Download Muscles Part 2 Powerpoint

Muscles Part 2
By the end of this class you should
understand:
• The behavior of muscle fibers and motor neurons
as a motor unit
• The comparative behaviors and roles of slow- and
fast-twitch muscles
• The types of exercise and the changes they
produce in the body
• The functions of cardiac and smooth muscle in
the body
• The structures that permit involuntary muscles to
contract regularly and continuously
Muscle Activation
• Recall that every muscle
fiber is stimulated by a
single motor neuron
• Together they form a
motor unit
– One neuron signals
multiple muscle fibers
distributed throughout the
whole muscle
– Every muscle fiber in a
muscle has one neuron it
responds to
Motor Unit
• Note that the fibers are spread
out so that the force is evenly
distributed through the whole
muscle
– If they were all on one side it
could cause the muscle to
contract unevenly
• The motor unit follows the allor-none principle
– When the neuron fires, all fibers
completely contract
All or None?
• This may seem a bit odd since we can clearly choose
to exert more or less force
• The trick is that each muscle is made of many motor
units
• Increasing the number of motor neurons firing
results in increased recruitment of muscle fibers
ion, Inc.
Increased Recruitment
Stimulus strength
Maximal
stimulus
Threshold
stimulus
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
• The more motor neurons are
firing, the more force the
motor units exert
• Each motor unit firing is called
a twitch
• As more twitches occur more
rapidly, their forces add
together until the muscle is at
maximum tension (tetanus)
– The disease tetanus is so named
because it is a bacterial toxin
that prevents relaxation of
motor neurons
Figure 9.16
Muscle Fiber Types
• Muscle fibers come in
two types:
– Fast-twitch fibers
– Slow-twitch fibers
• These are also described
by their appearance
– Fast-twitch = white
muscle
– Slow-twitch = red muscle
Fast-twitch Muscle
• Fast-twitch muscle fibers are
primarily anaerobic (use
glycolysis and creatine
phosphate)
• Produces lactic acid and
creatinine as waste products
• The cytoplasm contains many
glycosomes (vesicles that
contain glycogen)
• Called white muscle because
they lack myoglobin and
mitochondria and so are pale
in color
Slow-twitch Muscle
• Slow-twitch muscle contains
mitochondria (for aerobic
ATP production) and
myoglobin (hemoglobin-like
molecule that stores extra
oxygen)
– Myoglobin increases oxygen
concentration in muscles
• Can produce force constantly
without becoming fatigued
– Do not produce as much force
as white muscle
Exercise
• Exercise may be sustained
(aerobic) or unsustainable
(anaerobic)
– Aerobic exercise is also called
endurance training
– Anaerobic exercise is also
called strength training
• Exercise can alter the
composition of red and
white fibers
– Also influenced by genetics
Muscle Mass
• White muscle fibers are larger
than red muscle fibers, and so
strength training results in
more “bulk” than endurance
training
• Some hormones such as
testosterone increase the
number of muscle fibers
– “Steroids” are usually
testosterone analogs
Exercise
• Different people have different
natural levels of red and white
muscle
– Many athletes are advised to train both
unless they are in a very specialized
sport (marathon running or
weightlifting)
– Athletes are also recommended to use
stretching exercises to increase
circulation to ligaments and tendons,
preventing injury and increasing
flexibility
Involuntary Muscles
• Skeletal muscles are
voluntary muscles (part of
a motor unit with a neuron
under the control of the
conscious mind)
• Cardiac and smooth
muscle operate without
conscious thought
(involuntary)
Involuntary Muscle Contraction
• Both cardiac and smooth
muscle have one feature in
common that is absent from
skeletal muscle: gap junctions
• Gap junctions allow ions to
pass from one cell to another
– This means when one muscle
cell contracts it stimulates its
neighbors to contract as well
– Skeletal muscle does NOT have
gap junctions
Pacemaker Cells
• In cardiac and smooth muscle,
some cells are autorhythmic,
which means they contract
without any external stimulation
• Since they are also attached to
other cells via gap junctions,
they set the pace
– Known as pacemaker cells
– No external stimulation required
Involuntary Muscle Activity
• Note that when skeletal muscles
cramp (cannot relax) or spasm
(involuntary twitch) that is NOT
induced by pacemaker cells or
gap junctions
– Can be caused by ion imbalance,
energy depletion of the neurons
or muscles, or unusual brain
activity
• Only cardiac and smooth
muscles are truly
involuntary
Cardiac Muscle
• Cardiac muscle cells have
the same sarcomeres and
striped pattern as skeletal
muscles
• Cardiac muscle cells are
smaller and shorter and
some are branched
• They attach to each other
with intercalated discs
that include gap junctions
Smooth Muscle
• Smooth muscle contracts
differently than skeletal or
cardiac muscle
– Not striated
• Fibers running all through
the cell are attached to
dense bodies
• When the fibers contract the
cell shortens, producing
force in all directions rather
than just in one direction
Muscle Injuries
• Muscle fibers may be
depleted of energy and
unable to relax (cramp)
– Common cramp location is the
arch of the foot
• Muscles may tear from
excessive force placed upon
the sarcomeres
– Small tears produce soreness
– Large tears may require
surgery
Rhabdomyolysis
• Excessive overuse of a wellnourished muscle may cause the
muscle to literally explode
– This is very rare since usually a
muscle will become fatigued instead
– Typically only occurs in military
training and CrossFit activity
• The key sign is myoglobin in the
blood and urine (why?)
– Typically leads to kidney failure and
muscle scarring
Have a good weekend!
• It’s fun to end on a cheerful topic!