Chapter #9: Muscles & Muscle Tissue Flashcards
Muscle Functions
- Movement
- Body posture & body position
- Joint stability
- Maintaining body temperature
- Excitability
- Contractility
- Extensibility
- Elasticity
Movement
voluntary and involuntary
Body posture & body position
Muscles work to hold us up against gravity
Joint Stability
Muscles & tendons reinforce joints
Maintaining body temperature
Muscle contraction produces heat
Excitability
Membrane potential changes in response to stimulus
Contractility
Muscle cells shorten
Extensibility
Muscles cells can lengthen/stretch
Elasticity
Healthy muscle cells return to their original shape
Types of muscle tissue
- Skeletal muscle tissue
- Smooth muscle tissue
- Cardiac muscle tissue
Skeletal muscle tissue
-Voluntary muscle tissue
-Function: movement of body parts
-Striated & multinucleate
-Attaches to & uses skeleton
-Creates the most force
-But: needs the most rest
-Adaptable
Smooth muscle tissue
-Involuntary muscle tissue
-Function: moves fluids & substances through body
-No striations
-Uninucleate
Cardiac muscle tissue
-Involuntary muscle tissue
-Function: moves blood through body
*Rate of contraction set by pacemaker cells
-Striated
-Uninucleate
Gross Anatomy of Skeletal muscle tissue
-innervation
-vascularization
-connective tissue sheaths
Innervation
-Each muscle receives 1 motor nerve
-Function: nerve ending controls activity
-Motor neuron stimulates muscle fibers to contract
What neurotransmitter is released by motor neurons?
acetylcholine (always stimulatory)
Vascularization
-Each muscle receives 1 artery, 1+ vein
-Functions: Bring in nutrients, remove waste
Connective Tissue Sheaths
-Function: supports muscle, holds muscle together
-Layers:
-Endomysium: innermost layer
-Surrounds individual muscle fibers
-Perimysium: middle layer
-Discrete bundles of muscle fibers grouped together: form fascicles
-Epimysium: outermost layer
-Surrounds entire muscle
Skeletal muscle attachments
-For skeletal muscle to produce movement, it must attach to bone (or another tough structure)
-Why? Our muscles use our bones like levers
-When a muscle contracts, it pulls (or pushes) on a bone to produce movement
Direct vs. Indirect attachment of skeletal muscle attachments
-Direct: epimysium of muscle fuses directly to bone (or cartilage)
-Indirect: involves tendons
-Tendon: a band of dense fibrous connective tissue that connects a muscle to a bone
Is indirect or direct attachment of skeletal muscle attachment more common?
indirect because tendons are stronger and thicker
2 points of attachment for a muscle
1) Origin: where the muscle attaches to a less movable bone
-Always proximal
-Ex: biceps brachii: long head = lip of glenoid fossa, short head = coracoid process
2) Insertion: where the muscle attaches to a movable bone
-Always distal
-Ex: biceps brachii: radial tubersity
Microanatomy of skeletal muscles
Skeletal muscle cells (myocytes, muscle fibers) are among largest and longest cells in the body
Important terminology for skeletal muscle microanatomy
1) Sarcolemma
2) Sarcopla
3) Myofilaments
4) Myofibrils
Sarcolemma
plasma membrane of muscle fibers
Sarcoplasm
-cytoplasm of muscle fibers
-Contain high numbers of
A) Glycosomes: organelles that store glycogen
-Importance: glycogen is a polysaccharide that is converted to glucose for ATP production in skeletal muscle
B) Myoglobin: red pigment organelle that stores oxygen
-Importance: oxygen is needed for ATP production
Myofilaments
-protein filaments in muscle tissue
-Types of contractile myofilaments:
A) Thick filament (myosin)
B) Thin filament (actin)
-Function: actin and myosin interact during muscle contraction
Myosin FIlaments
-Composed of 6 chains
-4 light chains
-2 heavy chains
-Myosin head found at end of each heavy chain
-Each myosin head has 2 binding sites: 1 for ATP, 1 for actin
-Importance: myosin head uses ATP to link two types of myofilaments during contraction
Actin filaments
-Chains of G actin proteins with myosin binding sites
-Function: myosin head binds to the myosin binding site of actin during muscle contraction
-Regulatory proteins of actin control if/when myosin head can bind
2 regulatory proteins associated with actin filaments
1) Tropomyosin: arranged along length of thin filament
Function: blocks myosin binding sites on actin filament when muscle is relaxed
2) Troponin: globular protein associated with tropomyosin
Function: binds tropomyosin to position it on the actin filament
Myofibrils
-rod-like organelles of muscle cells
-Myofibrils are made up of bands of myofilaments
-Myofilaments overlap in some regions of the myofibril to produce dark bands
-This is what creates striations of skeletal muscle!
