Neurotransmitters—these are the substances released from the nerve terminals into synaptic cleft which then activate or inhibit post-synaptic neuron by binding to specialized receptors.
Neurochemical transmission—it is the transmission of nerve impulse across synapse located between the neurons or at neuro-effector junction.
Fundamental steps in the neuro-chemical transmission—
1. Synthesis of transmitters
2. Storage of transmitters
3. Release of transmitters by nerve action potential
4. Interaction of the released transmitter with receptors of the effector cell
5. Rapid removal of the transmitter
6. Recovery of the effector cell into previous state
*** the enteric nervous system is considered to be the third division of the ANS.
Autonomic transmitters—
Acetyl choline—
§ All preganglionic sympathetic and parasympathetic nerve terminals.
§ Parasympathetic postganglionic nerve terminals.
§ Somatic neuromuscular junction.
§ Secretory cells and smooth muscle of the ENS.
§ Some postganglionic fibers of the sympathetic such as the sweat glands.
Nor-adrenaline/nor-epinephrine—most sympathetic postganglionic nerve terminals.
{& Adrenaline/epinephrine}
Dopamine—sympathetic postganglionic transmitter of the renal blood vessels.
Others—Nitric oxide, Serotonin, Gastrin releasing peptide (GRP), Vaso-active intestinal peptide (VIP).
SYMPATHETIC SYSTEM
Biosynthesis of Nor-adrenaline— ↓ Tyrosin hydroxylase ↓ Dopa decarboxylase
Tyrosin Dopa Dopamine
(Rate limiting stage) ← Dopamine β-hydroxylase
Nor-adrenaline
Biosynthesis of Adrenaline—
Nor-adrenaline Adrenaline ↑ Phenylethanolamine N-Methyl
Transferase (PNMT)
*** this conversion occurs in the adrenal medulla.
Removal / inactivation of the nor-adrenaline—
§ Transport back into the Non-adrenergic neurons (reuptake) followed by vesicular storage or mitochondrial destruction by MAO (mono-amine oxidase). {more than 80%}.
§ Diffusion from the synapse into the circulation and ultimate enzymatic destruction in the liver by MAO & COMT (Catechol-ortho-methyl transferase).
§ Active transport of the released transmitter into the effector cells (extra-neuronal uptake) followed by enzymatic inactivation by COMT.
Classification of the adrenoceptors (adrenergic/nor-adrenergic receptors)—
Acetylcholinesterase—
True acetylcholinesterase—found in the synaptic cleft and RBC.
False acetylcholinesterase—found in plasma, liver and glial cells.
PARASYMPATHETIC SYSTEM
Anatomy of Parasympathetic outflow—
1. Cranial outflow.
2. Sacral outflow.
Cranial outflow—
o 4 cranial nerves (3, 7, 9, 10) carry these parasympathetic fibers to their destination.
o The parasympathetic component of 3rd nerve arises from the Edenger Westphal nucleus of the midbrain.
o The parasympathetic component of 7th nerve arises from the superior salivatory nucleus of the lower part of pons.
o The parasympathetic component of 9th nerve arises from the inferior salivatory nucleus of the medulla.
o The parasympathetic component of the 10th nerve arises from the dorsal nerve of vagus and nucleus ambiguous from the floor of the 4thventricle.
Sacral outflow / pelvic outflow—
Arises from the lateral horn of the sacral 2 or 3 segments of the spinal cord.
*** the parasympathetic center as a whole is controlled by the hypothalamus.
↓ acetyl transferase
Acetyl co-A + Choline Acetylcholine ↓ acetylcholinesterase
Acetyl choline Choline + Acetic acid
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