Excitation-Contraction-Coupling Flashcards
Explain the steps for initiating a signal for muscle contraction:
- The action potential travels the length of the axon of a motor neuron to an axon terminal.
- Voltage gated calcium channels open and calcium ions diffuse into the terminal.
- Calcium entry causes synaptic vesicles to release Acetylcholine via exocytosis.
- Ach diffuses across synaptic cleft and binds to Ach receptors which contain ligand gated cation channels.
- Ligan gated cation channels open. (Na+ and K+ channels) the ligand is Ach.
- Na+ enter and k+ leave muscle fibers, greater influx of Na+ related to the outward flux of K+ leads to membrane depolarization. (Membrane potential becomes less negative)
- When membrane potential reaches a threshold, an action potential propagates along the sarcolemma.
Neural transmission to muscle fiber stops when Ach is removed from the synaptic cleft.
What is the neuromuscular junction?
The site where a motor neuron excites a skeletal muscle fiber is called the neuromuscular junction. This junction is a chemical synapse consisting of the point of contact between the axon terminals of the motor neuron and motor end plate of the skeletal muscle fiber.
Neural transmission to muscle fiber stops when Ach is removed from the synaptic cleft. what are some ways to remove ACH from the synaptic cleft ?
- ACH diffuses away from the synapse
- ACH is broken down by the enzyme acetylcholinesterase to acetic acid and choline. Choline is transported back to the axon terminal for resynthesis of choline.
motor neuron
neuron that synapses with muscle.
In the excitation process, where is the ACH released from?
released from the motor neuron axon terminal into the synapse.
Ach binds to what receptors on the muscle fiber?
ACH binds to the Nicotinic ACH receptors on the motor endplate.
Where is the motor endplate?
Motor endplate is the region of the muscle fiber that is closely associated with the axon terminal. postsynaptic.
ACH Receptors are called ligand gated monovalent cation channels. Where does the associated labels come from?
ACH is the ligand that binds to the ligand gated nicotinic receptor, adjacent to the monovalent Na+ and K+ channels, this binding opens the channels causing Na+ to enter the cells and K+ to move out of the cell.
What happens when Na+ flows in and K+ flows out of the muscle fiber?
This causes a really large graded potential due to the strong driving force due to the electrical gradient of the Na+, making cells less negative. This graded potential will exceed threshold and fire action potential along the sarcolemma.
IN the neuromuscular junction:
- Motor neurons always depolarize skeletal muscle fibers.
- There are no ACHr voltage gated channels in the motor endplate. There are ONLY ACH receptors.
- Action potentials cannot be generated at the motor endplate.
- Along the sarcolemma, there are voltage gated sodium channels, and the graded potential can spread to those channels, which could generate action potentials.
- 1 action potential in the motor neuron always generates action potential in the skeletal muscle fiber.
- POSTIVE FEEDBACK
Why is there no need for a graded potential summation to reach threshold?
1 graded potential always reaches threshold through positive feedback.
How is the action potential in neurons different from action potential in skeletal muscles?
The difference is in the events leading to threshold potential. For skeletal muscles it is the net Na+ entry through ACH operated channels. In skeletal muscles graded potential always reaches threshold and in Neurons, graded potential can hyperpolarize, depolarize, doesn’t always reach threshold and may need summation.
What is Excitation-Contraction coupling?
It is a series of events that convert action potentials in a muscle fiber to a contraction.
