Lec 6
Physiology
Dr.HananLuay
Objectives
1-Define simple muscle twitch?
2-Describe the sequential events in muscle contraction according to
the sliding filament theory.
3- State the principles of walk along theory of skeletal muscle
contraction
4-Compare the two types of muscle contraction.
Electrical characteristics of skeletal muscles:
1- The resting membrane potential is – 80 to – 90 mill volt in skeletal
muscle fiber (same as in large mylinated nerve fiber).
2- The electrical changes of the ion fluxes are similar to those of the
nerve fiber during action potential.
3- Duration of the action potential is 1 to 5 milliseconds (5 times longer
than that in mylinated nerve fiber).
4- The conduction velocity is 3 to 5 m/ second (less than that in large
mylinated nerve fiber).
5-Due to the slight difference in the threshold between muscle fibers of
the same muscle and the difference in the distance between the
stimulation site and different muscle fibers, the action potential
recorded from the whole muscle after direct stimulation is proportional
to the intensity of the stimulus between threshold and maximum
intensity (do not obey all or none law for the whole muscle but not for a
single muscle fiber which obey this law).
6- Each single contraction is followed by a single relaxation in response
to a single action potential (simple muscle twitch).
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 (lag phase), then there is the relaxation phase.
The shape of the single muscle twitch is:
Excitation contraction coupling
1- Sliding filament theory:
The process by which depolarization of the muscle fiber initiates
contraction is called excitation- contraction coupling. It occurs in the
following steps:
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:
The binding of Troponin I to actin will be weakened.
This allows Tropomyosin to move laterally outside the groove and
uncover the binding sites for the myosin heads.
So Ca ions will act as an inhibitory factor on troponin –tropomyosin
attachment to actin.
ATP molecule will split to produce energy (degenerated to ADP) for the
contraction. 7 Myosin heads are uncovered for each molecule of
Troponin that binds to single Ca ion.
The formation of cross bridges between actin and myosin heads →
sliding of thin on thick filaments producing shortening (the sarcomere
will be shortened).
The width of A band is constant, whereas Z lines move closer when the
muscle contracted and apart when the muscle stretched.
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(then decreasing the angle because energy liberated)
producing movement by swiveling(pulling) and then disconnect and
reconnect at the next linking site repeating the process in a serial
fashion(i.e. after the head attaches to the active site, it produces
profound changes in the intramolecular forces between the head and
the arm , the new alignment of forces causes the head to tilt towards
the arm to drag the actin filament along with it, automatically after
tilting the head breaks away from the active site, then the head returns
to its extended direction , then it combines with a new active site
farther down along the actin filament , the head tilts again to form
another power stroke and then the actin filament moves another step
Each single cycle of attaching, swiveling and detaching shortens the
muscle fiber by 1%of its length.
Each thick filament has about 500 myosin heads, and each of these
cycle 5 times /second during rapid contraction. The pulling of the heads
of myosin to actin or the tilt of the myosin head is called the power
stroke.
Power stroke of myosin in skeletal muscle. The
myosin head detaches from actin (top), moves
several nm along the actin strand, and reattaches
(middle). The head then flexes on the neck of the
myosin molecule (bottom), moving the myosin along the actin strand.
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
(active transport, needs ATP i.e. both contraction and relaxation need
energy)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) this removal of calcium ions causes the muscle
contraction to stop.
If Ca ions stay in high concentration outside the SR, or if the Ca ions
transport to the SR is inhibited, there will be persistent contraction and
no relaxation even though there are no more action potentials and this
will result in what is called contracture (sustained contraction).
Characteristics of whole muscle contraction:
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. The muscle contracts against a force transducer without
decreasing the muscle length. e.g. trying to lift a heavy object. The
work done here is zero, because no movement.
The isometric contraction records the changes in force of the muscle
contraction itself, it is used to compare the functional characteristics of
different muscle types.
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 muscle shortens against a fixed load, and its characteristics depends
on the load against which the muscle contracts and on the inertia of the
load.