Chapter 4 - Neural Communication Flashcards
Electricity
electrons flowing from negative pole to positive pole via conducting medium
Current
flow of electrons from negative to positive pole
Electric Potential
difference in charge between negative & positive pole
- relative charge
- volts (V)
Linking Electricity & Neural Activity
-
History
- People (6)
1) Stephen Gray
2) Luigi Galvani
3) Gustv Fritsch & Eduard Hitzig
4) Bartholow
5) Wilder Penfield
6) Richard Caton
1) Stephen Gray
1731
- found that flying boy conducts electricity
- speculated that electricity is neural messenger
Findings from what suggested that neurons send electrical messages?
Electrical stimulation studies
Electrical Stimulation Studies suggesting neurons send electrical messages
2) Luigi Galvani
18th C
observed twitching frog legs on wire in market during lightning storm
- suspected that electricity was activating muscles
- confirmed speculation by stimulating nerves using electricity in lab
Electrical Stimulation Studies suggesting neurons send electrical messages
3) Gustav Fritsch & Eduard Hitzig
19th C
demonstrated that electrical stimulation of neocortex caused movement
- identified motor cortex & mapped motor homonculus in animals
Electrical Stimulation Studies suggesting neurons send electrical messages
4) Bartholow
1874
1st to electrically stimulate human brain
- patient with exposed parietal lobe
- reported pain & tingling sensation
Electrical Stimulation Studies suggesting neurons send electrical messages
5) Wilder Penfield
electrically stimulated people having surgery for epilepsy
- attempted to provoke “aura” (warning of impending seizure) that precedes seizure to find region of abnormal activity
- also identified areas associated with language to try to avoid removing these areas
- mapped out motor & sensory homonculus
Findings from Electrical _____ Studies also suggested neurons send electrical messages
6) Richard Caton
Electrical Recording Studies
20th C
1st to measure electrical currents of brain with voltmeter by placing electrodes on skull
→ Electroencephalogram (EEC)
Electroencephalogram (EEG)
standard tool for detecting electricaly activity in brain using electrodes attached to scalp
→ used to:
- monitor sleep stages
- record waking activity
- diagnose disruptions (such as in epilepsy)
Hermann von Helmholtz
stimulated nerve leading to muscle & measured time muscle took to contract
- 30-40 m/s
Microelectrodes
in brain to stimulate/record more precisely
- i.e. specific region/cell
Ion Movement & Electrical Charge
chemicals in ICF & ECF differ & are kept seperate by cell membrane
- chemicals are electrically charged (IONS)
Ions
electrically-charged chemicals
cations & anions
Cations
- (2)
positively-charged ions
- Sodium (Na+)
- Potassium (K+)
Anions
- (2)
negatively-charged ions
- Chloride (Cl-)
- Protein molecules (A-)
(3) factors influence movement of ions in/out of cell
1) Diffusion
2) Concentration Gradient
3) Voltage Gradient
Factors Influencing Ion Movement In/Out of Cell
1) Diffusion
movement of ions from area of [high] to area of [lower] through random motion
Factors Influencing Ion Movement In/Out of Cell
2) Concentration Gradient
differences in [substance] among regions that allow diffusion from area of [higher] to area of [lower]
Factors Influencing Ion Movement In/Out of Cell
3) Voltage Gradient
difference in charge between regions
- allow flow of current if regions are connected
Resting Potential
- definition
- due to?
electrical charge across membrane at rest
- greater negative charge on inside relative to outside
- store of PE
- - 70 mV
due to unequal distribution of ions inside & outside
Features of Cell Membrane Contributing to Resting Potential (4)
(How is the distribution of ions inside & outside of cell maintained?)
