The Muscular System
Chapter 8
All movement occurs because
muscles use energy to contract.
There are three types of muscle tissue,
cardiac, smooth, and skeletal. This chapter
will concentrate on skeletal.
Each skeletal muscle is a
complicated organ composed of
skeletal muscle tissue, nerves,
connective tissue, and blood.
Sometimes connective tissue
holds muscle into broad, fibrous
sheets called aponeuroses.
Each myofibril is surrounded by
a system of channels called the
sarcoplasmic reticulum.
Transverse tubules (the T
system) extends inward into the
fiber connecting the channels
with enlarged cisternae where
the actin and myosin meet.
Each muscle fiber is connected to a
motor neuron from the spinal cord at a
neuromuscular junction. At this motor
end plate, vesicles at the nerve ends
store neurotransmitters that cause the
muscle to contract when they are
secreted.
Mechanics of the Muscular Contraction
The actin and myosin filaments are made up of globular parts that are twisted
together. The actin fibers contain troponin and tropomysin which are important
in the contraction.
When acetylcholine is released at the motor end plate, Ca ions are released
into the T-system from the sarcolemma.
In the presence of Ca, troponin and tropomysin open binding sites that allow
the myosin to form cross bridges to the actin.
Globular parts of the myosin release ATPase which causes the breakdown of
ATP which gives the crossbridges the energy to curl and shorten the
sarcomere.
The muscle contracts
When production of acetylcholine stops, the Ca is reabsorbed by the
sarcolemma, the cross bridges disconnect, acetylcholinesterase decomposes
the acetylcholine at the end plate and the muscle relaxes.
• Fig. 8.07
Energy for the Muscle Contraction
ATP must be supplied to cause the changes in the cross bridges.
ATP→ADP + P + ENERGY
The ATP supply is quickly used up and the energy to rebuild the ATP is supplied
by creatine phosphate which muscles store.
CP →C + P + ENERGY
As the CP is used up, cellular respiration oxidizes glucose to supply energy.
As oxygen runs low in the muscle, anaerobic respiration changes glucose to
pyruvic acid for energy. The pyruvic acid is changed to lactic acid.
The lactic acid is converted by to glucose in the liver at the expense of energy.
An oxygen debt occurs and muscular fatigue and possibly cramps follow.
About half of the energy produced in these processes is lost as heat.
Muscle fibers donot contract
until a certain threshold stimulus
is reached at the end plate. At
this time, the fiber will contract
to it’s full potential. This is
called an “All-or-None”
response.
Muscle contractions may be studied on a
myogram as a muscle twitch.
Study of the twitch has led to
the discovery of two kinds of
muscle fibers: Slow twitch or
fast twitch.
Sometimes a muscle may be
restimulated before it has
completed relaxation. This
brings about a sustained
contraction called a tetanic
contraction.
A high intensity stimulation may
cause a muscle fiber to
stimulate fibers around it to
contract. This is called
recruitment.
Summation and recruitment
together brings about a
sustained contraction of
increasing strength.
The body maintains a certain
amount of sustained contraction
at all times. This is called
muscle tone and is very
important in maintaining
posture.
The long, tapered filaments of
smooth muscle are not as well
organized as those of skeletal
muscle thus there are no
striations.
Smooth muscle may be of two types:

Multiunit -- The fibers are separate as in the iris
of the eyes.

Visceral -- The fibers are organized into sheets.
They may stimulate each other with great
rhythmicity to generate peristalsis.
Smooth muscle contracts
slower than striated muscle but
can maintain the contraction
longer with a given amount of
ATP.
Cardiac muscle is arranged into
striated networks with
intercalated disks which help to
transfer stimuli from cell to cell.
This makes the contractions
rhythmic and self-exciting.
A number of terms are used
to describe the action of
muscles:







origin - the attachment of the muscle that doesn't
move
insertion - the attachment of the muscle that moves
in the action
flexion - bends a joint
extension - straightens a joint
prime mover or agonist - muscle that causes an
action
synergist - muscle that aids the prime mover
antagonist - muscle that resists the prime mover
and causes an opposite action.
Clinical Terms Related to the Muscular System:
convulsion - series of involuntary contractions of a
group of muscles
fibrillation - spontaneous, rapid, uncontrolled
activity of a muscle
fibrositis - inflammation of the muscle connective
tissue
muscular dystrophy - group of inherited disorders
in which the lack of certain glycoproteins causes
the collapse of muscle cells.
myalgia - any muscle pain
myasthenia gravis - chronic disease of the
neuromuscular junction where the muscles are
weak and easily fatigued
myoma - muscle tumor
myositis - muscle inflammation
paralysis - loss of the ability to move a body part
shin splints - inflammation of the anterior muscles
of the lower leg