9/11 and 9/14 (after - unit 1 (post quiz))

what drives a post-synaptic potential?

channel opening

Excitatory postsynaptic potential (EPSP)

raises Vm closer to AP threshold

Inhibitory postsynaptic potential (IPSP)

lowers Vm further from AP threshold

ionotropic receptor vs metabotropic receptor

ionotropic receptor

• ion channel

• neurotransmitter binds → channel immediately opens or closes → ions move down their concentration gradient → membrane potential changes

• fast and direct

metabotropic receptor

• NOT an ion channel

• usually a G-protein coupled receptor (GPCR)

• when neurotransmittor binds it starts a chain of events inside the cell that affects ion channels or other proteins

• slow (tens of ms to sec) but effects last longer

• provides amplification: many 2nd messenger molecules generate

metabotropic receptor pathway

• neurotransmittor binds to the metobotropic receptor

  • receptor has 7 transmembrane segments

• receotor activates a G protein

  • G protein is made up of three subunits: alpha, beta, and gamma

    • alpha separates and can interact with another protein

• G protein activates an enzyme

  • in this example it activates adenyl cyclase

• Adenyl cyclase produces a second messanger

  • converts ATP → cAMP

  • first messenger = neurotransmittor outside cell

  • second messenger = cAMP (molecule produced inside cell that carries signal forward)

• cAMP activates another enzyme (protein kinase A)

  • PKA → phosphorylkates proteins

• PKA phosphorylates the K+ channel

  • phosphorylation causes K+ channel to close

• K+ channel closes

  • less K+ leaves the cell

  • in this example causing depolarization but in other examples can also hyperpolarize

how can metabotropic receptors affect the cell?

depolarization or hypoerpolarization

GPCR structure

• 7 transmembrane domains that cross the membrane each time (so 7 total)

• N-terminus on outside (NH2)

• C-terminus on inside (COOH)

• ligand binds outside → receptor changes shape → G protein inside is affected

• 
  

what is a motor neuron, motor nerve, motor unit, muscle

• motor neuron carries signals from the CNS to the skeletal muscle

• one motor neuron can control multiple muscle fibers

• motor nerve contains multiple motor neuron axons

• motor unit - one motor neuron + ALL the muscle fibers that it innervates

• one muscle contains many motor units

neuromuscular junction

specialized synapse between the presynaptic cell = motor neuron and the postsynaptic cell = skeletal muscle fiber

• neuron→ muscle synapse

pathway of how the nervous system gets a signal to the muscle

• motor neuron fires an action potential, and it arrives at the motor neuron terminal depolarizing the presynaptic membrane

• depolarization opens voltage gated Ca2+ channels

  • Ca2+ moves from outside to inside

  • Ca2+ is the signal that triggers synaptic vesicle fusion

• vesicles release ACh by exocytosis into the synaptic cleft

• ACh binds to nicotine ACh receptors(ionotropic) located on the motor end plate

  • the channel opens allowing Na+ to flow into the muscle(greater influx) and K+ out of the muscle (smaller eflux)

    • causing local depolarization (end-plate potential)

• the depolarization spreads

  • positive current spreads and deploarizes nearby membrane containing voltage gated Na+ channels

• muscle AP is generated

  • if meets threshold*

  • now made transition from motor neuron AP to muscle AP (not the same thing)

• ACh has to be broken down

nicotine ACh receptor

• ionotropic receptor - fast and direct

  • ligand-gated ion channel

• pentamer (5 subunits, each with 4 transmembrane domains) crosses membrane 4 times; forms central pore ion flows thru

• non-selective cation channel

  • Na+ can enter

  • K+ can leave

for NMJ mechanism why does the membrane depolarize after AcH binds to nAChR if Na+ is going in and K+ is going out?

the inward pull (driving force) on sodium (\(\text{Na}^{+}\)) is much larger than the outward push on potassium (\(\text{K}^{+}\)), making the net movement of positive charge enter the cell

What happens to postsynaptic Vm when ACh binds to the channels at rest?

