CONTROL AND COORDINATION
MUSCLES
 Muscles can contract and relax
 do not write stretch and recoil- this property belongs to elas c
ssues, not muscles
 3 types of muscles
Syncy um = term used to describe
mul nucleate muscle fibre, muscle
has many nuclei
Neurogenic = need impulse
It only contracts when it is
s mulated to do so by impulses
that arrive via motor neurone
Stria ons = stripes on muscle fibres,
produced by the regular arrangement
of many microfibrils
Myogenic = heart only ini ates its own
pulse
It contracts and relaxes automa cally,
with no need for impulses arriving from
neurone
STRIATED MUSCLE FEATURES
Striated muscles have the following features:
 a ached to the skeleton via tendons
 neurogenic – contact when s mulated to do so by impulses that arrive
from motor neurones
 each muscle ‘cell’ is mul nucleate and is called syncy um
STRIATED MUSCLE STRUCTURE
the parts of the fibre are known by different terms
 sarcolemma – cell surface membrane
 sarcoplasm – cytoplasm;
lots of mitochondria present between myofibrils which generate the ATP
required for muscle contrac on
 sarcoplasmic re culum – endoplasmic re culum;
membranes of the SR have lots of protein pumps that transport Ca2+ to the
cisternae of the SR
 transverse system tubules/t-tubules –infolding/invagina on of the
sarcolemma
formed by the inward extension of the sarcolemma
func on of t-tubules:
 allows impulses from the sarcolemma to pass to the SR
 maintains the Ca2+ store in the SR
MYOFIBRILS
muscle fibres also contain tubular myofibrils, that run the length of the fibre
 myofibrils are rod-like organelles
 they are responsible for muscle contrac on
 these myofibrils are arranged in a very regular arrangement in the
sarcoplasm, and produces the stria ons of the muscle
each myofibril is made up of smaller components called filaments
two types of filaments, both made of proteins
thick filament- made of myosin
thin filament – made of ac n
example iden fica on of bands and lines :
what happens to bands when muscle contracts?
A band – stays same
HIS shorten
H – H band
I – I band
S – sacromere
Sarcomere = the part of a myofibril between
two Z lines is called a sarcomere
STRUCTURE OF THICK AND THIN FILAMENTS
MYOSIN
- makes up thick filament
- globular heads point away from
M line
- a fibrous protein
- has two globular heads and a tail
- heads point away from M line
- the heads a ach to myosin
binding sites that are present on
the thin filaments
- the heads contain the ATPase
enzyme which releases
energy/ATP to power muscle
contrac on
ACTIN
- ac n is a globular protein
- many ac n molecules are linked
to form a chain
- 2 such chains twist together to
form a thin filament
- Tropomyosin – a fibrous protein –
is also twisted along the ac n
chain
- Troponin – another protein –
a ached at regular intervals to
the chain
- Troponin is the binding site for
Ca2+ ions
- Ac n has binding site for myosin
head
The process of muscular contrac on can be summarised in following key steps:
a) depolarisa on and Ca2+ release
b) sliding filament model
c) sarcomere shortening (muscle contrac on)
a) DEPOLARISATION AND Ca2+ RELEASE
1. Ac on poten al arrives at pre-synap c membrane
2. Ca2+ channels in pre-synap c membrane open
3. Ca2+ enter pre-synap c neurone
4. This causes vesicles containing ACh/neurotransmi er to move and fuse
with presynap c membrane
5. ACh is released into synap c cle
6. ACh diffuse along synap c cle and bind with its receptor on sarcolemma
(cell membrane of muscle)
7. This causes opening of Na+ channels so Na+ enter and depolarize the
membrane
8. Depolarisa on spreads to T-tubules
9. Membrnes of Sarcoplasmic re culum are near T-tubules SR also
depolarised
10.Voltage-gated chnnels for Ca2+ open
11.Ca2+ diffuse out of SR into sarcoplsm surrounding myofibrils
b) SLIDING FILAMENT MODEL
Steps
1. Calcium ions are released by sarcoplasmic re culum (SR)
2. Calcium ions bind to troponin
3. Troponin molecules change shape
4. troponin and tropomyosin move
5. This exposes the myosin binding site on ac n
6. Myosin head binds to this site, forming cross-bridges between thick and
thin filaments
7. Myosin head moves, pulling ac n with it
8. Myosin head hydrolyses ATP
9. Head uses this energy to let go of the ac n molecule
10.The thin filaments have now moved
