Download IV. Conduction Across Synapses

IV. Conduction Across Synapses
synapses are gaps between neuron and target cell
refer to diagram of a synapse
identify: source of impulse:
target:
mechanism release of neurotransmitters that bind to receptors in target cell
if neurotransmitter stimulates Na+ channels to open – impulse generated
if neurotransmitter stimulates K+ channels to open – impulse suppressed
(chemo)receptors can only recognize specific neurotransmitters
can only respond to specific neurotransmitters
thus, response is dependent on neurotransmitter
respond to neurotransmitter as long as it is present.
neurotransmitters usually one type per neuron
effect usually due to type of receptor
A. Components
identify components listed in diagram “synaptic transmission”
B. Steps
go over briefly
then students attempt to complete handout. “synaptic transmission”
C. Termination of Synaptic Transmission
if neurotransmitters are present in synaptic cleft post-synaptic membrane will be
stimulated
must remove neurotransmitters to
1- end transmission
2- ready synapse for next transmission
1. diffusion
neurotransmitter floats out of cleft and into surrounding tissue
slow – long lasting – more potent – long to make
ex: neuropeptides like substance P
endophorins
will eventually be destroyed – takes minutes – hours
2. enzymatic destruction
neurotransmitter split by a specific enzyme
fragments re-absorbed by
presynaptic neuron
used to synthesize more neurotransmitter
ex: acetylcholine (Ach)
split by enzyme acetylcholinesterase (AChE)
3. transmitter re-uptake
neurotransmitter transported back into pre-synaptic neuron for re-use
ex: norepinephrine
dopamine
serotonin
D. Neurotransmitters
chemical messengers at synapses
most are excitatory – depolarize post-synaptic membrane
some are inhibitory – hyperpolarize post-synaptic membrane
effect of neurotransmitter determined by receptor
1. acetylcholine (ACh)
can be excitatory or inhibitory – depends on type of receptor and tissue
neurons that produce ACh are called cholinergic neurons
a) cholinergic neurons found in
• all neuromuscular junctions (ie all motor neurons)
• PNS neuron-neuron synapses
• brain areas: basal nucleus (near limbic system)
hippocampus (role in memory)
cerebral cortex
functions of ACh in brain
memory creation and retrevial / learning
pain conduction
regulation of neuroendocrine
regulation of REM cycles
b) 2 classes of receptors
• muscarine receptors in brain
• nicotinic receptors in brain, other neuron receptors and neuromuscular
junctions
c) termination
• with enzyme acetylcholinesterase (AChE)
• ACh binds to receptor – channel opens – ACh falls off receptor- ACh
degraded – receptor ready w/in milliseconds
d) effects of some chemicalsclass activity
nerve gas
rattlesnake venom
cobra venom BoTx
curare
black widow spider venom
2. serotonin
a) produced by neurons in brain stem
linked to and affects many areas
affected by many areas
limbic system
hypothalamus
cerebellum
spinal cord
cerebral cortex
b) termination method
re-uptake by transporters
c) implicated in many behaviors
mood
hunger
sleep
fear
learning and memory
sexual arousal
d) effects of some chemicals on serotonin synapses
• SSRI = selective serotonin re-uptake inhibitor
ex: Prozac; fluvoxamine, Paxil, Zoloft
bind to serotonin transporter
prevent termination
serotonin remains in synapse longer
• MDMA = ecstasy – an entacogen
blocks serotonin re-uptake and
causes release of serotonin (can deplete serotonin stores)
3. GABA gamma aminobutyric acid
a) inhibits by increases flow of Cl- into target cells
b) found in inhibitory neurons in brain and spinal cord
cerebellum
hippocampus
nucleus accembens
cerebral cortex
amygdale
c) associated with decreased anxiety, fear, stress
d) termination – re-uptake
many tranquilizers enhance GABA
ethanol, Valium, Xanon
4. dopamine
a) produced in ventral tegmental area (VTA) reward/pleasure pathway
excites targets in frontal lobe
produces feelings of euphoria
b) termination – re-uptake by transporters
5. nitric oxide NO
a gas
produced on demand (not stored in vesicles)
lasts 6-10 seconds after produced
causes blood vessels to relax and widen
roles: controls blood pressure
controls blood flow to specific regions
role in memory and learning
6. endorphins - opoids
a) produced in many areas of nervous system
in brain: basal ganglia, cortex, amygdale, pituitary, hypothalamus, parts
of medulla
b) role in: pain modulation
released when stressed or injured
if released in absence of pain – feelings of well-being/pleasure
also role in : motor coordination
learning and memory
gastrointestinal function
control of seizures
hormonal regulation of reproductive system
c) termination – diffusion
d) effect of some drugs
morphine, heroine, Demerol bind to endorphin receptors