Muscle and Innervation (histology and P&P)

Question 1

What is the role of the dystrophin-associated glycoprotein complex?

Answer 1

Structural: links f-actin (of cytoskeleton) to the ECM, providing stability during contraction cycles and transmitting force generated by the muscle through to the ECM

Question 2

What is dystrophin, and what is its role in disease?

Answer 2

A rod-like, triple helical protein that connects f-actin of the sarcoplasmic cytoskeleton to the dystrophin-associated glycoprotein complex. Deficiency of dystrophin due to deletions or mutations can lead to muscular dystophy. Muscular dystrophy is caused by poor stability of the muscle cell- each contraction disrupts the sarcolemma as the connection to it is defective, and force transmission throughout the muscle is weakened. This eventually leads to muscle wasting.

Question 3

Describe titin’s location, structure and function

Answer 3

Titin is a large protein that attaches at one end to the Z-disc (organiser and attachment site of thin filaments) and at the other to the M-line (organiser and attachment site of the thick filaments).
Elastic and holds contractile apparatus in place during contraction cycle.
Overlaps end-on with other titin molecules to form continuous titin filament system

Question 4

Describe a-actinin’s location, structure and function in muscle cells

Answer 4

It is an antiparallel dimeric actin binding/anchoring protein that is located in the z disc (skeletal muscle), dense bodies (smooth muscle) and zona adherens (cardiac muscle)

Question 5

Where are smooth muscle cells distributed?

Answer 5

In the visceral linings of most organs, such as blood vessels, GI tract and ciliary muscles/iris of eye

Question 6

What are the two types of smooth muscle?

Answer 6

Visceral and multi-unit

Question 7

Describe the denser type of smooth muscle (S,F and L)

Answer 7

Visceral - large sheets of SM densely packed and connected by gap junctions, with shared innervation from one nerve for, both to permit coordinated but coarse contraction.
Spontaneously contract (myogenic)
Contractions initiated by hormones but sometimes modulated by neurons
Visceral linings

Question 8

Describe the looser type of smooth muscle (S,F and L)

Answer 8

Multi-unit - individually innervated, loosely packed cells that act independently
No spontaneous contractions, mostly neurogenic although contractions can be modulated by circulating hormones, therefore finer contractions
Iris and ciliary muscles of eye, piloerector muscles in skin

Question 9

Outline the gross structure of smooth muscle cells

Answer 9

Non-striated, fusiform, elongated, mononucleate, central nucleus

Question 10

Outline the microscopic structure of smooth muscle

Answer 10

Myofilaments arranged in irregular lattice

Thin filaments attach to dense bodies (containing a-actinin) within sarcoplasm and dense plaques located on sarcolemma instead of z disc

No troponin

Intermediate filaments aid in force transmission between cells by attaching to dense bodies in sarcoplasm and focal densities in the sarcolemma

SR calcium store not very highly developed
Caveolae (sarcolemma invaginations) present

Question 11

Describe the connective tissues organise skeletal muscle tissue

Answer 11

Epimysium- sheaths bundles of fascicles, and thus the whole muscle
Perimysium- sheaths bundles of muscle fibres, which form fascicles
Endomysium- sheaths the individual muscle fibres, each containing many myofibrils

Question 12

Outline the development of skeletal muscle fibres

Answer 12

Differentiate from lateral plate mesodermal cells to myoblasts, which are stem cells which are unipotent. The myoblasts fuse end-on to form multinucleate myotubes, and as the myotubes mature they begin forming myofibrils (initially peripherally) and the nucleus migrates to the periphery of the cell. Once fully formed, the myotube is called a myocyte

Question 13

What is different between the gross structure and action of cardiac cells compared to skeletal muscle fibres?

Answer 13

Fibres are branched laterally, connected by gap junctions, and the cells are not fused but aligned end-on and connected via intercalated disks
Cells are myogenic

Question 14

What are the microscopic differences between cardiac and skeletal muscle cells?

Answer 14

Cardiac cells have a less extensive and irregular SR and T-tubule system that forms dyads rather than triads, and these occur over the Z-line, not at the junction of the A and I band.

Question 15

Describe the appearance of intercalated disks, outline their microscopic elements and relate that to their function.

