Chapter 12 Part 4 Flashcards
what are action potentials?
propagated changes in the transmembrane potential that affect an entire excitable membrane
voltage gated sodium channels are most abundant where?
on the axon, its branches, and synaptic terminals
what is the FIRST STEP in generating an action potential
the opening of voltage-gated sodium channels at one site (usually the initial segment of the axon)
the action potential is propagated (spread) along the length of the axon, eventually reaching….
the synaptic terminals
what does threshold mean? what is the typical threshold for an axon?
threshold is the transmembrane potential at which an action potential begins
average threshold for an axon is typically -60mV to -55mV
is the threshold for an axon is -60mV, what would be the depolarization?
10mV
will a stimulus that shifts the resting membrane potential from -70mV to -62mV produce an action potential?
no – only a graded depolarization
what causes the depolarization of the initial segment of the axon?
local currents resulting from the graded potential of the axon hillock
explain the “all or nothing” principle
as long as the depolarizing stimulus exceeds threshold, the properties of the action potential remain the same.
the force of depolarization doesnt matter. action potential remains the same regardless as long as threshold is reached
list the 4 steps of the generation of an action potential
- depolarization to threshold – this opens voltage-gated sodium channels
- activation of sodium channels and rapid depolarization – sodium ions rush into the cell and the transmembrane potential is now positive
- inactivation of sodium channels and activation of potassium channels – as the transmembrane potential approaches +30mV, sodium channels are inactivated and voltage gated potassium channels open, causing potassium ions to leave the cell. repolarization now begins
- closing of potassium channels – they begin closing as the membrane reaches the normal resting potential of about -70mV. Until ALL of these potassium channels have closed, potassium continues to leave the cell, producing a brief HYPERPOLARIZATION (-90mV)
as the voltage gated potassium channels close, what is happening to the transmembrane potential?
the transmembrane potential is returning back to normal resting levels. the action potential is now over
the resting potential depends on ___ channels
the graded potential depends on ___ channels
the action potential depends on ____ channels
resting – leak channels
graded – chemically gated
action – voltage gated
WHAT brings an area of excitable membrane to threshold?
graded depolarization of axon hillock that is large enough to open voltage-gated sodium channels
what is the refractory period
the time in which the membrane cannot respond to further stimulation from the moment the voltage-gated sodium channels open at threshold until sodium channel inactivation ends, because all the voltage gated sodium channels are either already open or inactivated
what are the 2 parts of the refractory period and how long do they last
part 1 = absolute refractory period. last 0.1msec-1msec (the smaller the axon diameter, the longer the duration)
part 2 = relative refractory period. begins when sodium channels regain their normal resting conditions and continues until the transmembrane potential stabilizes at resting levels
can another action potential occur during the relative refractory period? explain
yes
the depolarization, however, requires a larger than normal stimulus because…
-the local current must deliver enough Na+ to counteract the exit of K+ ions (through the voltage gated potassium channels)
-the membrane is hyperpolarized to some degree through most of the relative refractory period (due to the exit of K+ ions until every channel is closed)
in an action potential, depolarization results from the influx of Na+ ions and repolarization involves the loss of K+.
What returns these ion concentrations to prestimulation levels? Is it essential? explain
the sodium potassium exchange pump
it is NOT essential. The number of ions involved in a single action potential is insignificant compared to the total number of ions inside and outside of the cell
explain when the sodium-potassium exchange pump is ESSENTIAL
a maximally stimulated neuron can generate action potentials 1000/second
this is when the exchange pump is needed
explain how the sodium-potassium exchange pump works
each time the pump brings 2 potassium ions in and 3 sodium ions out, one molecule of ATP is broken down into ADP.
the transmembrane protein of the pump is sodium-potassium ATPase which gets the energy to pump ions by splitting a phosphate group from a molecule of ATP, forming ADP
action potentials may travel along an axon by ____ or ____
continuous propagation or saltatory propagatio
explain what continuous propagation is
continuous propagation occurs in UNMYELINATED axons.
an action potential moves across the membrane in a series of tiny steps.
the local current depolarizes adjacent portions of the membrane and continues in a chain reaction
explain how continuous propagation does not move backwards
because the previous segment of the axon is still in the absolute refractory period and cannot respond again to a stimulus
continuous propagation along an unmyelinated axon occurs at a speed of….
