Action potentials
I.
Neurons
a. CC neuron structures
II.
Membrane Potential
a. SEQ, IOV generating MP
b. CC, IOV changes to MP
III.
Action Potentials
a. SEQ, IOV AP
IV.
Conduction of Aps
a. CC, SEQ, IOV continuous and Saltatory conduction
--------------------------------------------------------------------------------------------------------------------I.
II.
Neurons
a. Structure and Function
i. Basic functional unit of NS
ii. Conduct electrical signals
iii. Various shapes and sizes
iv. 1. )A neurons is:
1. A cell body- organelles
2. Cytoplasmic connections
v. 2.) Dendrites
1. Receive information
2. Send signal to cell body
3. Short, branched
vi. 3.) Axons
1. 1 long axon per neuron
2. Transmit neural impulse away from cell body
3. Send signal to
a. Another neuron
b. An effector (muscle gland)
vii. 4.) Axon Hillock
1. Signals are generated here
2. Base of axon @ cell body
b. Neuron Anatomy
i. 5.) Synaptic Terminal
1. At end of axon branches
ii. 6.) Nerve- axons of many neurons w/connective tissue
Membrane Potential
a. introduction
i. selectively permeable membrane
1. all animal cells have
2. only certain ions cross
III.
ii. membrane potential (MP)= difference in %ions inside and outside
membrane
1. The possibility of doing work
b. Voltage
i. Only excitable cells can generate fast changes in MP
1. Nerves
2. Muscles
ii. Voltage= measures MP
c. Resting Potential (RP)
i. At rest, neuron has electrochemical gradient
1. (-) inside, (+) outside
ii. Concentration of K+ inside, Concentration of Na+ outside
iii. RP of cell= -70mV (varies, -60mV to -80mV)
iv. Process to achieve RP:
1. Neurons produce organic anions  inside slightly (-)
2. Na+/K+ pump pushes 3 Na+ ions out, 2 K+ ions in  net outflow
of positive ions
a. Increase membrane potential
b. Inside more (-)
3. Na+/K+ pump  concentration of K+ inside, concentration of
Na+ outside
4. Ion channels- ions diffuse across membrane  carry electrical
charge w/diffusion event
a. Membrane 100x more permeable to K+ thank Na+
b. K+ ions flow out  inside becomes more (-)
d. Changes to membrane potential
i. Hyperpolarization
1. Inhibitory
2. -70 mV to -90 mV
3. Membrane moves below resting potential
4. Lower ability to generate a signal
ii. Depolarization
1. Excitatory
2. -70 mV to -50 mV
3. Membrane becomes more positive
4. Greater ability to generate a signal
iii. Flashback
1. Fast block to polyspermy
2. Na+ ions channels opened
3. Na+ rushed into egg
4. Egg depolarized
e. Threshold Potential
i. MP required to trigger action potential
ii. -55 mV for most neurons
Action Potentials (AP= electrical signal within neuron)
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Either happens or doesn’t
Always the same voltage change
Intensity of sensation/ message depends on
o Number of neurons stimulated
o Frequency of stimulus
o Not strength of AP
A. Change in MP
a. Induces voltage- gated channels
b. Depolarization crosses threshold
c. Large change in MP
B. Voltage- Gated ion channels
a. Ion channels that open or close based on voltage
b. Allow passage of specific ions
c. Facilitated diffusion
C. Events of Action Potentials
a. Neuron at rest
b. Stimulus
c. Rising phase
d. Falling phase
e. Undershoot
f. 1.) Neuron at rest
i. Voltage-gated Na+ channels closed
ii. No stimulus
iii. MP= -70 mV
g. 2.) Stimulus
i. Voltage- gated Na+ channels open  Na+ enters axon
ii. Membrane depolarizes  MP gets closer to -55 mV
iii. Magnitude of change depends on strength of stimulus
h. Threshold isn’t always reached…
i. Small stimulus= few voltage-gated Na+ channels will open
ii. Strong stimulus= lots of channels open
iii. Threshold reached  action potential
iv. No threshold  stays at rest
i. 3.) Rising phase
i. Membrane very permeable to Na+
ii. Rapid influx of Na+  inside of cell (+)
iii. Membrane depolarizes  +35 mV
j. 4.) falling phase
i. Membrane impermeable to Na+ (channels close)
ii. K+ channels slowly open
iii. Membrane repolarizes
iv. Refractory period= wait period – Na+ channels must reset
k. 5.) undershoot
i. Na+ channels closed
ii. K+ channels partially open
IV.
iii. Hyperpolarization= MP is more (-) than at resting potential
iv. K+ channels close = return to RP
Conduction of action potentials
a. Introduction
i. Signal propagates as series of Aps along axon
ii. Axon hillock  terminal
iii. Voltage shifts in one region  triggers Na+ channels further down
iv. Area behind in refractory period
v. Unidirectional signal
b. Continuous conduction
i. Occurs in unmyelinated axons
ii. Every spot on axon depolarizes, repolarizes
c. Salutatory conduction
i. Occurs in myelinated axons
ii. Requires myelin sheath  faster conduction of AP
iii. Internodes: regions covered in myelin  no depolarization
iv. Nodes of ranvier: no myelin  depolarization
v. Salutatory conduction= depolarization only at nodes
1. Signal “jumps” from node to node
2. More energy efficient