Membrane Potential
Membrane potential: difference in charge inside and outside the cell
Plasma membrane barrier separating charges
Ion concentration differ between the inside and outside or outside cell
Polarized
Resting membrane potential: when neurons are not sending signals
Plasma membrane is not very permeable to cations and anions
Separates charge by keeping different ions largely inside or outside the cell
-70 mV resting potential inside cell
Interior more negative than exterior
Negative ions within the cell are drawn to the positive ions arrayed on the outer surface
NA⁺/K⁺ -ATPase (sodium-potassium pump): transports 3 Na+ out for every 2 K+ moved in
Ion specific channels: allow passive movement of ions
More ungated K+ channels than ungated Na+ channels
Membrane more permeable to K+ at rest
Negatively charged molecules such as proteins more abundant inside cell
Electrochemical gradient: combined effect of electrical and chemical gradient
Equilibrium potential: no net movement due to opposing forces of chemical and electrical gradients
Changes in membrane potential are changes in the degree of polarization
Depolarization: cell membrane less polarized and less negative relative to surrounding solution
Gated channels open allowing Na+ to flow in and membrane potential becomes more positive (less negative)
Hyperpolarization: cell membrane more polarized and more negative
K+ moves out of the cell making the cell membrane less positive (more negative)
All cells have a membrane potential
Only neurons and muscle cells are excitable
Excitable: capacity to generate electrical signals
Use gated ion channels
Voltage-gated ion channel: open and close in response to voltage changes
Ligand-gated ion channel: open and close in response to ligands or chemicals
Graded potentials
Depolarization or hyperpolarization
Varies depending on strength of stimulus
Occur locally on dendrites or cell body
Spreads a short distance and dies out
Act as triggers for action potential
Action potentials
Carry electrical signal along an axon
Always the large same amplitude depolarization
All-or-none - cannot be graded
Actively propagated - regenerates itself as it travels
Action potential begins when graded potential depolarizes to threshold potential (-50mV)
Voltage-gated Na+ channels, triggering action potential
Na+ rapidly diffuses into cell causing spike
Inactivation gate in Na+ channel shuts when membrane sufficiently positively polarized
Cannot reopen until resting potential is restored
Voltage-gated K+ channels also open at threshold potential, but 1 msec later than Na+ channels
K+ leave cell and membrane becomes negative again
So many K+ leave that membrane hyperpolarizes
Voltage-gated K+ channels close and resting membrane potential is restored
Evolution of K+ channels with a slightly slower opening time than Na+ channels was a key event that led to the formation of nervous systems
If both opened at the same time, they would negate each other’s effects
Absolute refractory period
While inactivation gates of Na⁺ channels are closed, cell is unresponsive to another stimulus
Places limits on the frequency of action potentials
Also ensures action potential does not move backward toward cell body
Speed varies depending on
Axon diameter
Broad axons provide less resistance and action potential moves faster
Myelination
Myelinated axons are faster then unmyelinated
Oligodendrocytes and Schwann cells make myelin sheath
Not continuous: gaps at nodes of Ranvier
Saltatory conduction: action potential seems to “jump” from node to node
Junction where nerve terminal meets a neuron, muscle cell, or gland
Presynaptic cell: sends signal
Synaptic cleft and postsynaptic cell: receives signal
Two types
Electrical synapses: electric charge freely flows through gap junctions from cell to cell
Chemical synapses: neurotransmitter acts as signal from presynaptic to postsynaptic cell
Presynaptic nerve cell contains vesicles of neurotransmitter
Exocytosis releases neurotransmitter into
synaptic cleft
Diffuses across cleft
Binds to channels or receptors in postsynaptic cell membrane
Binding of neurotransmitter changes membrane potential of postsynaptic cell
Excitatory postsynaptic potential (EPSP): brings membrane closer to threshold potential
Inhibitory postsynaptic potential (IPSP): takes membrane further from threshold potential (hyperpolarization)
Synaptic signal ends when neurotransmitter is broken down by enzymes or taken back into presynaptic cell for reuse
Synaptic integration: integrates multiple inputs to single neuron
Spatial summation: when two or more EPSPs or IPSPs are generated at one time along different regions of the dendrites and cell body, their effects sum together
Temporal summation: two or more EPSPs arrive at same location is quick succession
