A and P Polarization

Action Potential – electrical signal carried by an axon in a nerveoCaused by ionsoIs an electrochemical responseoResult from the disturbance of the potential difference across the axon plasma membrane

Excitability – the ability to create an action potential in response to a stimulus2 Types of excitable cells in body are muscle and nerve cells.

Potential differencemeasure of the charge difference across the plasma membrane. Measured in millivolts (mV)

Neuron Resting potential –electrical charge difference of -85 to - 70 mV.

inside

outside

A negative sign is used because inside is negative compared to outside of plasma membrane.

What creates the potential difference:1.Large negative proteins are trapped inside the cell2.Sodium-potassium pump - Pumps 3 Na+ out for every 2 K + it pumps into the cell.

What creates the potential difference:1. Large negative proteins2. Sodium-potassium pump

3.Potassium channels allow K + to diffuse out of the cell

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Electrical properties of the axon at rest:

othere is no action potential running down the axonoOutside + and inside is - comparativelyoWhen potential difference greatest

Characteristics of an action potential:

•All or none event

•There must be a stimulus

Characteristics of an action potential:

•Stimuli may include•Moving cell (touch)•Light (sight)•Change in temperature (hot/cold)•Chemicals (taste)•Electrical shock (not normal)

Steps of a nerve impulse:

1. Polarization

•Axon is at its resting potential.

• Axon is ready to do work, receive a stimulus. (Has potential energy)

Steps of a nerve impulse:1. Polarization

2. Depolarizationoccurs in response to a stimulus and is when there is no more separation of charge across the plasma membrane

Steps of a nerve impulse:1. Polarization2. Depolarization

How is a neuron depolarized?A.Stimulus hits channel proteins called gates located in the plasma membrane

B. These gates open allowing Na+ to rush into the cell


How is a neuron depolarized?

C. The action potential changes from -85mV to +30 mV

D. The inside of the plasma membrane is now + and outside – (reversed)

Depolarization


3. Repolarization – when the potential difference across the plasma membrane is restored to resting potential.

Steps of a nerve impulse:1. Polarization2. Depolarization3. Repolarization

How does repolarization occur?

A.Sodium gates close.

B. Potassium gates open; K+ rushes out of the cell resting potential is restored.

3. Repolarization

How does repolarization occur?

NOTE: It takes less than 1 millisecond for

depolarization and repolarization to take place.

Don’t write this!!!!

But wait!!! This is not what was originally our resting potential set up was it???

Don’t write this!!!

Very good! No it wasn’t. We had more Na+ on outside, now the sodium is on the inside.

Guess what will start to pump now?

Steps of a nerve impulse:1. Polarization2. Depolarization3. Repolarization

How does repolarization occur?A. Sodium gates close.B. Potassium gates openWrite now!!!

C.Sodium –potassium pump will re-establish original ion concentrations of resting potential.• Takes 1 millisecond• Axon can not receive next

stimulus

http://en.wikiversity.org/wiki/File:Action_potential_propagation_animation.gif

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Types of stimuli

1. Threshold stimulusIs strong enough to trigger the action potential.

Once it is reached there is no stopping the action potential from traveling.

Types of stimuli

1. Threshold stimulus

2. Subthreshold stimulus – Is a stimulus not strong enough to trigger the action potential.

Types of stimuli1. Threshold stimulus2. Subthreshold stimulus

3. Local potential – Caused by a subthreshold potential Causes a depolarization in the area of the stimulus but not strong enough to create an action potential to travel the axon

How does the action potential travel down the axon?

The action potential is strong enough to depolarize the adjacent area of membrane opening the sodium gates causing a wave of depolarization.

Refractory period – pump is bringing stimulate portion of membrane back to original resting potential ion concentration.

During this time this section cannot respond to another stimulus, keeps action potential from traveling wrong direction.

Two types of refractory periods:1.Absolute refractory period – no matter how strong the stimulus there is no action potential

2. Relative refractory period – if the stimulus is strong enough it will produce an action potential

Don’t writeRemember – action potential is all or none – so how do you feel different levels of intensity?

Level of intensity of senation is determined by:

1.Frequency of action potentials.The higher the frequency the greater the intensity.2. Whether or not the axon is myelinated determines type of conduction.

write

Level of intensity of senation is determined by:1.Frequency of action potentials2. Whether or not the axon is myelinated.

Continuous conduction –unmyelinated the wave of depolarization moves down the complete plasma membranedull, slow pain

Level of intensity of senation is determined by:1.Frequency of action potentials2. Whether or not the axon is myelinated.Continuous conduction

Saltatory conduction myelinatedimpulse jumps from node to node.

Sharper pain

Nerve impulses travel from one neuron to the next when the nerve impulse travels across the synapse.

NS Synapse – junction between two neuronsSynaptic cleft – space in the synapseNeurotransmitter –chemical that stimulates the dendrite of the next neuron to depolarize.

Pre -synaptic neuron – first neuron in a seriesPost – synaptic neuron – second neuron in a series

How does action potential travel across synapse?

1.Action potential reaches the pre-synaptic terminal.

2. Ca+ gates open and Ca+ rushes in.


3.Ca+ trigger release of the neurotransmitter from the synaptic vesicles.

4. Neurotransmitter travels across the synaptic cleft and binds to receptors on the post-synaptic neuron.


5. This stimulates ion gates to open.

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2 types of synapses

1.Excitatory – •Neurotransmitter stimulates Na+

gates to open at post-synapse •Enough Na+ gates must open for creation of an action potential in the post-synaptic membrane

2 types of synapses1.Excitatory –

Types of summation:A. Temporal summation – one pre-synaptic axon has a high enough frequency of action potentials to send enough neurotransmitter to open enough Na+ gates to continue the nerve impulse.

B. Spatial Summation –•have several pre-synaptic neurons that excite one post-synaptic neuron.

•enough Na+ gates will open for action potential to travel through next neuron.

2 types of synapses:1.Excitatory

2.Inhibitory Neurotransmitter opens K+ channels on post-synaptic membraneK+ will rush out of cell causing hyperpolarization; an increase in potential difference (-80mV to -90mV)

2 types of synapses:

1.Excitatory

2.Inhibitory

filters out unnecessary info

Mostly this type in body

Types of Neuron Arrangement:1.Convergent circuit– more than one pre-synaptic neuron goes to one post-synaptic neuron.

Ty pes of Neuron Arrangement:1. Convergent circuit

2. Divergent circuit one pre-synaptic neuron sends a signal to several post-synaptic neurons.This spreads the message to different parts of the body.Ex. To muscles and brain

Types of Neuron Arrangement:

3. Oscillating circuit – a neuron sends the message back to itself.A single stimulus can result in a long lasting signal.Ex. Alarm clock = stimulus, stay awake due to oscillating circuit.Circuit may stop due to fatigue of neuron or interference.

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A and P Polarization