Muscle Contraction
Sliding Filament Theory
Dr. Cox - April 30, 2014
Outline
1. Muscle structure
2. The Sarcomere
3. Action Potential and
Muscle Contraction
4. Energy for Muscle
Contraction
Skeletal Muscle
Structure
Skeletal muscles are
constructed similar to
nerves. They are organized
into bundles (right) and
each bundle is composed of
many muscle fibers called
fascicles. Each fascicle is
composed of many muscle
cells. In each muscle cell are
units that contract called
sarcomeres. There are 100s
in each skeletal muscle cell; these are the working units of the muscle.
The connective tissue of the muscle is fashioned very similar to nerves. The outermost
layer on each muscle bundle is called the epimysium. Each fascicle is surrounded by
another layer of connective tissue called the perimysium. Finally, each muscle cell is
surrounded by another layer of connective tissue called the endomysium. Those fuse on the
ends of each muscle to become a tendon. Tendons can attach to skin, bone, or another
muscle.
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Innervation
Each muscle is controlled by several motor neurons. The
axons of these motor neurons have many motor end plates
which are synaptic end bulbs containing neurotransmitter
(ACh – right). They function just like those innervating to
nerves. Upon stimulation (NAP), they will release
neurotransmitter which sets up an Action Potential (AP) on
the muscle and will result in its contraction.
Structure of a Muscle Cell
Refer to the diagram above as we walk through the structure of a single muscle cell. The cell
membrane of a muscle cell is called the sarcolemma (after plasmalemma used with other
cells). There are invaginations of the sarcolemma which are called traverse tubules or Ttubules. Each skeletal muscle cell has a multiple nuclei because of its extensive length.
Inside each muscle cell, there are bundles of myofilaments arranged in clusters called
myofibrils. Wrapped around each of these myofibrils is a network of smooth endoplasmic
reticulum called the sarcoplasmic reticulum. Within each bundle of
myofibrils, there are units of filaments called sarcomeres. The
sarcomeres are what gives skeletal muscle its striated appearance
when viewed under a microscope. Microscopically, the striations are
A Band
alternate units of A bands (darker) and I bands (lighter). The A bands
I Band
are overlapping filaments of myosin; the I bands - actin.
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Structure of a Sarcomere
The diagram to the right shows the basic structure of a
sarcomere. There are alternating rows of A bands and I
bands with a Z disc between each one. In the center of the
sarcomere is a line called the M line. Before we look at this
on a larger scale, let’s next look at the structure of the two
major myofilaments, myosin and actin, in the sarcomere (below).
The Thin Filament - Actin
The thin filament is probably the more complex of the two major myofilaments. It is made of
three major proteins. The main protein is actin as shown as the interlocking red spheres
above. Wrapped around the actin filament, is a another cable like protein called
tropomyosin. Each actin filament has many myosin binding sites which are hidden by the
tropomyosin filament. Attached to the tropomyosin is a third protein called troponin. On the
troponin molecule, there are special sites to which calcium ions may bind. When calcium
binds to these sites on troponin, the tropomyosin undergoes a conformational change and the
filaments slides and exposes the underlying myosin binding sites on the actin subunits.
The Thick Filament - Myosin
Myosin filaments (right) consists of
long filamentous proteins with a
rounded head. The tails are
wound around one another and
form a larger filament with the
heads projecting from it. On the
head portion of each myosin
molecule, there are 2 binding sites
for actin and also for ATP. Each
myosin molecule can bind to actin filaments in 2 places. The heads of the myosin are also
flexible, that is they swivel. This will be necessary for muscle contraction. So now, let’s next
look at the structure of the sarcomere on a much larger scale.
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In this diagram you can actually see the packets of myosin filaments and how they are
arranged in a sarcomere. The sarcomere runs from one Z disc to the next. Attached to the Z
disk are the actin filaments. Also connected to the Z disc are the myosin filaments which are
connected to the Z disk through another protein call connectin. Connectin extends from the
Z disk to the M line and extends through the core of each myosin filament. Its springlike
structure produces passive tension during muscle contraction. The M line is a protein
meshwork structure at the center of the H zone which attaches thick filaments (myosin) to
one another. The area in the center of the A band where only myosin filaments are present
and there is no actin filament is called the H zone. You might consider this the area between
adjacent actin filaments. When the sarcomere contracts, the thin filaments will be pulled
closer together and the distance of the H zone will shrink until the actin filaments opposite
one another are overelapping. The connectin coils will also be compressed.
Action Potential on a Muscle cell
When a nerve action potential is generated, it travels down the axon of the neuron to the
motor end plate or synaptic end bulb attached to the muscle cell. This of course, causes the
release of acetylcholine across the synapse between the neuron and muscle cell. The
acetylcholine initiates an action potential (AP) on the muscle cell. Like a neuron, sodium ions
rush into the muscle cell through voltage gated Na channels in the sarcolemma. The traverse
tubules help to carry this AP throughout the surface of the muscle cell to the sarcoplasmic
reticulum inside the muscle cell. When the action potential reaches the sarcoplasmic
reticulum, voltage gated Ca channels open, releasing Ca ions into the muscle cell sarcoplasm
and sarcomeres (see illustration below). The calcium binds to the calcium binding sites on
troponin. This causes the tropomyosin filaments to slide to one side and expose the myosin
binding sites on the actin filaments. The myosin heads then bind to the actin filament (at the
binding site) and ATP is hydrolyzed which causes the head to swivel toward its tail and pull
on the actin filament (power stroke). Immediately, ADP is released from the myosin head
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and binding of new molecule of ATP causes it to release from actin, to relax, and return to its
original confirmation. The myosin head is now free to attach to a another binding site on
the actin filament and the process is repeated. The actin filament does not slip because there
are other myosin heads attached to it which prevents slippage. So the myosin filaments
contracts over and over again ratcheting the actin filaments closer together until the H zone
has completely disappeared, i.e contraction (see Illustration below).
Muscle relaxation occurs when the nerve action potentials ceases, and the neurotransmitter
(ACh) in the synapse has been removed by AChE. The voltage gated sodium channels and
voltage gated calcium channels close. Sodium is pumped back out of the cell via the sodiumpotassium pump. Calcium is pumped out of the sarcoplasm back into the sarcoplasmic
reticulum via calcium pumps. As calcium is removed from the troponin protein, the
tropomyosin filament slides back over the myosin binding site and covers them. This causes
the release of the myosin from the actin filaments. Connectin helps to push the actin
filaments apart and reestablish the relaxed sarcomere (with an H zone).
Energy for Muscle Contraction
Muscle contraction requires energy in the form of ATP. ATP is supplied by three different
mechanisms: the phosphagen system; anaerobic respiration; and aerobic respiration.
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The phosphagen system provides energy for a very short period (10-15 sec) of time
primarily from stored ATP and the enzymes myokinase and creatinine kinase. Then the
muscle enters anaerobic respiration where it produces lactic acid as a byproduct from
glycolysis. This step is a bridge to long-term energy supply of aerobic respiration which
includes all of glycolysis thru the Krebs cycle, and electron transport for the production of
(36) ATP from the breakdown of sugars and fats. The reason that anaerobic respiration is
utilized as a bridge is that it takes time for the heart and respiratory rate to increase to
provide oxygen and increased sugar needed for aerobic respiration. The aerobic respiration
provides long-term energy, for example to run a marathon.
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