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Physiology - Foundation Module > Nerve Physiology > Flashcards

Nerve Physiology Flashcards

1
Q

What are the three parts of the nerve cell?

A

1.Cell body
- Has a single nucleus
2.Axon
- Long & cylindrical
- transmit impulses away from cell body
3.Dendrites
- Receive impulses from other nerves

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2
Q

Describe the Myelination of the axons in the PNS and the CNS.

A

*In PNS the myelin is formed by Schwann cells
*In CNS formed by oligodendrocytes.

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3
Q

Characteristic features of neurons?

A
  • Excitability - capability of generating electrical impulses (action potential) or ability to respond to stimuli;
  • Conductivity - ability of propagating the electrical impulses generated along the nerve fibers.
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4
Q

What makes neurone excitable?

A

The presence of an electrical difference between the inside and outside of the nerve cell called the RESTING MEMBRANE POTENTIAL; when disturbed by stimuli, it can generate a response.

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5
Q

What are the two types of responses that be result from the excitation of a neuron?

A
  1. Graded response
  2. Propagated potentials
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6
Q

What are Graded Responses?

A

also called local, nonpropagated potentials. They are several types depend on the function and location (e.g. synaptic, receptor or pacemaker potentials)

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7
Q

Define Conduction.

A

Conduction is an active, self-propagating process, and the impulse moves along the nerve at a constant amplitude and velocity specific to nerve fiber type

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8
Q

What are the two conditions required for the generation of a resting membrane potential?

A

– there must be an unequal distribution of one or more ions across
the membrane (ie, a concentration gradient).
– membrane must be selectively permeable to one or more of these ions.

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9
Q

What is the resting membrane potential?

A

An electrical (potential) difference between inside and outside of nerve cell membrane (-70mV)
– with the inside negative relative to the outside of the cell membrane at rest.

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10
Q

What are the two pumps required for the genesis and the maintenance of the RMP?

A
  1. The Na+/K+ pump
  2. The K+ “leak” channels
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11
Q

Describe the concentration gradient during RMP.

A

– Higher concentration of K+ inside the cell than outside.
- Higher concentration of Na+ outside than inside the cell

There is:
* Outward K+ concentration gradient
* Inward Na+ concentration gradient

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12
Q

Describe the role of K+ in the maintenance of the RMP.

A
  • Resting membrane potential in neuron is –70 mV;
  • This is close to the equilibrium potential for K+
  • This is because more K+ channels are open than Na + channels at rest, resulting more membrane permeability to K+
  • So intracellular and extracellular K+ concentrations are the main determinants of the RMP
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13
Q

How is the inside of the cell maintained at a negative potential at RMP?

A

RMP is largely because of the movement of K+ out of the cell down its concentration gradient through open K+ channels (called leak K+channels ).

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14
Q

Why is the resting membrane potential not equal to the K+ equilibrium potential?

A

because small number of Na+ channels that open pull the membrane potential slightly toward the Na+ eqquilibrium.

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15
Q

What does “polarised” mean in terms of RMP?

A

The resting membrane potential is “polarized,” simply meaning that the outside and inside of a cell have a different electrical charge.

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16
Q

What does it mean to say that a membrane is depolarised?

A

The membrane is depolarized when its potential becomes less negative (closer to zero) than the resting level

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17
Q

What is repolarising?

A

When a membrane potential that has been depolarized is returning toward the resting value, it is called repolarizing .

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18
Q

What does it mean to say that a membrane is “hyperpolarised”?

A

The membrane is hyperpolarized when the potential is more negative than the resting level.§

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19
Q

What is an Action Potential?

A

An action potential occurs when a neuron sends
information down an axon, away from the cell body (when it is activated).

*Action Potential is a sudden reversal of polarization

(Depolarization) followed by rapid returning to resting level (Repolarization)

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20
Q

What causes the depolarisation and the repolarization in an action potential?

A
  • Action potential (depoleraization) is due +
    to opening of voltage gated Na channels (Na+ influx)
  • Repolarization is due to opening of voltage gated K+ channels (K+ efflux)
21
Q

What is the principle behind the propagation of action potentials?

A

They are due to electrical current produced by changes in the conduction of ions across the cell membrane.

22
Q

Describe the basis of the threshold potential.

A

– When stimuli applied voltage-gated Na + channels open
– Na + enters the cell
– membrane is brought to the firing level

23
Q

What is the basis of the positive feedback loop in the depolarisation of the axon?

A

– Voltage-gated Na+ channels overwhelm the K+ eflux
– The entry of Na + causes the opening of more voltage-gated Na+ channels and further depolarization due to positive feedback loop

24
Q

Describe the Changes in membrane conductance of Na+ and K + that occur during the action potentials during depolarisation.

A
  1. due to cessation of Na+ influx
    – Na+ channels that was opened is abruptly close (channel opening is short lived) and enter into a closed state called the inactivated state and remain in this state for a few milliseconds before returning to the resting state to be activated again
  2. direction of the electrical gradient for Na + is reversed during the overshoot because the membrane potential is reversed resulting this limits Na + influx;
    3.opening of voltage-gated K + channels resulting K eflux
25
Q

Describe the Changes in membrane conductance of Na+ and K + that occur during the action potentials after depolarisation.

A

After the rapid initial repolarization (spike potential), the further repolarization occurs slowly. This is due to the fact that the rate of K+ efflux slows down as the electrical gradient responsible for initial rapid diffusion declines.

26
Q

Describe the Changes in membrane conductance of Na+ and K + that occur during the action potentials after hyperpolarization?

