ch. 48 part 2 Flashcards
action potentials
- carry electrical signals along an axon
- all-or-none - not graded
- actively propagated
actively propagated
regenerates itself as it travels
action potential sequences
- graded potential depolarizes to threshold potential (-50mV)
- voltage-gated Na+ channels open and trigger action potential
- Na+ rapidly diffuses into cell causing spike
- inactivation when gate in Na+ channel swings shut when membrane positively polarized
threshold potential
-50 mV
resting potential
-70 mV
when do voltage-gated K+ channels open
1 sec later than Na+ channels
opening of K+ channels for hyperpolarization
- voltage-gated K+ channels open by threshold potential
- K+ leaves cell and membrane becomes negative again
- membrane hyperpolarizes
- voltage-gated K+ channels close and resting potential restored
where do graded potentials reach threshold potential
axon hillock
what do graded potentials trigger just beyond the hillock region
opening of voltage-gated Na+ channels
what does the sequential opening of Na+ channels do?
conduct a wave of depolarization from axon hillock to axon terminal
what does the inactivation gate of Na+ channels do?
prevent backward movement toward cell body
absolute refractory period
- while Na+ gate is closed, cell is unresponsive to another stimulus
- limit on frequency of action potentials
- ensures action potential does not move backward toward cell body
relative refractory period
- K+ channels still open
- membrane hyperpolarizes
- graded potential would have to be huge to reach the threshold
what does speed of conduction depend on
- axon diameter
- myelination
axon diameter and speed
broad axons provide less resistance and action potential moves faster
myelination and speed
- myelination faster than unmyelinated
- oligodendrocytes and Schwann cells
- gaps at nodes of Ranvier
- saltatory conduction
saltatory conduction
action potential seems to “jump” from node to node
synapses
junction where nerve terminal meets a neuron, muscle cell, or gland
order at junction
- presynaptic cell sends signal
- synaptic cleft
- postsynaptic cell receives signal
2 types of synapses
- electrical
- chemical
electrical synapse
electric charge freely flows through gap junctions from cell to cell
chemical synapse
neurotransmitter acts as signal from presynaptic to postsynaptic cell
details of chemical synapse
- presynaptic cell contains vesicles of neurotransmitter
- exocytosis releases neurotransmitter into synaptic cleft
- diffuses across cleft
- binds to channels/receptors in postsynaptic cell membrane
what does binding of a neurotransmitter at a chemical synapse do
change membrane potential of postsynaptic cell
excitatory postsynaptic potential (EPSP)
- brings membrane closer to threshold potential
- less negative
- sodium channels opening
inhibitory postsynaptic potential (IPSP)
- takes membrane farther from threshold (hyperpolarization)
- more negative
- opening of chlorine channels (Cl- enters cells)
when does a synaptic signal end
when the neurotransmitter is broken down by enzymes or taken back into presynaptic cell for reuse
where does the cell bod of a postsynaptic neuron receive input from
can be hundreds or thousands of synaptic terminals
is a single EPSP strong enough to trigger an action potential in a postsynaptic neuron?
no - too small
summation
ind. postsynaptic potentials can combine to produce larger potential
temporal summation
if 2 EPSPs are produced in rapid succession
spatial summation
EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together
what can trigger an action potential
combination of EPSPs through spatial and temporal summation
how can an IPSP counter the effect of an EPSP?
through summation
what does the summed effect of EPSPs and IPSPs determine
whether an axon hillock will reach threshold and generate an action potential
after a response is triggered, what does the chemical synapse do
return to its resting state
- neurotransmitter molecules cleared from synaptic cleft
nerve gas sarin
tiggers paralysis and death due to inhibition of the enzyme that breaks down the neurotransmitter controlling skeletal muscles
metabotropic
movement of ions through a channel depends on one or more metabolic steps
- can be metabotropic receptor on postsynaptic membrane
what happens when a neurotransmitter binds to a metabotropic receptor
- activates signal transduction pathway in postsynaptic cell involving a second messenger
- leads to amplification where many channels are opened/closed in response
types of receptors for neurotransmitters
inotropic and metabotropic
how many receptors can a single neurotransmitter bind to
can be more than a dozen
- can excite postsynaptic cells expressing one receptor and inhibit postsynaptic cells expressing a different receptor
acetylcholine
common neurotransmitter in vertebrates and invertebrates
what is acetylcholine involved in
- muscle stimulation
- memory formation
- learning
2 major classes of acetylcholine receptors in vertebrates:
- ligand gated
- metabotropic
what can disrupt acetylcholine neurotransmission
number of toxins
- like nerve gas sarin and botulism toxin produced by certain bacteria
how many known neurotransmitters are there
over 100
4 classes of neurotransmitters
- amino acids
- biogenic amines
- neuropeptides
- gases
glutamate
one of several amino acids that can act as a neurotransmitter (vertebrates and invertebrates)
glycine
acts as inhibitory synapses in parts of CNS outside the brain
gamma-aminobutyric acid (GABA)
neurotransmitter at most inhibitory synapses in the brain
biogenic amines include:
- norepinephrine
- epinephrine
- dopamine
- serotonin
what do biogenic amines do
central role in number of nervous system disorders
Parkinson’s disease
associated with lack of dopamine in brain
neuropeptides
relatively short chains of amino acids that also function as neurotransmitters
what do neuropeptides include
- substance P
- endorphins
- both affect perception of pain
opiates
bind to same receptors as endorphins and can be used as painkillers
nitric oxide (NO)
local regulators in PNS
where is NO stores
- not stored in cytoplasmic vesicles like most neurotransmitters
- synthesized on demand
when can NO be broken down?
within a few seconds of production
can CO be used as a neurotransmitter?
yes - body makes it in small amounts
