Muscle Microstructure and Contraction

Question 1

What are the 3 main types of muscle?

What type of control are they under, where are they found… etc.?

Answer 1

1. Smooth - involuntary control from ANS, usually found around tube structures e.g. airways, blood vessels etc.
2. Cardiac - confined to the heart, under the control of the ANS and circulating chemicals, striated (striped) muscle
3. Skeletal - voluntary control (SNS), usually attached to bones, contract to bring about movement; striated (striped) muscle

Question 2

How are muscle fibres arranged? And why?

Answer 2

Arranged in different ways with respect to the tendons and bone to influence their function - different arrangements allow for different tensions to form in the muscle and pull in different directions

e.g. parallel = all fibres parallel to each other

fusiform = larger bulge in the middle

pennate = offset at an angle

Question 3

What are fasicles in muscles?

What are muscle fibres / myofibres?

What are myofibrils?

What are myofilaments?

Can you illustrate these in a diagram?

Answer 3

Fasicles = bundles of muscle fibres / myofibres

Myofibres are made of bundles of myofibrils

Myofibrils are made up of muscle filaments

Individual units (myosin and actin)

Question 4

What surrounds the muscles and what are it’s properties and roles?

What is compartment syndrome?

What is the name given to the layer that surrounds muscles?

What is the name given to the layer that surrounds muscle fascicles?

What is the name given to the layer that surrounds muscle fibres?

Answer 4

Collagen based connective tissue which creates compartments - tough / rigid, which doesn’t allow for expansion hence keeps the body in shape

When there is an increase in pressure in the muscle compartment e.g. from bleeding, causing increased pressure on the tissues, resulting in ischemia and necrosis of the tissue (extremely painful)

Epimysium

Perimysium

Endomysium

Question 5

What is the name given to cells from which skeletal muscle fibres arise?

What surrounds the myofibres?

What is found under the sarcolemma?

Answer 5

Myoblasts

Sarcolemma - muscle plasma membrane

T-tubules

Question 6

What are myofibrils? What are the 2 main proteins that are made up of?

What are sarcomeres?

What are the different bands / lines and which protein(s) are they made up of? Are they light or dark?

Answer 6

Thick (myosin) filaments and thin (actin) filaments

Sarcomere - distance between 2 Z lines

Dark = myosin = A band

Light = actin = I band

Question 7

Why are there dark and light bands in the myofibrils?

What gives muscles it striated (striped) appearance?

Answer 7

The different proteins reflect light to different extents - under the light microscopic, they polarised light different

The light and dark bands give the muscle it’s striated appearance

Question 8

How did the sliding filament theory come about?

Answer 8

Under a light microscope, when muscle contracted, certain light or dark bands disappeared

Question 9

What is the structure of myosin?

Answer 9

Has 2 globular heads, and a tail formed by two alpha-helices

All the heads tend to point / face the same direction, and same with their tails

Question 10

What is the structure of actin?

Answer 10

Made of actin, but also contains 2 filaments - troponin complex and tropomyosin

It has a myosin head binding site on the tropomyosin that is normally covered by the troponin complex

Question 11

What is the sliding filament theory?

Answer 11

Question 12

Which bands shorten, disappear and stay the same when the muscle contracts (AKA shortens)?

Answer 12

I band shortens

A band remains same length

H zone narrows or disappears

Sarcomere overall shortens

Question 13

How does is the signal sent by the lower motor neuron to the the NMJ?

What is the simulation that occurs at the NMJ?

Answer 13

Once the action potentials stop, the acetylecholine is broken down by acetylcholine esterase to stop muscle contraction

Question 14

What are the activation steps to muscle contraction / the sliding filament theory? (more detail)

Answer 14

1. Action potential propagates along surface membrane into the T-tubules
2. DHP (dihydropyridine) receptors in the t-tubule membranes sense the change in voltage and causes a conformational change to a protein linked to the ryanodine receptor
3. The ryanodine receptor is coupled with a Ca²⁺ channel, hence the change of shape in the protein causes the Ca²⁺ store in the sarcoplasmic reticulum to travel down the concentration gradient into the sarcoplasm
4. This allows for the rest of the sliding filament theory to take place

Question 15

What is rigor mortis?

Answer 15

A state after death, where ATP stores are depleted, therefore due to lack of ATP, myosin heads are unable to detach from the binding sites on the actin filaments

Question 16

What type of neuron attaches to skeletal muscle? Where does this neuron come from?

What type of neuron comes from the brain?

Answer 16

Lower motor neuron - comes from the brain stem / spinal cord, forms the final common pathway from the CNS to the muscle

Upper motor neuron - comes from the primary motor cortex (sits in front of the central gyrus)

Question 17

How does innervation to the muscle fibres change as we grow from foetuses to adults?

