Ion Channels
A brief description of ion channels is presented in this section.Ion channels are made up of proteins that are embedded in the lipid bilayer of the neuronal membrane across which they span (Fig. 6-1). They are characterized by the following general properties.
1. The flow of ions through the channels does not require metabolic energy; the flow is passive.
2. The electrochemical driving force across the membrane, but not the channel itself, determines the direction and eventual equilibrium of this flow.
3. The ionic charge determines whether a channel allows an ion to flow through; some channels allow cations, whereas others allow anions to flow through them.
4. Most cation-selective channels allow only one ion species (e.g., Na+ or K+ or Ca2+) to flow through them. However, some channels allow more than one ion species to flow through them. For example, when L-glutamate (an excitatory amino acid neurotransmitter) activates an N-methyl-D-aspartic acid (NMDA) receptor, both Na+ and Ca2+ ions flow through the NMDA receptor channel into the neuron.
5. Most anion-selective channels allow only Cl- to flow through them.
6. Some blockers can prevent the flow of ions through the ion channels. For example, phencyclidine (PCP, or Angel Dust) blocks the NMDA receptor channel.
Classification of Ion Channels
The ion channels have been divided into the following two major classes: nongated and gated channels. Nongated Channels. Although nongated channels are capable of opening as well as closing, most of the time they are open. They control the flow of ions during the resting membrane potential. They are also known as leak channels. Examples include nongated Na+ and K+ channels that contribute to the resting membrane potential. Gated Channels. These channels are also capable of opening as well as closing. All gated channels are allosteric proteins (i.e., they exist in more than one conformation, and their function is altered when they shift from one conformation to another). Each allosteric channel exists in at least one open and one closed state. The transition of a channel between the open and closed states is called gating. At rest, these channels are mostly closed, and they open in response to different stimuli (e.g., change in membrane potential, ligand-binding, or mechanical forces). Individual channels are usually most sensitive to only one of these stimuli. Subtypes of the gated ion channels are described in the following paragraphs.
The channels that are opened or closed by a change in the membrane potential are called voltage-gated channels. The opening and closing of the channel is believed to be due to the movement of the charged region of the channel back and forth through the electrical field of the membrane. Voltage-gated channels exist in three states: (1) resting state, in which the channel is closed but can be activated; (2) active state, in which the channel is open; and (3) refractory state, in which the channel is inactivated (See also the section, "Ionic Basis of the Action Potential"). Changes in the electrical potential difference across the membrane provide the energy for gating in these channels. Genes encoding for voltage-gated Na+, K+, and Ca2+ channels belong to one family. These channels are described as follows.
The voltage-gated Na+ channel is formed by a single long polypeptide (a string of amino acids containing peptide bonds) that has four domains (I-IV [Fig. 6-3A]). Each domain has six hydrophobic alpha helices (S1-S6) that span back and forth within the cell membrane. The four domains join together and form an aqueous pore of the channel (Fig. 6-3B). An additional hydrophobic region connects the S5 and S6 alpha helical segments, forming a pore loop (Fig. 6-3B). The presence of this pore loop makes the channel more permeable to Na+ than to K+. The membrane-spanning S4 alpha helical segment is believed to be voltage sensitive. At the resting membrane potential, the channel pore is closed. The S4 segment undergoes a conformational change when the membrane potential changes (e.g., when the neuron is depolarized), the S4 segment is pushed away from the inner side of the membrane, and the channel gate opens, allowing an influx of Na+ ions.
There are some cases in which Na+ permeability is blocked. Tetrodotoxin (TTX), a toxin isolated from the ovaries of Japanese puffer fish, binds to the sodium channel on the outside and blocks the sodium permeability pore. Consequently, neurons are not able to generate action potentials after the application of TTX. These channels are also blocked by local anesthetic drugs (e.g., lidocaine).
The basic structure of the voltage-gated Ca2+ channel is similar to that of the voltage-gated Na+ channel. Ca2+ ions enter the postsynaptic neurons through these channels and activate enzymes. Depolarization of presynaptic nerve terminals results in entry of Ca2+ ions into the terminal via these channels. An increase in the levels of intracellular Ca2+ results in the release of transmitters from presynaptic nerve terminals.
