Cells hate calcium!

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I always tell my students to remember these three things:

Cells hate sodium ions (Na⁺).
Cells hate calcium ions (Ca⁺⁺).
Cells love potassium ions (K⁺).

OK, that's not literally true.  As far as we know, cells are not conscious and therefore do not love or hate anything.  But they sure act like they do!

Think about it.  All living cells have Na-K pumps that pump Na⁺ out while at the same time pump K⁺ in. When Na⁺ leaks into the cell, out it goes.  Likewise, when K⁺ leaks out of a cell, it's pumped back in.

As far as cells are concerned, Na⁺ is like a rattlesnake and thus is repulsive and must be gotten rid of when it sneaks in.  And K⁺ is like a puppy that the must be brought in and cuddled.  Should K⁺ escape to the cold, cruel world outside a cell, it should be brought back inside and cuddled.

Knowing these facts about sodium and potassium ions is useful to A&P students.  Why?  Because it helps explain where these ions are likely to be found in the human body:

• If you're looking for Na⁺, look in the solution outside the cell (extracellular fluid). You won't find much Na⁺ inside the cell, because it is continually pumped out of the cell.
  
  
• If you are looking for K⁺, don't look in the extracellular fluid. You'll find very little K⁺ there. Most of the K⁺ will be inside the cell (intracellular fluid).
  
  

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The fact that there are these sodium and potassium ion concentration gradients help explain the concept of membrane voltage (membrane potential). This idea, then, is the foundation of understanding nerve impulses and muscle stimulation.

During a nerve impulse, Na⁺ rushes into the nerve cell because of the concentration gradient described above (most of the sodium is outside the cell). This gives the membrane a temporary inside-positive charge… and that's what a nerve impulse is. The normal membrane voltage is restored quickly when K⁺ is allowed to rush out of the nerve cell, thus moving the net positive charge to the outside of the cell membrane.

All living cells have calcium pumps that pump calcium out of the cell.  Some calcium pumps also pump calcium into sacks (the smooth ER).  To a cell, Ca⁺⁺ is like a cobra. When it leaks into a cell, and it will, it is pumped out quickly or pushed into a sack.

Knowing this fact about calcium ions is useful for understanding many different concepts in A&P.

For example, muscle fibers pump calcium ions out of the plasma membrane (sarcolemma) and into the sarcoplasmic reticulum (SR, a form of smooth ER). When the muscle membrane is stimulated (see the paragraphs above), the Ca⁺⁺ comes rushing into the intracellular fluid from the SR and/or from the extracellular fluid. Ca⁺⁺ immediately binds to the cytoskeleton, which then produces muscle contraction.

A similar thing happens at the end of a neuron when a nerve impulse (see the paragraphs above) gets to its farthest distance and permits Ca⁺⁺ to flow into the cell. The Ca⁺⁺ binds to the cytoskeleton and thereby triggers the movement of vesicles filled with neurotransmitter. These vesicles crash into the plasma membrane and release neurotransmitters by exocytosis, thus allowing them to signal another cell.

Ca⁺⁺ gradients are also key to understanding how many hormones trigger their target cells. It even helps explain some of the functions of sperm cells and egg cells during human reproduction.

So you can see that this idea of cells hating sodium and calcium ions and loving potassium ions comes in pretty handy when trying to understand many of the concepts of human physiology.