Download Muscular System

Muscles and Movement
Kate Phelan
Eleni Angelopoulos
Anastasia Matkovski
Fun Facts
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“Machines of the
Body”
We have over 700
muscles in our
body
Muscles account
for about 40% of
body weight
Allows the body to
move
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Types of Muscles
Smooth --->
Striated --->
Cardiac --->
Skeletal Muscles
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http://trc.ucdavis.edu/biosci10v/bis10v/media/ch21/myofibril.html
Important in maintaining posture, providing support, and allowing for movement
Important in homeostasis
– Constant body temp
• Contraction of muscle= ATP breakdown, releasing heat
Striated
– light and dark bands
• Overlapping bands of actin and myosin
Attached to the skeleton by tendons
– Bands of fibrous connective tissue
When muscles contract they shorten
Work in antagonistic pairs
– If one muscle of the pair flexes the joint and bends the limb, then the other one
must extend the joint and straighten the limb
Tone- when muscles appear to be at rest, some of their fibers are still contracting
– Important in maintaining posture
Structure
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Each muscle fiber is a cell with normal
components (nucleus, etc.) but some components
have special features
– T (because they are transverse) tubules from
the sarcolemma (plasma membrane) dip into
the sarcoplasmic reticulum (expanded
version of endoplasmic reticulum) and hold
Calcium ions, which, when nerve impulses
move down the tubules, are essential for
muscle contraction. Happens in the
sarcomere.
– Myofibrils- composed of two kinds of
myofilaments
– Thin filaments -two strands of actin
and a strand of regulatory protein
– Thick filaments -staggered arrays of
myosin molecules
• contractile parts of muscle fiber- actin and
myosin work together to create this
contraction
Myosin-Actin Interactions Underlying Muscle Fiber Contraction
Starting here the myosin head is
bound to ATP and is in its low energy
configuration
Binding of a new
molecule, ATP,
releases the myosin
head from actin, and a
new cycle begins
Releasing ADP and inorganic
phosphate, myosin returns to
its low energy configuration,
sliding the thin filament
The myosin head hydrolyzes the
ATP to ADP and inorganic
phosphate and is in its high
energy configuration
The myosin head binds to
actin, forming a crossbridge
Varying the Rate of Muscle
Fiber Stimulation
• If a second action potential arrives before the
muscle can relax from the first, then the two
twitches will sum resulting in greater tension.
• As the rate of stimulation increases, further
summation occurs.
• Tetanus- smooth sustained contraction when
twitches fuse because the rate is high enough
that the muscle fiber cannot relax at all
between stimuli.
Role of Calcium &Regulatory
Proteins in Muscle Contraction
• When is a muscle is at rest, the myosin binding sites on the thin
filament are blocked by the regulatory protein, tropomyosin.For
muscle fibers to contract, these sites must be uncovered.
• Calcium ions bind to the troponin complex, exposing the myosin
binding sites on the thin filaments.
• Thin &thick filaments slide past each other and the muscle fiber
contracts.
• After breaking down ATP, myosin heads bind to an actin filament
to move. When another ATP binds to myosin, the head detaches
from actin and the sycle begins again.
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter10/animation_
_action_potentials_and_muscle_contraction.html
Neural Control of Muscle
Tension
• When an action potential in a motor neuron
releases acetylcholine on a skeletal muscle
fiber, the muscle fiber twitches.
• We can voluntarily alter the extent and
strength of the contraction of a whole muscle.
– Two basic mechanisms:
• By varying the number of muscle fibers that contract
• By varying the rate by which muscle fibers are
stimulated.
Varying the Number of Muscle
Fibers
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Each muscle fiber is innervated by one motor neuron, though one
motor neuron may synapse with many muscle fibers.
– Hundreds of motor neurons controlling a muscle, each with its own
muscle fibers.
When a motor neuron produces an action potential, all the muscle
fibers in its motor unit contract as a group.
The strength of the contraction depends on ow many muscle fibers the
motor neuron controls.
Thus, the nervous system can regulate the strength of contraction in a
while muscle by determining how many motor units are activated and
by selecting large or small motor units to activate.
Smooth Muscles
• Involuntary movement (not under direct voluntary
control)- controlled by the autonomic nervous system
• Found in the internal organs, like the intestines and
the bladder
• Lack striations because their actin and myosin
filaments are not regularly arrayed along the cell.
• Instead the thick filaments are scattered throughout
cytoplasm and thin filaments are attached to structures
called dense bodies.
• Less Myosin than striated muscle fibers and myosin
isn’t associated with specific actin strands
• Does not have any troponin complex or T tubules
Some More Smooth Muscle
Stuff... Stay awake class...
• Smooth muscles contract slowly but over a greater
range of length than striated muscle.
• Some smooth muscle cells contracts only when
stimulated by neurons of the nervous system.
• Others can generate action potentials without neural
input and are electrically coupled to one another.
Cardiac Muscles
• Only found in heart (obvs)
• Characteristics of skeletal and smooth muscle
– striated like skeletal
• Cardiac muscles have ion channels in their plasma membrane that
cause rhythmic depolarizations, triggering action potentials without
imput from nervous system.
• Action potentials of cardiac muscle cells last up to 20 times longer
than those of skeletal muscle fibers
– Play important role in control of duration of contraction
– Interclated disks- specialized regions where plasma membranes of
adjacent cardiac muscle cells interlock
• Where gap junctions provide direct electrical coupling between the cells.
– Thus, an action potential generated by a cell in one part of the
heart will spread to all other cardiac muscle cells, and the
whole heart will contract.