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BIOL1412 > Topic 3: NS - Neurons, Impulse Generation, and Transmission > Flashcards

Topic 3: NS - Neurons, Impulse Generation, and Transmission Flashcards

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1
Q

what are electrochemical gradients within cells?

A

-differences in concentrations of ions between intracellular and extracellular fluid
-used for signalling (muscle + nerve cells)

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2
Q

what causes electrochemical gradients?

A

-ion concentration differences across the membrane
-permeability of the cell membrane to ions

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3
Q

what are the important ions in the cells intracellular + extracellular fluid?

A

-K+
-Na+
-Cl-
-Ca++
-org-

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4
Q

what are the concentrations of K+ and Na+ inside and outside the cell? how is this maintained?

A

-Na+ is high in ECF and low in ICF
-K+ is low in ECF and high in the ICF
-maintained by the activity of the Na+/K+ - ATPase pump in the cell membrane

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5
Q

what is the concentration of Ca++ inside and outside the cell? how is this maintained?

A

-Ca++ is high in ECF and low in ICF
-maintained by transporters in the cell membrane and the ER membranes (smooth ER = storage area for Ca++ to keep it out of cytosol)

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6
Q

what is the concentration of Cl- inside and outside the cell? how is this maintained?

A

-Cl- is high in ECF and low in ICF
-maintained due to the repelling by org-

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7
Q

what is the concentration of org- inside and outside the cell?

A

-org- are large organic ions (proteins)
-they are non-diffusible so they are only present inside the cell

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8
Q

how is the permeability of the cell membrane to ions determined?

A

-by ion channels
-ions diffuse through them down their concentration gradients
-2 types of channels that achieve this

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9
Q

what are the 2 types of ion channels?

A

-non-gated channels (leak channels)
-gated channels

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10
Q

what are non-gated (leak) channels?

A

-always open
-important in establishing the resting membrane potential (especially K+ leak channels)

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11
Q

what is important about the number of K+ non-gated channels vs the number of Na+ non-gated channels?

A

-there are more K+ than Na+ leak channels
-makes the cell membrane more permeable to K+ at rest (no stimulus present)
-help to establish resting membrane potential

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12
Q

what are gated ion channels?

A

-not involved at rest in the cell
-only open in response to stimuli
-variety of stimuli that trigger the opening of them

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13
Q

what are the different gated ion channels based on the stimuli that affect them?

A

-voltage gated channels (stimuli = membrane voltage changes)
-chemical/ligand gated channels (stimuli = binding of a hormone or neurotransmitter)
-thermal gated channels (stimuli = temperature)
-mechanical gated channels (stimuli = mechanical deformation (stretching))

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14
Q

what is membrane potential?

A

-the difference in electrical charge between the inside and outside of the cell
-created by ion movement
-measured in mV (millivolts)

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15
Q

what is membrane potential essential for?

A

-various cell processes
-nerve signalling
-muscle contraction
-maintaining homeostasis

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16
Q

what is resting membrane potential?

A

-the charge difference across the cell membrane of a resting cell (not being stimulated)
-approx. -70mV (inside of cell is more negative due to ions)

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17
Q

how does the diffusion of K+ establish the resting membrane potential?

A

-K+ will diffuse out of the cell down its concentration gradient (higher inside the cell)
-unlike charges will attract, as the inside of the cell becomes more negative K+ diffusion slows
-unlike charges also causes Na+ diffusion into the cell

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18
Q

how does the amount of K+ moving out of the cell and the Na+ moving into the cell compare before the cell reaches -70mV and after the cell reaches -70mV?

A

-until -70mV is reached the amount of K+ moving out is greater than the amount of Na+ moving in (K+ permeability is higher)
-at -70mV the amount of K+ moving out is equal to the amount of Na+ moving in

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19
Q

why does the K+ and Na+ movement at -70mV become equal?

A

-the electrical gradient increases the rate of Na+ entry and slows the K+ exiting
-making net movement of charge 0

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20
Q

what cell types are electrically excitable?

