I. Overview
The objective of this lab is for you to discover
some of the major features of vertebrate striated muscle (voluntary muscles).
We will use the frog gastrocnemius as a typical striated muscle, because
it is easy to get. The other major muscle types, cardiac muscle (from
the heart) and smooth muscle (from the gut and other involuntary organs),
have somewhat different properties from striated muscle. There is
also considerable variety in the properties of different striated muscles;
some are faster but fatigue more easily. We can get other muscles
for independent projects if you are interested in comparing their properties
to the gastrocnemius.
To make the muscle contract, you will stimulate
it electronically, triggering nerves in the muscle. Action potentials
in these nerves tell the muscle to contract. A weak stimulus will
only trigger a few nerves around the electrodes, a strong stimulus will
trigger a greater number of nerves. Increasing the frequency of stimuli
increases the frequency of action potentials in each nerve that is firing.
Your goal in this exercise is to experimentally
determine four features of muscle contraction. You will be able
to control the stimulus (thus the nerve firing pattern). Thus, you
can measure the force or velocity (contracting and relaxing) in response
to different types of stimulation. You can also measure the duration
of the twitch (time before onset, time of contraction, time of relaxation).
You can also investigate how factors such as fatigue affect the force or
velocity of contraction, or how load affects the velocity. You should
control different factors one at a time, and record the response of the
muscle. Your assignment will be to list three of your findings
and present the data to back them up (along with a brief description
of how you got the data if it is not obvious). I will base your grade
on the three best supported findings of your four.
Your grade will depend on how well your data
supports your conclusions, and how well you present the data. I'm
not talking neatness here; I'm talking about choosing an appropriate
format to display the data. Two of the more important factors to
consider when collecting data are: 1) how much data to take, and
2) how broad a range to test. If there is a lot of variability, then
you need to take a lot of data before claiming to detect a trend.
If there is little variability, and the results are very predictable and
repeatable, you don't need as much data. Also, you may see a trend
over the range of data you look at, but that doesn't necessarily mean it
continues beyond the range you consider. I am looking for you to
demonstrate a critical eye; can you analyze your data and see any
gaps in its support for your conclusions?
II. Instrumentation
You will use a DC amplifier connected to either
a force transducer or a movement transducer. For the force transducer
use single-ended mode on the amplifier. For the movement transducer
use differential mode.
In this exercise, you can use two separate computer programs
from the folder "physio lab". The "muscle" program
records a single twitch on a fine time scale. The "muscle multiple"
program allows you to output a series of stimuli over a longer period of
time. With this program, you can control the frequency in addition
to the amplitude of the stimuli.
"Muscle" program
The program "muscle" starts when you
hit the "start" button, then automatically stops after about 200
ms. There is a voltage control, which controls the strength
of the stimulus. When you hit start, the computer will output the
stimulus and begin recording simultaneously (thus you know that the stimulus
occurred at exactly zero seconds). The top recording shows the
voltage produced by the transducer (force or movement). A higher
voltage corresponds to a greater force. With the movement transducer,
you can set it so that zero volts corresponds to the resting length;
the further you are from resting length, the greater the voltage difference
from zero. The bottom recording shows the time derivative of the
top recording, thus, how fast the force or position changes (rate of position
change is velocity). The markers tell you the value of the data
at any point you choose (displayed above the marker as [seconds, volts]).
Punching the button marker values tells the computer to calculate and display
the difference in force and time between the two markers in the force display,
and to display the value of the velocity at the marker in the lower display.
Therefore, to determine the force of a single peak, put one marker at the
tip of the peak and the other at the baseline.
"Muscle multiple" program
The program "muscle multiple" works
just like the muscle program, except for three things. It records
for a longer period, it has a frequency control that determines how many
stimuli per second you send to the muscle, and it does not have a velocity
(rate of change) display.
NOTE: It is easier to adjust the baseline to zero using
the "muscle multiple" diplay.
III. Procedure
First, make sure your transducer is producing zero
volts at rest, and that it responds normally to movement/force.
Each lab group will get one frog leg.
Peel the skin off the same way you would peel a rubber glove off your hand.
This may require forceps to get a good grip, but once started, it comes
off easily. Once you have removed the skin remember to always
keep your muscle wetted with Ringers solution. If it dries out,
it won't work. Once your muscle is set up, keep a tray under it to
catch the Ringers as it drips off. Be careful not to get any of the
electronics wet!
Identify the gastrocnemius. This is
the muscle you will test. It is the major muscle of the lower leg,
and it is what you use when you stand on your tiptoes. Dissect out
the muscle, keeping a chunk of bone attached at each end to use as handles
(Fig. 1). This will involve stripping away the other muscles around
the shin, all the thigh muscles, and the central part of the shin bone.
Clamp the end of the femur in the lower clamp
as shown. Use a hook to connect the foot to a transducer. The
force
transducer can be connected directly above the muscle. When you
use it, make sure the muscle is stretched fairly tightly between the clamp
and transducer. This will give you a good strong output without any
delay as the contraction takes up slack. The movement transducer
must be connected through a lever so that the muscle lifts the central
rod, but doesn't drip on it. Balance the lever so that there is minimal
resistance to lifting the rod on the transducer. Clamp the force
transducer (lightly - it is delicate), or the movement transducer in place.
Before inserting the stimulating electrodes, hit start to reset the output.
Then, insert the two stimulating electrodes into the muscle, and run the
wires through a clamp to hold them in place.
