Chapter 3B: Neuropsychology: The Generation, Transmission, and Integration of Neural Signals Flashcards
Action potential
Originate in the axon hillock and propagate along the axon
“Firing” is how one cell sends a message to another cell
Action potential triggers
LESS LIKELY: Hyperpolarization: making the membrane potential of a neuron more negative by increasing the negative charge on the inside
Ex: -65mV → 70 mV
MORE LIKELY: Depolarization: making the membrane potential of a neuron more positive on the inside
Ex: -65 mV → -55mV
Hyperpolarization
Spreads passively from the point of stimulation
Response diminishes the further you get from the source
Response is graded: the stronger the stimulus, the stronger the response
Depolarization
If enough stimulation is applied, the threshold of activation (-40mV) is reached and an action potential occurs
Response is NOT graded: it’s all or nothing (it doesn’t vary in size, it fires fully or not at all)
Doesn’t diminish (full strength all the way down the axon)
What events explain the action potential?
Once activation is triggered, an action potential begins
Voltage-gated Na+ channels open and Na+ ions rush into the cell
This continues until the membrane potential reaches +40mV (the Na+ equilibrium potential)
Once this trigger point of +40 is reached, voltage-gated K+ channels open
K+ rushes out and the resting potential is restored
K+ channels close slowly, causing too many K+ ions to leave
Makes the cell more negative for a brief period
Then, membrane potential returns to resting
Happens all the way down the axon
Refractory periods
Absolute refractory phase: the point where you can’t fire any more action potential; the channels are already open
Relative refractory phase: right after the channels close; CAN fire an AP at this period but requires extra stimulation
How is the AP propagated along the axon?
The AP is a spike of depolarizing activity, so it strongly depolarizes the next adjacent axon segment
The next segment has voltage-gated Na+ channels, so the depolarization there causes them to open and produce another electrical spike
Occurs at Nodes of Ranvier all the way down the axon
Myelin insulates signal and pushes it from node to node
Conduction velocity
The speed of the action potential
Nodes of Ranvier
Small gaps in the insulating myelin sheath
Where the Na+ channels that propagate the AP are located
Saltatory conduction
The action potential travels inside the axon and jumps from node to node
Demyelinating disorder
Multiple sclerosis (MS) is a disorder that occurs when a body’s immune system produces antibodies that attack myelin
Action potentials cause release of…
Neurotransmitters
The point of firing an AP is to cause release of neurotransmitters from the presynaptic cell across the synapse to the postsynaptic cell
Steps for transmission at the synapse
1) The AP travels down the axon to the axon terminals
2) This opens voltage-gated calcium (Ca2+) channels at the terminal and causes influx of Ca2+
3) Calcium comes into the cell, which causes the vesicles to fuse with the presynaptic membrane (exocytosis)
4) Neurotransmitters spill into synaptic cleft
Blocking neural transmission
Animal toxins selectively block certain channels
Tetrodotoxin and saxitoxin block voltage-gated Na+ channels
Batrachotoxin forces Na+ channels to stay open
Botulinum toxin and tetanus toxin inhibit neural transmission by cutting up SNARE proteins and stopping exocytosis
Postsynaptic potentials
Neurotransmitters released into the synaptic cleft bind to receptors on the postsynaptic cell and briefly alter the membrane potential of the postsynaptic cell
They either depolarize or hyperpolarize it, making it more or less likely to fire an action potential
Excitatory postsynaptic potentials (EPSPs)
Some neurotransmitters cause an electrical change that make the membrane potential less negative
Depolarization
Makes it more likely that neuron will fire by bringing it closer to threshold of activation
Inhibitory postsynaptic potentials
Some neurotransmitters cause an electrical change that make the membrane potential more negative
Hyperpolarization
Makes it less likely that neuron will fire by bringing it further from threshold of activation
PSPs are local and graded
Local: the change in membrane potential spreads passively over the neuron and degrades in strength over time and distance
Graded: a big stimulus generates a big response, while a small stimulus makes a small response
Spatial summation
Inputs vary across space
Temporal summation
Inputs vary in timing
Question: How will the effect be different if a PSP is received at a far away dendrite versus one on the cell body? Which one will have a stronger effect and why?
Immediate action at the synapse might be the exact same, but the closer one yields a stronger change because it has less far to travel
Further away the input is, the less strong effect it has on the postsynaptic cell because the message diminishes as it spreads
Ligands
Fit receptors exactly and activate or block them
Endogenous ligands
Neurotransmitters and hormones
Inner origin
Exogenous ligands
Drugs and toxins from outside the body
Outer origin
Transmitters binding to receptors can control the opening of ion channels in two ways
Ionotropic and metabotropic
Ionotropic receptors
Ligand-gated ion channels
When a neurotransmitter binds to the receptor, it causes the receptor to change shape and allows ions to enter the cell
Quick
Metabotropic receptors
G protein-coupled receptors
When the transmitter binds, it activates G-protein molecules that open nearby channels or trigger other reactions
Utilizes second-messenger system: first the transmitter binds, then chemical signal activated by G protein that amplifies its effect
Slow
Autoreceptors
Located on the presynaptic membrane
Neurotransmitters can bind here to inform presynaptic cell about how much neurotransmitter is in the synapse, regulate future NT release with exocytosis, and can act as a shut-off mechanism
Axo-axonic synapses
Form near axon terminals, allowing the presynaptic neurons to regulate how much neurotransmitter will be released from that terminal
Dendro-dendritic synapses
Allow coordination of activities
Retrograde synapses
Use gas to signal from the dendrite of a postsynaptic cell to the axon terminal of the presynaptic cell to release more neurotransmitter
