Anatomy Muscles Flashcards Origins and Insertions Intended for entry level anatomy students.
Anatomy Muscles
-
Upload
mathios-tigeros -
Category
Documents
-
view
78 -
download
0
description
Transcript of Anatomy Muscles
(Chapter 9; pp. 276-311)
2.3.1 Describe the microscopic structure of skeletal muscle fibers and explain the cellular mechanisms of excitation-contraction
2.3.2 Describe the neuromuscular junction
2.3.3 Describe the contractile properties of skeletal muscle (motor unit, isotonic & isometric contractions, spatial & temporal
summation, etc)
2.3.4 Associate various muscle types with their metabolism and their speed of contraction and rate of fatigue
2.3.5 Compare the properties of smooth muscle with those of skeletal muscle
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Similarities Skeletal and smooth muscle cells are elongated
(not cardiac muscles…) and are called muscle fibers
Muscle contraction depends on two kinds of myofilaments – actin and myosin
Muscle terminology is similar Sarcolemma – muscle plasma membrane Sarcoplasm – cytoplasm of a muscle cell Prefixes – myo, mys, and sarco all refer to muscle
Pacemaker
Can contract rapidly;Tires easily & must rest;Strong but Adaptable
Neural input canIncrease rate
Slow, sustainedcontraction
Muscle Functions1. Generate movement: locomotion, manipulation, blood flow & pressure,
respiration, propelling of food, urine, etc..2. Maintain posture: constantly working against gravity3. Joint stabilization: eg: shoulders, knees when moving parts of skeleton4. Generation of heat: maintenance of body temperature
esp: skeletal muscle (at least 40% of body mass)
Functional Characteristics of Muscle1. Excitability (Irritability): “ability to receive and respond to a stimulus”
stimulus usually a chemical (NT, hormone, pH)response = AP along sarcolemma + muscle contraction
2. Contractility: “ability to shorten forcibly when adequately stimulated”3. Extensibility: “ability to be stretched or extended” 4. Elasticity: “ability to resume resting length after being stretched”
2.3.1.2 list & briefly describe 4 muscle functions as well as 4 2.3.1.2 list & briefly describe 4 muscle functions as well as 4 functional characteristics of musclefunctional characteristics of muscle
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle Each muscle is a discrete organ composed of
muscle tissue, blood vessels, nerve fibers, and connective tissue
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle: Nerve and Blood Supply Each muscle is served by one nerve, an artery, and
one or more veins Each skeletal muscle fiber is supplied with a nerve
ending that controls contraction Contracting fibers require continuous delivery of
oxygen and nutrients via arteries Wastes must be removed via veins
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structure and Organization of Skeletal Muscle
Table 9.1a
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle: 3 connective tissue sheaths
Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber
Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles
Epimysium – an overcoat of dense irregular connective tissue that surrounds the entire muscle
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structure and Organization of Skeletal Muscle
Table 9.1b
2 µm long
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Microscopic Anatomy of a Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma
Fibers are 10 to 100 µm in diameter, and up to 30 cm long Each cell is a syncytium produced by fusion of embryonic
cells
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Microscopic Anatomy of a Skeletal Muscle Fiber Sarcoplasm has numerous glycosomes and a unique
oxygen-binding protein called myoglobin Fibers contain the usual organelles, and myofibrils,
sarcoplasmic reticulum, and T tubules
Myofibrils
Myofibrils are densely packed, rodlike contractile elements
They make up most of the muscle cell volume (80%) The arrangement of myofibrils within a fiber is such that a
perfectly aligned repeating series of dark A bands and light I bands is evident (striated appearance)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sarcomeres
Thick filaments – extend the entire length of an A band
Thin filaments – extend across the I band and partway into the A band
Z-disc – coin-shaped sheet of proteins (alpha-actinin; desmin IF) that (i) anchors the thin filaments and (ii) connects myofibrils to one another
Thin filaments do not overlap with thick filaments in the lighter H zone (and no myosin head)
M lines appear darker due to the presence of the protein myomesin (holding thick filaments together)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sarcomeres
Copyright © 2010 Pearson Education, Inc. Figure 9.3
Flexible hinge region
Tail
Tropomyosin Troponin ActinMyosin head
ATP-bindingsite
Heads Active sitesfor myosinattachment
Actinsubunits
Actin-binding sites
Thick filamentEach thick filament consists of manymyosin molecules whose heads protrude at opposite ends of the filament.
