Transcript skeletal ms

‫بسم هللا الرحمن الرحيم‬
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‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
Skeletal Muscle
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Muscles
• Muscles are machines which convert the stored chemical
energy into mechanical energy (work) and heat.
• Ms constitutes 50% of the body weight.
• There are three types of muscles
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SKELETAL MS
• These ms are usually attached to the skeleton (skeletal ms)
• Functions of the skeletal ms
1) Movements of the body as a whole or part of it e.g. one
limb.
2) Maintenance of the body posture by their tonic
contraction and ms tone.
3) Control of body temperature as;
• 50% of heat production during rest is due ms activity
• During ms exercise heat production is much ↑ed.
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Functional Histology
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Sarcotubular System
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Sarcomere
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Excitation Contraction Coupling
(Mechanism of Sk. Ms
Contraction)
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Excitation Contraction Coupling
(Mechanism of Sk. Ms Contraction)
1) Def.,
• It is the process by which an action potential initiates the
ms contraction.
2) Steps:
• It involves the following steps:
1) Propagation of the AP and release of Ca ions:
• Propagation of the AP in the motor nerve → production of
an EPP at motor end plate → generation of AP at the
adjacent areas of ms cell membrane → AP spreads on both
sides of the motor end plate and spreads along the T
tubules which extend deep into the ms fiber → release Ca
ions from the terminal cisternae of SR.
Mechanism of Sk. Ms Contraction
2) Steps:
1) Propagation of the AP and release of Ca ions:
Mechanism of Sk. Ms Contraction
2) Steps:
2) Binding of the cross-bridges between myosin and actin:
 The released Ca ions combine with Troponin C molecules
→ causes the tropomyosin to move away from its blocking
position and thus exposing the binding sites present on
actin molecules.
 Cross bridges from the thick (myosin) filaments combine
with the binding sites on the actin.
Mechanism of Sk. Ms Contraction
2) Steps:
3) Cycling of cross-bridges:
It occurs in the following steps; a) Binding: b) Bending: c)
Detachment: d) Return to original position:
Mechanism of Sk. Ms Contraction
2) Steps:
4) Relaxation:
 It occurs when Ca ions are transported into the SR by an
active process and Ca-ATPase.
 Removal of Ca ions makes troponin to return to its original
state which causes tropomyosin to move back and cover
the binding sites on actin.
Mechanism of Sk. Ms Contraction
Changes that occur in the ms
during contraction
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I. Electrical Changes
Nerve fibre
Ms fibre
RMP
-70 m.v
-90 m.v
Firing level
55m.v.-
–50 m.v.
105 m.v.
130 m.v.
0.5- 1 m.sec
3-5 m. sec
-ve and +ve after potentials
short
long
Velocity of conduction of the
up to
3-5
120met/sec
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meters/sec
Magnitude
of
the
spike
potential
Duration of the AP
AP
I. Electrical Changes
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II- Excitability changes
• the excitability of the muscle fibers passes in the
following phases:
1. Absolute refractory period (ARP):
2. Relative refractory period (RRP):
3. Supernormal phase of excitability:
4. Subnormal phase of excitability:
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III. Metabolic (chemical) Changes
A) During contraction:
•Energy of contraction is derived from:
1) ATP:
It is the immediate source of energy in this process.
ATPase
ATP
ADP + P (phosphoric acid) + Energy.
2) Creatine phosphate (Cr-P):
Cr-P rapidly resynthesizes ATP from ADP by adding of phosphate group.
Creatine phosphokinase enzyme
ADP + Cr-P
Creatine + ATP
The amount of Cr-P is about 10 times larger than ATP
ATP and Cr-P allow the ms to contract only 50-100 times.
III. Metabolic (chemical) Changes
A) During contraction:
3) Oxidation of glucose and free fatty acids:
a) Anaerobic oxidation of glucose (or ms glycogen):
Anaerobic oxidation (Glycolysis)
Glucose (ms glycogen)
2ATP
Anaerobic glycolysis is rapid but not economic.
b) Aerobic oxidation of glucose and free acids:
Aerobic oxidation (Kreb's cycle)
Glucose + 6O2
6CO2+ 6H20 + 38ATP
It produces great amount of ATP (38 ATP).
It is economic but slow.
