Chapter 10 - Muslce Tissue Flashcards
What are the four key functions of muscular tissue?
- Produce body movement
- Stabilize body position
- Storing and moving substances within the body
- Generate heat
How does muscle tissue contribute to homeostasis?
By producing body movements, moving substances through the body and producing heat to maintain normal body temperature
What is myology?
The study of muscles
What are the three different kinds of muscle tissue?
Skeletal
Cardiac
Smooth
What is skeletal muscle tissue?
Moves the bones of the skeleton
Striated muscle
Work in a voluntary manner
However, some are unconscious … Muscles to maintain posture or stabilize body position, diaphragm for breathing
What is cardiac muscle tissue?
Contained only in the heart
Striated muscle
Involuntary actions
What is autorhythmicity?
Built-in rhythm of the heart
Several hormones and neurotransmitters can adjust heart rate by speeding or slowing the automatic pacemaker of the heart
What is smooth muscle tissue?
Located in the walls of hollow internal structures (blood vessels, airways), in the skin
Non striated muscle - hence … Smooth muscle
Usually involuntary
Some smooth muscles have autorhythmicity (gastrointestinal tract)
Part of autonomic nervous system
What is a sphincter?
Ring-like band of smooth muscle that prevents the outflow of the contents of a hollow organ
What is thermogenesis?
The heat produced from muscle tissue
What is shivering?
Involuntary contractions of the skeletal muscles can increase the rate of heat production
What are the four special properties of muscle tissue?
- Electrical excitability
- Contractility
- Extensibility
- Elasticity
What is electrical excitability?
Ability to respond to certain stimuli by producing electrical signals called action potentials
For muscle cells, what two main types of stimuli trigger action potentials?
- Electrical signals - autorhythmic, arising from the muscle tissue itself (heart’s pacemaker)
- Chemical stimuli - neurotransmitters released by neurons, hormones distributed by the blood or even local changes in pH
What is contractility?
The ability of muscular tissue to contract forcefully when stimulated by action potentials
What is extensibility?
The ability of muscular tissue to stretch, within limits, without being damaged
What kind of tissue allows muscle tissue to stretch?
Connective tissue - limits the range of extensibility and keeps it within the contractile range of the muscle cells
What kind of muscle tissue is subject to the greatest amount of stretching?
Smooth muscle tissue (example - stomach)
What is elasticity?
The ability of muscular tissue to return to its original length and shape after contraction of extension
What separates muscle from the skin?
Subcutaneous layer or hypodermis
Composed of areolar connective tissue and adipose tissue
Provides a pathway for nerves, blood vessels, lymphatic vessels
What is fascia?
A dense sheet of broad band of irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs of the body
What are the functions of fascia?
Holds muscles with similar functions together
Allows free movement of muscles
Carries nerves, blood vessels, lymphatic vessels
Fills space between the muscle
What is the epimysium?
The outermost layer of dense, irregular connective tissue, encircles the entire muscle
What is the perimysium?
A layer of dense irregular connective tissue, surrounds groups of 10-100 muscle fibres, separating them into bundles called fascicles
What is a fascicle?
A bundle of structures, such as nerve or muscle fibres
What is the endomysium?
Penetrates the interior of each fascicle and separates each individual muscle fibres from each other
Mostly reticular fibres
What is a tendon?
Connective tissue that attaches muscle to bone
What happens if the three connective tissue layers of muscle extend beyond the muscle fibres?
Forms a ropelike tendon
Attaches a muscle to the periosteum of a bone
What is the calcaneal (achilles) tendon?
Attaches the gastrocnemius (calf) muscle to the calcaneus (heel bone)
What is aponeurosis?
When the connective tissue of muscle extend as a broad, flat sheet
What is fibromyalgia?
Chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons and ligaments
What does skeletal muscle consist of?
Individual muscle fibers (cells) bundled into fascicles and surrounded by three connective tissue layers that are extensions of the fascia
Which connective tissue coat surrounds groups of muscle fibres, separating them into fascicles?
