Smooth Muscles & Muscle Metabolism Flashcards
composed of small fibers that are usually 1 to 5 micrometers in diameter and only 20 to 500 micrometers in length.
smooth muscle
In contrast to skeletal muscle fibers
are as much as 30 times greater in diameter and hundreds of times as long.
smooth muscle
has thick and thin filaments that are not arranged in sarcomeres; therefore, they
appear homogeneous rather than striated
smooth muscle
Most important, essentially the same attractive forces between myosin and actin filaments cause contraction in smooth muscle as in skeletal muscle, but the___________ is different
internal physical arrangement of smooth muscle fibers
is present in the iris, ciliary muscle of the lens, and vas deferens, arrector pili muscle
- behaves as separate motor units
Each fiber can contract independently of the others
and the control is exerted mainly by nerve signals
Multi-unit smooth muscle
has little or no electrical coupling between cells.
is densely innervated; contraction is controlled by neural innervation (e.g., autonomic
nervous system)
Multi-unit smooth muscle
In contrast, a major share of control
of unitary smooth muscle is exerted by ____________
non-nervous stimuli
unitary smooth muscle, syncytial
smooth muscle, and visceral smooth muscle
Single-unit smooth muscle
Muscle fibers contract together as a single unit
Cell membranes of adjacent fibers are connected
electrically by gap junctions
Single-unit smooth muscle
is spontaneously active (exhibits slow waves) and exhibits “pacemaker”
- which is modulated by hormones and neurotransmitters.
■ has a high degree of electrical coupling between cells and, therefore, permits coordinated contraction of the organ (e.g., bladder).
Unitary (single-unit) smooth muscle
is the most common type and is present in the uterus, gastrointestinal tract, ureter, and
bladder.
biliary tract, ureter , fallopian tube, blood vessels
Single-unit smooth muscle (unitary smooth muscle, syncytial smooth muscle, and visceral smooth muscle)
has properties of both multi-unit and single-unit smooth muscle
vascular smooth muscle
Smooth muscle does not have the same striated arrangement of actin and myosin filaments.
Actin filaments attach to dense bodies
Dense bodies are dispersed in linking one dense body to another or attached to the cell membrane and form bonds with dense bodies of adjacent cells allowing______
force to be transmitted from one cell to another
have similar functions to Z
disks in skeletal muscle.
Dense bodies
are interspersed among actin filaments.
its filaments have a diameter that is more
than twice as large as that of the actin filaments.
Myosin filaments
consist of actin filaments radiating from 2 dense bodies; these filaments overlap a single myosin filament that is located midway between the dense bodies.
Contractile units
Most smooth muscle contractions are prolonged __________ones that sometimes last hours or even days.
tonic
- Slow cycling of the cross-bridges
- Low energy requirement
- Slow onset of contraction and relaxation
- Increased maximum force of contraction
- “Latch Mechanism”
MUSCLE CONTRACTION
smooth muscle contracts 50-100
milliseconds after it is excited.
It has a long total contraction time of about ________, which is 30x stronger than skeletal muscle
1-3 seconds
Increased force of contraction is postulated to
resolve due to the long period of attachment of the _____
myosin cross bridges to the actin filaments.
Skeletal muscles have a useful distance contraction of only ¼-⅓ of its resting length, whereas smooth muscles often contract more than __________ of its stretch length.
⅔
facilitates prolonged holding contractions; muscle can maintain its full force of contraction even when there is reduction in the degree of activation from the initial level
“Latch Mechanism”
can maintain prolonged tonic contraction for hours with use of relatively low energy levels
“Latch Mechanism”
Calcium ions combine with ___________ to cause activation of myosin kinase and phosphorylation of the myosin head.
