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
General mechanism of smooth muscle contraction.
calcium ions combine with calmodulin to cause activation of myosin kinase and phosphorylation of the myosin head
How does calmodulin initiate contraction in smooth muscle?
calcium ions bind with calmodulin, calmodulin-calcium complex joins with and activates myosin light chain kinase (phosphorylating enzyme), one of the light chains (regulatory chains) of each myosin head becomes phosphorylated in response to ts myosin kinase; when this chain is not phosphorylated, the attachment-detachment cycling of the myosin head with the actin filament does not occur; when it is phosphorylated, it is able to bind repetitively with the actin filament proceeding through the entire cycling process of intermittent “pulls” (just like in skeletal muscle) and cause contraction
What is the role of myosin phosphatase in smooth muscle?
located in the cytosol of the smooth muscle cell, it splits the phosphate from the regulatory light chain, the cycling stops and contraction ceases, the time required for relaxation is determined to a great extent by the amount of active myosin phosphatase in the cell
Possible mechanism for regulation of the latch phenomenon?
as the myosin kinase and myosin phosphatase activation decreases, the cycling frequency decreases; at the same time, the deactivation of these enzymes, allows the myosin heads to remain attached to the actin filament for a longer and longer portion of the cycling period
What is one distinguishing feature of the NMJ of smooth muscle?
autonomic fibers that innervate smooth muscle branch diffusely on top of a sheet of muscle fibers, fibers do not make direct contact with the smooth muscle fiber membranes; instead they form diffuse-junctions
The vesicles in skeletal muscle contain Ach. In smooth muscle, the vesicles may contain Ach or?
norepinephrine
Can Ach and NE be secreted by the same nerve fibers?
no, Ach is excitatory and NE is inhibitory
Two forms of visceral smooth muscle contraction?
spike potentials and APs with plateaus
Does smooth muscle have more or less voltage-gated calcium channels than skeletal muscle?
more, less sodium channels
What is the cause of spontaneous APs in visceral smooth muscle when it is stretched?
the normal slow wave potentials and decrease in overall negativity of the membrane potential caused by the stretch itself; this stretch response allows the gut wall for example, when excessively stretched, to contract automatically and rhythmically
How do multi-unit smooth muscle fibers contract without APs?
contract in response to nerve stimuli, the nerve endings secrete Ach or NE which cause depolarization of the smooth muscle membrane, elicits a contraction; APs usually do NOT develop because the fibers are too small to generate them
How do small arterioles, which are normally in constant contraction, relax in order to allow blood flow?
a powerful local feedback control system controls the blood flow to the local tissue area by the following ways: 1. lack of oxygen in the local tissues causes smooth muscle relaxation, and therefore, vasodilation 2.excess carbon dioxide causes vasodilation 3. increased hydrogen ion concentration causes vasodilation 4. adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temp can all cause local vasodilation
Which hormones can cause smooth muscle contraction?
NE, epinephrine, Ach, angiotensin, endothelin, vasopressin, oxytocin, serotonin, histamine
How can hormones initiate or inhibit an AP?
by acting on the receptor affecting sodium and potassium channels
Can a hormone affect muscle contraction without causing a change in membrane potential?
yes hormone may activate the membrane receptor that causes an internal change in the muscle fiber, such as release of calcium from the sarcoplasmic reticulum (SR), calcium then induces the contraction
How does the cell inhibit contraction initiated by hormones affecting the SR and resulting in increased calcium?
activate adenylate cyclase (cAMP) or guanylate cyclase (cGMP) in the cell membrane, these changes the degree of phosphorylation of several enzymes that indirectly inhibit contraction, pump that moves Ca into the SR is activated, as well as the cell membrane that moves calcium out of the cell itself
The force of contraction of smooth muscle cells is usually dependent on:
extracellular fluid calcium concentration (SR is only slightly developed in smooth muscle so it is not primarily coming from that source)
