Action potentials are the means whereby information is passed from one neuron to an adjacent neuron. The balance between the excitatory and inhibitory impulses determines how many action potentials will reach the axonal terminal and, by releasing a specific type of neurotransmitter from the terminal, influence the adjacent neuron. Thus, in summary, chemical information in the form of small neurotransmitter molecules released from axonal terminals is responsible for changing the membrane potential at the synaptic junctions which may occur on the dendrites or directly on the cell body. The action potential then passes down the axon to initiate the release
of the neurotransmitter from the axonal terminal and thereby pass information on to any adjacent neurons.
A summary of the neurotransmitters and neuromodulators that have been identified in the mammalian brain is given in Table 2.1. The term neuromodulator is applied to those substances that may be released with a transmitter but which do not produce a direct effect on a receptor; a neuromodulator seems to work by modifying the responsiveness of the receptor to the action of the transmitter.
The metabolic unity of the neuron requires that the same transmitter is released at all its synapses. This is known as Dale's Law (or principle) which Sir Henry Dale proposed in 1935. Dale's Law only applies to the presynaptic portion of the neuron, not the postsynaptic effects which the transmitter may have on other target neurons. For example, acetylcholine released at motor neuron terminals has an excitatory action at the motor neuron junction, whereas the same transmitter released at vagal nerve terminals has an inhibitory action on the heart.
In addition to the diversity of action of a single transmitter released from a neuron, it has become well established that among invertebrates up to
Table 2.1. Some of the neurotransmitters and neuromodulators that have been identified in the mammalian brain
Distribution in brain
Involvement in CNS disease
Glutamate and aspartate
Most regions: long axons project from pons and brainstem
Most regions: short, medium and long axonal projections
Most regions: project from pons and brainstem
Most regions: long and short axonal projections from basal forebrain
Midbrain and brainstem
Spinal interneurons; modulates NMDA amino acid receptors in brain
Long neurons oîj receptors - inhibitory
/?i receptors - inhibitory
P2 receptors - excitatory?
Dj/D5 receptors - stimulatory
D2 receptors - inhibitory
5-HT1A receptors - inhibitory
Mj receptors - excitatory
M2 receptors - inhibitory
N receptors - excitatory
Possibly same as for noradrenaline
A receptors - hyperpolarize membranes
(inhibitory) B receptors - inhibitory Hyperpolarize membranes Strych-sensitive receptors - inhibitory Strych-insensitive receptors - excitatory
Quisqualate - depolarizes membranes NMDA - depolarizes membranes Kainate - depolarizes membranes
Learning and memory
Substances with a neuromodulatory effect on brain neurotransmitters by direct actions of specific receptors that modify the actions of the transmitters listed include: prostaglandins, adenosine, enkephalins, substance P, cholecystokinin, endorphins, endogenous benzodiazepine receptor ligands, and possibly histamine. CNS, central nervous system. NMDA, N-methyl-D-aspartate. Strych, strychnine.
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