Muscle Tissue Flashcards
Skeletal Muscle Tissue characteristics
attached to bones and skin voluntary (conscious control) striated powerful multi nucleated
cardiac muscle tissue characteristics
only in the heart
striated
involuntary
uni-or-binucleated
smooth muscle tissue characteristics
in the walls of hallow organs (stomach, GI, urinary, airways)
not striated
involuntary
uninucleated
what are the 4 special characteristics of muscle tissue?
excitability - ability to receive and respond to stimuli
contractility - ability to shorten when stimulated
extensibility - ability to be stretched
elasticity - ability to recoil to resting length
what are the 4 muscle functions?
movement of bones or fluids
maintaining posture and body position
stabilizing joints
heat generation
epimysium
dense irregular connective tissue surrounding entire muscle
connective tissue sheath
surrounds the whole muscle and is continuous with the tendon
perimysium
fibrous connective tissue surrounding fascicles (groups of muscle fibers)
endomysium
fine areolar connective tissue surrounding each muscle fiber
surrounds each cell
what are the 2 ways muscles attach?
- directly - epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage
- indirectly - connective tissue wrappings extend beyond the muscle as a rope-like tendon or sheetlike aponeurosis
skeletal muscle fiber characteristics
10 to 100um in diameter, up to 30cm long
multiple peripheral nuclei
results from the fusion of hundreds of embryonic precursor cells called myoblasts
a cell made from the fusion of many others is known as syncytium
glycosomes for glycogen storage, myoglobin for oxygen storage
what are myofibrils?
densely packed, rodlike elements that take up ~80% of muscle fiber
very fine contractile fibers, groups of which extend in parallel columns along the length of striated muscle fibers
made up of thick and thin myofilaments which give the muscle its striped appearance
what are sarcomeres?
the smallest contractile unit (functional unit) of muscle fiber
the region of a myofibril between 2 successive z discs
each sarcomere contains 2 types of
myofilaments
- thick filaments: composed of myosin
- thin filaments: composed to actin
thick filament characteristics
made up of myosin
myosin has a hinge that separates the tail and head
runs the entire length of an A band
dark band
thin filament characteristics
run the length of the I band and partway into the A band
associated with actin filaments
Z disc vs H zone vs M line
z disc - coin-shaped sheet of proteins that anchors thin filaments and connects myofibrils to one another
H zone - lighter midregion where the thick and thin filaments do not overlap
M line: line of protein myomesin that holds adjacent thick filaments together
describe what filaments are in each part of a sarcomere
I band - only thin filaments
H zone - only thick filaments
M line - thick filaments with accessory proteins
Outer edge of A band - thick and thin filaments overlap
what is the structure of a thick filament?
composed of myosin
has 2 myosin tail that are 2 interwoven heavy polypeptide chains
myosin heads are connected to the tails, and act as cross bridges during contraction
- binding sites for actin
- binding sites for ATP
- ATPase enzymes
structure of a thin filament
twisted double strand of filamentous protein F-actin
F-actin consists of globular actin subunits
G-actin bears active sites for myosin head during contraction
tropomyosin and troponin are regulatory proteins bound to actin
what are the 2 proteins bound to actin
tropomyosin and troponin
sarcoplasmic reticulum
network of smooth endoplasmic reticulum (SER)
pairs of terminal cisternae form perpendicular cross channels
functions in the regulation of intracellular calcium levels
t-tubules
continuous with sarcolemma
penetrate the cell’s interior at each A band - I band junction
associate with the paired terminal cisternae to form triads that encircle each sarcomere
what is in each triad
t-tubule
terminal cisternae of the SR (2)
what are the relationships associated with triads
t-tubules conduct impulses deep into muscle fiber
integral membrane proteins protrude into the intermembrane space between T-tubule and SR cisternae membranes
T-tubule proteins are voltage sensors
SR foot proteins are gated channels that regulate calcium release from the SR cisternae
sliding filament model of contraction
relaxed state - when thick and thin filaments overlap only slightly
contraction - myosin heads bind to actin, detach and bind again propelling thin filaments toward the M line
- contraction occurs when tension generated by cross bridges on the thin filaments exceeds the forces opposing shortening
voltage sensors - DHP receptor
DHP: dihydropyridine
calcium channel voltage sensor
interacts with proteins found in the cisternae
in the transverse tubule
- interact with sarcoplasmic reticulum protein
ryanodine receptors
interacts to bind with the sarcoplasmic protein
located in the sarcoplasmic reticulum membrane and responsible for the release of calcium from intracellular stores during excitatio-contraction coupling in both cardiac and skeletal muscle
what happens during contraction?
