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The Muscular System
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The Muscular System – an overview
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There are three types of muscular tissue:
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Skeletal muscle
Smooth muscle
Cardiac muscle
Muscular tissue is contractive tissue.
What is the function of the muscular system?
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The Muscular System Has Many Functions
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Allows movement through the environment
Stabilizes movement at the joints
Aids in the flow of lymph and blood through
the body
Protects our internal organs
Maintains homeostasis by producing heat
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Skeletal Muscles are Contractile Organs
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All human skeletal muscles have a similar
function and structure.
Function:
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They contract to produce movement.
They relax to their original length.
Structure:
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The origin is the end that remains stationary when
the organ shortens.
The insertion is the end that moves during
contraction.
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Figure 6.1 Muscle origin
and insertion
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Body movements
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To coordinate and control body movements,
most human skeletal muscles function as a
member of an antagonistic or synergistic pair.
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Antagonistic (synergistic) – muscles with opposing
actions working together to provide smooth and
controlled movements.
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Example: moving your hand to your shoulder requires:
1. simultaneous contraction of the prime movers (the
brachialis and biceps brachii muscles), and
2. relaxation of the antagonist (the triceps brachii).
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Figure 6.2 Anterior
view of the
superficial muscles
of the body
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Figure 6.3 Posterior
view of the superficial
muscles of the body
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Anatomy of a muscle
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Skeletal muscle is composed of numerous
elongated structures, one nested inside the
other.
Individual muscle cells are long (sometimes 30
cm), slender and fragile.
There are essentially 3 layers of the muscle:
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Epimysium – separates muscles
Perimysium –covers and supports muscle cells
Endomysium – covers muscle cells on top of
membrane
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Figure 6.4 Anatomy
of a muscle
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Organization of skeletal muscles
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Myofiber – a single muscle cell
Sarcolemma – cell membrane covering
myofiber
T tubules – specialized areas in the sarcolemma
that conduct the contraction messages
Myofibrils – linearly arranged groups of the
contractile proteins actin and myosin
Sarcomeres - contractile units which hold the
proteins in regular arrangements
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Sarcomeres are banded (striated)
Copyright 2008 John Wiley & Sons, Inc.
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Figure 6.5 Organization of skeletal muscles from gross to molecular
Copyright 2008 John Wiley & Sons, Inc.
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Figure 6.5 Organization of skeletal muscles from gross to molecular
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Figure 6.5 Organization of skeletal muscles from gross to molecular
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Actin and myosin are microscopic proteins that
interact and cause the entire muscle tissue to
shorten and, thus, move the skeletal tissue.
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Actin is a thin, globular protein
Myosin is a larger, heavier protein
Figure 6.6 Structure of thick and
thin filaments
Figure 6.7 Myosin filament
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Muscle Contraction Occurs as Filaments
Slide Past One Another
The contraction of a skeletal muscle begins
when an impulse reaches the neuromuscular
junction.
At this junction, the motor neuron ends very
close to muscle cells separated by the
synapse.
Nerves send a contraction impulse across the
synapse with neurotransmitters.
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Acetylcholine (ACh) is the most common.
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Muscle Contraction Occurs as Filaments
Slide Past One Another
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ACh is released from the axon terminal and
diffuses across the synaptic cleft and binds to
receptors on the muscle cell membrane.
The skeletal muscle will contract in response
to ACh binding.
The impulse to contract is then passed
through the entire muscle cell via T tubules.
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Figure 6.8
Neuromuscular
junction
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More details…
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Inside the muscle cell, sarcoplasmic reticulum (SR) stores
calcium ions and releases them when the ACh binds to
the surface of the cell.
Calcium is held in the SR by the enzyme calcium
sequestrin.
This enzyme on the surface of the SR stores and
releases calcium from the cytoplasm into the SR.
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The enzyme works by converting ATP to ADP.
Since free calcium in the cell is toxic, this ensures the
survival of the cell.
Neither actin nor myosin undergo chemical
transformations.
Actin merely slides over the myosin filament.
