Smooth Muscle

Question 0

Unitary Smooth Muscle

Answer 0

A mass of hundreds to thousands of smoooth muscle fibers that contract together as a single unit.
Cell membranes joined by many gap junctions through which ions can flow freely from one muscle cell to the next which helps the muscles fibers contract together through APs.
Visceral smooth muscle - found in the walls of most viscera of the body (gut, bile ducts, uterus, and many blood vessels)

Question 1

Multi-unit Smooth Muscle

Answer 1

Discrete separate smooth muscle fibers

Each fiber can contract independently of the others, and their control is exerted mainly by nerve signals.

Question 2

Contractile Mechanism

Answer 2

Similar to skeletal muscle, however, does not contain the normal troponin complex

This leads to a different mechanism for control of contraction

Question 3

Physical Basis for Smooth Muscle Contraction

Answer 3

Does not have the same striated arrangement of actin and myosin filaments
Force of contraction is transmitted through membrane dense bodies bonded together by intercellular protein bridges.
Actin filaments attached to these dense bodies
Dense bodies of smooth muscle serve the same role as the Z discs in skeletal muscle
Most of myosin filaments have sidepolar cross-bridges arranged so that the bridges on one side hinge in one direction and those on the other side hinge in the opposite direction.
This allows the myosin to pull an actin filament in one direction on one side while simultaneously pullin another actin filament in the opposite direction.
Allows smooth muscle cells to contract as much as 80% of their length instead of being limited to < 30% as occurs in skeletal muscle

Question 4

Slow Cycling of Myosin Cross-Bridges

Answer 4

Rapidity of cycling is much slower in smooth muscle than in skeletal muscle, yet the fraction of time that the cross-bridge remain attached to the actin filament, which is a major factor that determines the force of contraction, is believed to be greatly increased in smooth muscle.

Question 5

Energy Required

Answer 5

Much less energy is required to sustain the same tension of contraction in smooth muscle
Believed to result from the slow attachment and detachment cycling of the cross-bridges and because only one molecule of ATP is required for each cycle, regardless of its duration

Question 6

Slowness of Onset of Contraction and Relaxation

Answer 6

Caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments.

Question 7

Force of Muscle Contraction

Answer 7

The greater force of contraction results from the prolonged period of attachment of the myosin cross-bridges to the actin filaments

Question 8

Latch mechanism

Answer 8

Can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy

Question 9

Stress-relaxation / reverse stree-relaxtion

Answer 9

Allows a hollow organ to maintain about the same amount of pressure inside its lumen despite long-term, large changes in volume

Question 10

Ca++ with Calmodulin

Answer 10

1. The Ca++ ion binds with calmodulin
2. The calmomdulin-calcium combination joins with and activates myosin kinase
3. One of the light chains of each myosin head, called the regulatory chain, becomes phosphorylated. (When this chain is not phosphorylated, the attachment-detachment cycling of the myosin head with the actin filament does not occur.

Question 11

Cessation of Contraction

Answer 11

When Ca++ concentration falls below a critical level, the contraction mechanism automatically reverses, except for the phosph of the myosin head.
Reversal of this requries another enzyme, myosin phosphatase. This stops the cycling.

Question 12

Neuromuscular Junctions

Answer 12

Autonomic nerve fibers that innervate smooth muscle generally branch diffusely on top of a sheet of muscle fibers.

Form diffuse junctions that secrete their NT substance into the matrix coating of the smooth muscle; the transmiter then diffuses to the cells.

Where there are many layers, the nerve fibers often innervate only the outer layer and muscle excitation travels from this outer layer to the inner layers by AP conduction

Question 13

Acetylcholine and Norepinephrine

Answer 13

ACh = an excitatory transmitter substance for smooth muscle fibers in some organs but an inhibitory transmitter for smooth muscle in other organs.
When ACh excites a muscle fiber, NE ordinarily inhibits it, and vice versa.

Question 14

Membrane potential

Answer 14

The normal resting state, the intracellular potential is usually about -50 - -60 mV