Muscles

Question 1

Identify the different levels of skeletal muscle structure, being able to name structures and tissues from macroscopic to microscopic.

Answer 1

Muscle body
Fascicles (bundles of fibres)
Muscle fibre (cell)
Myofibrils
Protein filaments

Question 2

What is the neuromuscular junction?

Answer 2

Motor neuron forms presynaptic terminal, which contains acetylcholine
Presynaptic neuron is put up against muscle cell membrane, forming motor end plates which contains the Acetylcholine receptor channel - causes the flow of sodium ions when acetylcholine binds. Also contains voltage gated sodium channels - involved in conducting action potential, also found in muscle cell membrane - involved in excitation.

Question 3

Explain how muscle contraction is initiated at the neuromuscular junction

Answer 3

Action potential triggers the movement of vesicles to the presynaptic membrane, they fuse with the membrane - release of acetylcholine.
Some acetylcholine will bind to nicotinic ACh receptors, causing them to open, causing Na to move down concentration gradient into muscle cell - causing localised depolarisation.
If depolarisation is big enough, voltage gated Na channels will be activated, causing more influx of sodium which will be conducted along the muscle cell membrane.

Question 4

Skeletal muscle fibres features

Answer 4

Also known as muscle cells or myocytes
Very long cell (runs full length of the muscle)
Innervated by a motor neuron
Sarcolemma - plasma membrane of a muscle cell
Hundreds of myofibrils
Lots of sarcoplasmic reticulum and mitochondria
Multiple nuclei, located just inside the sarcolemma in healthy cells
Striated appearance

Question 5

What is a sarcomere? (also need to be able to draw)

Answer 5

Myofibrils are made up of sarcomeres

Smallest functional unit of contraction

Question 6

What are the key proteins of sarcomeres?

Answer 6

Contains two key contractile proteins
Actin (thin filament)
Myosin (thick filament)
Interaction of proteins generates muscle force

Question 7

Describe the sliding filament hypothesis of skeletal muscle contraction

Answer 7

Actin molecules slide over myosin molecules to cause contraction
When the muscle is contracted:
Z lines move closer
I band gets thinner
A band doesn’t change
H zone gets thinner

Question 8

Ways muscle cells have adapted to deal with stresses of contraction? - Contractile/structural protein - titin

Answer 8

Extends from the Z line to the thick filament
Keeps thick and thin filaments in alignment
Molecular spring
Restores optimal sarcomere length after contraction or stretching

Question 9

Appreciate ways muscle cells have adapted to deal with the stresses of contraction, using Duchenne’s Muscular Dystrophy as an example of a failure of one of these adaptations

Answer 9

Structural proteins - protect muscle cell from being damaged during contraction, involved in stabilising the sarcolemma - Dystrophin and the Dystrophin Associated Complex - responsible for muscular dystrophies.
Duchenne’s - genetic mutation leads to loss of functional dystrophin, sarcolemma not as stable - easily damaged. Repeated damage leads to degeneration of muscle fibre - muscles atrophy. Die in 20’s due to cardiac/respiratory failure.

Question 10

Understand the importance of connective tissues in transmitting force

Answer 10

Help maintain a regular structure - essential for effective transmission of force
Elastic - prevent damage due to over extension, like a bungee cord

Question 11

Sarcoplasmic Reticulum (SR)

Answer 11

Myofibrils are surrounded by a network of SR
The SR is an essential store of Ca2+
Ca2+ is a tightly controlled signalling molecule in the cytoplasm of cells
In muscle cells it initiates contraction
A healthy, resting muscle cell has a cytoplasmic Ca2 concentration in the nanomolar range
The SR has a Ca2+ concentration in the millimolar range compared to muscle cell concentration in nanomolar range = big driving force

Question 12

Describe the function of the T-tubular system in relation to the sarcoplasmic reticulum and explain how the process of excitation contraction coupling is thought to occur in skeletal muscle. (part 1)

Answer 12

T-tubules are continuous with the sarcolemma and go down into the cell interior
Near the T-tubule, the SR has enlargements called lateral sacs or terminal cisternae that store calcium
Each T-tubule is associated with two lateral sacs forming a triad - areas that specialise in detecting the depolarisation of the muscle cell membrane and cause the release of calcium

Question 13

Describe the function of the T-tubular system in relation to the sarcoplasmic reticulum and explain how the process of excitation contraction coupling is thought to occur in skeletal muscle. (part 2)

Answer 13

After contraction is initiated in the neuromuscular junctions and travels along the cell membrane, depolarisation spreads down into T-tubules and interacts with sarcoplasmic reticulum, causing calcium release.

The membrane of the T-tubule and the SR are physically linked by a complex of two proteins
The dihydropyridine receptor (DHP)
The ryanodine receptor - a calcium channel.

When the AP travels down the T-tubule, the voltage sensitive DHP receptor changes conformation, allows the ryanodine receptor to open
Ca2+ ions are released into the cell.

