Chapter 8 - Guyton Flashcards
General characteristics of smooth muscle.
smooth muscle is composed of smaller fibers as compared to skeletal muscle, same contractive forces (actin and myosin) that cause skeletal muscle to contract also cause smooth muscle contraction
Smooth muscle in each organ is distinct from other organs in these ways:
- physical dimensions
- organization into bundles or sheets
- response to different types of stimuli
- characteristics of innervation
- function
Explain multi-unit smooth muscle.
composed of separate smooth muscle fibers that operate independently and is innervated by a single nerve ending, control is exhibited mainly by nerve signals, outer surfaces of the fibers are covered by a membrane-like substance
a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another, Ex. ciliary and iris muscles of eye and piloerector muscles that cause the erection of hairs on your body
Explain single-unit smooth muscle.
aka syncytial smooth or visceral smooth muscle, control is exerted by non-nervous stimuli, hundreds of thousands of smooth muscle fibers contract together as a single unit, fibers are arranged in sheets/bundles, cell membranes are adherent to each other at multiple point which allows force generated at one muscle fiber to be transmitted to the next and are joined by gap junctions
Does smooth muscle contain troponin?
no
How is the force contraction in smooth muscle transmitted between one cell and the next?
intercellular protein bridges
What is the function of the dense bodies in smooth muscle?
dense bodies serve the same role as the Z discs in skeletal muscle
Why can smooth muscle cells contract as much as 80% of their length instead of being limited to <30% like in skeletal muscle?
myosin filaments have “side polar” crossbridges, this arrangement allows bridges on side to hinge in one direction and those on the other side to hinge in the opposite direction, the benefit of this is that it allows myosin to pull actin in both directions
What is the reason for the prolonged, tonic contraction in smooth muscle as compared to skeletal muscle?
slow cycling of the myosin crossbridges, low energy requirement to maintain contraction, slowness of onset of contraction and relaxation, maximum force of contraction is greater, “latch” mechanism, stress-relaxation of smooth muscle
Slow Cycling of the Myosin Cross-Bridges
the cross-bridge heads have far less ATPase activity than in skeletal muscle, degradation of the ATP that energizes the movements of the cross-bridge heads is greatly reduced
Low Energy Requirement to Sustain Smooth Muscle Contraction
only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle
Slowness of Onset of Contraction and Relaxation of the Total Smooth Muscle Tissue
the slow onset of contraction, as well as prolonged contraction, is caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments, the initiation of contraction in response to calcium ions is much slower than in skeletal muscle
Maximum Force of Contraction is Often Greater in Smooth Muscle than in Skeletal Muscle
this great force of smooth muscle contraction results from the prolonged period of attachment of the myosin cross-bridges to the actin filaments
What is the “latch” mechanism?
smooth muscle can maintain prolonged tonic contraction for hours with little use of energy
little continued excitatory signal is required from nerve fibers or hormonal sources
What is the significance of the stress-relaxation of smooth muscle?
smooth muscle has the ability to return to nearly its original force of contraction seconds or minutes after it has been elongated or shortened; when the volume is suddenly decreased, the pressure falls drastically at first but then rises in another few seconds or minutes to or near the original level and this allows a hollow organ to maintain about the same amount of pressure inside its lumen despite long-term, large changes in volume