Different varieties of voltage-gated K+ channels have been identified, and they serve different functions. The general scheme describing the components of this channel is similar to that of the voltage-gated Na+ channel, except that the voltage-gated K+ channel consists of four polypep-tides. It should be recalled that each polypeptide contributing to the formation of a large protein molecule is called a subunit. Each subunit of a voltage-gated K+ channel consists of six alpha-helical membrane-spanning segments (S1-S6). A pore loop makes the channel more permeable to K+ than to Na+. The S4 segment acts as an activation gate. The K+ channels are generally blocked by chemicals, such as tetraethylammonium or 4-aminopyridine.
The ligand-gated channels are opened by noncovalent binding of chemical substances with their receptors on the neuronal membrane. These chemical substances include: (1) transmitters or hormones present in the extracellular fluid that bind to their receptors on the extracellular side of the channel and bring about a conformational change to open the channel (e.g., acetylcholine, y-aminobutyric acid [GABA], or glycine), and (2) an intracellular second messenger (e.g., cyclic adenosine monophosphate, which is activated by a transmitter such as norepinephrine). Genes encoding for transmitter-gated channels (e.g., channels activated by acetylcholine, GABA, or glycine) and genes encoding for voltage-gated channels belong to different families. Ligand-gated channels are either directly gated or indirectly gated.
In a directly gated ligand channel, five protein subunits are typically arranged in such a way that the recognition site for the chemical substance is part of the ion channel. Each subunit contains four membrane-spanning alpha helices. This type of receptor is called an ionotropic receptor.A neurotransmitter binds to an ionotropic receptor and brings about a conformational change that results in the opening of the ion channel. Receptors of this type usually bring about fast synaptic responses that last for only a few milliseconds.
FIGURE 6-3 Voltage-gated Na+ (sodium) channel. (A) The channel is formed by a single long polypeptide that has four domains (I—IV). S1-S5 are hydrophobic alpha helices that span across the membrane. Note also the hydrophobic pore loop. The NH3+ (hydrogen carbonate) and COO- (carboxyl group) terminals are exposed on the cytoplasmic side of the membrane. (B) The four domains clump together to form a channel with a pore. The wall of the channel pore is formed by the pore loops. The domains are shown in clockwise fashion in A and B to facilitate orientation.
In an indirectly gated ligand channel, the ion channel and the recognition site for the transmitter (receptor) are separate. These receptors are called metabotropic receptors.Typically a metabotropic receptor consists of a single protein subunit with seven membrane-spanning alpha helices. When a transmitter binds to the metabotropic receptor, a guanosine-5′-triphosphate- binding protein (G-protein) is activated, which, in turn, activates a second-messenger system in the neuron. The second messenger can either act directly on the ion channel to open it or it can activate an enzyme that, in turn, opens the channel by phosphorylating the channel protein in the presence of a protein kinase. Dephosphor-ylation of the channel in the presence of a protein phos-phatase results in the closure of the channel. Activation of this type of channel elicits slow, long-lasting synaptic actions.
Mechanically gated channels open by a mechanical stimulus and include the channels involved in producing generator potentials of stretch and touch receptors.
Equilibrium Potentials
The equilibrium potential of an ion is the electrical potential difference at which the diffusional forces and electrical forces exerted on the ion are equal and opposite, and the net movement of the ion across the cell membrane ceases. The equilibrium potential of any ion that is present on the sides of the cell membrane, which is permeable to that ion, is calculated by the Nernst equation (Table 6-2). Using this equation, the value for the equilibrium potential of K+ is -80 mV. If the membrane were permeable only to K+, the resting membrane potential of a neuron would be equal to the equilibrium potential of K+ (-80 mV). However, actual recordings show that the resting membrane potential of a neuron is usually -65 mV. The reason for this discrepancy is that the neuronal membrane is permeable to more than one ion species. The resting membrane potential of the neuron under these conditions is calculated by the Goldman equation, which takes into consideration the contribution of the permeability of each ion and its extracellular and intracel-lular concentrations (Table 6-2).
Ionic Basis of the Resting Membrane Potential
When the neuron is at rest (i.e., when it is not generating action potentials), the cytosol along the inner surface of the cell membrane has a negative electrical charge compared with the outer surface of the cell membrane.
