Chapter 5 - Neurotransmitters Flashcards
acetylcholine (Ach)
activates skeletal muscles in the somatic nervous system, and excites or inhibits internal organs in the autonomic nervous system
epinephrine (EP)
- aka adrenaline
- speeds up the heartbeat in frog hearts in Loewi’s experiment
norepinephrine (NE)
chemical messenger that increases heart rate in mammals
neurotransmitters
chemical messengers that are released by a neuron on a target to cause an excitatory or inhibitatory effect
difference between neurotransmitters and hormones
hormones travel longer distances, so their effects are slower
structure of synapses
cell A is connected to cell B by a connection called the synapse
- cell A is the presynaptic cell (sender)
- cell B is the postsynaptic cell (receiver)
- synapse consists of the terminal button of the presynaptic axon, synaptic cleft (small gap between the 2 cells), and the postsynaptic membrane
- presynaptic neuron secretes a chemical into the synaptic cleft, a neurotransmitter, which then binds to the postsynaptic receptors on the postsynaptic membrane
synaptic vesicles
the neurotransmitters in the presynaptic cell
quantum
content of 1 synaptic vesicle
- there is the same number of neurotransmitters in each vesicle
five steps of neurotransmission
- synthesis
- packing + storage
- release
- receptor action at the postsynaptic membrane
- inactivation
synthesis
the neurotransmitter is synthesized somewhere in the neuron
- peptide transmitters are synthesized in the cell body (DNA, mRNA) through transcription and translation
- small-molecule transmitters are synthesized directly in the axon terminal, using food-derived substances
packing + storage
the synthesized neurotransmitters are packaged and stored in vesicles
release
the vesicles containing neurotransmitters dock near release sites on the presynaptic membrane
- amount of neurotransmitter released depends on the amount of Ca entering the axon terminal and the number of vesicles docked at the membrane
receptor action at the postsynaptic membrane
neurotransmitters that are released into the synaptic cleft reach the postsynaptic membrane, where they bind to its transmitter-activated receptors
inactivation
if neurotransmitters remained in the synaptic cleft indefinitely, they would continue to bind to receptors, so the cell could not respond to new signals
- thus, neurotransmitters must be inactivated after they have done their work
diffusion
neurotransmitters diffuse away from the synaptic cleft towards areas of lower concentration
degradation
neurotransmitters are broken down by enzymes in the synaptic cleft
reuptake
neurotransmitters (and/or the residues of their enzyme degradation) can return to the presynaptic cell, where they can be reused
astrocyte uptake
neurotransmitters can be taken up by astrocytes, which can then provide them once again to the presynaptic cell
axodendritic synapses
connect an axon (presynaptic) to a dendrite (postsynaptic)
axosomatic synapses
connect an axon (presynaptic) to a cell body (postsynaptic)
gap junction
two neuron’s intracellular fluids come into direct contact, and ions can pass from one neuron to the other in both directions
- can be either open or closed
- connections through gap junctions are faster and more efficient than those through chemical synapses
- allow groups of neurons to synchronize and fire rhythmically
- however they do not show plasticity
excitatory effect
brings the cell closer to the firing threshold, thus making it more likely to fire an action potential
inhibitory effect
brings the cell further away from the firing threshold, thus making it less likely to fire an action potential
excitatory synapses
typically found on dendrites, they have round vesicles, a wide synaptic cleft, a high density of materials on the membranes, and wide active zones
inhibitory synapses
typically found on the cell body, have flat vesicles, a thin synaptic cleft, a low density of materials on the membranes, and small active zones
4 criteria for a chemical to be a neurotransmitter
- the transmitter must be synthesized or be present in the neuron
- the transmitter must be released and produce an effect when the neuron is active
- the transmitter must produce the same effect when experimentally placed on the target
- there must be a mechanism for removing the transmitter after its effect
classes of neurotransmitters
- small-molecule transmitters
- peptide transmitters (= neuropeptides)
- lipid transmitters
- gaseous transmitters
- ion transmitters
small-molecule transmitters
- small molecules
- synthesized in the axon terminal from food-derived nutrients and are ready for use
- once they have been released in the synaptic cleft, they can be replaced quickly at the presynaptic membrane
peptide transmitters (neuropeptides)
chains of amino acids that function as neurotransmitters
- synthesized through DNA/mRNA transcription and translation
- typically synthesized in the cell body and are transported to the axon terminal by microtubules
- slow and not quickly replaced
lipid transmitters
main lipid transmitters are cannabinoids, which can either be generated by the body (endocannabinoids) or by plants (phytocannabinoids)
- endocannabinoids synthesized at the postynaptic membrane, where they are derived from arachidonic acid
- synthesized on demand rather than stored in vesicles , so effect is slow
acetylcholine (Ach)
small-molecule transmitter found at the junction between neurons and muscles, as well as in the CNS
- synthesized from acetate (vinegar, lemon juice) and choline (egg yolk, avocado, salmon, olive oil) by enzymes
dopamine (DA), norepinephrine (NE), and epinephrine (EP)
