

• pleasure (ie: eating) Schizophrenia Parkinson's

Seratonin

• sensory perception • mood control • temperature

Coma, death

(MDMA) depression

Endorphins • natural painkillers

• emotion addiction

depression, pain

Norepinephrine • readies body for stresshigh BP, anxiety, insomnia