How many muscle fibres are supplied by a single motor neuron?

Can muscle fibres be innervated by more than one motor neuron?

What is a motor unit?

Answer 17

During development - lots of muscle fibres are innervated by lots of nerves. One neuron wins out of them to innervate the collection of muscle fibres

600 muscle fibres

No - muscle fibres are only innervated by one motor neuron

The one motor neuron and all the muscle fibres it innervates = unit (smallest functional unit of muscle contraction)

Question 18

Why is there motor unit variability?

Answer 18

Muscles that require greater dexterity control = innervation ratio of fewer muscle fibres by a single motor neuron (e.g. hands)

Muscles responsible for simple movements / power = innervation ratio of greater muscle fibres by a single motor neuron

Question 19

What are the 3 types of motor units?

Answer 19

S (Type I) - Slow = smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity = usually for postural control

FR (Type IIA) - Fast, Fatigue resistant = larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduciton velocity = usually contract when called upon

FF (Type IIB) - Fast, Fatiguable = larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity = usually contract when called upon

Question 20

Fill in table:

Muscle tissue looks different colours depending on type of motor unit:

Answer 20

Greater myoglobin content = redder appearance

Slowly contracting = use continuous supply of oxygen

Fast muscles (fast contracting) = require on reserves to carry out respiration anaerobically

Question 21

Properties of motor units:

What are the 3 things to consider to classify the different motor unit types?

Answer 21

Amount of tension generated, speed of contraction, and fatiguability of the motor unit

Slow = takes a long time to generate its tension fully, low force, fatigue resistant

Fast = generates more power and more quickly but over more quickly

Fast a = moderate force, fatigue resistant, Fast b = high force, high fatigue

Question 22

What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?

What is the size principle?

What is summation?

Answer 22

1. Recruitment - gets more muscle fibres on board via more neurons = increases strength of contraction; uses the size principle so smaller units are recruited first = allows for fine control
2. Rate coding - send more impulses / change frequency of impulses arriving at the muscles = force builds up; slow units fire at a lower frequency

Slow units recruited first, then fatigue resistant, then fatiguable ones

Summation occurs when the units fire at a frequency that allows no time for the muscles to relax between the arriving action potentials (which are causing the contractions)

Question 23

This is a graph to show recuitment, explain how this supports the recruitment of muscle fibres?

Answer 23

Top graph read from left to right, bottom graph read from bottom to top

First motor unit is always firing, second motor unit begins to fire when the muscle reaches a certain percentage of contraction… etc.

The 5th motor unit comes on when the muscle has reached peak contraction, but also comes off quickly once the peak contraction has passed

Question 24

This is a graph to show rate coding, explain how this graph supports the rate coding of muscles?

Answer 24

Y axis is measuring the firing rate, X axis is measuring the amount of force being produce

At low force, the slow units begin to fire (i.e. at 8 hz); as the voluntary force increases, the slow units begin to fire more frequently and the fast fatigue resistant units begin to fire; then when the force is at its greatest, the slow, fast fatigue resistant and fast fatiguable all fire at their maximum frequency

Question 25

What happens if the nerve supply to the muscle fibres has been cut?

Neurons do more than just send impulses to the muscle fibres, what else does it do?

Answer 25

Muscular atrophy (muscle wastage)

The nerve must do more than just send signals to the muscle fibres if cutting off the nerve supply causes muscle wastage. So the nerve also has influence on the way the muscle behaves (i.e. slow, fast) - not just amount of myoglobin etc. as those things are changeable

Question 26

What was the outline of the experiment conducted that showed nerve supply to muscles also have influence over the behaviour of the muscle (i.e. slow or fast)?

Answer 26

Surgical procedure involving soleus (slow), flexor digitorium longus (fast), and flexor hallucis longus (fast)

The nerves innervating the soleus and FDL were switched, and the nerve innervating the FHL was kept the same (control)

Noticed the FDL started acting more like a slow twitch fibre, and soleus started acting more like a fast twitch fibre

Question 27

What are the 3 types of muscle contraction?

Answer 27

Concentric - active movement inwards

Eccentric - active movement backwards, lengthening contraction (muscle gets longer as force is generated)

Isometric - no movement, kept in the same place (muscle stays the same length but there is force generated)

Question 28

Can muscle fibre types be changed from one type to another?

Ageing is associated with which muscle fibre loss?

Answer 28

Change between fast types (i.e. between Type IIa and Type IIb) - with training

But cannot change between slow and fast types (unless in space or something where there is no gravity = no postural muscle use)

Ageing associated with loss of both types of fibres, but preferentially with Type II fibres (which is why ageing people have slower contraction times)