Unformatted text preview: '35:" mam G0 to the next screen: Now let’s consider the we“ Mannm-u-M-M movement of sodium. Sodium is in plentiful supply
52:: °mmmm-mmm outside the cell and m hmrted supp-1y. inside the cell.
__ °m~_____ﬂmm The chemical gradient for sodium is ms1de the cell
2:" (down its concentration gradient). Because sodium is
:__.. also an ion it has an electrical potential as well.
Go to the next screen: The inside of the cell is
more negative than the outside. The positive Na ion
will be attracted by the negative charge inside the cell.
Ell-Er] Therefore the electrical gradient for Na+ is inside the cell. The equilibrium potential for sodium KN, is +60
mV. At this point sodium will be at equilibrium — no net ﬂux inside or outside the cell. mum “mm G0 to the next screen; Now we will discuss
mus-u ommmu: how a typical resting membrane potential is —70mV 9:...— ommwmmmh—w— when E,K = -94mV and EN. = +60 mV. Both have 0::-' m-n-v gradients across the cell membrane. Both also have
'— channels and this makes the membrane permeable to 0: both ions. However there are more potassium 8:: channels than sodium channels making the Olav- membrane 25 times more permeable to potassium than to sodium. Both ions will move in the direction
of their chemical gradients — potassium out of the
; _ . cell and sodium into the cell. However, more
i I 8 potassium will move out per unit time because there
are more channels for potassium. Under these
conditions a net outward movement of positive charge will occur which give rise to the negative membrane
potential. As potassium continues to ﬂow out faster than sodium ﬂows back in the membrane potential
becomes more negative. After a period of time, however, the negative membrane potential will begin to
exert an electrical driving force on both potassium and sodium- it opposes potassium movement and
enhances sodium movement. Eventually the flow of both ions will become equal and opposite so that there
is no net movement of positive charge into or out of the cell. At this point the membrane potential holds steady at about —70mV, which is the typical resting membrane mtential for a neuron. a... mm.” . , . , . , $3,": Go to the next screen: Note that neither
W“ m“'°“‘"""°"""' sodium nor potassium ions are at equilibrium
2...... an‘m““m‘“m‘ because the membrane potential is not equal to the equilibrium potential of either ion. Therefore,
. . electrochemical forces are acting on both ions
causing sodium to leak into the cell and potassium
. to leak out of the cell. This will slowly alter the ion
. concentrations inside the cell and disturb the
I membrane potential and the neuron will not work.
N a+/K+ pumps in the cell membrane actively
a transport Na+ out of the cell and K+ into the cell
8 using ATP for energy. The Na+IK+ pump
maintains the electrochemical gradients. G0 to the next screen: Here are 4 points to remember.
1. Sodium and chloride are high outside cells (extracellular ﬂuid)
2. The permeability of a cell for ions depends on the number and type of ion channels in the cell
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- Spring '07