Thin filamentA thin filament consists of two strandsof actin subunits twisted into a helix plus two types of regulatory proteins(troponin and tropomyosin).
Thin filamentThick filament
In the center of the sarcomere, the thickfilaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap.
Longitudinal section of filamentswithin one sarcomere of a myofibril
Portion of a thick filamentPortion of a thin filament
Myosin molecule Actin subunits
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Ultrastructure of Myofilaments: Thick Filaments
Figure 9.4a,b
Thick filaments are composed of the protein myosin
Each myosin molecule has a rod-like tail and two globular heads Tails – two interwoven, heavy
polypeptide chains Heads – two smaller, light
polypeptide chains called cross bridges
Heads bear actin-binding site and ATPase activity
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Ultrastructure of Myofilaments: Thin Filaments
Figure 9.4c
Thin filaments are chiefly composed of the protein actin (microfilaments)
Each actin molecule is a helical polymer of globular subunits called G actin (F actin is the fibrous or filamentous form)
The subunits contain the active sites to which myosin heads attach during contraction
Tropomyosin and troponin (TnI, TnT and TnC) are regulatory subunits bound to actin
Sliding Filament Model of Contraction
• muscle fiber shortens because sarcomeres shorten; filaments remain same length
• thin filaments slide over thick filaments1° - Relaxed: only slight overlap of thick & thin filaments2° - Contracted: thin filaments penetrate more deeply into A band - Z discs pulled toward thick filaments
Overall:distance between Z discs reduced
I bands shortenH zones disappear
A bands move closer together, but stay same length
A. Endomysium; B. Epimysium; C. Fascicle; D. Fiber;
E. Myofilament; F. Myofibril; G. Perimysium; H. Sarcolemma;
I. Sarcomere; J. Sarcoplasm; K. Tendon
1. Connective tissue surrounding a fascicle.
2. Contractile unit of muscle.
3. A muscle cell.
4. Thin connective tissue investing each muscle cell.
5. Plasma membrane of the muscle cell.
6. Cordlike extension of CT beyond muscle - attaches it to bone.
7. A discrete bundle of muscle cells.
J. Carnegie, UofO
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Sarcoplasmic Reticulum (SR)Figure 9.5
SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril
Paired terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular calcium levels
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
T TubulesFigure 9.5
T tubules are continuous with the sarcolemma They conduct impulses to the deepest regions of the
muscle (at each A band–I band junction) These impulses signal for the release of Ca2+ from
adjacent terminal cisternae
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Triad Relationships
T tubules and SR provide tightly linked signals for muscle contraction A double zipper of integral membrane proteins protrudes into the
intermembrane space T tubule proteins act as voltage sensors SR foot proteins are receptors that regulate Ca2+ release from the SR
cisternae
• Skeletal muscles are stimulated by somatic motor neurons
• Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
• Each axon forms several branches as it enters a muscle
• Each axon ending forms a neuromuscular junction with a single muscle fiber
Copyright © 2010 Pearson Education, Inc. Figure 9.8
Nucleus
Actionpotential (AP)
Myelinated axonof motor neuron
Axon terminal ofneuromuscular junction
Sarcolemma ofthe muscle fiber
Ca2+ Ca2+
Axon terminalof motor neuron
Synaptic vesiclecontaining AChMitochondrionSynapticcleft
Junctionalfolds ofsarcolemma
Fusing synaptic vesicles
ACh
Sarcoplasm ofmuscle fiber
Postsynaptic membraneion channel opens;ions pass.
Na+ K+
Ach–
Na+
K+
Degraded ACh
Acetyl-cholinesterase
Postsynaptic membraneion channel closed;ions cannot pass.
1 Action potential arrives ataxon terminal of motor neuron.
2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.
4 Acetylcholine, aneurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
5 ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.
6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.
