BIOL 1051H Chapter Notes - Chapter 11: Soleus Muscle, Myocyte, Motor Neuron
Muscle Tissue Part 2
Muscles return to their resting length after contraction for two reasons. One is that
muscles generally have opposing muscle groups, so contraction of antagonist muscles would
cause the muscles in question to lengthen. The other, possibly more important reason is that
the connective tissue of muscles and tendons, as well as titin filaments in sarcomeres, have
elastic recoil, which causes the muscle to spring back to resting length after contraction.
As it turns out, muscles are strongest, or capable of exerting the most force when they
are at an optimum length. Therefore, the strength of muscle contraction can be influenced by
the frequency of stimulation, the thickness (or amount of actin and myosin) in each muscle fiber,
and the initial length of the muscle fiber. How the length is important can be understood by
taking another look at the sarcomere. When the sarcomere is short it really can’t get much
shorter, so it does not have much force generating power left. When a muscle is stretched, the
interaction between myosin heads and actin is very limited. I think of it as trying to hold on to
something with my fingers instead of my whole hand. When the muscle is at a normal resting
length it is much stronger. There is room for shortening, and the actin and myosin can engage
over a larger area.
Type 1 fibers are slow-twitch fibers meaning that they contract slowly. However, they are
very slow to fatigue so they can keep muscle tone for long periods of time. These fibers are
common in muscles involved in posture, or other muscles that must stay contracted for long
periods of time. These fibers are also called red slow oxidative fibers because they rely heavily
on aerobic respiration. The rich capillary supply, many mitochondria, and high levels of
myoglobin (an oxygen storage molecule like hemoglobin) make this muscle fiber type appear
red in color.
These fibers are generally innervated by small neurons. These neurons transmit
information more slowly than large neurons. Also, the motor units in muscles that are primarily
type 1 fibers tend to be on the small side. The motor units within the muscle can take turns
resting so they can go much longer as a whole than muscles that have fewer motor units. An
example is the soleus muscle which is an important postural muscle, shown as c in the graph
below.
Type 2 fibers are fast-twitch fibers that also fatigue quickly. Type IIX fibers are called
white fast glycolytic fibers. These fibers have very fast twitch but rely on anaerobic metabolism.
They have few capillaries and mitochondria and little myoglobin so the fibers appear white.
Because they are anaerobic they cannot hold a contraction for long without fatigue so they are
usually found in muscles that need to respond quickly but not for long periods. In the graph
below a = fast twitch muscle fiber found in extra ocular muscles (these are very fast twitch and
need to be if you will be able to quickly move your gaze from one place to another.
Document Summary
Muscles return to their resting length after contraction for two reasons. One is that muscles generally have opposing muscle groups, so contraction of antagonist muscles would cause the muscles in question to lengthen. The other, possibly more important reason is that the connective tissue of muscles and tendons, as well as titin filaments in sarcomeres, have elastic recoil, which causes the muscle to spring back to resting length after contraction. As it turns out, muscles are strongest, or capable of exerting the most force when they are at an optimum length. Therefore, the strength of muscle contraction can be influenced by the frequency of stimulation, the thickness (or amount of actin and myosin) in each muscle fiber, and the initial length of the muscle fiber. How the length is important can be understood by taking another look at the sarcomere.
