Download Lab 4-The Muscular System

The Muscular System
rev 2-11
Muscle cells are involved in every movement that
our bodies perform.
Muscles can:
• Shorten or contract to produce movement
• 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
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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 for our body
• Cardiac muscle
– found in the heart; they pump blood throughout the
body
• Smooth muscles
– found in most internal organs
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• Muscles may also be classified as
– voluntary (muscles over which we have
conscious control)
– involuntary (muscles over which we have no
conscious control)
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Skeletal Muscles
• are multinucleated
• cells are arranged in a parallel fashion
• will not contract unless stimulated by a neuron
• 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 discs
because of these connections, cardiac muscle
works as a single, coordinated unit
will contract without nervous stimulation
usually contracts at a steady rate set by the heart’s
pacemaker, but neural controls allow for a faster
beat
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
• Cells are spindle shaped; each cell has a centrally
located nucleus
• Do not require nervous stimulation for contractions
• Are found in the walls of hollow visceral organs
• 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
• many myofilaments bundled together are called
myofibrils
– The space between 2 Z-lines is called a sarcomere
• A sarcomere is the smallest 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
– ACh is a neurotransmitter--a chemical which
can either stimulate or inhibit another “excitable”
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
– the function of the T-tubules is to allow the electrical
impulse to travel to all cell parts
• The T-tubules are in close contact with the sarcoplasmic
reticulum
• The electrical impulse triggers the release of calcium from
the sarcoplasmic reticulum so the muscle can 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
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Calcium binds to troponin and causes the
• Troponin–tropomyosin protein complex to shift
position
– To expose the myosin binding site 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.
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So, in order to contract, a muscle must:
– Be stimulated by a nerve
– Cause an electric current along the sarcolemma
– Have a rise in intracellular calcium levels
Muscle Relaxation
• Nerve cell stimulation ends, contraction ends
• In order to stop the contraction, nerve stops
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Calcium pumped back into sarcoplasmic reticulum
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.
• ATP is the muscle’s energy source; typically muscle
cells store enough ATP for 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
– Glycogen is broken down by a process called glycolysis
or the Krebs Cycle or the Citric Acid Cycle
• 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. This is a fast process but only yields 2
ATP molecules per glucose molecule.
– It also produces lactic acid as a waste product which
can make muscles sore.
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• During vigorous exercise, typically the blood is unable to
carry enough oxygen for complete oxidation of glucose in
our muscles. So, the muscles will contract anaerobically.
• Lactic acid normally leaves the muscles and goes into the
blood. But, with continued exercise, the amount of lactic
acid in the blood increases so the lactic acid levels will
accumulate in the muscles.
• This will cause muscle fatigue and cramps.
• After exercise, the person rests and must take in enough
oxygen to allow the lactic acid to be changed into glucose.
• This oxygen is called the oxygen debt.
• So, the reason we breathe heavily after exercise is to
erase the oxygen debt.
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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.
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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.
• The perimysium layer is fibrous connective tissue that
surrounds the fascicles.
• The outermost connective tissue layer is called the
epimysium.
• At the ends of the muscle, all of the fascia come together
and form the tendon.
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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 insertion is on the bone which moves
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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
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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
• All-or-none principle: muscle cells are
completely under the control of their motor
neuron.
• 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.
• Slow twitch fibers: break down ATP slowly and
contract slowly
• Fast twitch: break down ATP quickly, contract more
quickly
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Exercise Training
• Strength training:
– Resistance training:
• Aerobic training:
– Builds endurance
– Increases blood supply to muscle cells
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Diseases and Disorders of the Muscular System
• Muscular dystrophy –inherited; loss of muscle fibers
resulting in muscle wasting and paralysis; death usually
from heart failure or respiratory failure
• 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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