GENERATION OF NERVE IMPULSES

The nervous system conveys information throughout the body in the form of nerve impulses. These impulses can travel very quickly at speeds up to 200 m/s.

How are these impulses created?

❖ There is an electrical difference that exists between the inside and the outside of each neuron. The

term for this difference is electrical potential (or voltage). Remember from science 9!

❖ Since it is electric, we can use an oscilloscope (a very sensitive type of voltmeter) to measure this

difference.

❖ When a neuron carries a message, all that seems to happen is that the electricity quickly reverses. This

reversal of electricity is known as the action potential (or nerve impulse).

❖ The cause of this electrical reversal is the movement of ions between the inside and outside of the

neuron. These ion are _______ & _______ and there movement is controlled by special membrane

_________________ & ______________.

There are THREE distinct phases in the generation of a nerve impulse: (Resting, Action, and Recovery).

(1) RESTING PHASE

• The potential difference across the membrane of the axon at rest normally equals mV.

• This negative polarity is caused by the unequal distribution of ions

• During the resting potential, _____ ions are more concentrated on the outside of the membrane and

_____ ions are more concentrated on the inside of the axon

• This uneven distribution of Na+ and K+ ions is maintained by _____________ transport across Na+/K+ pumps which operate whenever the neuron is not conducting an impulse (pumps require ATP)

Therefore, the cell is slightly on the inside and slightly on the outside

(2) ACTION PHASE

If nerve is stimulated by electric shock, pH change, a neurotransmitter or mechanical stimulation, a nerve impulse is generated, and a change in electrical potential can be seen on the oscilloscope.

This is called the action potential.

Nerve cells must be stimulated to threshold, the minimum stimulus required for the neuron to fire and send a message.

What happens during action potential?

When the axon becomes stimulated it causes voltage-gated ________ channels open at the initial site of stimulation.

1. Na+ ions rush from the outside of the neuron to the inside of the neuron (following concentration gradient)

2. "_____________________" occurs -- the inside of the axon becomes __________________ even though these channels only remain open for about 0.5-1.0 millisecond…so now more + ions trapped inside

3. Na+ can’t leave, but this change in voltage triggers the opening of the voltage-gated _______ channels and so K+ ions rush from the inside to the outside of the axon

4. “_______________________” has occurred -- as the inside of neuron becomes negative again (but now it has Na+ in/ K+ out – opposite of what it started with)

On the oscilloscope, the action potential can be broken into an and

During the upswing, the electrical potential in that section of the neuron changes from -60 mV to +40 mV (Depolarization)

During the downswing, the electrical potential in that section of the neuron changes from +40 mV back to -60 mV (Repolarization)

Oscilloscope reading:

(3) RECOVERY PHASE

After the action phase we are left with a build-up of Na+ ions inside the neuron and K+ ions outside the neuron. In order for another nerve impulse to travel along this neuron, we need to return these ions to their original positions. How can the neuron do this?

How does this impulse look along the axon of a nerve cell?

When a nerve cell carries the impulse, it doesn’t change electricity all at once. Only a small part of the nerve

cell changes at a time, moving along the cell like a wave!

Examine the diagram carefully. In which direction is the nerve impulse heading (left or right)? Explain.

Summary:

The speed of nerve impulses is quite fast, due to the structure of the nerves:

- They have a (covering), which is formed by tightly packed spirals of the

cell membrane of Shwann cells.

- Myelin acts as an , preventing ions from moving in and out (so this can only

occur in the gaps); these gaps in the sheath are called the nodes of Ranvier

- The speed of transmission is ~ m/s in myelinated fibers, but only m/s in

non-myelinated fibers. This difference in speed occurs because the nerve impulse "jumps" from node

to node in myelinated fibers, but must depolarize and repolarize along the entire nerve fiber in non-

mylelinated fibers

Step 1: Sodium moves in Step 2: Depolarization

Na+

Sodium channels open, Na+ ions diffuse into axon.

Na+

The inside of the axon has become positive in that

region. This is called depolarization.

+ + + + + + +

Step 3: Na+ channels close, K+ open Step 4: Repolarization

K+

Potassium channels open, K+ ions diffuse out of axon.

K+

The movement of K+ ions counters the depolarization.

The voltage differerence across the membrane returns to

the resting potential level (-60mV).

Step 5: Recovery Phase Step 6: Depolarization of adjacent part of axon

Na+

Na+ and K

+ actively transported back across

membrane until they are distributed in the same

concentrations as before the impulse was sent.

K+

Na+

Sodium channels open, Na+ ions diffuse into axon.

Na+

The impulse will continue to move down the axon until it

reaches the synapse (the junction between neurons).

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INTERESTING NOTE

Destruction of large patches of Myelin characterize a disease called Multiple Sclerosis. In multiple sclerosis, small, hard plaques appear throughout the myelin. Normal nerve function is impaired, causing symptoms such as double vision, muscular weakness, loss of memory, and paralysis.