INTRODUCTION TO NEUROSCIENCE

ION MOVEMENT

The open conformation allows the flow of ions (red circles) while the closed conformation does not allow the movement of ions across the cell membrane.

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• Glossary Terms
• Key Takeaways
• Test Yourself

Ion flow into and out of the neuron is a critical component of neuron function. Ions move in predictable ways, and the control of ion movement affects the cell at rest and while sending and receiving information from other neurons.

Phospholipid Bilayer Prevents Ion Movement

The cell membrane that separates the inside of the cell from the outside is a very effective boundary. It is described as being selectively permeable, which means that some molecules are able to travel across the membrane easily, other molecules have an intermediate ability to cross, and other molecules are completely incapable of passing. Generally, gases and molecules of water are able to pass through the cell membrane easily. Large molecules like glucose, and charged molecules like ions or amino acids, are unable to pass across the membrane.

The neuronal membrane is composed of lipid molecules that form two layers. The hydrophilic heads of the molecules align on the outside of the membrane, interacting with the intra- and extracellular solution of the cell, whereas the hydrophobic tails are arranged in the middle, forming a barrier to water and water-soluble molecules like ions. This barrier is critical to neuron function.

Figure 5.1. The neuronal membrane is composed of two layers of phospholipid molecules that form a barrier to water and water-soluble molecules due to the organization of the hydrophilic heads and hydrophobic ends of the molecules. ‘Phospholipid Bilayer’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial (CC-BY-NC) 4.0 International License.

Ion Channels Allow Ion Movement

Most cells of the body, including neurons, have specialized transmembrane proteins embedded in the cell membrane. These transmembrane proteins are huge protein complexes that span the entirety of the membrane, with an outer side and an inner side. In the middle of the protein is a pore, which is essentially a “tunnel” that allows molecules and ions to pass across the cell membrane. These proteins are called ion channels. These channels are passive since they do not use any cellular energy to move ions. Rather, they simply provide easy passage for ions. It may be useful to think of an ion channel as a “cellular door”. Ion channels are embedded throughout the neuronal membrane.

Channels can be opened and closed in a number of different ways. We can categorize ion channels into four major classes based on their opening and closing conditions.

1. Leak channels are persistently open. You can think of these leak channels as revolving doors that are never locked. Neurons usually have several leak channels.
2. Voltage-gated ion channels open in response to a change in membrane potential.
3. Ligand-gated ion channels open in response to chemical (ligand) binding, such as neurotransmitters.
4. The fourth class of ion channels is a catch-all category that includes a wide variety of channels that are used by the sensory systems. They open and close in response to unique stimuli depending on what they are able to sense. For example, some open and close depending when they are moved physically, such as a distortion or stretch (mechanoreceptors). We have these in the hair cells of our ears, in our skin, and in our muscles. Photoreceptors in our eyes have ion channels that close in response to being hit by photons of light, and this activity is necessary for us to be able to see in both brightly and dimly lit environments.

One important feature of ion channels is their ability to distinguish ions based on their chemical properties. For example, some channels are selective for Na+, while preventing the passage of all other ions. Each ion channel has special molecular characteristics that allow certain types of ions to pass through the pore while excluding other ions. Channels can be specific to one ion or allow the flow of multiple ions.

Figure 5.2. The phospholipid bilayer with embedded ion channels. The dotted, blue channels represent sodium channels; the striped, green channels represent potassium channels; the solid yellow channels represent chloride channels. ‘Membrane with Channels’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial (CC-BY-NC) 4.0 International License.

Ion channels control ion movement across the cell membrane because the phospholipid bilayer is impermeable to the charged atoms. When the channels are closed, no ions can move into or out of the cell. When ion channels open, however, then ions can move across the cell membrane.

Animation 5.1. When ion channels in the membrane are closed, ions cannot move into or out of the neuron. Ions can only cross the cell membrane when the appropriate channel is open. For example, only sodium can pass through open sodium channels. The dotted, blue channels represent sodium channels; the striped, green channels represent potassium channels; the solid yellow channels represent chloride channels. ‘Ion Movement’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial (CC-BY-NC) 4.0 International License. View static image of animation.

Gradients Drive Ion Movement

Ions move in predictable ways. Concentration (chemical) and electrical gradients drive ion movement. The chemical gradient refers to the natural process by which a high concentration of a substance, given enough time, will eventually diffuse to a lower concentration and settle evenly over the space. Ions will diffuse from regions of high concentration to regions of low concentration. Diffusion is a passive process, meaning it does not require energy. As long as a pathway exists (like through open ion channels), the ions will move down the concentration gradient.

In addition to concentration gradients, electrical gradients can also drive ion movement. The electrical gradient refers to the electrical forces acting on charged molecules, “pulling” opposite charges together while also “pushing” like charges away from one another—just like the polarity of magnets. Ions are attracted to, and will move toward, regions of opposite charge. Positive ions will move toward regions of negative charge, and vice versa.

