Muscles 3 Flashcards
Appearance of cardiac muscle
Striated
Where are myocytes joined in cardiac muscle?
Intercalated disks
Where does electric coupling happen?
At intercalated disk (between adjacent myocytes by means of gap junctions)
What initiates an action potential?
A pacemaker cell in the sinoatrial node
What causes a change in heart rate?
Changing sinoatrial node ap firing
What are multiunit and unitary classes of?
Smooth muscle
Apperance of smooth muscle
Non striated
Describe a multiunit smooth muscle.
Electrical isolation of cells (fine control of ACh muscle fibre)
Uses varicosities (synaptic contacts) to connect to autonomic neurons in each smc
Describe a unitary smooth muscle
Gap junctions for coordinated contraction
Autonomic neurons connect to only a few smc via varicosities (not every cell is innovated)
What is a dense body?
Multiple actin fibres joining a point
Do all smc generate action potentials?
No, some change depending on membrane potential
Describe the mechanism for increasing intracellular calcium (skeletal)
- Membrane depolarisation opens L type Ca2+ channel
- Coupling between L type and Ca2+ release channel triggers Ca2+ release channel to open
- Ca2+ exits sr (into cytoplasm) and activates troponin C, causing a muscle contraction
Ca2+ release can also activate the Ca2+ release channels.
What process occurs in T tubules?
Increasing intracellular calcium
What is a Ca2+ release channel also known as?
Ryanodine receptor
Describe the mechanism for increasing intracellular calcium (cardiac)
T tubule is only close to one branch of sr (dyad).
The influx Ca2+ through the T-tubule channel activates ryanodine receptors, triggering the release of Ca2+ from sr.
Calcium induced calcium release
Describe the mechanism from which calcium is removed from the cytoplasm
- Na-Ca exchanger and Ca2+ ATPase in the plasma membrane extrude Ca2+ from cell.
- Ca2+ pump sequesters Ca2+ within sr.
- Ca2+ is bound the sr by calreticulin and calsequestrin
Describe the mechanism for increasing intracellular calcium in smooth muscle.
- Change in Vm or action potentials activate L type Ca2+.
- This leads to CIRC via activation of ryanodine receptors in sr membrane.
- Activation of Gq coupled receptors leads to IP3 production and IP3 receptor stimulation in sr membrane.
Describe sarcomere contraction
Tropomyosin wraps around myosin and interacts with actin
Troponin complex sits over actin sites
Regulated by changes in Ca2+
Describe the events that follow calcium binding to troponin C in cross bridge formation.
A conformational change occurs, causing TnT to pull tropomyosin and TnI (troponin) out of the way.
Myosin can now interact with actin.
Describe the mechanism of contraction in smooth muscle
Ca2+ activates calmodulin to form calcium calmodulin complex.
Activation of myosin light chain kinase causes phosphorylation of mlc, removing c and c inhibitory effects.
This facilitates cross bridge formation and therefore contraction.
How is smooth muscle contraction inhibited?
Dephosphorylation of mlc
- using myosin light chain phosphatase
