NERVE EXERCISES
NERVE PRACTICAL
1)
Confusing terms glossary
This exercise is an extension of Appendix E on Confusing Terms in the No
Frills Generic Skills Guide. Read the introduction there, then scroll down to find the
terms which are relevant to the electrophysiology of nerve practical. Below are some
from that list, but you should be able to add more from your lectures and textbooks.
Refractoriness – refraction
Here are some additional ones:
Action potential – threshold potential
Capacitance - conductance
Conductance – conduction
Electronic conduction – saltatory conduction
Electrical potential – electrochemical equilibrium
Nernst potential – resting potential
Pacemaker potential – threshold potential
Write down your own definitions of 5 of these pairs of terms, then have a
look at the Confusing Terms list.
CONFUSING NERVE TERMS
1) Action potential – compound action potential
An action potential is recorded from a single excitable cell with one
electrode being intracellular, whereas a compound action potential is
recorded from a number of cells with both electrodes being extracellular.
2) Capacitance – conductance
Capacitance refers to the ability of the membrane to build up and hold
charge, while conductance is a measure of the ability of a current to pass
across the resistant membrane.
3) Conductance – conduction
The difference between these terms is that conductance is used to refer to
ions moving through membrane channels, whereas conduction is used to
refer to currents moving along excitable cells.
4) Electrotonic conduction – saltatory conduction
Electrotonic conduction occurs when the membrane potential changes
(depolarisations or hyperpolarisations) spread passively, depending only
on the membrane capacitance and the membrane and axoplasmic
conductances. Saltatory conduction occurs when the action potential
jumps from one node of Ranvier to the next due to the passive electrotonic
spread of the depolarisation from node to node.
5) Electrical potential – electrochemical equilibrium
Electrical potentials are the consequence of charge separation, so that
when there is no change in the charge stored on the membrane capacitance
the potential remains constant, whereas when ions flow as current through
membrane channels and alter the charge separation a change in potential
occurs, referred to as a depolarisation or a hyperpolarisation.
Electrochemical equilibrium for a particular ion occurs when the
membrane potential is equal to its Nernst potential, since this is when the
electrical force acting on the ion is exactly balanced by the chemical force
due to its concentration gradient.
6) Nernst potential - resting potential
Nernst potential for a particular ion is that potential where the electrical
and chemical forces acting on that ion are equal and opposite. Resting
potential refers to the steady potential across the membrane of an excitable
cell which is not being depolarized or hyperpolarized, and is the weighted
average of the Nernst potentials of all the ions which are able to cross the
membrane; although each one of them is not in equilibrium because the
resting potential is not at its Nernst potential, overall there is zero net
current flowing, a necessary condition for a steady potential.
7) Pacemaker potential – threshold potential
A pacemaker potential derives its name from the fact that the cell in which
it is occurring sets the pace for other linked cells; its characteristic feature
is a non-steady resting potential, so that gradual depolarisation to threshold
occurs periodically due to alterations in ion conductances. The threshold
potential is the potential at which the net flow of positive charge across the
membrane is inward, producing the escalating increase in Na conductance
which characterizes the Hodgkin cycle.
2)
Misconception MCQs
This exercise is designed to give you practice in logical thinking.
For each pair of statements, choose which one of the linking final statements
correctly completes the logical sequence.
1.
A. Increasing diameter of nerve fibres increases conduction velocity.
B. Measured velocity was that of the fastest fibres.
Therefore:
C. The largest fibres had the fastest velocities.
D. Measured velocity was that of the largest diameter fibres.
E The fastest fibres had the greatest velocity.
2.
A. Increasing diameter of nerve fibres reduces the length of their refractory
period.
B. Nerve excitability was reduced to 50% when a test stimulus was given 4
msec after the conditioning one.
Therefore:
C. The nerve fibres contributing to the test response were those of smaller
diameter.
D. Shorter refractory periods are associated with reduced excitability.
E. Larger diameter fibres are more likely to respond to a test stimulus 4 msec
after a conditioning one.
