Animal Physiology-Muscle

Question 1

What are the 3 types of muscle called? What do they do and are they under voluntary or involuntary control?

Answer 1

Skeletal muscle:
Movement of skeleton
Voluntary
Striated + multinucleated

Smooth muscle
Muscular component of visceral structures (digestive tract etc.)
Involuntary
Not striated + uninucleated

Cardiac muscle
Provides for the continuous, rhythmic contractility of the heart
Involuntary
Striated + uninucleated

Question 2

What is the ‘origin’ and ‘insertion’ of skeletal muscle?

Answer 2

Origin - The end of the muscle attached to the more stationary part of the skeleton (e.g origin of bicep would be at the shoulder/scapula)

Insertion - The end of the muscle attached to the skeletal part that moves (insertion of bicep is on radius- lower part of arm moves)

Question 3

What is extension and flexion?

Answer 3

Extension is movement that straighrens a joint

Flexion is movement that decreases a joint angle

Question 4

What is ‘isotonic’ and ‘isometric’ contraction?

Answer 4

Isotonic contraction – muscle tension remains constant as the muscle changes length; e.g. Bicep- when the tension in your bicep becomes great enough to lift something up to your mouth

Isometric contraction – the muscle is prevented from shortening, so tension develops at constant muscle length; e.g when you’ve lifted the food up to your mouth and you are holding it there (no longer shortening of muscle, but still working)

Question 5

How is muscle formed from myoblasts?

Answer 5

Myoblasts fuse and form myotubes which then form the muscle fibres

Question 6

What is skeletal muscle surrounded by?

Answer 6

Connective tissue known as epimysium (made up of collagen and elastin fibres that help protect the muscle and also have a rich supply of blood vessels)

Question 7

What are the muscle fascicles (bundles of fibres) surrounded by?

Answer 7

Perimysium (also connective tissue)

Question 8

The structure of the fascicles reflect the function of the tissue. What sort of structure does the muscle responsible for fine, highly controlled movements (e.g external muscles of the eyes) have? What about the structure of larger movements (e.g buttocks muscle)?

Answer 8

Finer movements are controlled by muscles which have small fascicles and a large proportion of perimysial

Larger movements are controlled by muscles which have larger fascicles and relatively little perimysial

Question 9

What gives the myofibrils and overall muscle a striated appearance?

Answer 9

Sarcomeres are regular contractile units which divide up a myofibril. Sarcomeres are easily identified on micrographs as transverse lines (Z-lines) that intersect the myofibril

Question 10

Sarcomeres are divided into thin and thick filaments. What are the thin filaments made up of?

What do the different components of the thin filaments do?

Answer 10

Actin as well as Tropomyosin and Troponin.

The backbone of the thin filament is formed by actin molecules joined into two strands and twisted together. Each actin molecule has a special binding site for attachment with a myosin cross bridge. Binding of actin and myosin molecules at the cross bridge results in energy consuming contraction of the muscle fibre. Actin and Myosin often called contractile proteins – even though, as you will see, actin and myosin do not actually contract. Thin strands of topomyosin wraps around the actin helix and then troponin molecules (which are made from 3 polypeptides) also hold it into place on the binding sites. This blocks myosin from the binding site

Question 11

Troponin is made up of 3 polypeptides. What do each bind to?

Answer 11

Actin, calcium and tropomyosin

• When troponin is not bound to Ca2+ this protein stabilises tropomyosin in its blocking position over actin’s cross bridge binding site
• When Ca2+ binds to troponin, the shape of this protein is changed in such a way that tropomyosin slides away from its blocking position
• With tropomyosin out of the way, actin and myosin can bind and interact at the cross bridges = resulting in contraction

Question 12

What is the thick filament made from?

What is the structure of the molecules?

What is found on each head?