Question 14

Closer look a thin filaments

Answer 14

Is an assembly of actin molecules and regulatory proteins
Tropomyosin - blocks the myosin binding site
Troponin complex - calcium sensor
When Ca2+ binds to the troponin complex it causes a conformational change
This moves the tropomyosin protein, exposing the myosin binding sites
A cross-bridge between actin and myosin can the form, which is the basis for the generation of force

Question 15

Closer look at thick filaments

Answer 15

Composed of hundreds of myosin molecules
Each myosin molecule is a dimer, with a twisted tail and two heads
Each head has:
An actin binding site (forms the cross bridge)
An ATPase site (generates the energy for movement)
Tails of myosin bind together, and adjacent molecules are staggered to form a helical pattern of heads in the thick filament

Question 16

Draw and describe the cross bridge cycle and the role it plays in muscle movement. Explain how the cross bridge cycle is initiated and terminated indicating the role of calcium in this.

Answer 16

1. Binding of myosin to actin
   Ca2+ concentration rises
   Binds to troponin complex
   Tropomyosin moves to expose the myosin binding sites
   → inorganic phosphate released
2. Power stroke (movement of actin filament)
   Actin gets pulled toward middle of sarcomere
   High energy to low energy conformation of myosin head
   → ADP is released
3. Rigor (myosin in low-energy form)
   Myosin head can’t unbind till ATP binds
   → new ATP binds to myosin head
4. Unbinding of myosin and actin
   → ATP is hydrolysed
5. Cocking of the myosin head (myosin in high energy form)
   → back to step 1

To stop the cross bridge cycle and allow muscle relaxation to occur calcium is removed - Tropomyosin would go back to blocking the myosin binding sites on the actin filament

Question 17

Using both lists and diagrams be able to catalogue the steps involved in skeletal muscle contraction and relaxation.

Answer 17

Muscle relaxation
1. Motor neuron stops firing
2. Muscle fibre membrane potential returns to resting levels
3. Voltage-sensitive DHP receptor returns to resting conformation
4. This blocks the ryanodine-sensitive calcium channel SR, stopping Ca2+ release
Ca2+ is actively pumped back into the SR by SERCA (Sarcoplasmic Endoplasmic Reticulum Calcium ATPase)
Without Ca2+, the troponin complex returns to its resting conformation, allowing tropomyosin to block the actin binding sites
As no more cross bridges are formed, the sarcomeres can passively return to their resting positions

Question 18

Explain isometric and isotonic contractions

Answer 18

Isometric - Load is greater than the force generated, muscle length doesn’t change but force is generated

Isotonic - Muscle length changes
Concentric contraction - Muscle shortens while force is produced
Eccentric contraction - Muscle lengthens while force is produced

Question 19

Be able to explain the mechanisms which control force output from skeletal muscle - more detail about each factor in notes

Answer 19

The number of cross bridges that can form directly affects force output - affected by three factors.
Frequency of stimulation (higher frequency = greater force output).
Changes in fibre length(if sarcomere is overstretched or compressed less force will be developed)
Fibre diameter (larger diameter = greater force)

Question 20

Explain the concept of fibre types and described what types of fibres are found in mammalian skeletal muscle. Describe how fibre types influence contraction.

Answer 20

Fast vs slow twitch
Key differences:
How quickly they contract and relax
How they generate their ATP
Rate of fatigue

Question 21

Features of smooth muscle

Answer 21

Tend to be long and slender (spindle shaped)
Ranging from 5-10µm in diameter and 30-200µm in length
Have a single, centrally located nucleus
No T-tubules and SR forms a loose network throughout the sarcoplasm
No myofibrils or sarcomeres
Hence the muscle doesn’t appear striated
But contraction still involved the thick and thin filaments interacting through the cross bridge cycle
Linked to each other by gap junctions
Permits the synchronous contraction of smooth muscle sheets
Can pass electrical excitation between cells in network
Insert image
Surrounded by the basal lamina
Layer of connective tissue which assists with force transmission between cells
Similar role to endomycium in skeletal muscle

Question 22

Dense bodies and intermediate filaments

smooth muscle

Answer 22

Attachment point for actin filaments
Equivalent of z-line in striated muscle
Transmit force to the exterior of the cell
Dense bodies are connected by a network of intermediate filaments
These are made by the protein desmin

Question 23

Contractile proteins in smooth muscle

Answer 23

Thin filament (actin)
Does not contain the calcium sensing troponin complex
So myosin binding are always available
Thick filament (myosin)
Interspersed among actin filaments (so lacks regular cross sectional structure of striated muscle)
“Side polar” cross bridges
Myosin heads can hinge in opposite directions on different sides of the filament
Allows myosin to pull actin in one direction on one side, while pulling in the opposite direction on the other
The myosin head needs to be phosphorylated before it can bind to actin
Smooth muscle contains Type II myosin
The myosin head is made up of two light chains and two heavy chains
The light chain must be phosphorylated to activate ATPase activity and expose the actin binding site
This is how Myosin Light Chain Kinase (MLCK) regulates smooth muscle activation
Key difference to skeletal muscle - calcium is targeting the thick filament rather than the thin filament for contraction

Question 24

Sliding of filaments in smooth muscle

Answer 24

Myosin heads orientated in “side polar” arrangement
Contraction pulls dense bodies together
Contraction is slow and sustained

Question 25

To understand the mechanism of smooth muscle contraction and explain how it is regulated.