Perimysium bundles groups of muscle fibres into fascicles
Which neurons stimulate skeletal muscles to contract?
Somatic motor neurons - each somatic neuron has a threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibres
What kind of blood vessels are plentiful in muscular tissue?
Capillaries - each muscle fibre is in close contact with one or more capillaries
How many arteries and veins accompany each nerve that penetrates the skeletal muscle?
1 artery
1 or 2 veins
What is the diameter of a mature skeletal muscle fibre?
Ranges from 10 to 100 micrometers
1 micrometer equals 1/25,000th of an inch
What is the length of a typical skeletal muscle fibre?
10 cm
How does a skeletal muscle fibre arise during embryonic development?
From the fusion of a hundred or more small mesodermal cells called myoblasts
Therefore, each skeletal muscle fibre has a hundred or more nuclei
Is a skeletal muscle fibre able to divide?
No! Once the myoblasts have fused to become a skeletal muscle fibre, it loses its ability for cellular division
The number of skeletal muscle fibres are set before you are born, most of these cells last a lifetime
Where are the nuclei of skeletal muscles located?
Just beneath the sarcolemma
What is the sarcolemma?
The plasma membrane of the muscle cell
What are transverse (T) tubules?
Thousands of tiny invaginations of the sarcolemma
Tunnel in from the surface toward the center of each muscle fibre
Filled with interstitial fluid
Where do muscle action potentials travel through? What does this arrangement ensure?
Along sacrolemma, through the T tubules, quickly spreading throughout the muscle fibre
Ensures that the action potential excites all parts of the muscle five at essentially the same instant
What is sacroplasm? Where is it located?
Within the sarcolemma
The cytoplasm of a muscle fibre
What is myoglobin?
Protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibres from interstitial fluid
Releases the oxygen molecules when it is needed by the mitochondria for ATP production
Where are mitochondria found in muscle fibre?
Lie in rows throughout the muscle fibre, strategically close to the contractile muscle proteins that use ATP during contraction so that ATP can be produced quickly
What are myofibrils?
Contractile organelles of skeletal muscle
2 micrometers in diameter, extend the entire length of the muscle fibre
Prominent striations make the entire skeletal muscle fibre appear striped (striated)
What is the sacroplasmic reticulum?
Fluid-filled system of membranous sacs that encircle each myofibril
What are terminal cisterns?
Dilated end sacs of the sacroplasmic reticulum that butt against the T tubule from both sides
What forms a triad?
A transverse tubule and two terminal cisterns on either side of it
What cation triggers muscle contraction?
Ca+2
Release of calcium from the terminal cisterns of the sacroplasmic reticulum triggers muscle contraction
What is muscular hypertrophy?
Muscle growth that occurs after birth by enlargement of existing muscle fibres
What is muscular hyperplasia?
An increase in the number of fibres
What causes muscular hypertrophy?
Due to increased production of myofibrils, mitochondria, sacroplasmic reticulum and other organelles
How can damaged muscle cells regenerate?
A few myoblasts persist in mature skeletal muscle as satellite cells
They retain the ability to fuse with one another or with damaged cells
What is fibrosis?
The replacement of muscle fibres by fibrous scar tissue
This occurs if the number of satellite myoblasts in not significant
What is muscular atrophy?
A wasting away of muscles
Individual muscle fibres decrease in size as a result of progressive loss of myofibrils
What are filaments?
Also called myofilaments
Contained within myofibrils, smaller protein structures
What is the difference between thin and thick filaments?
Thin filaments are 8 nanometers in diameter, 1-2 micrometers long
Thick filaments are 16 nanometers in diameter, 1-2 micrometers long
Thin filaments are mainly composed of the protein actin
Thick filaments are mainly composed of the protein myosin
What are filaments involved in?
Muscle contraction
What are sarcomeres?