ACh opens nAChRs, which are non selective cation channels. Na+ enters and K+ exits, but at resting Vm there is a net inward positive current, causing depolarization (EPSP) and driving Vm towards the nACHR reversal potential (around 0 mV)

• there is a net inward positive current because ENa (+60 mV) is far from Vm(-70 mV) causing a strong driving force inward and EK (-90 mV) is closer to Vm making it a smaller driving force

what are the three types of muscles?

• skeletal muscle

• cardiac muscle

• smooth muscle

skeletal muscle fibers

• large, long, striated, and multinucleated

• one skeletal muscle fiber = one muscle cell

• type involved in NMJ

cardiac muscle fibers

• striated, smaller, branched, and uninucleated

• intercalated disks are type of junction that joins cardiac cells

smooth muscle fibers

• small and lack striations

muscle fiber structure hierarchy

• one muscle fiber = one muscle cell = one myocyte

• sarcolemma - plasma membrane of muscle cell

• each fiber contains myofibrils (long cylindrical structures)

• each myofibril is made up of repeating units called sarcomeres

• each sarcomere contains thin (actin) and thick (myosin) filaments

sarcomere - contractile machinery + structure

• sarcomere is the fundamental unit of striated muscle

• thick (myosin) and thin (actin) filaments make up the contractile machinery

  • move relative to one another to contract muscle fiber and driven by depolarization and Ca2+ signalling

• zline → zline = one whole sarcomere

• zline - actin - myosin - actin - zline

A band - sarcomere

• corresponds to the entire length of the thick/myosin filaments

• regions with myosin only + regions where myosin + actin overlap

I band

• contains only thin flaments

• actin only

H zone

• center portion of A band where there is thick/myosin filament only

crossbridges

• when myosin heads attach to actin they form crossbridges

how do myosin and actin interact with each other in the sarcomere?

• thick filaments (myosin) pulls on thin filaments (actin) [not other way around]

  • myosin heads attach to the actin forming a crossbridge

  • the myosin head will pivot and pull actin toward the middle of the sarcomere

thin filament made of process?

• thin filaments are made of actin

  • individual actin proteins are called G actin and each one contains a myosin-binding site where the head will attach

  • G-actin molecules polymerize together gotming F-actin

  • two F-actin strands twist together to form double-helical actin strands

  • regulatory proteins (tropomyosin and troponin) modulate binding of motor of myosin to binding sites

thick filament made of process?

• thick filaments are made of myosin which is an ATPase (catalyze ATP and use released energy)

• single myosin molecule contains:

  • tail + 2 heads(molecular motors waiting to grab actin)

  • head contains: actin-binding site and ATPase sites

• two myosin molecules bound at theur tail ends

• several hundreds of myosin molecules assemble together to form one thick filament

Ca2+ importance in crossbridge cycling?

• In the absence of Ca2+:

  • relaxed muscle

  • muscle does not want myosin grabbing actin constantly so tropomyosin covers the myosin-binding sites on actin

  • myosin cannot bind to actin → no cross-bridge cycling → no contraction

• In the prescence of Ca2+:

  • contracting muscle

  • Ca2+ binds to troponin and causes troponin to change shape, which causes tropomyosin to move away from the myosin-binding sites on actin exposing them

  • binding sites exposed → myosin can bind to actin

The crossbridge cycle process

• requires ATP and Ca2+ to occur

• 1. A new ATP molecule binds to the myosin head causing myosin to release actin

• 2. myosin (ATPase) hydrolyzes ATP providing the energy to “cock” the myosin head

  • low energy myosin → ATP hydrolysis → high-energy/cocked myosin with ADP +Pi still attached to myosin head (ready to bind to actin form)

  • relaxed state bc myosin no longer bound to actin

• 3. Ca2+ must be present for this: myosin binds to actin and a coss-bridge has formed

  • Ca2+ binds to troponin → tropomyosin moves → actin binding sites exposed so the cocked myosin head can attach to the actin

• 4. high energy phosphate is released to provide energy for the power stroke

  • power stroke - as the myosin head pivots: myosin pulls actin toward the middle/M line of the sarcomere

• 5. ADP is released causing it to be in rigor state (myosin is in low-energy form)

  • myosin is still bound to actin therefore maintining tension hence rigor state

  • myosin can not detach from actin until another ATP binds

• (only 3-4 nm per stroke!)

