9.2 Electrochemical Impulse

Nerves impulses are similar to electrical impulses. Nerve impulses are slightly slower, however they stay the same strength through the entire length. Nerve impulses are electrochemical messages made by the movement of ions through the nerve cell membrane.

Experimented by putting a tiny electrode inside a squid’s large nerve cell: A fast change in electrical potential difference across the membrane was noted every time the nerve was excited. Resting membrane potential is about -70mV, but when the nerve was exited, it changed to +40mV. This change in potential is called the action potential. This change did not last more than a few milliseconds though before it returned to it’s resting potential.

How an impulse is transmitted The nerve cell membrane is permeable to ions which move across the membrane and set up an electrical chemical potential. When a nerve becomes excited a rapid change in the potential difference is detected. This potential difference is called the action potential (+40 mV) and it travels along the neuron from dendrite to axon. After the nerve impulse travels down the axon the neuron returns to its original potential difference called the resting potential (-70 mV). How do neurons work?

Resting Potential The neuron is polarized (more positive outside the membrane). -The Na+-K+ pump uses ATP to pump 3 Na+ ions out of the neuron and 2 K+ ions in. -The neuron membrane is “leaky” to K+ ions so some K+ ions diffuse out of the neuron. -Negative ions do not diffuse out of the neuron. http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf

Resting Potential The difference in charge between the outside and the inside of the neuron in most neurons it is –70 mV. When the neuron is at rest, it is polarized

Action Potential -The dendrites receive an electrical impulse resulting in a change in the permeability of the membrane. -Na+ ions diffuse in to the neuron through sodium channels and the polarity of the membrane is reversed. This is called depolarization and it moves as a wave down the neuron.

So how do the nerve cell membranes become charged? Due to the fact that neurons have a rich supply of positive and negative ions both inside and outside the cell. The negatives don’t really do much to help with the process as they are large and cannot move across the membrane, so they just stay inside the cell. The electrochemical event is caused by an imbalance of positive ions across the membrane. The highly concentrated potassium ions inside the nerve cells have a tendency to diffuse outside the nerve cells. Also, the highly concentrated sodium ions outside the nerve cell have a tendency to diffuse into the nerve cell. As potassium diffuses out of the neuron, sodium diffuses into the neuron. The resting membrane is about 50 times more permeable to potassium than it is to sodium, so more potassium ions diffuse out of the nerve cell than sodium ions into the cell. This makes the resting membrane potential unequal, with the inside of the cell being more negative and the outside being more positive, making it a polarized membrane.

So what happens when the neuron is excited? When the nerve cell is excited, the membrane becomes more permeable to sodium than potassium. It is believed that sodium gates in the membrane are opened and the potassium gates close. There is a rapid flow of sodium ions into the cell, creating a depolarization. Once the voltage inside the cell becomes positive, the sodium gates slam shut and the inflow of sodium is stopped.

Na+ channels open and Na+ ions flood into the neuron. K+ channels close at the same time and K+ ions can no longer leak out

The interior of the neuron in that area becomes positive relative to the outside of the neuron. This depolarization causes the electrical potential to change from –70 mV to + 40 mV

A sodium-potassium pump that is in the membrane restores the resting membrane by transporting sodium ions out of the neuron while moving potassium ions into the neuron in a ratio of 3 Na+ to 2 K+ ions. This pump requires energy from ATP and is called the process of repolarization Animation: The Nerve Impulse

Repolarization -Once the action potential has peaked a refractory period occurs where the neuron returns to its resting potential (polarized conditions). -The membrane becomes impermeable to Na+ ions and the Na+-K+ pump will pump the Na+ ions back out of the neuron. -During the refractory period the neuron cannot transmit another nerve impulse (about 1 to 10 ms).

Repolarization The spike in voltage causes the K+ pumps to open and K+ ions rush out The inside becomes negative again.

Nerves conducting an impulse cannot be activated until the condition of the resting membrane is restored. The period of depolarization must be completed and the nerve must repolarize before the next action potential is conducted. The time it takes for the cell to repolarize is called the refractory period.

The Refractory period So many K+ ions get out that the charge goes below the resting potential. While the neuron is in this state it cannot react to additional stimuli.

original resting potential Section of Graph Activity Description a original resting potential b gradual depolarization SODIUM CHANNELS OPEN c rapid depolarization (becomes positive in or negative on the outside) MORE SODIUM CHANNELS OPEN d excessive charge inside cell is rapidly lost SODIUM CHANNELS CLOSE & POTASSIUM CHANNELS OPEN e temporarily drops below original resting potential before restoration POTASSIUM CHANNELS CLOSE

MOVEMENT OF THE ACTION POTENTIAL In order for the impulse to move along the axon, the impulse must move from one zone of depolarization to adjacent regions. The positively charged ions that rush into the nerve cell during depolarization, are then attracted to the adjacent negative ions, which are aligned along the inside of the nerve membrane. A similar attraction occurs along the outside of the nerve membrane.

Why does the current move in one direction? The electric current passes outward over the membrane in all directions BUT the area to one side is still in the refractory period and is not sensitive to the current. Therefore, the impulse moves from the dendrites toward the axon.

