Electrical changes during muscular contraction
When the muscle is stimulated, electrical changes occur before onset of mechanical changes. Usually the electrical events in a muscle (or any living tissue) are measured by using a Cathode Ray Oscilloscope. Nowadays, sophisti cated electronic equipments like computerized polygraph are available to record and analyze the electrical activities of any tissue.
Resting membrane potential
The potential difference between inside and outside of the cell under resting condition is known as resting membrane potential.
Fig.2. Resting membrane potential.
When two electrodes are connected to a cathode ray oscilloscope through a suitable amplifier and placed over the surface of the muscle fiber, there is no potential difference. There is zero potential difference. But, if one of the electrodes is inserted into the interior of the muscle fiber, potential difference is observed across the sarcolemma (cell membrane). There is negativity inside the muscle fiber in relation to the outside. This potential difference is constant and is called resting membrane potential. The condition of the muscle during resting membrane potential is called polarized state. In human skeletal muscle, the resting membrane potential is -90 mV.
Action potential
When the muscle is stimulated, a series of changes occur in the membrane potential, which is called action potential. The action potential occurs in two phases.
1.Depolarization and
2.Repolarization.
Depolarization
When the impulse reaches the muscle, the polarized condition (-90 mV) is altered, i.e. the resting membrane potential is abolished. The interior of the muscle becomes positive and outside becomes negative. This condition is called depolarization. With other words, depolarisation is membrane potentials difference decreasing.
Repolarization
Within a short time, the muscle obtains the resting membrane potential once again. Interior of the muscle becomes negative and outside becomes positive. So, the polarized state of the muscle is re-established. This process is called repolarization. So, it is potentials difference restoration.
Action potential curve
Resting Membrane Potential
The resting membrane potential is recorded as a straight baseline at -90 mV .
Stimulus Artifact (local potential)
When a stimulus is applied, there is a slight irregular deflection of baseline for a very short period. This is called stimulus artifact.
Latent Period
The stimulus artifact is followed by a short period without any change. This period is called latent period, which is about 0.5 to 1 millisecond.
Firing Level or Critical Depolarization Level
Depolarization starts after the latent period. Initially, it is very slow. After the initial slow depolarization up to -15 mV, the rate of depolarization increases suddenly. The point at which, the rate of depolarization increases is called firing level.
Fig. 3. Action potential in a skeletal muscle.
Overshoot
From firing level, the curve reaches the isoelectric potential (zero potential) rapidly and then overshoots the zero line up to +55 mV.
After Depolarization or Negative after Potential
The rapid fall in spike potential is followed by a slow repolarization process. This is called after-depolarization, trace depolarisation or negative after potential. The duration of this is 2 to 4 milliseconds.
After Hyperpolarization or Positive after Potential
After reaching the resting level (-90 mV) it becomes little more negative than resting level. This is called after hyperpolarization, trace hyperpolarization or positive after potential. This lasts for more than 50 milliseconds. After this, the normal resting membrane potential is restored.
IONIC BASIS OF ELECTRICAL EVENTS
Resting Membrane Potential
The development and maintenance of resting membrane potential in a muscle fiber or a neuron are carried out by some mechanisms, which produce ionic imbalance across the cell membrane. This results in the development of more positivity outside and more negativity inside the cell. The ionic imbalance is produced mainly by two transport mechanisms in the cell membrane.
1.Sodium-potassium pump and
2.Selective permeability of cell membrane
Sodium-potassium Pump
Sodium and potassium ions are actively transported in opposite directions across the cell membrane by means of an electrogenic pump called sodium-potassium pump This moves three sodium ions out of the cell and two potassium ions inside the cell by using energy from ATP. Since more positive ions are pumped outside than inside, a net deficit of positive ions occurs inside the cell. This leads to negativity inside and positivity outside the cell.
Selective Permeability of Cell Membrane
The permeability of cell membrane depends largely on the transport channels. The transport channels are selective for the movement of some specific ions. Their permeability to these ions also varies. Most of the channels are gated channels and the specific ions can move across the membrane only when these gated channels are opened.
Channels for major anions like proteins: However, the channels for some of the negatively charged large substances such as proteins and negatively charged organic phosphate compounds and sulfate compounds are absent or closed. Such substances remain inside the cell and play a major role in the development of resting membrane potential.
Leak channels: In addition, the channels for three important ions — sodium, chloride and potassium — also play an important role in maintaining the resting membrane potential.
Since, the Cl- channels are mostly closed in resting conditions, these ions are retained outside the cell. Thus, only the positive ions, Na+ and K+ can move across the cell membrane. The Na ions are actively transported (against the concentration gradient) out of the cell and K+ is actively transported (against the concentration gradient) inside the cell. However, because of concentration gradient Na+ diffuses back into the cell through Na+ leak channels. And, K+ diffuses out of the cell through K+ leak channels.
In resting conditions, almost all the, K+ leak channels are opened but most of the Na+ leak channels are closed. Because of this, K+ ions transported actively into the cell and can diffuse back out of the cell in an attempt to maintain the concentration equilibrium. But only very little amount of Na+ ions transported actively out of the cell can diffuse back into the cell. That means in resting conditions, the passive K+ efflux is much greater than the passive Na+ influx. This results in resting membrane potential with negativity inside compared to outside.
After establishment of the resting membrane potential (i.e. inside negativity and outside positivity), the efflux of K+ ions stops in spite of concentration gradient. This is because of two reasons.
1. The positivity outside the cell repels the positive K+ ions and prevents the further efflux of these ions.
2. The negativity inside the cell attracts the positive K+ ions and prevents further leakage of these ions outside.