Lecture 9: Smooth Muscle

Question 1

Characteristics of smooth muscle vs. other types

Answer 1

Smooth muscle:
-Has no striations
-Is innervated by the ANS (involuntary)
-Also uses sliding filaments, Ca++, and cross bridges
-Filaments are organized differently
-Excitation-contraction coupling is different

Question 2

Characteristics of a smooth muscle cell

Answer 2

-Spindle shaped (much smaller vs sk. muscle fiber)
-Interconnected in sheet-like layers
-Mononucleated (maintain mitotic ability)
-No troponin (unknown tropomyosin function)
-Caldesmon on thin-f’s to regulate contraction(?)
-Myosin thick-f and actin thin-f

Question 3

Where do smooth muscle thin-f’s anchor?

Answer 3

Either to the plasma membrane or to cytoplasmic dense bodies (Z-line analog)

Question 4

How are smooth muscle filaments arranged? How does this shape contraction?

Answer 4

Thin-f’s are organized diagonal to the long axis of the cell; during contraction, plasma membrane between anchor points balloons out

Question 5

Smooth muscle isometric tension vs length

Answer 5

Smooth muscle has an optimal length but can generate significant force at a broader range of lengths, retaining tension while vessels/bodies change volume

Question 6

How is smooth muscle contraction activated/regulated?

Answer 6

Myosin light chain kinase vs myosin light chain phosphatase activity.

Ca++ binds calmodulin to activate MLCK; myosin-Pi moves away from thick-f and can bind to actin

Question 7

How does smooth muscle myosin compare to skeletal muscle?

Answer 7

Smooth muscle has much less myosin/actin and much slower myosin ATPase

Question 8

Smooth muscle force generation vs skeletal muscle

Answer 8

Smooth muscle has similar max tension per cross-sectional area but shortens much slower (more economical, less ATP per time) and does not fatigue

Question 9

Latch state

Answer 9

Smooth muscle can maintain tension without much cross-bridge activity for long periods of time (v. low ATP use); X-bridge actin release just occurs very slowly. Happens w/ persistent stimulation/elevated Ca++

Question 10

Sources of smooth muscle Ca++

Answer 10

1. Sarcoplasmic reticulum
2. Extracellular Ca++ via membrane channels

Question 11

Smooth muscle sarcoplasmic reticulum

Answer 11

Less present, no t-tubules. Responds to APs at associations near plasma membrane or to secondary messengers from EC chemical messenger binding

Question 12

How are smooth muscle APs mediated?

Answer 12

Voltage-gated Ca++ channels, not Na+

Question 13

Characteristics of smooth muscle cytosolic Ca++ removal

Answer 13

Pumped into SR or out of cell; occurs much slower so that 1 twitch may last several seconds

Question 14

Smooth muscle tension grading

Answer 14

Tension in smooth muscle can be graded by varying [Ca++], whereas in sk. muscle 1 AP = full troponin saturation

Question 15

Smooth muscle tone

Answer 15

When Ca++ is sufficient to maintain low-level but super-basal X-bridge/MLCK activity

Question 16

Pacemaker potentials

Answer 16

Some smooth muscle cells spontaneously generate APs without input, producing a rhythmic state of contractile activity

Question 17

How are pacemaker potentials generated?

Answer 17

1. Gradual depolarization to threshold, and again after repolarization
2. Slow waves, where regular variation in ion flux makes membrane potential drift up and down. A superimposed ion flux then reaches threshold.

Question 18

Varicosities

Answer 18

Swellings on axon branches of post-ganglionic autonomic nerves. 1 axon will have multiple varicosities for multiple cells

Question 19

Dual innervation of smooth muscle cells

Answer 19

1 smooth muscle cell might be near varicosities from multiple neurons, sympathetic and parasympathetic

Question 20

Local factors

Answer 20

Typically lead to smooth muscle relaxation. Allows local responses without long-distance signals. Includes:
-Paracrine signals
-pH
-pO2/pCO2
-Osmolarity
-Ionic comp. of ECF
e.g. nitric oxide

Question 21

Stretch responses

Answer 21

Stretch-gated ion channels open and are amplified by nearby voltage-gated channels, producing an opposing contraction. Stretch response is intrinsic and common to almost all muscle.

Question 22

Single-unit smooth muscle

Answer 22

Whole muscle tissue responds as a single unit; cells undergo synchronous electro-mechanical activity

Question 23

Features of single-unit smooth muscle

Answer 23

-Cells linked by gap junctions
-Often pacemaker cells connected to non-pacemakers
-Entire muscle can be controlled by innervating/regulating pacemaker AP frequency; fewer autonomic axons
-Influenced by nerves, hormones, local factors

E.g. GI tract, uterus

Question 24

Multi-unit smooth muscle

Answer 24

Muscle tissue acts as multiple units; each cell responds independently

Question 25

Features of multi-unit smooth muscle

Answer 25

-No gap junctions
-Richly innervated by autonomic branches
-Tissue contraction response depends on frequency of stimulation and number of activated cells
-Often no APs, no stretch response
-Influenced by circulating hormones

E.g. large airways/arteries, piloerectors

Question 26

Smooth muscle filaments

Answer 26

Thin-f’s are very long for unrestricted thick-f sliding and large % length changes

Thick filaments are sidepolar (sarcomeres have bipolar); myosin head orientation is always correct