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Figure 6.9 Contraction cycle
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Figure 6.10 Summary of
events in contraction and
relaxation of skeletal
muscle
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Whole-Muscle Contractions Emerge from
Tiny Impulses
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How does an entire large muscle like that of
your thigh contract and generate movement?
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Muscle cells are grouped in motor units
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1 motor neuron and the set of muscle cells it controls
The entire motor unit contracts when it receives a
signal from the motor nerve.
This causes calcium ions to be released which
triggers the sliding action.
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Whole-Muscle Contractions Emerge from
Tiny Impulses
Figure 6.12 Motor unit
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Muscle cells contract on an all-ornothing basis
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If the nerve stimulus is too weak, nothing
happens.
When the threshold stimulus is reached,
calcium is released and the entire muscle cell
contracts.
Single twitches are not effective in producing
body movement, because they last a fraction
of a second.
The motor unit requires multiple stimuli.
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Each contraction builds on the last until the
muscle cell is continuously contracted.
The buildup of contractions is called
summation.
Increased strength for similar stimuli is known
as treppe.
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Example – warming up for an athletic event takes
advantage of treppe
Once continuous contraction is achieved, the
muscle is in tetanus.
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Example – the neck muscles of an adult are in
tetanus most of the day
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Muscles Require Energy to Work
Smoothly and Powerfully
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What is the general source of energy inside
cells?
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ATP
The body can make ATP for muscular
contractions through either aerobic or
anaerobic pathways.
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Figure 6.16 Krebs cycle
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ATP production
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Aerobic pathway (highly efficient)
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Burns glucose, forming water, carbon dioxide and
ATP
Aerobic ATP is generated in the mitochondria
Produces the largest amount of ATP
Is the dominant method of energy production
During heavy muscle activity, oxygen cannot
keep up with the energy demands. ATP
production shifts to anaerobic pathways.
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ATP production
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Anaerobic pathway (less efficient)
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Produce fewer ATP molecules per glucose molecule
Produces lactic acid which is eventually removed
from the tissue by conversion to pyruvic acid
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This requires oxygen (which is one reason we breathe
heavily after exertion).
Oxygen is carried through the bloodstream to the lactic
acid-laden tissue, reacts with the acid converting it to
pyruvic acid and then to coenzyme A.
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Figure 6.17 Conversion of
lactic acid to pyruvate and
then on to ATP
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Creatine phosphate is important in the
anaerobic phase of muscle energy production
because it stores energy much like ATP, in a
phosphate bond.
Figure 6.18 Creatine phosphate reaction
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Muscle twitches can be fast or slow
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There are two types of muscle cells:
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Slow twitch (slow oxidative; aerobic)
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Appear red, have a large blood supply, have many
mitochondria, and store myoglobin.
Purpose is to supply oxygen to the mitochondria of the
cells and sustain the supply of ATP within sarcomeres.
Fast twitch (fast oxidative; anaerobic)
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Provide short bursts of incredible energy and contraction
power, but fatigue quickly.
These cells are thicker and usually contain larger
glycogen reserves and less developed blood supply.
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Figure 6.19 Fast twitch and slow twitch cells
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Toned muscles work better, look better
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Muscle tone is the constant partial contraction of
muscle when the body is “in shape”.
Toned muscles are more effective at burning
energy (using more ATP per gram).
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Therefore, people who are in shape can eat more
without gaining weight because the low-level
contractions of their toned muscles constantly burn
ATP.
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Exercise or chemical compounds can
change the size of a muscle.
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Scientists think that the total number of muscle fibers is
inherited; therefore, we alter the muscle by enlarging
cells (hypertrophy).
Hypertrophy is caused by the addition of new myofibrils
within the endomysium of muscle cells, which thickens
the myofibers.
Hypertrophic muscles should have thicker muscle cells,
packed with more sarcomeres.
Exercise that requires muscle to contract to at least
75% of maximum tension will cause hypertrophy.
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Summary
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Our muscles are designed to:
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Provide movement
Manipulate the environment
Maintain homeostasis
Protect our organs
Maintain our upright position
All movement requires the production of ATP,
either stored in the cell or produced via
metabolic pathways.
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