What is needed for cross-bridge cycling and why?

• ATP

  • allows myosin to detach → cock → cycle again

  • detaches and energixes myosin

• Ca2+

  • determines whether myosin has access to the myosin binding sites on actin

  • exposes actin

why does rigor mortis happen?

after death, ATP production stops:

• without ATP

  • myosin cannot detach from actin so cross bridges become stuck and muscles become stiff causing rigor mortis

where does the Ca2+ needed for cross bridge cycling come from?

• for skeletal muscle the major source is the sarcoplasmic reticulum

  • basically muscle cell’s specialized smooth ER and its main job is to store Ca2+

• T-tubules bring the action potential inside the muscle fiber for calcium release

  • invagination of sarcolemma allowing for deep activation

• AP travels along sarcolemma → enters T-tubules → electrical signal travels deep into the muscle fiber → eventually triggering Ca2+ release from the neighborinhg SR

sarcoplasmic reticulum and T tubules

excitation-contraction coupling

how does the muscle action potential cause the muscle to contract?

big pic sequence

• ACh released at NMJ

  • released fom axon terminal of a motor neuron and binds to receptors in the motor end plate

• ACh binds nAChRs

• end-plate potential

• muscle AP

• AP travels along sarcolemma

• AP travels down T-tubules

• SR releases Ca2+

• Ca2+ binds troponin

• tropomyosin moves

• cross-bridge cycling

• contraction

DHP vs RyR1 and their roles and how they work

• DHP receptor

  • Ca2+ channel protein located in the T-tubular membrane

  • functions as a voltage sensor for skeletal muscle excitation-contraction coupling

    • so when muscle AP depolarizes the T-tubule membrane; DHP detects the change in membrane voltage

• RyR1

  • located in the SR membrane

  • it functions as the Ca2+ release channel

• mechanically coupled to each other

Sr Ca2+ release pathway

• action potential depolarizes the T-tubule

• DHP changes conformation

• because DHP and RyR1 are mechanically coupled; RyR1 opens

• Ca2+ leaves SR

Where does Ca2+ move when RyR1 opens and why?

• SR has a very high Ca2+ concentration compared with the cytosol

• So when RyR1 opens: Ca2+: SR → cytosol

  • Ca2+ moves down its electrochemical/concentration gradient through RyR1

SERCA vs RyR1 vs Calsequestrin

• SERCA

  • pump (ATPase) in the SR membrane

  • Ca2+ moves against its gradient, requiring ATP (cytosol → SR)

• RyR1

  • Ca2+ moves down its gradient through a channel (SR → cytosol)

• Calsequestrin

  • is inside SR and binds Ca2+

  • helps SR store a large amount of Ca2+ and calsequestrin helps to do that by binding Ca2+ inside the SR

reversal process - contraction stopping (relaxation)

• Ca2+ stays bound to troponin and cross-bridge cycling continues for as long as cystolic Ca2+ remains high

• as cystolic Ca2+ decreases:

  • SERCA uses ATP (pumps Ca2+ cytosol → SR) → Ca2+ dissociates from troponin → troponin returns towards its resting conformation → tropomyosin moves back over the myosin-binding sites → new cross bridges can no longer form (cross-bridge cycling stopped) → muscle relaxes

ATP involvement - contraction vs relaxation?

ATP is used for:

• cross-bridge cycling: ATP binding allows myosin to detach, and ATP hydrolysis energizes/cocks the head

• relaxation: SERCA hydrolyzes ATP to pump Ca2+ back into the SR

why is the AP highly probable?

a motor neuron can form multiple axon-terminal contacts with a muscle fiber

• when motor neuron fires terminals release ACh onto the motor end plate

sliding filament model - what happens?