4) In exhausted ms:
There is an emergency mechanism for the supply of ATP
ADP + ADP
ATP + AMP
III. Metabolic (chemical) Changes
III- Chemical changes
IV- Thermal Changes
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III. Metabolic (chemical) Changes
B) During recovery:
At the end of ms activity, the energy stores in the ms (ATP, Cr-P
and ms glycogen) are depleted and the lactic acid is ↑ed in blood.
a) Part of the lactic acid is oxidized into CO2 and H2O:
The energy produced from this oxidation is used to reform ATP
and by turn Cr-P.
Oxidation (Kreb's cycle)
Oxidation
Lactic acid
Pyruvic acid
CO2+ H2O + ATP
ATP + Creatine
Cr-P + ADP
III. Metabolic (chemical) Changes
B) During recovery:
b) The other part of the lactic acid is converted into glucose:
•Lactic acid diffuses to the blood stream and then to the liver
where it is converted into blood glucose (through the Cori cycle
which is the reverse of glycolysis).
•Ms takes glucose from the blood stream and changes it into ms
glycogen.
•At the end of recovery the energy stores in the ms (ATP, Cr P
and ms glycogen) are reformed again, and the lactic acid is
removed.
III. Metabolic (chemical) Changes
N.B.
PH Changes in the ms during contraction:
1) At first, the pH becomes acidic due to the release of phosphoric
acid.
ATPase
ATP
ADP + Phosphoric acid + Energy
2) Then the pH becomes alkaline due to the release of creatine
from the Cr-P.
ADP + Cr-P
ATP + creatine
3) Lastly, the pH becomes acid due to the release of lactic acid
from the anaerobic oxidation of glucose.
Anaerobic oxidation
Glucose (ms glycogen)
2 lactic acid + 2ATP
IV. Thermal Changes
•During contraction, heat production in the ms is about 100-1000
times during rest.
•40-50% of the energy liberated is converted into work.
•50-60% of the energy is liberated as heat at the onset of and
during contraction of the muscle → so heat production occurs in
two phases:
1) Initial heat: includes:
i) Activation heat:
•It is a very rapid heat production which starts before any
shortening has occurred.
•It results from:
1. Release of Ca from the terminal cisternae.
2. Binding of Ca to troponin protein.
3. Movements of cross-bridges towards the binding sites on the
thin filaments.
IV. Thermal Changes
1) Initial heat:
ii) Shortening heat:
•It represents the heat produced by the process of shortening
(cross-bridge cycling).
•It is proportional to the degree of shortening.
iii) Work heat:
•It occurs when the ms performs work.
•It is proportional to the work done.
IV. Thermal Changes
2-Delayed (recovery) heat:
•It is the heat liberated from the ms after its relaxation.
•It results from the metabolic reactions needed to reform the
energy stores in the ms (ATP, Cr-P and ms glycogen) and to
remove lactic acid.
•It is nearly equals to the initial heat, and continues for about
30 minutes after the end of ms contraction.
V. Mechanical Changes
Types of muscle contraction:
There are two types of ms contraction→ isotonic and
isometric contractions.
Isotonic Contraction
Isometric Contraction
work is done
no work is done
Mechanical efficiency is It is zero i.e. all the energy
maximum (40 –50%).
is converted to waste
heat.
Tension ↑es at first, then
Tension is markedly
maintained constant
increased.
during the major part of
contraction isotonic
V. Mechanical Changes
V. Mechanical Changes
V. Mechanical Changes
Mechanism of isometric contraction:
•In isometric contraction, the contractile components of ms shorten at
the same time the elastic components (2 types, series and parallel
elastic elements) are stretched to the same degree.
•So the length of the ms remains constant but its tension is markedly
increased.
V. Mechanical Changes
Force velocity relationship
•The velocity of shortening is
inversely proportional to the
weight of the load.
•If the ms is unloaded, it shortens
with maximum velocity.
•As the weight of the load ↑es,
the velocity of shortening
decrease.
•When the load reaches a
maximum, the ms contracts
without shortening i.e. contracts
isometrically.
V. Mechanical Changes
Length tension relationship
•The tension developed during the isometric contractions
depends on the length of the ms.
•A maximum tension is obtained when the length of the ms is
nearly equals to the resting length of the ms in the body (optimal
length).
THANKS