Biology 2401 Anatomy and Physiology I
Exam 3 Notes- Muscular System Ch. 8
Functions of the muscular system: movement of body or body parts
and materials within the body
maintain posture and body position
support soft tissue
close openings, regulating filling and emptying
maintain body temperature by producing heat
Muscles composed of muscle tissue (skeletal, cardiac or smooth)
connective tissue (dense, loose, blood - well vascularized)
nervous tissue (well innervated)
Each muscle has layers of dense (fibrous) connective tissue. Together form “harness”.
epimysium - surrounds muscle and is continuous with tendon to bone
Fig. 8-1
perimysium - surrounds a bundle (fascicle) of muscle cells; continuous with
epimysium
endomysium - surrounds a muscle cell; continuous with perimysium.
facia - broad sheet of fibrous (dense) connective tissue around muscles;
continuous with tendon and aponeuroses
*List the connective tissue layers associated with muscles and describe their function.
Muscle cells : develop from embryonic cells called myoblasts
“myos” = muscle
become very specialized, lose the ability to divide
“blast” = produce
exaggerate the cytoskeleton, myofilaments
“sarco” = flesh
have unique traits: excitability - can be stimulated
contractility - can forcefully shorten
extensibility - can stretch and contract over wide range
elasticity - can stretch and rebound to original shape
conductivity - can generate and transmit electrical
impulse (nerve cells also do this)
Skeletal muscle cell anatomy:
Fig. 8.2-8.5
sarcolemma - cell membrane that encloses cell
sarcoplasm - cytoplasm of cell, fluid portion of cell
transverse tubule (T tubules) - tunnel-like extension of sarcolemma deep into cell
sarcoplasmic reticulum - specialized endoplasmic reticulum, an internal
membrane; forms expanded chambers and vesicles that store calcium (Ca++)
myofilaments - protein fibers that are contractile (actin, myosin for example)
myofibrils - long units composed of myofilaments; contracting units within cell
mitochondria - organelles that produce ATP energy for cell, aerobic respiration
*Make a drawing of a typical muscle cell and label each of the important structures listed
above. Describe the function of each.
*What is the importance of the T tubules?
*What ion is stored in the sarcoplasmic reticulum? Why are they close to T tubules?
*What is produced in the mitochondria? Why do muscle cells need many of these?
Contractile proteins in skeletal muscle cells:
Fig 8.2 through 8.8
Myofibrils are long cylindrical units that extend the length of the muscle cell, attached at each
end. They are the contracting units within the cell - when they shorten the cell shortens.
Each myofibril is a linear series of sarcomeres (about 10,000), each connected by a rigid Z line.
Each sarcomere can contract as a unit.
Each sarcomere is made of a series of myofilaments, primarily actin and myosin.
Actin filaments, also called thin filaments, are made of protein molecules called actin.
Thin filaments attach to the Z lines and extend toward the center of the sarcomere.
Each thin filament is wrapped by two proteins, troponin and tropomyosin. This wrapping covers
an active myosin bonding site on each actin molecule, preventing muscle contraction.
Myosin filaments, also called thick filaments, are made of protein molecules called myosin.
Each myosin molecule has a long tail and a flexible head. The thick filament is made of interwoven
myosin tails, with the heads exposed - similar to a frayed rope. The myosin heads, also called
cross-bridges, can attach to actin at the active myosin bonding site when the tropomyosin is not in
the way.
Thick filaments are suspended in the center of the sarcomere - they do not reach the Z lines, but
they do overlap the thin filaments.
The arrangement of myofilaments give the myofibril a banded, or striated, appearance.
*Make a drawing of a sarcomere, including all of the parts listed above. Label each part. (5
points)
(or tear Fig 8.3 from your text and glue it here)
The sliding filament model of muscle contraction: see Fig 8.6 - 8.8
Calcium is released into the cytoplasm from the sarcoplasmic reticulum and attaches to troponin.