For discussion of ion movement in this text, the combination of these two gradients will be referred to as the electrochemical gradient. Sometimes the concentration and electrical gradients driving ion movement can be in the same direction; sometimes the direction is opposite. The electrochemical gradient is the summation of the two individual gradients and provides a single direction for ion movement.

Animation 5.2. Concentration and electrical gradients drive ion movement. Ions diffuse down concentration gradients from regions of high concentration to regions of low concentration. Ions also move toward regions of opposite electrical charge. ‘Gradients’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial (CC-BY-NC) 4.0 International License. View static image of animation.

When Gradients Balance, Equilibrium Occurs

When the concentration and electrical gradients for a given ion balance—meaning they are equal in strength, but in different directions—that ion will be at equilibrium. Ions still move across the membrane through open channels when at equilibrium, but there is no net movement in either direction, meaning there is an equal number of ions moving into the cell as there are moving out of the cell.

Animation 5.3. When an ion is at equilibrium, which occurs when the concentration and electrical gradients acting on the ion balance, there is no net movement of the ion. The ions continue to move across the membrane through open channels, but the ion flow into and out of the cell is equal. In this animation, the membrane starts and ends with seven positive ions on each side even though the ions move through the open channels. ‘Ion Equilibrium’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial (CC-BY-NC) 4.0 International License. View static image of animation.

Important Ions for Neurons

Sodium, potassium, and chloride ions are found in different concentrations across the neuron cell membrane. The location of these ions across the cell membrane and their concentration gradients are important for the function of the neuron. Sodium (Na+) ions are more concentrated in the extracellular fluid and less concentrated within the intracellular fluid. Whereas potassium (K+) ions are more concentrated within the intracellular fluid and less concentrated in the extracellular fluid.

Table 5.1. Concentrations of sodium, potassium, and chloride inside and outside of the cell. 

Key Takeaways

• The phospholipid bilayer prevents ion movement into or out of the cell
• Ion channels allow ion movement across the membrane
• Electrochemical gradients drive the direction of ion flow
• At equilibrium, there is no net ion movement (but ions are still moving) 

Glossary

• ion channels- Transmembrane proteins that have a pore that allow for the passage of ions between the inside and outside of the cell
• Leak channels - Ion channels that are always open
• Voltage-gated ion channels- Ion channels that open/close in response to changes in voltage (membrane charge)
• Ligand-gated ion channels- Ion channels that open in response to the binding of a chemical ligand (such as a neurotransmitter)
• mechanoreceptors- Ion channels that open in response to physical distortion
• Photoreceptors- Channels that open in response to light
• impermeable- Does not allow passage
• chemical gradient - Process by which a high concentration of a substance, given enough time, will eventually diffuse to a lower concentration and settle evenly over the space
• Diffusion- Passive process when substances move from a higher concentration to a lower concentration
• electrical gradient - electrical forces acting on charged molecules, “pulling” opposite charges together while also “pushing” like charges away from one another
• electrochemical gradient- The combination of chemical and electrical gradients

Test Yourself! 

1. In Condition B, two chambers are separated by an impermeable membrane with an ion channel permeable only to X-, an anion. The left side of the chamber has four double-charged cations, C++, and four single-charged anions, X-. The right side has four uncharged ions, Z0, and four single-charged anions, X-.

Which way will the X- ions move once the channel opens?

What gradient is driving the force behind ion movement?

1. To the left; concentration gradient
2. To the right; electrical gradient
3. To the left; electrical gradient
4. To the right; concentration gradient

2. Which items are structural components of the cell that affect ion movement across the membrane? (Select all that apply)

1. Concentration gradient
2. Phospholipid bilayer
3. Electrical gradient
4. Ion channels

3. Which ion is more concentrated outside of the cell?

1. Sodium (Na+)
2. Potassium (K+)
3. Chloride (Cl-)

4. 

In Condition A, two chambers are separated by an impermeable membrane with an ion channel permeable only to Y0, an uncharged ion. The left side of the chamber has six Y0 ions with no charge. The right side has three Y0 ions with no charge.

Which way will Yo move once the channel opens?

What gradient is driving the force behind ion movement?

1. To the right; concentration gradient
2. To the right; electrical gradient
3. To the left; concentration gradient
4. To the left; electrical gradient

Answers 

1. To the left; electrical gradient -The negatively charged anions will move toward the region of positive charge. In this example, the left side of the chamber has an overall charge of +4, whereas the right side has an overall charge of -4. 
2. Phospholipid bilaye- The phosopholipid bilayer is a structural element that prevents the flow of ions into and out of the cell. 
3. Sodium (Na+)
4. To the right; concentration gradient- Ions move from regions of high concentration to regions of low concentration. In this example, Yo is more concentrated on the left side of the chamber. 

Attributions

Portions of this chapter were remixed and revised from the following sources:

• Foundations of Neuroscience by Casey Henley. The original work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
• Open Neuroscience Initiative by Austin Lim. The original work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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Next: Membrane Potential 

Adapted by Kateule Sydney based on Introduction to Neuroscience by Valerie Hedges is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.