chap 7,8- contraction of skeletal & smooth muscle (b1- SMS) Flashcards
sarcomere in smooth muscle cells
absent
- actin & myosin are not arranged in neat stripes (no striations)
arrangement of actin & myosin in smooth muscle cells + reason
actin & myosin are not arranged in neat stripes (no striations)
instead, actin filaments are attached to dense bodies (work like Z-lines in sarcomere) which are scattered throughout cell
myosin filaments are also longer than skeletal muscles’ & run b/w the actin filaments in a crisscross (diagonal) pattern
also have intermediate filaments that are not involved in contraction directly but help maintain shape/integrity of the smooth muscle cell
also have side-polar myosin arrangement
- the actin is not even looking like skeletal its more on the sides randomly
functional reason: allows greater flexibility & stretching (important in organs like bladder, uterus, or intestines), support longer, slower, & more sustained contractions w/ less energy
pattern of contraction of smooth muscle cell vs skeletal muscle cell
smooth muscle cell contracts in a twisting or corkscrew motion rather than straight like skeletal muscle
dense bodies & Z-lines relationship b/w smooth and skeletal muscle cells
in skeletal muscle: actin filaments anchor to Z-lines inside sarcomeres
in smooth muscle: actin filaments anchor to dense bodies (which are like anchor points in the cytoplasm and on the cell membrane)
5 structural differences b/w smooth & skeletal muscle
skeletal:
- striated
- long, cylindrical fibers
- multi-nucleated (peripheral)
- sarcomere present (organized actin & myosin)
- usually attached to skeleton
smooth:
- non-striated
- spindle shaped cells
- single nucleus (central)
- absent (actin & myosin arranged differently)
- usually covering wall of internal organs
6 physiological differences b/w smooth & skeletal muscle
skeletal muscle:
- voluntary (somatic nervous system)
- fast & brief contractions
- rapid fatigue
- movement of skeleton, posture
- higher energy requirement
- does not maintain tone (low level of continuous contraction) in response to stretch
smooth muscle:
- involuntary (autonomic nervous system)
- slow & sustained contractions
- resistant to fatigue
- visceral contraction (peristalsis, vasoconstriction)
- lower energy requirement
- maintains tone in response to stretch (when bladder fills & stretches, smooth muscle walls automatically adjust tone to keep pressure steady)
source + binding of calcium diff b/w smooth & skeletal muscle
skeletal: sarcoplasmic reticulum
- troponin is calcium binding protein
smooth: ECF (mainly), sarcoplasmic reticulum
- calmodulin is calcium binding protein
smooth muscle disorder
asthma
- hyper contraction of bronchial smooth muscle
red vs. white muscle fibers
red fibers (slow-twitch):
- red color (b/c lots of myoglobin)
- slow speed of contraction
- fatigue resistance
- many mitochondria
- rich in blood supply
- aerobic respiration (uses oxygen)
- endurance (posture muscles, long distance running)
white fibers (fast-twitch):
- white color (b/c of less myoglobin)
- fast speed of contraction
- fatigue quickly
- fewer mitochondria
- less extensive blood supply
- anaerobic respiration (glycolysis; no oxygen needed)
- quick powerful movements (sprinting, weightlifting)
most muscles have a mix of red & white fibers, but ratio depends on genetics & training
which muscle types are striated and which ones are smooth?