This causes troponin to change shape, moving tropomyosin away from the active sites of actin.
The myosin heads then attach to the active sites of actin. The myosin heads are normally in the
extended position, but when they attach to actin they change to the flexed position. This pulls the
actin filaments, and Z lines, toward the center of the sarcomere, shortening the sarcomere.
A molecule of ATP attaches to a myosin head, providing the energy to release actin and return to
the extended position. The attachment and flexing process is repeated, some myosin heads
attaching while others are releasing, until calcium is removed or the cell runs out of ATP (fatigue).
The contraction then stops.
*Describe the sliding filament model of muscle contraction.
*Where does the calcium come from (where is it being stored)?
*Where does the ATP come from?
*What starts the sarcomere contraction and what ends it?
*How does the length of the sarcomere change during this process? Make a drawing.
Steps in skeletal muscle contraction
see table 8-1 and Fig. 8.7
Nerve stimulation of muscle begins at neuromuscular junction where nerve cell (motor
neuron) releases a chemical that starts action potential on muscle cell at motor end
plate.
Action potential (electrical impulse) travels from this point along the sarcolemma,
including down into T tubules.
When action potential happens close to vesicles of sarcoplasmic reticulum calcium is
released into sarcoplasm around sarcomere.
Calcium binds with troponin, causing the tropomyosin to be moved away from the active site on
the actin molecules.
Myosin heads (cross bridges) bind to the actin active sites, then change shape to the flexed
position. This pulls the actin, and Z lines, toward the center of the sarcomere, shortening
the sarcomere. Shortening all of the sarcomeres shortens the myofibril and, therefore,
the muscle cell.
An ATP molecule, which provides energy, attaches to a myosin head, causing it to release
the actin and return to the extended position.
Myosin binding to actin, pulling, then releasing, binding to actin, pulling, then releasing
continues as long as calcium and ATP are present. When the nerve stimulation ends the
action potential ends and the calcium is pumped back into the vesicles. The
contraction ends.
Motor units
Fig. 8.13
A motor unit is one motor neuron and all of the muscle cells controlled by that neuron. A motor
unit works as an all-or-nothing unit. It is either on or off.
Motor units may be small, 2-3 muscle cells per motor neuron, in precision muscle such as
eye muscles, or they may be very large, 2000 muscle cells per motor neuron, in power
muscles such as the large leg muscles.
Muscles work in graded manner - more or less tension as needed- determined by the
number and frequency of motor units stimulated.
Muscle tone is maintained by having a few motor units stimulated at all times.
*Why is it beneficial for a precision muscle to have more, smaller motor units?
*Why is it beneficial for a power muscle to have fewer larger motor units?
*How is a muscle stimulated to work with just the power needed (graded)?
Muscle cell energy metabolism
Fig. 8.9 – 8.10
The energy source for muscle contractions is ATP (adenosine triphosphate). Energy is released by
the reaction ATP -----> ADP + P + energy. This reaction is reversible.
ATP is available from several sources as the muscle begins to work:
1) ATP is available in the cell. This supply is very limited and is used up quickly (a few
seconds)
2) Creatine phosphate (CP) is formed and stored in muscle cells. It is not used until ATP is used
up then quickly gives energy to rebuild ADP + P -----> ATP
CP and available ATP only last about 30 seconds. New ATP must then be made in
anaerobic and aerobic cellular respiration.
Glucose is the fuel molecule used to provide energy to rebuild ADP + P ----> ATP and
creatine phosphate.
Glucose is available from the blood (blood sugar) and in muscle cells it is stored as
glycogen (animal starch).
Glycolysis is the breakdown of glucose to 2 pyruvate molecules. Oxygen is not required in this
process in which 2 ATP molecules are produced.
The pyruvate molecules then go to the mitochondria where they are broken down to carbon
dioxide. Oxygen is required and 34 ATP molecules are produced per each 2 pyruvate
molecules. If oxygen is not available the pyruvate must be converted to lactic acid
and removed from the cell.
