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Is smooth muscle striated or non-striated?
Is smooth muscle under voluntary or involuntary control?
What nervous system is smooth muscle regulated by?
The autonomic nervous system
What are dense bodies and what do they do?
- Dense bodies are effectively the structural proteins seen in skeletal muscle. They anchor the thick and thin filaments.
- Dense bodies located on the outside of the cell will make contact with the adjacent cell. There are electrical gap junctions within dense bodies that allow electrical signals to pass between cells. Smooth muscle contacts in every direction it makes contact with dense bodies in.
Describe excitation contraction coupling in smooth muscle (Resting state vs Contraction)
- Resting State: In smooth muscle there is no tropomyosin blocking the myosin binding site. Calmodulin is present instead of troponin. It is a free floating molecule. The myosin in smooth muscle does not have ATPase activity - it cannot use ATP to activate the myosin.
- Contraction: An increase in intracellular calcium means calcium binds to calmodulin. The calcium-calmodulin complex binds to an enzyme called myosin light chain kinase which activates the enzyme. Myosin light chain kinase then activates myosin (thick filaments) so they can make contact with the binding site on the thin filaments. To inactivate the myosin ADP must be dephosphorylated by myosin phosphatase.
What is the main source of calcium for contractions in smooth muscle?
Extra cellular calcium
What is the affect of a) acetylcholine and b) noradrenaline on the activity of intestinal smooth muscle?
- a) acetylcholine increases activity
- b) noradrenaline decreases activity
What does a lack of O2 in tissues cause?
What does an excess of CO2 in tissues cause?
What does an increase in H+ in tissues cause?
Name two hormones that have an affect on smooth muscle
Oxytocin and seratonin
What are the two types of smooth muscle? Where are they found?
- Single Unit - action potentials are propagated from cell to cell via gap junctions (located in dense bodies). May develop spontaneous action potentials. Activity altered by hormones. Innervations varies, may be limited to areas containing pacemaker cells. Activated by stretch. This type of smooth muscle is found in the intestine, reproductive tract and small diameter blood vessels.
- Multi-unit - little or no propagation of action potentials. Activity closely regulated by neural inputs. Degree of contraction dependent upon number of units activated and summation of neural inputs. Activity altered by hormones. Not activated by stretch. This type of smooth muscle is found in the large airways to the lungs, large arteries, hairs on skin and pupil of the eye.
Islands of cells are present within smooth muscle which do not have a stable resting membrane potential. What is the difference between those that create spike APs and slow wave potentials?
- Spike APs: There is a net movement of positive charge into those cells. Eventually the cell reaches threshold, fires an AP and the cycle begins again. The gradual depolarisation of the cell is referred to as the pacemaker potential because it determines the pace at which there is another contraction within the muscle.
- Slow wave potentials: Cells must have a changing permeability to ions (the cause of which is unknown). If the wave goes above threshold then it induces an AP.
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