Basic functional unit of a myofibril
How thin and thick filaments are arranged in a myofibril
What is a Z disc?
Separates one sarcomere from another
Narrow, plate-shaped region of dense protein
To what extent do thin and thick ligaments overlap?
Depends on whether the muscle is contracted, relaxed or stretched
What is the A Band?
The darker middle part of the sarcomere
Extends the entire length of the thick filaments
What happens at the ends of each A band?
A zone of overlap
Where thin and thick filaments lie side by side
What causes the striations that can be seen in skeletal muscle?
The pattern of overlap between the thin and thick filaments
Can be seen in both single myofibrils and in whole muscle fibres
What is the I band?
A lighter, less dense area that contains the rest of the thin filaments but NO thick filaments
What passes through the centre of each I band?
Z disc
What is the H zone?
A narrow zone in the center of each A band that contains thick filaments but NO thin filaments
What is the M line?
The middle of the sarcomere
Supporting proteins that hold the thick filaments together at the center of the H zone
Myofibrils are built from what three kinds of proteins? Describe them.
- Contractile proteins - generate force during contraction
- Regulatory proteins - help switch the contraction process on and off
- Structural proteins - keep thick and thin filaments in the proper alignment, gives myofibril elasticity/extensibility, and link the myofibrils to the sarcolemma and extracellular matrix
What are the two contractile proteins in muscle?
- Myosin
2. Actin
What is myosin?
The main component of thick filaments and functions as a motor protein in all three types of muscle tissue
Has a head and tail
What do motor proteins do?
Pull various cellular structures to achieve movement by converting the chemical energy ATP to the mechanical energy of motion
The production of force
In skeletal muscles, how many molecules of myosin form a single thick filament?
About 300
What are anchored to the Z discs?
Thin filaments
What is actin?
The main component of thin filaments
Individual actin molecules join to form an actin filament that is twisted into a helix
One each actin molecule, there is a myosin-binding site, where a myosin head can attach
What are the two regulatory proteins in skeletal muscle fibres?
- Tropomyosin
- Troponin
Both components of thin filaments
What are the key structural proteins in skeletal muscle fibres?
Titin A-actinin myomesin Nebulin Dystrophin
What does tropomyosin do?
In a relaxed muscle, myosin is blocked from binding to actin b/c strands of tropomyosin block the binding sites on actin
What holds the tropomyosin strands in place?
Troponin molecules
What is tinin?
Structural protein that connects the Z disc to the M line of a sarcomere
Helping to stabilize the thick filament position
Can stretch and spring back unharmed
Accounts for much of the elasticity and extensibility of myofibrils
What is a-actinin?
Structural protein of Z discs that attaches to actin molecules of thin filaments to tinin molecules
What is myomesin?
Structural protein that forms the M line
Bonds to tinin molecules and connects adjacent thick filaments to one another
What is nebulin?
Structural protein that wraps around entire length of each thin filament
Helps to anchor thin filaments to the Z disc
Regulates the length of thin filaments during development
What is dystrophin?
Structural protein that links thin filaments of sarcomere to integral membrane proteins in sarcolemma
What are the five levels of organization of skeletal muscle?
- Skeletal muscle
- Fascicle
- Muscle fibre
- Myofibril
- Filaments (myofilaments)
What is the sliding filament mechanism?
Thin and thick filaments do not shorten during contraction of a muscle
Skeletal muscle shortens during contraction b/c the thick and thin filaments slide past each other
What happens to the I band and H zone as a muscle contracts?
I bands and H zones disappear during muscle contraction
Why does muscle contraction occur?
Occurs b/c myosin heads attach to and “walk” along the thin filaments at both ends of the sarcomere, progressively pulling the thin filaments toward the M line
Thin filaments slide inward
Describe the onset of the contraction cycle.
Sacroplasmic reticulum releases calcium ions into the sarcoplasm, there they bind to the troponin. Troponin moves tropomyosin away from the myosin-binding sites on actin. Once the binding sites are free, the contraction cycle begins
What are the four steps of the contraction cycle?
