tetanus, tonus, and summation

[virtualmaster999]

Hey everyone!

Kinda confused on these terms still. Do I have the right understanding here?:

-tetanus: a very forceful contraction, which does not allow relaxation
-tonus: partial contraction
-summation: repeated stimuli occur, so there is no time to relax

I'm just confused on the main difference between tetanus and summation, since they both inhibit relaxation.

Could someone clarify this for me?

Thank you!

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[cacajuate]

tetanus is a complete, sustained contraction.

Summation is like a stimulus activates one motor unit, then another stimulus activates another motor unit, so now there is a summation/increase of the contraction.

 

[tommyngu]

Summation is when muscle contractions are combined which produces increased contractile force. Summation occurs when action potential frequency is increased to a rate that is faster than a muscle fiber can relax. This causes the next action potential to propagate before the previous contraction has had time to relax, causing the two (and subsequent) contractions to combine. *this occurs within the same muscle fiber*

Tetanus is just a state of maximum contraction (the fiber can only contract so much). Summation can lead to tetanus.

Tonus is the default "relaxed" state of a muscle which is partially contracted since a muscle fiber cannot fully relax.

Be careful not to confuse summation with recruitment. Recruitment is when motor neurons with successively higher thresholds are activated by an increasing stimulus strength. *this occurs with different muscle fibers since muscles are composed of multiple motor units (a motor neuron and all muscle fibers attached to that neuron)*
For example... take one muscle. There are multiple motor units with each motor neuron having a different threshold. You send a weak stimulus and it activates the weakest motor unit. You increase the stimulus and it activates the weakest motor unit and a stronger motor unit. Increase the stimulus even more and you start recruiting more and more motor units with higher and higher thresholds.

What cacajuate described sounds more like recruitment to me.