[Answers: 1D 2E]
3)
Logical fallacies (and how to avoid them)
Following are statements written by students which do not pursue their arguments to a
correct conclusion. Explain as concisely as possible what you think is wrong with
each and how they could best be corrected.
a)
The population code means that different nerve fibres, with different
physical properties such as diameter and myelination, contribute different
amounts to a compound action potential.
b)
Conduction velocity depends on the distance between the stimulating and
recording electrodes, so it is greater in longer nerve fibres.
c)
Measuring the latencies to the beginning of the compound action
potentials for different inter-electrode distances gives values from which the
conduction velocity at each distance can be calculated.
Explanations of errors:
a)
This statement mixes up two ideas, both of which are correct on their own, but
cannot be linked giving one as the explanation for the other.
While it IS true that:
(i)
different fibres have different physical properties
(ii)
(iii)
these result in their contributing different amounts to a compound action
potential (since larger fibres have larger extracellular current flows),
it is NOT true that:
the population code means they contribute DIFFERENT amounts, only that
they contribute to a combined response.
b)
This statement confuses conduction TIME, which does depend on the interelectrode distance, with conduction VELOCITY, which is obtained by dividing
distance by time. It is predicated on the assumption that longer fibres “need” faster
velocities of conduction of the information in the form of action potentials,
presumably so that the “far reaches” of the organism can respond quickly enough.
However, for the actual distances involved, given that conduction velocities are of the
order of 50-100 metres/sec, or 5-10 cm/msec, the time it would take to reach the ends
of the limbs is of the order of a few msec, even in humans. What the conduction
velocity does depend on is diameter and myelination, which will be very close to
constant along the length of a nerve fibre, so that the velocity will also be constant.
c)
This statement suffers from an assumption that conduction velocity
varies along the length of a fibre, so that dividing inter-electrode distances by the
corresponding latencies will give a series of different numbers, when in fact they will
give the same number. This is the reason why the two variables are plotted against
each other; this yields a straight line and calculation of its SLOPE then gives the
velocity.
4)
Sequences which make sense
Although it is sometimes not very hard to arrange a sequence of steps in logical order,
what is difficult is avoiding errors in the correct use of scientific language. The
following sentences contain various errors in describing experiments on stimulating
and recording from nerves. What are they?
1. Stimuli were applied to the nerve with an inter-stimulus delay of 5 msec.
2. The inter-stimulus was 5 msec.
3. The criteria used was a response bigger than 1 mV.
4. The phenomena investigated was the response to increasing stimulation.
5. The data shows that there was a big increase in response.
6. Less fibres were active during the relative refractory period.
7. The conduction velocity is the time taken to travel from A to B.
8. The latency is the time it takes for the stimulus artifact to illicit a response.
9. The conduction velocity is proportional to the latency.
10. The distance traveled by the stimulus was determined by the length of the
nerve.
5)
The three most important facts
In this exercise you have to make a choice from all the relationships you have
covered in the previous exercises, which THREE are the most vital to
underpin a hypothesis about compound action potential variation with
inter-electrode distance. Since it is essential that the hypotheses evolve from
what you have written in the Introduction, these would constitute the three
most important sentences as they would be the ones from which your
hypotheses could then be predicted.
i)
ii)
iii)
Repeat this exercise now for the Discussion, where the THREE relationships
are those which best explain how your data have supported your hypotheses.
i)
ii)
iii)
Extra notes
When answering some of the questions on nerve activity, the concepts below may
be relevant. Consider the relationships between physical structure and different
aspects of physiological function, including any relevant mathematical equations.
1.
Structural variation in nerve fibre populations
2.
Correlation of structure with electrical properties
3.
Threshold for excitation
4.
Population code
5.
Shape of CAP
6.
Conduction velocity
7.
Reduced excitability (refractory periods) of single fibres
Concept maps
Concept Maps for Nerve prac
Map the nerve characteristics which
influence the shape of a CAP and how
they influence it.
Trigger: Did the CAP change shape when the
responses were obtained at different stimulus
strengths?
How do structural variations between groups
influence their functional behaviour?
Map the nerve characteristics which
influence the conduction velocities
derived from a CAP and how they
influence it.
Trigger: Did the CAP change shape when the
responses were obtained with different
conduction distances?
How do structural variations between groups
influence their functional behaviour?