Neuromodulators and hormones
A still further level of complexity is provided by non-classical neurotransmitter substances. Some of these are released by neurons like the conventional neurotransmitters already described, but they can have longer-lasting actions and (like paracrine neurotransmitters) act at greater distances from their release sites. Some may have no directly measurable effects on their targets, but they may change the target neuron’s responsiveness to its other classical neurotransmitter inputs. There is a more or less indefinable boundary between these substances – often called neuromodulators – and hormones. For example, cholecystokinin (CCK) is a peptide that is released as a hormone by the duodenum (part of the digestive tract), but is also released like a neurotransmitter from dopaminergic neurons in the brain, where it modifies the responses of dopamine autoreceptors. So the same molecule can operate as a neuromodulator in the brain and as a hormone in the gut. Hormones are molecules that are released into the bloodstream from specialized endocrine glands (such as the pituitary gland or the adrenal gland) and can therefore, in principle, act anywhere in the body. For hormones, specificity of action results from the presence of chemically specific receptors on the target structures that are bathed by the bloodstream. The hormonal receptors are activated when the hormones pass by in the blood. Hormones can affect neuronal function in a similar way to neuromodulators, changing sensitivity to other inputs and altering the release of neurotransmitters.
A still further level of complexity is provided by non-classical neurotransmitter substances. Some of these are released by neurons like the conventional neurotransmitters already described, but they can have longer-lasting actions and (like paracrine neurotransmitters) act at greater distances from their release sites. Some may have no directly measurable effects on their targets, but they may change the target neuron’s responsiveness to its other classical neurotransmitter inputs. There is a more or less indefinable boundary between these substances – often called neuromodulators – and hormones. For example, cholecystokinin (CCK) is a peptide that is released as a hormone by the duodenum (part of the digestive tract), but is also released like a neurotransmitter from dopaminergic neurons in the brain, where it modifies the responses of dopamine autoreceptors. So the same molecule can operate as a neuromodulator in the brain and as a hormone in the gut. Hormones are molecules that are released into the bloodstream from specialized endocrine glands (such as the pituitary gland or the adrenal gland) and can therefore, in principle, act anywhere in the body. For hormones, specificity of action results from the presence of chemically specific receptors on the target structures that are bathed by the bloodstream. The hormonal receptors are activated when the hormones pass by in the blood. Hormones can affect neuronal function in a similar way to neuromodulators, changing sensitivity to other inputs and altering the release of neurotransmitters.
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