1 meter/second (2 miles an hour)
explain saltatory propagation
occurs in myelinated axons of the PNS and CNS. much faster than continuous propagation
when an action potential appears at the initial segment of a myelinated axon, the local current SKIPS the myelinated internodes and depolarizes the closest node.
“jumps” from one node to another instead of moving in a series of tiny steps (continuous)
which uses less energy — continuous or saltatory propagation?
saltatory. because less surface area is involved and fewer sodium ions must be pumped out of the cytoplasm (and into the cell)
the larger the diameter, the ___ the speed of the action potential
greater (faster)
classify axons into 3 groups according to the relationships between diameter, myelination, and propagation speed
Type A fibers = largest myelinated axons. (4-20micro meters) carry action potentials 120m/sec or 268mph
Type B fibers = smaller myelinated axons. diameter (2-4 micrometers) 18m/sec or 40mph
Type C fibers UNMYELINATED less than 2 micrometers in diameter. 1m/sec or 2mph
what is the FUNCTION of type A fibers
type A fibers carry sensory information about position, balance, and delicate touch and pressure senses FROM THE SKIN SURFACE TO THE CNS
what is the FUNCTION of types B and C fibers
carry information to the CNS and carry instructions to smooth muscle, cardiac muscle, glands, and other peripheral effectors
why isnt every axon in the nervous system large and myelinated?
physically impossible – too large
action potentials can also be called
nerve impulses
to be effective, an action potential must not only be propagated along an axon, but also…..
be transferred in some way to another cell (transfer takes place at synapses)
a synapse may be _____, involving DIRECT PHYSICAL CONTACT between the cells, or it may be ______, involving a neurotransmitter
electrical synapse – direct physical contact
chemical —- involving a neurotransmitter
at electrical synapses, the presynaptic and postsynaptic membranes are locked together at…..
gap junction
the lipid portions of opposing membranes at a synapse are held in position by binding between….
integral membrane proteins called CONNEXONS
What do connexons do?
form pores that permit ions to pass between cells
explain the advantage of the 2 cells being linked in this way (gap junctions, binding between connexons)
changes in the transmembrane potential of one cell produce local currents that affect the other cell as if the 2 shared a common membrane.
therefore, an electrical synapse propagates action potentials quickly and efficiently from 1 cell to the next
are electrical synapses common?
NO – they are extremely rare in both the PNS and CNS.
they occur in some areas of the brain, the eye, and in at least 1 pair of PNS ganglia
what is the most abundant type of synapse
a chemical synapse
neurotransmitters are either classified as ___ neurotransmitters or ____ neurotransmitters
excitatory or inhibitory
explain the difference between excitatory and inhibitory neurotransmitters
excitatory neurotransmitters – cause depolarization and promote the generation of action potentials
inhibitory neurotransmitters – cause hyperpolarization and suppress the generation of action potentials
in which synapse is an action potential ALWAYS propagated to the next cell — chemical or electrical?
electrical
explain how an action potential MAY OR MAY NOT be propagated to the next cell at a chemical synapse
-an action potential may or may not release enough neurotransmitter to bring the postsynaptic neuron to threshold
-other factors may intervene and make the postsynaptic cell more or less sensitive to arriving stimuli
the effect of a neurotransmitter on the postsynaptic membrane depends on ____ NOT ____
give a specific example
effect of a neurotransmitter on a postsynaptic membrane depends on THE PROPERTIES OF THE RECEPTOR, NOT the neurotransmitter itself
ACh typically causes depolarization in the postsynaptic membrane, but ACh released at neuromuscular junctions in the heart has an inhibitory effect
Synapses that release ACh (acetylcholine) are known as……
cholinergic synapses
the ___ junction is an example of a cholinergic synapse
neuromuscular
name 4 areas in which ACh is released
- ALL neuromuscular junctions involved with skeletal muscle fibers
- at many synapses in the CNS
- at ALL neuron-neuron synapses in the PNS
- ALL neuromuscular and neuroglandular junctions in parasympathetic divisions of the ANS
describe the events in the functioning of a cholinergic synapse
- action potential arrives and depolarizes the synaptic terminal
- extracellular calcium enters the synaptic terminal. this triggers the exocytosis of ACh
- ACh binds to receptors and depolarizes the postsynaptic membrane
- ACh is removed by acetylcholinesterase (AChE)
what exactly is ACh binding to on the postsynaptic membrane?
