Transcript The Twitch Contraction

Lindsey Bily
Anatomy & Physiology
Austin High School
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Have certain
characteristics that
allow them to do their
job.
◦ Excitability (aka
irritability): the ability to
be stimulated
◦ Contractility: they can
contract or shorten
which allows them to
pull on bones and
produce movement
◦ Extensibility: they than
extend or stretch letting
them return back to
their original shape after
contracting.
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Remember a muscle is composed of bundles
of muscle fibers (cells).
Can range in length from 1mm to 40 mm
long!
They share many of the same structural parts
as other types of cells, but of course, have
will have different names since they are in a
muscle fiber.
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Sarcolemma: plasma membrane of a muscle
fiber.
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Sarcoplasm: cytoplasm of a muscle fiber.
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Sarcoplasmic Reticulum (SR): network of
tubules and sacs similar to the ER of other
cells.
They contain many mitochondria and have
multiple nuclei.
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Fibers have structures that are only present in
muscle fibers.
Myofibrils: very thin fibers within the cell that
extend lengthwise and almost fill up the
sarcoplasm.
Sarcomere: basic contractile unit of the muscle
fiber.
T Tubules (transverse tubules): they are tubes
that run perpendicular to the myofibrils.
◦ Formed by extensions of the sarcolemma
◦ Allow nerve impulses (electrical signals) move deeper
into the cell.
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Each muscle fiber contains about a thousand
myofibrils. Shockingly, these myofibrils are
composed of even smaller fibers called
myofilaments.
Myofilaments can be either thick or thin.
Proteins that make up myofilaments…
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Myosin
Actin
Tropomyosin
Troponin
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Thin Filaments are made up of actin,
tropomyosin, and troponin.
Thick Filaments are up of myosin. They are shaped
like golf clubs so that they can “grab” the actin on
the thin filament.
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Contraction of a muscle requires several
processes to happen in a sequence.
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Excitation of the Sarcolemma
◦ A motor neuron is a specialized nerve cell that
connects to the sarcolemma of a muscle fiber at the
motor endplate. However, they don’t touch
completely, there is a gap.
◦ This connection is called a neuromuscular junction
(also a synapse).
◦ Neurotransmitters are chemicals that transmit
signals.
◦ When a nerve impulses reaches the end of a
neuron, it releases the neurotransmitter,
acetylcholine into the synaptic cleft (gap between
the muscle fiber and the neuron).
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Acetylcholine diffuses across the synapse and
binds to receptors on the sarcolemma on the
muscle fiber.
Binding to the receptors causes them to sent
an electrical impulse into the sarcolemma.
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The electrical impulse travels across the
sarcolemma and down the T tubules.
The impulse triggers the sarcoplasmic reticulum
(SR) to release stores of Ca2+.
The Ca2+ combine with the troponin on the thin
filaments and cause the active sites of actin to
become exposed.
The myosin heads on the thick filament can now
bind to the actin and pull the thin filaments past
them.
The myosin then releases itself and binds to
another actin active site, there by shortening the
muscle.
This is the SLIDING FILAMENT THEORY.
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Click here to see an animation.
http://msjensen.cehd.umn.edu/1135/Links/
Animations/Flash/0011swf_breakdown_of_a.swf
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Almost immediately after the SR releases Ca2+
into the sarcoplasm, it actively pumps it back
into its sacs.
Within a couple of milliseconds, the Ca2+ has
been recovered.
The SR steals the Ca2+ from the troponin
which then causes the active sites of the actin
to become blocked again.
The contraction process automatically shuts
itself off within a small fraction of a second
after initial stimulation.
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Muscles use ATP for energy.
The myosin head will bind to the ATP and it is “at
rest” waiting for actin to be able to bind. Imagine
pulling back on a slingshot.
Once the myosin has bound to the actin, a new ATP
molecule comes in and the myosin goes back in the
“rest” position therefore pulling the actin towards
it.
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Remember, in order for ATP to be used, the
muscle must have an ample supply of glucose
and oxygen.
Some muscle fibers store glycogen so they
have enough glucose readily available.
Muscle fibers also store oxygen in a molecule
called myoglobin (similar to hemoglobin).
Fibers with lots of myoglobin are called red
fibers and those with few myoglobin are
called white fibers.
Most muscles have a mixture of the two.
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If there is not enough oxygen present, then
the body will undergo anaerobic respiration.
This will produce a buildup of lactic acid in
the muscles and they might start to cramp or
fatigue.
The lactic acid will gradually get back into the
blood and travel to the liver where it will be
converted back to glucose.
Remember the ULTRAMARATHON MAN?
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A functional unit that includes a motor
neuron and all of the muscle fibers it attaches
to.
So, each muscle fiber DOES NOT have to have
their own neuron. One neuron could
stimulate hundreds of fibers.
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“muscle graphing”
This is done to see the contraction of a
muscle.
The force of the contraction is shown as a
line on a graph.
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Threshold stimulus is the minimum amount
of electrical stimulus that needs to be applied
to get a muscle to contract.
Applying a single, brief threshold stimulus to
a muscle will produce a quick jerk of the
muscle (twitch contraction).
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The entire process takes 1/10 of a second!
1. Latent Period: the nerve impulse travels
through the sarcolemma and T tubules and
causes the release of Ca2+ by the SR.
2. Contraction: The Ca2+ binds to the
troponin, so the myosin can bind to the actin.
3. Relaxation: the Ca2+ is reabsorbed by the
SR and the contraction stops.
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Our muscles rarely have twitch contractions.
The nervous system makes sure that we move
in fluid, smooth movements.
However, if we ever get an electrical shock or
for some reason there is overactivity of the
nervous system, we may experience twitch
contractions.
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If a series of twitches
occur, you will see a
gradual increase in the
strength of each
contraction.
Due to several factors, but
mainly Ca2+ diffuses
through muscle fibers that
are warm.
This is why athletes “warm
up” before they exercise.
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Occurs when there is repeated stimulation of
the muscle. Over time, the muscle responds
less and less to the same stimulation.
Usually doesn’t occur over the whole body,
just an isolated muscle.
Physiological muscle fatigue is due to low
levels of ATP, oxygen and glycogen, high
levels of lactic acid.
We physically cannot contract our muscles.
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Different from psychological fatigue, where we feel
exhausted or tired. During psychological fatigue, we
will not contract our muscles as opposed to cannot.
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This is normal.
Tetanus is sustained
contractions. This is how
we move.
Multiple motor units will
stagger their stimuli so
that the muscle never has
time to relax before it
gets stimulated again.
This produces nice,
smooth movements.
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A continual , partial
contraction of a
muscle.
When people go
unconscious, they
become limp, or lose
their muscle tone.
Muscles that don’t
have enough tone are
called flaccid.
Muscles with too
much tone are called
spastic.
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If you don’t use your muscles, you’ll lose them
(atrophy).
Strength training will make the muscles bigger
(hypertrophy) by increasing the number of
myofilaments within each muscle fiber. You do not
get more muscle fibers!
Aerobic training will not make your muscles bigger,
you will however, increase the number of
mitochondria in each fiber as well as the number of
blood vessels in the muscle and it will be more
efficient at using oxygen.