- ATP hydrolysis
- Attachment of myosin to actin to form cross-bridges
- Power stroke
- Detachment of myosin from actin
What would happen if ATP suddenly were not available after the sarcomere had started to shorten?
The cross-bridges would not be able to detach from actin. The muscles would remain in a state of rigidity, as occurs in rigor mortis
Describe the first step of the contraction cycle.
ATP hydrolysis:
Myosin head hydrolyzes ATP and become reoriented and energized
Describe the second step of the contraction cycle.
Attachment of myosin to actin to form cross-bridges:
Myosin head binds to actin, forming cross-bridges
Describe the third step of the contraction cycle.
Power stroke:
Myosin cross-bridges rotate toward center of sarcomere
Slides the thin filament past the thick filament towards the M line
Describe the fourth step of the contraction cycle.
Detachment of myosin from actin:
As myosin heads bind to ATP, the cross-bridges detach from actin
Why will the contraction cycle continue?
Continues as long as ATP is available and the calcium levels near the thin filament is sufficiently high
Why does the contraction cycle not always result in shortening of the muscle fibres?
If the tension generated by the thin filaments is not enough to move the load on the muscle
(Example - trying to lift a whole box of books with one hand)
What is excitation-contraction coupling?
As a muscle action potential propagates along the sarcolemma and into the T tubules, it causes the calcium release channels in the SR to open. The calcium combine with the troponin, causing it to change shape. This shape change causes tropomyosin to move away from the myosin binding site on actin. The contraction cycle beings.
What are the three functions of ATP in muscle contraction?
- Hydrolysis by an ATPase activates the myosin head so it can bind to actin and rotate
- Binding to myosin causes detachment from actin after the power stroke
- Powers the pumps that transport calcium from the cytosol back into the sacroplasmic reticulum
Why is tension maximal at a sarcomere length of 2.2 micrometers?
2.2 micrometers gives a generous zone of overlap between the parts of the thick filaments that have myosin heads and the thin filaments, without the overlap being so extensive that sarcomere shortening is limited
What is the NMJ?
Neuromuscular junction
The synapse between a somatic motor neuron and a skeletal muscle fibre
What is a synaptic cleft?
The gap between two neurons or between a neuron and its target cell
How does communication occur across the synaptic cleft?
Neurotransmitters
What is the end of a motor neuron called?
Axon terminal - divides into a cluster of synoptic end bulbs
What is a synaptic vesicle?
Membrane enclosed sacs that are suspended in the cytosol within each synaptic end bulb (there are hundreds of these in each synaptic end bulb) Contains ACh (acetylcholine)
What is acetylcholine?
ACh
The neurotransmitter released at the NMJ
What is the motor end plate?
The region of the sarcolemma opposite the synaptic end bulbs
Muscle fibre part of the NMJ
How many acetylcholine receptors are within each motor plate?
30-40 million receptors
What is a junctional fold?
Deep grooves in the motor end plate
Provide a large surface area for ACh
Describe how a nerve impulse (nerve action potential) elicits a muscle action potential.
- Release of acetylcholine
- Activation of ACh receptors
- Production of muscle action potential
- Termination of ACh activity
What part of the sarcolemma contains acetylcholine receptors?
Motor end plate
Describe in more detail the first step of how a nerve impulse elicits a muscle action potential.
Release of acetylcholine:
Arrival of the nerve impulse at the synaptic end bulbs stimulate voltage gated channels to open.
Calcium ions from the extracellular fluid flow through the open gates.
Calcium ions stimulate the synaptic vesicles to undergo exocytosis.
Synaptic vesicles release ACh into synaptic cleft.
ACh diffuses across the synaptic cleft between the motor neuron and the motor end plate.
Describe in more detail the second step of how a nerve impulse elicits a muscle action potential.
