Lecture 7 and 8 - movement and muscle physiology

lecture 7,

what parts of the cytoskeleton are involved in movement

1. microtubules

2. microfilamants

what are the 3 components of the cytoskeleton and what is their composition

1. microtubules

   1. made of tubulin a and b heterodimers and 13 protofilaments

2. microfilaments

   1. made of 2 actin subunits twisted together

3. intermediate filaments

   1. fibrous subunit of keratins colled together

what are motor proteins

enzymes that convert the chemical energy released from ATP hydrolysis into mechanical energy ATPases

• they undergo conformational changes to move along cytoskeleton

  • kinesins and dyneins move along tubulin

  • myosin moves along actin

what are the four ways to use the cytoskeleton for movement

1. active reorganization (no motor protien)

2. anchored motor protein pulls/pushes cytoskeleton

3. anchored cytoskeleton/motile motor proteins

4. complex arrays - cytoskeleton elements slide past eachother (cilia, flagella, muscle

what is the strucutre of microtubules

• tube like polymers of tubulin

• multiple isoforms and form spontaneously

• anchored at both ends

  • - near nucleus

  • integral proteins (+) in PM

  • radiate from centromere

how do mictotubules assembly

1. a and b tubulin form a heterodimer

2. multiple dimers form a protofilament non covalently

3. 13 protofilaments line up to forma sheet which roles into a tube

4. microtubules grow by monomer addition to positive end (GTP bound) and shrinks by monomer removal from - end ( GDP bound )

factors affecting microtubule growth and shrinkage

1. tubulin concentration (primary factor)

   1. high tubulin conc. promotes growth

   2. cc (critical concentration) lower on + end then - end; making it grow faster

      1. grows asymmetrically ; + end more likely to grow

2. microtubule associated proteins (MAPs)

   1. bind to surface of microtubules; stabilizing or destabilizing

   2. polarization= addition

   3. depolmerzation = removal

what is critical concentraion (Cc)

concentration of tubulin where growth and shrinkage in balance

• below concentration at + and - end causes tread milling which allows movement

how do motor proteins move

ATP hydrolysis

• kinesins move in + direction

• dynein moves in - direction

*rate determines by the ATPase domain of motor protein and regulatory proteins

does kinesin or dynein move faster

dynein moves 5x faster due to being larger and is activated by asymmetric activation

what are cilia and flagella composed of

microtubules arranged into axoneme (bundle of parallel microtubules)

how do microfilaments move

1. actin polymerization

   1. b actin- microfilaments (cell motility)

2. sliding filaments using myosin

   1. a actin- thin filaments (muscle contraction)

how do microfilaments form and how do they move

• activated G actin monomers polymerize (ATP) to form F actin (a polymer)

• assymetric growth ; faster at + end

• tradmilling

  • assembly and disassembly occr and the same time and no growth occurs

• capping proteins stabilize F actin preventing depolimerization

acrosome of sperm

The acrosome is a specialized cap-like structure that covers the anterior portion of the head of a sperm cell.

• It originates from the Golgi apparatus during spermatogenesis and contains hydrolytic enzymes which are crucial for fertilization.

• During fertilization, the acrosomal vesicle binds to a receptor on the egg jelly coat and releases acrosomal vesicle contents.

binding tiggers actin polymerization and fusion with the egg PM and then transfer of sperm DNA

cellular microfilament tangled networks

microfilaments linked by filamin protein

cellular microfilament bundles

cross linked microfilaments by fascin protien

what does dystrophin protein do

it attaches netowrks and bundles of microfilaments to cell membrane ECM inside cell which helps to maintain cell shape and can be used for movement

what is myosin; what is its strucutre; what is I/V and II used for

• an ATPase that binds and converts ATP to mechanical energy (movement)

• 18 classes of myosin with multiple isoforms in each class (all isoforms have a similar strucutre)

  • head- ATP/actin binding site

  • tail - can bind to subcellular components

  • neck- regulation of myosin

    • hinge - allows flexibility

I and V myosin are for vesicle transport

II myosin is for cytokinesis and muscle contraction

which direction do actin and myosin move

direction of movement depends on which element is immobile

what are the two processes involved in sliding filament model

1. chemical reaction → myosin binds to actin to form the cross bridge

2. mechanical change → myosin bends to create the power stroke

what are sarcomers composed of

myosin (thick) and actin (thin) filaments

explain sliding filament theory

Myosin heads bind to actin filaments, forming cross-bridges.

