Sensory and motor nerves (wk 3) Flashcards
What are the passive electrical properties of neurons?
Wires conduct current by electron flow in metal. Neurons conduct current by ion flow in fluid. In both cases, current flow is impeded by resistance.
How can neurons be modelled as a wire?
-> Each segment in model has a resistance. This causes an applied voltage to decay as it travels along the neuron. Ultimately, the voltage will decay to zero (just as in wire e.g. transatlantic cables). This decay is defined by 2 parameters:
* Space constant -> i.e. how far the voltage travels
* Time constant -> i.e. how fast the voltage travels
+ Conduction velocity
-If you apply a fixed voltage, to a neuron, you are able to identify the time and space constant of it
What is the space constant and time constant with neuronal conductance?
-Space constant -> Distance for voltage to reach ~37% of original value. Determined by axonal resistance. 100’s of miles for a telephone wire ~2-4mm for a neuron. Therefore they need amplification. You want the space constant to be high and this is done through long longitudinal and ?
-Time constant -> Time for voltage to reach ~37% of original value. Determined by resistance and capacitance. Determined by medial resistance and medical capacitance.
What is passive conduction velocity (with conduction velocity)?
-Passive conduction velocity -> Conduction velocity – Anything that increases the space constant OR reduces the time constant will therefore increase conduction velocity.
+ This is important for understanding effects of myelination and axon diameter – both factors increase the space constant. Conduction speed of a conventional wire can reach 3-99% the speed of light. Fast neuronal conduction speed is ~120 m/s.
What are chemical and electrical gradients?
The voltage across a cell membrane (membrane potential) is determined by a balance of electrical and chemical forces. i.e. a balance between electrical charge and osmotic force.
What are chemical and electrical gradients (extracellular charges)?
Chemical and electrical gradients -> Net extracellular positive charge under resting conditions. Membrane potential of -60 to -70mA in mammalian neurons. Hyper-polarisation increases this potential (makes action potential less likely). De-polarisation reduces this potential (makes action potential more likely). Neurons generate this base line state through potassium pumps.
What is the sodium-potassium pump?
-> For one ATP molecule -> 2 K+ pumped inward and 3 Na+ pumped outward. Causes -> High extracellular (Na+) and High intracellular (K+). The Na+ pump is crucial for establishing the membrane potential necessary for the action potential. Otherwise, Na+ and K+ would gradually leak across the membrane, causing potential to drop to zero.
What are the voltage-gates ion channels in the action potential?
AP is mediated by voltage-gated ion channels embedded in cell membrane. Open when a threshold voltage is reached. Na and K channels are the primary mediators of the action potential. Na channel has 3 states: open/closed/deactivated. K has 2: open/closed.
Describe the 7 step positive feedback cycle:
- Cell membrane becomes depolarised
- Once depolarisation reaches a threshold, Na+ channels open
- Resulting influx of Na+ leads to further depolarisation
- Adjacent Na+ channels are opened, causing a chain reaction
- Meanwhile, K+ channels open (more slowly), causing an outflow of K+
- Resulting outflow of K+ repolarises membrane potential
- Na+ channels become closed, and temporarily deactivated
How do Na and K channel dynamics differ in the action potential and speed limitation by permeability changes?
Na and K channels have different dynamics. Na channels open quickly; K channels open more slowly. This difference is crucial for the AP.
Speed limited by permeability changes -> The Na/K ion channel opening/closing is a rate-limiting factor. Higher temperatures facilitate this process. Hence, the AP becomes faster and shorter.
What is unidirectional?
The refractory period prevents the positive feedback cycle continuing indefinitely. This ensures that the action potential only travels in one direction (during natural activation of neuron; not so for artificial stimulation).
What are the effects of neuron diameter?
-Increased diameter causes increased time constant
-Write equation down
What are the factors affecting conduction velocity?
-Temperature, myelination (improves conduction velocity in axons), axon diameter (fatter axons carry action potentials faster)
What is myelin and saltatory conduction?
-Myelin and Saltatory conduction -> Myelin causes saltatory conduction. This insulation increases the space constant (just like diameter does). The ion current can travel much further before it decays. Passive ionic conduction occurs between nodes of ranvier. Amplified at nodes by Na channels. Hence, the AP ‘jumps’ between nodes (‘saltare’ = jump).
What are the effects of myelin and neuron diameter?
Large diameter axons are faster for both myelinated and unmyelinated fibres. But – unmyelinated fibres take up more space.
