Lecture 2: Action Potentials + Propagation Synaptic Transmission Flashcards
why are neurons electrically polarized like all cells?
Due to an unequal distribution of ions on each side of the plasma membrane
are neurons electrically excitable?
yes unlike most cells
the membrane potential can deviate from -70m
what is the change in membrane potential (voltage)
the signal neurons use to communicate with other neurons or other targets (muscles, glands).
what are graded potentials (GPs)
changes in membrane potential are typically small, slow and gradual and either decrease (depolarize) or increase (hyperpolarize) membrane potential
in graded potentials, what is the activity of the neuron related to
the voltage at any point in time
what type of communication are neurons capable of only generating GPs involved in
local or short distance communication, as signal transmission distance is limited (typically to a few mm).
what are action potentials (APs)
changes in membrane potential are large and typically repetitive, with rapid alternating depolarization and hyperpolarization (“spikes”) of membrane potential
in action potentials, what is the activity of the neurons related to
not the voltage, but the number of action potential spikes over time
what type of communication are neurons that generate APs capable of
long-range signaling
do all neurons that generate APs procuce GP as well?
yes because the generation of APs depends on the generation of GPs
what would happen if there were no sodium Na channels
the resting membrane would be more negative
How are graded potentials generated?
with GPs, the permeability of Na+ channels can change (go up or down) by neurotransmitters or mechanical forces.
how are action potentials generated?
Starting from rest (-70 mV), a stimulus first produces a graded depolarization. Then, at a certain voltage, there is a rapid, but brief, depolarization to +35 mV. The voltage drops quickly, and there is a period of calm before it happens again.
during stimulation of an action potential, does the membrane potential return back to -70 mV?
not during the period of stimulation, but at the end of the stimulation, there is a gradual hyperpolarization back to rest.
what makes APs special?
they are produced by ion channels that are “voltage-gated.”
why do voltage-gated Na channels drop and not keep increasing
they are subject to intrinsic inactivation. Soon after activation, they inactivate, causing the falling phase after the peak
Does sodium moving into the cell cause depolarization or repolarization
depolarize (move towards positive)
what is a refractory period and what are the two types
Voltage-gated Na+ channel inactivation imposes a refractory period on AP generation.
absolute refractory period
relative refractory period
what is an absolute refractory period
the period during which, no matter how strong the stimulus, another AP cannot be generated. It is due to the inactivation of voltage-gated Na+ channels.
what is a relative refractory period
the time following Na+ channel reactivation, but when voltage-gated K+ channel are still sufficiently active to oppose depolarization to threshold. However, a sufficiently strong stimulus could overcome such opposition.
what 2 things impose limits on the frequency that action potentials can be generated
The refractory periods
the after hyperpolarization
Why are graded signals limited to local or short-distance?
Electrical current flows passively both down the interior of the axon and leaks through the membrane. With distance, the amplitude of the potential change decreases.
How can we make the action potential travel further?
active regeneration of the signal
there are two mechanisms
what is the less ideal mechanism to regenerate the signal?
Cover unmyelinated axons with voltage-gated channels so that the action potential is constantly being boosted
what are the disadvantages to Cover unmyelinated axons with voltage-gated channels
its slow (takes time for each segment of the axon to reach threshold)
costs a lot of energy (the extensive movement of Na+ and K+ along the axon)
what is the ideal mechanism to regenerate the signal?
Insulate the axon, with myelin leaving some gaps (nodes), to decrease decay of signal.
what happens in the gaps (nodes) of myelinated axons?
Voltage-gated Na+ and K+ channels are only found in the nodes. Myelin reduces the decay of the electrical signal so that the voltage is above threshold when it reaches the node. At the node the signal is regenerated.
why are myelinated axons faster?
fast speed of transmission because only the voltage-gated channels within the nodes need to be activated
how is conduction velocity affected by axon diameter?
larger axons have less longitudinal resistance and thicker myelin so they’re faster
what are the 2 basic types of synapses
Electrical synapses (gap junctions)
Chemical synapses (release neurotransmitter)
what are electrical synapses (gap junctions)
Transmembrane channels that join to connect the interior of one cell with the interior of another cell.
Bidirectional and with essentially no synaptic delay.
what are chemical synapses?
neurotransmitter released at axon terminals, crosses synaptic cleft, acts on receptors on postsynaptic membrane
is neurotransmission unidirectional or bidirectional?
unidirectional: presynaptic→postsynaptic.
is there ever partial fusing of vesicles
no. If a vesicle fuses, all of the neurotransmitter in the vesicle is released.
what is a quantum
The amount of neurotransmitter released by a single vesicle
quanta-vesicles released over time
The total amount of neurotransmitter released by a single action potential corresponds to what?
the number of quanta
what is Glutamate
the most common excitatory neurotransmitter in the CNS.
Vesicles are loaded with glutamate by a vesicular glutamate transporter
what are the 2 types of post-synaptic receptors that glutamate acts as
AMPA receptors
NMDA receptors
what is the duration of an excitatory post-synaptic potential produced by AMPA alone vs with AMPA and NMDA
Just AMPA = brief
AMPA + NMDA = longer
what do GABA b receptors do
metabotropic receptors that increase efflux of K+
What will happen if sodium ions are not allowed to leave the membrane?
Returning to a resting membrane potential is not possible
The most common type of synapse are what kind
chemical
What enables neurotransmitters to be released?
Introduction of ions
Chemical synapses perform the following 2 functions:
Provide cell to cell communication
Produce secondary current in post synaptic neuron
true or false:
The “amount of brain” devoted to a particular body part is related to tactile acuity. This would generally mean that “more brain area” is equivalent to a smaller 2 point discrimination (less distance between the two points of stimulus).
true
what are Nodes of Ranvier
The location of ions channels
true or false:
A neuron can use multiple neurotransmitters at each of its synapses
true
what is Depolarization
Going from a negative resting membrane potential to a more positive charge
Opening of K+ channels causes
Hyperpolarization
