BISC 307L 2nd Edition Lecture 5 Current Lecture Nature of Neuronal Signals All living cells generate an electrical potential difference Vm across their plasma membranes due to an unequal distribution of ions across the membrane because of selective permeability and transporters like the sodium potassium pump Neurons and muscle cells are unique in that their electrical properties are used to generate signals in the form of brief changes in membrane potential There are two general types of signals 1 graded local signals Graded because the size of the signal varies with the strength of the stimulus And local because they are restricted to the point they originated from on the membrane The amplitude exponentially decays with distance as it spreads away from the starting point so it doesn t get very far 2 all or none regenerative signals All or none because once the stimulus brings the Vm up to the threshold value it causes a very fast positive peak called the action potential whose amplitude is generally the same Regenerative because as it spreads it regenerates itself continually No matter how long or big the axon is once you generate an AP it goes all the way to the end of the cell undiminished in amplitude until it hits the end where it stops Stimulus anything done to the cell that provokes a response such as release of neurotransmitters exposure to light etc Going up positive inside Down negative inside Vm when one ion is permeable 2 important conditions behind the mechanisms of membrane potential 1 There is an unequal distribution of a particular ion across the membrane 2 The membrane of the cell has to be permeable to the ion due to the presence of selective membrane ion channels Left Only K is permeable Potassium will diffuse outwards down its concentration gradient leaving the inside of cell negative Simultaneously potassium also diffuses inwards down its electrical gradient so you have a flux of potassium moving in two directions oppositely across the membrane The rates of movement will continue until they reach equilibrium At equilibrium the flux of an ion down its concentration gradient will equal the flux of ions down its electrical gradient and net movement will be zero In membranes with only one ion permeable equilibrium is achieved instantaneously Right Only Na is permeable Now we have a sodium selective channel As sodium diffuses in down its concentration gradient it makes the inside positive As a result Na also travels outwards down the electrical gradient One way to calculate the membrane potential when only one ion is permeable is through the Nernst Equation If a cell is only permeable to potassium then the membrane will be negative inside But if it is only permeable to sodium it would be positive inside You can begin to see how cells control their membrane potential changes in membrane potential signals by having gated ion channels Vm when several ions are permeable What if your membrane is permeable to different ions and to different degrees What would the membrane potential of the whole cell be General mammalian values 90 Ek 70 Ena The membrane potential depends on 2 things relative permeability of each ion the more permeable an ion is the more influence it will have and the relative concentrations of each ion across the membrane This is seen in the GoldmanHodgkin Katz equation where P is the permeability constant Notice that Chloride is in over out opposite of others you have to flip it because it is an anion while the others are cations Key Rules If the permeability for a particular ion increases the membrane potential will change tending to move toward the equilibrium potential for that ion In other words if the permeability of a particular ion increases then it will have a more important effect on the Vm If the permeability of a particular ion is very low approaching 0 then that term just drops out and becomes 0 and doesn t contribute to membrane potential Ion Leakage and Na K pumps The membrane of a cell is pictured above It has two open ion channels one for K and one for Na The concentrations are shown A typical cell will have both open Na and open K channels So what is the membrane potential going to be If the actual Vm 70 inside is negative neither ion is at equilibrium The potential at which K s outward chemical diffusion and inward electrical diffusion is equal and opposite is 90 not 70 At 70 the concentration gradient driving K out remains the same but the electrical gradient is weaker Outward movement becomes greater than inward movement and so there is net outward movement of potassium What about for Na The membrane would have to be 60 inside in order for the inward movement of Na down its concentration gradient to be balanced by an outward electrical gradient So there is a huge net inward current for sodium If you are not at equilibrium potential then there is a force due to the difference between membrane potential and the equilibrium potential for the ion This force Vm Eion is the driving force making the ion move For Na the driving force is 70 MINUS a 60 130 A negative driving force means the resulting cation movement will be inwards 130mV is the force driving a strong inward movement of Na into the cell For K it would be 70 90 20 so we have outward movement of the cation If anion it s the opposite So there is a force of 130 pushing Na inwards and one of 20 pushing K outwards meaning ions are typically leaking in and out all the time The cell has a finite small volume however so to combat this it has a pump that moves sodium back out and gets K back in The Na K pump plays a vital role in maintaining distribution of ions concentration gradient that makes this possible More sodium coming in and K getting out stimulates the pump to work faster as long as there is ATP Bonus Hererra Tangent Neurons which use the leakage of ions through channels to generate signals have a lot of Na K pumps Babies have active neurons that are growing and going under rapid cell division and it is important they get enough energy to power the metabolism of their brain This is why at 6 months babies have to be given cereal because even if they are breastfeeding well they cannot get enough calories to keep up with ATP necessary to power brain development In adults brain consumes 20 of the body s calories In a baby up until age 4 brain consumes 50 of that energy Simplest ion channel is on the upper left hand corner consists of 2 transmembrane segments with a loop that