A

– Slower and prolonged opening of voltage-gated K +
channels
– This results slow return of the K + channels to the closed state

27
Q

Describe the “All of non law” regarding action potentials.

A

Action potentials occur only when the initial stimulus open sufficient voltage gated Na+ channels to elevate the membrane potential beyond the threshold potential (- 55mV).

28
Q

What are sub threshold stimuli?

A

These weak depolarizations are called subthreshold potentials , and the stimuli that cause them are subthreshold stimuli.

29
Q

Explain what it means to say that “Action Potentials obey the all-or-nothing-law”

A
  • Stimuli stronger than those required to reach threshold elicit action potentials, but the action potentials resulting from such stimuli have exactly the same amplitude as those caused by threshold stimuli.
  • This is because once threshold is reached, membrane events are no longer dependent upon stimulus strength
  • This is because the depolarization generates an action potential due to the positive feedback cycle is operating.
  • That is an action potentials either occur maximally or they do not occur at all.
30
Q

What is the refractory period?

A

During depolarizing stage and much of the hyperpolarizing stage of the action potential , the neuron is refractory to stimulation That is another action potential cannot be produced .

30
Q

What is the refractory period?

A

During depolarizing stage and much of the hyperpolarizing stage of the action potential , the neuron is refractory to stimulation That is another action potential cannot be produced .

31
Q

This refractory period is divided into…

A

– Absolute refractory
– Relative refractory period

32
Q

Absolute refractory period?

A

– Correspond to the period from the time the firing level is reached until Repolarization is about one-third complete
– During the absolute refractory period, no stimulus, no matter how strong, will excite the nerve

33
Q

Relative refractory period?

A

– from the end of absolute refractory period to the start
of after-depolarization
– during this period, stronger than normal stimuli can cause excitation.

34
Q

What is the importance of the refractory period?

A
  1. Limit the number of actions potentials that can be produced.
  2. Separates the action potentials so that they can be passed down.
  3. Blocks backward transmission
  4. Discriminate the intensity of the stimulus.
35
Q

Describe the supernormal period during action potentials.

A
  • During supernormal period, the membrane is hyperexcitable.
  • i.e. the threshold of stimulus is decreased.
  • This period corresponds with the after depolarization phase of the action potential.
36
Q

Explain why the membrane is at a “hyper excitable” state during the relative refractory period.

A
  • During ‘after depolarization’ phase the Na+ channels have come out of inactivated state but the K+ channels (voltage gated) which are open are slow channels and the membrane potential is still nearer to the firing level
  • So a stimulus of low intensity will be able to excite action potential. In other words, the threshold level of stimulus is decreased during this stage resulting hyperexictability.
37
Q

Describe the subnormal period during an action potential.

A
  • This period corresponds with the after
    hyperpolarization stage of the action potential.
  • Low excitability of membrane is because membrane potential has moved away from firing level ( hyperpolarized)
38
Q

What are graded potentials?

A

Graded potentials are changes in membrane potential that are confined to a relatively small region of the plasma membrane.

39
Q

What are the three features of graded potentials?

A
  • Graded potentials
    (a) can be depolarizing or hyperpolarizing
    (b) can vary in size (not obey all or none law) * (c) are conducted decrementally.
40
Q

Describe what happens at the membrane during subthreshold potentials.

A
  • Not all membrane depolarizations in excitable cells trigger the positive feedback process that leads to an action potential.
  • If the firing level does not reach positive feed back will not occur and no action potential will be produced
  • In such cases, the membrane will return to its resting level as soon as the stimulus is removed and no action potential will be generated.
41
Q

How can the graded potential cause an action potential?

A
  • If stimuli strength gradually increases size of the local response increases
  • When depolarization reaches firing level during any stage action potential can occurs
42
Q

What do graded potentials do?

A
  • Although subthreshold stimuli do not produce an action potential, they can produce localize change in the resting membrane potential.
  • This can be either depolarizing or hyperpolarizing
43
Q

Relationship between graded potentials and nerve excitability?

A
  • Hyperpolarizing responses elevate the threshold ( reduce the nerve excitability)
  • Depolarizing potentials lower it as they move the membrane potential closer to the firing level
    ( increase nerve excitability) .
44
Q

Describe the propagation of a nervous impuls.

A
  • Nerve cell membrane is polarized at rest ( has a RMP)
  • Positive charges along the outside of the membrane and negative charges along the inside.
  • During the action potential this arrangement is abolished and for a brief period is reversed
  • Positive charges from the membrane ahead of and behind the action potential flow into the area of negativity represented by the action potential (“current sink”).
  • As positive charges accumulate, membrane polarity ( membrane potential) ahead of the action potential becomes depolarised
  • Such depolarization initiates a local response, and when the firing level is reached, another action potential occurs that in turn produces current sink and depolarizes the membrane in front of it.
45
Q

Describe Saltatory Conduction.

A

As myelin is an insulator, and current flow through it is negligible.
*So current sink and depolarization in myelinated axons occurs from one node of Ranvier to the next
*This “jumping” of depolarization from node to node is called saltatory conduction.

46
Q

How does Local Anesthesia work?

A

blockade of voltage- gated Na+ channels on the nerve cell membrane.
– This causes a gradual increase in the threshold for electrical excitability of the nerve
– reduction in the rate of rise of the action potential – slowing of axonal conduction velocity.

47
Q

Nerve fiber types and their functions?

A
  • Type A - Touch, pressure
  • Type B -Preganglionic autonomic
  • Type C - Pain, temperature, Sympathetic Postganglionic

Physiology - Foundation Module (6 decks)

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