Muscle Contractions

Do I know the neuromuscular junction???

YES, if not REVIEW!!

initial simulation of a muscle contraction (KNOW ORDER OF STEPS FOR TEST)

• electric signal at END of a motor neuron releases chemical signal ACETYLCHOLINE

• Acetylcholine binds to RECEPTORS in the MOTOR and PLATE of the muscle, allowing Na+ ions to pass through (permeate) the channel proteins

  • acetylcholine needed to trigger channel proteins to open, allowing sodium ion to permeate membrane and go into the muscle fiber

• electric signal triggers diffusion of Ca2+ from the SARCOPLASMIC RETICULUM into the CYTOPLASM of the muscle fiber

  • calcium then binds with TROPONIN (a protein in muscles) and steps of muscle contraction begins

how to remember what chemical happens first

Acetylcholine - starts with A

• order is Acetylcholine, sodium, calcium, and troponin

origin and insertion

origin: muscle part that attaches to fixed part of the bone

insertion: muscle part that attaches to bone that will move (will pull on)

agonist/antagonist

agonist- main muscle doing movement

antagonist- opposite of movement

parts within the sarcomere (KNOW WHERE TO LABEL TOO)

myosin: thick filaments

actin: thin filaments

z-line: where filaments attach

filaments in the sarcomere

when muscles contract, which parts of sarcomere move? and what is process called

Sliding Filament model:

actin slides into middle, z lines move closer together (myosin do not move.)

how does myosin pull actin in when muscles contract?

myosin have myosin heads,

myosin heads break away from myosin with ATP, heads bind to the actin and pulls actin (thin filaments) to slide inside.

tropomyosin

regulatory protein that blocks binding sites in myosin (if myosin doesn’t bind the actin, muscle will not contract)

• calcium and sodium ions that come in during initial steps bind to tropomyosin and unblock the myosin so the muscle can contract

CAN I LABEL THE ANATOMY OF A SARCOMERE???

YESSSSS if not review

sliding Filament model sciencey process

• muscle fiber contraction involves an interaction in which myosin bind to acting (forming a CROSS-BRIDGE) and exert a pulling force

• filaments slide past each other, increasing area of overlap in order to shorten (contract) the muscle fiber *Filaments do not shorten*

• myosin can release and then binds again with another site further down the actin filament

myosin

two twisted proteins with globular heads projecting from one end

• contains ATPase (enzyme that breaks ATP into ADP + P)

actin

molecules twisted into a double helix

• tropomyosin: regulatory protein that blocks binding of myosin

  • troponin: binding site for Ca2+ and pulls tropomyosin aside to allow formation of cross bridges

steps of muscle contraction

• initial steps listed above relating to sodium and calcium.

• release of Ca2+ from SARCOPLASMIC reticulum

• Ca2+ binds to TROPONIN, pulling tropomyosin aside and exposing binding sites on actin

• myosin heads bind to acting, forming cross-bridges

• ADP and phosphate are released from myosin, performing a POWER STROKE (pulling of actin)

• ATP binds to myosin, breaking the cross-bridges

• ATP splits into ADP + P (assistance from ATPase), which powers the “cocking” of the myosin heads storing energy for the next power stroke

ACTION: take piece of paper, write down steps of muscle contraction (skipping lines), cut out steps and order them then check

I did this!

Major events of muscle contraction

1. impulse travels down a motor neuron axon

2. motor neuron releases the neurotransmitter acetylcholine (ACh)

3. ACh binds to ACh receptors in the muscle fiber membrane

4. sarcolemma is stimulated. an impulse travels of the surface of the muscle fiber and deep into the fiber throughout the transverse tubules

5. impulse reaches the sarcoplasmic reticulum, and calcium channels open

6. calcium ions diffuse from the sarcoplasmic reticulum into the cytosol and bind to TROPONIN molecules

7. tropomyosin molecules move and expose specific sites on actin where myosin heads can bind

8. cross-bridges form, linking thin and thick filaments

9. thin filaments are pullled towards the center of the sarcomere by pulling of the cross-bridges

10. The muscle fiber exerts a pulling force on its attachments as a contraction occurs

muscle relaxation

• requires active transport (using ATP) of calcium in into sarcoplasmic reticulum, allowing tropomyosin to conver binding sites on actin, which acts as an inhibitor

rigor mortis

skeletal muscles partially contract and become rigid after death

• increase in calcium ion permeability

• decrease in atp in muscle fibers

• can last for a couple hours until the pathways cannot be stimulated

removal of stimulus (muscle relaxation)

when nervous simulation ceases:

• acetylcholine is decomposed by ACETYLCHOLINESTERASE (enzyme present at the neuromuscular junction, ending impulse in the muscle)

• Calcium ion is actively transported back into sarcoplasmic reticulum

• ATP binds to myosin, breaking cross bridges and relaxing the fiber

OVERVIEW: steps of muscle relaxation (write out on paper, cut, and order)

• acetylcholinesterase decomposed acetylcholine, and the muscle fiber is no longer stimulated

• calcium ions are actively transported into the sarcoplasmic reticulum

• ATP breaks cross-bridge linkages between actin and myosin filaments without breakdown of the ATP itself

• breakdown of ATP “cocks” the myosin heads

• TROPONIN and tropomyosin molecules block the interaction between myosin and actin filaments

• the muscle fiber remains relaxed yet ready until stimulated again

all or none response in muscle contraction

single muscle fiber contracts with maximum force whenever stiumlated

• TWITCH- contractile response of a single muscle fiber to a single impulse

  • LATENT PERIO- delay between the time of stimulation and contraction

myogram

graphical recording of muscle activity (KNOW HOW TO READ)

strength of muscle contractions can be determined by: (IMPORTANT FOR TEST)

• frequency at which individual muscle fibers are stimulated

• how many fibers take part in the overall contraction of the muscle

summation and partial and complete tetany

as frequency of stimulation of a muscle fiber increases, fiber may be unable to completely relax between stimuli (force of individual twitches combine)

• PARTIAL TETANY: high frequency of muscle stimulation, resulting in very brief periods of relaxation between contractions (KNOW WHAT THIS GRAPH LOOKS LIKE)

• COMPLETE TETANY: rapid stimulation of the muscle fiber, so that relaxation does not occur at all (only can occur in a lab) KNOW WHAT THIS GRAPH LOOKS LIKE

motor unit

motor neuron and all of the muscle fibers that it controls

• an impulse from a motor neuron will cause all muscle fibers that it innervates to contract at the same time

whole muscle composed of:

many motor units controlled by different motor neurons

• some motor neurons are more sensitive to stimulation than others (and contract independently)

isotonic contraction

contraction of a muscle where the length of the muscle changes (crunches, sit-ups)

• CONCENTRIC contraction: muscle shortens (ex: biceps during a curl)

  • ECCENTRIC contraction: muscle lengthens (ex: triceps during a curl)

ISOMETRIC contraction

muscle generates force without changing length

• ex: abdominal planks, holding leg lifts, wall sits