Lecture 9 - Skeletal Muscle Neurophysiology

Question 1

what is the appearance of skeletal muscle?

Answer 1

a striated appearance due to the arrangement of thick filaments (myosin) and thin filaments (actin)

Question 2

what does skeletal muscle interact with?

Answer 2

interacts with the skeleton

Question 3

what is the origin of skeletal muscle?

Answer 3

the most proximal skeletal muscle insertion

Question 4

what is the insertion of skeletal muscle?

Answer 4

the most distal skeletal muscle insertion

Question 5

where do the skeletal muscle origin and insertion insert into?

Answer 5

insert into the bone via tendons

Question 6

what is the function of tendons?

Answer 6

connect muscle to bone to allow the muscle to exert force on the skeleton

Question 7

what is fascicles?

Answer 7

a group of muscle fibres that are bundled together

Question 8

what are muscle fibres?

Answer 8

multi nucleated muscle cell that is between 10-80um but up to 25cm in length

Question 9

what are myofibrils?

Answer 9

long filaments composed of myosin (thick filament) and actin (thin filament) that make up the inside of muscle fibres

Question 10

what is the sacromere?

Answer 10

the contractile unit of skeletal muscle that is the region between the ‘Z’ lines

Question 11

what is a ‘Z’ line?

Answer 11

a zig zag arrangement of actin (thin filaments) that project to the centre of the sarcomere

Question 12

what is the ‘I’ band?

Answer 12

the region between the myosin filaments (thick) that only contains actin (thin filament)

Question 13

what is the ‘A’ band?

Answer 13

the region in-between each I band region that contains both actin and myosin filaments

Question 14

what is the ‘H’ zone?

Answer 14

when the muscle is at resting length, the actin filaments do not project all the way into the centre of the sarcomere creating a gap where there is only myosin and no actin

Question 15

what is the ‘M’ line?

Answer 15

the centre of the myosin that contains proteins which organise and align myosin allowing for the ordered arrangement

Question 16

what is the relationship between the bands/zones/lines and skeletal muscle contraction?

Answer 16

all of the different bands change in length depending on whether the muscle is at rest or shortening as it contracts

Question 17

what occurs at the sarcomere as skeletal muscle contracts in term of the Z line, H zone, I band and the A band?

Answer 17

As the muscle starts to contract, the Z lines come close together by shortening.
Myosin slides over actin and the H zone becomes much smaller because the actin filaments slide across each other.
The I band becomes smaller as myosin partially comes together.
The A band stays the same

Question 18

when does maximally contracted skeletal muscle occur?

Answer 18

occurs when actin filaments have slide so far they can not go any further and are pushed right up against the myosin.

• the H zone is gone
• the I band is really tiny
• the A band remains the same

Question 19

what is the relationship between motor neurons in the spinal cord and skeletal muscle?

Answer 19

motor neurons in the spinal cord innervate skeletal muscle to generate voluntary control (non intrinsic spontaneous activity)

Question 20

what does a single motor neuron innervate?

Answer 20

can innervate up to a thousand muscle fibres at once

Question 21

what is the 1:1 relationship between an AP in a motor neuron and an AP in a muscle fibre?

Answer 21

a single AP in a motor neuron drives a single AP in a muscle fibre and that single AP drives a twitch

Question 22

what is the NMJ?

Answer 22

neuromuscular junction

Question 23

where is the NMJ located?

Answer 23

located on a motor neuron axon on a muscle fibre

Question 24

what is another name for the NMJ?

Answer 24

motor endplate

Question 25

what is the NMJ?

Answer 25

the synapse between a motor neuron axon terminal and a muscle fibre

Question 26

what do AP’s in the motor neuron lead to?

Answer 26

depolarisation of a muscle fibre via the release of Ach

Question 27

what does depolarisation of muscle fibre trigger?

Answer 27

triggers an AP

Question 28

what does the muscle fibre AP trigger?

Answer 28

triggers contraction through excitation-contraction coupling

Question 29

what is the synthesis of acetylcholine?

Answer 29

acetyl CoA + choline –> Ach

catalysed by the enzyme choline acetyltransferase (ChAT)

Question 30

what is the breakdown of acetylcholine?

Answer 30

Ach –> acetic acid + choline

catalysed by the enzyme acetylcholineasterase

Question 31

what is the lifecycle of acteylcholine?

Answer 31

Begins with the synthesis of Ach through the reaction of choline with acetyl coenzyme A. Much of the choline used in this process is recycled but it can also be synthesised in the neuron. This reaction is catalysed by the enzyme choline acetyl transferase.
Once Ach has combined with the receptor site, it must be rapidly removed from the synapse for the repolarisation to occur. It is therefore catalysed by the enzyme acetylcholinesterase to produce choline and acetate.
The choline is then recycled back into the terminal to produce new molecules of Ach