Transmission of information in the nervous system
(Heavily from Encyclopaedia Britannica)
In the nervous system of animals at all levels of the evolutionary scale, the signals containing information about a particular stimulus are electrical in nature. In the past the nerve fibre and its contents were compared to metal wire, while the membrane was compared to insulation around the wire. This comparison was erroneous for a number of reasons. First, the charge carriers in nerves are ions, not electrons, and the density of ions in the axon is much less than that of electrons in a metal wire. Second, the membrane of an axon is not a perfect insulator, so that the movement of current along the axon is not complete. Finally, nerve fibres are smaller than most wires, so that the currents they can carry are limited in amplitude.
The Ionic Basis of Electrical Signals in the Neuron
The electrical events that constitute signaling in the nervous system depend upon the distribution of ions on either side of the nerve membrane. Underlying these distributions and their change are crucial physical-chemical principles.
The plasma membrane of the neuron is semipermeable, being highly permeable to K+ and slightly permeable to Cl− and Na+. In the extracellular fluid, electroneutrality is preserved by a balance between a high concentration of Na+ on the one hand and a high concentration of Cl−, as well as small quantities of impermeant anions such as bicarbonate, phosphate, and sulfate, on the other. In the cytoplasm, where K+ concentration is high, the concentration of Cl− is much below that necessary to balance the sum of the positive charges. Electroneutrality is maintained there by negatively charged impermeant proteins and phosphates. Osmotic balance is maintained between the extracellular fluid and the cytoplasm by movement of water through the plasma membrane when the total concentration of particles on one side is not equal to that on the other.
These three characteristics of the neuron—semipermeability of the membrane, osmotic balance, and electroneutrality on each side—create an equilibrium electrical potential at which the inside of the membrane is more negative than the outside. In most neurons this potential, called the membrane potential, is between −60 and −75 millivolts (mV; or thousandths of a volt; the minus sign indicates that the inner surface is negative). When the inside of the plasma membrane has a negative charge compared to the outside, the neuron is said to be polarized. Any change in membrane potential tending to make the inside even more negative is called hyperpolarization, while any change tending to make it less negative is called depolarization.
The Nernst potential is the potential difference that exists across a membrane when a particular ion, having reached equilibrium between the tendency to diffuse down its concentration gradient and the tendency to be drawn back by other ions, is in a state of no net flux. The plasma membrane of the neuron is highly permeable to K+, and in fact the recorded membrane potential of most neurons (−60 to −75 mV) is close to that predicted by the Nernst equation for K+. However, it is not exactly the same, because K+ is not the only ion affecting the membrane potential. The membrane is also slightly permeable to Na+ and Cl−. The permeability to Na+ may be low, but the high concentration of this cation outside the cell and the slightly negative electric charge inside the cell tend to drive Na+ inward. This in turn causes the inside of the cell to depolarize, placing K+ out of equilibrium. As a consequence, K+ leaves the cell until an equilibrium state is reached in which the leak inward of Na+ is equaled by the leak outward of K+ and there is no net flux of ions. There is also a tendency for Cl− to permeate the membrane, since that ion is at higher concentration outside the neuron than inside. Therefore, for an equilibrium state to be produced, the sum of all three net currents must equal zero.
Given the concentrations of all three ions on each side of the membrane and the relative permeability of the membrane to each ion, researchers can calculate the combined effect of K+, Na+, and Cl− on the membrane potential by using the so-called constant-field equation. This equation, by including relative permeability as an important factor, takes into account the phenomenon that the more permeable a membrane is to a particular ion, the greater is the influence of that ion on the membrane potential. The permeance of Na+, for example, is only a fraction of that of K+, and the permeance of Cl− is lower yet; therefore, while the membrane potential is highly sensitive to changes in the concentration of K+, it is less affected by changes in Na+ and almost unaffected by changes in Cl−.
Transmission in the neuron
The discussion above demonstrates that the electrical potential existing in neurons is based on the distribution of ions across the plasma membrane and that this distribution comes about through permeation of the membrane. In fact, ions are almost always hydrated in the form of ion-water complexes, which have great difficulty in penetrating the hydrophobic lipid bilayer of the plasma membrane. Permeation actually occurs through protein structures embedded in the lipid bilayer and spanning the membrane from cytoplasm to extracellular fluid. These structures, sometimes pumping ions from one side to the other and sometimes merely providing channels through which diffusing ions can flow past the lipid molecules, maintain the ionic distribution that keeps the membrane polarized, and they also allow the abrupt changes in distribution that create nerve impulses. The protein structures are described in detail in the section Ion transport. Following is a discussion of the electrical events, created by movement of ions, that lead to nervous transmission in the neuron.
Resting potential
The electrical potential across the nerve membrane can be measured by placing one microelectrode within the neuron (usually in the soma) and a second microelectrode in the extracellular fluid. The microelectrode consists of a sharp-tipped glass capillary tube filled with conducting solution. Upon penetration of the neuron, the potential at the tip of the electrode becomes electrically negative in relation to the outside of the electrode. As described above and shown in the graph, the value of this negative charge is usually between −60 and −75 mV. This is the membrane potential of the neuron at rest (i.e., when it is not generating a nerve impulse), and for this reason it is called the resting potential.