Excitation-Contraction (E-C) coupling in smooth muscle

Answer 25

1. Intracellular Ca2+ concentration increases
   Ca2+ channels in the sarcolemma
   Ca2+ induces calcium release from the sarcoplasmic reticulum
2. Ca2+ binds to calmodulin
   Triggers a conformational change
3. Ca2+-Calmodulin complex to activate the enzyme myosin light chain kinase (MLCK)
4. MLCK phosphorylates the myosin heads
   Increase myosin ATPase activity
   Exposes actin binding site
5. The cross bridge cycle is initiated

Question 26

Why is Excitation-Contraction coupling slower in smooth muscle compared to skeletal muscle?

Answer 26

Takes longer for Ca2+ levels to peak
Relying more on extracellular Ca2+ influx
Less sarcoplasmic reticulum Ca2+ release
Takes longer for relaxation to occur
Requires more than just Ca2+ re-uptake
Phosphorylation must be removed by phosphatase enzymes
Myosin ATPase activity is slower
10-100 times slower so cross bridge cycling takes longer
Also mean that less energy is needed to maintain a contraction

Question 27

Innervation of smooth muscle

Answer 27

Nervous control of smooth muscles is via automatic neurons
There is no neuromuscular junction
No specialised motor end plate
Neurotransmitter is still released
Autonomic nerves have varicosities
Swellings where neurotransmitter is released
Many of these along axon length
Sympathetic or parasympathetic may cause contraction or relaxation
Depends on the receptor expressed and its effect on Ca2+

Question 28

Activation of smooth muscle

Answer 28

Smooth muscle contraction or relaxation doesn’t need to be directly stimulated by neural activity
Also activated by hormones or stretch
E.g. peristalsis
Series of wave like contractions which move food throughout the gut
Stretch activated mechano-gated Ca2+ channels in the membrane
Whether being directly activated by a neuron or not, response of the cell is determined by the receptors expressed on the sarcolemma and how this modulated intracellular Ca2+ levels

Question 29

Multi-unit vs single unit smooth muscle

Answer 29

Multi-unit
Each cell can contract independently and is innervated by a single neuron
E.g. iris muscle of eye, piloerector muscles for “hair raising” effect
Unitary/single unit
Usually arranged in bundles or sheets with hundreds of cell contracting simultaneously
Adhere at multiple points and contain many gap junctions
Mostly non-nervous stimuli initiate contraction (e.g. stretch hormones)
Also referred to as visceral smooth muscle, e.g. GI tract and blood vessels

Question 30

Features of cardiac muscle

Answer 30

Like skeletal muscle, cardiac muscle has an organised arrangements of thick and thin filaments
Also referred to as striated muscle
Same organised myofibrils and sarcomere structure
Contractions are regulated by the troponin/tropomyosin system
Their major similarity to the smooth muscle is that the cells are connected by gap junctions
Electrical excitation is passed between cells

Question 31

Unique features of cardiac muscle

Answer 31

Each cell contains two nuclei and is usually branched
T-tubules are shorter, not as broad
Form a diad with the sarcoplasmic reticulum
Sarcoplasmic reticulum is not as extensive
Does not form direct connection with T-tubule
Stores fewer Ca2+ ions
Lots more mitochondria
Each cell contacts others at a specialised sight called an intercalated disc
Stabilises the positions of adjacent cells to maintain 3D structure
Allows transmission of ions (electrical excitation - cells beat together)
Efficient transmission of force as myofibrils of cells are anchored to the plasma membrane

Question 32

Triggering contractions in cardiac myocytes

Answer 32

Because cardiac myocytes are physically, chemically and electrically coupled to each other
Kind of like a single enormous cell
Makes sense as function is to pump blood rhythmically through different chambers
Cardiac myocytes do no need neuronal input to contract, they have an internal rhythm
Specialised cells (pacemaker cells) determine the rate of contraction
Nervous system can modulate this rate (autonomic)
The main difference in excitation contraction coupling to skeletal muscle
No direct link between T-tubules and SR
Reliance on Ca2+ channels in the sarcolemma (much slower to open and close)

Question 33

Excitation Contraction coupling in cardiac muscle

Answer 33

1. An action potential depolarises the cardiac cell membrane
2. This activates voltage sensitive calcium channels (VSCC) causing a Ca2+ influx from the extracellular space
3. This triggers further Ca2+ release from the sarcoplasmic reticulum
4. Remaining steps for contraction and relaxation are the same as skeletal muscle

Question 34

Cardiac action potentials and contraction

Answer 34

Cause a contraction ~10x as long as that in skeletal muscle
Much larger refractory period
Can not produce tetanic contractions