Cellular Physiology (Part 2) Flashcards
1
Q
What is resting membrane potential?
A
The potential difference that exists across the membrane of excitable cells, such as nerve and muscle, in the period between action potentials
2
Q
The resting membrane potential is established by what?
A
diffusion potentials, which result from the concentration differences for various ions across the cell membrane
3
Q
Which ions will make the greatest contributions to the resting membrane potential?
A
Ions with the highest permeabilities at rest
4
Q
Resting membrane potential of excitable cells falls in the range of −__ to −__mV
A
−70 to −80mV
5
Q
What is the resting membrane potential close to?
A
The equilibrium potentials for K+ and Cl- because the permeability to these ions at rest is high
6
Q
What is the resting membrane potential far from?
A
The equilibrium potentials for Na+ and Ca2+ because the permeability to these ions at rest is low
7
Q
What role does the Na+ - K+ APTase play in creating the resting membrane potential?
A
It creates and maintains the K+ concentration gradient, which establishes the resting membrane potential
8
Q
What is an action potential?
A
A phenomenon of excitable cells, such as nerve and muscle, and consists of a rapid depolarization (upstroke) followed by repolarization of the membrane potential
9
Q
What is the basic mechanism for transmission of information in the nervous system and in all types of muscle?
A
an action potential
10
Q
What is depolarization?
A
the process of making membrane potential less negative
11
Q
What is repolarization?
A
The process by which the membrane potential returns the membrane potential to a negative value after the depolarization phase of an action potential
12
Q
What is hyperpolarization?
A
the process of making the membrane potential more negative
13
Q
What is inward current?
A
The flow of positive ions from the ECF into the ICF
14
Q
What is an example of inward current?
A
The flow of sodium into the cell during the upstroke of the action potential
15
Q
What is outward current?
A
The flow of positive ions from the ICF to the ECF
16
Q
What is an example of outward current?
A
The flow of potassium out of the cell during the repolarization phase of the action potential
17
Q
Inward currents _____ the membrane potential and outward currents ______ the membrane potential.
A
depolarize
hyperpolarize
18
Q
What is threshold potential?
A
the membrane potential at which occurrence of the action potential s inevitable
19
Q
An ____ current is required to depolarize the membrane potential to threshold
A
inward
20
Q
The portion of the action potential where the membrane potential is positive is called what?
A
overshoot
21
Q
The portion of the action potential, following repolarization, where the membrane potential is actually more negative than it is at rest is called what?
A
undershoot
22
Q
What is the refractory period?
A
a period during which another normal action potential cannot be elicited in an excitable cell
23
Q
Each normal action potential for a given cell type looks ____, depolarizes to the same potential, and repolarizes back to the ___ resting potential.
A
identical
same
24
Q
Describe the propagation of an action potential
A
An action potential at one site causes depolarization at adjacent sites, bringing those adjacent sites to threshold.
- This propagation is nondecremental
25
What channel is responsible for the upstroke of the action potential in nerve and skeletal muscle?
the voltage-gated Na+ channel
26
What are the 2 types of gates on the Na+ channel?
activation and inactivation gates
27
In order for Na+ to move through the Na+ channel what gate must be open?
both
28
How do the gates on the Na+ channel respond throughout the course of an action potential?
1) during rest the activation gate is closed and the inactivation gate is open
2) during the upstroke of the action potential, the activation gate opens quickly (the inactivation gate is still open) allowing Na+ to flow through the channel
3) at the peak of the action potential the inactivation gate closes, stopping the passage on Na+ and repolarization begins
29
What are the 2 types of refractory periods?
Absolute or Relative
30
The absolute refractory period overlaps with how much of the action potential?
Almost the entire duration
31
What is the basis of the absolute refractory period?
Closure of the inactivation gates of the Na+ channel in response to depolarization
32
How long are the inactivation gates in the closed position?
Until the cell is repolarized back to its resting membrane potential
33
When does the relative refractory period begin?
It begins at the end of the absolute refractory period and overlaps primarily with the period of hyperpolarization afterpotential
34
Can an action potential be elicited during the relative refractory period?
Yes, but only if a greater than usual depolarizing (inward) current is applied
35
What is the basis of the relative refractory period?
The higher K+ conductance than is present at rest
36
Why is more inward current needed to bring the membrane to threshold for the next action potential to be initiated after the relative refractory period?
Because the membrane potential is closer to the K+ equilibrium potential
37
Describe the concept of accommodation
When a nerve or muscle cell is depolarized slowly or is held at a depolarized level, the usual threshold potential may pass without an action potential having been fired.