The resting potential is maintained by the sodium-potassium pump, which steadily discharges more positive charge (i.e., Na+) from the cell than it allows in, and by the relatively high permeance of K+, which leaks out of the cell through its membrane channels faster than Na+ leaks in.
Localized potential
When a physical stimulus, such as touch, taste, or colour, acts on a sensory receptor cell specifically designed to respond to that stimulus, then the energy of the stimulus (e.g., mechanical, chemical, light) is transduced, or transformed, into an electrical response. This response is called the receptor potential, a type of local potential that, when it reaches high enough amplitude, generates the nerve impulse. (Another type of local potential is the postsynaptic potential, which originates in chemical receptors at the synaptic cleft. See the section Transmission at the synapse: Chemical transmission.)
Sensory receptors transduce stimuli into electrical responses by activating ion channels in their membranes. For example, in the stretch receptors of neurons attached to muscle cells, the stretching action of the muscle is thought to put a mechanical stress on protein filaments of the cytoskeleton, which in turn alter the shape of ion channels, inducing them to open and allowing cations to diffuse into the cell. Receptor cells sensitive to chemical and light energy, on the other hand, activate ion channels through the second-messenger system. In this system, stimulated receptor molecules on the surface of the cell membrane catalyze a series of enzymatic reactions within the cytoplasm; these reactions in turn release energy, which activates the ion channels.
By permitting a flux of Na+ into the cell, the opening of ion channels slightly depolarizes the membrane. The extent to which the membrane is depolarized depends upon the extent to which the sodium channels are activated, and this in turn depends upon the strength and duration of the original stimulus at the receptor. If depolarization reaches what is called the threshold potential, it triggers the nerve impulse, or action potential (see below). If it does not reach that amplitude, then the neuron remains at rest, and the local potential, through a process called passive spread, diffuses along the nerve fibre and back out through the membrane.
When it is of the postsynaptic type, the local potential usually begins in the dendrites and spreads toward the soma and axon. It is at the initial segment of the axon where, if the local potential is of threshold amplitude, the nerve impulse is generated.
Action Potential
Depolarization
Because it varies in amplitude, the local potential is said to be graded. The greater the influx of positive charge—and, consequently, depolarization of the membrane—the higher the grade. Beginning at the resting potential of a neuron (for instance, −75 mV), a local potential can be of any grade up to the threshold potential (for instance, −58 mV). At the threshold, voltage-dependent sodium channels become fully activated, and Na+ pours into the cell. Almost instantly the membrane actually reverses polarity, and the inside acquires a positive charge in relation to the outside. This reverse polarity constitutes the nerve impulse. It is called the action potential because the positive charge then flows through the cytoplasm, activating sodium channels along the entire length of the nerve fibre. This series of activations, by propagating the action potential along the fibre with virtually no reduction in amplitude, gives the nerve impulse its regenerative property.
Researchers call the nerve impulse an
all-or-nonereaction since there are no gradations between threshold potential and fully activated potential. The neuron is either at rest with a polarized membrane, or it is conducting a nerve impulse at reverse polarization. The reverse polarity of active neurons is measured at about +30 mV. This is close to the Nernst potential for Na+—that is, the membrane potential at which electrochemical equilibrium would be established if the membrane were completely permeable to Na+.Repolarization
As instantaneous as the opening of sodium channels at threshold potential is their closing at the peak of action potential. This is called sodium inactivation, and it is caused by gates within the channel that are sensitive to depolarization. Following sodium inactivation is the opening of potassium channels, which allows the diffusion of K+ out of the cell. The combined effect of sodium inactivation, which blocks the influx of cations, and potassium activation, which causes the efflux of other cations, is the immediate return of the cell membrane to a polarized state, with the inside negative in relation to the outside. After repolarization there is a period during which a second action potential cannot be initiated, no matter how large a stimulus current is applied to the neuron. This is called the absolute refractory period, and it is followed by a relative refractory period, during which another action potential can be generated, but only by a greater stimulus current than that originally needed. This period is followed by the return of the neuronal properties to the threshold levels originally required for the initiation of action potentials.
Conduction
The sequence of sodium activation–sodium inactivation–potassium activation creates a nerve impulse that is brief in duration, lasting only a few milliseconds, and that travels down the nerve fibre like a wave, the membrane depolarizing in front of the current and repolarizing behind. Because nerve impulses are not graded in amplitude, it is not the size of the action potential that is important in processing information within the nervous system; rather, it is the number and frequency with which the impulses are fired.
