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L3 Flashcards

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1
Q

What happens when a patch of excitable membrane generates an action potential?

A

It causes an influx of Na+ and reverses the potential difference across the membrane

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2
Q

What is the direction of potential change during depolarization?

A

From “-“ on the inside to “+” on the inside

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3
Q

What serves as the source of depolarizing current for adjacent membrane?

A

The local reversal in potential

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4
Q

What voltage change must occur for an adjacent patch of membrane to generate another AP?

A

From -70 mV to -55 mV (threshold)

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5
Q

What is the nature of the depolarization current?

A

Pure electromagnetism that can occur in anything that conducts current

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6
Q

Why are most cells not ‘excitable’?

A

They lack voltage-gated Na+ channels

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7
Q

What types of cells generate propagating action potentials?

A

Neurons with long axons and muscle cells

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8
Q

What is an axon?

A

A long extension of the cell body (like a wire) that carries AP away to another location

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9
Q

Can non-excitable cells conduct any type of current?

A

Yes, they can conduct passive currents but cannot generate APs

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10
Q

What is the difference between passive current and action potential?

A

Passive current is electromagnetic spread while AP requires voltage-gated Na+ channels to actively generate a signal

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11
Q

Where is the trigger zone located in a neuron?

A

Between the axon hillock and initial segment

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12
Q

Where is the action potential first generated in a neuron?

A

In the trigger zone

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13
Q

What is the path of an action potential after generation?

A

It propagates from the trigger zone to the axon terminal

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14
Q

How does biological tissue compare to copper wire in conducting electricity?

A

Biological tissue is a poor conductor, losing significant signal over distance

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15
Q

What happens to a voltage signal as it travels along a biological membrane?

A

It loses amplitude due to current leakage across the membrane

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16
Q

How does the capacitive property of the membrane affect signal transmission?

A

It causes loss of sharp, high-frequency components, making signals rise and fall more gradually

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17
Q

What are two main challenges in signal transmission along neurons?

A

Signal loss and distortion as current travels along the membrane

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18
Q

What is the key question regarding signal transmission in neurons?

A

How to prevent signal loss and move signals without degradation

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19
Q

What is the length constant (λ) in neural conduction?

A

A measure of how quickly a potential difference disappears as a function of distance

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20
Q

What does conduction velocity of an action potential depend on?

A

The membrane length constant (λ)

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21
Q

What happens when λ is larger?

A

Potential differences are carried further without losing their original value

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22
Q

How does increasing axon diameter affect λ?

A

Increases λ by reducing internal resistance, resulting in less voltage loss

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23
Q

What’s the analogy used to explain increased axon diameter?

A

Multiple straws for orange juice - wider passage means less resistance

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24
Q

How does increasing membrane resistance affect λ?