PLAY A&P Flix™: Events at the Neuromuscular Junction
Copyright © 2010 Pearson Education, Inc. Figure 9.9
Na+
Na+
Open Na+
Channel
Closed Na+
Channel
Closed K+
Channel
Open K+
Channel
Action potential++++++
++++++
Axon terminal
Synapticcleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
2 Generation and propagation ofthe action potential (AP)
3 Repolarization
1 Local depolarization: generation of the end plate potential on the sarcolemma
K+
K+Na+
K+Na+
Wave of dep
olar
izat
io n
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 1
Axon terminalof motor neuron
Muscle fiber Triad
One sarcomere
Synaptic cleft
Sarcolemma
Action potentialis generated
Terminal cisterna of SR ACh
Ca2+
Events of Excitation-Contraction (E-C) Coupling
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 2
1 - Action potential is propagated alongthe sarcolemma and down the T tubules.
Steps in E-C Coupling:
Troponin Tropomyosinblocking active sites
Myosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Myosincross bridge
Ca2+
Sarcolemma
2 - Calcium ions are released.
3 - Calcium binds to troponin andremoves the blocking action oftropomyosin.
4 - Contraction begins
The aftermath
Events of Excitation-Contraction (E-C) Coupling
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 3
Steps inE-C Coupling:
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Ca2+
Sarcolemma
Action potential ispropagated along thesarcolemma and downthe T tubules.
1
Events of Excitation-Contraction (E-C) Coupling
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 5
Troponin Tropomyosinblocking active sitesMyosin
Actin
Ca2+
The aftermath
Events of Excitation-Contraction (E-C) Coupling
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 6
Troponin Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Calcium binds totroponin and removesthe blocking action oftropomyosin.
The aftermath
3
Events of Excitation-Contraction (E-C) Coupling
Figure 9.11
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 7
Troponin Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Myosincross bridge
Calcium binds totroponin and removesthe blocking action oftropomyosin.
Contraction begins
The aftermath
3
4
Events of Excitation-Contraction (E-C) Coupling
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12
Each power stroke =1% shortening
Routine contraction = 30-35% shortening
Many suchCross Bridge Cycles
are required!
(High Energy)
(Bent Low Energy)
Pulls on actinfilament…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)
Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening (load)
When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends
Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced (Ca2+ removal => blockage restored)
http://www.youtube.com/watch?v=DeTen4GVb-Q
PUT IN THE CORRECT ORDER:
1. Myosin heads bind to active sites on actin molecules
2. ATP is hydrolyzed
3. Myosin heads return to high-energy shape, ready for the next power stroke
4. Calcium binds to troponin
5. Cycling continues until calcium ions are sequestered by the SR
6. Myosin cross brides detach from actin
7. Troponin changes shape
8. ADP and Pi (inorganic phosphate) are released from the thick filament
9. Myosin heads pull on the thin filaments (power stroke) and slide them toward the centre of the sarcomere
10. ATP binds to the thick filament
11. Tropomyosin is moved exposing active sites on F-actin J. Carnegie, UofO
http://www.youtube.com/watch?v=DeTen4GVb-Q
IV. CONTRACTION OF A WHOLE SKELETAL MUSCLE2.3.3.1 define motor unit; describe the influences of wave summation and motor
unit summation on the contractile response of skeletal muscle; define tetanus in terms of muscular contraction
The Motor Unit
motor nerve (at least 1/muscle)
hundreds of motor neuron axons
each axon to many axonal terminals
each axonal terminal to NMJ of a single muscle fiber (cell)
• avg. no muscle fibers/neuron = 150 (range = 4 to hundreds)
• when neuron fires, all fibers contract
MOTOR UNIT = 1 motor neuron + all muscle fibers it supplies
Fig. 9.12
Innervated fibers notclustered
1 motor unit = weak contractionof the entire muscle!Depends on specific muscle function…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Phases of a Muscle Twitch
A muscle twitch is the response of a muscle to a single, brief threshold stimulus
Latent period – first few msec after stimulus; EC coupling taking place
Period of contraction – cross bridges form; muscle shortens
Period of relaxation – Ca2+ reabsorbed; muscle tension goes to zero
Figure 9.14a
Myogram
The force exerted by a contracting muscle on an object is called muscle tension