small-molecule transmitters all synthesized from the precursur chemical tyrosine
- tyrosine is an amino acid found in food (e.g. hard cheese, bananas)
- an enzyme turns tyrosine into L-dopa, which is then turned into dopamine, then norepinephrine, and finally epinephrine
- the enzyme that converts tyrosine into L-dopa is limited, so it limits the rate at which dopamine is produced
- orally administering L-dopa can bypass this rate-limiting factor (dopamine cannot pass the blood-brain barrier, but L-dopa can)
- thus L-dopa is used in the treatment of Parkinson’s disease, which is caused by low dopamine
serotonin
small-molecule transmitter synthesized from L-tryptophan, an amino acid that is found in pork, turkey, milk, and bananas
- it regulates mood, aggression, appetite, arousal, respiration, and pain perception
GABA
small-molecule transmitter formed by a modification of the glutamate (Glu) molecule (removal of COOH)
- in the forebrain and cerebellum, GABA is found in inhibitory synapses and glutamate is found in excitatory synapses
- in the brainstem and spinal cord, glycine (Gly) is a more common inhibitory transmitter
histamine
small-molecule transmitter neurotransmitter that regulates excitement and waking
- it can cause the narrowing of smooth muscles
- when activated in allergic reactions, narrowed airways contribute to asthma
opioids
a type of peptide transmitter which can be endogenous (produced by the body) or exogenous (opium, morphine, heroin)
ionotropic receptors
have 2 parts: a binding site for neurotransmitters, and an ion channel
- when the neurotransmitter binds to the receptor, the channel quickly changes shape (open or close)
- there is a direct and rapid change in the membrane voltage, due to the change in ion flow
- effect is usually direct, and excitatory, and may trigger an action potential
metabotropic receptors
have a binding site for neurotransmitters, but they do not have an ion channel
- their effect is therefore indirect and slower
- metabotropic receptors are linked to a G-protein and have an indirect effect
- when a neurotransmitter binds to the receptor, the G-protein can either activate an ion channel or activate a second messenger, which activates other cell processes
- this may lead to an amplification cascade, in which a single neurotransmitter can start a chain of many proteins being activated (not present in ionotropic receptors)
cholinergic neurons
another name for motor neurons because acetylcholine (Ach) is their main neurotransmitter
4 activating systems in the central nervous system
- cholinergic system
- dopaminergic system
- noradrenergic system
- serotonergic system
cholinergic system
(acetylcholine, ACh) is important for waking behavior, attention, and memory
- people suffering from Alzheimer’s disease have a lack of cholinergic neurons
- an Alzheimer’s treatment is therfore a medicine that stimulates the cholinergic system
dopaminergic system
(dopamine, DA) is in the basal ganglia has 2 pathways (nigrostriatal pathway and mesolimbic pathway)
nigrostriatal pathway
includes the substantia nigra and it plays a role in coordinating movements
- patients with Parkinson’s disease have a decreased amount of dopamine, which reduces their movement
mesolimbic pathway
related to reward and pleasure, it plays a role in addiction and loss of impulse control
- overactivity in this pathway is related to symptoms of schizophrenia (e.g. hallucinations, delusions, disorganized speech, agitation), while decreased activity is related to ADHD
noradrenergic system
(noradrenaline = norepinephrine, NE) plays a role in learning, healthy brain development, and organizing movement
- problems in the noradrenergic system involve emotions
- decreased noradrenergic activity is linked to depression and to ADHD
- conversely, higher levels of noradrenergic activity are linked to mania
serotonergic system
(serotonin, 5-HT) plays a role in wakefulness, like the cholinergic system
- it also plays a role in learning, like the noradrenergic system
- increased serotonergic activity is linked to schizophrenia
- decreased serotonergic activity is linked to depression, obsessive-compulsive disorder (OCD), sleep apnea, and sudden infant death syndrome (SIDS)
habituation
the reduction of response following repeated exposure to a stimulus
saccades
small random eye movements done in order to counteract habituation to visual stimuli
neurological level of habituation
- with a repeated stimulus, there is a decreased influx of calcium ions (Ca2+) in the presynaptic cell
- this leads to fewer neurotransmitters being released in the synaptic cleft
- thus, the EPSPs at the postsynaptic membrane are smaller, leading to less depolarization; the postsynaptic cell is likely to fire
sensitization
the opposite of habituation: it is an increased response to stimuli
- sensitization is always linked to a specific event
- a related disorder is PTSD
neurological basis of sensitization
- an interneuron interferes with the communication between 2 neurons by releasing serotonin on the presynaptic membrane (this makes the K+ ion channels less responsive on the presynaptic membrane
- there is reduced efflux of K+; the presynaptic cell stays depolarized longer, so it fires a longer action potential
- as a result, the voltage-sensitive Ca channels on the presynaptic membrane remain open longer, so there is an increased influx of Ca2+ in the presynaptic cell
- this leads to more neurotransmitters being released in the synaptic cleft
- thus, the EPSPs at the postynaptic membrane are larger, leading to more depolarization; the postynaptic cell is more likely to fire