• during contraction the filaments don’t shorten, the whole sarcomere does

  • action and myosin DO NOT get shorter

  • actin slides past myosin

• thin actin filaments are attached to the Z lines so during the power stroke, myosin pulls actin toward the M line, which id the center if the sarcomere

  • actin pulled inward → z lines move closer together → sarcomere shortens

sliding filament model - what happens to the bands?

• sarcomere - shortens during contraction

• I band - shortens during contraction

• H zone - shortens during contractio

• A band - stays the same during contraction

• Thick filament - stays same length during contraction

• Thin filament - stays same length during contraction

• actin-myosin overlaps - increases during contraction

force-length relationship

• a muscle produces different amounts of force depending on how stretched/shortened it is

• the amount of force depends on the starting length of the sarcomere, because starting length determines how much actin and myosin overlap

  • actin - myosin overlap → determines how many cross-bridges can form → determines force

• general relationship: appropriate overlap → many cross-bridges → high force

  • can be too little or too much overlap of thick and thin filaments in resting muscles which results in decreased tension

• graph - descriptions

  • a and b

    • sarcomere is too short (increased overlap)

    • filaments begin interfering with one another and the thick filament approaches/compresses against the z discs; thin filaments from opposite sides also overlap excessively

    • interferes with effective cross-bridge production so force falls

  • c - (2-2.3 um)

    • actin and myosin have optimal amount of overlap

      • many myosin heads can interact with actin → lots of cross bridges → maximum active tension

  • d and e

    • sarcomere is too stretched (less overlap)

    • fewer myosin heads can reach actin→ fewer cross bridges → less force

Immediately following an action potential at the NMJ that causes a muscle twitch, what three things have to happen to stop excitation-contraction coupling?

1. neurotransmitter clearance

   1. the ACh the motor neuron released has to be cleared or else it wuld continue activating receptors

      1. acetylcholinesterase (AChE) - breaks doen ACh in the synaptic cleft

2. myocyte repolarization

   1. muscle membrane was depolarized during its AP so it has to return toward resting membrane potential

      1. voltage gated K+ channels open → K+ leaves the muscle cell → membrane repolarizes

3. Ca2+ clearance

   1. SERCA uses ATP to pump Ca2+ from the cytosol back into the SR (once Ca2+ back inside SR → calquestrin helps bind/store it there)

      1. as cytosolic Ca2+ decreases: Ca2+ comes off troponin → troponin returns toward resting conformation → tropomyosin moves back over the myosin-binding sites → myosin can no longer continue forming productive cross bridges → contraction stops/muscle relaxes

what determines the amount of force produced in a contraction?

• actin-myosin cross bridge number is proportional to force

  • force-length relationship

where is force generated during sarcomere contraction?

• myosin power stroke

  • myosin binds actin → power stroke → actin pulled → force generated

• elastic structures

  • elastic elements within the sarcomere → titin

  • elastic elements at muscle attachment points → tendon recoil

If myosin pulls on actin inside the cell, how does that force get transmitted to structures outside the muscle fiber?

dystophin helps connect the internal contractile machinery to the sarcolemma and extracellular matrix, allowing force to be transmitted while stabilizing the muscle fiber

where does muscles need energy (which processes in excitation-contraction coupling)?

Muscles need energy to:

1. maintain concentration gradients (Na+/K+ ATPase)

2. drive myosin power stroke (ATPase)

3. pump calcium into the SR (SERCA ATPase)

During active cross-bridge cycling is the entire muscle getting shorter or longer?

• the muscle could be getting longer or shorter or staying the same

does contraction always generate movement?

contraction generates force, but not necessarily movement

isotonic contraction

• muscle contracts, shortens, and create enough force to move this load

• muscle contracts → fore exceeds required load → muscle shortens → load moves

isometric contraction

• muscle contracts but does not shorten. force cannot move this load.

• muscle generates force but it doesnt generate enough force to move the load

  • so cross bridges are active and force is being generated but muscle length does not shorten