The opposing force exerted on the muscle is called the load
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Twitch Comparisons
Figure 9.14b
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Muscle Responses Graded muscle responses are:
Variations in the degree of muscle contraction Required for proper control of skeletal
movement
Responses are graded by: Changing the frequency of stimulation Changing the strength of the stimulus
Speed of stimulation: wave summation & tetanus:• rapid rate of stimuli: each contraction builds on the previous one ([Ca2+]; Heat)• but AP refractory period is always honored (repolarization)• tetanus: fused contractions - inter-stimulus interval too short to allow
inter-twitch muscle relaxation - eventually followed by muscle fatigue• Affects force, but primary goal is to generate smooth, continuous contractions
Fig. 9.15
(Rare…)
Treppe – Staircase Effect
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Response: Stimulation Strength Threshold stimulus – the
stimulus strength at which the first observable muscle contraction occurs
Beyond threshold, muscle contracts more vigorously as stimulus strength is increased
Force of contraction is precisely controlled by multiple motor unit summation
This phenomenon, called recruitment, brings more and more muscle fibers into play
In the lab
In the body…
Not random…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Size Principle
Figure 9.17
Motor units with the smallest fibers are controlled by highly excitable motor neurons – activated first
As contractile strength increases – larger and larger muscle fibers are recruited
controlled by motor neurons that are least excitable (highest threshold)
Usually asynchronous in the body… delays fatigue!
Also explains wide rangeof possible force…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle tone: It is the constant, slightly contracted state of all muscles, which
does not produce active movements Keeps the muscles firm, healthy, and ready to respond to
stimulus Spinal reflexes account for muscle tone by:
Activating one motor unit and then another Responding to activation of stretch receptors in muscles and
tendons (Chapter 13)
1. After ACh attaches to its receptors at the neuromuscular junction, the next step is:a) K+-gated channels openb) Ca++ binds to troponinc) the T tubules depolarized) cross bridges attache) ATP is hydrolyzed
2. Which of these bands or lines does NOT narrow when skeletal muscle contracts?
a) H zoneb) A bandc) I bandd) M linee) b and d
3. is a continuous contraction that shows no evidence of relaxation.a) tetanusb) all or nonec) isometricd) treppe J. Carnegie,
UofO
2.3.3.2 differentiate between isotonic and isometric contractions, giving an example of each
(1) isotonic: muscle changes in length & moves load - what are concentric vs eccentric isotonic contractions?
NB: eccentric contractions are 50% more forceful, use less ATP/O2 & fewer muscle fibers but more prone to delayed-onset soreness than concentric
contractions
2.3.3.2 differentiate between isotonic and isometric contractions, giving an example of each
(2) isometric: tension increases but muscle remains same length• most body movements are a mix of isotonic and isometric contractions• realize that in isotonic contractions, the thin filaments are sliding, but in
isometric contractions the cross bridges are generating force, but do not move thin filaments
2.3.3.3 Define the optimal length-tension relationship for muscle in terms of muscle anatomy
Fig. 9.21
Fig. 9.22
Factors influencing the Force ofMuscle Contraction
Degree of muscle stretch – length/tension relationship
J. Carnegie, UofO
2.3.3.4 Indicate the influence of load on the velocity and duration of skeletal muscle contraction
Fig. 9.23
Velocity & Duration of Contraction depends on:Load: greater load = longer latent period = slower contraction = shorter
contraction duration
Also depends on Fiber type and Recruitment
The more the merrier!
Copyright © 2010 Pearson Education, Inc. Figure 9.19a
Coupled reaction of creatinephosphate (CP) and ADP
Energy source: CP
(a) Direct phosphorylation
Oxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 seconds
Creatinekinase
ADPCP
Creatine ATP
Creatine Kinase
ATP is the only source used directly for contractile activity
As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by:
Anaerobic glycolysis (Fast Glycolytic Fibers):
• only 2 ATP/glucose but O2 not used and is 2.5x faster than aerobic pathway
• Together, ATP, CP and Anaerobic glycolysis can support strenuous muscle activity for ~ 1 min.
