Action Potential 6.5

How do neurons communicate and relay information. http://www

Action Potential ~1900 Julius Berstein suggested that nerve impulses were an electrochemical message created by the movement of ions through the nerve cell membrane. Through experimentation it was found that every time a nerve is excited, there was a rapid change in electrical potential difference (electric potential)

Some important terms…. Resting potential: the voltage difference (mV) across a nerve cell membrane during the resting stage (when it is not relaying information) -70 mV (average) (The area outside the neuron is more positive than the inside.) Action potential: a rapid change in nerve cell membrane potential (mV) when the nerve is excited. Consists of depolarization, and repolarization

Resting Potential Along the neural membrane there are many Na+/K+ pumps Neurons pump Na+ out of the neuron and K+ into the neuron to generate the resting potential Remember, 3 Na+ are pumped out for every 2 K+ that are pumped in.

How do nerve cells become charged? This creates an unequal charge, making the extracellular space more positive in comparison to the intracellular space There are also negative ions that line the inside of the neuron However, the negative ions are not really involved in creating the charged membrane – they are large and cannot easily cross the membrane.

Depolarization: Repolarization A change in the membrane potential from negative to positive Repolarization a change in the membrane potential from positive to negative

Pre-Excitation Before an action potential is initiated, there is a high concentration of Na+ outside the neuron and a high concentration of K+ inside the neuron Negative ions (Cl-) will want to move out of the axon (where it is more positive) but they cannot get through the membrane, so they will accumulate along the inside the membrane.

At Resting Potential

Pre-Excitation / Resting Potential The membrane is now a polarized membrane - charged by the unequal distribution of ions. the membrane has the potential to do work – expressed in mV.

Excitation 1. When the nerve cell becomes excited (due to a stimulus), the membrane becomes more permeable to sodium than potassium. i.e. Na+ gates open and K + gates are closed 2. Na+ moves into cell following a concentration gradient (diffusion) and also an electrical potential gradient. The positive charge moving into the neuron reduces the potential difference of the membrane . This is depolarization.

When the potential difference is above zero, Na+ movement will now only be driven by diffusion. 3. When the membrane potential is ~+30mV, the sodium gates will close. And the K+ gates will open.

4. K+ will now exit the neuron following the rules of diffusion and following the electric potential gradient. As the positive K+ leave the neuron, the potential difference across the membrane will start to decrease: Repolarization When the potential difference falls below zero again, the movement of K+ is due to diffusion alone.

5. The K+ pores will close when the resting potential (of -70mV) is reached again. However, the Na+ and K+ ions are on the “wrong” side of the cell membrane.

6. Refractory Period - a resting period; during this time, the neuron cannot start another action potential. -The ions will move back to their original positions through sodium/potassium pumps (active transport requiring ATP) http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__voltage-gated_channels_and_the_action_potential__quiz_2_.html http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__sodium-potassium_exchange_pump__quiz_1_.html

Analysing Oscilloscope Traces “Undershoot” = Hyperpolarization: excess K+ diffuse out of cell (more K+ leave than Na+ enters). The excess K will diffuse away.

Movement of Action Potential Movement of Na+ ions into the neuron causes depolarization and action potential. For the impulse to be conducted along the axon, the impulse must move from the zone of depolarization to an adjacent region.

When the Na+ ions move in, they will be attracted to the adjacent negative charges and cause depolarization in the adjacent area. This previously resting membrane would have had negative charges lining the inside membrane. With depolarization, it becomes positive. This electrical disturbance causes the Na+ channels in the adjoining area to open resulting in another action potential.

A nerve impulse is an action potential that starts at one end of a neuron and is propagated along the axon to the other end of the neuron

Unmylenated vs Mylenated? http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__action_potential_propagation_in_an_unmyelinated_axon__quiz_1_.html

Synaptic Transmission A single neuron many branch many times and at its end join with many different neurons. (The end of a neuron may be referred to as an “axon end plate” or a synaptic knob” or a terminal button”) Small spaces between neurons or between neurons and effectors are known as synapses.

Small vesicles containing neurotransmitters are located in the end plates of axons. Ex of a neurotransmitter: acetylcholine As the impulses moves along the axon, the axon releases neurotransmitters from the end plate which will diffuse into the dendrites of an adjacent neuron and create a depolarization of the dendrites. The neuron that releases the neurotransmitters is the presynaptic neuron The neuron that receives them is the postsynaptic neuron.

Synaptic Transmission When an action potential reaches a synaptic knob, it causes the membrane to be more permeable to calcium ions Ca2+ As Ca2+ move into the neuron, this causes the axon to release neurotransmitters into synaptic cleft. The synapses are very small (~20nm), however nerve transmission slows across a synapse. The greater the number of synapses, the slower the transmission.

The neurotransmitters move across the synaptic cleft and will bind to receptors on the cell membranes of adjacent neuron (post-synaptic neuron) This will cause Na+ channels to open up and cause an action potential to start in that neuron. But with the Na+ channels open, the neuron would remain in a constant state of depolarization. The neurotransmitter needs to be released from the receptor sites.

5. The neurotransmitter is rapidly broken down and removed from the synaptic cleft. Enzymes such as acetlycholinesterase are released by the postsynaptic neuron to destroy acetylcholine, thus closing the Na+ channels.

This increases the “distance” to the threshold value (Acetylcholine is changed into choline and ethanoic acid, and will diffuse back to the presynaptic neuron to be reused). Inhibitory neurotransmitters make postsynaptic membranes more permeable to K+ and Cl- K+ will diffuse out the neuron (and Cl- in) creating an even more negative resting membrane which is said to be hyperpolarized. This increases the “distance” to the threshold value To achieve depolarization, even more Na+ channels must be opened. http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter11/animation__transmission_across_a_synapse.html

Threshold Levels and the All or None Response Experiments with neurons have shows that there is a threshold level Threshold level – is the minimum level of stimulus required to produce a response. A nerve impulse is only initiated if the threshold potential is reached. Experiments also show that increasing the intensity of the stimuli above the threshold value does not change the intensity of speed of transmission – this is known as the all-or-non-response.

So, how to we detect intensity of stimuli if nerve fibres fire completely or not at all? Frequency! The more intense the stimulus, the greater the frequency of the impulse. When you put your hand on a warm surface – fewer impulses are sent to your brain than if you were to put your hand on a hot surface.

Also, different threshold levels of neurons provide a way for intensity to be detected. Each nerve is compose of many individual neurons each with different threshold levels. Ex: touching a surface at a temp of 40°C may cause 1 neuron to reach threshold and fire a signal, but touching a surface at a temp of 50 °C may cause 2 neurons to reach threshold temps and fire a signal.

Side note… The interaction of excitatory and inhibitory neurotransmitters is what allows you to do everything you do: throwing a ball, talking…. Inhibitory impulses in your CNS allows for prioritization of information received by your brain. Ie: when you are listening to a biology lecture, your sensory info should be directed to Ms. De Souza. Information from other sensory nerves (ie, temp in the room, the pressure receptors confirming you are wearing clothes) should be suppressed.