Muscle contraction
1
Introduction
Living muscle : is highly specialized tissue
that is capable of converting chemical
energy into mechanical energy through its
contraction .
Muscles are positioned and attached to the
skeleton in such a way that their contraction
and relaxation lead to movement and
locomotion .
2
Introduction
The ability to contract and relax is lost
when muscle is converted to meat.
The biochemical processes that provide
energy for muscle function in the living
animal are the same processes that cause
lactic acid production and loss of water
holding capacity during the postmortem
period.
3
Muscle how to contract and generate
force and perform work
At first, a muscle contraction is initiated by
a stimulus that arrives at the surface of the
muscle fiber.
This stimulus is coming from the brain and
spinal cord and is transmitted to muscle via
a nerve
Membrane potential
4
Membrane potential
Divided into two, one is the resting
membrane potential and another one is
action membrane potential.
A electrical potential always exists between
the inside and outside of the cell.
Vary from 10-100 millivolts.
Nerve and muscle fibers exhibit membrane
potential
5
The resting membrane potential
Generally, a slight excess of negative ions
accumulates in the intracellular fluid along
the inner surface of the membrane and a
slight excess of positive ions is present
along the extracellular surface of the
membrane. This condition is exist in the
normal resting nerve fiber
6
7
The resting membrane potential
Is the result of
The active transport of iron through the
membrane.
Te selective permeability characteristics of
the membrane to the diffusion of ions and
small molecular.
The unique ionic composition of the
intracellular and extracellular fluids.
8
The ionic change
Extracellular fluid contains high concen. Of Na+
and Cl- , but very low concen. Of K + and
nondiffusible negative ions, in contrast, K + and
nondiffusible negative ions are very high in
intracellular fluid and Na+ and Cl- are quite low.
Concentration gradient, maintained by active
transport. Need a Na+ and K + pump – which is
located in the plasma membrane.
Energy required for pump action
9
Action potentials : the stimuli
An electrical impulse, can along muscle and nerve
membrane surface.
An action potential is transferred from a motor
nerve to muscle fibers.
Is a wave of reversing electrical polarization that
results from chemical changes in membraneelectro-chemical process.
Is initiated by a several thousandfold increase in
the sodium ion permeability of the membrane .
10
Action potentials : the stimuli
If Na+ permeability increase to a higher value than
that which exists for K+ , the high concentration
of Na+ in the extracellular fluid cause s a more
rapid rate of diffusion of Na+ into the cell, than of
the K+ out of the cell.
This results in an excess of positive charge inside
the cell membrane and negative charges on its
outside, thus reversing the resting membrane
potential.
11
12
Action potentials : the stimuli
The entire sequence of events in the
transmission of an action potential past any
point on the nerve fiber requires about 0.51.0 millisecond.
13
Myoneural Junction
The stimulus that initiates muscle
contraction in transferred from the nerve
fiber to the muscle fiber at the myoneural
junction .
Structure as figure
14
15
Action potential -- chemical
A special mechanism amplifies – the electronic
signal and transfers it to the muscle fiber.
The action potential arrives at the end of the
myoneural junction , it causes a chemical
transmitter -acetycholine ( 乙醯膽鹼 ) to be
released.
Acetycholine will act on sarcolemma then make
more permeable to Na+ , its polarity reverses and
an action potential is propagated along its length.
16
Acetycholine
Acts on the sarcolemma for only a few
milliseconds , then destroyed after it release
by cholinesterase.
17
Action potentials in muscle
That occurs in muscle fibers when
acetylcholine contacts the sarcolemma is
nearly identical to that which occurs in
nerve fiber.
The entire sequence of events in the
transmission of an action potential past any
point on the muscle fiber requires about 510 millisecond.
18
Contraction of skeletal muscle
Involve four myofibrillar proteins : actin, myosin,
tropomyosin and troponin.
Contractile protein: actin and myosin
Crossbridges formed between the filamentscontractile force
No crossbridges between actin and myosin
filaments-relaxed state.
If a permanent crossbridges form and prevent the
sliding of these filaments-rigor mortis
19
20
Contraction of skeletal muscle
Relax state: Calcium concentration is less
than 10-7 moles/liter. Total of Ca+2
concentration is about 1000 times 10-7
moles/liter( about 10-4 moles/liter). Need
pump into SR, so need higher level of
energy-ATP
Most of ATP is found in Mg-ATP, must be
present in order to prevent interaction of
actin and myosin (crossbridges).
21
Inhibit crossbridge (Contraction
of skeletal muscle)
Two factors :
sarcoplasma Ca+2 concentration is low (less
than 10-7 moles/liter).
the Mg-ATP concentration is high.
22
Contraction of skeletal muscle
When action potential is transmitted from
the sarcolemma to muscle and T tubles
system. Ca+2 concentration is released
from SR and also increased sarcoplasm Ca+2
concentration. (10-5 ~10-6 moles/liter), the
contractile mechanism will be started.
Ca+2 will bound by troponin (activated )
and interacts with tropomyosin causing it to
shift its position along the actin filament.
23
Contraction of skeletal muscle
The shift by tropomyosin allow the myosin
head s to form crossbridges between the
myosin and actin filaments= a contractile
force : pulled toward the center of the
sarcomere. (figure) The change of the
sarcomere (distance of two Z line).
Crossbridges=actomyosin
24
25
Energy-ATP
Muscle contraction required energy, which
is derived from ATP by a reaction catalyzed
by the enzyme myosin ATPase.
ATP ADP +Pi
High Ca+2 concentration also can increase
ATP splitting .
26
Relaxation of skeletal muscle
sarcoplasma Ca+2 concentration is low (less
than 10-7 moles/liter).
the Mg-ATP concentration is high.
As the troponin loss these Ca+2 , it is again
able to inhibit the formation of crossbridge
then relaxation state will be occurred .
27
28
Source of energy
Muscle glycogen
Blood glucose
Aerobic metabolism
29
How to supply energy
30
Energy , lactic acid and heat
31
The end of this chapter
32