11.Myosin head now binds to ac n further along the thin filament, closer to
Z line
12.Myosin heads move again
13.Myosin heads hydrolyse more ATP, let go of ac n
14.This con nues as long as
- troponin and tropomyosin are not blocking the binding sites
- ATP molecules are available
 each myosin head is an ATPase – when ATP is hydrolysed to ADP and Pi, the
energy released is used to carry out the power stroke
 power stroke – myosin head a aching to ac n binding sites while ADP
and Pi remain bound
c) SARCOMERE SHORTENING
1) the repeated reorienta on of the myosin heads drags the ac n filaments
along the length of the myosin
2) as ac n filaments are anchored to Z lines, the dragging of ac n pulls the Z
lines closer together, shortening the sarcomere
3) as the individual sarcomeres become shorter in length, the muscle fibres as
a whole contracts
providing enery for muscle contrac on
Role of ATP - allows actomyosin cross bridge to detach and hydrolyze so that
myosin can return to its original posi on and allows the reabsorp on of
calcium ions via ac ve transport (a er s mula on has ended)
muscular contrac on needs a lot of ATP
this comes from following resources
1) aerobic respira on in mitochondria
2) lac c acid fermenta on in sarcoplasm
3) crea ne phosphate – it is stored in sarcoplasm and acts as immediate
energy source when ATP runs out
crea nine phosphate + ADP  ATP + crea ne
- when ATP demand is less, ATP recharges crea ne (adds phosphate to
crea ne)
ATP + crea ne  crea ne phosphate + ADP
- If energy is s ll in high demand, there is no spare ATP so body
converts crea ne to crea nine and excretes it in urine
PAST PAPER QUESTIONS :
Q. b) Describe how the response of sarcoplasmic re culum to arrival of ac on
poten al leads to the contrac on of striated muscle. (MJ 17 P42 Q3)
A. Ac on poten al causes opening of voltage-gated calcium ion channels so
calcium ions diffuse out of SR into the sarcoplsm near myofibril. Calcium ions
bind to troponin, causing it to move so myosin binding site on ac n is exposed.
Myosin binds to ac n, forming a cross-bridge
c) With refernece to sliding filament model, suggest why a lack of ATP affects
func oning of striated muscles.
A. Hydrolysis of ATP is required for the detchment of myosin heads to break
cross-bridge
1. no detachment of myosin heads ;
2. so no, energy transferred to myosin / ATPase ac vity / hydrolysis of ATP ;
3. so no, cross bridge forma on ;
4. so no, power stroke / pulling of ac n ;
5. so no recovery stroke / myosin head does not return to original posi on ;
6. no pumping of calcium ions into SR ;
Q. Describe how tropomyosin and myosin are each involved in sliding filament
model of muscle contrac on. (MJ17 P41 Q6)
Tropomyosin
- it covers myosin binding sites on ac n
- when Ca2+ bind to troponin, it moves/changes shape to expose myosin
binding site
- myosin heads can now bind at the site
Q. Explain why mitochondria are needed in a neuromuscular junc on. (MJ 18
P42 Q8).
A. Mitochondria produce ATP which is needed for




Recycling of ACh
Movement of vesicles containing neurotransmi er/Exocytosis
Contrac on of sarcomere
For sodium potassium pumps
Q. (a) Describe the roles of the neuromuscular junc on, transverse system
tubules (T-tubules) and the sarcoplasmic re culum in s mula ng contrac on in
striated muscle. [7] (ON 19 P43 Q10)
any seven from:
1. ac on poten al / depolarisa on / impulse, at pre-synap c membrane ;
2. (voltage-gated) calcium ion channels open / calcium ions enter (cell /
cytoplasm /(motor) neurone / pre-synap c knob) ;
3. vesicles fuse with pre-synap c membrane ;
4. acetylcholine / ACh, released, by exocytosis / into synap c cle ;
5. (ACh) binds to receptors on, muscle cell membrane / sarcolemma /
motor end plate ;
6. sodium ion channels open / sodium ions enter (muscle cell / sarcoplasm)
;
7. depolarisa on of, (muscle) cell surface membrane / sarcolemma ;
8. (depolarisa on) spreads / transmi ed, to / down / via, T-tubules ;
9. depolarisa on of (adjacent) sarcoplasmic re culum (membrane) ;
10.(voltage-gated) calcium ion channels open ;
11.calcium ions, move / diffuse, out of SR / out of cisterna(e) ;
12.calcium ions, move / diffuse, into, sarcoplasm / cytoplasm ;
13.calcium ions, start contrac on / bind to troponin ;