Neurotransmitters:
Chemicals substance for communication between neurons.
Acetylcholine (ACh):
major transmitter in the peripheral nervous system
released by all neurons that stimulate skeletal muscles and some in ANS and CNS
binds to post-synaptic receptors briefly – released and degraded to acetic acid and choline by the enzyme acetylcholinesterase. Choline is recaptured by presynaptic terminal resynthesised to acetylcholine
excitatory to skeletal muscle or inhibitory to visceral effectors (depends on receptors bound)
Biogenic amines:
includes the catecholamines (dopamine, norepinephrine, epinephrine) and the indolamines (serotonin and histamine)
broadly distributed in brain play role in emotional behaviours and regulation of the biological clock
catecholamines are released by some motor neurons in ANS
Amino acids:
include gamma-aminobutyric acid (GABA), glycine, aspatate and glutamate
so far only reported in CNS; glycine is only in spinal cord; up to 70% of brain synapses are glutamate
GABA and glycine is inhibitory; antianxiety drugs (eg benzodiazepines) try to mimic action of GABA
glutamate is generally excitatory – main transmitter in CNS;
Peptides:
endorphins, dysnorphin, enkephalins – generally inhibitory
tachykinins, substance P, neurokinin A (NKA) – generally excitatory
somatostatin – generally inhibitory
Action of neurotransmitters:
Excitatory or inhibitory:
Some are excitatory cause depolarisation; some are inhibitory cause hyperpolarisation; some are both (depends on receptor type)
Direct or indirect:
Direct – open ion channels; provoke a more rapid response in postsynaptic cells (eg ACh, amino acid neurotransmitters)
Indirect – have a longer lasting effect; act through intra-cellular second messengers (eg biogenic amines, peptides)
Neurotransmitter Receptors:
Two types:
1) Channel linked or ionotropic receptors:
mediate the faster direct transmitter action
after binding, receptor proteins change shape opens ions channel ions pass into cell alters membrane potential
2) G protein linked receptors (metabotrophic receptors):
mediate the slower and more prolonged indirect transmitter action (by allowing the ion channel to stay open longer)
after neurotransmitter binds to G protein, it is activated produces affect by releasing second messenger (eg cyclic AMP, cyclic GMP, Ca+) these regulate ion channels or activate enzyme controlled pathways.
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