Answer 12

Myosin. A myosin molecule is a protein consisting of two identical subunits, protein’s tails are intertwined around each other. The two halves of each thick filament are mirror images made up of myosin molecules lying lengthwise with their tails orientated toward the centre of the filament and their globular heads protruding outward at regular intervals. These heads form the cross bridges between the thick and thin filaments.

An actin binding site
A myosin ATPase (ATP splitting) site

Question 13

What are the Z-lines, M-line, H-band, A-band and I-band?

Answer 13

Z-lines show a sarcomere and thin filaments are anchored towards them
M-line (middle line) shows where the myosin molecules are anchored towards
H-band is the width of just the myosin
A-band is the width of both the actin and the myosin
I-band is just the actib filaments

Question 14

What happens when an action potential arrives at a muscle cell/ how does a muscle contract?

Answer 14

Under the influence of energy released from ATP – the thick and thin filaments slide over one another causing shortening of the sarcomere;

• Calcium ions are concentrated within the lumen of the sarcoplasmic reticulum
• Depolarisation of the cell membrane (sarcolemma) is rapidly disseminated throughout
• This promotes the release of calcium ions into the sarcoplasm surrounding the myofibrils
• Calcium ions activate the sliding filament mechanism resulting in muscle contraction
• tropomyosin and troponin are pulled out of the way by Ca2+
• The myosin cross bridge from a thick filament can bind with the actin molecules in the surrounding thin filaments
• Myosin heads (or cross bridges) walk along an actin filament to pull it inwards relative to the stationary thick filament
• When myosin and actin make contact at a cross bridge, the bridge changes shape, bending 45⁰ inwards as if it were on a hinge “stroking” toward the middle of the sarcomere, like the stroking of a boat oar

This is the so-called power stroke

Question 15

What happens after an action potential has left?

Answer 15

Calcium ions taken back up, allowing muscle to relax. Tropomyosin binds back onto the actin

Question 16

What happens to the size of the H, I and A bands during contraction?

Answer 16

I band shortens, H band also decreses and A band stays the same

Question 17

How does the action potential generate excitation in each of the myofibrils?

Answer 17

At each junction of an A band and an I band, the surface membrane dips into the muscle fibre to form a transverse tubule (T tubule) which runs perpendicularly from the surface of the muscle cell membrane into the central portions of the muscle fibre

Question 18

What are lateral sacs?

Answer 18

These are structures that hold calcium ions until an action potential arrives at the muscle fibre

Question 19

Can you describe how an action potential works?

Answer 19

• Action potential arrives at axon terminal (neuromuscular junction)
• Triggers opening of Ca2+ channels
• Ca2+ triggers the release of Ach from vesicles
• Ach diffuses across synaptic cleft and binds with receptor channels
• Binding leads to opening of channels - large amounts of -Na+ into the cell small amounts of K+ out
• Results in endplate potential - opens Na+ channels – initiates action potential

Question 20

What are the 3 key areas that require ATP?

Answer 20

1) When the myosin heads need to carry out the ‘stroke’ by bending
2) When myosin heads detach from the actin
3) When calcium ions are actively transported back into the lateral sacs

Question 21

What does ATPase (on myosin head) work? What does it do?

Answer 21

It is an enzymatic site that can bind the energy carrier ATP and split it to ADP and Pi yielding energy in the process. The break down of ATP occurs on the myosin cross bridge before the bridge links with an actin molecule. The ADP and Pi remain tightly bound to the myosin, and the generated energy is stored within the cross bridge to produce a high energy form of myosin. The energy is released when there is contact between actin and myosin, causing the cross bridge bending that produces the power stroke. It is not know how the energy released from ATP is stored within the myosin and then converted to the mechanical energy of the power stroke

Question 22

What are the 3 sources of ATP, in order of immediate to long term stores?

Answer 22

3 biochemical mechanisms that produce ATP

Use of phosphagen creatine phosphate

Anaerobic glycolysis

Aerobic catabolism

Question 23

So how can phosphagen creatine phosphate generate ATP?