As stated above, the action potential is propagated along the axon without any decrease in amplitude with distance. However, the velocity of conduction along the nerve fibre is dependent upon several factors. The first factor is the outside diameter of the nerve fibre. The fastest conduction velocity occurs in the largest diameter nerve fibres. This phenomenon has formed the basis for classifying mammalian nerve fibres into groups in order of decreasing diameter and decreasing conduction velocity. Another factor is the temperature of the nerve fibre. Conduction velocity increases at high temperatures and decreases at low temperatures. Indeed, nerve conduction can be blocked by the local application of cold to a nerve fibre. Conduction velocity is also affected by myelination of the nerve fibre. Since ions cannot cross the lipid content of the myelin sheath, they spread passively down the nerve fibre until reaching the unmyelinated nodes of Ranvier. The nodes of Ranvier are packed with a high concentration of ion channels, which, upon stimulation, propagate the nerve impulse to the next node. In this manner the action potential jumps quickly from node to node along the fibre in a process called saltatory conduction (from Latin saltare, “to jump”).
Transmission at the synapse
Once an action potential has been generated at the axon hillock, it is conducted along the length of the axon until it reaches the terminals, the fingerlike extensions of the neuron that are next to other neurons and muscle cells. At this point there exist two methods for transmitting the action potential from one cell to the other. In electrical transmission, the ionic current flows directly through channels that couple the cells. In chemical transmission, a chemical substance called the neurotransmitter passes from one cell to the other, stimulating the second cell to generate its own action potential.
Electrical transmission
This method of transmitting nerve impulses, while far less common than chemical transmission, occurs in the nervous systems of invertebrates and lower vertebrates, as well as in the central nervous systems of some mammals. Transmission takes place through gap junctions, which are protein channels that link the cellular contents of adjacent neurons. Direct diffusion of ions through these junctions allows the action potential to be transmitted with little or no delay or distortion, in effect synchronizing the response of an entire group of neurons. The channels often allow ions to diffuse in both directions, but some gated channels restrict transmission to only one direction.
Chemical transmission
Neurotransmitter release
Two factors are essential for the release of the neurotransmitter from the presynaptic terminal: (1) depolarization of the terminal and (2) the presence of calcium ions (Ca2+) in the extracellular fluid. The membrane of the presynaptic terminal contains voltage-dependent calcium channels that open when the membrane is depolarized by a nerve impulse, allowing Ca2+ to diffuse into the terminal along its concentration gradient. Following the entrance of Ca2+ is the release of neurotransmitter.
It is uncertain what happens in the time between Ca2+ entry and transmitter release. Ca2+ is known to be sequestered by certain organelles within the terminal, including the endoplasmic reticulum. The ions may attach to the membranes of synaptic vesicles, in some way facilitating their fusion with the nerve terminal membrane. They may also be removed from the terminal by exchange with extracellular Na+—a mechanism known to occur at some neuronal membranes. What is certain is that when the concentration of Ca2+ is increased within the terminal, the probability of transmitter release is also increased.
Neurotransmitters are packed into small, membrane-bound synaptic vesicles. Each vesicle contains thousands of neurotransmitter molecules, and there are thousands of vesicles in each axon terminal. Once stimulated by Ca2+, the vesicles move through the cytoplasm and fuse their membranes with the plasma membrane of the terminal. The transmitter molecules are then expelled from the vesicles into the synaptic cleft. This expulsion process is called exocytosis. Vesicle membranes are then recovered from the plasma membrane through endocytosis. In this process the membranes are surrounded by a protein coat at the lateral margins of the synapse and are then transferred to cisternae, which form in the terminal during nerve stimulation. There the vesicles lose their coats, are probably refilled with neurotransmitter, and pinch off from the cisternae to become synaptic vesicles once more.
Because the neurotransmitter chemicals are packed into separate, almost identically sized vesicles, their release into the synaptic cleft is said to be quantal—that is, they are expelled in parcels, each vesicle adding its contents incrementally to the contents released from other parcels. This quantal release of neurotransmitter has a critical influence on the electrical potential created in the postsynaptic membrane.
Postsynaptic potential
After neurotransmitter is released from the presynaptic terminal, it diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane. Some receptors are ion channels that open or close when their molecular configuration is altered by the binding action of the neurotransmitter. Others are membrane proteins that, upon activation, catalyze second-messenger reactions within the postsynaptic cell; these reactions in turn open or close the ion channels. Whether acting upon ion channels directly or indirectly, the neurotransmitter molecules cause a sudden change in the permeability of the membrane to specific ions. Exactly which ions now permeate the membrane vary according to the neurotransmitters and their receptors, but the net result of a change in ion diffusion is a change in electrical potential across the membrane. This change is called the postsynaptic potential, or PSP. (In reference to the neuromuscular synapse, it is called the end-plate potential, or EPP.)
The most common potential change is depolarization, caused by a net influx of cations (usually Na+). Because this infusion of positive charge brings the membrane potential toward the threshold at which the nerve impulse is generated, it is called an excitatory postsynaptic potential (EPSP). Other neurotransmitters stimulate a net efflux of positive charge (usually in the form of K+ diffusing out of the cell), leaving the inside of the membrane more negative. Because this hyperpolarization draws the membrane potential farther from the threshold, making it more difficult to generate a nerve impulse, it is called an inhibitory postsynaptic potential (IPSP). The interaction of competing EPSPs and IPSPs at the hundreds or even thousands of synapses on a single neuron determines whether the nerve impulse arriving at the presynaptic terminals will be regenerated in the postsynaptic membrane.