CHEMICALLY GATED SODIUM ION CHANNELS.
This will initiate an action potential if threshold is reached at the initial segment
the depolarization of the synaptic terminal causes calcium (Ca2+) ions to rush in. What do they rush in through?
voltage gated calcium channels
the depolarization of the postsynaptic membrane due to the influx of sodium ions through chemical-gated channels is a ___ potential.
explain
a GRADED potential
the greater the amount of ACh released at the presynaptic membrane, the greater the number of open channels in the postynaptic membrane, THUS, THE LARGER DEPOLARIZATION
Where does AChE reside?
in the synaptic cleft and in the postsynaptic membrane
around what percent of acetylcholine released at the presynaptic membrane is broken down before it reaches receptors at the postsynaptic membrane?
around half is broken down
AChE breaks down molecules of ACh by ____ into ___ and ____
by hydrolysis into acetate and choline
how is more ACh synthesized?
the choline resulting from the breakdown of ACh by AChE is actively absorbed by the synaptic terminal and used to synthesize more ACh
also acetate is used too and is provided by CoA (coenzyme A)
the acetate that results from the hydrolysis reaction by AChE goes where?
it can be absorbed and metabolized by the postsynaptic cell or by other cells and tissues
what is synaptic delay and how long is it
0.2-0.5msec
occurs between the arrival of an action potential (depolarization) at the synaptic terminal and the effect on the postsynaptic membrane
most of the synaptic delay is contributed by…
calcium influx and the RELEASE of neurotransmitter (not its diffusion across the cleft)
the fastest reflexes have how many synapses?
ONE
the more synapses, the more synaptic delay
when does ACh release from the presynaptic cell cease?
when active transport mechanisms quickly remove calcium from the synaptic terminal
is the synaptic terminal totally dependent on the ACh synthesized in the cell body of the neuron?
NO
ACh is made from recycled choline and acetate
when under intensive stimulation, resynthesis and transport mechanisms may not be able to keep up with the demand for neurotransmitter.
What is the term for this?
synaptic fatigue
during synaptic fatigue, the synapse weakens until….
Acetylcholine has been replenished
name some important neurotransmitters
norepinephrine
dopamine
serotonin
GABA (gamma-aminobutyric)
if the neurons that produce ______ are damaged or destroyed, the result can be the characteristic stiffness or rigidity that is Parkinson’s disease
(this is a _____ effect)
DOPAMINE
inhibitory
Explain the excitatory effect of dopamine
cocaine inhibits the removal of dopamine from synapses in specific areas of the brain. This rise in dopamine concentration creates a “high”
inadequate ____ production causes severe chronic depression
serotonin
GABA has an ___ effect
inhibitory
what do SSRI’s do? (selective serotonin reuptake inhibitors)
inhibit the reabsorption of serotonin by synaptic terminals, increasing the concentration of serotonin
what are 4 major CATEGORIES of neurotransmitters
-biogenic amines
-amino acids
-neuropeptides
-dissolved gasses
what are adrenergic synapses?
synapses that release norepinephrine
norepinephrine has a __ effect
excitatory, depolarizing effect
norepinephrine (NE) is widely distributed where>?
in the brain and portions of the ANS
Name 2 “dissolved gases” neurotransmitters
nitric oxide (NO)
carbon monoxide (CO)
Nitric oxide is generated by synaptic terminals that innervate ________
smooth muscle in the walls of blood vessels of the PNS, and at synapses in several regions of the brain
where is CO generated
similar to NO, at specialized synaptic terminals in the brain
what are neuromodulators?
active compounds that are released from synaptic terminals along with neurotransmitters.
they alter the rate of neurotransmitter release by the presynaptic cell OR change the post synaptic cell’s response to neurotransmitters
neuromodulators are typically _____
neuropeptides (small peptide chains synthesized and released by the synaptic terminal)
explain how neuromodulators act
they bind to receptors in the pre or post synaptic membranes and activate cytoplasmic enzymes