38
Why does accommodation occur?
Because depolarization closes inactivation gates on the Na+ channels, and if this depolarization occurs slowly enough the Na+ channels close and remain closed. So the upstroke of the action potential cannot occur because there are not enough Na+ channels available to carry inward current.
39
Accommodation is seen in what type of people?
Persons who have an elevated serum K+ concentration, or hyperkalemia
40
Propagation of action potentials down a nerve or muscle fiber occurs by the spread of ____ currents from active regions to adjacent inactive regions
local
41
As the initial segment of the nerve axon is depolarized as the result of an inward Na+ current what happens to the polarity of the membrane potential?
It is reversed and the cell interior becomes positive
42
Describe how propagation of the local current down the axon
At the negative site, positive charges inside the cell flow toward negative charges at the adjacent inactive site. This current flow causes the adjacent region to depolarize to threshold. With the transmission of each sequential action potential down the axon, the original active region is restored to its inside-negative polarity.
43
What is the time constant?
The amount of time it takes following the injection of current for the potential to change to 63% of its final value.
44
What are the 2 factors that affect the time constant?
- membrane resistance
| - membrane capacitance
45
When the membrane resistance is high the time constant _____.
increases
46
What is membrane capacitance?
The ability of the cell membrane to store charge
47
When the membrane capacitance is high, the time constant _____.
increases
48
What are the 2 mechanisms that increase conduction velocity?
- increasing the size of the nerve diameter
| - myelinating the nerve fiber
49
Myelin increases membrane _____ and decreases membrane _______ which results in increased conduction velocity
resistance
capacitance
50
What would occur if the entire nerve were coated with lipid myelin?
No action potentials could occur because there would be no low resistance breaks in the membrane across which depolarizing current could flow.
51
Nodes of Ranvier occur at intervals of _ to _ mm along the axon
-2 mm
52
What are the two ways in which information can be transmitted from one cell to another across a synapse?
either electrically or via chemical transmitter
53
Electrical synapses allow current to flow from one excitable cell to the next via what?
low resistance pathways between the cells called gap junctions
54
In what types of tissue are electrical synapses found?
in cardiac muscle and some types of smooth muscle
55
In chemical synapses there us a gap between the presynaptic cell membrane and the postsynaptic cell membrane known as what?
the synaptic cleft
56
Information is transmitted across the synaptic cleft via what?
Neurotransmitters
57
Describe the sequence of events that occur at a chemical synapse
1) an action potential travels down the motor neuron to the presynaptic terminal
2) depolarization of the presynaptic terminal opens Ca2+ channels, and Ca2+ flows into the terminal
3) an influx of Ca2+ into the presynaptic terminal causes the neurotransmitter, which is stored in the synaptic vesicles, to be released by exocytosis
4) The NT diffuses across the synaptic cleft, binds to receptors on the postsynaptic membrane, and produces a change in the membrane potential on the postsynaptic cell
58
If the neurotransmitter that crosses the synaptic cleft and binds to the postsynaptic membrane is excitatory what will happen?
It causes depolarization of the postsynaptic cell
59
If the neurotransmitter that crosses the synaptic cleft and binds to the postsynaptic membrane is inhibitory what will happen?
It causes hyperpolarization of the postsynaptic cell
60
In contrast to electrical synapses, neurotransmission across the chemical synapse is ___directional
unidirectional
61
What is the synaptic delay?
The time required for the multiple steps in the chemical neurotransmission to occur
62
What are motoneurons?
the nerves that innervate muscle fibers
63
What comprises a motor unit?
A single motor neuron and the muscle fibers it innervates
64
____ motor units are involved in fine motor activities and ____ motor units are involved in large motor activities.
Small
Large
65
The synapse between a motor neuron and a muscle fibers is called what?
the neuromuscular junction
66
Describe the sequence of events in neuromuscular transmission
1) 1) an action potential travels down the motor neuron to the presynaptic terminal
2) depolarization of the presynaptic terminal opens Ca2+ channels, and Ca2+ flows into the terminal
3) ACh is extruded into the synapse by exocytotsis
4) ACh binds to its receptor on the motor end plate
5) channels for Na+ and K+ are opened in the motor end plate
6) depolarization of the motor end plate causes action potentials to be generated in the adjacent muscle tissue
7) ACh is degraded to choline and acetate by acetylcholinesterase and choline is taken back up into the presynaptic terminal on the Na+ - choline cotransporter
67
How is acetylcholine formed?
