Neural Physiology - AP Biology

An electrical stimulus causes voltage-gated sodium channels in a neuron to open. Sodium then travels down its concentration gradient through the channels, into the cell. With the movement of sodium into the cell, the cell depolarizes (its membrane potential becomes more positive). The gradient that drives depolarization is established by the sodium-potassium pump, which causes two primary effects: the resting membrane potential is negative and there is a large concentration of sodium outside of the cell. When sodium channels open, sodium ions flow down both the electrical gradient formed by the negative membrane potential and the chemical gradient formed by ion concentrations.

Graded potentials reflect stimulus strength because they do not propagate via saltatory conduction (as action potentials do), and thus decrease in amplitude down the axon. All other answers listed are characteristics of action potentials.

Once the cell reaches threshold, an action potential is fired. Sodium, a positive ion, enters the cell, and causes the charge on the membrane to rise, or depolarize. After a certain point, sodium gates close. Potassium, another positive ion, then leaves the cell, and the charge on the membrane decreases. As the potassium ions exit, the membrane potential plunges even lower than the resting potential, causing it to become hyperpolarized. At this point, the sodium/potassium pump works to repolarize the cell to return to the resting membrane potential.

1. The cell reaches threshold

2. Sodium gates open, and sodium floods into the cell

3. The cell depolarizes

4. Sodium gates close

5. Potassium gates open and potassium leaves the cell

6. The cell hyperpolarizes

7. Potassium gates close

8. Na/K pump repolarizes the cell during refractory period

During the generation of an action potential, the cell will undergo two refractory periods. The first is referred to as the absolute refractory period, during which no stimulus, regardless of size, will generate another action potential. This is followed by the relative refractory period, during which an action potential will be generated only if an abnormally large stimulus is encountered. During the relative refractory period, the cell is hyperpolarized due to the removal of potassium ions from the cell interior, which results in a more negative membrane potential than the cell would have at rest.

By pumping two positively-charged potassium molecules in for every three positively-charged sodium molecules that are pumped out of the cell, the sodium-potassium pump maintains a resting potential of –70mV relative to outside of the cell. This function is important for creating an electrochemical gradient along the neuron.

Remember that sodium flows down its gradient to enter the cell during depolarization, while potassium flows down its gradient to exit a cell after an action potential, causing hyperpolarization during the refractory period.

After the sodium channel is opened, sodium rushes into the cell down its concentration gradient (as previously created by the sodium-potassium pump). This causes depolarization of the membrane as its potential reaches a value of +35mV, which is eventually lowered by the opening of the potassium channels. This leads to hyperpolarization, which prevents the signal from traveling backwards.

The refractory period, a phase in which action potentials cannot be fired, is the result of hyperpolarization, during which the membrane potential drops below –70mV. The membrane potential is at this –70mV level while the threshold, which needs to be reached to fire action potential, is slightly higher at –50mV. During the period of extreme hyperpolarization, an action potential will not form.

The resting membrane potential is approximately –70mV, while the threshold potential is roughly –55mV. When a neuron receives a stimulus, the binding of neurotransmitters elicits small, localized influxes of sodium known as postsynaptic potentials. These small potentials must sum together in order to raise the local region of the neuron to –55mV. Once this threshold potential is reached, an action potential is generated and the neuron perpetuates the signal.

Threshold potential is defined as the potential that must be reached in order for an action potential to be initiated by a neuron. Threshold potential is around -55mV in humans, which is slightly higher than the resting potential of -70mV. Once this threshold is reached, the electrical signal will propagate as the membrane depolarizes to a positive potential.

Sub-threshold stimuli, such as stimulus causing depolarization to -65mV, will not trigger action potentials. Muscle contractions can result from action potentials or provide sensory feedback, but the contractions themselves do not play a role in initiating action potentials.

The refractory period is the span of time during which the neuron "recovers" and generally does not respond to a second stimulus as strongly as it did to the first. As the name implies, the absolute refractory period refers to the time when no stimulus, no matter how strong, can provoke a second action potential. This occurs because sodium channels, the opening of which causes depolarization, are sealed by a gating mechanism.

The relative refractory period follows the absolute refractory period. During the relative refractory period, the cell will not respond to normal stimuli, but can generate an action potential if an exceptionally large stimulus occurs. During this period the sodium channels are closed, but not sealed by the gating mechanism; they are essentially normal. The relative refractory period is caused by hyperpolarization as potassium rushes out of the cell after the action potential. Because the potential is lower than normal, only a very large stimulus can overcome the threshold.

Though threshold, saltatory conduction, and depolarization do relate to nervous system potentials, they do not refer to this specific period. Apoptosis is a completely unrelated process referring to a type of cell death.

Temporal summation refers to the phenomenon that an individual neuron will fire with such a high frequency that previous changes in potential have not yet normalized before a new one begins. This summative effect can cause the generation of an action potential, once the threshold potential is surpassed.

