Week 4 Flashcards
(84 cards)
1
Q
Two types of nervous system cells
A
Neurons
Glial cells
2
Q
What is a neuron
A
Basic unit of the nervous system
3
Q
What do neurons do?
A
transmit electrical impulse signals for communication
4
Q
Important neuron properties
A
Excitability
Conductivity
Synaptic Transmission
Plasticity
5
Q
What is synaptic transmission?
A
Communicate with each other through specialized connections
6
Q
What is plasticity?
A
Form new connections or modifying existing ones
7
Q
What is excitability?
A
Respond to stimuli and generate electrical signals (action potentials) in response to these stimuli
8
Q
What do glial cells do?
A
(Neuroglia) provide structural support, insulation and nutrients for neurons
Increase speed of impulse transmission
9
Q
Cell body
A
biosynthetic center and receptive region
10
Q
What is conductivity?
A
Transmit electrical impulses over long distances, from dendrites to axon terminals
11
Q
Dendrites
A
receptive regions
12
Q
Axon
A
impulse conducting region
13
Q
Axon hillock
A
site of impulse generation
14
Q
Axon terminals
A
secretory region
15
Q
Structural classifications of neurons and where are they commonly found
A
Multipolar (brain, spinal cord – major neuron type in central NS)
Bipolar (retina, olfactory mucosa)
Unipolar (peripheral NS)
16
Q
Multipolar neurons
A
Many processes extend from cell body; all are dendrites except axon.
Most abundant (99%)
17
Q
Bipolar neurons
A
two processes extend from cell body; one is fused with dendrite, the other is an axon
rare
18
Q
Unipolar neurons
A
one process extends from cell body and forms central and peripheral processes, together comprising an axon
function as sensory neurons (in PNS)
19
Q
Functional classifications for neurons
A
Sensory (afferent) neurons
Motor (efferent) neurons
Interneurons (association neurons)
20
Q
Sensory neurons function
A
conduct impulses from sensory impulses into CNS
21
Q
Motor neurons function
A
conduct impulses away from CNS to effector organs (muscles and glands)
22
Q
Interneurons function
A
processing and integration of signals
lie between sensory and motor neurons or other interneurons
most abundant (99%)
23
Q
Aδ neuron type: mean diameter, mean conduction speed (m/s) and function
A
4µm
3-15
sharp/localised pain and temperature sensations
24
Q
B neuron type: mean diameter, mean conduction speed (m/s) and function
A
3µm
1-7
autonomic motor signals to regulate internal organs
25
C neuron type: mean diameter, mean conduction speed (m/s) and function
1µm
<2
dull/aching pain and temperature sensations
26
Aα neuron type: mean diameter, mean conduction speed (m/s) and function
15µm (thick myelin sheath)
80-120
motor signals to skeletal muscles
27
Aβ neuron type: mean diameter, mean conduction speed (m/s) and function
8µm
33-75
touch and proprioception sensations
28
Neuron negative membrane potential in a resting neuron
typically around -70 millivolts (mV) inside the cell relative to the outside
-ve = unequal distribution of charge across the membrane. relatively more negative charge on inner side of membrane
29
What do neurons use changes in membrane potential for?
Neurons use rapid changes in their membrane potential as signals to receive, integrate and send information
30
Two key factors that influence negative membrane potential of neurons
1. The high concentration of potassium (K+) ions inside and high concentration of sodium (Na+) ions outside the cell, which is setup by the sodium-potassium pump.
2. The cell membrane is selectively permeable, allowing specific ions to move across through ion channels (impermeable to ions). At rest, the neuron membrane is more permeable to potassium (K+) ions through leak channels that allow K+ to move down its concentration gradient, leading to an efflux of K+ from the cell.
31
What is pumped in and out of the cell by a sodium-potassium pump?
Continuously pumps Na+ out and K+ into cell
32
What powers the sodium-potassium pump and where does this energy come from?
ATP (hydrolysis of ATP)
33
How many ions in sodium potassium pump moved for every ATP?
For each ATP broken down, 3 Na+ ions moved out and 2 K+ ions in.
34
What creates a concentration gradient in sodium potassium cell and what type of movement is it?