From ecetyl conenzyme A (acetyl CoA) and by choline by the action of the enzyme choline acetyltransferase
68
What are 4 agents that alter neuromuscular function?
- botulinus toxin
- curare
- neostigmine
- hemicholinium
69
What does botulinus toxin do?
It blocks the release of ACh from presynaptic terminals, causing total blockade of neuromuscular transmission, paralysis of skeletal muscle, and eventually death from respiratory failure
70
What does curare do?
It competes with ACh for the noctinic receptors on the motor end plate, decreasing the size of the end plate potential (EPP) which can cause paralysis and death when administered in maximal doses
71
What does neostigmine do?
It prevents degradation of ACh in the synaptic cleft, which prolongs and enhances the action of ACh at the motor end plate
72
Neostigmine can be used in the treatment of what disease?
myasthenia gravis, which is characterized by skeletal muscle weakness and fatigability, in which ACh receptors are blocked by antibodies
73
What does hemicholinium do?
It blocks the reuptake into presynaptic terminals, thus deleting choline stores from the motor neuron terminal and decreasing the synthesis of ACh
74
What are the 3 types of relationships between the input to a synapse and the output?
one-to-one
one-to-many
many-to-one
75
Describe one-to-one synapses
A single action potential in the presynaptic cell causes a single action potential in the postsynaptic cell
76
One-to-one synapses are illustrated by what?
the neuromuscular junction
77
Describe one-to-many synapses
An action potential in the presynaptic cell causes a burst of action potentials in the postsynaptic cells
78
One-to-many synapses are illustrated by what?
motor neurons on Renshaw cells of the spinal cord
79
Describe many-to-one synapses
An action potential in the presynaptic cell is insufficient to produce an action potential in the postsynaptic cell, so many presynaptic cells converge on the postsynaptic cell, these pulses summate, and the sum of the inputs determines whether the postsynaptic cell will fire an action potential
80
Many-to-one synapses are illustrated by what?
Nervous system cells
81
Excitatory postsynaptic potentials are synaptic inputs that ___ the postsynaptic cell, beinging the membrane closer to threshold and closer to firing an action potential
depolarizes
82
What produces an EPSP?
Opening of Na+ and K+ channels
83
What are some examples of excitatory NTs?
- Ach
- norepinephrine
- epinephrine
- dopamine
- glutamate
- serotonin
84
Inhibitory postsynaptic potentials are synaptic inputs that ___ the postsynaptic cell, taking the membrane potential away from threshold and farther from firing an action potential
hyperpolarize
85
What are some examples of inhibitory NTs?
- gamma-aminobutyric acid (GABA)
| - glycine
86
What are the 2 ways in which presynaptic information arrives at the synapse?
- Spatially
| - Temporally
87
When does spatial summation occur?
When two or more presynaptic inputs arrive at a postsynaptic cell simultaneously
88
When does temporal summation occur?
When two presynaptic inputs arrive at a postsynaptic cell in rapid succession
89
What are 3 other phenomena that alter synaptic activity?
- Facilitation/Augmentation/Post tetanic potentiation
- Long-term potentiation
- Synaptic fatigue
90
Describe facilitation, augmentation and post tetanic potentiation
An increased release of NT into the synapse causes the response of the postsynaptic cell to be greater than expected
91
Describe long-term potentiation
Occurs in the storage of memories and involves both the release of NT from presynaptic terminals and increased sensitivity of postsynaptic membranes to the transmitter
92
Describe synaptic fatigue
Occurs where repeated stimulation produces a smaller than expected response in the postsynaptic cell, possibly resulting from the depletion of NT stores from the presynaptic terminal
93
3 criteria used to formally designate a substance as a neurotransmitter
- The substance must be synthesized in the presynaptic cell
- The substance must be released by the presynaptic cell upon stimulation
- If the substance is applied exogenously to the postsynaptic membrane at physiologic concentration, the response of the postsynaptic cell must mimic the in vivo response.
94
What are the 3 groupings of neurotransmitter substances?
- acetylcholine
- biogenic amines (norepinephrine, epinephrine, and dopamine)
- amino acids
- neuropeptides
95
3 functions of ACh
- only NT that is utilized at the neuromuscular junction
- released from all preganglionlic and most postgnglionic in the PNS
- released from presynaptic neurons in the adrenal medulla
96
What is the common precursor of the biogenic amines (norepinephrine, epinephrine, and dopamine)?
tryosine
97
What are the 2 enzymes that degrade norepinephrine, epinephrine, and dopamine?,
- catechol-O-methyltransferase (COMT)
| - monoamine oxidase (MAO)
