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Muscle Ii Notes

Pharmacology Notes > BIOL10832 Excitable Cells Notes

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Excitable Cells - Lecture 16 (22/03/2018)

Muscle II
Action Potential vs Contraction

The duration of a neuronal action potential is typically just a few ms. The same holds true of the muscle action potential that results at the neuromuscular junction.

The force developed by a muscle fibre after a single stimulus,
termed a 'twitch', however, lasts much longer - on the order of 10 to 100 ms.

This is a very important feature of skeletal muscle. As it allows the force to be maintained without having to continuously expend energy to fire the action potentials at a high rate (as much of the body's ATP is used by the sodium pump).

This occurs as muscle contraction is dependent on the concentration of cytoplasmic calcium rather than being directly dependent on the electrical activity of the muscle membrane and here the process of calcium release and uptake is simply slower than the processes involved in an action potential.

Twitch and Tetanus

A single action potential causes a twitch.
Summation and unfused tetanus occurs when there are more than one action potentials observed over a short period of time.
Higher rates of action potential results in fused tetanus.
As the duration of the action potential stimulus for contraction, is much shorter than the duration of the force generated by the muscle, this means that another action potential can be fired well before the first twitch has finished.
What happens under these circumstances is that if a second stimulus arrives before the first muscle twitch has fully relaxed, a second twitch will occur.
However, if this second twitch is larger than the first then 'summation' is observed.
The key here lies in remembering that muscle contraction is due to calcium release and reuptake.
Re-uptake actually starts simultaneously with calcium release, so the effect of repeated stimulation is that calcium concentrations can reach a higher level than with a single twitch.
The force generated by the muscle is proportional to the calcium concentration so repeated stimulations produce a bigger force of contraction. 

A second reason for the increased force is that in a single twitch is that some of the force is absorbed by elastic proteins in the muscle.

As the stimulation frequency is increased still further, the twitches begin to merge with each other and eventually merge together into a state called 'tetanus' (or 'tetany').

This is not to be confused with the muscle disease 'tetanus' (caused by a bacterial toxin).
The two terms have similar etymologies in that they come from the Greek tetanos, meaning taut. However, in physiology, tetanus simply means a state in which a muscle is maximally stimulated.

Henneman's Size Principle

We've seen how the control of force development operates at the level of the single muscle fibre (frequency of stimulation).
However, the smallest contractile unit of a muscle is not a muscle fibre, but rather a motor unit.
A motor unit is the set of muscle fibres that are controlled by a single motor neurone.
The number of fibres in a motor unit can vary considerably and are related to the size of the neuron: the bigger the motor neuron (cell body size), the more muscle fibres it innervates.
So, the Henneman's size principle states that as a muscle is stimulated to contract by a neurone (W) in the CNS, motor units become recruited in order of size, where the smaller units are recruited first
(because for a given stimulus by neuron W, motor neuron X will be depolarised more because it is smaller), followed by the intermediate ones (Y) and then the largest motor units (Z).

This means that the force generated by a muscle can be finely controlled by the CNS.

Diversity of Skeletal Muscle
These include:

 Fast-twitch glycolytic.
 Fast-twitch oxidative.
 Slow-twitch oxidative.

Slow fibres - Slow fibres are used for posture maintenance, etc. They have myoglobin
(red) as an oxygen store and many mitochondria. 

Fast fibres - Fast fibres allows rapid shortening of the muscle but at a higher energy cost as the ATP is hydrolysed quickly. This contains a fast myosin isoform with a high-rate SR
Ca pump.

Glycolytic fibres: Glycolytic fibres cause lactate accumulation & acidosis, which can limit the contraction.

Skeletal muscle fulfills diverse roles around the body. The muscles of the legs, for example,
need to contract for long periods of time in order to maintain posture.

On the other hand, the extraocular muscles that move the eye need to contract much less frequently, but need to do so very rapidly in order to maintain an object in the field of view.

These varied roles place different energy demands on the muscles and it is perhaps not surprising that evolution has endowed mammals with different subtypes of skeletal muscle to take on specialised roles.

 There are two main classes of skeletal muscle fibres: slow twitch and fast twitch.
 The fast twitch type is further subdivided into oxidative and glycolytic types.

Slow Fibres

Slow fibres (also known as Type I) contain large amounts of myoglobin, that act as an oxygen store and mitochondria, and are richly supplied with blood vessels. This allows these fibres to make use of oxidative breakdown of glucose and they are used for slow,
sustained contractions and are resistant to fatigue.

Fast Fibres

Fast fibres are subdivided into Type IIa and Type IIb depending on their type of metabolism.

Type IIa fibres are fast twitch fibers capable of rapid contraction due to the type of myosin they contain and a high concentration of SR Ca pumps, but rely on oxidative metabolism.
Type IIa have lots of mitochondria and abundant stores of glycogen, which allows them to resist fatigue, albeit at a high energy cost.

Type IIb fibres on the other hand, fatigue easily. They are fast twitch type, but differ from
Type IIa in that they rely on glycolytic metabolism. They do not have as many mitochondria as other fibre types, and have lower levels of glycogen and mitochondria as well as a poorer blood supply.

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