Cell is depleted of sodium, creates an electrical gradient and concentration gradient --> many physiological functions
Active transport --> moves substance across membrane against concentration gradient (low to high concentration from inside to outside cell)
35
What enzyme is found in the sodium potassium pump?
Na+/K+ -ATPase
36
How does the sodium potassium pump work?
1. 3 cytoplasmic sodium (Na+) ions bind to the pump
2. Na+ binding promotes hydrolysis of ATP, into ADP, which releases energy
3. The Na+/K+ pump changes shape within the membrane and releases Na+ outside of the cell
4. 2 extracellular K+ bind to pump
5. K+ binding triggers release of phosphate. The dephosphorylated pump resumes its original shape
6. Releases K+ inside the cell
37
What is the resting membrane potential?
Voltage that exists across the membrane of all cells (inc. neurons)
38
What does a voltmeter measure?
The electrical potential inside the cell relative to the outside.
39
In a typical neuron at rest, what is the concentration of sodium and potassium ions in the intracellular environment?
Sodium -- 15mM (lower concentration)
Potassium -- 140mM (higher concentration)
40
In a typical neuron at rest, what is the concentration of sodium and potassium ions in the extracellular environment?
Sodium -- 140mM (higher concentration)
Potassium -- 5mM (lower concentration)
41
How do ions move through the phospholipid bilayer?
Protein channels
Carrier Proteins (Na+/K+ pump)
42
What is the resting membrane potential primarily due to the activity of?
1. Sodium/potassium pump
2. Leak channels (always open), selectively permeable to potassium or sodium -- more potassium leak channels than sodium so more potassium diffuses out of the cell
43
Why is the electrical charge inside the cell more negative than outside the cell in a resting neuron?
There is a high level of sodium ions outside the cell compared with potassium inside the cell.
Leaky channels allow the movement of more potassium than sodium to diffuse across the membrane and therefore more positive potassium ions will leave the cell.
The potassiums are brought back inside while sodium ions are removed from the cell by the Na+/K+ pump. This helps maintain the unequal distributions of ions inside and outside the neuron and establishes a negative potential across the neuron of ~-70mV at rest.
44
What is an action potential?
long-distance impulse signals that travel along axons
45
total amplitude and strength of action potentials
brief reversals of membrane potential with a total amplitude (change in voltage) of about 100 mV (from 270 mV to 130 mV). They are always have the same strength.
46
What causes an action potential?
results from a rapid change in the permeability of the neuronal plasma membrane to Na+ and K+. permeability changes as voltage gated ion channels open and close. (changes in the number of open channels)
47
Where is the action potential generated?
axon hillock (density of voltage gated sodium channels are at greatest)
48
What two processes end depolarisation?
1. Inactivation of the sodium voltage gated channels
2. Opening of potassium voltage gated channels
49
What does repolarisation restore?
The electrical balance, but not the uneven distribution of ions
50
What does the Na+/K+ pump restore during hyperpolarisation?
redistributes the uneven distribution of ions
51
What is the absolute refractory period?
A neuron cannot generate an action potential (even with a very strong stimulus) because sodium cannot move in through inactive channels and potassium continues to move out through voltage gated channels.
52
What is the relative refractory period?
A strong stimuli can generate an action potential (depolarised to a value more positive than normal threshold). This can occur since some of the sodium channels are still inactive, but some have reset and returned to their resting state, and some potassium channels are still open.
53
What is the order of neuronal action potential?
Threshold initiation
Depolarisation
Repolarisation
Hyperpolarisation
54
Threshold initiation
Signals from the dendrites and cell body reach the axon hillock. As the axon hillock depolarises, voltage-gated channels for sodium open rapidly, increasing membrane permeability to sodium. Sodium diffuses down its concentration gradient into the cell. If the stimulus at the axon hillock is great enough, the neuron depolarises by about 15mV to a point called "threshold" (-55mV). At threshold, an action potential is generated.
55
Depolarisation
More sodium voltage-gated channels open. This causes more sodium to flow into the cell, which in turn causes the cell to depolarise further and opens more voltage-gated sodium channels. This positive feedback loop produces the rising phase of the action potential. This ends with the inactivation of the Na+ voltage-gated channels and the opening of K+ voltage-gated channels.
