The Muscular System
rev 12-12
Muscle cells are involved in every movement that our bodies
perform.
– They are found in every body organ and tissue
Muscles can:
• Shorten or contract to produce movement (prime mover
or agonist)
• Relax or be pulled back to their original length by gravity
or by opposing muscle groups, called antagonistic
muscles
• Work with other muscle groups, called synergistic
muscles, to produce movement
• Resist movement to maintain our posture
• Generate heat to maintain our body temperature
– Shiver, sweat
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• Muscle tissue is made up of tightly packed cells
called muscle fibers. The muscle fiber cytoplasm
contains proteins which allow the cell to contract
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• There are 3 types of muscles:
– Skeletal muscles
– Cardiac muscles
– Smooth muscles
• Skeletal muscles
– attach to the bones of our skeleton and provide strength
and mobility (movement) for our body
• Cardiac muscle
– found in the heart; pumps blood throughout the body
• Smooth muscles
– found in most internal organs; generally they work to
push something (fluids or other body substances)
through a body part
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• Muscles may also be classified as
– voluntary (muscles over which we have
conscious control and requires peripheral
nervous stimulation to contract)
– involuntary (muscles over which we have no
conscious control; require autonomic nervous
stimulation to contract)
• The term sarcolemma refers to the cell membrane
of a muscle cell
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Skeletal Muscles
• are multinucleated
• cells are arranged in a parallel fashion
• are responsible for all locomotion and
manipulation
• enable us to respond quickly to changes in the
external environment
• compared to other muscle types, their speed of
contraction is fast
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Skeletal Muscle
• Is also called Striated or Voluntary muscle
– they have striations (or stripes) which are
caused by alternating dark and light “bands”
– bands are composed of tightly packed
contractile proteins called myofilaments which
are made up of thicker myosin filaments and
thinner actin filaments
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Cardiac Muscle
• Cells are striated, short, fat, branched and interconnected
• Have specialized areas called intercalated disks where the
cells connect with each other
– Intercalated disks contain gap junctions that permit one cell to
electrically stimulate the next one.
• because of these connections, cardiac muscle works as a
single, coordinated unit
• usually contracts at a steady rate set by the heart’s
pacemaker, but neural controls allow for a faster beat for
brief periods (i.e. when you perform intense activities)
• compared to other muscle types, their speed of contraction
is moderate
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Smooth Muscle
• Cells are shorter than skeletal and cardiac muscle cells
• Because the cells have fewer contractile proteins
they do not have striations (thus their name,
smooth muscle)
• Cells are spindle shaped (thicker in the middle and
tapered at each end); each cell has a centrally
located nucleus
• Are found in the walls of hollow visceral organs
(i.e. stomach, intestines, bladder, blood vessels)
• role is to force fluids and other substances through
body channels
• compared to other muscle types, their speed of
contraction is slow and sustained
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Muscle Contraction
Contractile proteins or myofilaments, called actin and myosin,
slide past each other using energy from ATP molecules.
• These myofilaments produce alternating light and dark areas
called striations
– Dark areas are called A-bands
– Lighter areas are called I-bands
– The Z-line is a thin, dark line where sets of actin
myofilaments are woven together
– The space between 2 Z-lines is called a sarcomere
• A sarcomere is the smallest functional (contractile)
unit of a muscle fiber
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Mechanism of Muscle Contraction:
Nerve Activation of Individual Muscle Cells
• In order for a muscle to contract, its cells must be
stimulated by a nerve
• The motor neuron secretes acetylcholine (ACh) at
the neuromuscular junction (the space where the
motor neuron and muscle cell meet).
– ACh is a neurotransmitter--a chemical which
can either stimulate or inhibit another “excitable”
cell (either a nerve cell or a muscle cell)
• The ACh diffuses across the space between the
neuron and the muscle cell (called the synaptic
cleft) and binds to receptor sites on the muscle cell
membrane.
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• The ACh binding causes the muscle cell membrane to generate
an electrical impulse which travels along the cell membrane
and along the T-tubules (cylindrical extensions of the cell
membrane [or sarcolemma] which travel into the interior parts
of the cell and activate the sarcoplasmic reticulum);
– the function of the T-tubules is to allow the electrical
impulse to quickly travel to all cell parts
• The electrical impulse triggers the release of calcium from the
sarcoplasmic reticulum .
– Muscles require calcium in order to contract.