via channels, pumps & gates
- proteins cannot leave cell (large & - charged)
- channels allow K+ & Cl- to move in & out of cell more freely
- Gated channels prevent Na+ from entering cell
- Na/K Pumps → 3 Na+ out & 2 K+ in
Movement of K+ & Cl- across membrane
K+ → in
- attracted by (-) charge
Cl- → out
- stays in more (+) environment
Concentration of Sodium in ICF & ECF
[Na]out = 10x [Na]in
10x more Na+ outside than inside cell
Graded Potentials
small voltage fluctuations restricted to vicinity where ion concentrations change
-
change is proportional to stimulation
- Δ[ion] → Δ membrane potential
(2) types of Graded Potentials
1) Excitatory Postsynaptic Potentials (EPSPs)
2) Inhibitory Postsynaptic Potentials (IPSPs)
1) Excitatory Postsynaptic Potentials (EPSPs)
- define
- effects (2)
- due to?
GPs (brief graded depolarization) of membrane in response to stimulation
- makes membrane potential more positive
- inside more + than outside
- ↑ increase likelihood of AP
- due to opening of Na+ channels → Na+ enters
2) Inhibitory Postsynaptic Potentials (IPSPs)
- define
- effects? (2)
- due to? (2)
GPs (brief, graded hyperpolarization) of neural membrane in response to stimulation
- makes membrane potential more negative
- inside even more (-) than outside
- ↓ decreases likelihood of AP
- due to: (caused by binding of NT)
- opening of K+ channels → K+ out
- OR opening of Cl- channels → Cl- in
Hyperpolarization
small increase in charge across membrane due to
Cl- influx & K+ outflow
- IPSPs → ↓ likelihood of firing
Depolarization
small decrease in charge across cell membrane due to Na+ influx through open gated channels
- EPSP → ↑ likelihood of firing
GABA
major inhibitory NT
- receptors have Cl- channels
- Cl- influx
alcohol binds to GABA sites
Glutamate
major excitatory NT
Graded Potentials in:
a) Sensory Neuron
b) Interneurons & Motor Neurons
a) GP produced by external stimuli → only EPSPs
b) GP produced by other neurons
* EPSPs & IPSPs
Action Potential
large, brief reversal in polarity of membrane
- neuron that is stimulated to point of transmitting a signal is in action
- 200 /s
Threshold Potential
voltage on neural membrane at which an AP is triggered by opening of Na & K vs-channels
~ -50mV
Phases of an AP (5)
1) Resting
2) Depolarization
3) Repolarization
4) Hyperpolarization
5) Resting
Phases of AP
1) Resting
vs-Na channels & vs-K channels are closed
Phases of AP
2) Depolarization
enough EPSPs & Na+ inflow
membrane potential reaches threshold
- inside charge = ~-50 mV relative to outside
→ AP triggered
- vs-Na & K channels open
- vs-Na channels open faster → Na+ influx
- +30 mV
- vs-Na channels open faster → Na+ influx
Phases of AP
3) Repolarization
- membrane potential of 30 mV triggers closing of non-vs Na channel
- vs-K+ channels slower to open but remain open longer
- K+ outflow reverses depolarization
- - 70 mV
Phases of AP
4) Hyperpolarization
vs-Na channels closed (non-vs Na channels reopened)
Since vs-K channels close slower than Na channels, too much K+ leaves, causing hyperpolarization
→ -73 mV
Phases of AP
5) Resting
Resting potential restored by Na/K pump
- active transport of 3Na+ out & 2K+ in
Refractory Periods (2)
- Absolute RF
- Relative RF
Absolute Refractory Period
period during repolarization phase in which another AP cannot be triggered
- since Gate 2 of Na+ channel (non-vs) is closed
Relative Refractory Period
period during hyperpolarization in which more stimulation is needed to trigger another AP
- since membrane potential is further from threshold
- ~ -73 mV
- non-vs Na+ channel (gate 2) is open
Nerve Impulse
propogation of AP along axon membrane
→ each AP causes adjacent point on membrane to reach threshold potential
- APs are of a constant size
(2) Practical Uses of Refractory Periods
1) limits maximum rate of AP to ~ 200 per second
2) prevent AP from reversing direction & returning to origin
* creates single, discrete impulses traveling away from point of initial stimulation
Backpropagation
- role in?
