- Chemical substances which act as synaptic transmitters in the nervous system are called as neurotransmitters. Other synaptic transmitters include;
- Neuromodulators.. These are substances other than neurotransmitters which are released by a neuron and transmit information to other neurons. Neuromediators also are transmitters.
- Co-transmitters are the substances which are released along with neurotransmitters.
- Neurotransmitters can be rapidly acting which include acetylecholine, norepinephrine, epinephrine GABA(gamma amino butyric acid) and nitric oxide.
- Neurotransmitters can also be slowly acting called as neuropeptides. These include Thyrotropin, Oxytocin, Vasopressin, Angiotensin2 and calcitonin. Neuropeptides cause prolonged actions such as long term opening or closing of the ion channels.
- Upon stimulation, neurotransmitters are released in the synaptic cleft and stimulate the receptors on other neurons or effector organs. Even exogenously given transmitter will work in the same way. Nitric oxide is classified as class4 neurotransmitter. It works rapidly.
These are reactive chemical groups present on macromolecules(proteins) which bind with reactive chemical groups of the drugs or transmitters. Binding Is chemical. This binding stimulates biological activity.
Agonist.. It has affinity of binding and ability to produce effect(Intrinsic activity) as well.
Antagonist.. It has affinity but no intrinsic activity.
Types of Receptors:
- Ion channel associated receptors which are responsible for flow of ions across cell membrane.
- Enzyme linked receptors.
- G-protein coupled receptors
- Intracellular receptors.
Cholinergic receptors are those which are related to acetylcholine. These are of two types.
Nicotinic receptors are present in ganglions and skeletal muscles. Nicotinic receptors present on autonomic ganglion is called Nn and present on skeletal muscles is called Nm. Muscle contraction takes place by stimulation of Nm receptors. Adrenal gland also contains nicotinic receptors. Upon stimulation, adrenaline is released.
Adrenergic receptors are present on post synaptic membranes of neuroeffector organs like smooth muscles, cardiac muscles and exocrine glands. These are class of G-protein coupled receptors. These are furthure divided into α and β receptors. The α receptors are divided into α-1 and α-2 while β receptors are divided into β-1 and β-2 receptors. Both 1 are excitatory and 2 are inhibitory.