-Each muscle fiber has several myofibrils
Myofibrils composed of alternating A bands and I bands
1) A band: region of myofibril where actin and myosin filaments overlap
-H zone at the center of A band has only myosin filaments
2) I band: region of myofibril with only actin filaments
-Z disc at center holds the actin filaments in place
What does an A band and I band create?
Sarcomere
Sarcomeres
-A sarcomere is the region of a myofibril found between 2 successive Z discs
-Importance: the sarcomere is the smallest contractile unit of skeletal muscle tissue
What part of the A band and I band make up a single sarcomere?
1 A band and half an I band
What happens to a muscle when the sarcomere shortens?
the myofibril becomes shorter, so the muscles cell becomes shorter, so the entire muscle becomes shorter
Other intracellular structures that regulate muscle contraction
1) T-Tubules
2) Sarcoplasmic reticulum
T-Tubules
-extensions of the sarcolemma that wrap around deeper myofibrils
-Function: increase surface area of muscle fiber sarcolemma
-Importance: changes in membrane potential can reach myofibrils not in direct contact with sarcolemma
Sarcoplasmic reticulum
-smooth endoplasmic reticulum of muscle
-Highly branched, wraps around myofibrils
-Form terminal cisterns around T-tubules
-Function: stores and releases intracellular Ca2+ for muscle relaxation & contraction
-Notice: 2 terminal cisterns surround 1 t-tubule to form a triad
The Neuromuscular Junction
-Definition: site of synapse between a somatic motor neuron and a muscle fiber
-Neurotransmitter released: Acetylcholine (ACh)
-Sarcolemma at synaptic cleft folded to form junctional folds
-Function: folds increase surface area of muscle fiber sarcolemma
What effect does acetylcholine have on a muscle fiber?
Always stimulatory, always causes contraction
Why is it advantageous for the sarcolemma at the synaptic cleft to form junction folds?
increases the likelihood that receptor can bind with acetylcholine since surface area is increased
For stimulation of muscle fiber to occur, to following steps must take place
1) Events at the neuromuscular junction
2) Generation of action potential across sarcolemma
3) Excitation-Contraction Coupling
4) Cross Bridge Formation & Muscle Contraction
Step 1 for stimulation of muscle fiber: Events at the neuromuscular junction
-transmission of action potential
-Result: ACh is released & binds to chemically-gated ion channels on the sarcolemma
Step 2 for stimulation of muscle fiber: Generation of action potential across sarcolemma
-ACh binds to and opens ion channels on sarcolemma to create end plate potential (EPP)
-EPP is a graded potential specific to muscle tissue
-EPP depolarizes sarcolemma
-If strong enough: action potential generated on sarcolemma
Step 3 for stimulation of muscle fiber: Excitation-Contraction Coupling
-Occurs when action potential spreads from sarcolemma to T-tubules
-When action potential arrives at T-tubules: voltage-gated proteins in T-tubules change shape
-When T-tubule proteins change shapeCa2+ channels in terminal cistern forced open
-Result: Ca2+ released from sarcoplasmic reticulum & flows into cytosol of muscle fiber
Step 4 for stimulation of muscle fiber: Cross Bridge Formation & Muscle Contraction
-Once Ca2+ enters cytosolinteraction between actin and myosin filaments can begin
-Cross bridge: the attachment of myosin to actin
-The Process of Cross Bridge Formation:
A) Ca2+ binds troponintroponin changes shape
B) Change in troponin shape causes tropomyosin to roll to the side
C) When tropomyosin is movedmyosin binding site on actin is exposed
-Myosin head will bind to actin binding site with use of ATP
D) Myosin head splits ATP into ADP + Pi
-This allows myosin head to bind to actin
E) ADP + Pi is released from myosin head, causing the myosin head to bend
-Effect: myosin head “pulls” actin filament
-This is called the power stroke
F) Myosin head binds to another ATP: myosin head detaches from actin binding site
G) The myosin head binds to a different actin binding site
-Steps D-G repeat until muscle contraction ends or ATP/Ca2+ run short
Ending cross-bridge formation & muscle contraction
-Motor impulses no longer sent to muscle fiber
-Action potential to muscle fiber ends
-Ca2+ is returned to sarcoplasmic reticulum
-When Ca2+ levels in sarcoplasm drop, it can no longer bind to troponin
-Troponin returns to original shape
-Result: Tropomyosin movescovers actin binding sites
Sliding Filament Model of Contraction
-During contraction, actin filaments “slide” over myosin filaments
-Why? Myosin heads form & break multiple cross-bridges with actin
-Myosin heads “slide” thin filaments toward the center of the A band -Note: filaments do not change length!