contraction occurs when tension generated by cross bridges on the thin filaments exceeds the forces opposing shortening
brings the 2 z-discs together to basically close in the H zone
relaxes sarcomere - some overlap between the thick and thin filaments, and H zone is easily seen
characteristics of a contracted sarcomere
I band is very small
H zone can disappear
the filaments are closer as they are contracted together
Z discs become closer together
Dark A band does not change (constant)
Neuromuscular junction characteristics
skeletal muscles are stimulated by somatic motor neurons
each motor neuron axon forms several branches as it enters a muscle and its ending forms a neuromuscular junction (NMJ) with a muscle fiber
NMJ is situated midway along the length of a muscle fiber
axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft
synaptic vesicles in axon terminal have the neurotransmitter acetylcholine (ACh)
what are the 6 events that occur at the neuromuscular junction?
- action potential arrives at axon terminal of motor neuron
- voltage-gated calcium channels open and calcium enters the axon terminal
- calcium causes synaptic vesicles to release ACh
- ACh diffuses across the synaptic cleft and binds to receptors in the sarcolemma
- ACh binding opens ion channels that allow simultaneous passage of sodium into the muscle fiber and potassium out
- ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase
what are the 3 events of the generation of an action potential?
- local depolarization: generation of the endplate potential on the sarcolemma
- generation and propagation of the action potential
- repolarization
repolarization
once the action potential happens, need to repolarize the channel for the next action potential
sodium channels will close, and then potassium channels reopen which leads to repolarization
what is resting potential?
what do you need to reach for the channels to open?
-70mV
once activated, the voltage gated sodium channels open when you reach a threshold of -55mV
what is the generation of the action potential accredited to?
sodium entry
sodium flows in, potassium flows out
non selective
what is the role of calcium in contraction? *low calcium concentration
low intracellular calcium concentration - tropomyosin blocks the active sites on actin
myosin heads cannot attach to actin
what is the role of calcium in contraction? *high calcium concentration
calcium binds to troponin and changes shape and moves tropomyosin away from active sites
events of the cross bridge cycle occur
when nervous stimulation ceases, calcium is pumped back into the SR and contraction ends
calcium-activated troponin undergoes a conformational change that moves tropomyosin away from actins binding sites
what are the 6 actions that happen with excitation-contraction (EC) coupling?
- Action potential is propagated along the sarcolemma and down the T-tubules
- Activate DHPR – voltage sensitive tubule protein
a. Activate the receptors in the T-tubule - Calcium ions are released by the Ryanodine receptor (RyR1)
a. Found on SR
b. Goes into the cytoplasm to lead to muscle contraction - Calcium binds troponin and removes the blocking action of tropomyosin
- Binds to troponin and ship the tropomyosin out of the area
- Add calcium, shifts tropomyosin away and active sites are exposed and ready for myosin binding
- Myosin can bind to actin molecules and continue to contract the muscle until maximal contraction
latent period
time between action potential starting and the beginning of a contraction
what is excitation contraction coupling?
