Skeletal muscle contraction
Lecture 6
Objectives:
1-Define simple muscle twitch?
2-Describe the sequential events in muscle contraction
according to the sliding filament theory.
3-Compare the two types of muscle
contraction.
4-Recognize the meaning of summation of contraction
and its types.
5-Detrmine the effect of changing length on
skeletal muscle tension.
6-Differntiate between the 2 types of muscle fibers and
motor units.
7-Outline the abnormalities in muscle fiber contractions.
Simple muscle twitch:
Is a single contraction followed by single relaxation in response
to action potential .It is measured usually by a device called
Myogram.
The shape is consisted of contraction phase which is preceded by
latent period ,then there is the relaxation phase.
The shape of the single muscle twitch is:
Excitation contraction coupling
The process by which depolarization of the muscle fiber
initiates contraction is called excitation- contraction coupling.
1- Sliding filament theory
:
1 – The discharge of motor neuron.
2- An action potential travels along the motor
nerve to its ending in the muscle fiber.
3- Secretion of small amounts of
neurotransmitter substance Acetylcholine
(Ach) at the motor end plate.
4-Ach binds to nicotinic receptors on muscle
fiber membrane to open Ach gated channels.
5- Increase in Na and K ions conductance (Na ions
diffuse to the interior of the muscle fiber
membrane) and this will initiate a local end plate
potential, and when firing level is reached, action
potential is generated and spread along the
whole muscle fiber.
6- The inwards spread of the action potential by the
T system of tubules.
7- Release of calcium ions from the terminal
cisterns of the sarcoplasmic reticulum.
8-Calcium will bind to Troponin C molecule this will
lead to conformational changes:
This allows Tropomyosin to move laterally outside the groove
and uncover the binding sites for the myosin heads.
The formation of cross bridges between actin and myosin heads
→ sliding of thin on thick filaments producing shortening (the
sarcomere will be shortened).
So during muscle contraction 1- the Z lines move closer to each
other, 2- the I band becomes shorter and 3- the A band stays at
the same length.
2- The walk- along or Rachet theory of
contraction:
This theory suggests that the sliding
during muscle contraction is produced by
attaching, breaking and reforming of the
cross linkages between actin and myosin
heads,
the intensity of the interaction
depends on the number of cross linkages .
-
After uncovering of the active sites of the actin.
- Myosin head link to actin at 90 degrees angle, the
head tilts towards the arm to pull the actin filament
along with it,
- Producing movement by pulling.
- Disconnect(breaks away from the active site, then
the head returns to its extended direction ,
-The head combines with a new active site and the
process is repeated.
The pulling of the heads of myosin to actin is
called the
power stroke.
Steps in relaxation:
1- After a fraction of a second, the calcium ions are pumped
actively back into the sarcoplasmic reticulum by a Calcium
membrane pump
,
they are going to diffuse into the terminal
cisterns to be released by the next action potential.
2- The release of calcium ions from Troponin C,
3- Then cessation of binding between actin and myosin (i.e.
tropomyosin returns to its site.
4- Contraction stops.
Types of contraction:
1- Isomertic contraction
:
is when the muscle does not shorten during
contraction i.e. no change in muscle length, but the
tension will increase.
e.g. trying to lift a heavy object. The work done here is zero,
because no movement.
2- Isotonic contraction:
It is the contraction
that causes shortening of the muscle
length and the muscle has the same tension. e.g. lifting an object
by contracting the biceps muscle.
Here there is work done because there is movement.
The summation of contraction
It means the adding together of individual twitch
contractions to increase the intensity of overall
contraction.
Because the contractile mechanism has no
refractory period.
Repeated stimulation, before relaxation can
produce additional activation of the contractile
elements .
The response will be added to that already present,
this is called
"summation of contraction".
It depends on the frequency of stimulation. It occurs in
2 ways:
1 – Multiple fiber summation
2- Frequency summation and tetanization, with
rapid repeated stimulation, before any
relaxation occurs called Tetanus (
by increasing
the frequency of contraction).
During tetanus the tension developed is 4 times than
the individual contraction.
If a lower frequency is used, there will be a
period of incomplete relaxation between the
summated stimuli; this condition is called
incomplete tetanization or clonus.
