L34, L35: Membrane Excitability / Transmission of nerve signals Flashcards
Types of transmembrane ion channel
- Non-gated
- Ligand-gated
- Voltage-gated
What contributes to resting membrane potential
- Selective permeability of membrane to different ions, impermeable to anion
- Concentration gradient of ions across membrane
- Electrical potential gradient
- Concentration gradient: K+ passively diffuse out, Na+ diffuse in via non-gated channels
- Electrical potential gradient: oppositely charged anion attract K+ inwards
- maintained by Na/K pump: ATP dependent, 3 Na out: 2 K in
Describe action potential
- all-or-none phenomenon
- positive feedback
- sequence of events:
- Depolarisation to threshold:
- Na permeability increase, Na influx
- fast Na channel: activating m gate and inactivating h gate which closes quickly - Overshoot: self-reinforcing Na influx
- Repolarisation:
- progressive increase in permeability to K—> K outflux (slow K channel (n gate))
- decrease in permeability to Na by inactivating gate —> decrease Na influx - Na/K pump restore ion content
Experimental evidence of patch-clamp
- Measure ionic current of individual channels
2. Use of channel blockers
Absolute vs Relative refractory period
Absolute: complete inexcitability due to inactivating h gate of sodium channel closed
Relative:
- incomplete restoration of membrane permeability, inactivating h gate of sodium channel reopens again —> allow depolarisation again
- higher stimulus is required to generate another AP
Calcium in membrane excitability
Alter excitability of membrane by affecting threshold and ionic conductance of Na and K
Cardiac muscle: L-type
Smooth muscle: slow type Ca channel
Clinical application of membrane excitability
- ECG
- EEG
- BAER (brainstem auditory evoked response)
Equilibrium potential
Voltage required to maintain the concentration gradient of ions across the membrane
As more Na outside, the voltage need to drive Na out of cell is +ve: therefore Na: +60 mV
more K+ inside: -90 mV to attract K back
Graded potential vs Action potential
Graded:
- passive event: a result of current flow
- spread of electrical signals locally (dendrites —> cell body / hillock)
- passive membrane properties
- V=IR, resistance unchanged, V proportional to I (current)
- can be depolarising / hyperpolarising
- local and transient, decay rapidly with time and space
- integrated at soma —> whether over threshold —> action potential
Action:
- driven by voltage-gated channels open/close
- generation and propagation along axon (initial segment —> axon terminals)
- active membrane properties: involve active ion channels activity
Integration of graded potentials
- Temporal (same branch firing)
- Spatial (signals from different branches)
- both occur at axon hillock
Membrane resistance and Membrane capacitance
Membrane resistance: Inversely proportional to membrane permeability
Membrane capacitance: excess of opposite charges stored on both sides of membrane
What is Time constant
- Resistance x Capacitance
- indicate how fast cell membrane can be charged/discharged
- determine whether temporal summation can occur: longer the time constant —> more possible
What is Length constant
- Square root of Membrane resistance / Axial resistance
- Indicate how far the local voltage change can spread along the membrane
- determine whether spatial summation can occur: higher the length constant —> more possible
How Propagation of action potential is achieved
*Electrotonic spread of ions:
Electrical attraction and diffusion / local current move along the interior side of membrane and depolarise adjacent membrane areas —> action potential generated in adjacent membrane
However, electrotonic spread makes signal die down quickly, need boost from action potential generated from sodium channel opening in nodes of Ranvier
Unidirectionality: due to refractory period / inactivated Na channels in the back
Factors affecting conduction velocity of action potential
- Diameter: increase diameter —> decrease in axial resistance —> increase length constant + decrease time constant
- Myelin sheath:
- increase membrane resistance —> increase length constant
- decrease capacitance —> decrease time constant - Distribution of Na channels: more in nodes of Ranvier —> lower threshold for generating AP —> action potential only generated in nodes of Ranvier (myelinated axons)
For unmyelinated axons, action potential is generated in all segments of axon (by opening Na channel) since local current leaks out of axon
*Opening of Na channel is slower than electrotonic spread of ions
