Muscle Microstructure

Question 1

Three types of muscle

Answer 1

Smooth muscle
Cardiac muscle
Skeletal muscle

Question 2

Smooth muscle function

Answer 2

Smooth muscle is under involuntary control, from the autonomic nervous system

Question 3

Cardiac muscle

Answer 3

Contract autonomously , under the influence of the autonomic nervous system and circulating chemicals

Question 4

Skeletal muscles

Answer 4

Voluntary control , usually attached to bones and contract to bring about movement

Question 5

Skeletal muscle function

Answer 5

Skeletal muscles are under voluntary control from the somatic nervous system
They are usually attached to the bones
They contract to bring about movement

Question 6

Arrange,ent of muscle fibres

Answer 6

Parallel
Fusiform
Triangular

Pennate - unipennate, Bipennate and multipennate

Question 7

What is a fascicles

Answer 7

Bundles of muscle fibres ( myofibres

Question 8

Sarcolemma and T tubules

Answer 8

Covered by plasma membrane - sarcolemma

T tubule tunnels into centre and this has many ion channels and so the whole area of the muscle can be excited

Question 9

Sarcoplasm

Answer 9

Myoglobin and mitochondria present

Question 10

Sarcoplasmic reticulum

Answer 10

Network of fluid filled tubules

Question 11

Structure of myofibrils

Answer 11

1-2 um in diameter
Extend along the entire length of the myofibres
Composed of two main types of protein - actin and myosin

Question 12

What gives muscles the striated appearance

Answer 12

Light and dark bands
However the actin and myosin do not extend along length of myofibres
They overlap and are arranged in compartments called sarcolemma

Question 13

Dark bands

Answer 13

A band and are composed of myosin

Question 14

Light bands

Answer 14

I band and are composed of actin ( thin)

Question 15

What separates the sarcomeres

Answer 15

Z discs

Question 16

What moves to cause the muscle contraction

Answer 16

The binding of myosin filaments to the actin heads causes the muscle contraction and force generation

Question 17

Structure of myosin

Answer 17

Two globular heads
Single tail formed by two alpha helices
Tails of several hundred molecules form one filament

Question 18

Structure of actin

Answer 18

Actin molecules twist into helix
Each molecule has a myosin binding site
Filaments also contain troponin and tropomyosin

Question 19

Sliding filament theory of muscle contraction

Answer 19

During contraction the I band becomes shorter
A band remains the same length
H zone narrows or disappears

Question 20

Myasthenia Graves

Answer 20

Where the antibodies attack the ACh receptor , this residual ACh can be prolonged and so remains at the receptor

Question 21

Initiation of muscle contraction

Answer 21

Action potential along surface membrane and T tubule

Dihydropridine ( DHP) receptor in T tubule membrane senses change in voltage and changes shape of the protein linked to the ryanodine receptor

This opens ryanodine receptor calcium channel into sarcoplasmic reticulum

Calcium released from ST into space around filaments

Question 22

What happens in the initiation of muscle contraction after the calcium ions have been released from the SR into the space around the filaments

Answer 22

Calcium binds to troponin and tropomyosin moves which allows cross ridges to attach to actin

Calcium ions are actively transported into the SR continuously while AP continue .

ATP driven pump and so uptake rate =/< release rate

Question 23

Excitation contraction coupling

Answer 23

In the presence of calcium ions movement of troponin from tropomyosin chain
Movement exposes myosin binding site on the surface of the actin chain
Charged myosin heads bind to exposed site on actin filament
This binding and discharge of ADP causes myosin head to pivot ( the power stroke) - pulls actin filament towards the centre of the sarcomeres
ATP binding - releases myosin head from actin chain
ATP hydrolysis - provides energy to recharge myosin head

Question 24

What is a motor unit

Answer 24

Name given to a single motor neuron together with all the muscle fibres that it innervates . Stimulation of one motor unit causes contraction of all muscle fibres in that unit

Question 25

How can fine control be maintained in terms of motor units

Answer 25

Having only one nerve innervate has

Question 26

Three types of motor units

Answer 26

``` Slow, ( S type 1) Fast fatigue resistant( FR, type IIA) Fast fatiguable ( FF , type II B) ```

Question 27

Which muscle contains the greates proportion of slow twitch type muscle fibres

Answer 27

Muscles with postural muscle control tend to have a greater proportion of slow twitch muscle fibres

Question 28

What are motor unit types classified by

Answer 28

Amount of tension generates Speed of contraction Fatiguability of the motor unit

Question 29

Type IIB

Answer 29

Fast twitch High force High fatigue

Question 30

Type IIa

Answer 30

Fast twitch Moderate force Fatigue resistant

Question 31

Type I

Answer 31

Slow twitch Low force Fatigue resistant

Question 32

Regulation of muscle force

Answer 32

Recruitment - there is an order in which muscle fibres are recruited . ‘ size principle’ . As more force is required , more units are recruited. This allows fine control ( e.g. when writing ) where low force levels are required Rate coding - a motor unit can fire at a range of frequencies , slow units fire at a lower frequency. As the firing rate increases, the force produced by the unit increased.

Question 33

What are neurotrophic factors

Answer 33

Are types of growth factors Prevent neuronal death Promote growth of neurones after injury

Question 34

Effect of neurotrophic factors

Answer 34

Motor units and fibre characteristics are dependent on the nerve which innervates them If a fast and slow twitch muscle are cross innervates , the slow one becomes fast and vice versa The motor neurone has some effect on the properties of the muscle fibres it innervates

Question 35

Three types of muscle contraction

Answer 35

Isometric Concentric Eccentric

Question 36

Isometric

Answer 36

Produces force but doesn’t change length

Question 37

Concentric

Answer 37

Shortens muscle to produce movement

Question 38

Eccentric

Answer 38

Contraction in which muscle is producing fforce but muscle is getting longer - dumbbell which is heavier than the person can handle and so the muscle is still contracting but overall the movement is outwards. This is when MUSCLE DAMAGE can occur and the muscle can get torn as it’s too heavy

Question 39

What is the most common change in fibre type following training

Answer 39

Change from type IIB ( fatiguable) to IIA ( less fatiguable) So from fast fatiguable to fatigue resistant

Question 40

Microgravity and changing of muscle fibre types

Answer 40

Type 1 to II possible in cases of severe de conditioning or spinal cord injury. Microgravity during space flight results in shift from slow to fast muscle fibre types

Question 41

Ageing and change in muscle fibre types

Answer 41

Ageing associated with loss of type 1 and II fibres but also preferential loss of type II fibres. This results in a larger proportion of type I fibres in aged muscle ( evidence from slower contraction times )