-Effect: when the filaments “slide,” the sarcomere shortens and generates tension in the muscle
-This is why muscle fibers shorten when they contract!
Motor Units
-A single motor neuron can serve multiple muscle fibers
-BUT: A single muscle fiber is served by only ONE motor neuron
-Motor unit: a single motor neuron and all the muscle fibers it innervates
What is the result of stimulation by 1 motor neuron?
cause a widespread weak contraction; if all stimulated at the same time, it would be a much stronger contraction in one part of the muscle
Motor Unit rule 1
-when the motor neuron fires: all fibers it innervates will contract
-Fibers innervated by a single motor neuron are spread out over entire muscle: not clumped together!
Motor Unit rule 2
-number of muscle fibers a single motor neuron innervates influences movement
-Motor neuron innervating few fibers
VS.
-Motor neuron innervating many fibers
What is the result on movement for each innervating fiber type?
-few fibers = very precise, fine-tuned control
-many fibers = not as precise
Muscle Twitch
-Definition: response of a muscle to a single stimulus
-Measured with a twitch myogram
3 phases of a myogram
1) Latent period: first few milliseconds following stimulation
-Excitation-contraction coupling occurs, but no tension observable in muscle
2) Period of contraction
-Active cross-bridge formation with increasing tension
3) Period of relaxation
-Cross bridge formation declines, muscle tension declines to resting value
Graded Muscle Contractions
-Definition: sustained muscle contraction that is modified by the nervous system to produce varying amounts of force
-Muscle contractions can be graded 2 ways:
1) Temporal summation: increasing the frequency of stimulation of a muscle
2) Motor Unit summation: increasing the strength of stimulation of a muscle
Temporal Summation
-Increasing the firing rate of a motor neuron can generate more force at the muscle
-How does it happen?
-Fire stimuli in rapid successionthe second twitch hits the muscle before the first twitch has ended
-Effect: muscle tension increases
Temporal summation can lead to 2 forms of tetanus
1) Unfused (incomplete) tetanus: rate of stimulation creates a sustained & quivering muscle contraction
-ex. when you lift something heavy and your muscles shake
-some relaxation occurs
2) Fused (complete) tetanus: rate of stimulation creates smooth, sustained muscle contraction
-No relaxation occurs
-most force you can possibly produce by the muscle
-can be damaging overtime because the muscle fibers use large amounts of ATP at once
Motor unit summation
-Definition: recruitment of additional motor units in a muscle to generate more force during contraction
-How does it happen?
-Increase the number of motor units used over time during contraction
-Size principal of motor unit summation
-Motor units with smallest muscle fibers recruited first
-Motor units with largest muscle fibers recruited last: create most force
-Motor units recruited asynchronously: some contracting, others relaxing
Why are motor units recruited asynchronously?
-if you use every muscle fiber at the same time, you will fatigue that muscle really quickly and not be able to use it
-trade-off which ones are resting and working
-allows you to vary the force exerted and duration of exertion
Muscle Tone
-Relaxed muscles are always slightly contracted: creates muscle tone
-Does not produce movement
-So why is it important?
-Muscle tone keeps muscle tissue healthy and responsive, stabilizes joints, maintains posture (ex. holding your neck up)
-allows muscles to remain excitable
-Loss of muscle tone leads to loss of responsiveness
-Muscle will not respond to stimuli
-Ex: stroke victims
Types of Muscle Contraction
-Isotonic contraction
-Isometric Contraction
Isotonic contraction
-muscle tension develops to overcome the load, muscle shortening occurs
-Two subtypes:
1) Concentric contraction: muscle shortens and does work
-Ex: upward motion of a bicep curl
2) Eccentric contraction: muscle generates force as it lengthens
-Ex: downward motion of a bicep curl
-50% stronger than the concentric contraction
Isometric Contraction
-tension develops in a muscle, but the length of the muscle does not change
-Occurs when the load is not moved
-Cross-bridge formation still occurs, but: the actin filaments do not slide
-Ex: most muscles responsible for body posture, plank position, wall sits, etc.
