Download We have seen how the Nervous System plays an important role in

PPT
Nerve Impulses
This needs A LOT of help. Good diagrams are a must for this topic!!!
I’m not sure if this is too specific and needs to be more general, or if it is too general and
needs to be more specific????????
We have seen how the nervous system plays an important role in reaction time, stability
and balance, hearing, and vision. The nervous system contains special cells called
neurons that respond to and send messages. These “messages” are actually electrical. We
can use our knowledge of physics to understand how they are transmitted!
Different types of neurons respond to different stimuli. A stimulus is anything that
generates a nerve response. For example, light is a stimulus that generates a response in
specific neurons called photoreceptors (rods & cones). Neurons are peculiar looking
cells with a main cell body and a long tail called the axon. The entire cell is enclosed by
a cell membrane.
When a neuron is not responding to a stimulus it is called a resting neuron. There is a
difference in charge between the inside and outside of a resting neuron. The inside of a
resting neuron has a net negative charge while the outside has a net positive charge. How
can this be? Don’t opposite charges attract? Wouldn’t the charges want to be in
equilibrium? The difference in charge results from different amounts of positive and
negative ions (charged atoms) inside and outside the cell. This difference is maintained
by energy using protein “pumps” that transport ions across the cell membrane. Without
the action of the pumps the amounts of positive and negative ions inside and outside the
cell would be equal; there would be no net charge. However, with the action of such a
pump, along with diffusion, 3 positive sodium ions (Na+) go outside the cell while only 2
positive potassium ions (K+) come inside the cell. The inside of the cell loses 3 positive
charges and only gains 2. The outside of the cell only loses 2 positive charges while it
gains 3. Thus, there is a net negative charge inside the cell and a net positive charge
outside the cell. Transporting only positive ions may seem silly. Why not just separate
the individual positive and negative ions? The negative ions found in our cellular fluids
are relatively large and do not pass through cell membranes easily. The positive ions are
much smaller and easier to move!
Since there is a separation of charges, there is a difference in electric potential (a
voltage!). Think about it! Work has to be done to separate the charges, thus increasing
potential energy. If a small positive charge were put outside the cell it would have a high
potential, it would be repelled and travel toward the inside of the cell where it would lose
potential energy and gain kinetic energy. Since this difference in potential occurs in a
resting neuron, it is referred to as the resting potential.
A stimulus disrupts the resting potential of a neuron. At the location of the stimulus,
channels in the cell membrane open and allow Na+ to freely enter the cell. This causes a
change in the resting potential (voltage) at the area of stimulation. This change is called
an action potential. (The neuron is now active)! How does the signal then travel along
the neuron to relay the message? Na+ channels further along the neuron will open in
response to a change in voltage (they are called “voltage-gated channels”). So one action
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PPT
Nerve Impulses
potential causes adjacent Na+ channels to open, this causes another action potential that
causes more channels to open…and the chain reaction keeps going! The wave of action
potentials along the neuron constitutes the “signal.” What happens when the signal
reaches the end of a neuron? The neuron will release a chemical (neurotransmitter) that
will act as a stimulus on the next neuron. (In the heart and digestive system, one neuron
can stimulate the next one directly with action potentials).
What happens after the action potentials have been generated? The neuron quickly reestablishes its resting potential.
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