– Sliding Filament Mechanism: muscle contracts when the
sarcomeres shorten. This occurs when the thick and thin
filaments form cross bridges and slide past each other
resulting in shortening of the sarcomere.
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Calcium binds to troponin, a protein molecule, and causes
the
• Troponin–tropomyosin protein complex to shift position
– this exposes the actin-myosin binding sites and allow
the myosin heads and actin filaments to make contact,
forming cross-bridges.
– The actin filaments are pulled toward the center of the
sarcomere and the muscle contracts.
• In order to stop the contraction, nerve cell
stimulation stops
– calcium is no longer secreted and the troponin–
tropomyosin protein complex shifts position so the
myosin heads are no longer exposed. They can no
longer make contact with the actin filaments and
– the muscle will be unable to contract
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• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animation__actio
n_potentials_and_muscle_contraction.html
•
• http://www.brookscole.com/chemistry_d/templates/student_resources/
shared_resources/animations/muscles/muscles.html
• http://www.dnatube.com/video/1310/Action-potential
• GOOD ONE:
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animation__break
down_of_atp_and_crossbridge_movement_during_muscle_contraction.html
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animation__sarco
mere_contraction.html
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Muscle Relaxation
• Nerve activation ends, contraction ends
• Calcium pumped back into sarcoplasmic reticulum
(requires ATP)
– Calcium removed from actin filaments
– Myosin-binding site covered
– No calcium = no cross-bridges
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Energy Use by Muscle Cells
• Muscle contraction requires energy
– In the presence of calcium, myosin acts as an
enzyme to split ATPADP + inorganic phosphate
to release energy.
• So, ATP is the muscle’s energy source
• Typically muscle cells store enough ATP for
approximately 10 seconds of heavy activity
• After this, ATP can be replenished by:
– Creatine phosphate which makes enough ATP for
~ 20-30 seconds
– After this short amount of time, energy must be
obtained from stored glycogen
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– For long term energy, ATP can also be obtained via
aerobic metabolism of glucose, fatty acids, and
other high-energy molecules which are constantly
supplied by the blood
• Glycogen is broken down by a process called
glycolysis. (The end products of glycolysis begin the
Krebs Cycle [Citric Acid Cycle] and the electron
transport chain to generate energy for the organism.)
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• Glucose molecules are removed from the glycogen
and the cell uses the glucose to synthesize more ATP.
– Part of the glucose breakdown process can be done
anaerobically (without oxygen). This is a fast
process but only yields 2 ATP molecules per
glucose molecule.
– Or glucose 2 ATP + lactic acid
– It also produces lactic acid as a waste product
which can make muscles sore.
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---The most efficient, but much slower, process for energy
production is aerobic metabolism. This yields 36 ATP
molecules from 1 molecule of glucose. Carbon dioxide is
produced as a waste product.
or
glucose + O2  36 ATP + CO2 + H2O
• When you perform strenuous activity, it usually
takes a few minutes to start breathing heavily. The
increase in respirations indicates that aerobic
metabolism is now occuring.
– During strenuous exercise, typically the blood is unable
to carry enough oxygen for complete oxidation of
glucose in our muscles. So, the muscles will also
contract anaerobically and produce lactic acid.
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• The lack of oxygen and subsequent production of lactic acid is
called oxygen debt or oxygen deficit.
– So, oxygen debt occurs when the muscles have consumed
oxygen at a faster rate than it can be replaced. The body
must take in an extra amount of oxygen to replace the
oxygen in the muscles and return the muscles to their
resting state.
• Therefore, the reason we breathe heavily after exercise is to
erase or pay back the oxygen debt.
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Gross Anatomy of Skeletal Muscle
Individual muscle fibers are wrapped and held together by
several different layers of connective tissue
• The individual muscle fibers are surrounded by a fine
sheath of connective tissue called the endomysium.
• The fibers within the muscle are grouped into fascicles,
bundles of muscle fibers with a connective tissue covering
and look like a bunch of sticks with an outer wrapping
• The perimysium layer is fibrous connective tissue that
surrounds the fascicles.
• The outermost connective tissue layer is called the
epimysium and surrounds the whole muscle.
• Portions of the epimysium project inward and divide the muscle
into compartments
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• Skeletal muscles can be made up of hundreds or
thousands of muscle fibers bundled together
• Myofibrils are contractile units made up of long
protein molecules called myofilaments
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• Fascia, connective tissue outside the epimysium,
surrounds the entire muscle
• At the ends of the muscle, all of the connective
tissues come together and form the tendon that
attaches the muscle to bone.
