The Chemical Messengers in Hospice
The communication of information between neurons is accomplished by the movement of small signaling molecules across a small gap called the synapse. Neurotransmitter molecules are made in the cell body of the neuron and then transported down the axon to the axon terminals. Once released the neurotransmitter spills across the synaptic gap and connects (binds) to a receptor molecule on the surface of an adjoining neuron. Neurotransmitter molecules have different molecular shapes and bind best to the receptor molecule sites that have shapes that match.
Binding to the active site on the receptor will cause the next neuron to either depolarize and fire or hyperpolarize and not fire. In that way, the neurotransmitter is the key and the receptor is the ignition switch.
Some neurotransmitters are commonly described as “excitatory” or “inhibitory”. However, the purpose of a neurotransmitter is to signal another neuron via a target receptor. The effect on the postsynaptic cell depends, therefore, on the properties of those receptors. It happens that for some neurotransmitters (for example, glutamate), the most important target receptors all have excitatory effects: that is, they increase the probability that the target cell will fire an action potential. For other neurotransmitters, such as GABA, the most important receptors all have an inhibitory effect: that is, they decrease the probability that the target cell will fire.
The number of receptor sites on an individual neuron can vary over time, increasing in number if the amount of neurotransmitter molecule is in abundance and decreasing in number when fewer neurotransmitter messengers are around. An increase in receptor sites increases the likelihood that action potential will be reached and the message that relays pain will be relayed.
Types of Neurotransmitters
The following neurotransmitters play an active role in the transmission and perception of physical pain:
- Glutamate is used at most of the fast excitatory synapses in the brain and spinal cord. It is also used at most synapses that are involved in the modulation (the increase or decrease of strength) of a chemical message.
- GABA is used at the great majority of fast inhibitory synapses in virtually every part of the brain. Many sedative/tranquilizing drugs act by enhancing the effects of GABA. Correspondingly glycine is the inhibitory transmitter in the spinal cord.
- Dopamine has a number of important functions in the brain. It plays a critical role in the reward system, but dysfunction of the dopamine system is also implicated in Parkinson’s disease and schizophrenia.
- Substance P is a peptide responsible for transmission of pain from peripheral nociceptor neurons to the central nervous system.
- Serotonin is a monoamine neurotransmitter. In the central nervous system the amount of serotonin influences the regulation of mood, appetite, sleep, memory and learning.
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