HN220 Lecture 7: HN220 Lecture 7 - Nervous & Electrical Signalling Part 2 - Winter 2019
HN220 Lecture 7
Nerve Cells & Electrical Signalling
Neuron
Recall…
Transmembrane Potential
• Ion Movements and Electrical Signals
• All cell membranes produce electrical signals by ion movements
• Different membrane permeability (more to K+)
• Determined primarily by K+ gradient and the cell’s resting permeability to K+, Na+,
Cl-
• Particularly important to neurons
• 2 Factors Influence Membrane Potential:
1. Uneven distribution of ions across cell membrane
2. Different membrane permeability to those ions
Establishing Membrane Potential
• 3 Requirements for Membrane Potential:
1. ICF and ECF differ markedly in ionic concentration
• ECF [Na+] > [K+]
• ICF [K+] > [Na+]
2. Selectively permeable through channels
• more permeable to K+
3. Cell’s passive and active transport do not ensure an equal distribution of charges
across its membrane
Resting Membrane Potential
• -70mV
o inside relative to outside
• Largely depends on:
1. Ratio of the concentrations of K+ and Na+ on either side of the membrane
2. Specific permeability to each ion
• Other ions must be contributing other than K+
Passive Forces Acting Across the Membrane
• Chemical gradients
o Conc. gradients of ions (Na+, K+)
o Permeability K+ > Na+ or Ca2+
• Electrical Gradients
o Separate charges of (+) and (-) ions
o Result in potential difference
• Electrochemical Gradient
o For a particular ion (Na+, K+) is the sum of chemical and electrical forces
o Ions have an equilibrium potential
• Equilibrium Potential
o The transmembrane potential at which there is no net movement of a
particular ion across the cell membrane
o Two factors are critical in determining resting membrane potential:
▪ Ion concentration gradients
▪ Membrane permeability to these ions (ion channels)
• K+=–90mV
• Na+ = +66 mV
Resting Membrane Potential of a Neuron
• Typical neuron
• Permeable to potassium and sodium
• 25x more permeable to potassium
▪ ion distribution
•
outside
cell: sodium & chloride
•
inside
cell: potassium & organic anions
Active Forces Acting Across the Membrane
• Sodium–potassium ATPase (exchange pump)
• Powered by ATP
• Carries 3Na+ out and 2K+ in
• Balances passive forces of diffusion
• Helps maintain resting potential (–70 mV)
Sodium-Potassium Pump
• ATPase enzyme
• ATP→ ADP + Pi
• Moves both Na+ and K+ against their gradients - costs
energy!
• Goal is to maintain a steep gradient for Na+ and K+
• Minor contribution to membrane potential
• ~3mV
Membrane Potential Results from Both Active & Passive Forces
• More K+ moves out than Na+ moving in
• -ve potential
• Membrane ~25 times more permeable to K+ than Na+
• More K+ channels
Changes in Membrane Potential
• Changes in membrane permeability to Na+ and K+ alters transmembrane potential
• Resulting from opening or closing specific membrane channels
o Passive channels:
▪ Leaky channels are always open (non-gated)
▪ Gated channels may be open or closed in response to stimuli
▪ At resting potential, most gated channels are closed
Gated Channels
• Chemically gated channels
o AKA Ligand-gated channel