Membrane potential: difference in charge inside and outside the cell
Plasma membrane barrier separating charges
Ion concentration differ between the inside and outside or outside cell
Polarized
Resting membrane potential: when neurons are not sending signals
Plasma membrane is not very permeable to cations and anions
Separates charge by keeping different ions largely inside or outside the cell
-70 mV resting potential inside cell
Interior more negative than exterior
Negative ions within the cell are drawn to the positive ions arrayed on the outer surface
NA⁺/K⁺ -ATPase (sodium-potassium pump): transports 3 Na+ out for every 2 K+ moved in
Ion specific channels: allow passive movement of ions
More ungated K+ channels than ungated Na+ channels
Membrane more permeable to K+ at rest
Negatively charged molecules such as proteins more abundant inside cell
Electrochemical gradient: combined effect of electrical and chemical gradient
Equilibrium potential: no net movement due to opposing forces of chemical and electrical gradients
Changes in membrane potential are changes in the degree of polarization
Depolarization: cell membrane less polarized and less negative relative to surrounding solution
Gated channels open allowing Na+ to flow in and membrane potential becomes more positive (less negative)
Hyperpolarization: cell membrane more polarized and more negative
K+ moves out of the cell making the cell membrane less positive (more negative)
All cells have a membrane potential
Only neurons and muscle cells are excitable
Excitable: capacity to generate electrical signals
Use gated ion channels
Voltage-gated ion channel: open and close in response to voltage changes
Ligand-gated ion channel: open and close in response to ligands or chemicals
Graded potentials
Depolarization or hyperpolarization
Varies depending on strength of stimulus
Occur locally on dendrites or cell body
Spreads a short distance and dies out
Act as triggers for action potential
Action potentials
Carry electrical signal along an axon
Always the large same amplitude depolarization
All-or-none - cannot be graded
Actively propagated - regenerates itself as it travels
Action potential begins when graded potential depolarizes to threshold potential (-50mV)
Voltage-gated Na+ channels, triggering action potential
Na+ rapidly diffuses into cell causing spike
Inactivation gate in Na+ channel shuts when membrane sufficiently positively polarized
Cannot reopen until resting potential is restored
Voltage-gated K+ channels also open at threshold potential, but 1 msec later than Na+ channels
K+ leave cell and membrane becomes negative again
So many K+ leave that membrane hyperpolarizes
Voltage-gated K+ channels close and resting membrane potential is restored
Evolution of K+ channels with a slightly slower opening time than Na+ channels was a key event that led to the formation of nervous systems
If both opened at the same time, they would negate each other’s effects
Absolute refractory period
While inactivation gates of Na⁺ channels are closed, cell is unresponsive to another stimulus
Places limits on the frequency of action potentials
Also ensures action potential does not move backward toward cell body
Speed varies depending on
Axon diameter
Broad axons provide less resistance and action potential moves faster
Myelination
Myelinated axons are faster then unmyelinated
Oligodendrocytes and Schwann cells make myelin sheath
Not continuous: gaps at nodes of Ranvier
Saltatory conduction: action potential seems to “jump” from node to node
Junction where nerve terminal meets a neuron, muscle cell, or gland
Presynaptic cell: sends signal
Synaptic cleft and postsynaptic cell: receives signal
Two types
Electrical synapses: electric charge freely flows through gap junctions from cell to cell
Chemical synapses: neurotransmitter acts as signal from presynaptic to postsynaptic cell
Presynaptic nerve cell contains vesicles of neurotransmitter
Exocytosis releases neurotransmitter into
synaptic cleft
Diffuses across cleft
Binds to channels or receptors in postsynaptic cell membrane
Binding of neurotransmitter changes membrane potential of postsynaptic cell
Excitatory postsynaptic potential (EPSP): brings membrane closer to threshold potential
Inhibitory postsynaptic potential (IPSP): takes membrane further from threshold potential (hyperpolarization)
Synaptic signal ends when neurotransmitter is broken down by enzymes or taken back into presynaptic cell for reuse
Synaptic integration: integrates multiple inputs to single neuron
Spatial summation: when two or more EPSPs or IPSPs are generated at one time along different regions of the dendrites and cell body, their effects sum together
Temporal summation: two or more EPSPs arrive at same location is quick succession