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The Motor Unit 3

 

E. Spatial and Temporal Summation:

1.

The summation discussed above, in which several motor neurons work together to induce stronger contractions is called spatial summation. The term “spatial” refers to the fact that muscle fibers from different regions in the same muscle are activated together and contract together.

2.

In another page, we have also discussed a different type of summation (see Twitch & Tetanus). In that situation, one single motor unit was repeatedly activated. This also caused summation and this type of summation can now be called temporal summation (=summation in time).

3.

In daily life, both types of summation, spatial and temporal, are simultaneous active and work together to perform contractions and movements in some parts of our skeleton.

 

F. Why do we need motor units? Would it not be simpler if a motor neuron would innervate and activate a single muscle fiber?

1.

If every muscle fiber was connected to its own nerve cell, then there would be as many motor neurons as there are muscle fibers.

2.

The problem is that there are too many muscle fibers in the body. If every muscle fiber had its own motor neuron, then there would be an enormous increase in the number of motor neurons and hence in the size of the spinal cord. The spinal cord would then be very much larger (wider) than it now is.

3.

So, to be more efficient, one motor neuron typically innervates 100-10,000 motor fibers.

4.

In fact, the ventral horns (those that contain the motor neurons) are indeed larger in two areas along the spinal cord; namely at those locations where the motor neurons innervate the upper and the lower limbs. In those locations, there are more motor neurons to innervate more muscles, in the limbs, then in other parts of the body (such as thorax or abdomen).

 

 

G. A few more notes on motor units :

1.

Motor units provide for a more efficient way of stimulating and contracting muscles. If the brain needs a lot of control to perform delicate movements, then there will be many neurons involved in this contraction and hence the motor units will be small (less muscle fibers connected to each motor neuron).

However, if in some muscles, the brain does not need a lot of control, then less motor neurons will be dedicated to these muscles.

This is often the case in large muscles such as in the legs or the abdomen. Motor neurons that control these muscles often connect to thousands of muscle fibers each.

2.

Motor units also provide the possibility of sustaining contractions for longer times. In the diagram for example, if motor units A alternate with B and C, then contraction of the whole muscle would be summated in time and each motor unit, in turn, gets some rest while the others are working.

3.

These motor units and regulation of contraction is only true for skeletal muscles. The situation is very different in cardiac muscles and in smooth muscles.

 

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