Answer 15

Zig-zag/ruffled
Longitudinal portion contains fasciae adherentes for anchoring actin filaments and desmosomes for binding cells together
Horizontal portion contains gap junctions for ionic coupling of cells so cells act as functional syncytium

Question 16

Describe the membrane potential of smooth muscle cells

Answer 16

Unsteady, constantly drifting- usually around -50mV

Question 17

Describe and explain a smooth muscle action potential compared to that of skeletal muscle

Answer 17

Much slower upstroke, sometimes spiked but often with a plateau
They last much longer as they are totally dependant on the opening of voltage gated calcium channels rather then just simply depolarisations, and they don’t have T-tubules or a very extensive SR to enable a quicker, more efficient response, and the relaxation is also largely dependant on slow potassium efflux through calcium controlled voltage gated k channels

Question 18

What is significant about multi-unit smooth muscle action potentials?

Answer 18

They don’t occur

Question 19

What are slow waves (in relation to smooth muscle) and what do they cause?

Answer 19

oscillating and opposing calcium and potassium currents causing graded depolarisations

Question 20

What is the one necessary event for initiation of contraction in smooth muscle, and in what ways can it occur?

Answer 20

Increase in intracellular [Ca2+] caused by:

• influx during an action potential
• influx during slow waves
• IP3 mediated release of Ca2+ from the SR in response to NTs, hormones or membrane potential changes

Question 21

What is IP3 and what does it do?

Answer 21

IP3 is a secondary messenger released intracellularly in response to extracellular signals. It binds to the IP3 receptor, which is a calcium release channel found in the SR of smooth muscle cells

Question 22

Describe the contraction cycle in smooth muscle

Answer 22

1. intracellular [Ca] rises
2. Ca binds to calmodulin
3. Ca-calmodulin complex activates myosin light chain kinase
4. MLCK phosphorylates myosin regulatory light chain of the myosin head which allows binding to actin
5. this allows myosin head binding to actin and also activates ATPase activity in myosin head
6. ATP hydrolysed, energy released enables powerstroke to take place
7. cross-bridge cycling continues until intracellular [Ca] levels falls, decreasing MLCK activity and increasing myosin light chain phosphatase activity, which dephosphorylates the myosin head.

Question 23

How is MLCK activity regulated?

Answer 23

Increase in intracellular cAMP and cGMP due to hormonal agonist will increase intracellular PKA and PKG respectively. These both inhibit MLCK

Question 24

Name an inhibitor of MLCP

Answer 24

PKC

Question 25

Describe the excitation pathway in a heart beat and give rough timings

Answer 25

1. myogenic SAN cells generate action potentials which conduct throughout the atrial walls (40ms)
2. excitation not able to pass directly into ventricular wall due to the presence of a non-conducting fibrous ring called the annulus fibrosus, so passes to only conducting cells available found in the AVN, where the excitation wave is held (100ms)
3. Excitation then passes through the bundle of His and the Purkinje fibres (30ms)
4. Once it reaches the base of the ventricles, the excitation wave spreads throughout the ventricular walls (30ms)

Question 26

Why is the excitation wave held in the AVN?

Answer 26

To allow time for blood to pass from the atria into the ventricles

Question 27

Outline the events of the ventricular action potential, giving the phase numbers and the gross actions of the heart (contraction/relaxation)

Answer 27

Phase 0 - upstroke is rapid. Depolarisation past -65mV opens v-gated na channels causing membrane potential to overshoot to 20mV.

Phase 1 - Na channels inactivate, then there is an incomplete repolarisation to about 0mV caused by rapid activation and inactivation of Ito1 outward potassium current channels.

Phase 2 - AP enters plateau phase as delayed rectifier potassium channels and Ito2 chloride channels activated by Calcium are counteracted by L-type v-gated ca channels (slow to open, at around -45mV) which allow influx of ca. In addition, the 3 Na in 1 Ca out Na/Ca antiporter contributes a net 1 inward positive charge which also sustains the depolarisation. Heart contraction occurs during the plateau.