sequence of events by which transmission of an action potential along the sarcolemma leads to sliding of the myofilaments
action potential is propagated along sarcomere to T-tubules
voltage sensitive proteins stimulate calcium release from SR (calcium is necessary for contraction)
cross bridge formation
high-energy myosin head attaches to thin filament
working (power) stroke of the cross bridge cycle
myosin head pivots and pulls thin filament toward M-line
cross bridge detachment of the cross-bridge cycle
ATP attaches to myosin head and the cross bridge detaches
cocking of the myosin head of the cross bridge cycle
energy from hydrolysis of ATP cocks the myosin head into the high-energy state
cross bridge cycle steps
- cross bridge formation (myosin attaches to the thin filament)
- the power stroke happens, and ADP + P are released
- cross bridge detachment
- cocking of myosin head causes ATP hydrolysis
isometric contraction
no shortening; muscle tension increases but does not exceed the load
the load is greater than the tension the muscle is able to develop
muscle neither shortens nor lengthens
isotonic contraction
muscle shortens because muscle tension exceeds the load
what is a motor unit
a motor neuron and all (4-several hundred) muscle fibers it supplies
a motor unit extends from the motor neuron cell body (spinal cord), extends by the motor neuron axon, to the muscle fibers to activate it
muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle
small motor unit
muscles that control fine movements (fingers, eyes)
what are the 3 types of troponin?
troponin I - interacts with actin
Troponin C - interacts with calcium
Troponin T - interacts with tropomyosin
large motor unit
large weight-bering muscles (thighs and hips)
what are the 3 phases of a muscle twitch?
latent period: events of excitation-contraction coupling
period of contraction: cross-bridge formation; tension increases
period of relaxation: calcium reentry into the SR; tension declines to 0
what is a muscle twitch?
response of a muscle to a single, brief threshold stimulus
simplest contraction observable in the lab
why are there differences in muscle twitch?
different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles
extraocular muscle - eye movements (want a fast twitch that goes on and off quickly)
what are graded muscle responses? what are they graded by?
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
what is the response of a single stimulus?
single stimulus - single twitch
muscle contractions and relaxes
what is the response of an increase in frequency of stimulus?
muscle does not have time to relax between stimuli
calcium release stimulates further contraction -> temporal summation (wave)
further increase in stimulus frequency - unfused tetanus
low stimulation frequency - causes unfused tetanus
what is the response of multiple quick stimuli?
high stimulation frequency causes fused (complete) tetanus
threshold stimulus
stimulus strength at which the first observable muscle contraction occurs
contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
size principle
motor units with larger and larger fibers are recruited as stimulus intensity increases
what is the force of concentration affected by?
frequency of stimulation - increased frequency allows time for more effective transfer of tension to noncontractile componetns
number - the number of muscle fibers controlled (recruitment)
length-tension relationship - muscles contract most strongly when muscle fibers are 80-120% of their normal resting length
relative size of fibers - hypertrophy of cells increase length
what increases contractile force?
large number of muscle fibers activated
large muscle fibers
high frequency of stimulation
muscle and sarcomere stretched slightly over 100% of resting length
muscle tone
constant, slightly contracted state of all muscles
due to spinal flexes that activate groups of motor units alternately in response to input from stretch receptors in muscles
keep muscles firm, healthy, ready to respond
isotonic contractions are either….
concentric - the muscle shortens and does not work
eccentric - the muscle contracts as it lengthens
describe the resistance and speed of contraction
there is an inverse relationship between the amount of resistance and the speed of contraction
when the load exceeds the ability of the muscle to move it, the velocity of shortening becomes zero and the contraction is isometric
isotonic vs isometric
isotonic - concentric contraction and eccentric contraction (muscle changes shape)
isometric - muscle does not change length
what are 3 ways ATP is regenerated?