A

Increases λ by reducing current leakage, forcing current down the membrane

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25
What's the analogy used to explain increased membrane resistance?
Wrapping a leaky straw to prevent leaking and drink faster
26
What are the two main mechanisms to improve λ in neurons?
Increasing axon diameter and increasing membrane resistance
27
What happens to current flow when membrane resistance is higher?
Less current leaks out, and more is forced down the axon along the membrane
28
What is the length constant (λ) in neurophysiology?
The distance at which voltage drops to about 37% of its original value
29
What components define the length constant?
Internal resistance, extracellular fluid resistance, and membrane resistance
30
Why is extracellular fluid resistance often dropped from the length constant equation?
Because it's not adjustable and is relatively low compared to other factors
31
What are two ways to increase lambda (λ)?
Decrease internal resistance (by increasing axon diameter) or increase membrane resistance
32
How does increasing axon diameter affect the length constant?
It decreases internal resistance, providing more space for current to pass through, thus increasing λ
33
Why does axon B have a higher lambda than axon A in the diagram?
Axon B has greater membrane resistance, reducing current leakage
34
What causes voltage loss as signals travel down an axon?
Internal resistance from organelles in the cytoplasm and leakage across the membrane
35
Why is increasing λ important for neural signaling?
It allows depolarizing current to spread a greater distance without significant signal degradation
36
What happens to the signal in an axon with low membrane resistance?
The signal loses voltage rapidly due to increased current leakage across the membrane
37
What is the most efficient means of increasing conduction velocity?
Increasing membrane resistance through myelination
38
What are glial cells and what is their function?
Cells that assist the nervous system by providing nutrition and increasing membrane resistance
39
Which specialized glial cells form myelin in the PNS?
Schwann cells
40
Which specialized glial cells form myelin in the CNS?
Oligodendrocytes
41
How many layers typically wrap around a myelinated axon?
50-100 layers
42
How does myelin affect current flow in an axon?
It reduces leakage of current by increasing membrane resistance
43
What percentage of axons are myelinated?
About 20%
44
Which types of signals benefit most from myelination?
Signals that need to travel fast or far
45
How does a Schwann cell differ from an oligodendrocyte in myelination?
Schwann cells wrap around a single portion of one axon, while oligodendrocytes wrap around multiple axons individually
46
By how much does myelination increase conduction efficiency?
By approximately 25-fold
47
Why can't all axons be myelinated?
It takes up too much space and resources
48
What happens to the Schwann cell cytoplasm during myelination?
It gets squeezed out as the cell wraps around the axon
49
Why is changing axon diameter not the preferred method to improve conduction?
It's physiologically very difficult to accomplish
50
How do oligodendrocytes extend their processes for myelination?
They extend multiple processes "like an octopus" to wrap around multiple axons
51
What are Nodes of Ranvier?
Small gaps between adjacent portions of myelin sheath where the axon membrane is exposed
52
Why are Nodes of Ranvier important for action potentials?
They contain voltage-gated sodium channels necessary for generating action potentials
53
Is myelination continuous along an axon?
No, myelination occurs in sections with gaps (Nodes of Ranvier) between them
54
What happens at the Nodes of Ranvier during signal transmission?
Action potentials are generated here as currents cross the membrane and cause depolarization
55
What disease is associated with myelin sheath damage?
Multiple Sclerosis (MS)
56
What symptoms can result from myelin damage in MS?
Visual disturbances, numbness, thinking/memory problems, and slowed neural transmission
57
What is saltatory conduction?
The "jumping" mode of conduction where APs jump from one Node of Ranvier to the next
58
In myelinated axons, where are action potentials generated?
Only at the Nodes of Ranvier where the membrane is excitable
59
What happens in the myelinated portions between Nodes of Ranvier?
No APs are generated; current spreads passively
60
Why is saltatory conduction more efficient than continuous conduction?
It's faster because APs "jump" between nodes rather than traveling along the entire axon
61
What makes the Nodes of Ranvier excitable compared to myelinated regions?
They contain voltage-gated sodium channels necessary for AP generation
62
How does current travel in the myelinated portions between nodes?
Passively, without generating new action potentials
63
How far can the depolarizing current from one node travel?
It can travel down the axon for 5-10 nodes
64
What voltage must the nodes reach to generate new action potentials?
-55 mV (threshold potential)
65
What happens between nodes during saltatory conduction?