Phase 3 - L-type ca channels close, delayed rectifier potassium channel remains open, so rapid repolarisation (also, inward rectifier potassium channel reopens to counter repolarisation)

Phase 4 - overshoot caused by delayed inactivation of delayed rectifier current corrected by inward rectifier, Na/Ca antiporter and Na/K pump. This is the diastolic phase

Question 28

Outline the SAN action potential, with phase numbers and actions they cause

Answer 28

Phase 0 - slow (because ca influx not rapid) upstroke representing the opening of L type v-gated Ca channels in response to depolarisation (funny current and other ca channel current stop)

Phase 1 and Phase 2 - non existent as there is no short, premature repolarisation stage or plateau

Phase 3 - delayed rectifier potassium channels repolarise and L type v-gated ca channels close

Phase 4 - hyperpolarised cell state (necessary for spontaneous APs) cause slow inward Na currents through non selective ion channels (called a funny current) and cell begins a slow depolarisation. Also inactivates potassium rectifier and after -50mV activates a short lasting inward current T-type Ca channels.
Phase 4 not stable as SAN cells lack background K channels which maintain resting membrane potential

Question 29

Differences between SAN and ventricular APs

Answer 29

SAN cause spontaneous depolarisation, ventricles need action potential to be induced

Diastolic potential in SAN cells is not steady, ventricular diastolic potential is due to background potassium conductance

No plateau in SAN

In ventricles, upstroke caused by inward Na, in SAN its inward Ca

No funny current in Ventricles

Question 30

Which cells in the heart can assume pacemaker role (have latent rhythmicity) when SAN is blocked, and how?

Answer 30

AVN, if not purkinje fibres. Both have funny currents

Question 31

How is cardiac contraction regulated, and why is these mechanisms not possible in skeletal muscle?

Answer 31

Frank-Starling length-tension relationship: length of heart muscle determined by the pre-load (blood volume returning to heart). Cross-bridge formation not optimized in heart therefore stretching of the fibres will increase tension. Not possible in skeletal as cross-bridge formation optimised

Autonomic/hormonal modulation: sympathetic nerves releasing noradrenaline/ blood with adrenaline will bind to receptors which activate g-proteins to cause adenylate cyclase to make cAMP, initiating phosphorylation cascade making PKA.
In ventricles:
–> PKA phosphorylates LTCC to increase Ca plateau
–> PKA phosphorylates phospholamban which increases SERCA activity
In SAN:
–> PKA phosphorylates LTCC to increase rate of pacemaker potential decay and therefore increase AP frequency
–> PKA phosphorylates delayed rectifier k channel to increase k efflux, shortening AP
–> cAMP bind funny current producing channel to increase open probability and increase rate of pm potential decay

Parasympathetic nerves release ACh, which bind receptor, activate opposite activating g-protein which inhibits cAMP synthesis

Not possible for skeletal muscle as nerve stimulation is necessary for contraction initiation, and causes all or nothing response

Question 32

What are the words used to describe the modulation of force and the modulation of frequency of contraction?

Answer 32

Inotropy and chronotropy

Question 33

Why can the mechanism for modulation of frequency of contraction in skeletal muscle not be used in cardiac muscle?

Answer 33

Tetany cannot be achieved in cardiac muscle due to slow calcium influx in SAN and long plateau and refractory period in ventricles

Question 34

Why is recruitment modulation not possible in the heart muscle?

Answer 34

All cardiomyocytes are recruited as they act as a functional syncytium, all receiving the same signal not contract.

Question 35

Give the steps of the cross-bridge cycle

Answer 35

Step 1 - begin with myosin head attached to actin filament

Step 2 - ATP binding to head causes detachment of the head from the actin

Step 3 - ATPase in the head hydrolyses ATP to ADP and Pi, which yields the energy required to change the angle of the myosin head into the cocked state.

Step 4 - when calcium is present, it binds troponin complex, which moves tropomyosin away from the myosin binding site on the actin, allowing the myosin head (bound to ADP and Pi, in its high energy configuration) to bind actin

Step 5 - the myosin-actin complex is sufficient to cause Pi release, and this induces a conformational change to myosin’s low energy state, bound only to ADP. The change causes a power stroke, pulling the thin filament towards the M line

Step 6 - ADP is released, but in order for myosin head to release from actin it must return to its high energy state. This is caused by ATP binding

Question 36

Label the action potential graphs in notability

Answer 36

Revision–>P and P–>ventricular and SAN AP