- direct phosphorylation of ADP by creatine phosphate (CP)
- Anaerobic pathway (glycolysis)
- Aerobic respiration
direct phosphorylation of ADP
-Coupled reaction of creatine phosphate (CP) and ADP
-Requires creatine kinase – to generate ATP
oCan buffer the ATP generation
- Creatine phosphate gives its phosphate to ADP to form ATP
- Energy source: CP
- Oxygen use: None
- Products: 1 ATP per CP, creatine
- Duration of energy provision: 15 seconds
aerobic pathway
-Produces 95% of ATP during rest and light to moderate exercise
-Fuels:
oStored glycogen
oThen bloodborne glucose, pyruvic acid from glycolysis and free fatty acids
- Duration of energy provisions: Hours
- Glucose comes from glycogen or blood, then converted to pyruvic acid (no oxygen)
-Respiration in the mitochondria requires oxygen, and can generate a lot of ATP per glucose molecule
- Can also use fatty acid and amino acids as an energy source
- Can be used for running, long duration exercise
-Exercise gets easier here after 5 minutes, once the aerobic pathway kicks in
anaerobic pathway: lactic acid and 70% maximum contractile activity
at 70% maximum contractile activity
- bulging muscles compress blood vessels
- oxygen delivery is impaired
- pyruvic acid is converted into lactic acid
Lactic acid
- diffuses into bloodstream
- used as fuel by the liver, kidneys, heart
- converted back into pyruvic acid by the liver
anaerobic pathway characteristics
energy source - glucose
derived from what is in the muscle cell or from glycogen
- breaking down glucose decreases blood glucose, then enters the metabolic pathway to glycolysis
- oxygen use: none (not oxygen required)
products: 2 atp per glucose, lactic acid
duration: 60 seconds
muscle fatigue occurs when..
ionic imbalances (potassium, calcium, phosphate) interfere with E-C coupling
prolonged exercise damages the SR and interferes with calcium regulation and release
low intensity exercise vs high intensity exercise muscle fatigue
high intensity-short duration exercise produces rapid muscle fatigue (with rapid recovery)
- sodium potassium pumps cannot restore ionic balances quickly enough
- dysregulation of ionic substances
Low intensity long duration exercise produces slow developing fatigue
- SR is damaged and calcium regulation is disrupted
muscle fatigue characteristics
physiological inability to contract
rarely because of a lack of ATP
total lack of ATP occurs rarely, during states of continuous contraction and causes contractures (continuous contractions)
oxygen deficit
-Extra oxygen needed after exercise for:
oReplenishment of oxygen reserves, glycogen stores, ATP and CP reserves, and conversion of lactic acid to pyruvic acid, glucose, glycogen
-EPOC – excess post-exercise oxygen consumption
oWhen you are done exercising, still feel winded with some energy
oNeed to replace oxygen reserves and glycogen stores
velocity and duration of contraction is
muscle fiber type
load
recruitment
what are the 2 characteristics of muscle fiber type
Speed of contraction: slow or fast, according to:
- Speed at which myosin ATPases split ATP
- Pattern of electrical activity for the motor neurons
Metabolic pathways for ATP synthesis
- Oxidative fibers – use aerobic pathways
- Glycolytic fibers - use anaerobic glycolysis
slow oxidative fibers
Soleus muscle Aerobic pathway High myoglobin in muscle cells Low glycogen stores Do not need a lot of contraction to activate Red colour
fast oxidative fibers
Intermediate between the slow and fast Can be both aerobic and anaerobic Intermediate glycogen stores High myoglobin Pink colour
fast glycolytic fibers
Rapid on, rapid off Does not use oxygen primarily Low myoglobin High glycogen stores Need a lot of stimulation to be able to contract White in colour Use glycogen store levels (high levels)
aerobic (endurance) exercise results in
Leads to increased:
- Muscle capillaries
- Number of mitochondria
- Myoglobin synthesis
Results in greater endurance, strength, and resistance to fatigue
May convert fast glycolytic fibers into fast oxidative fibers
Little hypertrophy but biochemical adaptations within muscle fibers
Concentration and activities of oxidative enzymes (succinate dehydrogenase, SDH)
SDH activity
low activity is light
high activity is dark
effects of resistance exercise
Resistance exercise (typically anaerobic) results in:
- Muscle hypertrophy (increase in fiber size)
- Increased mitochondria
- Myofilaments
- Glycogen stores and connective tissue
overload principle
forcing a muscle to work hard promotes increased muscle strength and endurance
muscles adapt to increased demands
