Question

The ion flows across neuronal membranes at rest and during an action potential do not significantly change bulk ion concentrations, except for that of \(\mathrm{Ca}^{2+}\) ions. Resting \(\mathrm{Ca}^{2+}\) ion concentrations in cells are usually about \(10^{-7} \mathrm{M}\), and \(\mathrm{Ca}^{2+}\) ions exert physiological effects at concentrations of perhaps \(10^{-5} \mathrm{M}\). Explain why relative changes of intracellular \(\left[\mathrm{Ca}^{2+}\right]\) are much greater than for, say \(\left[\mathrm{Na}^{+}\right](12-50 \mathrm{mM})\).

Short answer

Relative changes in the concentration of \(\mathrm{Ca}^{2+}\) ions in cells are much greater than those of \(\mathrm{Na}^{+}\) ions due to the much lower initial concentration of \(\mathrm{Ca}^{2+}\) and its significant physiological effects at slightly increased levels. Because \(\mathrm{Na}^{+}\) ions are more abundant, large quantitative changes have less relative impact on their overall concentration.

Step-by-step solution

Understand the Concentration Levels

Consider the provided ion concentrations. Note that the resting concentration of \(\mathrm{Ca}^{2+}\) ions is about \(10^{-7} \mathrm{M}\). These ions exert physiological effects at concentrations of around \(10^{-5} \mathrm{M}\). The concentration of \(\mathrm{Na}^{+}\) ions, on the other hand, ranges between 12 and 50 \(\mathrm{mM}\), or \(0.012 - 0.05 \mathrm{M}\). This means there are many more \(\mathrm{Na}^{+}\) ions than \(\mathrm{Ca}^{2+}\) ions in the cell to start with.

Consider the Ion's Functions

Next, consider the different functions of the two ions in nerve cells. \(\mathrm{Na}^{+}\) ions are involved in generating and carrying electrical signals along the neuron, while \(\mathrm{Ca}^{2+}\) ions are important for neurotransmitter release at synaptic terminals. The roles these ions play in neuronal activity require different concentration changes.

Relating Ion Concentration to their Physiological Effects

Since the number of \(\mathrm{Ca}^{2+}\) ions needed to cause physiological effects is fairly little in absolute terms, small changes in this ion’s concentration can cause significant functional changes. In contrast, \(\mathrm{Na}^{+}\), being more abundant, would require much larger absolute changes in concentration to exert equivalent functional effects.

Conclusion

In conclusion, the reason that relative changes of intracellular \(\mathrm{Ca}^{2+}\) are much greater than for \(\mathrm{Na}^{+}\) has to do with the initial concentration of these ions and their specific roles in cell physiology. A small increase in the resting \(\mathrm{Ca}^{2+}\) concentration can substantially alter the physiological function of the ion due to its low quantities to begin with and its role in important cell functions.

Key concepts

Neuronal Activity

Neuronal activity refers to the complex processes that occur within a neuron that allow it to communicate with other neurons and tissues. This involves both electrical and chemical changes. Neurons use electrical signals, such as action potentials, to quickly transmit information across their lengths.
During an action potential, ions like sodium (\(\mathrm{Na}^{+}\)) and calcium (\(\mathrm{Ca}^{2+}\)) move across the neuron's membrane. These ion flows change the membrane potential, allowing the neuron to carry electrical signals.
When a neuron receives a stimulus, the membrane potential changes as \(\mathrm{Na}^{+}\) channels open, allowing positive ions to flood into the cell, which depolarizes it. Shortly after, \(\mathrm{Ca}^{2+}\) channels open, and the influx of these ions can trigger neurotransmitter release, communicating the signal to the next neuron.
Neurons have specialized structures, such as the cell body, axons, and synaptic terminals, to facilitate this communication. Each part ensures effective transmission of electrical impulses and chemical signals.

Ion Concentration Changes

Changes in ion concentrations across neuronal membranes are vital for neuronal function. Resting ion concentrations establish a baseline from which dramatic changes can occur during neuronal activity. For example, the typical intracellular concentration of \(\mathrm{Na}^{+}\) is quite high, ranging from 12 to 50 mM, while \(\mathrm{Ca}^{2+}\) is much lower, around \(10^{-7}\) M when a neuron is at rest.
The dramatic difference in initial concentrations of these ions means that any movement of \(\mathrm{Ca}^{2+}\) results in a larger percentage change compared to \(\mathrm{Na}^{+}\). Even a slight increase of \(\mathrm{Ca}^{2+}\), from \(10^{-7}\) M to \(10^{-5}\) M, represents a significant alteration in concentration, impacting physiological functions.
In contrast, the larger pool of \(\mathrm{Na}^{+}\) requires a substantial influx to cause a similar percentage change. This demonstrates why small changes in \(\mathrm{Ca}^{2+}\) concentration can have profound effects on neuronal activity.

Physiological Effects

Physiological effects of ion concentration changes in neurons are vital for functions such as communication between neurons and muscular contractions. \(\mathrm{Ca}^{2+}\) is particularly important because of its role in causing fast and precise physiological changes. When \(\mathrm{Ca}^{2+}\) concentrations rise from baseline levels, it often triggers the release of neurotransmitters at synaptic terminals.
Neurotransmitter release is crucial for neuron-to-neuron communication. This release translates electrical signals into chemical messages, which then propagate across synapses to the next neuron. \(\mathrm{Ca}^{2+}\)'s involvement in this process underlines its vital role in physiological processes.
Unlike \(\mathrm{Na}^{+}\), whose role is more related to generating and propagating action potentials, \(\mathrm{Ca}^{2+}\) serves as a key signaling molecule due to its ability to activate various cellular processes even with minor changes in its concentration. This highlights why relative changes in intracellular \(\mathrm{Ca}^{2+}\) can lead to significant physiological outcomes.