The PSP is a type of local potential, having properties similar to the electrical potential set up at sensory receptor neurons (see the section Transmission in the neuron: Localized potential). Like the receptor potential, the PSP is a graded response, varying in amplitude according to the duration and amount of stimulation by neurotransmitters. At the neuromuscular junction, brief depolarizations measuring no more than one millivolt can be observed in the postsynaptic muscle membrane, even when it is at rest. These tiny electrical events, called miniature end-plate potentials (MEPPs), or miniature postsynaptic potentials (MPSPs), are caused by the random release of single quanta of neurotransmitter from a resting presynaptic terminal. The EPP is actually made up of multiple MEPPs, which arise when an activated terminal releases hundreds of neurotransmitter quanta. A series of EPPs, or a number of them stimulated simultaneously at many synapses, can then bring the cell to the threshold of the action potential. This combined action of EPPs is called summation.
In contrast to electrical transmission, which takes place with almost no delay, chemical transmission exhibits synaptic delay. Recordings from squid synapses and neuromuscular junctions of the frog reveal a delay of 0.5 to 4.0 milliseconds between the onset of action potential at the nerve terminal and action potential at the postsynaptic site. This delay may be accounted for by three factors. First, diffusion of the neurotransmitter across the synaptic cleft takes approximately 0.05 millisecond. Second, the response of the postsynaptic receptor takes about 0.15 millisecond. This leaves 0.30 to 3.80 milliseconds for other processes. A third process, called mobilization of the transmitter, is traditionally postulated as taking up the remaining time, but evidence suggests that the time is occupied at least partially by the opening of calcium channels to allow the entry of Ca2+ into the presynaptic terminal.
Inactivation
A series of nerve impulses arriving in rapid succession at the axon terminal is accurately reproduced as a series in the postsynaptic cell because the quanta of neurotransmitter released by each impulse are inactivated as soon as they stimulate the receptor proteins. Neurotransmitter inactivation is carried out by a combination of three processes. First, the neurotransmitter molecules simply diffuse out of the narrow synaptic cleft. Second, they are taken back into the presynaptic terminal by transmitter-sensitive transport molecules. Third, they are metabolized into inactive compounds by enzymes in the synaptic cleft.
Ion transport
As is stated above, the lipid bilayer of the neuronal membrane tends to repel electrically charged, hydrated ions, making virtually impossible the movement across the membrane that is necessary for the generation of nerve impulses. The transmembrane movement of ions is actually carried out by molecular mechanism—specifically, by protein molecules embedded in the lipid layers. One mechanism, the sodium-potassium pump, maintains the resting potential, and another, the various ion channels, helps create the action potential.
Active transport: the sodium-potassium pump
Since the plasma membrane of the neuron is highly permeable to K+ and slightly permeable to Na+, and since neither of these ions is in a state of equilibrium (Na+ being at higher concentration outside the cell than inside and K+ at higher concentration inside the cell), then a natural occurrence should be the diffusion of both ions down their electrochemical gradients—K+ out of the cell and Na+ into the cell. However, the concentrations of these ions are maintained at constant disequilibrium, indicating that there is a compensatory mechanism moving Na+ outward against its concentration gradient and K+ inward. This mechanism is the sodium-potassium pump. Actually a large protein molecule that traverses the plasma membrane of the neuron, the pump presents receptor areas to both the cytoplasm and the extracellular environment. That part of the molecule facing the cytoplasm has a high affinity for Na+ and a low affinity for K+, while that part facing the outside has a high affinity for K+ and a low affinity for Na+. Stimulated by the action of the ions on its receptors, the pump transports them in opposite directions against their concentration gradients.
If equal amounts of Na+ and K+ were transported across the membrane by the pump, the net charge transfer would be zero; there would be no net flow of current and no effect on the membrane potential. In fact, in many neurons three sodium ions are transported for every potassium ion; sometimes the ratio is three sodium ions for every two potassium ions, and in a few neurons it is two sodium ions for one potassium ion. This inequality of ionic transfer produces a net efflux of positive charge, maintaining a polarized membrane with the inner surface slightly negative in relation to the outer surface. Because it creates this potential difference across the membrane, the sodium-potassium pump is said to be electrogenic.
The sodium-potassium pump carries out a form of active transport—that is, its pumping of ions against their gradients requires the addition of energy from an outside source. That source is adenosine triphosphate (ATP), the principal energy-carrying molecule of the cell. ATP is formed by an inorganic phosphate molecule held in high-energy linkage with a molecule of adenosine diphosphate (ADP). When an enzyme in the pump, called sodium-potassium-ATPase, splits the phosphate from the ADP, the energy released powers the transport action of the pump.
Passive transport: membrane channels
The sodium-potassium pump sets the membrane potential of the neuron by keeping the concentrations of Na+ and K+ at constant disequilibrium. The sudden shift from a resting to an active state, when the neuron generates a nerve impulse, is caused by a sudden movement of ions across the membrane—specifically, a flux of Na+ into the cell. Given the relative impermeability of the plasma membrane to Na+, this influx itself implies a sudden change in permeability.