A

-muscle and nerve cells only

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21
Q

why are muscle and nerve cells able to be electrically excitable?

A

-they are capable of producing departures from the resting membrane potential in response to stimuli (change in external or internal environment)

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22
Q

what is the basis of what happens when a neuron is stimulated?

A

-gated ion channels open (type depends on the stimulus)
-the membrane potential changes to produce a graded potential
-will either go back to the RMP or an action potential will be triggered

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23
Q

what must occur for an action potential to be triggered?

A

-the membrane must meet the threshold potential of -55mV

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24
Q

what is a graded potential?

A

-caused by a stimulus that creates a small change in the resting membrane potential
-does so by opening gated channels
-typically on a dendrite or cell body
-short distance signal (dies away quickly)
-can either be a hyperpolarization and depolarization
-can summate

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25
how does ion movement contribute to the graded potential?
-ions move passively (unlike charges will attract) - = current flow (movement of charged particles) -cause a depolarization or hyperpolarization
26
what is the magnitude and distance travelled of a graded potential?
-varies with the strength of the stimulus -ex: larger stimulus will lead to a larger graded potential and therefore travel further
27
how can graded potentials summate?
-if the first graded potential is still present when a second stimulus occurs they will add together -this creates a larger graded potential as a result (more gated channels open)
28
what can occur after a graded potential?
-repolarization (bringing the cell back to RMP) -an action potential (if threshold of -55mV is reached due to a depolarization graded potential)
29
what is depolarization?
-a membrane potential less than -70mV (more negative)
30
what is hyperpolarization?
-a membrane potential greater than -70mV (more positive)
31
what causes a depolarization graded potential that is strong enough to result in an action potential?
-a critical stimulus -multiple graded potentials summate
32
what are the basic steps to reaching an action potential?
-critical stimulus or summating stimuli (gated channels open) -GP reaching -55mV -action potential
33
what is an action potential?
-a nerve impulse/signal -large change in the membrane potential that moves quickly along an axon (communication device) -has no change in its intensity (only frequency) -will initiate at a trigger zone
34
what is a trigger zone for an action potential?
-axon hillock of multipolar + bipolar neurons -just past the dendrites of unipolar neurons
35
what are the different events within an action potential?
-depolarization phase -repolarization phase -after - hyperpolarization phase
36
what is the depolarization phase of an action potential?
-voltage gated Na+ channels respond to the MP change (graded potential) and open (Na+ permeability increases) -influx of Na+ further changes the MP and causes more Na+ gated channels to open -Na+ continues to diffuse and depolarize the membrane until it reaches +30mV (inside of the cell is now positive)
37
what type of feedback mechanism is the entry of Na+ into the cell causing more Na+ voltage gated channels to open?
-positive feedback mechanism (increasing the change, not reverting it)
38
what is the repolarization phase of an action potential?
-Na+ voltage gated channels close and become inactivated (Na+ permeability decreases) -Na+ movement now returns to resting levels -voltage gated K+ channels open (K+ permeability increases) -K+ diffuses out and decreases the MP (inside of the cell is becoming more negative)
39
what is the after-hyperpolarization phase of an action potential?
-K+ voltage gated channels are slow to close and remain open longer than necessary (MP goes below RMP) -Na+ voltage gated channels are reactivated at -70mV and can now respond to stimuli
40
is the Na+/K+ - ATPase pump still working during an action potential?
-yes -it is always working to maintain gradients
41
are the ion concentration changes that occur during an AP measureable?
-no -it would take 10000s of APs to cause a measurable change
42
what are the 2 refractory periods of an AP?
-absolute refractory period -relative refractory period
43
what is the absolute refractory period?
