Yesterday we discussed the differences between the SNS and PNS; today we are diving a bit deeper into the neurotransmitters and receptors that make the changes happen. Since the neurotransmitter cannot enter the cell itself, all receptors are coupled to G proteins which serve as secondary messengers when a neurotransmitter attaches.
Sympathetic neurotransmitters include norepinephrine (epinephrine too, but focused on norepinephrine); these neurotransmitters bind to 1 of 5 receptors: alpha 1, 2 and beta 1, 2, or 3. This is where most stimulants act. Things like DMAA are highly adrenergic, causing bronchodilation, etc.
Alpha 1: stimulation leads to increased intracellular calcium, causing an excitatory response and contraction in smooth muscle.
Alpha 2: stimulation leads to an inhibitory effect; relaxation of smooth muscle, and inhibition of further norepinephrine release via a negative feedback loop (norepinephrine inhibits its own release). Beta 1: primary adrenergic receptor on the heart and kidney, increases heart rate and contractility and renin secretion in the kidney (necessary in the formation of angiotensin 1 from angiotensinogen, key in blood pressure regulation). Beta 2: located in smooth muscle, and lungs; serves as a vasodilator, relaxes smooth muscle and bronchodilates the lungs allowing for more air flow.
Beta 3: located in adipocytes, stimulation causes increased lipolysis for energy usage.
Parasympathetic neurotransmitters include acetylcholine; this neurotransmitter binds to either nicotinic or muscarinic receptors. Known for its impact on muscular health, acetylcholine is vastly important for the body.
Nicotinic: causes rapid increase in cellular permeability to sodium and calcium leading to depolarization in cells, necessary for muscular contraction.
Muscarinic: linked to G proteins which carry out the intracellular effect; can either be inhibitory or excitatory depending on tissue where it is found. In the heart, muscarinic receptors are inhibitory, decreasing heart rate. In the lungs, these receptors are excitatory, causing airway smooth muscle contraction and bronchoconstriction.