Physiology 3120 Study Guide - Endoplasmic Reticulum, T-Tubule, Axon Terminal
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What to know about muscles?
- should know the structure from the whole muscle down to the myofilaments, as well as the structures
within the muscle cell, the transverse tubule, sarcoplasmic reticulum and muscle cell membrane
(sarcolemma) and how they function
In the last module you learned about how neurons fire action potentials and how they travel down the
axon to the axon terminal.
But how do action potentials make muscles work?
The neuromuscular junction is where neuron meets muscle (hence the name). It is at this region that the
action potentials in the axon are “transferred” to the muscle cells.
A) Ca++ Voltage Gated (V.G.) Channels
Once the action potential has reached the axon terminal, there are Ca++ V. G. channels located in
the terminals which open. Because Ca++ is higher on the outside of the cell, Ca++ flows into the
cell (axon terminal). When Ca++ flows into the axon terminal, it causes synaptic vesicles
(containing the neurotransmitter acetylcholine (Ach)) to move and fuse with the cell membrane.
B) The Neurotransmitter Acetylcholine and Ligand/Chemical Ion Channels
Ach can then leave the axon terminal via exocytosis. Ach can cross the synaptic cleft and bind to
receptors on the end plate and cause ion channels (non-specific) to open. These are not V.G.
channels but are called ligand/chemically-gated channels because it requires a ligand or chemical
to open them. The opening of these channels allows mostly Na+ to flow in (some K+ can also
leave) but this causes a depolarization that is called an end plate potential. This can eventually
lead to an action potential on the muscle cell membrane. Ach is then broken up by an enzyme
called acetylcholinesterase and taken back into the axon terminal to be recycled. In a healthy motor
unit, an action potential in a neuron will always lead to an action potential in the muscle cell.
C) Excitation-Contraction Coupling
Excitation-Contraction coupling is the process that turns an action potential into muscle activity. As
the action potential is occurring on the muscle cell membrane, the action potential travels deep into
the muscle by way of transverse tubules (T-tubules) (they are really a continuation of the
sarcolemma). As the potential travels down the T-tubule, Ca++ is released by the sarcoplasmic
reticulum by diffusion.
D) Troponin and Tropomyosin
Recall that the myofilament is made up of the protein actin and regulatory proteins, Troponin and
Tropomyosin (know their function). So as Ca++ diffuses out of the sarcoplasmic reticulum, it binds
to troponin which releases the tropomyosin. The tropomyosin then rolls out of the way, uncovering
the myosin binding sites. This allows the thick filament (made mostly of myosin) to interact with the
actin. Ca++ is not enough; we also need energy in the form of ATP (adenosine triphosphate).
Check out the home page, animations section and watch the actin-myosin-ATP cycle for a better
Myosin needs to take an ATP molecule and split it into ADP+ and inorganic phosphate (Pi)(which
actually both stay bound to myosin at this point) in order to interact with actin. The energy from
ATP becomes stored by the myosin and fuels the power stroke. Stored energy in myosin also
increases the affinity of myosin for actin. But remember that myosin cannot interact with actin
unless tropomyosin gets out of the way (which can’t happen unless Ca++ is released, which can’t
happen unless an action potential occurs). When myosin and actin bind, they form a crossbridge
which then allows for the powerstroke (the myosin head swings over and binds to another actin
molecule, releasing Pi). After the powerstroke, myosin is still attached to actin but releases ADP,
until another ATP molecule binds to myosin; this causes the crossbridge to dissociate, allowing the
process to repeat itself.
As long as Ca++ is still floating inside the cell, the muscle will contract. To remove Ca++, it needs
to be actively pumped back into the sarcoplasmic reticulum (up its concentration gradient). This set
of events is called excitation-contraction coupling.
What does only one action potential do?
This is called the muscle twitch--a single, small contraction.
How do we vary the strength of a contraction? It involves the process known as summation and
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