Lecture 13 - Muscles (1)

Question 1

What are the 2 different kinds of muscle and any subdivisions they may have?

Answer 1

Striated Muscle (Skeletal + Cardiac)
Non-Striated Muscle (Smooth)

Question 2

What are the main differences between skeletal, cardiac and smooth muscle?

Answer 2

Skeletal: myoglobin present, voluntary control, direct nerve-muscle communication.
Cardiac: myoglobin present, involuntary control, indirect nerve-muscle communication
Smooth: myoglobin absent, involuntary control, no direct nerve-muscle communication.

Question 3

What is myoglobin & its function?
When does Hb give up oxygen to myoglobin?
What occurs during muscle necrosis regarding myoglobin? + What can this cause?

Answer 3

• Myoglobin is a red protein structurally similar to 1 sub-unit of Hb, and provides oxygen to striated muscle.
• Hb gives up oxygen to myoglobin, particularly at low pH
• Myoglobin is released after muscle necrosis, leads to myoglobinaemia + myoglobinuria (dark brown/tea coloured urine)
• This can lead to renal damage.

Question 4

Describe the basic structure of skeletal muscle.

Answer 4

• Individual muscle fibres (muscle cell is called a fibre) join to form a fascicle
• A number of fascicles together make up the muscle
• Around each cell layer of muscle fibres is a sheath of connective tissue called the endomysium
• Around each fascicle is layer of loose connective tissue called the perimysium
• Around the entire muscle is the thicker dense connective tissue called the epimysium.

Question 5

Which way does movement of muscle occur along?

Where is tension and movement created?

Answer 5

• Movement is always along the direction of a fibre
• Tension is created at origin tendon point (attachment of a muscle on a bone which is stable/doesn’t move)
• Movement is created at insertion tendon point (attachment on the muscle of a moveable bone)

Question 6

How can you recognise skeletal muscle in a transverse section and longitudinal section (on histological slide)?

Answer 6

• In TS there is peripheral nuclei
• In LS there are row of nuclei
• Each fascicle is surrounded by a perimysium layer
• Can see the individual fascicles surrounded by perimysium

Question 7

What is the singular contractile unit of skeletal muscle?

Describe its structure.

Answer 7

• A sarcomere
• Dark bands = A band, Light band = I band. I band - where Z-disc joins thin fibres.
• Z-discs = ends of sarcomere.
• A-band is where actin + myosin overlap, but not within the H-zone where the M-line resides.
• Thin filaments = originate at the Z-band, form the I-band and pass between ends of thick filaments in A-band.
• Thick filaments = contracted within the A-band.

Question 8

What are the 3 types of muscle fibre?

Answer 8

1) Slow-twitch (Type 1)
2) Fast-twitch (Type 2A + 2B)

Type 1 = red
Type 2A = red/pink
Type 2B = white

Question 9

Describe the main differences between type 1, 2A & 2B fibres in terms of: respiration, myoglobin, cytochromes, fatigue resistance and function

Answer 9

Type 1 (red) = Aerobic, Rich capillary supply, High myoglobin, High cytochromes, High resistance to fatigue, Endurance activities, e.g.: running.
Type 2A (red/pink) = Aerobic, Rich capillary supply, High myoglobin, High cytochromes, Moderate resistance to fatigue, Assist both type 1 and 2B fibres.
Type 2B (white) = Anaerobic, Poor capillary supply, Low myoglobin, Few cytochromes, Low resistance to fatigue, Strength/Anaerobic activities (sprinting, weightlifting)

Question 10

What is the fuel source of type 1 and type 2 fibres = which one can produce more ATP?

Answer 10

Type 1 = Glucose - 36 mol ATP (per glucose) via oxidative PP - lots of ATP/CO2 from glucose.
Type 2B = Glycogen - 2 mol ATP (per glycogen) via Anaerobic glycolysis - produces lots of lactate from glycogen

Question 11

How can you distinguish cardiac muscle cell fibres on a histological slide?

Answer 11

• Striated muscle
• 1 or 2 centrally positioned nuclei
• Intercalated discs
• ANP granules on TEM picture.

Question 12

Where is ANP and BNP released and what are there effects?

Answer 12

ANP = atria, BNP = ventricles - reduce blood pressure via effects on the RAA system. (reducing GFR, blood volume and thus pressure) - released in response to HF.

Question 13

What are the 2 ways in which tissues (including muscle) can increase their size?

Answer 13

1) Hyperplasia - multiplication of cell number

2) Hypertrophy - enlargement of individual cells

Question 14

Give a brief summary of the electrical activity of the heart (path of action potential) in each beat.

Answer 14

1) SAN in right atrium fires AP - causes atria to contract and blood is pushed into ventricles.
2) AP passes round septum and reaches AVN (small delay to allow final filling of ventricles)
3) AP goes down bundle of his and into left and right divisions
4) Reaches apex of heart and meets purkinje fibres where AP fires up towards base of heart.

Question 15

Purkinje fibres conduct AP’s rapidly - they have large cells with what in abundance?
What does this allow for?

Answer 15

• Glycogen
• Gap junction sites
• NB - have sparse myofibrils.
• Allows for rapid synchronous contraction of the ventricles.

Question 16

Name the main similarities and differences between cardiac and skeletal muscle.

Answer 16

Similarities:
- Both striated muscles
- Contraction mechanisms similar
Differences:
- Nuclei in cardiac muscle central - not peripheral
- Sarcomere not so developed
- No T-tubules in SR
- Contractile cell type is cardiomyocyte

Question 17

Describe the structure and contraction mechanism of smooth muscle.

Answer 17

• Spindle (fusiform) shape with single central nucleus
• Not striated - no sarcomeres or t-tubules
• Contraction still relies on actin & myosin overlap but is slower, more sustained and requires less ATP (can last for hours to days)
• Forms sheets, bundles or layers containing thousands of cells.

Question 18

What do smooth muscles often form?
What kind of clinical disorders are they important in?
Why can smooth muscle contract in all directions?

Answer 18

• Contractile walls of passageways or cavities (e.g.: gut, respiratory tract etc)
• They are involuntary so important in disorders such as hypertension, painful menstruation, bronchospasm (asthma), IBS and incontinence.
• Due to the criss-cross pattern in which the cells come together.

Question 19

How are smooth muscle cells innervated?

Answer 19

• Via the ANS fibres that release NT’s into the from varicosities into a wide synaptic cleft. They have many varicosities which allow for maximum release of NT to make sure all cells can fire together.

Question 20

Skeletal, cardiac and smooth muscle tissue can all tear - which ones have repair mechanisms and how do they work?

Answer 20

Skeletal = cannot divide but regenerate by mitotic activity of satellite cells (hyperplasia). Satellite cells can also fuse with existing muscle cells to increase mass (hypertrophy).

Cardiac = cannot regenerate, after damage fibroblasts invade and lay down scar tissue.

Smooth = retain their mitotic activity and form new cells (hyperplasia) - e.g.: in pregnant uterine wall, thickening of myometrium via hyperplasia and hypertrophy.