CSCS Practice Questions – Exercise Science Part 7

Posted by on Apr 21, 2013 in Exercise Science, Practice Questions, Review Topics | 2 comments

The key to understanding the last question is knowing the components of muscles.  Again, let’s start with re-reading the question:

When you move a muscle, neurons in your brain fire to recruit motor units.  When a motor unit is activated by a neuron, every muscle fiber in that motor unit fully contracts because:

To get this question reliably correct in every form that it may be presented to you, you need to be able to distinguish what motor units, muscle fibers, and motor neurons are, and what they do.

motor unit is a collection of muscle fibers, all innervated by a single motor neuron.
muscle fiber is a single muscle cell, sometimes running as long as the entire muscle.

Some motor units are huge and are comprised of hundreds of thousands of muscle fibers, while others may consist of a single muscle fiber (think of the eye).  The number of muscle fibers in a motor unit usually correlates with how fine of movements the muscle makes.

If you’re an engineer like me you can think of the motor unit as a collection of muscle fibers connected in parallel to a motor neuron, or displayed graphically:

Circuit Diagram of a Motor Unit

Circuit Diagram of a Motor Unit

So as you can see, when a motor neuron activates all muscle fibers connected to it contract.  This is known as the all-or-none law, so the correct answer is B.  A question like this could trip you up even if you knew how motor neurons, motor units, and muscle fibers were related and grouped…because you could fall for something like “the principle of total recruitment”.

 

Next Question!
During the Excitation-Contraction Coupling Phase of a muscle, calcium ions are released so that they can bind to ______, a protein located at regular intervals along the actin filament.

A.  Myosin
B.  Troponin
C.  Leucine
D.  Tropomyosin

Hint: read chapter 1!

2 Comments

  1. Julian,

    It is to my understanding that the All-or-None law refers to the concept that there is a specific threshold stimulus required to achieve contraction. Whether that stimulus is very high or just above the threshold, the contraction is of the same strength. If the stimulus is NOT sufficient however, there is no contraction at all. Thus this is why it is called the All-or-None principle. Can you explain to me your reasoning behind applying it to all of the fibers of an individual motor unit firing? A motor unit fires in totality because each motor unit, and all of its individual fibers, are innervated by the same motor Neuron… This is a different concept than the All-or-None principle.

    Thanks,
    Thomas.

    • Hey Thomas,

      Great point – I think I misapplied a different concept here. It’s been a while since I wrote this post, looks like it needs some editing! Great catch.

Leave a Reply