Lecture 4: Neural Communication 1 Flashcards
The synapse
• Site of neural communication: between axon terminal and dendrites of another neuron
Resting Membrane Potential
A healthy neuron has a resting membrane potential (or membrane voltage) of between -60 and -80 mV (the voltage inside the neuron is 60-80 mV less than outside the neuron). usually -65mV
- Sticking a needle inside a cell (intracellular electrode), then stick a needle outside the cell (extracellular electrode), & compare the charges (potential energy)
- potential is always a comparison between 2 sides: inside and outside of the cell
- inside is slightly negative compared to the outside of the cell
- mV = millivolts
- right along the membrane* ionic changes/voltage differences
The membrane potential … originally performed with invertebrates
o On average can have very large neurons, as compared to humans
o The solution of myelin (fatty sheathes that wrap around the axon), is only seen in vertebrate animals (have spinal cord)
o Their solution: built really big axons, more efficient conduction of the action potential
o In some, their neurons/axons are much bigger/easier to study
o how big? about 1mm in diameter - can drop an electrode directly in there
Neuronal communication is chemical
1) primarily the result of two ions, sodium (Na+) and potassium (K+)
2) ions move into or out of the cells, but not freely
Neuronal communication is electrical
1) ions are positively and negatively charged (Na+ and K+ are both positive - missing an electron)
2) as they move into or out of cell, they change the potential (voltage) at the membrane
o note: absence of pos. is neg.
o i.e. remove a pos., leave a neg.
Chemical gradients
- Ions want to flow from high concentration to low concentration
- Chemical force, actual thing pushing on these ions
• Swimming pool:
o down at the corner there is this bubble, where you’re supposed to enter the pool… the stairs
o separate that area will a wall, and put dye into the entrance/stairs portion
o remove the wall, what happens to the dye?
move out from that little area into the rest of the pool
spread out equally; in all directions
• lots and lots of ions inside cell: chemical force pushing them to the outside & vice versa
Electrical gradients
- Charge/potential wants to flow from high concentration to low concentration, too
- Sometimes electrical and chemical gradients are at odds, causing an equilibrium that =/= 0mV
- Cells are resting at a negative; positive ions want to move into the cell
- Sometimes they will be pushing on the same ions, in different directions (electrical and gradient)
The cell membrane - guardian
• lipid bilayer is tightly packed, both hydrophobic and hydrophilic, keeping out all dangerous entities
o phospholipid bilayer
“head” — hydrophilic (inside and outside the cell)
- happy to interact with molecules similar to water (polar), anything non-polar/fatty (like oil) will bounce off these heads
“tails” — hydrophobic (pointing inward towards each other)
- lipid tails; fatty
- don’t like to interact with water; will bounce off the tails
- will interact with fatty-like/non-polar things
Effective barrier: keeps almost everything out
- Foreign invading bodies; viruses
Channels and pumps
• only certain molecules and ions permitted via channels and pumps
channels
o allow PASSIVE diffusion (i.e. along chemical gradient)
proteins that cause a pore (hole); allow a passageway from one side to the other
don’t allow just any ions; only certain ones
when they open; ions are going to move according to the law of nature (e.g. lot on the outside, going to want to move to the inside)
useful, limited in the directions that the ions will move “forces of nature”
Pumps
o actively push ions against their chemical gradient
requires energy (ATP - adenosine triphosphate)
not just a hole; “double-door” — allow things to travel
things come into a little out-cove, from there the outer door closes, then the inner door opens and the ions can move in
will actively pump ions against their concentration gradients; mechanistic action; require ATP
A cell with no pumps or channels
- nothing moves in or out of the cell
- draw in a phospholipid bilayer
- roughly the same amount of Na+ and K+ — concentration is the same
- even if tore a hole, concentration would remain the same
- no chemical gradients, no electrical forces
The sodium-potassium pump
- active process —requires ATP; mechanistic action; can push against concentration gradients (much slower than a channel)
- embedded in cell membrane
• extremely important
o consumers 2/3 of all neuronal energy
• pushes/pumps 3 Na+ out, and 2 K+ in ***
o i.e. active process that requires energy
• how does this affect the chemical gradients?
o more Na+ (sodium) on the outside, means pushing from the outside to the inside
o pumping more K+ (potassium) inside; building up the concentration inside… higher concentration inside — chemical gradient pushing from the inside to the outside
• how does this affect the electrical gradients?
o leaving the inside charge with a -1 with each pump… building up a chemical gradient (with respect to the outside)
o leaving the inside slightly negative compared to the outside
Potassium “leak” channels
- K+ can move freely via K+ “leak” channels that are always open
- Na+ cannot move freely across the membrane: it has channels, but they are usually closed (resting conditions)
- (Chemical gradient strong; from the inside to the outside — as K+ is leaving, making the inside more negative (building up the gradient) to push them back into the cell)
Cells are POLARIZED
• Na+/K+ pump pushing more Na+ out of the cell than K+ into cell
o Result: inside out of cell slightly more negative, than outside
• But, K+ can move freely through its leak channels
o Result: K+ wants to move with chemical ingredient, out of the cell
• But, this moving K+ is making the cell even more negative
o Result: flow of K+ stops, when force of electrical gradient equals force of chemical gradient
• End result: cell has resting membrane potential of roughly -70mV (equilibrium