Many negatively charged fixed ions (associated with proteins and amino acids) make the interior of the cell more negative relative to outside. The potential difference across the cell membrane during resting state is called the resting membrane potential. The lipid bilayer of the neu-ronal membrane maintains this separation of charges by acting as a barrier to the diffusion of ions across the membrane. The ion concentration gradients across the neuro-nal membrane are established by ion pumps that actively move ions into or out of neurons against their concentration gradients. The selective permeability of membranes is due to the presence of ion channels that allow some ions to cross the membrane in the direction of their concentration gradients.
The ion pumps and ion channels work against each other in this manner. If the neuronal membrane is selectively permeable to only a K+ ion, this ion will move out of the neuron down its concentration gradient. Therefore, more positive charges accumulate outside the neuron. The fixed negative charges inside the neuron impede the efflux of positively charged K+ ions, and excess positive charges outside the neuron tend to promote influx of the K+ ions into the neuron due to the electrostatic forces. It should be recalled that opposite charges attract, while similar charges repel each other. Thus, two forces are acting on the flow of K+ ions out of the neuron; a higher concentration inside the neuron (concentration gradient) tends to expel them out of the neuron, whereas the electrostatic forces tend to prevent their flow out of the neuron.
TABLE 6-2 Nernst and Goldman Equations to Determine Equilibrium Potential of Ions and Membrane Potentials
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Description |
Components of the Equation |
Ionic Equilibrium or Membrane Potentials |
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Nernst equation |
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This equation is used to calculate equilibrium potential of an ion that is present on both sides of the cell membrane. |
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Where: |
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Substitution of the values for R, T, F, and extracellular and intracellular concentrations of the ion (see Table 6-1) in the equation yields the value of equilibrium potential of that ion. |
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T = temperature in Kelvin scale (273.15 + temperature in centigrade degrees) |
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Z = valence of the ion |
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If the membrane was permeable to only K+, the neuronal membrane potential would be equal to Ek (-80 mV). Actual membrane potential of a neuron is usually -65 mV (see below). |
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Goldman equation |
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Because the neuronal membrane is permeable to more than one ion, the Goldman equation is used to calculate membrane potential. |
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This equation takes into consideration the contribution of the permeability of each ion and its extracellular and intracellular concentrations. |
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Where: |
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P = permeability of the specific ion |
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Other abbreviations = same as in Nernst equation |
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Note that the valence factor (Z) present in the Nernst equation is absent in this equation. Therefore, the concentration of negatively charged ion (Cl-) has been inverted relative to the concentrations of positively charged ions; recall that -log(A/ B) = log (B/A). |
When the two opposing forces are equal, K+ concentrations inside and outside the neuron are in equilibrium. The value of the membrane potential at this time is the K+ equilibrium potential. Thus, if the neuronal membrane contained only K+ channels, the resting membrane potential would be determined by the K+ concentration gradient and would be equal to the equilibrium potential for K+ ions (approximately -80 mV). However, as stated earlier, the resting membrane potential of a neuron is usually -65 mV. This is because neurons at rest are permeable to the Na+ ion also. The Na+ ions tend to flow into the neuron due to two forces: (1) concentration gradient of Na+ ions (extracellular Na+ concentration is much higher than its intrac-ellular concentration) and (2) electrostatic forces (there is an excess of positive charges outside and an excess of negative charges inside the neuron). Due to the influx of Na+ ions, the resting membrane potential deviates from that of the K+ equilibrium potential (i.e., it becomes -65 mV instead of -80 mV).
FIGURE 6-4 Configuration of an action potential. (A) Phases of an action potential. (B) Inward and outward current flows due to the influx of Na+ (sodium) and efflux of K+ (potassium) during the rising and falling phases of the action potential, respectively.
However, the membrane potential does not reach the equilibrium potential for Na+. The reason for the neuron’s inability to attain a resting membrane potential closer to the Na+ equilibrium potential is that the number of open nongated Na+ channels is much smaller than the number of open nongated K+ channels in the resting state of a neuron. Therefore, the permeability of Na+ is small despite large electrostatic and concentration gradient forces tending to drive it into the neuron. To maintain a steady resting membrane potential, the separation of charges across the neuro-nal membrane must be maintained at a constant. This is accomplished by the Na+-K+ pump described earlier.
