NEUR30080 L11 JoC smooth muscle: mechanical properties

smooth muscle

• involuntary muscle

• innervated by Autonomic NS (ANS) (visceral efferent fibres)

• found primarily in the walls of hollow organs and tubes

• spindle-shaped cells typically arranged in sheets

• cells do not have T-tubules, and have very little SR

smooth muscle cells

• do NOT contain sarcomeres, not striated

• but made of thick and thin filaments

  • thin filaments in smooth muscle do NOT contain troponin

  • Ca binds to CaM instead of troponin

  • Ca-CaM complex activates myosin which then binds to actin and contraction begins

contraction

the swivelling of cross-bridges

2 types of smooth muscle

1. visceral (single-unit)

2. multi-unit

visceral smooth muscle

• single-unit/unitary

• found in walls of hollow organs

• e.g. small blood vessels, digestive tracts, urinary & reprod systems

• multiple fibres contract as a unit (impulse travels easily across gap junctions from cell to cell)

• self-excitable in some cases (gen spontaneous APs & contractions)

multi-unit smooth muscle

• consists of MUs that are activated by nervous stimulation

• found in walls of large blood vessels

• e.g. in the eye (adjusting the shape of the lens to permit accommodation & size of pupil)

• e.g. at base of hair follicle (‘goose-bump’ muscles)

which is responsible for ‘goose bumps’?

multi-unit smooth muscle

single- vs multi-unit smooth muscle image

contractile activity of different smooth muscle

red: bladder, normally relaxed, then contracts

green: the opposite, anal sphynctus

time course events in smooth muscle contraction

A: force follows phasic contraction

B: continuous stimulation

1. relationship between Em and force generation (F) in diff smooth muscle cells

2. relationship between Em and force generation (F) in diff smooth muscle cells RHYTHMIC

• rhythmical activity depolarising then firing Ca APs

• depending on the amount of depol from Ca levels corresponds to force being gen

• more depol = more force

3. relationship between Em and force generation (F) in diff smooth muscle cells TONIC

• might be from some hyperpol

• force gen directly correlated to amount of RMP

4. relationship between Em and force generation (F) in diff smooth muscle cells PHARMACO-MECHANICAL COUPLING

• bypassing Ca, so NOT via myosin-actin kinase

• may cause contraction of muscle, and memb potential does NOT change

length-tension relationships

• if you stretch a muscle making it bigger, adding passive tension to active tension = total tension

• e.g. stretch stomach but still able to contract when it is stretched

• diff for skeletal - NOT able to contract while stretched (L0 different from skeletal)

length-tension continued

• usually called force-length relationship

• v different in smooth vs skeletal

• skeletal: decrease in active tension and increase in passive tension

• smooth: a small, temporary increase in passive tension, but that rapidly returns to normal

• the ability of smooth muscle cell to gen active tension remains normal over a v wide range of changes in length

length-tension relationships continued image

length-tension: smooth muscle histology

• NO striations

• the thick and thin filaments are not lined up in the way that they are in skeletal

• this lack of rigid structure means that i can stretch the muscle fibre and there will still be some myosin head able to bind to the active site

• may not be the same myosin head, but doesnt matter

• Z line to Z line actin, sarcomere in skeletal (possible MQ)

length adaptation of smooth muscle

• both skeletal (A) and smooth (B) muscle exhibit a bell-shaped length-tension relationship, although the length-tension relationship of smooth muscle can vary

• within a short period after stretching smooth muscle, there is a rightward shift in the length-tension relationship such that maximal force gen occurs at longer muscle length (B)

• likewise, within a brief time after shortening of smooth muscle, there is a leftward shift in the length-tension relationship (B)

• the mechanism(s) underlying this adaptation are hypothesised to reflect a change in the no. of contractile units in series (C)

force-velocity curves

• Velocity of shortening would get less and you pull and pull

• More stress on muscle = slower velocity

• Highest stress = lowest

• If draw straight line would get away w it  (instead of exponential)

• Skeletal = all or none, a twitch - Smooth not like that

modelling of smooth muscle length-adaptation

skeletal vs. smooth muscle curves

summary

key take-aways

• not just Ca2+ APs

• to do with receptors

• skeletal muscle not good at dealing w/ hypoxia but smooth is

• be aware of pharmacology of smooth muscle contraction too