The positively charged sodium ions of the resting membrane are attracted to the negative charge that has accumulated along the outside of the membrane in the area of action potential. The flow of the positively charged ions from the area of the action potential toward the adjacent regions of the resting membrane causes a depolarization in the nearby areas. The electrical disturbance causes sodium channels to open in the adjoining area and results in the movement of the action potential.

What causes the inside of a neuron to become negatively charged? This makes the resting membrane negative, relative to the outside of the membrane. Positively charged ions are lost from inside of the resting membrane faster than they are added. 23

A wave of depolarization moves along the nerve membrane and then enters the refractory period as the membrane causes the sodium channels to close and potassium channels to open. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Why does the polarity of a cell membrane reverse during an action potential? http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html

The action potential causes ion gates in the cell membrane to open, allowing ions to flow across the membrane, effectively reversing the polarity of the membrane. When the nerve cell becomes excited, the membrane of the cell becomes more permeable to sodium than to potassium. The sodium gates of the membrane are opened and the potassium gates are closed. The highly concentrated sodium ions rush into the cell via diffusion and charge attraction. This rapid inflow of sodium causes the charge reversal.

ANIMATIONS Neurons and Synapse Action 9.2 Electrochemical Impulse p.418 – 426 ANIMATIONS Neurons and Synapse Action McGraw-Hill http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html# Channel gating during an action potential http://www.blackwellpublishing.com/matthews/channel.html Propagation of the action potential. http://www.blackwellpublishing.com/matthews/actionp.html Synaptic vesicle fusion and neurotransmitter release. http://www.blackwellpublishing.com/matthews/nmj.html

THRESHOLD LEVELS AND THE ALL-OR-NONE RESPONSE The threshold level is the minimum level of a stimulus required to produce a response. The all-or-none response is the fact that a nerve or muscle fibre responds completely or not at all to a stimulus. If there is not enough potential to reach the threshold level, no response is noted. If there is more potential than the threshold level, there will be a response. The more intense the stimulus, the greater the frequency of the impulses.

Threshold Levels & the All-or-None Response A potential stimulus must be above a critical value (threshold level) to produce a response.

Threshold Levels & the All-or-None Response Increasing the intensity of the stimuli above threshold will not produce an increased response. Intensity of impulse & speed of transmission remain the same. Known as the all-or-none response. Neurons either fire maximally or not at all. Neurons, Synapses, Action Potentials, and Neurotransmission - The Mind Project

Threshold Levels & the All-or-None Response Differentiating Between Warm & Hot The more intense the stimulus, the greater the frequency of impulses. Intense stimuli excite more neurons. Different neurons will have different threshold levels. This affects the number of impulses reaching the brain.

SYNAPTIC TRANSMISSION There are small spaces between neurons and between a neuron and an effector. These spaces are called synapses. Small vesicles that contain chemicals called neurotransmitters are found in the end plates of nerves. Impulses move along the axon and release neurotransmitters from the end plate.

Neurotransmitters Chemicals that are produced within a neuron, are released by a stimulated neuron, and cause an effect on adjoining neurons. There are two types of neurotransmitters: 1. Small molecule neurotransmitters: 2. Neuropeptides

These neurotransmitters are released from the presynaptic neuron and diffuse across the synaptic cleft, creating a depolarization of the dendrites of the postsynaptic neuron. This does slow down the nerve transmission, so the greater number of synapses, the slower the speed of transmission.

Synaptic Transmission Spaces between two neurons or a neuron & an effector is called a synapse. Synaptic vesicles containing neurotransmitters (NTs) found in the end plates of axons. Impulse down axon  NTs released from presynaptic neuron  NTs diffuse across synaptic cleft  depolarizes postsynaptic neuron.

Synaptic Transmission Neuronal communication

Acetylcholine is an example of a neurotransmitter that is found in the end plates of many nerve cells. It can act as an excitatory neurotransmitter on many postsynaptic neurons by opening the sodium channels. Once acetylcholine has done it’s job, and the action potential has moved to the postsynaptic neuron, we need cholinesterase to break down the acetylcholine so that the sodium channels can close. Acetylcholine is known as an excitatory neurotransmitter

But not all neurotransmitters are excitatory because we also have inhibitory neurotransmitters. Inhibitory neurotransmitters work by opening the potassium channels, letting out even more potassium from the postsynaptic cell and then the neuron is said to be hyperpolarized because it becomes more negative in the cell. These inhibitory neurotransmitters help prevent postsynaptic neurons from being active.

Synaptic Transmission Acetylcholine opens Na+ channels on postsynaptic neuron, causing depolarization. Cholinesterase (from postsynaptic neuron) destroys acetylcholine. Inhibitory NTs make the postsynaptic membrane more permeable to K+. Neuron becomes hyperpolarized. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter45/animations.html# Animation: Chemical Synapse (Quiz 1)

Close to Home Animation: Cocaine Close to Home Animation: Alcohol

Synaptic Transmission Summation: Effect produced by the accumulation of NTs from two or more neurons.

The interaction of excitatory and inhibitory neurotransmitters is what allows you to throw a ball. As the triceps receive excitatory impulses and contracts, the biceps receive inhibitory and relaxes.