Energy Needs for Contraction
-ATP is the only energy source used directly for contractile activity
-Skeletal muscle stores energy in the form of glycogen
-ATP regenerated as fast as it is used
-3 Pathways used to regenerate ATP:
1) Direct phosphorylation
2) Anaerobic pathways
3) Aerobic pathways
Direct Phosphorylation
-Creates ATP from ADP + Pi using creatine phosphate (CP)
-Catalyzed by creatine kinase
-1 ATP produced per CP molecule
-Does not require oxygen
-Supplies ~15 s worth of ATP
Anaerobic Pathways: Glycolysis & Lactic Acid Formation
-Glucose broken down to form 2 ATP & pyruvic acid
-In absence of O2pyruvic acid converted to lactic acid
-Lactic acid not used by musclediffuses into blood
-Does not require oxygen
-Fast method to create ATP
-Drawbacks:
-A lot of glucose used for low ATP yield (2 ATP)
-Lactic acid build-up results in DOMS (delayed onset muscle soreness)
-Stored glycogen, creatine phosphate, & glycolysis provide ~1 minute of ATP
Aerobic Pathway: Cellular Respiration
-Creates 95% of ATP used by muscle during rest and light to moderate long-term exercise
-Requires pyruvic acid produced in anaerobic pathway
-Requires oxygen and mitochondria
-Produces ~30 ATP, H20, and CO2
-Drawbacks:
-Slow process
-Requires constant O2 and glucose
Muscle Fatigue
-We can produce ATP: but it is not unlimited
-Muscle fatigue occurs: muscle is physiologically incapable of contracting
-Rate & duration of fatigue depends on activity
-High intensity exercise (ex. HIT workouts) VS.
-Low intensity exercise (ex. marathon running)
Why is muscle fatigue necessary and important? What happens to muscle fibers if ATP is completely depleted?
-muscle fatigue prevents a muscle cell from running out of ATP because if it runs out of ATP, it dies
-your skeletal muscle cells have other activities to keep them alive that require ATP
How quickly will high-intensity vs low-intensity exercise cause fatigue? What is the duration of fatigue for each?
-High-intensity exercise = muscle tires out quickly, rate of fatigue is fast, but the duration of fatigue is short
-Low-intensity exercise = muscle tires out at a slower rate, rate of fatigue is slow, but the duration of fatigue is long
Muscle Contraction: Force
-Force of contraction is determined by the number of cross-bridges formed between myosin and actin filaments
-More cross bridges = more force
Force of muscle contraction is influenced by 4 factors
1) Frequency of stimulation: temporal summation
2) Number of muscle fibers recruited: motor unit summation
3) Size of muscle fiber
-Bulkier muscle generates more tension, creates more force
-Hypertrophy: increase size of muscle fibers in muscle to increase force generated -Rate of hypertrophy dependent on genetics, sex, nutrition, etc.
4) Degree of muscle stretch
-Force a muscle creates varies with how much the muscle is stretched
-Length-tension relationship: the maximal force produced will differ based on the degree of muscle stretch or contraction (optimal overlap is going to generate the most force)
Why can bulkier muscles generate more tension and force?
larger muscle fibers have more myofibrils, more myofibrils have more myofilaments, and more myofilaments mean more cross-bridges that can be formed
Why can muscle generate less tension when the sarcomeres are stretched? Shortened?
