Heart cells spontaneously contract in unison. They contract at all in response to what is called an action potential -- a positive feedback loop
across the cell membrane (that is, between the inside of the cell, and the outside of the cell). The cell membrane is a layer of phospholipids
(basically a double headed chain of fat), cholesterol, and embedded proteins. These proteins can open to allow ions to flow in and out of a cell.
These ions will want to flow because there is a charge difference: Na+, or sodium ion, will want to flow into a cell if it is more negative than its
surroundings. Conversely, Cl-, or chlorine ion, wants to move from a more positive space to a more negative space. Normally, a cell is kept at a
resting voltage of around -70mV. Simply, this means that the inside of a cell is more negative than the space outside of the cell -- positive ions
will want to enter the cell, and chlorine will want to exit the cell. To maintain this charge, proteins embedded in the cell membrane actively, and
using energy (which is why you die without food), pump positive ions out of the cell to maintain this gradient with its environment.
I just mentioned an ion pump with is active, but there are also ion channels which allow, without energy and down a gradient (relying on this charge
difference), ions to enter or leave the cell. This is often specific to certain ions -- one channel may let potassium flow, but not chlorine. These
will open in response to a variety of factors: chemicals, light (channelrhodopsin/Chr2 in some organisms but not humans), or, most importantly
cellular voltage (in excitable cells).
So if your cell is resting at -70mV, and you open a channel which allows positive ions to enter the cell, it will depolarize in a graded fashion.
That is, the cell voltage will rise (become LESS negative) in a way which is proportional to the amount of positive ions which enter the cell. This
may bring the cell to -60mV. After the channel is closed, the cell will move back to -70mV. But if you keep depolarizing the cell to its action
potential threshold in an excitable cell -- that is, a muscular or nervous cell, or something else capable of generating this action potential (your
skin for example will not do this) -- it will open ion channels which cause the cell to FURTHER depolarize. In nerve cells this is ~ -55mV. Upon
hitting -55mV, the proper ion channels will open and cause the cell voltage to very abruptly rise to +40mV to almost +100mV. The cell will then reset
(passing through a period of hyperpolarization, where it is more negative than -70mV, before retuning to 70mV).
This picture from Wikipedia helps:
Sorry for that long winded explanation but this is the basis of electrical communication. Chemical communication is arguably more common but I care
less about that because I'm not a biochemist.
Anyway, heart cells contract in response to this change in voltage. Read about actin and myosin for more details (wikipedia is nice). Essentially,
in a muscle cell, the massive rise in voltage due to the action potential either directly or indirectly releases calcium from a storage organelle
inside of the muscle cell called the sarcoplasmic recticulum. The calcium causes a protein called tropomyosin, which is blocking the long molecular
filament of actin from connecting other parallel filaments of myosin, to move out of the way so myosin can bind. Both actin and myosin are anchored
to parts of the muscle cell called Z-disks; a myosin filament is connected to two Z-disks, while an actin filiament connects to only one. Since I'm
bad at this, here's a picture, also from wikipedia:
Note that the M-line is right between both the Z-disks and actin filaments.
After they connect, the end of myosin connected to actin shifts, moving the actin. This is done such that the actin chains are pulled toward one
another as in the picture. This shortens the space between the Z-disks. As this is done throughout the muscle cell in the same direction, therefore
shortening, or contracting, the muscle.
Now on to what you were talking about, which is a little less exciting to explain since I already described what an action potential was. If you put
a bunch of baby heart cells (I see this done with cells from neonatal rat ventricles) next to one another in culture, they will connect to one another
through what is called a gap junction. A gap junction is just essentially an anchored pore connecting two adjacent cells. It's a hole in the cell
membrane that connects to the other cell. So, when you have an action potential in one cell, it spreads to the other via this gap junction.
Why do cells beat on their own? Because in the aftermath of returning to resting voltage, calcium, Ca2+ is allowed to enter the cell faster than
positive ions, particularly potassiun K+ can leave it, creating a net influx of positive ions. This reaches -55mV and stimulates another action
[edit on 27-8-2010 by Johnmike]
[edit on 27-8-2010 by Johnmike]