In Review Stw
In mammals, the principal form of communication between neurons is chemical. Chemical neurotransmission appears to be an adaptation of processes used by single-celled organisms to immobilize, ingest, and digest food. When an action potential is propagated on an axon terminal, a chemical transmitter is released from the presynaptic membrane into the synaptic cleft. There the transmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane, after which the transmitter is deactivated. The nervous system has evolved a variety of synapses, bet ween axon terminals and dendrites, cell bodies, muscles, other axons, and even other synapses. One variety of synapse releases chemical transmitters into extracellular fluid or into the bloodstream as hormones, and still another connects dendrites to other dendrites. Chemical synapses, though slower and more complex than electrical synapses, more than compensate by greatly increasing behavioral flexibility. Even though synapses vary in both structure and location, they all do one of only two things: excite their targets or inhibit them.
H OW D O N E U R O N S C O MM UN ICATE A ND A DA PT?
VARIETIES OF NEUROTRANSMITTERS_
Subsequent to the discovery that excitatory and inhibitory chemicals control heart rate, many researchers of the 1920s thought that the brain must work under much the same dual-type control. They reasoned that there must be excitatory and inhibitory brain cells and that norepinephrine and acetylcholine were the transmitters through which these neurons worked. They could never have imagined what we know today: the human brain may employ as many as 100 neurotransmitters, which may be excitatory at one location and inhibitory in another, and more than one neurotransmitter may be active at a single synapse.
Although neuroscientists are now certain of only about 50 substances that act as neurotransmitters, discovery in this field continues. Few scientists are willing to put an upper limit on the eventual number of transmitters that will be found. In this section, you will learn how neurotransmitters are identified and how they fit within three broad categories based on their chemical structure. The functional aspects of neurotransmit-ters interrelate and are intricate, with no simple one-to-one relation between a single neurotransmitter and a single behavior.
O Visit the Brain and Behavior Web site (www.worthpublishers.com/kolb)
and go to the Chapter 5 Web links for information about current research on neurotransmitters.
Identifying Neurotransmitters
Among the many thousands of chemicals in the nervous system, which are neurotransmitters? Figure 5-7 presents four identifying criteria:
1. The chemical must be synthesized in the neuron or otherwise be present in it.
2. When the neuron is active, the chemical must be released and produce a response in some target.
3. The same response must be obtained when the chemical is experimentally placed on the target.
4. A mechanism must exist for removing the chemical from its site of action after its work is done.
The criteria for identifying a neurotransmitter are fairly easy to apply when examining the somatic nervous system, especially at an accessible nerve-muscle junction with only one main neurotransmitter, acetylcholine. But identifying chemical transmitters in the central nervous system is not so easy. In the brain and spinal cord, thousands of synapses are packed around every neuron, preventing easy access to a single synapse and its activities. Consequently, for many of the substances £ thought to be CNS neurotransmitters, the four criteria have been met only to varying degrees. A suspect chemical that has not yet been shown to meet all the criteria is called a putative (supposed) transmitter.
Researchers trying to identify new CNS neurotransmitters use mi-croelectrodes to stimulate and record from single neurons. A glass microelectrode is small enough to be placed on specific targets on a neuron. It can be filled with a chemical of interest and, when a current is passed through the electrode, the chemical can be ejected into or onto the neuron to mimic the release of a neurotransmitter onto the cell.
New staining techniques can identify specific chemicals inside the cell. Methods have also been developed for preserving nervous system tissue in a saline bath while experiments are performed to determine how the neurons in the tissue communicate. The use of "slices of tissue" simplifies the investigation by allowing the researcher to view a single neuron through a microscope while stimulating it or recording from it.
Figure 5-7
Criteria for Identifying Neurotransmitters
Chemical must be synthesized or present in neuron
When released, chemical must produce response in target cell.
Chemical iL
Same response must be obtained when chemical is experimentally placed on target.
Figure 5-7
Chemical must be synthesized or present in neuron
When released, chemical must produce response in target cell.
Chemical
- There must be a mechanism for removal after chemical's work is done.
^Acetylcholine
Figure 5-8
Axon collateral
^Acetylcholine
Figure 5-8
Renshaw Loop (A) Location of spinalcord motor neurons that project to the muscles of the rat's forelimb. (B) Circular connections of a motor neuron in a Renshaw loop, with its main axon going to a muscle and its axon collateral remaining in the spinal cord to synapse with a Renshaw interneuron there. The terminals of both the main axon and the collateral contain ACh. The plus and minus signs indicate that, when the motor neuron is highly excited, it can modulate its activity level through the Renshaw loop.
Axon collateral
Small-molecule transmitters. Class of quick-acting neurotransmitters synthesized in the axon terminal from products derived from the diet.
Acetylcholine was the first substance identified as a CNS neurotransmitter. A logical argument that predicted its presence even before experimental proof was gathered greatly facilitated the process. All the motor-neuron axons leaving the spinal cord use ACh as a transmitter. Each of these axons has an axon collateral within the spinal cord that synapses on a nearby CNS interneuron. The interneuron, in turn, synapses back on the motor neuron's cell body. This circular set of connections, called a Renshaw loop after the researcher who first described it, is shown in Figure 5-8.
Because the main axon to the muscle releases acetylcholine, investigators suspected that its axon collateral also might release ACh. It seemed unlikely that two terminals of the same axon would use different transmitters. Knowing what chemical to look for made it easier to find and obtain the required proof that ACh is in fact a neurotransmitter in both locations. The loop made by the axon collateral and the interneuron in the spinal cord forms a feedback circuit that enables the motor neuron to inhibit itself from becoming overexcited if it receives a great many excitatory inputs from other parts of the CNS. Follow the positive and negative signs in Figure 5-8B to see how the Renshaw loop works.
Today the term "neurotransmitter" is used more broadly than it was when researchers began to identify these chemicals. The term applies to substances that carry a message from one neuron to another by influencing the voltage on the postsynaptic membrane. And chemicals that have little effect on membrane voltage but rather share a message-carrying function, such as changing the structure of a synapse, also qualify as neurotransmitters. Furthermore, neurotransmitters can communicate not only by delivering a message from the presynaptic to the postsynaptic membrane but by sending messages in the opposite direction as well. These reverse-direction messages influence the release or reuptake of transmitters.
The definition of what a transmitter is and the criteria used to identify one have also become increasingly flexible because neurotransmitters are so diverse and active in such an array of ways. Different kinds of neurotransmitters typically coexist within the same synapse, complicating the question of what exactly each contributes in relaying or modulating a message. To find out, researchers have to apply various transmitter "cocktails" to the postsynaptic membrane. And some transmitters are gases that act so differently from a classic neurotransmitter such as acetylcholine that it is hard to compare the two.
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