• usually pyruvic acid enters aerobic pathway
When muscle contractile activity reaches 70% of maximum:
Bulging muscles compress blood vessels Oxygen delivery is impaired Pyruvic acid is converted into lactic acid
The lactic acid: Diffuses into the bloodstream Is picked up and used as fuel by the liver,
kidneys, and heart Is converted back into pyruvic acid by the
liver
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Metabolism: Energy for Contraction
Figure 9.19
(Slow Oxydative Fibers)
Energy systems used during sports activities:• weight lifting, diving, sprinting: ATP & CP - very short surge of power• tennis, soccer, 100 m swim: almost entirely anaerobic - on-and-off; burstlike• marathon runs, jogging: mainly aerobic; but anaerobic may function until
aerobic reaches full efficiency – Mainly endurance rather than power
aerobic endurance: length of time a muscle can use aerobic anaerobic threshold: point at which muscle converts to anaerobic
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Fatigue – there’s an end to every good thing…
Muscle fatigue – the muscle is in a state of physiological inability to contract
Muscle fatigue occurs when: ATP production fails to keep pace with ATP use There is a relative deficit of ATP, causing contractures
(cramps! = myosin head can not be released) Lactic acid accumulates in the muscle Ionic imbalances are present (e.g. dehydration; sweat)
Na+-K+ pumps cannot restore ionic balances quickly enough (intense exercise)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Oxygen Debt Vigorous exercise causes dramatic changes in muscle
chemistry For a muscle to return to a resting state:
Oxygen reserves must be replenished Lactic acid must be converted to pyruvic acid (liver) Glycogen stores must be replaced ATP and CP reserves must be resynthesized
Oxygen debt – the extra amount of O2 needed for the above restorative processes
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Heat Production During Muscle Activity Only 40% of the energy released in muscle activity is
useful as work The remaining 60% is given off as heat Dangerous heat levels are prevented by radiation of heat
from the skin and sweating What is shivering then?
Copyright © 2010 Pearson Education, Inc. Figure 9.24
FO
FG
SO
Copyright © 2010 Pearson Education, Inc.
Table 9.2
(O2)
a motor unit consists of just one muscle fiber type…but one muscle usually contain a mix of fiber types…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Effects of Exercise Aerobic exercise (endurance) results in an increase of:
Muscle capillaries Number of mitochondria Myoglobin synthesis
Resistance exercise (typically anaerobic) results in: Muscle hypertrophy Increased mitochondria, myofilaments, and glycogen
stores
Check the appropriate boxes to characterize each type of skeletal muscle fiber:
Characteristics Slow Oxidative Fast Glycolytic Fast Oxidative
Rapid twitch rate
Fast myosin ATPases Use mostly aerobic metabolism
Large myoglobin stores
Large glycogen stores
Fatigue slowly
Fibers are white
Fibers are smallFibers contain many capillaries & mitochondria
J. Carnegie, UofO
Copyright © 2010 Pearson Education, Inc. Table 9.3
http://www.youtube.com/watch?v=DeTen4GVb-Q
Copyright © 2010 Pearson Education, Inc. Table 9.3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Smooth Muscle Composed of spindle-shaped fibers with a diameter of 5-10 µm and lengths of
several hundred µm – Sk. M. are ~10x wider; 1000x longer Lack the coarse connective tissue sheaths of skeletal muscle, but have fine
endomysium Organized into two layers (longitudinal and circular) of closely apposed fibers
Found in walls of hollow organs (except the heart) Have essentially the same contractile mechanisms as skeletal muscle
OrganDilates
& Contracts
OrganElongates
Peristalsis
Parallel to organ…
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Innervation of Smooth Muscle Smooth muscle lack neuromuscular junctions Innervating nerves have bulbous swellings called varicosities Varicosities release neurotransmitters into wide synaptic clefts called
diffuse junctions
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Microscopic Anatomy of Smooth Muscle SR is less developed than in skeletal muscle and
lacks a specific pattern T tubules are absent Plasma membranes have pouchlike infoldings
called caveolae
Ca2+ is sequestered in the extracellular space near the caveolae, allowing rapid influx when channels are opened
SR still releases some Ca2+ and contributes to its removal
1 nucleus per cell!