Calmodulin (Counterpart of Troponin)
The calcium ions bind with calmodulin; the
calmodulin-calcium complex then joins with and
activates ______________
myosin kinase (phosphorylating enzyme)
One of the light chains of each___________ becomes phosphorylated in
response to myosin kinase
myosin head (regulatory chain)
When the “regulatory chain” is phosphorylated, the
HEAD has the capability of binding with the actin
filament, causing _________
muscle contraction
is required for cessation of contraction
Myosin Phosphatase
Once ________ falls below a critical level, processes automatically reverse except for phosphorylation of the myosin head.
calcium ion concentration
Reversal of this step requires ___________, which splits the phosphate from the regulatory chain
— relaxation then occurs
myosin phosphatase
neuromuscular junctions of the highly structured
type found on skeletal muscle fibers are not present in smooth muscles
due to the presence of:
- Diffuse junctions formed by the autonomic fibers
- Varicosities on the axon terminals distributed along the axis (loaded with transmitter substance) (- axon do not have typical branching)
- Contact junctions in the multi unit of smooth
muscles (similar to neuromuscular junction)
Neurotransmitters _________ & ____________ can have
excitatory or inhibitory effects at the smooth muscle neuromuscular junction
acetylcholine and norepinephrine
○ Secreted by the autonomic neurons that innervate the smooth muscles
○ Subs are never secreted at the same time
acetylcholine and norepinephrine
excitatory transmitter for smooth
muscle fibers in some organs but inhibitory in others
Acetylcholine
would inhibit some excitatory
actions of the acetylcholine
Norepinephrine
depends of the smooth muscle and the momentary condition of the muscle
is usually about -50 to -60 millivolts, or about 30
millivolts less negative than in skeletal muscle
RMP (Resting membrane potential)
Action Potentials occur in single-unit smooth muscle, such as visceral smooth muscle, in a manner similar to that of skeletal muscle, occurs in 2 forms:
- Spike potentials
- Action potentials with plateaus
—occurs in most types of
single unit smooth muscle and can be
elicited by electrical stimulation, stretch or
the action of hormone/ transmitter subs,
spontaneous generation from muscle fiber
Spike potentials
onset is similar of the spike, however
depolarization is delayed for about several
100 milliseconds
Action potentials with plateaus
accounts for the prolonged
period of contraction in the
uterus, ureter, and some types of
vascular smooth muscles
Action potentials with plateaus
are required for generating smooth muscle action potentials
○ Movement of Ca ions to the interior of the fiber is mainly responsible for the action potential
Calcium Ions
_______ in single unit smooth muscle can lead to generation of action potentials
Slow-wave potentials
are slow oscillations in membrane
potentials.
is not an action potential , but can LEAD or can CAUSE an action potential —> making muscle contract
Slow waves
Causes of slow waves:
- The oscillation of sodium pump activity
which causes membrane potential to
become more negative (sodium is
pumped rapidly) and less negative (sodium
is pumped slowly) - The conductance of the ion channel may
increase & decrease rhythmically
sodium is pumped rapidly
membrane potential become more negative
Action potential can be initiated when the potential of a slow wave raises above the threshold ___________
-35 millivolts
__________are often generated when
visceral (Single-Unit) smooth muscle is stretched
Spontaneous Action Potentials
Spontaneous action potentials result from a combination of the normal SLOW WAVE potentials in addition to a decrease in the negativity of the
membrane potential caused by the ____________
stretch (self-stimulus)
The response to stretch allows the GI wall when excessively stretched, to ______________ thereby resisting the stretch
contract rhythmically
______________ in blood vessels occurs in response
to local tissue factors
Smooth Muscle Relaxation
is required for local control of blood flow
Vasodilatory response
Many circulating ________in the body affect smooth muscle
contraction to some degree
hormones
A hormone causes contraction when the muscle cell membrane contains ____________ for the
respective hormone
excitatory receptors
Conversely, the hormone causes relaxation if the membrane contains _________________
inhibitory receptors
Fibers are stimulated by certain:
neurotransmitters
(acetylcholine & norepinephrine),
hormones, or
autorhythmic signals (eg in the GIT system to move food along)
Action Potentials are transmitted via
gap junction
- Walls of arteries and veins
○ Walls of Hollow organs
○ Walls of airway to the lungs
○ (Arrector pili) Muscles that attach to hair follicles
○ Muscles that adjust pupil diameter
○ Muscles that adjust focus of the lens in the eye
smooth mucles
Layers of Smooth Muscle (2) oriented at right angles to each other:
○ Longitudinal layer — causes organ dilation & shortening
○ Circular layer — causes organ constriction & elongation
in skeletal muscle cell, which has neuromuscular junction, in smooth muscle cells you
see these _________
varicosities
in skeletal muscle you have somatic nerve fibers, in smooth muscle -
autonomic nerve fibers
series of neurotransmitter-filled bulges
an axon courses through smooth muscle, loosely forming motor units ([link]).