phenomenon in which AP of neuron creates voltage spike at end of axon & back through dendrites (from which original input current originated)
- may play role in plastic changes that underlie learning
Rate of Nerve Impulses
- affected by? (2)
1) Width of axon
2) Amount of myelin
Rate of Nerve Impulse →
1) Width of axon
↑ diameter → ↑ total volume for charges to flow through & ↓ resistance
Rate of Nerve Impulse
2) Amount of Myelin
Myelin creates insulating barrier to flow of ionic current
Nodes of Ranvier are close enough that AP at one node can trigger opening of vs-gates at adjacent node, allowing AP to “jump” from node to node (saltatory conduction)
Saltatory Conduction
propagation of AP at successive nodes of Ranvier
Why are APs not produced on motor neuron’s cell body?
cell body membrane of most neurons does NOT contain vs-channels
- stimulation must reach axon hillock, which is rich in vs-channels
Whether or not threshold is reached & AP is generated depends upon? (2)
Temporal Summation
Spatial Summation
For summation of inputs to occur….
Both Temporal & Spatial Summation must occur
Temporal Summation
GPs that occur at approximately the same time on membrane are summated
Spatial Summation
GPs that occur at approximately the same location on membrane are summated
Role of IONS in Summation
EPSP → Na+ influx
added to
IPSP → K+ outflow
if spatially & temporally close together
- If summed EPSPs & IPSPs charge membrane to threshold level at axon hillock, AP travels down axon membrane
Temporal & Spatial Summation are how neurons…
integrate info recieved from other neurons
Where does summation occur?
Soma & Dendrites recieve EPSPs & IPSPs
- inputs are summated here
Where is AP initiated?
Axon Hillock is rich in vs-gated channels
→ AP is initiated here
How does sensory stimuli produce APs?
Sense receptors detect physical energy (chemical & mechanical stimuli) & convert E into APs
- ion channels chemically/mechanically opened, which activate vs-gated ion channels to produce APs
Sense Receptors
sensory nerve endings that responds to stimuli by converting its energy into an AP (sensory transduction)
What type of senses are:
a) olfaction (smell)
b) gustation (taste)
c) vision
d) somatosensation
e) audition
olfactory, gustation & vision → chemical senses
somatosensation & audition → mechanical senses (produced by physical movement)
Process of Touch Receptors
mechanical displacement of hair causes dendrite of touch neuron encircling base of hair to stretch
- opens stretch-sensitive channels in dendrite membrane
- Na+ influx depolarizes dendrite to threshold
-
vs-channels (Na & K) activated
- open & initiate nerve impulse
- conveys touch info to brain
- open & initiate nerve impulse
Process of Receptors in Eye
light particles strike chemicals in receptors
cause chemical change → activates ion channels in relay neurons’ membrane
Stretch-Sensitive Channel
ion channel on tactile sensory neuron that activates in response to stretching of membrane & initaties nerve impulse
Why are Somatosensory Neurons special?
Unipolar → 1 pole off of cell body
dendrites are myelinated & propogate APs
How do Motor Neurons facilitate muscle movement?
axon of motor neurons synapses with target muscle
axon terminal releases ACh onto end plate of muscle membrane
- opens transmitter-sensitive channels
- Na+ in, K+ out → depolarizes muscle to threshold
- adjacent vs-channels open → produce AP
- muscle contracts
End Plate
Receptor-Ion complex on muscle that is activated by release of ACh from terminal of motor neuron
Transmitter-sensitive channel
Receptor complex with both a receptor site & pore through which ions can flow
Neurons communicate using?
Electrochemical messages
- ‘Electric’ aspect caused by change in [C] of ions found inside & outside cell