skeletal & cardiac are striated
visceral (smooth) muscle is smooth
its literally in the name
structural organization of skeletal muscle cells
Muscle
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Fascicle: bundle of muscle fibers thats surrounded by the perimysium
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Muscle fiber: single muscle cell, is multinucleated, surrounded by membrane called sarcolemma
- contains t-tubules, sarcoplasm, sarcoplasmic reticulum, & mitochondria
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Myofibrils: long thread like structures that run parallel, made of repeating sarcomere unit, responsible for the striated appearance under microscope
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Myofilaments: thin & thick filaments inside each myofibril that contain the actual contractile proteins
going from outside of muscle to the inside
I band, A band, H zone, M line, Z line
I band: less dense, contains only thin (actin) filaments
- have the Z line in the center
- shortens during muscle contraction
A band: more dense, contain both actin & myosin
- stays same length during muscle contraction
H zone: in the center of A band, contains myosin only (no overlapping actin)
- becomes shorter/disappears during contraction
M line: located in the center of the H zone
- contains proteins that hold myosin filaments in place
Z line: between Z lines is the entire sarcomere (basic contractile unit of muscle)
- anchors the actin filaments
structure of actin (3 components)
not just actin, is actually a complex of 3 different proteins
-
F-actin (filamentous actin): made of G-actin units (globular actin), look like circles looped together
- contains the active sites where the myosin heads will attach during contraction - Tropomyosin: long, rope like protein that sits on top of F-actin & covers the active sites, so that myosin cannot bind at rest
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Troponin: little balls that sit on top of the tropomyosin, have 3 parts:
- troponin C: binds calcium
- troponin I: inhibitory, binds to actin to prevent actin-myosin binding
- troponin T: binds troponin to tropomyosin
structure of myosin
2 main parts:
Tail: made of 2 heavy chains that twist around each other
Heads (globular part): active part of molecule, 2 heads on each myosin that stick out and face the thin filament
- they also have ATPase activity - break down ATP to release energy for contraction
- and have actin-binding sites - the parts that bind to specific sites on actin to form cross bridges that then repeatedly attach & pull actin filaments to shorten the muscles
titin
giant elastic protein in the sarcomere that binds thick filaments to the Z lines
contributes to muscle elasticity
- largest known protein in the human body (why they call it titin)
sliding filament mechanism
explains how sarcomeres shorten during contraction
thin filaments (actin) slide over thick filaments (myosin) - cause the Z lines to move closer together, shortening the sarcomere & producing contraction
the filaments themselves don’t shorten - only move past each other
Ca2+ binds to troponin C → tropomyosin pulls away exposing actin-binding sites → myosin heads attach to exposed site on actin to form cross-bridge → power stroke (bending of cross bridge & inward movement of actin), **ADP + Pi are released from myosin head during this movement → new ATP molecule binds causing dettachment → ATP bound to myosin is then split into ADP + Pi & myosin in energized state ready for next contraction
3 different sources of energy for muscle cells
-
Creatine phosphate (5-8 sec): readily available energy, very short but quickly use up, broken down to ATP right away
- but has limited supply (for quick bursts- sprinting, weightlifting) -
Glycolysis (1-2 min): glucose from blood or oxygen is broken down to pyruvate
- slower than creatine phosphate system but produces more acid
- then pyruvate is converted to lactic acid which builds up & causes muscle fatigue/burning sensation - Oxidative phosphorylation (many hours): takes many hours, for long term energy, best for sustained, endurance activity (jogging, walking, posture)
length-tension relationship
tension (force) that a muscle fiber generates is dependent on its sarcomere length before contraction begins
optimal length: 2.0 to 2.2 μm
- at this length, actin & myosin overlap perfectly and cross-bridge formation is ideal
anything less or more = tension generated is low
isometric vs isotonic contraction
isometric: length stays the same, tension increases (muscle doesn’t shorten but tension develops)
- ex. holding a heavy object without moving it, pushing against wall, posture muscles
isotonic: tension stays same, muscle length changes
- muscle can either shorten (concentric) or lengthen (eccentric) while generating
- ex. biceps contracting to lift a dumbbell or lengthening to put it down
what is a motor unit (small & large ones too- this bit is for own knowledge)
1 motor neuron + all the muscle fibers that it innervates (can innervate 3 or 300)
small motor unit: fewer muscle fibers are innervated, weaker contraction but more fine control
large motor unit: many muscle fibers, strong contraction but less precision
multiple motor unit summation
process by which more motor units are recruited to increase the strength of muscle contraction
CNS controls how much force a muscle generates by deciding how many motor units are activated - weaker contractions will activate the smaller motor units & as force increases, start recruiting more - called size principle
frequency summation & tetanization
frequency summation: increase in the force of muscle contraction when a muscle is stimulated repeatedly in a short amount of time
muscle keeps getting stimulated again before it can fully relax (Ca2+ uptake not fully occurring so more Ca2+ stays in sarcoplasm)
- eventually the muscle contractions starts to add up and get even stronger
tetanization (tetanus): as stimulus frequency increases further, contractions become closer together
- eventually, no time for muscle to relax at all b/w contractions
- all individual twitches fuse into 1 smooth sustained contraction
occurs bc Ca2+ levels remain elevated in sarcoplasm
Tetany vs tetanization
tetanization/tetanus: normal, sustained muscle contraction caused by high-frequency stimulation of a muscle
tetany: pathological condition where involuntary & abnormal muscle spasms occur due to low calcium (hypocalcemia)
Lambert-Eaton syndrom
autoantibodies against voltage gated calcium channels = impaired release of ACh
symptoms:
- weakness in proximal muscles
- reduced reflexes
- muscle weakness improves w/ activity (key sign of this)
2 types of smooth muscle (UQ)
single unit: function as 1, connected by gap junctions, present in walls of hollow viscera
- ex. lining of organs (GIT, reproductive, urinary), small blood vessels
multi unit: more precision/control, each muscle contracts separately, no gap junctions (so contraction does not spread)
- ex. iris (eye) muscles, hair follicles, walls of large blood vessels
smooth muscle components that are diff from skeletal muscle
smooth muscles have caveoli instead of T-tubules (skeletal) & calmodulin instead of troponin (skeletal) - but smooth muscles still have tropomyosin tho
also have capability to divide/proliferate (unlike skeletal muscles that just dies and leaves a hole)
dense bodies contain the protein ________
alpha-actinin
process of smooth muscle contraction
calcium stimulus causes calcium to enter the cell →
calcium binds to calmodulin →
Ca2+-calmodulin complex activates enzyme called MLCK (myosin light chain kinase) →
phosphorylation of myosin light chains and myosin heads can attach to actin filaments→
activation of myosin ATPase →
attachment of myosin head w/ actin
NMJ in smooth muscle
- no structured NMJs like in skeletal muscle!