Known as “voltage dependent” when activated by changes in the membrane potential and “neurotransmitter sensitive” when activated by neurotransmitter substances, these channels are protein structures that span the membrane from the extracellular space to the cytoplasm. They are thought to be cylindrical, with a hollow, water-filled pore wider than the ion passing through it except at one region called the selectivity filter. This filter makes each channel specific to one type of ion.
Sodium channels
Voltage-sensitive sodium channels have been characterized with respect to their subunit structure and their amino acid sequences. The principal protein component is a glycoprotein containing 1,820 amino acids. Four similar transmembrane domains, of about 300 amino acids each, surround a central aqueous pore through which the ions pass. The selectivity filter is a constriction of the channel ringed by negatively charged carbonyl oxygens, which repel anions but attract cations. Also within the channel are thought to be two types of charged particles forming the gates that control the diffusion of Na+. One gate closes at polarization and opens at depolarization; the other closes at depolarization.
It is thought that the resting, activated, and inactivated states of the sodium channel are due to voltage-dependent conformational changes in the glycoprotein component. These changes result from effects of the electrical field on the charges and dipoles of the amino acids within the protein. With a large electrical field applied to it, the protein has been observed to change its conformation from a stable, closed resting state to a stable, open state in which the net charge or the location of the charge on the protein is changed.
Potassium channels
There are several types of voltage-dependent potassium channels, each having its own physiological and pharmacological properties. A single neuron may contain more than one type of potassium channel.
The best-known flow of K+ is the outward current following depolarization of the membrane. This occurs through the delayed rectifier channel (IDR), which, activated by the influx of Na+, counteracts the effect of that cation by allowing the discharge of K+. By repolarizing the membrane in this way, the IDR channel restricts the duration of the nerve impulse and participates in the regulation of repetitive firing of the neuron.
Another outward K+ current, occurring with little delay after depolarization, is the A current. IA channels are opened by depolarization following hyperpolarization. By increasing the interval between action potentials, they help a neuron to fire repetitively at low frequencies.
Another type of potassium channel, the IK(Ca) channel, is activated by high concentrations of intracellular Ca2+. The opening of these channels results in hyperpolarization of the membrane, so that they appear to slow the repetitive firing of nerve impulses.
The IM channel is opened by depolarization but is deactivated only by the neurotransmitter acetylcholine. This property may serve to regulate the sensitivity of neurons to synaptic input.
A final type of potassium channel is the anomalous, or inward, rectifier channel (IIR). This channel closes with depolarization and opens with hyperpolarization. By allowing an unusual inward diffusion of K+, the IIR channel prolongs depolarization of the neuron and helps produce long-lasting nerve impulses.
Calcium channels
As with potassium channels, there is more than one type of calcium channel. The inward calcium current is slower than the sodium current. There are at least two types of current in certain neurons of the central nervous system—a long-lasting current activated at positive potential and a transient current activated at more negative potential. There are two corresponding types of calcium channels: a large conductance channel that gives rise to a long-lasting current at positive membrane potentials and a low conductance channel that gives rise to a transient current at more negative potentials. In some neurons a third channel current occurs that is transient and can only be activated at high negative potential.
Anion channels
There may be channels that pass anions such as Cl−, but their existence is difficult to prove. Single-channel recordings of cultured tissue have shown selective Cl− channels that are voltage dependent and of high conductance. Channels with lower conductance have been demonstrated in reconstituted artificial membranes as well as in neurons.
Neurotransmitters and neuromodulators
The traditional models for the study of neurotransmitter release are either the neuromuscular junction of the frog, crayfish, and rat or the giant synapse of the squid. These synapses are relatively simple in their structure, with a single axon terminal forming an identifiable synapse at the postsynaptic membrane of a muscle fibre or neuron. Recordings can be obtained from these single-synaptic junctions in response to the release of a single neurotransmitter. At neurons of the central nervous system, on the other hand, the situation is more complex. Each central neuron has several synapses with other neurons at various locations, such as on the dendrites, soma, and initial segment of the axon. Several neurotransmitters, therefore—some excitatory and others inhibitory—may be involved in the final integrated response of a central neuron, making their identities difficult to determine.
Further complicating neurotransmitter action is the presence not only of multiple transmitter substances but also of neuromodulators. Neuromodulators are substances that do not directly activate ion-channel receptors but that, acting together with neurotransmitters, enhance the excitatory or inhibitory responses of the receptors. It is often impossible to determine, in the presence of many substances, which are transmitters and which are modulators. Such is the case with many of the neuropeptides (see the section Neuroactive peptides).
In addition to the multiplicity of transmitters and modulators there is a multiplicity of receptors. Some receptors directly open ion channels, while others activate the second-messenger system, any of a number of reactions that take place in the cytoplasm or plasma membrane and indirectly act upon the ion channels. One second-messenger system involves the activation by receptor proteins of linking proteins, which move across the membrane, bind to channel proteins, and open the channels. Another system is the cyclic adenosine monophosphate (cAMP) system. In this chain reaction, receptor proteins activate linking proteins, which then activate the enzymes that synthesize cAMP. The cAMP molecules activate other enzymes that, in turn, activate ion channels.
