Learning: Module 02: Nervous System Organization and Signaling - Electrical Potentials and Signaling

In myelinated axons, .

Allof the listed responses are correct. Once initiated, action potentials (AP) travel along the length of the axon with no decrease in voltage, regardless of the distance travelled, and in one direction only. In myelinated cells, this electronic conduciotn is saltatroy conduction. It is largely due to the manner in which the AP is first generated and then propagated at each node of Ranvier before the membrane potential falls below the threshold due to some current leakage across the membrane. Saltatory conduction is represented in the figure below.

In neurons that generate many action potentials, why don't the ion gradients across the neuron's cell membrane dissipate?

The Na+/K+ pump continuously helps reestablish the gradients. The Na+/K+ pump continuously helps reestablish the gradients. Even though the membrane potential can change dramatically, very few ions actually move across the neuronal membrane during each action potential. The neuron is brought to the threshold for producing an action potential by the movement of a relatively small amount of sodium ions, which in turn triggers the opening of more sodium channel gates and a dramatic increase in voltage. Potassium ions leak out of the cell next, and it is then up to the Na+/K+ pump to help reestablish the gradients for sodium and potassium by actively transporting these ions back across the neuronal membrane. Votlage-gated channels close during establishment of the gradient and remain closed until a threshold stimulus is present. Leak channels are always open and cannot close. Additionally, ions are not held in any place by carrier proteins.

Action potentials are unidirectional. Why do they travel only from the cell body of a neuron to the terminal and never go backward?

The sodium channel inactivation gates close once an action potential passes.

The neurotoxin called tetrodotoxin, or TTX, .

blocks voltage-gated sodium channels The neurotoxin called tetrodotoxin, or TTX, blocks voltage-gated sodium channels. Tetrodotoxin is a well known poison that interferes with neuromuscular transmission and results in death in those who ingest it by blocking the action of voltage-gated sodium ion channels. This interferes with the generation and propagation of vital nerve signals. TTX does not inhibit sodium leak channels, potassium leak channels, or ligand-gated potassium channels.

Action potentials propagate down the axon .

by ion diffusion across sections of axon membrane

Graded potentials .

can be either depolarizing or hyperpolarizing events Graded potentials can be either depolarizing or hyperpolarizing events. Depolarizing (excitatory) graded potentials bring a post-synaptic cell closer to threshold. Hyperpolarizing (inhibitory) graded potentials bring a post-synaptic cell further from threshold. If the voltage produced in a single graded depolarization or the summed effect of multiple graded potentials does not bring the voltage to threshold, an action potential will not be produced. In these instsances, once the sub-threshold stimulus is removed, the membrane will return to its resting membrane potential. Individual graded potentials are not measured by their frequency but instead by the degree of change from resting membrane potential. The figure below illustrates two graded depolarizations and one graded hyperpolarization

When a cell's membrane potential becomes less negative or even positive, the cell is said to be .

depolarized When a cell's membrane potential becomes less negative or even positive, the cell is said to be depolarized. A neuron in its resting membrane potential has a negative charge due to the uneven separation of positively charged sodium ions outside and negatively charged anionic proteins and less positively charged potassium ions inside of the cell. The outside is more positively charged overall compared to the more negatively charged inside of the cell, in a polarized state. As a stimulus triggers influx of sodium ions across the cell membrane, the separation is lessened as the voltage becomes less negative (than rest) and the polarity of the cell is less. Thus, depolarization occurs. Polarized is a term to describe a voltage difference across the membrane and is not specific to the degree of difference. Inhibition nor hypersensitivity do not refer to polarity changes. The relationship of membrane potentials are illustrated in the image below.

The amplitude of the peak of the action potential depends on the .

electrochemical gradients for Na+ and K+ The amplitude of the peak of the action potential depends on the electrochemical gradients for Na+ and K+. Amplitude, in this case, refers to the highest voltage achieved in the production of an action potential. The highest point is achieved when the maximum number of sodium ions have diffused into the cell. At resting membrane potential, the concentration gradient difference of sodium ions to potassium ions inside is the key to achieving an action potential. Neither stimulus strength nor duration are of consequence once the threshold voltage is achieved. Additionally, the Na+/K+ pump does not change its rate of pumping.