56
Repolarisation
Potassium diffuses out of the cell as the potassium voltage-gated channels open. With less sodium moving into the cell and more potassium moving out of the cell, the membrane potential becomes more negative, moving back towards the resting value. Repolarisation restores the electrical balance.
57
Hyperpolarisation
Excessive potassium continues to diffuse out of the cell, causing the membrane potential to become more negative than the resting membrane potential. All the potassium channels are closed and the sodium-potassium pump redistributes the ions to their original, resting state levels.
58
What is the same for all action potentials?
Same size/amplitude (all or none)
59
How does the central nervous system tell the difference between a weak stimulus and a strong one?
Neurons code the intensity of information by the frequency of action potentials.
60
How does the central nervous system control different strengths of muscle contraction?
Neurons code the intensity of information by the frequency of action potentials.
61
Action potential frequency in sensory neurons
mild stimulus = low frequency with few action potentials
strong stimulus = high frequency with many action potentials
62
Action potential frequency in motor neurons
The motor cortex of the brain controls the strength of muscle contraction by regulating the frequency of action potentials sent along motor neurons
higher frequency = greater force of contraction
63
What is conduction velocity?
speed that an action potential is propagated along an axon
64
Where is fast conduction velocity?
Essential neural pathways
Reflexes
65
Where is slow conduction velocity?
Serve internal organs
Digestive tract, glands and blood vessels
66
What two things does conduction velocity depend on?
1. diameter of axon
2. how well axon is insulated with myelin
67
Diameter of axom
Fast = less resistance to flow of local current
Slow = conduct signals slower
68
How well axon is insulated with myelin
Continuous conduction = non-myelinated and slow
Saltatory conduction = myelinated = fast = acting as insulated and prevents nearly all of charge from leaking from the axon, allowing membrane voltage to change more rapidly
charge only flows at nodes of ranvier (gaps in myelin sheath), where action potential is generated, nearly all voltage gated sodium channels in gaps
when action potential generated, local depolarising current does not dissipate and moves rapidly to next myelin sheath, triggering new action potential
electrical signal appears to jump from gap to gap
69
how fast do action potentials travel?
The conduction velocities of neurons vary widely. Nerve fibres that transmit impulses most rapidly (100 m/s or more) are found in neural pathways where speed is essential, such as those that mediate postural reflexes. Axons that conduct impulses more slowly typically serve internal organs (the gut, glands, blood vessels) where slower responses are not a handicap
70
What is a synapse?
A synapse is a junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell.
71
What happens at a synapse between two neurons?
ACtion potential translated into a chemical message by presynaptic neuron. Chemical diffuses across synaptic cleft until it reaches second neuron. Second neuron then translates chemical signal back into action potential. CHemical signal degrades and new chemical signal can now travel down the neuron until it reaches another nerve, muscle or gland.
72
What happens at the synapse between two neurons?
1. Action potential arrives at axon terminal
2. Voltage gated calcium channels open and calcium enters the axon terminal
3. Calcium entry causes synaptic vesicle to release neurotransmitter
4. Neurotransmitters diffuse across synaptic cleft and bind to specific receptors on postsynaptic membrane
5. Binding of neurotransmitter opens ion channels, resulting in graded potentials
6. Neurotransmitter effects are terminated by reuptake, degradation or diffuse away.
73
What are post synaptic membranes also known as?
Chemically (neurotransmitter) gated ion channels
74
What to post synaptic membranes do?
Convert chemical signals to electric signals
75
What are graded potentials?
Local changes in membrane potential that are graded (vary in strength)
76
What happens when a post synaptic channel is opened?
Effect can be:
- Excitatory (depolarises)
- Inhibitory (hyperpolarises)
77
EPSP
Excitatory PostSynaptic Potential
78
What are Excitatory PostSynaptic Potential?
Excitatory ion channels are permeable to Na+ and K+
79
IPSP
Inhibitory PostSynaptic Potential
80
What is inhibitory PostSynaptic Potential?
Inhibitory ion channels are permeable to Cl- and K+
81
Summation
additive (EPSP and IPSP) changes to the post synaptic membrane potential
82
Two types of summation
spacial summation
temporal summation
83
Spacial summation
stimuli from multiple synapses
84
Temporal summation
Increased frequency of stimuli at the same synapses