Whether they activate channels directly or through a second-messenger system, neurotransmitters are considered to be primary messengers. Described below are the principal proved or suggested neurotransmitters of the mammalian nervous system and their corresponding receptors.
Acetylcholine
Although early studies of acetylcholine were undertaken at neuromuscular junctions, where it is especially concentrated, the concept leading to the identification of the substance as a neurotransmitter of the central nervous system is a landmark in neuroscience. The concept is called Dale’s principle after Sir Henry Dale, a British physiologist who, in 1935, stated that a neurotransmitter released at one axon terminal of a neuron can be presumed to be released at other axon terminals of the same neuron. (Dale’s principle refers only to the presynaptic neuron, as the responses of different postsynaptic receptors to a single neurotransmitter can vary in the same or different neurons.) The first application of Dale’s principle was at the mammalian spinal cord, from which motor neurons send their axons to striated muscles, where the terminals are observed to release acetylcholine. According to Dale’s principle, all the branches of a single motor neuron axon should release acetylcholine—including the terminals in the spinal cord. In fact, it was found that some collateral branches leave the motor axons and reenter the gray matter of the spinal cord, where they synapse onto spinal interneurons. The neurotransmitter released at these terminals is acetylcholine. High concentrations of the acetylcholine-synthesizing enzyme, choline acetyltransferase, and the enzyme for its breakdown, acetylcholinesterase, are also found in motor neuron regions of the spinal cord.
Acetylcholine receptors (also called cholinergic receptors) appear in clusters on muscle-cell membranes opposite the active zones of presynaptic terminals. Their density at these receptor regions is between 7,000 and 30,000 sites per square micrometre (micron; millionth of a metre). The number drops drastically even a few nanometres (billionths of a metre) away from the receptor region, so that sensitivity to acetylcholine is about 50 to 100 times less one millimetre from the receptor region than it is at the receptor site itself. Cholinergic receptors also exist on the presynaptic terminals of neurons that release acetylcholine as well as on terminals that release other neurotransmitters. These receptors are called autoreceptors, and they probably regulate the release of neurotransmitter at the terminal.
There are two main categories of cholinergic receptor, nicotinic and muscarinic. The nicotinic receptor is a channel protein that, upon binding by acetylcholine, opens to allow diffusion of cations. The muscarinic receptor, on the other hand, is a membrane protein; upon stimulation by neurotransmitter, it causes the opening of ion channels indirectly, through a second messenger. For this reason, the action of a muscarinic synapse is relatively slow. Muscarinic receptors predominate at higher levels of the central nervous system, while nicotinic receptors, which are much faster acting, are more prevalent at neurons of the spinal cord and at neuromuscular junctions in skeletal muscle.
The nicotinic receptor channel is a glycoprotein composed of five subunits. Two alpha- (α-) subunits contain the two acetylcholine-binding sites associated with the channel. Three other subunits—a beta- (β-) subunit, a gamma- (γ-) subunit, and a delta- (δ-) subunit—complete the protein. High-resolution electron microscopy with optical image reconstruction, as well as freeze-fracture electron microscopy, reveal a highly symmetrical structure, looking from the top somewhat like a life belt, with the presumed channel in the centre. About one-third of the protein protrudes from the plasma membrane, while the rest is embedded in the membrane or protruding into the cell.
Patch-clamp techniques give information on single channel currents and, therefore, on the conductance and kinetics of the cholinergic receptor channel. At the neuromuscular junction, approximately 20,000 univalent ions carry the charge across a single activated channel, and a quantum of acetylcholine activates about 1,500 channels. The time constant for the decay of the MEPP is the same as that for channel closing. The time constant for channel closing is voltage dependent, with depolarization shortening the duration of open channels and hyperpolarization lengthening the duration.
Studies show that nicotinic acetylcholine-activated channels allow cations to permeate the membrane with no specificity—that is, all cations can diffuse through the channels indiscriminately. Because the resting membrane is already near the equilibrium potential of K+, this means that much more Na+ and Ca2+ diffuse into the cell than K+ out, causing depolarization and excitation of the neuron or muscle cell. However, in certain molluscan neurons, nicotinic acetylcholine receptors can also activate Cl− channels, causing hyperpolarization of the postsynaptic membrane and inhibition of excitability. With respect to muscarinic receptors, the situation is not clear. Second messengers may be involved, and potassium channels may be activated.
Epinephrine and norepinephrine
These related hormones, also called adrenaline (epinephrine) and noradrenaline (norepinephrine), act to increase the heart rate, blood pressure, and levels of sugar and fat in the blood. They are secreted into the bloodstream by the adrenal glands in response to stress, but they are also synthesized and released as neurotransmitters by axon terminals in the central nervous system and in sympathetic fibres of the autonomic nervous system.