Spatial summation refers to the simultaneous activation of several unique neurons to affect another. Numerous individual inputs sum together on the target neuron to stimulate an action potential.

At the end of an action potential, the voltage-gated potassium channels are slow to close. This allows both the normal "leaky" potassium channels and the voltage-gated potassium channels to be open simultaneously. Large amounts of potassium are able to flow down their concentration gradient, exiting the cell. The exiting of these positively charged ions results in the negative dip in cell membrane potential, known as hyperpolarization.

During hyperpolarization, voltage-gated potassium channels close and the sodium-potassium pump is activated to return the cell to the resting potential by moving potassium back into the cell, and sodium out of the cell.

The resting potential of a cell is roughly –70mV. When the potential rises above this level, the cell is considered "depolarized." When the potential delves below this level, the cell is considered "hyperpolarized." If the cell is depolarized above –55mV, the threshold potential, then an action potential is triggered.

Hyperpolarization occurs because potassium channels are slow to open and close, and thus the cell polarizes itself beyond its usual membrane potential. After an action potential depolarizes a cell there is a build-up of positive charge in the cell interior. The late opening of potassium channels causes an abrupt rush of potassium out of the cell, propelled by its electrochemical gradient. This rush lowers the cell potential below its normal resting state, resulting in hyperpolarization. The cell then returns to its resting state via repolarization. Sodium is removed from the cell and potassium is reintroduced through action of the sodium-potassium pump.

The axon hillock is the region that is the first to respond to changing sodium levels by triggering an action potential, should threshold potential be reached. This is due to the presence of voltage-gated channels that can detect the voltage difference caused by the influx of sodium making the inside of the cell more positive.

Axons are responsible for the propagation of the action potential down to the axon terminal, but do not initiate the signal. The cell body has no voltage-gated channels, and therefore cannot respond to the influx of sodium ions. Myelin sheaths also do not have voltage-gated channels, and only assist in speeding up the conduction of the signal. Axon terminals form the interface used to innervate a muscle, a gland, or another neuron.

The period of time immediately after an action potential passes will be characterized by an immediate reduction in membrane potential, followed by hyperpolarization. At this point in time, the sodium voltage-gated channels are inactivated, halting the overshoot and influx of sodium ions into the cell. These channels remain inactivated, in order to avoid having the voltage-gated sodium channels open again to trigger another action potential. This is considered the absolute refractory period. The voltage-gated potassium channels are slower to open than the voltage-gated sodium channels. By the time the sodium overshoot has peaked, the voltage-gated potassium channels are open, allowing an efflux of potassium out of the cell. The efflux is responsible for lowering the membrane potential and eventually causing hyperpolarization.

Remember that the sodium potassium pump is an integral membrane protein that is essential in maintaining the resting membrane potential in neurons. This protein pumps three sodium ions out of the cell and two potassium ions into the cell, creating an electrochemical gradient. The activity of this pump creates a higher concentration of sodium ions outside the cell and a higher concentration of potassium ions inside the cell. This creates a chemical gradient for sodium to flow into the cell and potassium to flow out of the cell. The net transfer of three ions out of the cell and two ions into the cell also generates an electrical gradient, increasing the potential for positive ions to enter the cell. Together, the chemical gradient and electrical gradient are called an electrochemical gradient, and are responsible for the influx of sodium during the depolarization of an axon.

The question states that the person is malnourished. This means that he/she is not getting enough nutrients and energy to fuel the body, which directly affects the production of ATP. The correct answer will be a protein that requires energy to transport the molecules (active transport).

Out of the three proteins presented in the question, only one uses ATP to transport molecules: the sodium-potassium pump. It requires ATP because the pump transports sodium (Na) and potassium (K) ions against their respective concentration gradients. The leak channels and the voltage-gated channels use facilitated diffusion and the electrochemical gradients of the ions as the driving force for transport.

Eventually, as ion concentrations fluctuate in the individual, all three types of proteins may be affected, but only as an indirect consequence. Malnourishment will directly affect the available ATP, reducing functionality of the sodium-potassium pump.

The sodium-potassium pump moves sodium to the outside of the cell and potassium to the inside of the cell. Since the pump moves the ions in opposite directions, the pump is classified as an antiporter. If the ions moved in the same direction it would be classified as a symporter.

The activity of the sodium-potassium pump creates an electrochemical gradient. There are more sodium ions outside the cell and more potassium ions inside the cell. Recall from diffusion that molecules will always want to go from a region of high concentration to a region of low concentration; therefore, diffusion will drive sodium ions into the cell and potassium ions out of the cell.

However, these ions can only traverse through the cell membrane through specialized channels, called leak channels. The sodium leak channels will facilitate the movement of sodium ions into the cell and potassium leak channel will facilitate the movement of potassium ions out of the cell. The sodium-potassium pump and the leak channels move ions in opposite directions, which is why the pump requires ATP input and the leak channels are examples of passive diffusion.