• ATP hydrolysis provides energy for myosin heads to pivot and pull actin filaments toward the sarcomere center (power stroke).

• ATP binds again to myosin, causing it to release from actin and reset for another cycle.

• Coordinated cross-bridge cycling shortens sarcomeres, leading to muscle contraction.

• Key points:

• Actin and myosin filament lengths remain constant; they slide past each other.

• Calcium ions trigger contraction by exposing myosin binding sites on actin.

• Muscle relaxes when stimulation ends or calcium is removed, allowing filaments to slide back.

what two factors infulence actin-myosin activity and what do they depend on

1. unitary displacement

   1. distance myosin steps during each cross bridge cycle

      1. depends on; myosin neck length, location of myosin binding sites on actin, helical structure of actin

2. duty cycle

   1. cross bridge time/cross bridge cycle time (typically 0.5)

   2. use of multiple myosin dimers to maintain contact

two types of muscle cells and explain each arrangement, where they are found

• striated

  • skeletal and cardiac

  • actin and myosin in parallel repeating functional untis

  • e.g. sarcomeres

• smooth

• actin and myosin filaments less organized

explain cardiac muscle

• straited

• controls invuluntary and rhythmical and pumps blood

• short, branched, narrow

• limited and replaced by scar tissue

explain skeletal muscle

• striated

• attached to bones

• voluntary, conscious movement in response to neural stimulation

• long cylindrical fibers

• can regenerate

explain myocyte straited muscle cells

• contractile cell

• polarized contractile elements within myocutes

  • thick filaments

    • polymers of myosin II

    • two halves are mirror images

  • thin filaments

    • polymers of a actin

    • ends capped by topomodulin (-) and capz (+) to stabilize

    • proteins troponin and topomyosin on outer surface to mediate interactions with myosin

explain sarcomer strucutre

• Z disk

  • anchor thin filament

  • links adjacent myofibril

• A band

  • spans entire length of thick filament

  • H zone- middle section; composed only of thick filaments

    • M line- proteins cross linking myosin II tails together

  • I band

    • spans a Z disk

    • occupied by thin filaments only

-thick and think filaments overlap in two regions of each sarcomere and are tension generated

how is sarcomere organization maintained

by structural proteins

• nebulin

  • aligns thin filaments

  • protein ruler

• titin

  • keeps thick filaments centered in sarcomere

  • attaches to Z disc and M line

  • region along I band is folded and elastic

muscle diameter = , muscle length =

# of myofibrils in parallel, # of sarcomeres in series

explain muscle actino-myosin activity

• myosin II cant drift away from actin due to sarcomere strucutre and each head is only attached from a breif time to not impede other myosins from pulling. displacement is also short

• therefore short quick pulls

what does contractile force (tension) depend on

• degree of overlap between thick and thin filaments

  • more overlap=more cross bridges = more force

  • amount of overlap depends on sarcomere length

• maximal force occurs at optimal length

  • declines as shortens

  • declines as lengthens

what is force proportional to

cross sectional area of the sarcomere

• more myofibirls in parallel = potential for more force

• it is independent on length

what is sarcomere shortening proprotional to

the length of myofibril

• more myofibrils in series = potential for greater shortening

what regulates contraction - excitation contraction coupling

1. depolarization (excitation) of the muscle PM (sarcolemma)

2. elevation of intracellular calcium

3. contraction

   1. sliding filament model (cross bridge cycles)