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Movememt
• If the muscle spans a joint, one bone moves while
the other one remains stationary
– the muscle’s origin is on the bone which does
not move
• the muscle’s origin is generally closer to the midline
of the body than its insertion
– the insertion is on the bone which moves
• All muscle cells in a muscle have the same origin
and insertion
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Muscle Contractions
• Isotonic contractions: occur when the muscle shortens and
movement occurs
• Isometric contractions occur when muscle tension develops
but the muscle doesn’t shorten and no bony movement
occurs. These contractions help stabilize the skeleton.
• Degree of nerve activation influences force generated by the
muscle
– A single muscle consists of many individual muscle cells
typically organized into groups of cells that work together
cooperatively.
– Each group of cells is controlled by a single nerve cell
called a motor neuron.
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Terms to know:
• Motor unit: the motor neuron and all the muscle fibers it
supplies. Is the smallest functional unit of muscle
contraction.
• Muscle tension: force generated by a contracting muscle
upon an object
• How much tension is generated by a muscle depends on
– The number of muscle cells in each motor unit
– The number of motor units active at the time
– The frequency of stimulation of individual motor units
• All-or-none principle: muscle cells are completely under
the control of their motor neuron. Muscle fibers always
respond with a complete cycle of contraction and
relaxation every time they are stimulated.
• Muscle tone: low level of contractile activity in a relaxed
muscle.
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Muscle Activity
• Muscle twitch: a complete cycle of contraction and
relaxation
• Humans have 2 types of skeletal muscle fibers: slowtwitch and fast-twitch fibers. The difference is based
on how quickly the fibers can produce a contraction
and whether the muscle contracts aerobically or
anaerobically.
– Most muscles contain a mixture of both types of fibers
– Ratio of fibers depends on the function of the muscle
– Abnormal muscle twitches occur when almost all muscle
spindles are excited simultaneously
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• Slow twitch fibers: break down ATP slowly and
contract slowly
– Tend to make ATP aerobically, as they need it
– endurance, long duration contraction, contain myoglobin
to store oxygen
– Jogging, swimming, biking
• Fast twitch: break down ATP quickly, contract more
quickly
– Store large amounts of glycogen and tend to rely on
anaerobic metabolism for quick bursts of high energy
– Rapid, powerful contractions but can’t be sustained
– strength, short duration contraction:
– Sprinting, weight lifting, tennis
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Exercise Training
• Strength training:
– Resistance training:
• Short, intense
• Builds more fast-twitch myofibrils
• Aerobic training:
– Builds endurance
– Increases blood supply to muscle cells
– Target heart rate at least 20 minutes, three times
a week
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Muscle Contraction: Myogram
• Latent period-the time
•
•
between stimulation and the
start of a contraction
Contraction-time when the muscle
actually shortens
Relaxation-muscle returns to its
original length
• Summation vs. tetanus
Summation: an increase in the frequency
with which a muscle is stimulated so
that the muscle doesn’t relax
completely. This causes a
“summation” of the contractile force
so total force produced is greater than
the force produced by one twitch.
Tetanus: If muscle stimulation is so rapid
that the muscle can’t relax at all, it
will remain in a state of maximal
contraction.
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Figure 6.10
Diseases and Disorders of the Muscular System
• Muscular dystrophy –group of inherited muscle
diseases in which muscle fibers are unusually
susceptible to damage.
– Muscles, primarily voluntary muscles, become
progressively weaker. In the late stages of muscular
dystrophy, fat and connective tissue often replace
muscle fibers OR
– loss of muscle fibers due to muscle’s inability to
create some proteins it needs to function normally
OR
– Leak of calcium into the muscle cell which
damages muscle proteins and may kill the cell
– results in muscle wasting and paralysis; death
usually from heart failure or respiratory failure
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Tetanus or “lock jaw” –bacterial infection resulting in
overstimulation of nerves and therefore muscles resulting in
tetanic contractions; death from exhaustion or respiratory
failure
Muscle cramps – painful, uncontrollable muscle
contractions; caused by dehydration and ion imbalances
caused by heavy exercise
Pulled muscles –caused by stretching a muscle too far
causing some fibers to tear apart
Fasciitis –inflammation of the connective tissue fascia that
surrounds a muscle
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