Receptors sensitive to norepinephrine and epinephrine are called adrenergic receptors. They are divided into two types, α and β. These are further classified into subtypes α1, α2, β1, and β2.
Both types of adrenergic receptors produce changes in the postsynaptic membrane potential by acting upon ion channels specific to K+ and Ca2+. They differ in the mechanisms that, upon stimulation by neurotransmitter, they employ to activate those channels. Stimulated β1 receptors bind to linking proteins that in turn bind to calcium channels, changing their shape and altering their permeability to the cation. More important, the linking proteins stimulate the synthesis of cAMP, which, through another series of reactions, opens potassium channels. The efflux of K+ tends to hyperpolarize the postsynaptic membrane, inhibiting the generation of a nerve impulse. The β2 receptor has been found on glial cells.
The α2 receptor activates potassium channels in both the postsynaptic and presynaptic membranes, probably via linking proteins and the synthesis of cAMP. The α1 receptor acts on calcium channels through a series of reactions linked to the lipid molecules of the plasma membrane.
Both epinephrine and norepinephrine are terminated by uptake back into the presynaptic terminals, where they are enzymatically degraded or inactivated.
Dopamine
Dopamine is a precursor of norepinephrine that acts as a neurotransmitter at certain synapses of the brain. Disorders at these synapses have been implicated in schizophrenia and Parkinson disease.
There are two types of dopaminergic receptors, called the D1 and the D2. The former catalyzes the synthesis of cAMP, and the latter inhibits its synthesis. These reactions then regulate calcium and potassium channels in the postsynaptic membrane. Dopaminergic receptors also exist on the presynaptic membrane. The neurotransmitter is terminated by uptake into the presynaptic terminal.
Serotonin (5-hydroxytryptamine)
Although the brain has only a small percentage of the serotonin found in the human body, there appears to be a strong relationship between the levels of this neurotransmitter at some regions of the brain and certain behavioral patterns, including sleep, sexual urge, and mood. At synapses of the peripheral nervous system, serotonin seems to prime muscle cells for an excitatory response to other neurotransmitters.
Serotonin receptors, or 5HT receptors, activate calcium and potassium channels through linking proteins and the cAMP second-messenger systems. After acting on the postsynaptic receptors, the neurotransmitter is taken up by the presynaptic terminal and enzymatically degraded.
Amino acids
Several amino acids exist in the central nervous system in extremely high concentrations, but their ubiquity makes their identification as true neurotransmitters difficult. Furthermore, because some of them are essential components of metabolic reactions, their presence within a neuron does not prove that they function as neurotransmitters. Nevertheless, there is enough evidence that some amino acids act as either excitatory or inhibitory transmitters. The excitatory amino acids include glutamic acid (or glutamate) and aspartic acid (or aspartate), and the inhibitory amino acids include gamma-aminobutyric acid (GABA) and glycine.
Glutamate is the most abundant amino acid in the brain. Unlike acetylcholine, glutamate does not vary greatly in concentration from one region to the next. However, the dorsal gray matter of the spinal cord, which contains terminals of incoming dorsal roots, has large concentrations of glutamate. Aspartate, on the other hand, is believed to be concentrated in the interneurons of the ventral gray matter.
At postsynaptic receptor sites glutamate depolarizes the membrane by opening nonspecific cation channels, which allow a net influx of Na+ and Ca2+. Of the excitatory amino acid receptors, the N-methyl-D-aspartic acid (NMDA) receptor has been thoroughly characterized. Patch-clamp studies show that this receptor is influenced by the presence of magnesium ions (Mg2+). In the absence of Mg2+, activated NMDA receptors open nonspecific cationic channels with no variation when the voltage is changed. With Mg2+ added to the extracellular medium, though, the frequency of channel openings is reduced when the membrane is hyperpolarized. Both glutamate and aspartate are probably inactivated by uptake systems at the presynaptic terminals and at glial cells surrounding some of the synaptic junctions.
GABA and glycine cause hyperpolarization of the postsynaptic membrane. GABA is widely distributed in the brain, being especially prevalent at higher levels of the central nervous system. It is produced from glutamate by the enzyme glutamic acid decarboxylase (GAD). Consequently, the concentrations of GABA and GAD parallel each other in the nervous system.
At postsynaptic receptor sites GABA opens chloride channels, causing in most cells a hyperpolarization of the membrane as Cl− diffuses inward to reach its equilibrium potential. However, GABA inhibits presynaptic nerve fibres as well. At certain synaptic junctions the release of neurotransmitter is modulated by the binding to presynaptic receptors of neurotransmitter released from other neurons. An example of this is at the axon terminals of incoming dorsal roots in the dorsal gray matter. Projecting onto these terminals are other terminals that release GABA. Although GABA causes an increased Cl− conductance at these terminals, the result is depolarization, not hyperpolarization, of the membrane. This is because the resting membrane potential of the receiving nerve terminal is much more negative than the Cl− equilibrium potential. This means that as Cl− flows into the terminal to reach equilibrium, the membrane is actually depolarized. The effect at the terminal is a decrease in neurotransmitter release.
