1. Biology 211 Anatomy & Physiology I
Electrophysiology of Nervous Tissue

2. Recall: A neuron carries an electrical signal produced by the movement of ions across its plasma membrane
The mechanism by which it does this should be familiar –
it is very similar to how the plasma membrane of a muscle cell (its sarcolemma) generates and carries electrical signals as we have previously discussed …

3. When resting, the
plasma membrane
of a neuron is
polarized.
Sodium ions are
concentrated on
its outer surface.
Potassium ions and
large negative ions (proteins, phosphate, sulfate, etc)
are concentrated on its inner surface.
Sodium channels and potassium channels are closed so very few ions are passing across the membrane.

4. An action potential
begins when the
plasma membrane
begins to depolarize
Sodium gates
(or "gated channels")
open on one section
of the membrane.
(later, we’ll discuss
what causes this to happen)
Large amounts of sodium ions flow into the cell carrying
their positive charges, making the inner surface of the
plasma membrane more positive. .

5. A few milliseconds
later, potassium
gates open as
the sodium gates
close.
Potassium ions, with
their positive charges,
flow out of the cell,
again making the
outer surface of the plasma membrane more positive.
The plasma membrane has begun to repolarize.

6. The potassium
gates then also
close.
The cell quickly
pumps sodium ions
back to the outside
of the membrane
and potassium ions
back to the inside
of the membrane.
The plasma membrane
becomes fully repolarized.

7. This depolarization / repolarization starts at one point on the membrane, then spreads to nearby regions of the
membrane, causing them
to depolarize / repolarize.
This, in turn, stimulates
regions a little further out
to depolarize / repolarize,
and these events spread
away from the original location

8. This movement of
depolarization &
repolarization Is an
action potential
which travels along
the plasma membrane
of the neuron.

9. This polarization of the membrane is measured as its voltage
This polarization of the membrane is measured as its voltage. This can be increased or decreased by changing how many positive and negative ions are separated from each other.
A voltage of “0”
means that positive
and negative ions
are mixed together
and not separated
from each other…
No separation of + and - ions
Millivolts

10. This polarization of the membrane is measured as its voltage
This polarization of the membrane is measured as its voltage. This can be increased or decreased by changing how many positive and negative ions are separated from each other.
If the membrane
separates those ions
by pushing positive
ions to the outside of
the cell, the voltage
drops below 0.
No separation of + and - ions
Millivolts
+ and – ions separated
with
more + ions outside

11. This polarization of the membrane is measured as its voltage
This polarization of the membrane is measured as its voltage. This can be increased or decreased by changing how many positive and negative ions are separated from each other.
If there are more
positive ions inside
the cell, the voltage
rises above 0.
+ and – ions separated
with
More positive ions inside
No separation of + and - ions
Millivolts

12. A normal resting voltage for a neuron (that is, how much are the positive ions and negative ions separated across its plasma membrane) is between -60 and -70 millivolts.
The “ - ” means that more positive ions are on the outside and more negative ions are on the inside of the membrane.
Millivolts

13. A normal resting voltage for a neuron is between -60 and -70 millivolts.
As Na+ flows into the cell (the membrane depolarizes), the voltage moves closer to “0” and will go past it (more positive ions on the inside)
As K+ flows out of the cell (the membrane repolarizes), voltage returns toward its resting value (more positive ions on the outside)

14. Although the voltage has returned to the resting level of approximately
-70mv after the Na+ and K+ channels close, it is not ready to depolarized again because there is a mixture of Na+ and K+ ions both inside the cell and outside the cell.
A “pump” on the membrane uses active transport to push Na+ ions back outside the cell and push K+ ions back inside the cell. Since both of these ions have a positive charge, this doesn’t change the voltage much.
The membrane is now ready to depolarize again.

15. In reality, the plasma membrane of a neuron does not depolarize (lots of Na+ flowing in) as quickly as those earlier diagrams imply since not all of the sodium gated channels open at the same time.
Instead, a few Na+ gates open first, then a few more, and a few more….
Each time this raises the voltage of the membrane a little bit, until the voltage reaches a point, called the threshold voltage, which causes all of the remaining Na+ gates to open as well, causing rapid depolarization.

16. Those small depolarizations are called Excitatory Postsynaptic Potentials (EPSP)
Each of these raises the voltage of the neuron’s plasma membrane closer to its threshold voltage
If enough EPSPs occur in a short period of time, the full action potential begins.

17. Inhibitory Postsynaptic Potentials (IPSPs) can also occur, which INCREASE in the separation of positive and negative ions across the
plasma membrane of the
neuron (making it even
more polarized.)
These IPSPs make it less likely that the plasma membrane of the neuron will reach threshold voltage.

18. Remember when we discussed excitatory synapses and inhibitory synapses affecting the axon hillock?
Those synapses are producing EPSPs and IPSPs.
The dendrites (and body) of every neuron are constantly receiving both stimulatory signals which move its membrane voltage closer to threshold (EPSPs), and inhibitory signals which move it membrane voltage further away from threshold (IPSPs).
Those travel along the membrane until they reach the axon hillock, and if there are enough more EPSPs than IPSPs to reach threshold voltage, it will depolarize.

19. While some neurons use this continuous type of action potentials to carry their electrical signals, most neurons use a more efficient method of carrying action potentials called saltatory conduction.
This is much more rapid and requires much less energy.

20. Saltatory conduction can only occur on neuron processes which have been myelinated by oligodendrocytes or Schwann cells
The action potential (depolarization then repolarization) occurs only at nodes of Ranvier, so the action potential skips from node to node to node .....

21. Whether the action potential travels along an axon by continuous or saltatory conduction, it eventually spreads along telodendria and reaches the axon terminals.
From here, the signal can be passed to another cell at a synapse

22. Two types of synapses:
a) Ions can pass directly from one neuron to another if their
plasma membranes are connected by gap junctions, thus
starting a new action
potential on the second
cell. This is an
electrical synapse;
it is rare.
The action potential can
cause the axon terminal of
the first neuron to release a neurotransmitter, which
binds to the plasma membrane of the second cell and
stimulates a new action potential on it.
This is a chemical synapse; it is very common

23. Chemical Synapse
Axon terminal releases
neurotransmitter from
synaptic vessicles

24. Chemical Synapse
2. Neurotransmitter diffuses
across synaptic cleft and
binds onto receptors
located on the plasma
membrane of second cell

25. Chemical Synapse
Binding of neurotransmitter to
its receptors opens the sodium
channels, starting depolarization

26. More Definitions
Presynaptic Neuron:
The neuron which secretes the neurotransmitter at a synapse.
Postsynaptic Neuron:
The neuron to which this neurotransmitter binds, thus creating a new action potential on its plasma membrane.

27. Notice that the same neuron can be the
postsynaptic neuron at one synapse
and the presynaptic neuron at the
next synapse.

28. There are dozens of different chemicals which act as neurotransmitters, some of which are listed in this table from Saladin.

29. However: any neuron can only secrete one type of neurotransmitter from all of its axon terminals

30. Additionally, at each synapse there must be a perfect match between neurotransmitter and receptor:
The postsynaptic cell must have receptors which are specific for the neurotransmitter which is secreted by the presynaptic cell:
For example:
If the presynaptic neuron secretes acetylcholine,
the postsynaptic neuron must have acetylcholine receptors.
If the presynaptic neuron secretes serotonin
the postsynaptic neuron must have serotonin receptors
etc,

31. Electrophysiology is one of the most complex and difficult concepts you will face in this A&P class.
Please take the time to understand it.
It is important.