-when the sarcomeres are shortened, there is so much overlap between myofilaments that they cannot move anywhere and cannot form cross-bridges
-when the sarcomeres are stretched, there is no overlap between myofilaments, the myosin heads have nothing to physically attach themselves to and cannot form cross-bridges
-cannot generate a lot of force in either of these situations
Muscle Contraction: Velocity & Duration
-Fiber type influences velocity & duration of muscle contraction
1) Speed of contraction
-Dependent on:
A) How fast ATP is split: how fast cross bridges can form & break
B) Electrical activity of motor neurons: fast neurons = fast contraction
2) Load and recruitment Small loads allow faster contraction More motor unit recruitment = faster contraction 3) Pathway of ATP production Oxidative fibers: use aerobic pathways Glycolytic fibers: use anaerobic pathways
-Three fiber types created from different combinations of the above: fast glycolytic, fast oxidative, & slow oxidative
Slow Oxidation Fibers
-speed of contraction & ATP breakdown = slow
-myoglobin content = high
-glycogen content = low
-fatigue = slow
-mitochondria = many
-capillary supply = many
-example activities = anything endurance
Fast Oxidative Fibers
-speed of contraction & ATP breakdown = fast
-myoglobin content = high
-glycogen content = intermediate
-fatigue = intermediate
-mitochondria = many
-capillary supply = many
-example activities = sprinting, walking
Fast Glycolytic Fibers
-speed of contraction & ATP breakdown = fast
-myoglobin content = low (doesn’t use oxygen)
-glycogen content = high
-fatigue = fast
-mitochondria = few
-capillary supply = few
-example activities = intense, powerful movement
Gross Anatomy of Smooth Muscle Tissue
-Hollow organs in the body have smooth muscle tissue
-Most organs have 2 layers of smooth muscle tissue that never contract simultaneously in the same area:
1) Longitudinal layer: muscle fibers run the
length of the organ
-More superficial
2) Circular layer: muscle fibers run the circumference of the organ
-Deep to longitudinal layer
What organ does not have smooth muscle tissue?
heart has cardiac muscle tissue
What happens to the organ when the longitudinal layer contracts?
leads to the organ becoming shorter and wider
What happens to the organ when the circular layer contracts?
organ becomes long and thin
Differences between smooth muscle fibers and skeletal muscle fibers
1) Smooth muscle fibers are short, spindle-shaped
2) Covered only by endomysium
3) No neuromuscular junctions
4) Smooth muscle fibers have no T-tubules & less sarcoplasmic reticulum
5) Muscle fibers have gap junctions
6) No striations or sarcomeres
7) No troponin
8) Thick and thin filaments arranged diagonally
No neuromuscular junctions (smooth muscle tissue)
-innervation forms varicosities
-Autonomic fibers have bulb-like swellings scattered over smooth muscle tissue surface
-Importance: creates diffuse junctions
-Wide synaptic clefts that release neurotransmitter to multiple muscle fibers
Smooth muscle fibers have no T-tubules & less sarcoplasmic reticulum (smooth muscle tissue)
-Sarcoplasmic reticulum releases only a small amount of Ca2+
-Caveolae: invaginations of of sarcolemma of muscle fiber
-Have Ca2+ ion channels
Where does the calcium come from in caveolae?
extracellular fluid (outside of the cell)
Muscle fibers have gap junctions (smooth muscle tissue)
Depolarization spreads from cell to cell
No striations or sarcomeres (smooth muscle tissue)
-There are still thick and thin filaments, but:
-Fewer thick filaments overall
-Myosin heads found along entire length of thick filament
No troponin (smooth muscle tissue)
Calmodulin: protein that acts as Ca2+ binding site
Thick and thin filaments arranged diagonally (smooth muscle tissue)
-Filaments spiral down axis of muscle fiber
-Effect: when a muscle fiber contracts: it twists
Types of Smooth Muscle
-Unitary smooth muscle
-Multi-unit smooth muscle
Unitary smooth muscle
-Everything described so far are characteristics of unitary smooth muscle
-Much more commonfound in hollow organs
Multi-Unit smooth muscle
-Have no gap junctions or spontaneous depolarization
-Muscle fibers are structurally independent
-Forms motor units
-Have graded contractions with recruitment
-Found in arrector pili, smooth muscle of airways, internal eye muscles
Regulation of Contraction
-Neural Regulation
-Hormones & Local Chemical Factors
Neural Regulation
-Neurotransmitter can excite or inhibit smooth muscle tissue
-Response is dependent on receptor molecules on sarcolemma
Hormones & Local Chemical Factors
-Some smooth muscle has no innervation: respond only to local chemicals
-Others spontaneously depolarize
-Act by enhancing or inhibiting Ca2+ entry into sarcoplasm
Unique Features of Smooth Muscle
1) Response to stretch
2) Length and tension changes
Response to stretch (smooth muscle tissue)
-Smooth muscle responds to stretch by contracting
-Importance: increases ability to push substances through organ
-Stress-relaxation response: if an organ is filled slowly, it will not contract strongly
Length and tension changes (smooth muscle tissue)
-Smooth muscle can stretch more & can generate more tension while stretched
-Can still contract when stretched 150% its length