-period when no AP can be generated regardless of the stimulus size -results from all Na+ channels being open or all Na+ channels being inactivated
44
what does the absolute refractory period prevent?
-AP summation
45
what is the relative refractory period?
-period when an AP can only be generated by a greater than normal stimulus due to the membrane being hyperpolarized (threshold is further) -Na+ channels are reactivated (but closed), can be reopened if the threshold is reached
46
what is the all-or-none principle of APs?
-ALL = if threshold is reached an AP is produced -NONE = below the threshold there is no AP
47
what is the process of AP propogation?
-when Na+ ions move into the cell during the depolarization phase of an AP, they will move towards the negative charge on the adjacent membrane -this causes the adjacent membrane to depolarize and reach threshold (AP created on the adjacent membrane) -charge movement occurs in both directions -APs only move in one direction (preceding membrane is in the absolute refractory period) -APs occur as a sequence along the membrane (each is the same) -goes through this process until the axon terminal is reached
48
what does the rate of propagation depend on?
-axon/fiber diameter (larger diameter = faster b/c less resistance to ion flow (current)) -myelination (unmyelinated vs myelinated)
49
how do unmyelinated fibers propagate an action potential?
-APs occur all along the fiber -Na+ channels are adjacent to one another -conduction is continuous -propagates slower
50
how do myelinated fibers propagate an action potential?
-APs occur at nodes of ranvier -Na+ channels are only present at the nodes -saltatory (leaping) conduction -propagates faster
51
what are the 2 fiber/axon types?
-Type A -Type C
52
what are the characteristics of a Type A fiber/axon?
-large diameter -myelinated -propagates APs faster -most sensory neurons and motor neurons to skeletal muscles
53
what are the characteristics of a Type C fiber/axon?
-small diameter -unmyelinated -propagates APs slower -found in the autonomic NS and some pain fibers
54
what is the junction between chains of neurons that the nervous system depends on?
-synapses -presynaptic neuron to postsynaptic neuron transmission
55
what are the steps in a synaptic transmission at a neuronal junction?
-AP arrives at the axon terminal (synaptic end bulbs) -Ca++ voltage gates open and Ca++ enters the cell (higher Ca++ outside) -rise in Ca++ triggers exocytosis of neurotransmitter vesicles -nt diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane -chemically gated ion channels open and allows ions to move in or out of the postsynaptic membrane -graded potential occurs (postsynaptic potential)
56
what are the receptors on the postsynaptic membrane?
-chemically gated ion channels that open in response to the binding of a neurotransmitter
57
what are the two types of postsynaptic potentials?
-excitatory PSPs (depolarization GP) -inhibitory PSPs (hyperpolarization GP)
58
what causes an excitatory PSP?
-opening of Na+ or Ca++ channels -closing of K+ channels -neurotransmitter is typically acetylcholine (ACh) or glutamate -excites the neuron towards the -55mV threshold
59
what causes an inhibitory PSP?
-opening of K+ or Cl- channels -neurotransmitter is typically glycine or GABA -inhibits the neuron from reaching the -55mV threshold
60
where do PSPs occur on the neuron?
-cell body or dendrites
61
how do postsynaptic potentials summate?
-many neurons can synapse onto one neuron (if many EPSPs, summation occurs and a large area of the membrane is depolarized, possibly leading to an AP) -some neurons may also be IPSPs -the sum of all EPSPs and IPSPs determines if an AP will occur at the axon hillock
62
what are the steps in a synaptic transmission at a neuromuscular junction?
-AP arrives at the axon terminal (synaptic end bulbs) -Ca++ voltage gates open and Ca++ enters the cell (higher Ca++ outside) -rise in Ca++ triggers exocytosis of neurotransmitter vesicles (nt always=ACh) -nt diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane (motor end plate of the muscle) -Na+ chemically gated ion channels open on the motor end plate -graded potential occurs (end plate potential) -EPP will trigger an AP on the sarcolemma
63
what is the motor end plate?
-sarcolemma of a muscle fiber
64
will an AP always occur from an EPP?
-YES -lots of ACh is released, making an AP occur everytime -AP is wanted/needed
65
what are the different types of graded potentials?
-end plate potential -excitatory postsynaptic potential -inhibitory postsynaptic potential

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