Smooth Muscle

Question 1

How does the innervation of single unit smooth muscle vary from that of multi unit smooth muscle?

Answer 1

. In single unit SM, one varicosity supplies a few units

. In multi unit SM, one varicosity supplies one unit

Question 2

Give 4 examples of where multi-unit SM can be found.

Answer 2

Bronchi, large arteries, ciliary body, iris (all places that need graded fine contraction)

Question 3

Give 4 examples of where single-unit SM can be found.

Answer 3

Arterioles, lymph vessels, GI and uterine tracts

Question 4

Describe the main differences between single-unit and multi-unit SM.

Answer 4

. In single unit one varicosity supplies a few units, but in multi unit one varicosity supplies one unit only
. In single unit, the units are electrically coupled by gap junctions and non-selective channels. In multi unit the units are independent (not coupled by gap junctions)
. In single unit the units act in a united synchronous manner as a functional syncytium, in multi unit there is fine control and graded contraction (similar to skeletal muscle)
. In multi unit there is no NMJ, instead NT diffuses and binds to receptors found on the units
. Single unit is myogenic and multi unit is neurogenic (normally quiescent until stimulated)

Question 5

How is over-stretching prevented in smooth and skeletal muscle?

Answer 5

. Prevented by connective tissue in smooth muscle

. Prevented by elasticity of fibres in skeletal muscle

Question 6

What is the stretch relaxation response? Give an example of where this response is important.

Answer 6

. When smooth muscle is stretched, tension initially develops rapidly
. Then expansion occurs so that the muscle relaxes to its original tone (tension) but at a new stretched length (‘relaxes’)
. The smooth muscle will then contract when stimulated to do so
. Important in the bladder where muscle tension (tone) is maintained while bladder fills with urine without expulsing the urine when stretched- will only contract and release the urine when stimulated

Question 7

How is graded contraction achieved in smooth muscle? How does this compare with skeletal muscle?

Answer 7

. Smooth muscle strength of contraction depends on concentration of calcium ions
. Skeletal muscle strength of contraction depends on the number of fibres involved in the actin-myosin interactions

Question 8

Describe EC coupling in smooth muscle

Answer 8

. Calcium influxes into cells and binds to calmodulin
. Calmodulin activate myosin light chain kinase
. Myosin light chain kinase phosphorylates myosin light chain so actin-myosin cross bridges can be formed

Question 9

Describe the mechanism by which smooth muscle relaxes.

Answer 9

. Myosin phosphatase acts in opposite way to MLCK

. Dephosphorylates myosin light chain, inactivating it so actin-myosin cross-bridges not formed

Question 10

What effect does noradrenaline have on calcium ion concentration in smooth muscle

Answer 10

Decreases calcium ion concentration because smooth muscle contraction not essential during sympathetic response.