Physiology of Skeletal Muscle Contraction

Question 1

What is considered the muscle cell, things to note about its structure

Answer 1

muscle fibres
large with multiple nuclei (up to 30 cm long)
membrane is sarcolemma
cytoplasm is sarcoplasm
packed full of rod like structures - myofibrils within them sarcomeres

Question 2

what accounts for the striation of the muscle fibres (what makes them)

Answer 2

the sarcomeres being joined end to end

Question 3

what does a contracting myofibril result in

Answer 3

a contracting muscle

Question 4

three components of sarcomeres and what they’re made of

Answer 4

thick filaments made of 100s of myosin molecules
thin filaments made of actin
elastic filaments

Question 5

anatomy of thick filaments

Answer 5

made of myosin molecules which have to heads and a long tail
which has a binding site for actin and ATP
the central part of the myosin is bare (no heads)

Question 6

anatomy of thin filaments

Answer 6

string made of actin (blueberry looking molecules) joined by tropomyosin rope like protein and troponin molecules tying those two together

actin has binding site for myosin attachment (active sites)
forms two chains that interwire

tropomyosin blok myosin binding sites in relaxed muscles

troponin - binds actin and tropomyosin to stabilise thin filament
has binding cites for Ca2+

Question 7

what is the smooth endoplasmic reticulum called in muscle fibres and where is it located
+what element does it store

Answer 7

sarcoplasmic reticulum and it surround each myofibril like a papaya mesh sleeve (and T tubules)
stores Ca2+

Question 8

what is a t tubule where are they located and what is their role

Answer 8

a tube of sarcolemma (membrane) that runs down the sarcoplasmic reticulum
they take action potentials deep down into the muscle fibre to cause the release of the Ca2+ molecules stored in the sarcoplasmic reticulum

Question 9

steps in skeletal muscle contraction

Answer 9

ap travels down motor neuron
acetylcholine (always Ach) released at neuromuscular junction (synapse between neuron and muscle fibre)
AP runs along sarcolemma and down along T tubules causing release of Ca2+
Ca+ allows actin and myosin to bind
myosin pulls on actins to middle of sarcomere causing the sarcomeres to shorten

Question 10

what neurons control skeletal muscles

Answer 10

somatic motor neurons

Question 11

what is the space between the somatic motor neuron axons and the sarcolemma called (the neuromuscular junction - synapse)

Answer 11

synaptic cleft

Question 12

difference between synapse at neuron neuron and neuron muscle

Answer 12

with muscle always acetylcholine

otherwise everything same calcium at axon triggers Ach to be released release of Ach triggers Na Chanel of muscle to open causes graded potational (endplate potetional in skeletal muscle cells)

Question 13

how and why I Ach broken down

Answer 13

to stop contraction of muscle
can be done by enzyme called acetylcholinesterase
or by defusing away by itself

Question 14

what kind of polarisation does the endplate potential cause

Answer 14

depolarising one which would spread to voltage gated Na and K channels and IMPORTANTLY along sarcolemma and down T tubules

Question 15

how does the action potential reach the muscle fibre

Answer 15

AP travelling down T tubules causes voltage-sensitive tubule proteins to change shape which opens the Ca2+ channels in the sarcoplasmic reticulum

Question 16

how does Ca2+ facilitate muscle contraction

Answer 16

Ca2+ bind to troponin causing tropomyosin to pull away from the active sites of the actin allowing it to bind to myosin to form cross-bridges

Question 17

step 4 of muscle contraction

Answer 17

myosin as an ADP and Phosphate molecule attached (ATP has broken down into adenosine triphosphate has broken down into diphosphate) so the myosin head now has energy and moves into a cocked position, which is high energy and ready to bind to actin

Question 18

step 1 of muscle contraction

Answer 18

once Ca2+ released myosin and actin can bind together (cross-bridge)

Question 19

What is the power stroke in muscle contraction

Answer 19

when the myosin head bends towards the centre of the sarcomere

Question 20

step 2 of muscle contraction

Answer 20

ADP and P are detached from the myosin than it performs the power stroke pulling the actin towards the midline

Question 21

step 3 of muscle contraction

Answer 21

a new ATP molecule attaches to the myosin which reduced the affinity of myosin for actin causing the cross-bridge to break