Passive spread of depolarizing current occurs in the myelinated portions
66
What is the role of myelin in saltatory conduction?
Myelin prevents leakage of current across the membrane between nodes
67
Why is the furthest node (10th node) important in saltatory conduction?
It serves as the depolarizing force for the next 10 nodes, continuing the propagation
68
What is the "safety factor" in saltatory conduction?
The ability of the depolarizing current to skip past poisoned/damaged nodes and reach the next healthy patch of membrane
69
How much of the axon must be damaged to stop an action potential?
A fair length of the membrane must be destroyed to stop the AP in its track
70
Why do unmyelinated axons have slower conduction velocity?
They lack extensive wrapping, resulting in more current leakage, small axon diameter, and low membrane resistance
71
How are voltage-gated channels arranged in unmyelinated axons?
Na+ and K+ voltage-gated channels are intermixed throughout the membrane.
72
What percentage of axons are unmyelinated?
The majority of axons
73
How do Nodes of Ranvier differ from unmyelinated axons in channel distribution?
Nodes of Ranvier have mainly voltage-gated sodium channels, while unmyelinated axons have intermixed sodium and potassium channels
74
What is a Remak Bundle?
A structure where Schwann cells and oligodendrocytes engulf multiple unmyelinated axons (5-30) without winding around them
75
How does a Remak Bundle affect membrane resistance?
It improves membrane resistance somewhat, providing limited insulation
76
What is the conduction velocity of the fastest myelinated axons?
About 80 meters per second
77
What is the conduction velocity of unmyelinated axons?
About 2 meters per second
78
How fast can action potentials travel in cats compared to humans?
Up to 120 meters per second in cats (humans have more axons instead of faster ones)
79
What happens to an action potential at the end of an axon?
It reaches the axon terminal and continues generating depolarizing currents
80
Why can't an action potential travel backward after reaching the axon terminal?
Due to the refractory period where voltage-gated Na+ channels are inactivated
81
What is the final fate of an action potential at the axon terminal?
It dies out because it can only travel in one direction (forward)
82
Why does current from an action potential only move forward toward the terminal?
The region behind is undergoing refractory period, preventing AP generation in that direction
83
What is a synapse?
A functional association of a neuron with another neuron or effector organ
84
What are effector organs in the context of synapses?
Muscles or glands
85
What are the two main types of synapses?
Electrical and chemical.
86
What distinguishes electrical synapses from chemical synapses?
Electrical synapses use direct connections while chemical synapses use neurotransmitters
87
What are gap junctions in electrical synapses?
Connections where adjacent membranes are about 35Å apart, bridged by connexins that allow small ions to cross directly
88
What protein forms the bridge in gap junctions?
Connexin proteins
89
What is a key advantage of electrical synapses?
Rapid bidirectional communication between cells
90
Where are gap junctions commonly found in the body?
Between neurons, glial cells, and in cardiac muscle cells
91
Why do cardiac muscle cells use gap junctions?
For quick and reliable coordinated pumping of blood
92
What is the width of a synaptic cleft?
About 200Å wide
93
What are the two main components of a chemical synapse?
Presynaptic surface (bouton with vesicles) and postsynaptic membrane with receptors
94
What happens when an action potential arrives at the axon terminal?
It results in the release of neurotransmitters into the synapse
95
What specialized structures are found on the postsynaptic membrane?
Specific protein receptors that bind neurotransmitter molecules
96
What are boutons?
Endings of axons filled with vesicles containing neurotransmitters
97
How are neurotransmitters released from vesicles?
Through exocytosis triggered by action potentials
98
What are vesicles in the context of neurons?
Tiny organelles containing neurotransmitters that release into extracellular fluid
99
What is the primary trigger for exocytosis?
Ca++ ions
100
What type of channels in the bouton membrane allow Ca++ entry?
Voltage-gated Ca++ channels
101
At what threshold voltage do voltage-gated Ca++ channels open?
-50mV
102
What is the state of vesicles before release?
Docked in preparation for fusion
103
What triggers vesicle fusion with the membrane?
Calcium
104
What is the probability range of an AP releasing a vesicle?
10-90% chance of releasing 1 vesicle
105
Why are chemical synapses considered "processing stations"?
Because vesicle release is probabilistic, not guaranteed
106
Can neurotransmitter release be 100% guaranteed?
No, it's never 100%
107
What happens to signal transmission at chemical synapses?
Despite careful signal transmission, release at the end is chance-based
108
How does the probabilistic nature affect synaptic function?
Synapses function more as processing stations than secure signal relays

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