Copyright © 2010 Pearson Education, Inc. Table 9.3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Proportion and Organization of Myofilaments in Smooth Muscle Ratio of thick to thin filaments is much lower (1:13)
than in skeletal muscle (1:2) Thick filaments have heads along their entire
length (more strength) There is no troponin complex
Thick and thin filaments are arranged diagonally (no sarcomere; no striations), causing smooth muscle to contract in a corkscrew manner
Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals (linked to thin filaments)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Contraction of Smooth Muscle Whole sheets of smooth muscle exhibit slow,
synchronized contraction They contract in unison, reflecting their
electrical coupling with gap junctions Action potentials are transmitted from cell to
cell
Some smooth muscle cells (stomach and small intestine): Act as pacemakers and set the contractile pace for whole sheets
of muscle Are self-excitatory and depolarize without external stimuli
Figure 9.29
Stimuli can vary Different autonomic nerves
release diff. NT NT may excite or inhibit a
particular group of smooth muscle cells
e.g. ACh (+) vs NE (-) in the bronchioles
vs NE (+) in most blood vessels…
Non-neuronal signals: hormones (gastrin), histamine, hypoxia, excess CO2, low pH
Relaxation requires:
• Ca2+ detachment from calmodulin
•Active transport of Ca2+ into SR and ECF
•Dephosphorylation of myosin to reduce myosin ATPase activity
Copyright © 2010 Pearson Education, Inc. Table 9.3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Special Features of Smooth Muscle Contraction Unique characteristics of smooth muscle include:
Smooth muscle tone (24h maintenance in blood vessel; visceral organs) Slow, prolonged contractile activity – 30x longer than Sk. M. Low energy requirements – 1% the energy cost Few mitos; most ATP generated aerobically Sluggish ATPases (Myosin heads)
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Special Features of Smooth Muscle Contraction Response to Stretch: Sk. M. can still contract when stretch up to
60%; Smooth muscle can still generate tension if stretch up to 150%! Smooth muscle exhibits a phenomenon called stress-relaxation
response in which: Smooth muscle responds to stretch only briefly, and then adapts
to its new length The new length, however, retains its ability to contract This enables organs such as the stomach to temporarily store
contents
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Hyperplasia Certain smooth muscles can divide and increase their
numbers by undergoing hyperplasia (vs Hypertrophy) This is shown by estrogen’s effect on the uterus
At puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
During pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus
Secretory functions: collagen, elastin - make their own CT
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Smooth Muscle: Single Unit The cells of single-unit smooth muscle, commonly called
visceral muscle: Contract rhythmically as a unit Are electrically coupled to one another via gap
junctions – no recruitment Often exhibit spontaneous action potentials (presence
of some Pacemaker cells) Few nerve terminals – usually where pacermaker cells
are… Are arranged in opposing sheets and exhibit stress-
relaxation response… and pretty much what we saw so far!
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Smooth Muscle: Multiunit Multiunit smooth muscles are found:
In large airways to the lungs, large arteries In arrector pili muscles, attached to hair follicles In the internal eye muscles (focus)
Their characteristics include: Rare gap junctions Infrequent spontaneous depolarizations Structurally independent muscle fibers A rich nerve supply (still Autonomic/Involuntary), which, with a
number of muscle fibers, forms motor units Graded contractions in response to neural stimuli – recruitment Can also reponsd to Hormonal control
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
1. The first energy source used to regenerate ATP when muscles are extremely active is:
a) fatty acidsb) glucosec) creatine phosphated) pyruvic acid
2. When paying back the oxygen debt:a) lactic acid is formedb) lactic acid is converted to pyruvic acidc) ATP formation requires creatine phosphated) muscle cells utilize glycogen reserves
3. Holding up the corner of a heavy couch to vacuum under it involves which type(s) of contraction?a) tetanicb) isotonicc) isometricd) tonee) a and c
J. Carnegie, UofO