Neurosciences 2-3-4
HC 3
Chapter 2
Nerve cells generate electrical signals, which transmit and store information, based on the
flow of ions across their plasma membranes. When recording the voltage inside versus
outside of neurons, the resting (negative) membrane potential can be identified – which is
overcome and made positive when an action potential arises. All of the electrical signals,
which travel across the neuron as an action potential to carry information from one place to
another, are generated by ion fluxes. These, in turn, are caused by selective ion permeability
and the nonuniform distribution of these ions across the membrane (caused by active
transporters), so a concentration gradient. Electrical signalling must be fast, be able to cross
long distances, and should not loose strength over distance, to work.
So, electrical signals of nerve cells can best be recorded using an intracellular micro-
electrode. When inserted, the machinery will display a negative potential, meaning the
neurons maintain and generate a constant voltage over the membrane when at rest – the
resting membrane potential. There are various other types of signals or potentials:
- Receptor potential: due to activation of sensory neurons, which then confer an
electrical signal that relays to the somatosensory system and generates sensation.
- Synaptic potential: a signal associated with synaptic communication, which upon
activation, allows transmission of information from one neuron to the next.
- Action potential: also known as spikes or impulses, these are the signals that travel
along axons of neurons which are responsible for long range transmission of
information. The 5 phases:
o Resting state: Na+ channels closed, K+ channels closed.
o Threshold: a few Na+ channels open, K+ channels closed. If one Na+ channel
opens, the others are accelerated to also open.
o Depolarization: Na+ channels open, K+ channels closed.
o Repolarization: as you approach the reverse potential Na+ channels are
inactivated, K+ channels open.
o Undershoot: Na+ channels closed and inactivated, K+ channels slowly closing,
causing hyperpolarization. Also, because the Na + channels are still
inactivated, they need to recover, refractory period.
The impulse takes shortest in action potentials, then synaptic potentials and then receptor
signalling.
An action potential is generated by passing an electrical current across the membrane,
which is normally done by synaptic or receptor potentials but can also be done in the lab
with a microelectrode. A second microelectrode can be inserted to measure the changes in
the membrane potential as the current is applied. If the membrane potential becomes more
negative, there is hyperpolarization, which does not elicit any dramatic response, but just
changes the membrane potential because of the current. When no response occurs, this is a
passive electrical response. However, if the current applied causes the membrane potential
to become positive, and cross a certain threshold, depolarization occurs, and the action
potential is generated – an all or nothing phenomenon once the threshold potential has
been passed. The action potential on the other hand is an active electrical response, as
selective changes in membrane permeability take place. Also, the amplitude of the action
, potential is independent of the current magnitude; because if the magnitude increases, the
frequency of action potentials increases, not the amplitude. For receptor potentials, the
amplitude does depend on the magnitude of the sensory stimulus, while for synaptic
potentials the amplitude depends on the number of synapses activated, strength of each
synapse and previous synapse activity.
Neurons, though they can passively conduct electricity, essentially are not good electrical
conductors. If the current pulse is below the threshold potential, the magnitude of the
resulting potential will decay over the length it travels. This decrease in amplitude over the
length occurs because there is leakage of the current across the membrane. The leakage
prevents passive conduction of the electrical signal. To compensate, axons have a so-called
booster system. If a depolarizing current pulse is injected, so one which passes the
threshold, an action potential of constant amplitude is observed over the entire length of
the axon. Active conduction thus counteracts the leakiness of neurons.
Electrical potentials are generated because there are differences in ion concentration across
the neuronal membrane, and because membranes are selectively permeable to specific
ions. These conditions in turn depend on:
- Active transporters: establish ion concentration gradients, as they actively move ions
in and out of the cell against their concentration. An example is the ATPase Na+/K+-
pump, which moves three Na + ions out of the cell when phosphorylated (as this
changes its configuration), whereby after dephosphorylation, it moves to its original
configuration by taking two K+ ions with it. Because these numbers are unequal, an
electrical charge is created.
- Ion channels: confer selective permeability, as only certain ions can pass the
membrane through designated channels along their concentration gradient. They
may also exchange ions via antiport or co-transport, coupling ions transport
channels.
For example, take a membrane selectively permeable to only K +, with equal concentrations
inside and outside – then there is no electrical potential. However, if the concentration
inside is higher than outside, then the electrical potential inside is negative relative to the
outside; this so, because the ion is electrically charged and hence create an electrical
difference. The gradient is established because active transporters actively pump K + into the
cell. From this point onward, the development of the resting membrane potential is started,
which impedes further flow of K+:
1. K+ ions move out of the cell due to passive diffusion.
2. The electrical charge outside becomes more positive.
3. Because K+ ions are positively charged, they are repelled to travel to the outside
where the positive charge dominates.
4. The net movement or flux of K+ ions will thus stop; this point is right when the
potential change across the membrane equals the force from the concentration
gradient. Electro-chemical equilibrium is reached whereby there is an exact balance
of opposing forces; the concentration gradient and the opposing electrical gradient.
To generate the above, a very small amount of K + ions to flow outside because of diffusion is
enough – the concentration of permeant ions remains relatively the same and the tiny
fluxes do not disrupt the chemical electroneutrality. This last point is the case because each
ion has an oppositely charged counter ion; K + has Cl– ions. So, in the end, the equilibrium or
resting membrane potential is the potential at electro-chemical equilibrium. Hereby, there is
a balance between diffusion and electrical force, leading to no net movement of K + ions.