Effect of muscle length on the force of
contraction:
When the length of the unstimulated muscle
fiber is changed (the muscle stretched but not
stimulated) this is called passive tension. It
exists because the muscle and its connective
tissues have some elasticity.
Passive tension curve
. shows that, as the length
increases, it rises sharply.
The active tension
recorded when the muscle
contracts.
The maximum tension occurs when the muscle length is at its
resting length i.e. the sarcomere is at the relaxed state.
Shortening or contraction of the muscle will reduce this
tension. This is because the maximum number of
interaction between actin and myosin occurs at the resting
length .So shortening smaller sarcomere and actin
filament overlap, so less actin to myosin interaction sites.
Stretching of muscle fiber (increase in its length) less
actin to myosin interaction i.e. bigger sarcomere with a
large gap in the middle ..
The oxygen debt mechanism:
During exercise
the blood vessels dilates to provide
enough O 2 for the muscle, the energy is supplied by
aerobic glycolysis.
If the exercise is sever
or continues for longer
periods,
the anaerobic glycolysis
contribute
also to provide energy.
It is self limiting ,because lactic acid will diffuse
to the blood it will lower the PH ,accumulate
in the muscle causing muscle exhaustion ,
So after the exercise there will be a period of
hyperventilation to produce the extra amount of
O2 in order to remove the lactic acid and to
rebuild the storage of ATP and phosphorylcreatinin
. This extra amount of O2 taken to replace the
demand required more than that supplied by the
aerobic glycolysis during exercise is called the
oxygen debt
.
mechanism
Trained persons need less period of
hyperventilation because they
have
smaller
oxygen
debt
mechanism
(i.e.
he
has
endurance, he can use the
muscle to perform the job
better).
Types of muscle fibers
3 types according to the differences in enzyme
activity, metabolism and contractile properties:
1- Type I fibers
: these are darker than other muscles
called
Red muscles
or the slow fibers, they
respond slowly and have longer duration of
action(resist fatigue). They are specialized for
long slow sustained contraction, supplied by slow
conducting fibers. e.g. muscles in the back and in
the lower limbs which are used to maintain
posture.
2-
Type II b:
called the white muscles or the fast
fibers, they are innervated by large mylinated
fibers, they have short duration of action(fatigue
quickly) .So they are specialized for fine skilled
movement e.g. muscles of the hand and the
extra-ocular muscles. They are large fibers
3-
Type II a
: this is rare type in human, has
properties similar to type I, and other properties
to type II.
The motor units
:
The motor unit means all the muscle fibers
innervated by a single nerve fiber .. There are 2
types of motor units:
1- Small motor units
: contain 3-6 muscle fibers,
concerned with fine graded, precise movement,
like movements of the hand. They are Small
muscles that react rapidly and whose control
must be exact have more nerve fibers for fewer
muscle fibers (2-3 muscle fibers for each motor
unit, e.g. laryngeal muscles).
2-
large motor units
:contain usually 120-165
muscle fibers ,like muscles of the back ,for the
sustained form of activity. These are large
muscles that do not need fine control (e.g.
soleus muscle).
Each motor unit is of one type i.e.
innervates one type of muscle, but when a
nerve to slow muscle is cut and replaced by a
nerve to fast muscle ,the slow muscle after a
period of time becomes fast.
Abnormalities in muscle contraction
.
Denervation
Means the deprivation of muscles from the nerve supply ,the
following will happen:
1- Immediate loss of muscular activity.
2- Abnormal excitability.
3- Atrophy of the muscle
If after 2 months the nerve supply back, full return of function .but after 3 months
the return back of muscle function is less
Contracture.
Rigor.
Muscle fatigue.
-
Depletion of glycogen stores in the muscle (inability of the contractile and
metabolic processes to continue supplying the same work).
-Diminished transmission in the NMJ.
- Interruption of blood supply or flow results in nutrient and O2 deficiency.
•
Summary
:
-Theories of skeletal muscle contraction includes sliding filament
theory and walk along theory.
-There are 2 types of muscle contraction: isometric and isotonic-
-The contractile mechanism can be summated.
-There are 2 types of muscle fibers :type I (red fibers) and type II
(white fibers).
-There are 2 types of motor units :small and large.
-Abnormalities of the contractile mechanism : denervation
,contracture , rigor, muscle fatigue
.