Properties of an Isolated Sartorius muscle – Isotonic Contraction – Science Report Assignment Answer

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Science Report Assignment Answer
Task:
This demonstration will look at the properties of an isolated Sartorius muscle from the cane toad undergoing 
isotonic contraction
. In an isotonic contraction, the muscle shortens and therefore a weight/load is lifted.
Increasing load/weight will be hung from the muscle.
With each weight, the muscle will be given a maximum stimulus – i.e., one sufficient for all the muscle fibers to reach threshold and undergo cross-bridge cycling.
We will examine the following, together with the underlying mechanisms:

Does the muscle lift the load?

What happens to the latency period with the increasing load? In this case, the latency period is the time between stimulating the muscle to the time at which the load starts to lift.

What is the maximum velocity of lift and how does it change with the load?

When the load is maximally displaced upwards (we limit the shortening of the muscle to 2 mm), why does it stay up there? And how does ‘hang time’ change with increasing load and why?

Your goal will then be to plot this relationship as a Force – Velocity graph and understand the underlying molecular principles involved.
From this information, you will be able to understand the following:

Lifting velocity.

Net force generation.

Mechanical power output.

Understand the molecular characteristics of the muscle which contribute to maximum lifting velocity and force of contraction.

Be able to describe the sequence of 
in vivo
 events from motoneurone stimulation (excitation) to muscle force generation and/or movement (contraction) in terms of the key chemicals, including acetylcholine, Ca

2+

, and ATP.

Data collection

The experiment begins with a 5 g weight, and the size of the weight is progressively increased.

Electricity in the form of closely spaced pulses is delivered from the power source to the stimulating electrodes to induce the muscle to contract. These stimuli will be shown in the top trace on the screen.

The position-sensing device records the distance of lift with time (i.e., a plot of distance against time) – this will be the middle trace on the screen.

From this trace, it is easy to look at changes in latency period, how far the weight is lifted, how long the weight remains lifted to 2 mm (if indeed it is lifted that far), and when the weight is dropped. It will also give you a pretty good idea of changes in the velocity of lift as the size of the weight is increased.

The first differential of this middle trace is then calculated (bottom trace). The first differential is a mathematical transformation of the middle trace which will give the moment-to-moment rate of change of distance. The distance over time is, of course, velocity.

This plot is useful because it is very easy to quantify the 
maximum
 velocity of the lift. It is this maximum velocity of lift that you will use for your Force – Velocity graph.

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