Activation of ACh receptors:
Two ACh molecules bind to the ACh receptor on the motor end plate.
This opens the ion channel of the ACh receptor.
Sodium ions flow across the membrane.
Describe in more detail the third step of how a nerve impulse elicits a muscle action potential.
Production of muscle action potential:
Inflow of sodium ions makes the muscle fibre more positively charged.
This triggers a muscle action potential.
The muscle action potential then propagates along the sarcolemma into the system of T tubules.
This causes the sacroplasmic reticulum to release its stored calcium ions into the sarcoplasm and the muscle contracts.
Describe in more detail the fourth step of how a nerve impulse elicits a muscle action potential.
Termination of the ACh activity:
The effect of ACh binding lasts only briefly as it is broken down by acetylcholinesterase (AChE).
This enzyme is attached to collagen fibers in the extracellular matrix of the synaptic cleft.
AChE breaks down ACh into acetyl and choline, products that cannot activate the receptor.
What two drugs can block certain events at the NMJ?
Botulinum toxin - blocks exocytosis of synaptic vesicles, muscle contraction does not occur
Curare - binds to and blocks the ACh receptors
What is an EMG test?
Electromyography
Test that measures the the electrical activity in resting and contracting muscles
What are the three ways that muscle fibres can produce ATP?
- From creatine phosphate
- By anaerobic cellular respiration
- By aerobic cellular respiration
What is creatine phosphate?
An energy rich molecule, found in muscle fibres
When muscle fibres are relaxed, they produce more ATP than they need, so excess ATP is used to synthesize creatine phosphate
How is creatine phosphate created?
The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP.
What is anaerobic cellular respiration?
A series of ATP-producing reactions that do not require oxygen
When muscle activity continues and the supply of creatine phosphate within the muscle fibres is depleted, glucose is catabolized to generate ATP.
What is glycolysis?
A series of reactions that quickly breaks down each glucose molecule into two molecules of pyruvic acid
Occurs in the cytosol and produces a net gain of two molecules of ATP
What happens to pyruvic acid during times of heavy exercise?
Anaerobic reactions convert most of the pyruvic acid to lactic acid in the cytosol.
80% of the lactic acid diffuses out of the skeletal muscle fibres into the blood.
Liver cells can convert some of the lactic acid back to glucose
What is aerobic exercise?
Occurs during periods of light to moderate exercise
Where a sufficient amount of oxygen is available to skeletal muscle fibres
Pyruvic acid enters the mitochondria, where it is completely oxidized in reactions that generate ATP, carbon dioxide, water and heat.
Slower than glycolysis, but yields more ATP (36 molecules)
What are the two main sources of oxygen for muscle tissue?
- Oxygen that diffuses into muscle fibres from the blood
2. Oxygen released by myoglobin within muscle fibres
What are two oxygen binding proteins and where are they found?
Myoglobin - muscle cells
Hemoglobin - red blood cells
What is muscle fatigue?
The inability of a muscle to maintain force of contraction after prolonged activity
What factors contribute to muscle fatigue?
Inadequate release of calcium ions from the sacroplasmic reticulum
Low oxygen
Depletion of glycogen
Buildup of lactic acid and ADP
What is oxygen debt?
Refers to the added oxygen, over and above the resting consumption, that is taken into the body after exercise
Used to restore metabolic conditions to the resting level
In what three ways is the oxygen debt restored?
- Convert lactic acid back into glycogen stores in the liver
- Resynthesize creatine phosphate and ATP in muscle fibres
- Replace the oxygen removed from myoglobin
How is oxygen use boosted AFTER exercise?
- Elevated body temperature - faster chemical reactions, use more ATP, more oxygen is needed to generate ATP
- Heart and muscles used in breathing are still working harder, they consume more ATP
- Tissue repair are occurring at an increased pace
What does the total force or tension that a single muscle fibre can produce, depend on?