4. relaxation

   1. sarcolemma repolarizes and calcium returns to resting levels

all or none law

a muscle fiber or nerve will respond or not to a stimulis- no inbetween

how does calcium permit myosin to bind actin

at rest, calcium concentragion is low → troponin-tropomyosin covers myosin binding sites on actin

as calcium concentration increases → calcium binds to TnC (calcium binding sites on troponin) and troponin-tropomyosin moves, exposing myosin-binding site on actin and myosin binds to actin to begin the cross-bridge cycle

• the cross bridge cycles continue as long as calcium conc is high

• relaxation occurs when the sarcolemma repolerizes and intracellular calcium returns to resting levels

what does strength and duration of contraction depend on

strength depends on calcium oncentration and duration depends on the length of time clacium remains elevated

explain the initial causes of depolerization

• myogenic → beginning in the muscle; intrinsic

  • e.g. cardiac muscle pacemaker cells

• neurogenic→ begining in the nerve

  • e.g. vertebrate skeletal muscle; excited by neurotransmitters

what is effective refractory period (ERP) and how does it vary across fast twitch, slow twitch and cardiac muscle

time during which another contraction cannot be stimulated

• fast → rapid de and re polarization causes rapid contraction

• slow → less rapid de and re polarization leading to more sustained contraction that doesn’t use as much energy

• cardiac → has an extended ERP that lasts almost as long as entire muscle contraction; this ensures multiple contractions and action potentials from occurring and keeping the heart contracted

  • on the graph ERP flows into contraction curve

compare skeletal vs cardiac muscle refractory periods

skeletal

• temporal summation (contractions add on to previous contraction

• tetanus

  • maximum, sustained contraction

  • twitch fibres typically operate at near maximal tetanic force

• refractory period is short compared to time required to develop tension

cardiac

• temporal summation/tetanus prevented by long refractory periods and a plateau phase

• twitch only type of muscle; never reaches tentnus

• refractory period is almost as long as muscle twitch time to ensure no constant contraction

what is AP conductance facilitated by (2)

• transverse tubules (t tubules)

  • invaginations of sarcolemma that enhance penetration of AP into myocyte and synchronize calcium release

• sarcoplasmic reticulum (SR)

  • network of tubules that surround myofibrils

  • store caclium bound protein calsequestrin; works on principle of mass action

  • terminal cisternae increase storage

where would you find more developed T tubules and terminal cisternae and where would you find less developed

more → larger, faster twitching muscles

less → cardiac muscle

how does depolarization effect calcium concentration

it increases calcium concentration

• channels allow calcium to enter the cytoplasm

  • channels in cell membrane → dihydropyridine receptor (DHRP) - L type

  • channels in SR membrane → ryanodine receptor (RyR)

how does repolarization effect calcium concentration

calcium concentration decreases as calcium is removed

• transporters remove calcium from cytoplasm

  • transporters in SR membrane → ATPase (SERCA)

  • transporters in cell membrane → calcium ATPase, Na/Ca exchanger

how does depolarization effect DHPR and RyR in skeletal and cardiac muscle

skeletal- depolarization changes conformation of DHPR and opens it which then physically interacts with and triggers the opening of RyR calcium channel

cardiac- calcium enters cytosol from ECR and changes the DHPR conformation, the increase in calcium concentration binds to RyR and triggers opening of RyR

• positive feedback loop

what happens during relaxation

• repolarization of sarcolemma

• reestablish calcium gradients

• calcium dissociates from toponin (TnC)

• myosin can no longer bind to actin

what does parvalbumin do

it is a cytosolic calcium binding protein that buffers calcium (fast twitch muscles) to help reestablish the calcium gradient during relaxation

explain the process of relaxation

1. calcium binds parvalbumin

2. calcium is pumped across the sarcolemma and into the SR

3. calcium is released by TnC

4. weakened TnC causes TnI interaction which stengthens actin interaction

5. Tn tropomyosin returns to inibitory position

what are the three phases of muscle twitch → explain each

1. latent period → events of excitation-contraction coupling; no muscle tension

2. period of contraction → cross bridge cycles; tension increases and builds to peak