HC 3
Chapter 2
Nerve cells generate electrical signals, which transmit and store information, based on the
flow of ions across their plasma membranes. When recording the voltage inside versus
outside of neurons, the resting (negative) membrane potential can be identified – which is
overcome and made positive when an action potential arises. All of the electrical signals,
which travel across the neuron as an action potential to carry information from one place to
another, are generated by ion fluxes. These, in turn, are caused by selective ion permeability
and the nonuniform distribution of these ions across the membrane (caused by active
transporters), so a concentration gradient. Electrical signalling must be fast, be able to cross
long distances, and should not loose strength over distance, to work.
So, electrical signals of nerve cells can best be recorded using an intracellular micro-
electrode. When inserted, the machinery will display a negative potential, meaning the
neurons maintain and generate a constant voltage over the membrane when at rest – the
resting membrane potential. There are various other types of signals or potentials:
- Receptor potential: due to activation of sensory neurons, which then confer an
electrical signal that relays to the somatosensory system and generates sensation.
- Synaptic potential: a signal associated with synaptic communication, which upon
activation, allows transmission of information from one neuron to the next.
- Action potential: also known as spikes or impulses, these are the signals that travel
along axons of neurons which are responsible for long range transmission of
information. The 5 phases:
o Resting state: Na+ channels closed, K+ channels closed.
o Threshold: a few Na+ channels open, K+ channels closed. If one Na+ channel
opens, the others are accelerated to also open.
o Depolarization: Na+ channels open, K+ channels closed.
o Repolarization: as you approach the reverse potential Na+ channels are
inactivated, K+ channels open.
o Undershoot: Na+ channels closed and inactivated, K+ channels slowly closing,
causing hyperpolarization. Also, because the Na + channels are still
inactivated, they need to recover, refractory period.
The impulse takes shortest in action potentials, then synaptic potentials and then receptor
signalling.
An action potential is generated by passing an electrical current across the membrane,
which is normally done by synaptic or receptor potentials but can also be done in the lab
with a microelectrode. A second microelectrode can be inserted to measure the changes in
the membrane potential as the current is applied. If the membrane potential becomes more
negative, there is hyperpolarization, which does not elicit any dramatic response, but just
changes the membrane potential because of the current. When no response occurs, this is a
passive electrical response. However, if the current applied causes the membrane potential
to become positive, and cross a certain threshold, depolarization occurs, and the action
potential is generated – an all or nothing phenomenon once the threshold potential has
been passed. The action potential on the other hand is an active electrical response, as
selective changes in membrane permeability take place. Also, the amplitude of the action
, potential is independent of the current magnitude; because if the magnitude increases, the
frequency of action potentials increases, not the amplitude. For receptor potentials, the
amplitude does depend on the magnitude of the sensory stimulus, while for synaptic
potentials the amplitude depends on the number of synapses activated, strength of each
synapse and previous synapse activity.
Neurons, though they can passively conduct electricity, essentially are not good electrical
conductors. If the current pulse is below the threshold potential, the magnitude of the
resulting potential will decay over the length it travels. This decrease in amplitude over the
length occurs because there is leakage of the current across the membrane. The leakage
prevents passive conduction of the electrical signal. To compensate, axons have a so-called
booster system. If a depolarizing current pulse is injected, so one which passes the
threshold, an action potential of constant amplitude is observed over the entire length of
the axon. Active conduction thus counteracts the leakiness of neurons.
Electrical potentials are generated because there are differences in ion concentration across
the neuronal membrane, and because membranes are selectively permeable to specific
ions. These conditions in turn depend on:
- Active transporters: establish ion concentration gradients, as they actively move ions
in and out of the cell against their concentration. An example is the ATPase Na+/K+-
pump, which moves three Na + ions out of the cell when phosphorylated (as this
changes its configuration), whereby after dephosphorylation, it moves to its original
configuration by taking two K+ ions with it. Because these numbers are unequal, an
electrical charge is created.
- Ion channels: confer selective permeability, as only certain ions can pass the
membrane through designated channels along their concentration gradient. They
may also exchange ions via antiport or co-transport, coupling ions transport
channels.
For example, take a membrane selectively permeable to only K +, with equal concentrations
inside and outside – then there is no electrical potential. However, if the concentration
inside is higher than outside, then the electrical potential inside is negative relative to the
outside; this so, because the ion is electrically charged and hence create an electrical
difference. The gradient is established because active transporters actively pump K + into the
cell. From this point onward, the development of the resting membrane potential is started,
which impedes further flow of K+:
1. K+ ions move out of the cell due to passive diffusion.
2. The electrical charge outside becomes more positive.
3. Because K+ ions are positively charged, they are repelled to travel to the outside
where the positive charge dominates.
4. The net movement or flux of K+ ions will thus stop; this point is right when the
potential change across the membrane equals the force from the concentration
gradient. Electro-chemical equilibrium is reached whereby there is an exact balance
of opposing forces; the concentration gradient and the opposing electrical gradient.
To generate the above, a very small amount of K + ions to flow outside because of diffusion is
enough – the concentration of permeant ions remains relatively the same and the tiny
fluxes do not disrupt the chemical electroneutrality. This last point is the case because each
ion has an oppositely charged counter ion; K + has Cl– ions. So, in the end, the equilibrium or
resting membrane potential is the potential at electro-chemical equilibrium. Hereby, there is
a balance between diffusion and electrical force, leading to no net movement of K + ions.