On the rate at which nerve impulses arrive at the neuromuscular junction - “Frequency of stimulation”
Number of muscle fibres contracting in unison
Amount of stretch in the muscle fibre before contraction
Nutrient and oxygen availability
What is a motor unit?
Consists of a somatic motor neuron and all the skeletal muscle fibres it stimulates.
The muscle fibers of a motor unit are dispersed throughout a muscle rather than clustered together
Approximately how many skeletal muscle fibres does a single motor neuron make contact with?
150 - and they all contract in unison
What is a twitch contraction?
The brief contraction of all the muscle fibres in a motor unit in response to a single acton potential in its motor neuron
What is a myogram?
The record of muscle contraction
What is the effect of the size of a motor unit on its strength of contraction?
Motor units having many muscle fibres are capable of more forceful contractions than those having only a few fibres
What is the refractory period?
The period of lost excitability of a muscle fibre
When a muscle fibre receives enough stimulation to contract, it temporarily loses its excitability and cannot respond for a time
What are the three periods of a twitch contraction? Describe them.
- Latent period - muscle acton potential sweeps over the sarcolemma and calcium ions are released from the SR
- Contraction period - calcium ions bind to troponin, myosin binding sites on actin are exposed, cross-bridges form. Peak tension develops
- Relaxation period - calcium ions are actively transported back into the SR, myosin binding sites are blocked by tropomyosin, myosin heads detach from actin, tension decreases
What is wave summation?
When a second stimulation occurs after the refractory period of the first stimulus is over, but before the skeletal muscle has relaxed, the second contraction will actually by stronger than the first
What is an unfused (incomplete) tetanus?
Sustained but wavering contraction
Occurs when a skeletal muscle fibre is stimulated at a rate of 20 to 30 times per second
It can only partially relax between stimuli
What is fused (complete) tetanus?
A sustained contraction in which individual twitches cannot be detected
When a skeletal muscle fibre is stimulated 80-100 times per second
It does not relax
What is motor unit recruitment?
The process in which the number of active motor units increases
- responsible for producing smooth movements rather than a series of jerks
What does the motor unit recruitment process delay?
Muscle fatigue
Weakest motor units are recruited first, with progressively stronger motor units added if the task requires more force
Allows contraction of the whole muscle to be sustained for long periods of time
What is muscle tone?
A small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units
What happens if a muscle is flaccid?
A state of limpness in which muscle tone is lost
What is hypotonia?
Refers to decreased or lost muscle tone
What is hypertonia?
Refers to increased muscle tone as in spasticity or rigidity
What is an isotonic contraction?
The tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length Used for body movements and for moving objects
What are the two kinds of isotonic contractions?
- Concentric isotonic contractions - if the tension generated is enough to overcome the resistance of the object to be moved, the muscle shortens and pulls on another structure (such as a tendon), to produce movement
- Eccentric isotonic contractions - if the length of the muscle increases during a contraction, (lowering your arm to place a book back on the table)
What is isometric contraction?
The tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change in length
(Holding a book steady on an outstretched hand)
- maintains posture
Do not result in body movement, energy is still expended
How are skeletal muscle fibres classified?
- Slow oxidative fibres
- Fast oxidative - glycolytic fibres
- Fast fast glycolytic fibres
Describe slow oxidative (SO) fibres.
Smallest in diameters, least powerful muscle fibre
Appear dark red b/c they contain large amounts of myoglobin and many blood capillaries
Generate ATP mainly by aerobic cellular respiration (b/c they have many large mitochondria)
Slow speed of contraction
Very resistant to fatigue
ATPase in the myosin heads hydrolyzes ATP relatively slowly
Used for marathon running
Describe fast oxidative-glycolytic fibres.
Intermediate in diameter
Large amounts of myoglobin and blood capillaries
Dark red appearance
Generate ATP by aerobic cellular respiration
Moderate resistance to fatigue
Also generate ATP by anaerobic glycolysis
Hydrolyzes ATP faster than the myosin ATPase in SO fibres
Used in waking and jogging
Describe fast-glycolytic fibres.
