Chapter 5 Part II (MT2) Flashcards
How do Neurons Communicate and Adapt?
Four criteria for identifying neurotransmitters
(1) The transmitter must be synthesized in the neuron or otherwise be present in it
(2) When the neuron is active, the transmitter must be released and produce a response in some target
(3) The same response must be obtained when the transmitter is experimentally placed on the target
(4) A mechanism must exist for removing the transmitter from its site of action after its work is done
Putative transmitter
A suspect chemical that has not yet been shown to meet all the criteria (of a neurotransmitter)
What chemicals are classified as neurotransmitters?
Chemicals that:
(1) Carry a message from the presynaptic membrane of one neuron to another by influencing postsynaptic membrane voltage
(2) Change the structure of a synapse
(3) Communicate by sending messages in the opposite direction. These retrograde (reverse-direction) messages influence the release or reuptake of transmitters on the presynaptic side
Classes of neurotransmitters
(1) Small-molecule transmitters (ACh, DA, Glu)
(2) Peptide transmitters (oxytocin)
(3) Lipid transmitters (endocannabinoids)
(4) Gaseous transmitters (CO, NO)
(5) Ion transmitter (Zn)
Small-molecule transmitter
The quick-acting small-molecule transmitters, such as acetylcholine, are typically synthesized from dietary nutrients and packaged ready for use in axon terminals
- diet can influence abundance/activity
- acetylcholine
- amines
- serotonin
- amino acids
- purines
What is acetylcholine (small-molecule transmitter) made from?
How is it broken down?
Made from choline (fats in foods, egg yolk, salmon, avocado) and acetate (acidic foods, lemon juice, vinegar)
Broken down by enzyme acetylcholinesterase (AChE)
How is amine synthesized?
Tyrosine (precursor, from food like cheese)
[enzyme 1]
L-Dopa
[enzyme 2]
Dopamine
[enzyme 3]
Norepinephrine
[enzyme 4]
Epinephrine
Serotonin synthesis and role
Synthesized by amino acid L-tryptophan
Role in mood, aggression, appetite, arousal, respiration, pain
Amino acid transmitters (examples)
Glutamate (main excitatory transmitter)
GABA (main inhibitory transmitter)
Purine synthesis
Synthesized as nucleotides
Neuropeptides (peptide transmitters)
Multifunctional chains of amino acids that act as neurotransmitters
– Synthesized through translation of mRNA from instructions in the neuron’s DNA
– Act slowly and are not replaced quickly
Functions of peptide transmitters
Wide range of functions: act as hormones that respond to
stress, enable a mother to bond with her infant, regulate eating and drinking, pleasure and pain, contribute to learning
What do opioids (like morphine and heroine) do?
Mimic the actions of natural brain peptides
Endocannabinoids (lipid transmitter)
Synthesized at the postsynaptic membrane to act on receptors at the presynaptic membrane
- CB1 receptor is the target of all cannabinoids
Anandamide and 2-AG (2-
arachidonoylglycerol)
(Endocannabinoids)
– Both derived from arachidonic acid, an unsaturated fatty acid (found in poultry and eggs)
– Once synthesized, it diffuses across the synaptic cleft and interacts with its receptor on the presynaptic membrane
– Can reduce the amount of small-molecule transmitter being released
Gaseous transmitters
Neither stored in synaptic vesicles nor released from them
Synthesized in cell as needed; easily cross the cell membrane
Ion transmitters
Actively transported, packaged into vesicles—usually with another transmitter like glutamate—and released into the synaptic cleft
Two classes of neurotransmitter receptors
Ionotropic receptor
Metabotropic receptor
Ionotropic receptor
Binding site (for neurotransmitter) and pore/channel (for ion) which either opens or closes when the neurotransmitter binds
- rapid changes in membrane volatage, and usually excitatory
Metabotropic receptor
Lack their own pore for ions, so they indirectly produce changes in nearby ion channels or in the cell’s metabolic activity
- slower, longer-lasting, and widespread effects
What is a metabotropic receptor linked to?
A G protein, which can affect other receptors or act with second messengers to affect other cellular processes
What happens to a G protein (guanyl nucleotide-binding protein) when a neurotransmitter binds?
3 subunits of G protein = alpha, beta, and gamma
The alpha subunit detaches
Subunit = a protein that assembles with other proteins
Second messenger
Chemical that initiates a biochemical process when activated
What can a second messenger do?
(1) Bind to a channel & alter ion flow
(2) Form new ion channels
(3) Bind to DNA to alter protein production
Can many transmitters coexist in one vesicle?
Yes, more than one type of transmitter is packaged in a single vesicle
SNS neurotransmission
Motor (cholinergic) neurons send axons to body
- ACh is main neurotransmitter
- transmitter activated ionotropic channel = nicotine acetylcholine receptor (nAChr)
ANS neurotransmission
CNS neurons synapse with parasympathetic neurons
- ACh receptors on the heart are inhibitory
- ACh receptors on the gut are excitatory
CNS neurons synapse with sympathetic neurons
- NE receptors on the heart are excitatory
- NE receptors on the gut are inhibitory
ENS neurotransmission
Can act without input from CNS – “the second brain”
Sensory neurons detect mechanical and chemical conditions in GI system
Motor neurons control internal muscles for mixing intestinal
contents & secreting digestive enzymes
CNS activating systems
Cholinergic system
Dopaminergic system
Noradrenergic system
Serotonergic system