The two types of electrical signals transmitted through neurons are .

graded potentials and action potentials

At rest, a voltage-gated sodium channel .

has the activation gate closed while the inactivation gate is open At rest, a voltage-gated sodium channel, has the activation gate closed while the inactivation gate is open. Once a depolarizing stimulus is presnt, then a voltage-gated sodium activation gate will open, while the inactivation gate remains open. When this happens, it allows sodium ions to diffuse through the membrane to the inside of the cell. The resulting voltage change then causes the inactivation gate to close to prevent excessive diffusion of sodium ions before the gates reset to their resting position. The voltage-gated sodium channel activation and inactivation gates are illustrated in the image below.

During the after-hyperpolarization phase of the action potential, .

potassium channels are closing. During the after-hyperpolarization phase of the action potential, potassium channels are closing. Inactivation gates for potassium ion channels are closing, but not completely closed during this time, allowing for excessive potassium to diffuse out of the cell. The membrane potential undershoots resting membrane potential so that the voltage is even more negative.

The resting membrane potential of a cell is produced by ion diffusion through ________.

potassium leak channels and sodium leak channels The resting membrane potential of a cell is produced by ion diffusion through potassium leak channels and sodium leak channels. In a neuron poised to conduct an action potential, a resting membrane potential is established and maintained by the sodium-potassium pump to create a higher concentration of sodium ions outside and potassium ions inside. These ions have separate, dedicated channels for each. Leak channels are open so that each ion can diffuse down their electrochemical gradient and contribute to the resting membrane potential. Although the sodium-potassium pump is interal for the generation of resting membrane potential, it is a form of active transport. Therefore, ions cannot diffuse through it. Voltage-gated potassium and voltage-gated sodium channels will allow the resepective ions to flow through them when they are open. However, these voltage-gated channels are closed during the resting membrane potential.

Action potential conduction velocity is the slowest in .

small, unmyelinated axons Action potential conduction velocity is the slowest in small, unmyelinated axons. The velocity of action potential conduction refers to the distance traveled per unit of time, such as meters /sec. Two factors enhance conduction velocity in neuron axons: 1) presence of a myelin sheath and 2) an axon of increased diameter. Therefore, small diameter unmyelinated axons would produce the slowest conduction velocities. Sample conduction velocities in various nerve fiber types are summarized in the table below. The conduction velocities are in the far right column.

The site of information integration in a neuron is the .

trigger zone The site of information integration in a neuron is the trigger zone. The trigger zone is located on the proximal end of the axon near the base of the soma. This is where a depolarizing stimulus can produce a voltage change to the cell's threshold voltage due to the number of voltage-gated ion channels in this area. Neuronal dendrites and the soma are subject to depolarizing stimuli but are less likely to produce a threshold voltage due to the density of sodium and potassium ion channels in these regions. Chemical and electrical synapses are found at the distal end of the axon and not involved as directly in initating an action potential.

The positive feedback loop during the depolarization phase of the action potential is "turned off" during repolarization because .

voltage-gated Na+ channels inactivate and voltage-gated K+ channels open

If the Na+/K+ pump were turned off, the membrane potential would eventually become equal to .

zero mV If the Na+/K+ pump were turned off, the membrane potential would eventually become equal to zero mV. Normally, the resting membrane potential has a negative voltage of -70mV and is a product of the uneven separation of sodium and potassium ions across the neuron cell membrane. The Na+/K+ pump is responsible for actively creating this concentration gradient. The result is an equilibrium potential for sodium of +60 mV and an equilibrium potential for potassium of -90 mV. Without the pump and the resulting gradients, sodium and potassium would reach equilluibrium between the ICF and ECF. Therefore, there would be no membrane potential, and the cell would have a neutral voltage of zero mV.