Skeletal muscle AP II

Question 1

Are muscle fibres self activated?

Answer 1

No they are under voluntary control of the CNS

Question 2

Where are the nerve cells that activate the skeletal muscle fibres?

Answer 2

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

Question 3

What are motor neurons activated by?

Answer 3

Activated by cells in the brain called the motor cortex.

Question 4

What is spcialised synapse NMJ?

Answer 4

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

Question 5

What kind of synapse is NMJ

Answer 5

It’s an excitatory synapse (no skeletal muscles are inhibitory)

Question 6

What is a motor unit?

Answer 6

A motor neuron and all of the muscle fibres it controls

Question 7

Does one motor neuron branch to many different muscle fibres?

Answer 7

Yes

Question 8

Where are motor neurons found?

Answer 8

Ventral part of the spinal cord

Question 9

How are spinal nerves formed?

Answer 9

The motor axons branch out from the spinal cord to form vental nerve roots –> spinal nerves

Question 10

How are muscle fibres innervated?

Answer 10

Axons project together and then branch out in the muscle and connect to the muscle fibres

Question 11

Is a whole muscle a collection of motor units?

Answer 11

Yes, motor neurons whose axons project to the same muscle lie close together on the spinal cord

Question 12

Function of very big motor units

Answer 12

Provide a lot of force but can’t provide fine increment control

Question 13

What is recruitment?

Answer 13

Amount of motor neurons activated at any one time that can be varied to change the amount of force produced

Question 14

Where is the NMJ located?

Answer 14

Usually at the middle 3rd of the fibres length so that the AP can spread over the sarcolemma away from NMJ in both directions

Question 15

What is myosin?

Answer 15

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

Question 16

What is Actin?

Answer 16

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

Question 17

What is tropomyosin?

Answer 17

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)

Question 18

What is troponin?

Answer 18

Globular protein found throughout tropomyosin and actin, controls the rolling of tropomyosin that makes the binding sites of actin available

Question 19

What is excitation-contraction coupling?

Answer 19

aka EC - coupling
*Plasma membrane excitation –> Ca2+ release –> muscle contraction

Question 20

What is the protein that detects a change in the Voltage of the transverse tubule?

Answer 20

DHPR - Dihydropyridine

Question 21

How does EC occur?

Answer 21

*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)

Question 22

What is SERCA

Answer 22

Sarco(endo) plasmic reticulum Ca2+ ATP ase

Question 23

What is cross bridge cycling?

Answer 23

Process of contraction in the myofilaments

Question 24

What happens in the cross bridge cycling process?

Answer 24

*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”

Question 25

Muscle tension depends on two things:

Answer 25

*Rate of stimulation i.e the amount of APs
* No. of muscle fibres recruited for the task

Question 26

Frequency of stimulation - Twitch

Answer 26

*One AP generates a single twitch. If MF restimulated after complete relaxation, second twitch is the same magnitude as the first

Question 27

Frequency of stimulation - Twitch summation

Answer 27

If muscle fibre has not relaxed before restimulation, sum of twitches = twitch summation = larger force

Question 28

Frequency of stimulation - Tetanus

Answer 28

If there is no time to relax (like me haha :”) ), maximal sustained contraction of Tetanus due to continuous release of Ca2+

Question 29

No. of fibres in recruitment

Answer 29

• Force produced by each fibre depends on no. of fibres activated and the sarcomeres length

Question 30

Length-tension relationship (amount of force produced by each fibre)

Answer 30

Each muscle sits at an optimal length n body at rest
*Too stretched, cross bridges can’t occur
* Too slack cause its too contracted, can’t have more contraction of the sarcomeres

Question 31

Recruitment (no. of fibres activated)

Answer 31

*force produced by each fibre + no. of fibres activated

No. of fibres activated is regulated by how many neurons activated at one time
Process of activating more motor units is recruitment = more force