Chapter 3 and 4 - Nerve Activity Flashcards
The cell membrane is made up of…
Mostly water
Phospholipids
The molecules that make up the cell membrane
Phospholipid Bilayer
Hydrophilic heads (will dissolve in water)
Hydrophobic tails
Protein Chain
Dna (transcription ->) RNA (translation ->) Amino Acid Chain (folding ->) Protein
Amino Acid
Basic building block of protein
- 20 different kinds (human)
- Vary on the ‘R’ group
R Groups
Can have hydrophilic or hydrophobic properties
Lead to different structure when left alone in the cellular environment
Protein folding
The Cell Membrane
Semipermeable Phospholipid Bilayer
Causes difference in electrical charge between, inside and outside of cell
Ionic Basis of the Resting Potential
Ions, charged particles, are unevenly distributed
4 Ions Contributing:
- Sodium (Na+)
- Chloride (Cl-)
- Potassium (K+)
- Negatively charged proteins
(synthesized within the neuron)
(found primarily within the neuron)
What 4 factors influence the resting potential and the unequal ion distribution?
Two factors are homogenizing (distribute ions equally)
Two factors counteract the homogenization
(contribute to uneven distribution)
Random Motion
Particles move down their concentration gradient
Contributing to even distribution.
Electrostatic Pressure
Like forces repel, opposites attract
Contributing to even distribution.
Selective Permeability
Ions move in and out through ion-specific channels
K+ and Cl- pass readily
Little movement of Na+
Negatively charged proteins don’t move at all, trapped inside
To calculate the equilibrium of one ion -> Nerst equation
All relevant ions -> Goldman equation
Contributing to uneven distribution.
Sodium-Potassium Pump
Moves 3 Na+ out for every 2K+ in
Active process – much of the energy consumed by the brain (asleep or awake) goes toward maintenance of these ionic differences across the membrane
Contributing to uneven distribution.
Membrane Potential
The end result of all of these forces is that we have a resting membrane potential – the difference in charge between the inside and outside of a cell – of -70mv
Neurons in action
Neuron at rest -> resting membrane potential
- Homeostatic balance of ions
- -70mv
Neuron in action -> action potential (AP)
- Ions in flux
- Changing potentials depending on stage
The Action Potential
All-or-none:
- if threshold is reached the neuron “fires” and AP occurs
- If threshold not reached – no AP
- Through APs - message can be transmitted from one neuron to another
When threshold is reached, voltage activated ion channels are opened
When summation = threshold of excitation
(-55mV)…
When summation = threshold of excitation
(-55mV), voltage-activated Na+ channels
open and sodium rushes in
Voltage Gated Sodium Channel
Immediate response when threshold is reached
Allows near full permeability to sodium
The Ionic Basis of Action Potentials
Remember, all forces were acting to move Na+ into the cell
And now permeability isn’t an issue
Membrane potential moves from -70 to +50mV
The Action Potential - Steps
Rising Phase:
- Na+ in, K+ out
- Potassium channels open
- Sodium channels open
Repolarization:
- Sodium channels close
Hyperpolarization
- Potassium channels start to close
Rising phase (Depolarization):
Na+ moves membrane potential from -70 to +50mV
End of rising phase:
After about 1 millisec, Na+ channels close
Repolarization:
Concentration gradient and change in charge leads to efflux of K+
Hyperpolaization: Channels close slowly - K+ efflux leads to membrane potential <-70mV40
Depolarization in one location leads to…
An increase in membrane potential in an adjacent location
- This opens voltage gated sodium channels along the next part of the membrane
- This propagates the action potential
Refractory Periods
Prevent the backwards movement of APs and limit the rate of firing (to 1000 times/sec!)
ABSOLUTE → impossible to initiate another action potential
RELATIVE → harder to initiate another action potential
Refractory periods occur because of the ion channel properties, and ion concentrations
Overshoot:
Membrane potential is below the resting potential
Greater jump is required to reach threshold
Conduction in Myelinated Axons: Saltatory Conduction
Passive conduction along each myelin segment to next node of Ranvier
New action potential generated at each node
Faster conduction than in unmyelinated
axons -Saltatory conduction