Largest in diameter Contain the most myofibrils Most powerful contractions Low myoglobin content Few blood capillaries, few mitochondria Appear white in colour Generate ATP by glycolysis Contract strongly and quickly Fatigue quickly Used for weight lifting
If an individual has a higher percentage of SO fibres, what activity would they excel at? FG fibres?
SO fibres - long distance running
FG fibres - weight lifting or sprinting
What is strength training?
Refers to the process of exercising with progressively heavier resistance for the purpose of strengthening the musculoskeletal system
What are the benefits of strength training?
Stronger muscles
Increase bone strength
Increase resting metabolic rate
Reduced feelings of stress and fatigue
How are cardiac muscle fibres arranged?
The same as skeletal muscles
Same arrangement of actin and myosin, the same bands, same zones and Z disc.
What disc is unique to cardiac muscle fibres?
Intercalated disc
Irregular thickenings of the sarcolemma that connect the ends of cardiac muscle fibres to one another
What two layers does cardiac muscle fibres have? Which is it lacking?
Endomysium and perimysium
Lacking epimysium
What do the intercalated discs contain?
Desmosomes - hold the fibres together
Gap junctions - allow muscle action potentials to spread from one cardiac muscle fibre to another
Why does a cardiac muscle contraction last much longer than a skeletal muscle twitch?
Prolonged delivery of calcium ions into the sarcoplasm
Calcium ions enter the sarcoplasm from both SR and the interstitial fluid
What is the major physiological difference between skeletal muscle fibres and cardiac muscle fibres?
Cardiac muscle tissue contracts when stimulated by its own autorhythmic muscle fibres
Depends largely on aerobic cellular respiration to generate ATP
What are the two types of smooth muscle tissue?
- Visceral (single-unit) smooth muscle tissue
2. Multiunit smooth muscle tissue
Where is visceral smooth muscle found?
In the skin
In tubular arrangements that form part of the walls of small arteries and veins
Hollow organs
What are the characteristics of visceral smooth muscle?
Autorhythmic - connect to one another by gap junctions
Involuntary
When a neurotransmitter, hormone, or autorhythmic signal stimulates one fibre, the muscle action potential is transmitted to neighbouring fibres, which then contract in unison as a single unit
What does multiunit smooth muscle tissue consist of?
Individual fibres, each with its own motor neuron terminals and with few gap junctions between neighbouring fibres
Where are multiunit smooth muscle tissues found?
Walls of large arteries
Airways to the lungs
Arrector pili muscles that attach to hair follicles
Muscles of the iris that adjust pupil diameter
Ciliary body that adjusts focus of the lens of the eye
What is the microscopic anatomy of smooth muscle?
Thickest in the middle, tapers at each end
Single, oval, centrally located nucleus
Contains both thick and thin filaments, not arranged in orderly sarcomeres
Do not exhibit striations
No transverse tubules
What are caveolae?
Small pouchlike invaginations of the plasma membrane that contain extracellular calcium ions that can be used for muscle contractions
What are dense bodies?
The structures that thin filaments attach to in smooth muscle, similar to Z discs
How is the contraction of smooth muscle different than skeletal muscle?
Starts more slowly and lasts longer
Can shorten and stretch to a greater extent than the other muscle types
How does a smooth muscle contract?
An increase in the concentration of calcium ions in the cytosol initiates contraction. (Just as in striated muscle)
Calcium ions flow into smooth muscle cytosol from both the interstitial fluid and the SR.
There are no T tubules, so it takes longer for the calcium ions to reach the filaments in the center of the fibre to trigger the contraction process.
What is calmodulin?
A regulatory protein that binds to calcium ions in the cytosol
(Troponin takes this role in striated muscle)
What happens after calmodulin binds to calcium ions?