3. period of relaxation → depolarization and calcium reuptake into SR; tension declines

what are phosphagens

alternative high energy phosphate compounds, don’t require oxygen and are reversible

• phosphocreatine (PCr)

• creatine kinase (CPK)- enzyme

they imporve the efficiency of energy transfer

what is the rate of diffusion formula

rate of diffusion = SA - conc grad - membrane perm / membrane thickness

smoothed muscle

• specialized for slow, prolonged graded contractions

• invuluntary

• found in walls of hollow or tubular organs

• 2 types

  • multi-unit → neurogenic ; acts like skeletal, each cell on its own and doesn’t communicate

  • single unit → myogenic and stretch activated ; contract as one

differences of smooth muscle from straited muscle

• no sarcomeres

• no T tubules, no troponin and minimal SR

• slow cross-bridge cycling

• different mechanism of EC coupling

  • myosin heads along entire length and opposite facing heads; side polar filaments

• no defined NMJ

• autonomic nerve fibers innervate smooth muscle at diffuse junctions

  • varicosities of nerve fibers store and release neurotransmitters into diffuse junctions

smooth muscle contraction is calcium independent - what does it directly activatie/inhibit

MLCK (myosin light changin kinase)or MLCP (myosin light chain phosphatase) and phosphorylation of caldesmon

type I, type IIa and type IIb muscle

type I → slow twitch oxidation

type IIa → fast twitch oxidative-glycolytic

type IIb → fast twitch glycolytic

explain how aerobic vs resistance exercise effects muscles

aerobic → improves endurance

• increase miochondrial size and number, capillary density, myoglobin, FA oxidation

• more type I

ressitance → improves strength

• increase myofibril synthesis, cross sectional area

• more type II

trans-differentiation of muscle cells

the process by which a fully differentiated muscle cell changes its identity and function to become another specialized cell type without first reverting to a multipotent or stem cell state. does this through changes in gene expression

invertebrate muscle types

1. smooth

2. striated

3. intermediate obliquely striated

   1. sarcomeres not connected side by side, individual sarcomeres attached to dense body and myofilaments are staggered

what are excitatory postsynaptic potentials do (EPSP) and the principles

change variation in contraction force

• polyneuronal innervation

• multi-terminal innervation

• EPSP can summate to give stronger contraction

how do superfast muscles work

due to isoforms of excitation-contraction machinery

1. low calcium troponin affinity and unbinds calcium rapidly

2. increased density of SERCA and parvalumin in cytoplasm causes rapid calcium relaxation

3. rapid cross bridge cycling trough increased myosin ATPase activity and rapid detachment

asynchronous insect skeletal flight muscles

asynchronous muscle contractions → one contraction doesn’t = 1 excitation

• intracellular calcium remains elevated

• more space for myofibrils

• stretch activated

• antagonistic muslces

  • sensitivity of myofibirl to calcium changes during contraction/relaxation cycle

    • contracted muscle is calcium insensitive → muscle relaxes

    • stretched muscle is calcium sensitive → muscle contracts

molluscan catch muscle

has long duration contraction with little expenditure of energy

• high calcium conc triggers contraction and contraction force is maintained as calcium declines; untile serotonin

direct vs indirect insect flight muscles

Direct flight muscles

• attached directly to the bases of the wings

• When these muscles contract, they move the wings up or down by pulling directly on the wing base

• each contraction of a direct muscle results in one wingbeat

Indirect flight muscles

• not attached directly to the wings but instead connect to parts of the thorax

• The main indirect flight muscles are called dorsal-ventral and longitudinal muscles

• When dorsal-ventral muscles contract, they pull the top (dorsal) part of the thorax downward, causing the wings to go up

• When longitudinal muscles contract, they shorten and arch the thorax from front to back, making it bulge upward so that the wings move down

• allows for very rapid wingbeats because a single nerve impulse can trigger multiple contractions through stretch activation.