Activation of nicotinic receptors by ACh causes depolarization and thus excitation of the postsynaptic cell which can be a postganglionic neuron, and autonomic effector, or a skeletal muscle fibre. Muscarinic Receptors : present in the plasma membrane of all effectors (smooth muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic axons and most most sweat glands receive innervation from cholinergic sympathetic postganglionic neurons and possess muscarinic receptors. These receptors are named after the mushroom poison called Muscarine. It mimics the actions of ACh by binding to them. Activation of muscarinic receptors by ACh sometimes causes depolarization and excitation depending upon the particular cell.
2) Adrenergic Neurons and Receptors
In the ANS, Adrenergic Neurons release norepinephrine (NE) also known as Noradrenalin. Most sympathetic postganglionic neurons and equine sweat glands are adrenergic. NE is also synthesized and stored in synaptic vesicles and released by exocytosis where it diffuses across the cleft to bind with specific adrenergic receptors on the postsynaptic membrane causing either excitation or inhibition.
Adrenergic Receptors
These receptors bind either norepinephrine or epinephrine . NE can be released as a NT or as a hormone into the blood by cells of the adrenal medullae. Epinephrine is always released as a hormone.
There are 2 main receptor types :
Alpha (α) and Beta (β): found on the visceral effectors innervated by most sympathetic postganglionic axons. These receptors are further classified into subtypes: α1, α2, β1, β2, β3 based on the specific responses.
α1, β1 generally produce excitation
α2, β2 cause inhibition of effector tissues
β3 are present only on special cells where activation causes thermogenesis
NE stimulates alpha receptors more strongly than beta receptors, and epinephrine is a potent stimulator of both receptors.
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