Action Potentials (AP) in Cardiac Muscle- What made cardiac AP longer

Action Potentials (AP) in Cardiac Muscle

The Cardiac Muscle action potential averages about 105 mv.

It rises from about -85 mv (between beats) to about +20 mv (during each beat).

After the initial spike, the membrane remains depolarized for about 0.2 second (plateau phase).

Plateau is followed by abrupt repolarization The presence of plateau in AP causes 15 times longer  ventricular contraction than in skeletal muscle
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What made cardiac AP longer

Skeletal muscle AP is caused by sudden opening of fast Na channels (open for a few thousandths of a second).

In cardiac muscle AP is caused by opening of two types of channels:

(1) fast Na channels (same as those in skeletal muscle)

(2) slow Ca channels (also called Ca-Na channels).

Ca channels are slower to open, remain open for tenths of a second, Large numbers of Ca/Na flow inside causing plateau.

Ca entering during plateau activates contraction (in skeletal muscle activated by Ca from sarcoplasmic reticulum (SR)).

The second major difference between cardiac & skeletal muscle is the immediate decrease in “K” ions permeability after onset of AP.

“K” permeability decreases about fivefold (does not occur in skeletal muscle), preventing early return of the AP voltage.

When slow Ca-Na channels close (0.2 to 0.3 second) & Ca – Na ions influx ceases, membrane permeability of K ions increases rapidly.

This rapid loss of K immediately returns the membrane potential to its resting level (repolarization) & ending the AP.

The presence of plateau in AP causes 15 times longer  ventricular contraction than in skeletal muscle
 

Muscle Tone-Muscle Attachments-Muscle Shape and Form

Muscle Tone

If a muscle is flaccid, then either the afferent, the efferent, or both neurons involved in the reflex arc necessary for the production of muscle tone have been interrupted. For example, if the nerve trunk to a muscle is severed, both neurons will have been interrupted.  SO Determination of the tone of a muscle is an important clinical examination. If poliomyelitis has involved the motor anterior horn cells at a level in the spinal cord that innervates the muscle, the efferent motor neurons will not function. If, conversely, the muscle is found to be hypertonic, the possibility exists of a lesion involving higher motor neurons in the spinal cord or brain.

Muscle Attachments

The importance of knowing the main attachments of all the major muscles of the body need not be emphasized. Only with such knowledge is it possible to understand the normal and abnormal actions of individual muscles or muscle groups. How can one even attempt to analyze, for example, the abnormal gait of a patient without this information

Muscle Shape and Form
The general shape and form of muscles should also be noted, since a paralyzed muscle or one that is not used (such as occurs when a limb is immobilized in a cast) quickly atrophies and changes shape. In the case of the limbs, it is always worth remembering that a muscle on the opposite side of the body can be used for comparison


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