Tema 10

Question 1

What is an action potential

Answer 1

an abrupt and transient change of the membrane potential. In a few milliseconds the potential reverses from negative to positive and returns to the resting potential.

The basic mechanism is fairly constant in the different cells: changes in membrane permeability mediated by the opening and closing of ion channels operated by voltage.

Question 2

What are the 3 basic properties of an a.potential

Answer 2

1. Threshold (“all or nothing”)
2. Refractory period
3. Conduction

Question 3

a. potential properties that can vary

Answer 3

Duration: usually very short (0.5 - some milliseconds), can arrive up to a few seconds

Amplitude: from 20 to 120 mV of amplitude.

Start: by stimuli or spontaneously.

Question 4

Phases of an a.potential

Answer 4

1) Depolarization: the potential rises in a positive direction, first gradually to a threshold and then abruptly, becoming inverted. The peak of inverted potential (positive) is called excess or overshoot.

2) Repolarization: the potential falls rapidly in a negative direction towards the resting potential.

3) Post-potential hyperpolarization: the potential is placed transiently in values slightly more negative than the resting potential.

Question 5

Hodgkin-Huxley model

Answer 5

The squid giant axon has been used as a model because:
A. It is very similar to the axons of mammals.
B. It has few types of channels compared to cardiac or smooth muscle and neurons.

It uses uses only 3 ion channels.
1. Leak channels of K+
2. Voltage-dependent Na+ channels
3. Voltage-dependent K+ channels

Question 6

Ionic pases of the a.potential

Answer 6

Voltage-gated membrane channels intervene.
- Na+ channels: open at the beginning of depolarization and close when repolarization begins.
- K+ channels: open from the beginning of depolarization to the end of hyperpolarization.

Question 7

voltage dependent Na channel during A.P

• what blocks Na channels

Answer 7

At the resting membrane potential, the channel activation gate is closed

A depolarizing stimulus reaches the channel

With the activation gate open, Na+ enters the cell.

The gate is closed and the Na+ stops entering.

During the repolarization produced by the K+ exit of the cell, the two gates return to their original position.

*Tetrodotoxin was used by Hodgkin and Huxley to block voltage-gated Na+ channels.

Question 8

What is conductance

Answer 8

The conductance (g) or permeability is the easiness with which ions flow through the membrane. The conductance of the membrane is the summation of the conductances of all individual channels.

The changes in the membrane potential during the action potential are due to selective changes in the conductance of the membrane for Na+ and K+ and the resulting variation in the ratio between the conductances for both ions.

The predominance of conductance for one of the two ions draws the membrane potential towards the equilibrium potential (Nernst potential) of said ion.

Question 9

drawing showing conductance of Na an K during an a.potential

Question 10

Drawing of an action potential and explaining what happens each step

Question 11

Regulation of the action potential

Answer 11

Na+ channel gate opens fast, letting Na+ go into the cell, by POSITIVE FEEDBACK, more Na+ enters (cell depolarises more, so more Na+ enters).
This stops when the na channels close.
K channels open, so K exits the cell - repolarizing it

Question 12

How is the gradient maintained

Answer 12

The Na+ -K+ pump is responsible for maintaining chemical gradients by consuming ATP. The pumping capacity increases exponentially with the intracellular concentration of
Na+

The generation of an action potential (entry of Na+ and exit of K+) produces an inappreciable effect on the gradient of concentrations of these ions. Hundreds of thousands or even millions of action potentials must be produced so that these gradients are significantly reduced and the capacity to generate more action potentials is lost.

Question 13

Why are electrical signals used?

Answer 13

Because it is the fastest process that happens in cells: it connects long distances in a very short time
They must be able to reach any part of the body and produce an immediate response.

2. Because it is energetically very cheap: it uses electric gradients that already exist in the cells.
   They have to be continuously active.

Question 14

What is the threshold

Answer 14

The excitation threshold is a minimum level of depolarization that must be overcome by the stimulation to trigger the action potential.
All suprathreshold stimuli cause an action potential of the same size and shape (it is an all-or-nothing process).

Threshold = -65mV

A stimulus that is unable to produce an action potential is a subthreshold stimulus.

A stimulus that produces an action potential is a suprathreshold stimulus.

The action potentials are independent of the stimulus; all or nothing, you get an action potential or not, but they are all the same intensity.

Question 15

Types of stimualtion/ excitation

Answer 15

A. Mechanical stimulation (pressure on sensitive nerve terminals in the skin)
B. Chemical stimulation (neurotransmitter release)
C. Electrical stimulation (transmission of excitation between neighboring muscle
cells in the heart or intestine)

Question 16

STIMULUS INTENSITY AND DURATION RELATIONSHIP

Answer 16

Rheobase: Minimum intensity that, applied for an indefinite time, gives rise to a response.

Chronaxia: time during which a stimulus (with double intensity than rheobase) has to be applied in order to produce a response.

*draw graph

Question 17

What is the refractory period

Answer 17

Time during which no other action potential can be generated due to the inactivation of the Na+ channels

2 types:
1. Absolute refractory period: includes the depolarization phase and the beginning of repolarization. It limits the maximum frequency of triggering of potentials and, therefore, of signal transmission.
*When the inactivation gate of Na+ channels is closed - no other A.P can occur

2.Relative refractory period: in which it is possible to generate an action potential with an intensity higher than normal. It comprises the end of repolarization and hyperpolarization.

Question 18

Where does the A.P generate
–>

Answer 18

Action potentials can only be generated in specific regions of the membrane; only parts with a high concentration of voltage-dependent ion channels are capable of generating an action potential.

In a classical neuron, the site of origin of an A.P is usually the axonic cone
Other possible sites depend on the type of neurons:
-Dendritic potentials in cerebellar Purkinje cells.
-Dendritic potentials in cortical pyramidal cells.

All muscle cells (skeletal, cardiac and smooth) can generate action potentials over their entire surface.

Question 19

SUMMATION TYPES

Answer 19

Spatial summations: signals coming from multiple simultaneous inputs.
Temporal summation: signals coming from repeated inputs

*DRAW THEM

Question 20

Different shape of A.P

Answer 20

Many cells vary their membrane potential.
* Skeletal, cardiac and smooth muscle
* Neurons

Activates the contraction or secretion

The signal travels quickly throughout great distances.

The shape influences the maximum frequency at which the neuron can generate action potentials and the number of action potentials generated for a given stimulus.

The shape will influence the amount of Ca2+ that will enter during the action potential (or a train of action potentials) and will influence the amount of transmitter that will be released.

The shape of an action potentials depend on the channels involved and how they work.
There are channels with different opening characteristics for the same ion.

Question 21

Rhythmicity of some excitable tissues.

Answer 21

Some excitable cells can discharge repeatedly. Repeated discharges occur normally in the heart, smooth muscle and in some neurons of the CNS (thalamic and hypothalamic).
The membrane must be permeable to Na+, so its membrane potential oscillates between - 60 and - 70 mV, close to the threshold.