Topic 6 Nervous System: Transmission of a Nerve Impulse

Question 1

• Electrical signal that is transmitted along a nerve fiber, allowing us to send signals to perform actions like raising an arm to catch a ball
• We can do this because the membrane of an unstimulated neuron is polarized – a high concentration of Na+ is presented outside the cell and a high concentration of K+ inside

Answer 1

Nerve Impulse

Question 2

• Normal unstimulated state of a neuron

Answer 2

Resting Membrane Potential

Question 3

1. Neuron membranes are selectively permeable to K+, but only minimally permeable to Na+, which helps maintain the polarization. There are many K+ channels open, allowing the ion to freely flow outward, creating a charge differential across the membrane
2. There are negatively charged proteins and nucleic acids residing in the cell
3. An Na+/K+ ATPase pump maintains the resting potential; 3Na+ are pumped out for every 2K+ brought in, resulting in the net removal of one positive charge from the intracellular space

Answer 3

Resting Membrane Potential is negative because:

Question 4

1. Resting Potential
2. Action Potential
3. Repolarization
4. Hyperpolarization
5. Refactory Period

Answer 4

Steps of Nerve Impulse Transmission

Question 5

• the normal polarized state of neuron, -70 mV

Answer 5

Resting Potential

Question 6

• a stimulus causes gated ion channels to open and Na+ ions enter the axon, depolarizing the neuron. If the threshold level is reached (-50 mV), an action potential is caused that will result in the opening of voltage gated Na+ channels down the entire length of the neuron All or nothing event!

Answer 6

Action Potential

Question 7

• in response to the Na+ flow in, more gated ion channels let K+ out of the cell, restoring polarization
• Note that Na+ are now IN and K+ are OUT

Answer 7

Repolarization

Question 8

• by the time channels close, too much K+ is released (about -80 mV)

Answer 8

Hyperpolarization

Question 9

• period where neuron will not respond to a new stimulus until Na+/K+ pumps return the ions to their resting potential locations

Answer 9

Refactory period

Question 10

1. Absolute Refactory Period

2. Relative Refactory Period

Answer 10

Types of Refactory Period

Question 11

• where Na+ channels are inactivated and there is no chance of responding to a new stimulus; sets upper limit to action potential frequency

Answer 11

Absolute Refactory Period

Question 12

• an abnormally large stimulus can create an action potential

Answer 12

Relative Refactory Period

Question 13

• a refractory period is what prevents an action potential from moving backwards, even though ions are theoretically rushing in and diffusing in both directions

Answer 13

Note

Question 14

• From -70 mV to the threshold, or from -70 mV and downward, is the graded potential that cannot travel. However, if this stimulus surpasses the threshold, it can potentially open the voltage gated channels, which can lead to an action potential that travels by opening other voltage gated channels. Other types of gated potentials cannot spread unless they trigger an action potential
• An action potential is an all or nothing event: it either occurs or doesn’t. The strength of a neural signal is based on other factors such as frequency of action potential firing or how many nervous cells contribute to result in the action potential
• The K-ATP sensitive channel will close in the presence of ATP, causing K+ to be unable to escape, thus resulting in depolarization. In beta cells, this depolarization leads to the voltage dependent calcium channel to open, causing the exocytosis of insulin

Answer 14

Note