diffuse junctions: nerve endings dont directly touch each muscle cell, but rather release neurotransmitters into surrounding ECF
- this diffuses over a wide area reaching many smooth muscle cells at once
varicosities on axons: swellings/bulges on the autonomic nerves that contains vesicles filled w/ neurotransmitters
- as nerve travels, neurotransmitters released from these
but sometimes, these come really close in contact with the muscle that they form contact junctions that directly contact smooth muscle, similar to skeletal muscle (but still not the same level of organization)
length-tension relationship & stretch adaptability in smooth muscle cells + reason for why
smooth muscle can still generate strong contractions even when stretched far beyond its resting length — up to 2.5× its original length
- unlike skeletal muscles that need the optimal overlap b/w actin & myosin
reasons:
1. shorter resting length: much shorter at rest so a lot more room for stretch, can lengthen significantly (like bladder or uterus stretches a lot but can still contract powerfully)
- sustained overlap of thick & think filaments: thick filaments are longer & arrangement is not sarcomere-based so overlap is preserved even through long distances
this is stretch adaptability
latch mechanism in smooth muscle
unique feature of smooth muscle that allows it to maintain prolonged contraction w/o need for continuous ATP consumption or neural stimulation
steps:
contraction & cross bridge formation (b/w myosin & actin) to generate tension
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need for ATP consumption decreases after initial contraction
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cross-bridges “latch” & stay attached longer: myosin stays attached to actin - muscle can maintain tension even though ATP tension is low
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slow detachment of cross bridges - so smooth muscles can maintain tension for long periods, slower Ca2+ removal
caveolae in smooth muscle
lipid-rich invaginations of the sarcolemma (plasma membrane)
involved in signal transduction pathways (regulate smooth muscle contraction/relaxation)
- can store & release Ca2+ ions
structure/appearance of cardiac muscle (key features)
-
branched fibers: unlike long cylindrical fibers of skeletal muscle, cardiac are branched (like tree branches)
- branches connect w/ neighboring cells to help coordinate contractions - single central nucleus (sometimes bi-nucleated): older or larger cells can sometimes have 2 nuclei
- striated appearance: like skeletal muscle, striations are due to the sarcomeres
intercalated discs (only in cardiac muscles) + components
intercalated discs: specialized junctions where 1 cardiac cell connects to another
- hold cells together & allows them to communicate quickly
components:
1. desmosomes - for strength: give mechanical strength, prevent heart cells from pulling apart when heart contracts forcefully
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gap junctions - for communication: allow ions & small molecules to pass directly from 1 cell to another
- allows electrical signals to pass rlly fast - fascia adheres - for anchoring: anchoring sites where actin filaments attach to inside of cell membrane
excitation-contraction coupling in cardiac muscle
dependent on diad
both Na+ & Ca2+ ions are responsible
cardiac action potential (Na+ enters) →
Ca2+ enters cell during plateau →
Ca2+ induced calcium release from SR →
Ca2+ binds to troponin-C→
cross-bridge cycling →
tension
diad vs triad
refers to structures involved in muscle contraction
triad: in skeletal muscle
- consists of 1 T-tubule & 2 sarcoplasmic reticulum tubules
- found at junction of A & I band
diad: in cardiac muscle
- consists of 1 T-tubule & 1 sarcoplasmic reticulum tubule
- found at the Z line
AV nodal delay (in heart)
SA node starts the electrical signal
AV nodal delay: slight pause/delay in electrical signal as it passes through the AV node of the heart
significance: gives time for the atria to fully contract & empty their blood into ventricles before the ventricles contract
