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Tropomyosin
alpha-1 chain
Tropomyosin
beta chain
G protein
complex
Guanine
nucleotide-
binding protein
G(i) subunit
alpha-1
Adenylyl
Cyclase
Funny channel
(If)
Sodium/calcium
exchanger 1
Inositol 1,4,5-
trisphosphate
receptor type 1
Voltage-
dependent
T-type calcium
channel subunit
alpha-1H
Voltage-
dependent
T-type calcium
channel subunit
alpha-1G
Guanine
nucleotide-
binding protein
G(I)/G(S)/G(T)
subunit beta-1
Guanine
nucleotide-
binding protein
G(I)/G(S)/G(O)
subunit
gamma-12
cAMP
Ca
+
Ca
+
Na
+
Na
+
Ca
+
Ca
+
Na
+
Na
+
K
+
K
+
Ca
+
Ca
+
GDP
GTP
ATP
GTP
Ca
+
Muscarinic
Receptor
Troponin
Troponin
Muscle
Contraction
Cardiac Myocyte
Sarcoplasmic Reticulum
Calcium is unable to bind to
troponin to displace
tropomyosin from the myosin
binding sites on the actin
filaments
Myosin and actin are not
bound allowing the filaments
to slide further resulting
in muscle relaxation
Actin Filament
Myosin Filament
Cytosol
T-tubule
GDP is replaced with GTP on
the alpha subunit
The G protein separates into
alpha and beta/gamma
subunits
The G(i) alpha subunit
inhibits adenylyl cyclase,
lowering the production of
cAMP
There is an overall decrease
in calcium levels in the
cytosol
The muscarinic receptor that
activates Gi signalling
causing cardiac muscle
contraction is the M2
receptor, acetylcholine is
an agonist of this receptor
Lower cAMP concentrations
means the open probability
of funny channels are
decreased, meaning there is
less influx of ions
Voltage dependent calcium
channels are unable to open
TPM1
TPM2
GNG2
GNAI1
ADCY9
HCN4
SLC8A1
ITPR1
CACNA1H
CACNA1G
GNB1
GNG12
cAMP
Calcium
Calcium
Sodium
Sodium
Calcium
Calcium
Sodium
Sodium
Potassium
Potassium
Calcium
Calcium
Guanosine
diphosphate
Guanosine
triphosphate
Adenosine
triphosphate
Guanosine
triphosphate
Calcium
Muscarinic
Receptor
Troponin
Troponin
Muscle
Contraction