3) Aerobic metabolism is glycolysis followed by pyruvate breakdown in the mitochondria. This is
the most efficient method and produces 95 % of the ATP in resting muscle cells. However,
this only works if oxygen is available. Muscle cells have a method of storing oxygen with
a molecule called myoglobin, so when the muscle cells are working hard and the blood
supply of oxygen is limited, some oxygen is still available.
4) With higher levels of exertion the muscle cells do not receive enough oxygen and must
use anaerobic metabolism. Glucose is broken down to pyruvate which is then
converted to lactic acid, producing only 2 ATP molecules per glucose. Anaerobic
metabolism can sustain maximum exertion for about 30 seconds.
Muscle fatigue begins when the muscle can no longer contract because it lacks sufficient ATP or
lactic acid has accumulated. The recovery period (repaying the oxygen debt) requires oxygen to
convert lactic acid back to pyruvate and to glycose and glycogen and to regenerate the supply of
ATP and creatine phosphate.
Heat is also a product of metabolism. About 60 % of the energy released is heat energy and 40 %
is trapped in ATP. Heat is used to maintain body temperature, but excess heat must be removed
during muscle exertion.
*Describe the sources of energy to make muscles work, in order that they are used.
*Describe 3 features that muscle cells have, that other cells lake, to increase energy.
Muscular Responses
Fig. 8.11 – 8.12
A muscle twitch is a single stimulation - the shortest event that can occur. 10 to 100 msec.
This is the neural stimulation of the muscle to release calcium, causing myofilaments to slide,
causing the sarcomere to shorten, and calcium to be pumped back into vesicles of sarcoplasmic
reticulum.
The muscle fiber (cell) contracts all-or-nothing. It is either on or off. The tension generated is at
a peak when the most calcium is available and the most myosin cross-bridges are binding.
If the twitches are repeated before the preceding twitch completely relaxes, a process called
summation, the tension, or force, produced increases with each twitch until a maximum is reached.
First treppe (the stair step increase of each contraction) then tetanus.
If the rate of stimulation increases the fibers cannot relax. This is complete tetanus.
*Make a drawing of a myogram showing a single muscle twitch and summation (treppe and
tetanus).
*In view of the myogram, why do muscles work better if they are “warmed up”.
*What is the value of a skeletal muscle going into complete tetanus?
Muscle contraction can be classified as either isometric or isotonic.
In isometric (“same length”) contraction the force changes but the overall length stays
about the same.
In isotonic (“same force”) contractions the force stays about the same but the overall
length changes.
*Is body posture and balance maintained by isometric or isotonic contractions?
*Is raising your hand caused by isometric or isotonic contractions?
Muscles can contract and shorten with great force, but cannot elongate on their own.
They must elongate passively due to an outside force.
The outside forces are:
1) elastic recoil of the muscles, tendons, etc,
2) opposing muscles,
3) gravity and
4)fluid pressure.
*Why must skeletal muscles be stretched by an outside (of the muscle cell) forces?
*Describe four examples of the forces that stretch muscles.
Muscle fibers contract with the most force when they are at their normal resting length ( + 30% to
- 30%). This is due to amount of overlapping of actin and myosin filaments.
*Explain why muscles contract most forcefully when they are at about their normal resting length.
*Make a drawing of a sarcomere showing the myofilament positions at normal +30%.
Muscle performance can be measured in two ways, power (maximum tension produced) and
endurance (the maximum time the muscle can exert tension).
Conditioning can increase the power and endurance of muscle contractions:
Anaerobic endurance involves short intense activities supported by anaerobic metabolism.
The muscles respond by enlarging (hypertrophy) to increase power.
Aerobic endurance involves sustained low levels of activities supported by aerobic metabolism.