Answer 23

Phosphagens temporarily store high energy phosphate bonds. The high energy phosphate of creatine phosphate can be donated to ADP to produce ATP. Creatine phosphate is produced in the resting muscle from creatine and ATP

(we can measure muscle mass by how much creatinine is in the blood)

Question 24

Skeletal muscle (aka twitch) is made up of 3 different types of fibres. Can you name them and describe their levels of mitochondria and myoglobin?

Answer 24

Slow oxidative (red) fibres (aka Type 1)- high levels of mitochondria and myoglobin
Fast oxidative glycolytic (pink) fibres (Type 2b)- intermediate levels of mitochondria and myoglobin
Fast glycolytic (white) fibres (Type 2a)- low levels of mitochondria and myoglobin

(tells us speed of contraction and whether oxygen is needed for contraction)

Question 25

Which twitch/skeletal muscle types are adapted for the following?

1) Adapted for isometric postural functions and for small, slow movements,Do not generate much tension, Operate efficiently and without fatigue?
2) Adapted for repeated movement such as locomotion,Generate more tension and faster contraction, Fatigue resistant?
3) Adapted for forceful fast movements such as leaps or bursts of speed, Generates large tension and rapid contractions but lack endurance, Fatigue rapidly?

Answer 25

1) Slow oxidative
2) Fast oxidative (2b)
3) Fast glycolytic (2a)

Question 26

Using calf muscles as an example, explain how all the skeletal muscles may work together.

Remember- Trade off between speed and energy costs
Very fast muscles require large amounts of ATP
Slow muscles perform less rapidly but require less energy

Answer 26

Soleus – contracts slowly and made up of only slow oxidative fibres
It is most active in postural standing

The medial and lateral gastrocnemii are faster muscles of mixed fibre composition
45% fast glycolytic, 25% fast oxidative and 25% slow oxidative

Fast oxidative glycolytic and slow oxidative fibres produce tension for walking and running

Fast glycolytic fibres are reserved for short burst of actions - jumping

Question 27

Imagining you are a drop of blood, what happens after you enter the right atrium?

Answer 27

Pumped through the atrioventricular valve into the right ventricle then out of the aortic valve into the pulmonary valve to the lungs. blood then returns back to the heart into the left atria then ventricle then out the aorta to the rest of the body

Question 28

How does cardiac muscle (which is striated like skeletal muscle) differ from skeletal muscle?

Answer 28

uninucleated and have interclated discs made up of gap junctions for action potential travel and desmosomes for mechanical strength between cells.

Question 29

What do pacemaker cells do in the heart?

Answer 29

Fires an action potential at a constant rate. A heart beat consists of a rhythmic contraction (systole) and relaxation (diastole) of the whole cardiac muscle mass. An action potential is initiated in the pacemaker region (sinoatrial node which is in the right atrium) and spreads over the heart from one cell to another.

Question 30

What 3 types of filaments does smooth muscle have?

Are there any differences in structure to cardiac and skeletal muscle?

Answer 30

myosin, actin and tropomyosin – not organised into sarcomeres

Also no t-tubules but they do have a sarcoplasmic reticulum. In addition, smooth muscle does not contain troponin and are uninucleated
Organised in bundles – contraction fibre changes from a spindle shape to a globular shape
ATPase activity is much slower than in skeletal muscle
Contract slowly for long periods of time

Question 31

As smooth muscle does not have troponin, what blocks myosin from binding with actin in the absense of an action potential?

Answer 31

Myosin light chain which blocks site on myosin head not actin. Calcium binds to calcium calmodulin which activates myosin light chain kinase enzyme which phosphorylates and remove

Question 32

What can smooth muscle be divided into?

Answer 32

Phasic smooth muscle
Contracts in bursts, triggered by action potentials

Tonic smooth muscle
Is usually partially contracted at all times

Multi-unit smooth muscle
Discrete units that function independently from each other
Single-unit
Contracts as a single unit