It activates an enzyme called myosin light chain kinase
Uses ATP to add a phosphate group to a portion of the myosin head
Once the phosphate group is attached, the myosin head can bind to the actin and contraction can occur
- works slowly, therefore it contributes to the slowness of smooth muscle contraction
What is smooth muscle tone?
A state of continued partial contraction
B/c calcium ions move out of the muscle fibre slowly, delays relaxation
Where do action potentials come from for smooth muscle contractions?
Autonomic nervous system Stretching Hormones Changes in pH levels Changes in oxygen and carbon dioxide levels Changes in temperature
What is the stress-relaxation response?
When smooth muscles are stretched, they initially contract, developing increased tension.
Within a minute or so, tension decreases
This allows the smooth muscle to undergo great changes in length while retaining the ability to contract effectively
What is hypertrophy?
The enlargement of existing cells
Not due to cellular division (mature skeletal muscles have lost that ability)
What is hyperplasia?
Increase in the number of fibres or cells (due to cellular division)
Can smooth muscle undergo hypertrophy? Hyperplasia?
Hypertrophy - yes
Hyperplasia - certain smooth muscle tissue can undergo hyperplasia, (uterus)
What are pericytes?
Stem cells found in association with blood capillaries and veins, able to produce new smooth muscle fibres
Which muscle tissue has a greater ability to regenerate? Skeletal, cardiac or smooth muscle tissue?
Smooth!
What kinds of muscle tissue are derived from mesoderm?
All kinds - except those of the iris and the acceptor pili muscles attached to hairs
What is a somite?
A series of cubelike structures
As mesoderm develops it becomes arranged in dense columns on either side of the developing nervous system
These columns undergo segmentation into somites.
The cells of a somite differentiate into what three regions?
- Myotome - forms the skeletal muscles of the head, neck and limbs
- Dermatome - forms the connective tissues, including the dermis of the skin
- Sclerotome - gives rise to the vertebrae
How does cardiac muscle develop?
From mesodermal cells that migrate to and envelop the developing heart while it is still in the form of endocardial heart tubes
How do smooth muscles develop?
From mesodermal cells that migrate to and envelop the developing gastrointestinal tract and viscera
What happens to your muscle tissue between the ages of 30-50?
Slow, progressive loss of skeletal muscle mass that is replaced largely by fibrous connective tissue and adipose tissue Approx. 10% of the muscle mass is lost Decrease in muscle strength Slowing of muscle reflexes Loss of flexibility
What happens to muscle between the ages of 50-80?
Loss of another 40%
What is myasthenia gravis?
Autoimmune disease that causes chronic, progressive damage of the NMJ (neuromuscular junction)
Antibodies bind and block some ACh receptors
What is muscular dystrophy?
Refers to a group of inherited muscle-destroying diseases that cause progressive degeneration of skeletal muscle fibres
What is a spasm?
A sudden, involuntary contraction of a single muscle in a large group of muscles
What is a cramp?
A painful spasmodic contraction
Caused by inadequate blood flow, overuse of muscle, dehydration, injury or holding a position for a long period of time
What is a tic?
A spasmodic twitching made involuntarily by muscle that are ordinarily under voluntary control
What is a tremor?
Rhythmic, involuntary, purposeless contraction that produces a quivering or shaking movement.
What is a fasciculation?
Involuntary, brief twitch of an entire motor unit that is visible under the skin
Occurs irregularly and is not associated with movement of the affected muscle
What is a fibrillation?
A spontaneous contraction of a single muscle fibre that is not visible under the skin but can be recorded by electromyography
May signal destruction of motor neurons
What is myalgia?
Pain in or associated with muscles
What is myoma?
Tumor consisting of muscle tissue
What is myomalacia?
Pathological softening of muscle tissues
What is myositis?
Inflammation of muscle fibres (cells)
What is myotonia?
Increased muscular excitability and contractility
Tonic spasm of muscle
What is a volkmann’s contracture?
Permanent shortening of a muscle due to replacement of destroyed muscle fibres with fibrous connective tissue, which lacks extensibility