The number of mitochondria and myoglobin increases in muscle cells and the
cardiovascular system (the oxygen delivery system) responds by increasing blood vessel
growth, heart output capacity, etc.
Lack of conditioning causes the muscles to decrease in size (atrophy).
Muscle fibers differ metabolically, and can be classified as fast fibers and slow fibers.
Fast fibers: - respond quickly with large numbers of myofibrils
- have much glycogen and few mitochondria and little myoglobin
- use anaerobic metabolism
They are, therefore, fast and strong but fatigue quickly.
Slow fibers: - respond slowly with fewer myofibrils
- have many mitochondria and myoglobin
-have a much more extensive blood supply (many capillaries)
They are, therefore, slower and weaker but fatigue very slowly.
Comparison of types of muscle tissue
Table 8.3, 5.5, Fig. 5.21-5.22
Skeletal (striated or voluntary) muscle: found in muscles that move skeleton
myofibrils are arranged into sarcomeres - pull in line, point-to-point, strong
voluntary (stimulated by somatic nervous system)
have transverse tubules and calcium stored inside cell
lack gap junctions so each cell is stimulated only by a nerve
have fast contraction and a short (5 msec) refractory period - experience tetanus
Smooth (involuntary, visceral) muscle: found in internal organs
myofibrils are not arranged into sarcomeres, at many angles in cell - weaker
no transverse tubules, calcium mostly outside cell
slow contraction, much weaker but require very little energy (1/50 ATP as other)
multiunit smooth - walls of blood vessels - stimulated by nerves and hormones
visceral smooth - internal organs - stimulated by nerves, hormones, stretch, local
factors and autorhythmicity (can contract as wave)
Cardiac muscle: found in heart
myofibrils are arranged into sarcomeres - strong
cells branch and form 3 dimensional network
involuntary (stimulated by autonomic nervous system), have autorhythmicity
have transverse tubules and calcium stored inside cells and outside cells
have gap junctions so cells can stimulate neighboring cells
have moderately fast contraction and long (300 msec) refractory period - do not
experience tetanus
use only aerobic metabolism
Kinesiology
Skeletal system and muscular systems work as levers and motors
Levers systems have four basic parts:
lever - rigid bar (bones)
fulcrum - pivot point around which the lever rotates (joints)
effort - force applied to move lever (muscle)
resistance - force of load to be moved (large coffee mug)
Fig 7. 7
torque is rotating force applied to lever - effort force opposes resistance force
torque is equal to force applied * distance from fulcrum
Example: torque of E = 1Ft * 100Lb = 100Ft Lb
FE * DE
=
FR * DR
or
torque of R = 5Ft * 20Lb = 100FtLb
torque E = torque R
mechanical advantage (leverage) - less E force required to move resistance
range of motion - more E force required but resistance moved through greater range
Both important in body.
First class levers - fulcrum between E and R
Second class levers - R between F and E
Third class levers - E between R and F
*Give an example (a joint, bones and muscles) of each of the three type levers in the body.
*Calculate the force required of the biceps brachii muscle to pick up a 50 pound object. The
biceps muscle attaches to the radius 2 inches from the elbow and the hand (where the object is
located) is 20 inches from the elbow.
What class lever is this?
*Third class levers never produce mechanical advantage, yet they are very common in the
body. Explain why.
*Write a 5,000 word essay explaining the importance of kinesiology to life on Earth.
Muscles of the muscular system
Be able to identify the following muscles. For each muscle know its origin, insertion and action
(see tables 8.4 through 8.15)
The origin is the end that does not move, in relation to the body, when the muscle contracts.
The insertion is the end that moves, in relation to the body, when the muscle contracts.
masseter
temporalis
trapezius
sternocleidomastoid
deltoid
biceps brachii
triceps brachii
pectoralis major
latissimus dorsi
rectus abdominis
external oblique
gluteus maximus
semitendinosus
biceps femoris
quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis)
gastrocnemius
tibialis anterior