I'm reading my Physio book over again and there are some statements i would really appreciate getting cleared up!
1) The contraction force of a single fiber of a Smooth or Cardiac muscle is GRADED. Why is this? Doesn't Na+ enter to reach threshold to fire an AP in all-or-none fashion?
2) The contraction force of a single fiber of a Skeletal muscle is ALL OR NONE. Why is this? Isn't the contraction force dependent on the length of skeletal muscle? (too long of skeletal muscle means thick and thin are barely overlapped and forms few crossbridges, the sliding filaments interact only minimally and not generate much force. Too short of skeletal muscle means they are overlapped too well and thick filaments can't move the thin filaments too far)
Thanks!!!
All or none isn't referring to contraction force. You are mixing up a bunch of the concepts. First, lets start with what a motor unit is. It is the alpha motor neuron and all the muscle cells it innervates. A skeletal muscle can contract with varying degrees of strength. However, a single motor unit with in that skeletal muscle is an all or none phenomenon. So a muscle cell will either contract or not. So, all muscle cells of a given motor unit contract in unison if excited. An action potential will be sufficient to excite muscle cells in the motor unit to contract. Spatial summations will increase force production by recruiting more motor units. Temporal summation allows for tetanus, which is a sustained muscle contraction versus individual twitches. So, force adjustment in skeletal muscle consists of the length-tension relationship, spatial summation (recruitment of more motor units) and temporal summation.
Cardiac cells are physiologically different than skeletal muscle cells. They cannot undergo spatial summation because every cardiac cell is interconnected via gap junctions in the intercalated discs. Since they are interconnected, you cannot selectively recruit individual cardiac muscle fibers without activating the other fibers. The electrophysiology doesn't allow for temporal summation as well. So the force of contraction will depend on the contractility,which is the ability to develop contractile force at a given muscle fiber length, and the length-tension relationship. I'm not going to going in to cardiac electrophysiology, you can read more about that. However, you should note that there are differences and similarities between smooth, cardiac and skeletal muscle.
Smooth muscle has gap junctions, similar to that seen in cardiac muscle. They are not organized in motor units like skeletal muscle is. Usually, only a few cells are innervated, and the rest are activated or inactivated via gap junctions. Both cardiac and smooth muscle exhibit pacemaker activity. I wont go into the electrophysiology because that would be beyond the scope of what you would need to know for the MCAT. Grading will involve the frequency of neuron action potentials that reach threshold for contraction. In smooth muscle, for example, more neural signals that reach threshold for contraction will generate a stronger contraction. Generally speaking, the more action potentials the greater the contraction or tone. In terms of cardiac muscle, increasing sympathetic tone and withdrawing parasympathetic tone will increase contractility of the cardiac muscle, and more force generated.
It sounds like you are confusing some of the concepts that apply to skeletal muscle with the physiology of cardiac and smooth muscle. These three types of muscles, despite sharing some similarities, do function differently from one another. I hope this gave you a little better insight.
