2.1 Neuron Properties Flashcards
Dendrites
Receive synaptic INPUTS from neuronal axons
Cell Body (neuron)
Same organelles as any other cell
Trigger zone
Action potentials are started here, axon runs with them
Axons
Fire rapid impulses (APs) to synaptic terminals
Pre-synaptic terminal
Releases output signal to “post,” releases neurotransmitters at synapse
Multipolar neurons are found in…
Brain, spine, motor
Bipolar neurons are found in…
Retina, ear, nose
Unipolar neurons are…
Sensory receptor neurons
3 things necessary for cellular membrane potentials
1) Ions can be pumped through ion carriers embedded in cell membrane
2) Ions are maintained at different concentrations on either side of cell membrane (concentration gradient)
3) The ion (thus the potentials) can change by flowing through ion channels
Major ions that are pumped in neurons
Na+, K+, Cl-, Ca2+
What happens if there is not a constant source of energy for ion pumps?
Gradients erode very quickly, if you die from running out of energy it is probably because these pumps stopped working
Passive / Leakage (nongated channels)
Open randomly, important in maintaining the baseline neuronal membrane potential ; ion flow sensitive to concentration gradients
POTASSIUM
Ligand-gated channels
Open in response to binding of a ligand (neurotransmitters, hormones, etc)
Voltage-gated channels
Respond to changes in transmembrane potential (when it deviates from ‘normal’ resting membrane potential”
Mechanically gated ion channels
Respond to mechanical vibration; pressure
Activated by physical stretching of plasma membrane
Resting membrane potential
Baseline, status quo, where neuron sits most of the time, -70mV
Depolarization
Reduces the potential difference, brings it closer to zero.. Describes the state of flux when going more positive
Repolarizing
Potential is fluxing, getting more negative and returning to the reference point
Hyperpolarization
Increasing the potential difference; making it more negative (further away from 0) goes beyond -70mV
What does an RMP of -70 mV mean?
It’s more negative inside the neuron’s plasma membrane
Three compartments that matter to the RMP
1) Aqueous extracellular fluid (ECF) (aka interstitial fluid)
2) Non-conducting lipid barrier
3) Aqueous cytoplasm
What causes the potential difference?
1) Ion differences between ECF and cytoplasm
2) Ion pumps
3) Selective ion leakage
Ion differences
Phosphates and proteins are anions at physiological pH, they stay trapped in the cell
THEIR (-) CHARGES DOMINATE OVER THE IONS (Na/Cl/K)
-Primary reason the cytoplasm stays more negative
Are anions more concentrated inside or outside cell?
Inside
Is K+ more concentrated inside or outside cell?
Inside
Is Cl- more concentrated inside or outside cell?
Outside
Is Na+ more concentrated inside or outside cell?
Outside
Intracellular ions create…
An electrostatic force field (both attracts and repels more mobile ions)
Sodium-potassium pump
-3 Na+ ions out
-2 K+ ions in
ATP -> ADP +Pi
*Cl- also gets pumped out
What ion as many leak channels?
K+, the membrane is very permeable to K+
Inward forces on K+
1) Pumps push K+ into the cell (against gradient)
2) K+ tends to follow the anions into the cell (attracted to negative charge)
3) High Na+ in the ECF repels K+
Outward Force on K+
1) Leak channels (moving down the concentration gradient)
Result of forces on K+
K+ going inside offsets anions slightly, pushes RMP towards (+)
(RMP is at -70 rather than -80 b/c of potassium)
Movers of Na+
Sodium potassium pumps
Very, very slow leak channels
Hyperkalemia
Increase in bloodstream (ECM) potassium
- Decreases gradient
- K+ doesn’t diffuse out of cells as rapidly
- Membrane DEPOLARIZATION
Hypokalemia
Decrease in extracellular K+
- Increase in concentration gradient
- More diffusion of K+ out of the cell
- Membrane hyperpolarization
What keeps neurons at a resting membrane potential of -70 mV?
Electrostatic forces, Ion concentration gradients, Leakage channels and Pumps
What can cause a flux or change in the RMP of a neuron?
Physical stimuli or input from other neurons
Do all parts of the cell experience flux in RMP?
No, only regions near the plasma membrane
Graded Potentials
Have an amplitude (height)
-Results from a variable degree of divergence (big, med, small) from the RMP
Why does RMP change? (graded potentials)
It is changes in local ion permealbility
- Na+ going in will make the neuron RMP more positive (stimulatory)
- Cl- going in will make the RMP more negative (inhibitory)
What’s the stimulus that elicits the Graded Potential?
- Ligands (neurotransmitters, hormones) bind ligand gated ion channels
- Mechanical stimulation
- Temperature change, etc
Summation
An additive effect on the amplitude when two or more stimuli occur at the same membrane locale at nearly the same time
Spatial summation
Many / multiple synapses release stimulus or inhibitory signal in the same locale on the target neuron
Temporal Summation
A single synapse may fire rapidly at a single point on the neuron
This is the consequence to RMP of a graded potential that causes the opening of Cl- channels
Hyperpolarization
Where are graded potentials in the neurons?
Dendrites, where summation and propagation of GPs to the trigger zone
Trigger zone
Decision point.. If the GPs are pushing to threshold (~50 mV) then ACTION POTENTIAL
Action potentials
Rapid, local reversals of the membrane potential
-All-or none (always same shape and amplitude)
What is the main mechanism for rapid communication along nerve axons?
Action potentials!
Why are action potentials triggered?
Because voltage-gated ion channels activated by crossing threshold in trigger zone
Sequence of AP
- Stimulus (often graded potentials
- Depolarizing phase
- Repolarizing phase
- Hyperpolarizing phase
What first happens when graded potentials cross threshold?
Voltage gated ion channels open, causes sharp depolarization
What happens after sodium channels are opened?
(+30mV) Na+ channels close and voltage gated K+ channels open
What happens when voltage gated K+ channels are opened?
Repolarization!
Why does hyper polarization occur?
Goes too far on downstroke, close K+ channels, returns to -70mV, prevents immediate location from having another action potential
Action potentials propagate…
UNIDIRECTIONALLY
Action potentials cannot go backwards due to…
Refractory period, membrane can’t immediately fire an action potential due to channels being wide open & ion gradients exhausted
Saltatory conduction
Voltage gated ion channels are only located at nodes (between myelin sheath), action potential jumps from node to node
Information is encoded (strength of stimulus) in…
The FREQUENCY of action potentials
What is the final output of a neuron
Release of a neurotransmitter at synapse (NOT action potentials)
First step of synaptic transmission
AP triggers V-gated Ca2+ channels to open
Calcium influx into presynaptic neuron causes… (synaptic transmission)
Exocytosis of secretory vesicles carrying neurotransmitters
Are neurotransmitters stimulatory or inhibitory?
Both! (Could be either)
Once neurotransmitters are released into the synaptic cleft…
They commonly bind to ion channels on the post-synaptic neuron
What happens in an Excitatory Post-synaptic potential (EPSP)?
Postsynaptic neuron experiences depolarizing graded potential
ex: neurotransmitter (glutamate) opens Na+ channels
What happens in Inhibitory Postsynaptic Potential (IPSP)?
Hyper-polarizing graded potential
ex: Neurotransmitter GABA opens Cl- channels (Cl- rushes into post-synaptic cell)