muscles must be overloaded to produce further gains
smooth muscle
found in walls of most hollow organs (except heart)
- usually 2 layers (longitudinal and circular)
Longitudinal - looks like cubes
- along the length
- shortening of the tube when contracted
Circular - very stretched out
- interior of the smooth muscle cell
- pinches of the tube diameter smaller
peristalsis
Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs
Longitudinal layer contracts; organ dilates and shortens
Circular layer contracts: organ constricts and elongates
Pinching of the layer
Moves the bolus movement along the tube through shortening (like pushing toothpaste out)
innervation of smooth muscle
Pouch like structures as the neuron moves into the layer: Varicosities
In each varicosity, have a lot of neurons and mitochondrion
Varicosities: release their neurotransmitters into a wide synaptic cleft (diffuse junction)
Norepinephrine gets released by the sympathetic nerve on the alpha-adrenergic receptors (smooth muscle cell)
myofilaments in smooth muscle
Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:6)
Thick filaments have myosin heads along their entire length
No troponin complex; protein calmodulin binds calcium
- Troponin is associated with thin filaments
Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
Dense bodies: protein that anchor non-contractile intermediate filaments to sarcolemma at regular intervals
- Membrane structures in SR
contraction of smooth muscle
Slow, synchronized contractions
Some cells are electrically coupled by jap junctions
Some cells are self-excitatory and act as pacemakers for sheets of muscle
Rate and intensity of contraction may be modified by neural and chemical stimuli
Sliding filament mechanism
- Overlap between thick and thin filaments, pulling of actin filaments along the length of the thick filament which contains myosin
- Final trigger is an increase in intracellular calcium
- Calcium is obtained from the SR and extracellular space
Calcium enters the cytosol from the extracellular fluid via voltage-dependent or voltage-independent calcium channels or from scant SR
Calcium binds to and activates calmodulin
Activated calmodulin activates MLCK
The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases
- 2 phosphate molecules added to the myosin head
Activated myosin forms cross bridges with actin of the thin filaments and shortening begins
- Very energy efficient (slow ATPases)
Myofilaments may maintain a latch state for prolonged contractions
role of calcium ions in smooth muscle
- Calcium binds to and activates calmodulin
- Activated calmodulin activates myosin (light chain) kinase (MLCK)
- Activated kinase phosphorylates and activates myosin 2
- Cross bridges interact with actin
- Source of calcium comes from extracellular sources to lead to muscle contraction, and internal stores
- Unlike skeletal and cardiac (only get calcium from SR), smooth muscle has 2 sources of calcium to utilize
- Outside the cell and internal stores
regulation of contraction
Neural regulation
- Neurotransmitter binding – increases concentration of calcium in sarcoplasm; either graded (local) potential or action potential
- Response depends on neurotransmitter released and type of receptor molecules
Hormones and local chemicals
- May bind to G-protein-linked receptors
- May either enhance or inhibit calcium entry
special features of smooth muscle contraction
Stress-relaxation response
- Responds to stretch only briefly, then adapts to new length
- Retains ability to contract on demand
- Enables organs such as the stomach, and bladder to temporarily store contents
Length and Tension changes
- Can contract when between half and twice its resting length
Hyperplasia
- Smooth muscle cells can divide and increase their numbers
- Example: estrogen effects on uterus at puberty and pregnancy
types of smooth muscle: single unit
The cells of single-unit smooth muscle: visceral muscle
- Contract rhythmically as a unit
- Are electrically coupled to one another via gap junctions
- Spontaneous AP
- Are arranged in opposing sheets and exhibit stress-relaxation response
types of smooth muscle: multiunit
Multiunit smooth muscles are found
- In large airways to the lungs and large arteries
- In arrector pili muscles attached to hair follicles
- Eye muscles of the iris
Characteristics include
- Rate gap junctions
- Infrequent spontaneous depolarizations
- Structurally independent muscle fibers
- A rich nerve supply, which, with several muscle fibers, forms motor units
- Graded contractions in response to neural stimuli
