Muscles Are Organized Into Motor Units

• When a single nerve enters a muscle it splits and makes neuromuscular junctions (NMJs) with several muscle cells
• A nerve and the muscle cells it makes NMJs with is called a motor unit
• When the nerve fires the whole motor unit is stimulated and the muscle cells contract together
• Muscles with large motor units have coarse movements
• Muscles with small motor units give fine, graded movements
  
  
• This is a small motor unit with only 3 muscle fibers

Two Basic Types of Contraction Are Isotonic and Isometric

• In an isotonic contraction the muscle shortens, keeping a constant tension
• In an isometric contraction the muscle does not shorten and tension builds up
• Most real contractions are mixtures of the 2 types

A Single Nerve Impulse Produces a Muscle Twitch

• Single stimuli usually release enough acetylcholine in the NMJs of the motor unit to produce action potentials in the muscle membranes
• This will cause the muscle to contract after a short delay
• Order of events: ACh release -> muscle action potential -> Ca release -> contraction
• A simple twitch gives only 20-30% of the maximum tension possible- the muscle starts to relax before the maximum is reached
• In the figure below a muscle is stimulated at 0.5 seconds and again at 2.5 seconds; there is complete relaxation between the stimuli and the tension reaches only 25% of maximum
  
• These graphs are Madonna computer simulations of muscle contraction. It is assumed that tension is proportional to the amount of Ca bound to troponin

Muscle Contractions Can Summate to Produce More Force

• If a second stimulus is given before a muscle relaxes the muscle will shorten further, building up more tension than a simple twitch- this is called summation
  
• In the graph above the muscle is stimulated at 0.5 seconds and again at 0.7 seconds. The muscle does not completely relax between stimuli and the tension summates to 35% of maximum
• If many stimuli are given very close together the muscle will go into a smooth continuous contraction called tetanus
  
• In this computer experiment the muscle was given 20 stimuli 0.1 seconds apart (lower trace). The contractions fuse to produce a tetanus that rises to over 90% of maximum
• Tetanus gives the maximum tension, about 4X higher than a simple twitch (isometric contraction)

Another Way to Increase the Force of Contraction is to Recruit More Motor Units

• Each muscle is made up of tens of thousands of motor units
• Force generated by a muscle can be increased by firing more and more motor units

Different Types of Skeletal Muscle Fibers Specialize for Endurance or Speed

• Muscle cells (fibers) specialize for their type of activity
  • Athletes have fiber types that match their activities
• Endurance fibers (type I)
  • Have many mitochondria- the mitochondria give these fibers a red appearance because one of the mitochondrial enzymes contains Fe.
  • Also contain a red pigment called myoglobin which stores O2.
  • Contract slowly but resist fatigue
• Fast twitch fibers (type II)
  • Fibers specialized for fast contractions are white- they contain few mitochondria
  • Relying on glycolysis to supply energy (glycolysis is faster than respiration).
  • Contract rapidly but fatigue quickly
• Fiber type is mostly genetically determined, but some experiments have shown conversion of one fiber type into another

Muscle Produces the Greatest Isometric Tension at Intermediate Lengths

• If you measure the isometric tension of a muscle when it is fixed at different lengths you will find that there is an optimum length for producing tension
  • At rest many of the body's muscles are close to their optimum lengths
• There is a connection between the chemical anatomy of actin and myosin and the amount of tension produced when they interact
• The chemical connection is based upon 2 principles:
  • 1) actin and myosin connect through crossbridges- the more crossbridges the more tension
    • Suppose the muscle is stretched so far that actin and myosin hardly overlap- then there will be few crossbridges and little tension
    • As the muscle is shortened from this extreme length more and more overlap will occur and the tension will rise
  • 2) when the muscle proteins interfere with crossbridges it will weaken the tension
    • If the muscle is shortened too much the actin filaments will bump into each other and bend- this distorts the sarcomere and weaken the contraction

• Sarcomere at point C:

This figure corresponds to point C on the graph. The muscle is stretched to a point where there is very little overlap between actin and myosin. The isometric tension will be low.

• Sarcomere at point B:

At point B on the graph there is considerable overlap between actin and myosin. There are many active crossbridges, so the isometric tension will be high.

• Sarcomere at point A:

At point A there is a lot of overlap between actin and myosin, but the actin filaments are pushing on each other. This distorts the filaments, weakening the crossbridges.

Muscle Refractory Periods are Related to Function

• The refractory period of the muscle membrane controls how rapidly a muscle can be fired
  • Muscle must recover before it can be fired a second time
• Examples:
  • Flight muscles of insects and hummingbirds can contract about 1000 times a second
    • To do that they must recover very rapidly so that they can fire again (very short refractory period)
  • The heart needs to slow down the firing rate so that it has time to fill
    • Hearts usually have quite long refractory periods that limit the maximum heart beat

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