releases neurotransmitters into the synaptic cleft.
varicosities
With sympathetic innervation,
usually the neurotransmitter is ________
norepinephrine
parasympathetic innervation, generally the
neurotransmitter will be _________.
acetylcholine
Contains both thick filaments and thin filaments
- Not arranged in orderly sarcomeres
- Arranged diagonally
- No regular pattern of overlap - not striated
SMOOTH MUSCLE TISSUE
_________in the sarcolemma instead of T tubules
Caveolae
why smooth muscles are not striated?
thick filaments and thin filaments are not arranged in orderly sarcomeres
No regular pattern of overlap
__________in the sarcolemma instead of T tubules
Caveolae
not may SR in smooth muscle tissue so most of the calcium will come out from _____________
extracellular fluid
instead of Z disks, it has _______
dense bodies
functions same as Z disks,
During contraction the filaments pull on the _________ causing a shortening of the muscle fiber
dense bodies
protein used to bind calcium in the cytosol (instead of troponin)
then activates myosin kinase, which
phosphorylates myosin head
Calmodulin
During contraction, the __________ do get pulled closer to each other
dense bodies
There is an ion channel that allows calcium inside.
Remember, most of your calcium will come from the _________, but some will still come from the SR
extracellular fluid
calcium makes its way into the ______
cytosol
calcium becomes available, __________ will
activate
calmodulin
This activated calmodulin can then activate __________.
and they can then catalyze the transfer of this phosphate from ATP onto myosin
myosin light chain kinases
the activated kinase enzymes catalyze transfer of phosphate to myosin, activating the
myosin ATPases
Starts slower and lasts much longer than skeletal
muscle contraction
SMOOTH MUSCLE TISSUE - physiology
Prolonged presence of calcium in the cell provides for a state of continued ___
partial contraction
calcium moves slowly out of the muscle fiber, delaying _________
relaxation
delay relaxation is important in ___________ where a steady pressure is maintained on the contents of the tract
Gastrointestinal tract
delay relaxation is important in the _________ which maintain a
steady pressure on blood
walls of blood vessels
Smooth muscle fibers contract or relax in response to:
- Action potentials
- Stretching
- Hormones
- Changes in pH, oxygen and carbon dioxide levels
- Temperature
Action potentials is from the ________
unlike skeletal system that uses somatic nervous system
autonomic nervous system
cold temperature can cause muscle contractions
true
Pupil constriction due to increased light energy is an example of
action potential
Food in digestive tract stretches intestinal walls
initiating peristalsis
stretching
_________ causes relaxation of smooth muscle in the air-ways and in some blood vessel walls
Epinephrine
______________of the cell membrane opens voltage-gated Ca2+ channels and Ca2+ flows
into the cell down its electrochemical gradient, increasing the intracellular [Ca2+].
Depolarization
Hormones and neurotransmitters may open ligand-gated Ca2+channels in the cell membrane.
They also directly release Ca2+ from the SR through_______
inositol 1,4,5-triphosphate
(IP3)-gated Ca2+ channels
Ca2+ binds to calmodulin.
The Ca2+–calmodulin complex binds to and activates myosin light-chain kinase.
When activated, myosin light-chain kinase phosphorylates myosin and allows it to bind to actin, thus initiating cross-bridge cycling.
The amount of tension produced is proportional to the intracellular Ca2+ concentration.
A decrease in intracellular [Ca2+] produces relaxation
Upstroke of action potential in skeletal muscle and smooth muscle is Inward Na+ current
in cardiac muscle:
Inward Ca2+ current (SA node)
Inward Na+ current (atria,
ventricles, Purkinje fibers)
Plateau in skeletal muscle and smooth muscle is NO (absent)
in cardiac muscle:
No (SA node)
Yes (atria, ventricles, Purkinje
fibers; due to inward Ca2+ current)
Duration of action potential in skeletal muscle is 1msec
smooth muscle is 10msec
cardiac muscle:
150 msec (SA node, atria)
250–300 msec (ventricles and
Purkinje fibers)
Excitation–contraction
coupling in smooth muscle:
Action potential opens
voltage-gated Ca2+
channels in cell membrane
Hormones and transmitters
open IP3– gated Ca2+
channels in SR
Molecular basis for
contraction
Ca2+–calmodulin ↑ myosin light-chain kinase