Unlike GABA, glycine is found mostly at lower levels of the central nervous system, including the spinal cord, medulla oblongata, and pons. It is a major inhibitor released by interneurons to suppress motoneuronal activity. Like GABA, glycine acts by increasing Cl− conductance at the postsynaptic membrane, although it acts at a clearly different receptor.
It appears that at least two molecules of glycine and GABA must bind to their respective receptors to activate a chloride channel. The action of both neurotransmitters is terminated by uptake back into the presynaptic terminal or into surrounding glial cells.
Neuroactive peptides
Neuroactive peptides are sequences of amino acids, usually longer than amino acid neurotransmitters yet shorter than hormones or proteins. Unlike the classic neurotransmitters described above, which are formed by enzymes near the presynaptic terminals, neuroactive peptides are assembled by ribosomes attached to the endoplasmic reticulum. From there they are transferred to the Golgi apparatus, where they are packed into secretory vesicles and transported to the terminals. Some peptides are secreted by neuroendocrine cells of the hypothalamus or pituitary gland. Because they are released into the capillary system of the bloodstream and act at distant sites of the body, these are called neurohormones. Other peptides are released into the synaptic cleft between neurons of the central nervous system (including the hypothalamus). Many of these neuropeptides fulfill some criteria of neurotransmitters, evoking excitatory or inhibitory responses in postsynaptic ion channels, yet it is still uncertain to what extent they act as true neurotransmitters or as neuromodulators.
Distinguishing neuropeptides from the classic neurotransmitters is the longevity of their action. While acetylcholine, for example, acts upon synaptic receptors for only a few milliseconds, neuropeptides have a course of action lasting from several seconds to several days. Also, neuropeptides are released in much lower concentrations than are transmitter substances, although the peptides have a much higher potency.
The list of neuropeptides is not yet complete. Among those peptides known to affect synaptic transmission are substance P, neurotensin, somatostatin, vasoactive intestinal peptide, cholecystokinin, and the opioid peptides. The best-studied are the opioid peptides, so called because opiate drugs, such as morphine, are known to bind to their receptors and mimic their painkilling and mood-altering actions. All opoid peptides belong to three genetically distinct families: the β-endorphins, the enkephalins, and the dynorphins.
It has long been known that opioids and opiate drugs have varied and powerful effects on pain, mood, sleep, sedation, and the cough reflex—apart from effects on the gastrointestinal tract and the cardiovascular system. It is not surprising, therefore, that there are multiple receptors for these substances. There may in fact be as many as eight different types of opioid receptors, but the four best-described are designated mu (μ), kappa (κ), delta (δ), and sigma (σ). The μ receptors, which readily bind morphine, are thought to mediate euphoria, respiratory depression, and physical addiction and to block pain pathways in the brain. The κ receptors bind preferentially to dynorphin and are thought to mediate analgesia and sedation at the spinal cord. The δ receptors, located primarily in the limbic portions of the brain, bind enkephalin. They may be responsible for dysphoria (extreme depression), hallucination, and respiratory and vasomotor stimulation. The σ receptors, found in the hippocampus, may be involved in alterations of affective behaviour, but their functions are unclear.
The opioid receptors mediate their effects mainly by inhibiting regeneration of the nerve impulse at the postsynaptic membrane. They accomplish this by opening potassium channels or closing calcium channels, causing a net outflow of positive charge that keeps the postsynaptic membrane from reaching threshold potential. As with other neuropeptides, it is not known whether all the opioid receptors are activated by the opioids alone or by a combination of opioid and other transmitter substances. For this reason it is uncertain whether the opioid peptides are true neurotransmitters or are neuromodulators.
The presence of peptides within certain structures of the central nervous system is well established; more important, peptides are often found in the same neurons with classic neurotransmitters or with other peptides. For example, substance P can be found in the same neurons of the brainstem as serotonin. In the sympathetic system, norepinephrine is found with somatostatin in some neurons and with enkephalin in others. Because some neuropeptides and neurotransmitters are stored in the same vesicles and secreted together in response to stimulation, a form of interaction between the substances appears likely. The interaction may take place at presynaptic terminals, altering the release of neurotransmitter, or it may take place postsynaptically, altering the effect of neurotransmitter. At the neuromuscular junction of the lobster, for example, the neurotransmitters serotonin and octopamine and the neuropeptide proctolin can act presynaptically to alter the amounts of GABA or glutamate released from the nerve terminals. In a similar manner, at some regions of the central nervous system opioid peptides inhibit the release of norepinephrine, acetylcholine, dopamine, and substance P. The discovery of more than one type of neuroactive substance in one set of axon terminals has disproved an assumption implied by Dale’s principle—that a single neuron synthesizes and secretes a single neurotransmitter. Also called into doubt is another assumption—that a single neuron secretes a single set of neurotransmitters at all of its synapses. Researchers are finding evidence that different synapses of the same neuron act somewhat independently. This may mean that different areas of a single neuron synthesize different neuroactive substances. Such a phenomenon would be another example of the metabolic and functional complexity of the nervous system.