T9

Question 1

An action potential is

Answer 1

An action potential is a rapid, temporary change in a cell’s membrane potential, allowing it to transmit electrical signals along a neuron

Question 2

The Action Potential and its Propagation
- resting potential, threshold, key phases

Answer 2

Resting Potential: the neuron starts at a resting potential of around -70mV, with more Na+ (sodium) ions outside and more K+ (potassium) ions inside

Threshold: when a stimulus reaches the neuron and the membrane potential reaches approximately -55mV, the action potential is triggered

Key Phases:
1. Depolarization: Na+ channels open, Na+ rushes in, making the inside of the cell more positive
2. Repolarization: K+ channels open, K+ flows out, restoring a more negative internal charge

Question 3

PROPAGATION

Answer 3

The action potential travels down the axon by sequential depolarization of neighbouring segments.

Hyperpolarization: the cell briefly becomes more negative than the resting potential as K+ channels close slowly
Return to Resting State: The Na+/K+ pump restores the original ion distribution, bringing the neuron back to its resting state
All-or-None Principle: the action potential either occurs fully or not at all, ensuring consistent signal transmission
Refractory Period: during this phase, the neuron cannot fire another action potential, preventing signal overlap

Question 4

FORMATION OF THE RESTING POTENTIAL

Answer 4

In a mammalian neuron at resting potential, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside the cell
Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane
These concentration gradients represent chemical potential energy
In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady.

Question 5

ACTION POTENTIAL

Answer 5

Are the signals conducted by axons. Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli
Large negatively charged protein molecules remain inside the cell
Gates keep out positively charged Na+ ions and channels allow K+ and Cl- ions to pass more freely
Na+ – K- pumps extrude Na+ from the intracellular fluid

Question 6

GRADED POTENTIALS

Answer 6

Small voltage fluctuations that are restricted to the neighborhood of the axon where ion concentrations change
Any change in ion concentration can change the membrane potential, which varies according to the Na+ and K+ equilibrium
Occurs when the permeability of the membrane to a specific ion change: change in conductance

Question 7

Hyperpolarization + Depolarisation

Answer 7

Hyperpolarization (intracell space more negative) → when A- ion go into the cell / or + go out
Small increase in electrical charge across a membrane

Depolarization (intracell space more positive) → when A– ion go out of the cell / or + go in
Small decrease in electrical charge across a membrane

Question 8

RESTING MEMBRANE POTENTIAL

Answer 8

Most voltage-gated sodium (Na+) channels are closed
Most of the voltage-gated potassium (K+) channels are also closed

Question 9

DEPOLARIZATION

Answer 9

Voltage-gated Na+ channels open first and Na+ flows into the cell
During the rising phase, the threshold is crossed, and the membrane potential increases
During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

Question 10

THRESHOLD POTENTIAL

Answer 10

The voltage on a neural membrane at which an action potential is triggered by the opening of Na+ and K+ voltage- sensitive channels
An action potential is triggered when the cell membrane is depolarized to about -50 millivolts.
At this threshold potential, the membrane charge undergoes a remarkable further change with no additional stimulation.
The relative voltage of the membrane drops to +30 mV (total change 100 mV) generating the nerve impulse after, returns to the original stage (-70 mV).

Question 11

During the action potential, the membrane is …

Answer 11

absolute refractory, if the axon membrane is stimulated during the action potential, another action potential will not occur

Question 12

Relatively refractory period:

Answer 12

after the action potential, the membrane is hyperpolarized, and a more intense stimulation (than the initial one) is needed to generate a new action potential.

Question 13

CHARACTERISTICS OF THE AP

Answer 13

Depends on the presence of ionic channels in the membrane
All or none
All AP are equal in amplitude and duration for each type of fiber
Travel at constant velocity along the axon
The AP cannot sum due to the refractory periods

Question 14

PROPAGATION OF THE AP

Answer 14

When a part of the membrane of an axon reaches the threshold, induce a change in the voltage of other parts still further along the axon, and so on and on.
The propagation of action potentials constitutes the nerve impulse
The nerve impulse is the propagation of an action potential across the membrane of an axon.
But importantly, refractory periods impair that the nerve impulse can reverse the direction, permitting the transmission of the message across the axon to other neurons.
Refractory periods limit the capacity to generate actions potentials to about 200 per second.

Question 15

SPEED OF PROPAGATION

Answer 15

Internal resistance (ri): not so variable
Axon diameter: the higher the diameter the more propagation speed (less resistance)
Cytoplasm conductibility
Membrane resistance(rm) → is more variable
Depend on the number of open channels
Amount of myelin
Longitude constant (λ) → distance to what a change of voltage decrease 1/3 from the initial value
Speed of propagation

Question 16

SALTATORY CONDUCTION

Answer 16

Propagation of an action potential at successive nodes of Ranvier; saltatory is understand as “jumping” or “dancing”.
Glial cells → Schwann cells in PNS and Oligodendroglia in CNS
play a key role in speeding the nerve impulse.
Glial cells wrap around each axon, forming the myelin that insulate it.
Node of Ranvier → part of an axon that is not covered by myelin.
Sufficiently close to one another that an action potential occurring at one node can trigger the opening of voltage-sensitivity gates at an adjacent node

Question 17

EXCITORY AND INHIBITORY POSTSYNAPTIC POTENTIALS

Answer 17

1. Excitatory postsynaptic potential (EPSP) → brief depolarization of a neuron membrane in response to stimulation, making the neuron more likely to produce an action potential
2. Inhibitory postsynaptic potential (IPSP) → brief hyperpolarization of a neuron membrane in response to stimulation, making the neuron less likely to produce an action potential

Question 18

TYPES OF INHIBITION

Answer 18

Presynaptic inhibition: also called lateral synapsis.
The inhibition is produced to a presynaptic terminal button of an excitatory synapsis
Decrease of NT

Postsynaptic inhibition (PIPS) → the NT causes a hyperpolarization of the postsynaptic membrane

Question 19

SUMMATION OF INPUTS

Answer 19

According to this theory, the neuron sums all inputs that are close together in time and space. The decision is made at the axon hillock, the region that initiates the action potential.
The axon hillock is a specialized part of the cell body of a neuron that connects to the axon… integrating the signal
Differentially, in sensitive neurons the key point is located in the sensitive terminal
Lines of data now indicate that, contrary to theoretical expectations, there is not location or spatial dependence…
The postsynaptic potential increases the amplitude with distance from the soma, reducing the effect of temporal and spatial summation.
More research is needed to know the exact principles that regulates how neurons integrate information.