Skeletal muscle AP II Flashcards
Are muscle fibres self activated?
No they are under voluntary control of the CNS
Where are the nerve cells that activate the skeletal muscle fibres?
In the spinal cord, they are called motor neurons. They form a very long axon that extends into the periphery and makes synaptic contact with muscle fibres at a single pt called the NMJ
What are motor neurons activated by?
Activated by cells in the brain called the motor cortex.
What is spcialised synapse NMJ?
It is the myelinated axon termination point of a motor neuron where the brain sends an AP (membrane potential change) to the muscle fibre sarcolemma (excitable cell) to initiate contraction in the muscle fibre
What kind of synapse is NMJ
It’s an excitatory synapse (no skeletal muscles are inhibitory)
What is a motor unit?
A motor neuron and all of the muscle fibres it controls
Does one motor neuron branch to many different muscle fibres?
Yes
Where are motor neurons found?
Ventral part of the spinal cord
How are spinal nerves formed?
The motor axons branch out from the spinal cord to form vental nerve roots –> spinal nerves
How are muscle fibres innervated?
Axons project together and then branch out in the muscle and connect to the muscle fibres
Is a whole muscle a collection of motor units?
Yes, motor neurons whose axons project to the same muscle lie close together on the spinal cord
Function of very big motor units
Provide a lot of force but can’t provide fine increment control
What is recruitment?
Amount of motor neurons activated at any one time that can be varied to change the amount of force produced
Where is the NMJ located?
Usually at the middle 3rd of the fibres length so that the AP can spread over the sarcolemma away from NMJ in both directions
What is myosin?
The myofilament inbetween actin, it has a long tail and a globular head that can flex i.e its site of binding and “walking” along Actin
What is Actin?
It is a globular protein (G-actin) that assemble to form a filamentous protein strand of (F-Actin)
In the sarcomere, the strands that conect to the ends of the z line are two of the filamentous protein strands
What is tropomyosin?
It is a thin strand that runs along each of the two Actin strands, it blocks the sites where myosin interacts with actin (at rest)
What is troponin?
Globular protein found throughout tropomyosin and actin, controls the rolling of tropomyosin that makes the binding sites of actin available
What is excitation-contraction coupling?
aka EC - coupling
*Plasma membrane excitation –> Ca2+ release –> muscle contraction
What is the protein that detects a change in the Voltage of the transverse tubule?
DHPR - Dihydropyridine
How does EC occur?
*AP from nerves causes snaptic transmission at NMJ to trigger AP in muscle fibre
*Muscle AP spreads over surface of sarcolemma and invades the T - tubular system
*Detection of membrane potential change causes the Ryanodine receptor (RyR), which releases Ca2+ from reserves into the Cytoplasm of the cell
*Ca2+ binds to the troponin C on the actin filaments, changes the shape of troponin and causes the tropomyosin to roll due to change in shape.
*Exposure of actin to myosin, filaments slide against each other to contract and produce force
*For relaxation, Ca2+ unbinds, goes back to cytoplasm from the contractile apparatus, and goes through SERCA (uses ATP to pump Ca back)
What is SERCA
Sarco(endo) plasmic reticulum Ca2+ ATP ase
What is cross bridge cycling?
Process of contraction in the myofilaments
What happens in the cross bridge cycling process?
*When Ca2+ levels are low, ADP is found to the myosin and is kept in a cocked state, ready to bind to Actin
*Ca2+ released from the EC coupling
*Ca2+ binds to troponin, rolling away the tropomyosin and exposing the myosin binding sites on F-Actin
*Myosin head makes contact with Actin and rotates its head while Actin contracts –>
* ADP released causeing flexion (change in shape of the myosin head) –> “power stroke” of the myosin head. The contact between A and M is called “Cross bridge”
*If ATP available, it binds to free myosin head, causes lowering of affinity of myosin to actin, breaks the cross bridge.
*ATP hydrolised by myosin ATP ase –> ADP + Pi, the energy released is used to bring back the myosin head to the “cocked phase”
