Properties of Sensory and Motor Neurons

Question 1

what are the passive electrical properties of neurons

Answer 1

• 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

Question 2

how can neurons be modelled as a wire

Answer 2

• each segment in model has a resistance
• this causes an applied voltage to decay as it travels along neuron
• ultimately the voltage will decay to zero (just as in a wire e.g. transatlantic cables)
• this decay is defined by two parameters
  
  • space constant
    
    • i.e. how far the voltage travels
  • time constant
    
    • i.e. how fast the voltage travels
• conduction velocity

Question 3

what is space constant

Answer 3

• distance for voltage to reach ~37% of original value
• determined purely by axonal resistance
• 100’s of miles for a telephone wire
  
  • 2-4mm for a neuron
• therefore need amplification

Question 4

what is time constant

Answer 4

• time for voltage to reach ~37% of original value
• determined by resistance and capacitance

Question 5

what is passive conduction velocity

Answer 5

• conduction velocity
• anything that increases the space constant or reduces the time constant will therefore increase conduction velocity
• 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
• fastest neuronal conduction speed is ~120m/s

Question 6

what are the chemical and electrical gradients

Answer 6

• 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
• 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)

Question 7

what is the sodium-potassium pump

Answer 7

• 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
• for one ATP molecule
  
  • 2 K+ pumped inward
  • 3 Na+ pumped outward
  • causes:
    
    • high extracellular [Na+]
    • high intracellular [K+]

Question 8

what happens in the voltage gated ion channels in the action potential

Answer 8

• 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

Question 9

what is the positive feedback cycle of the action potential

Answer 9

• 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

Question 10

how do Na and K channel dynamics differ in the action potential

Answer 10

• Na and K channels have different dynamics
• Na channels open quickly; K channels open more slowly
• this difference is crucial for the AP

Question 11

how is the action potential uni directional

Answer 11

• 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 far for artificial stimulation)

Question 12

how come speed is limited by permeability changes in the action potential

Answer 12

• the Na/K ion channel opening / closing is a role - limiting factor
• higher temperatures facilitate this process
• hence, the AP becomes faster and shorter

Question 13

what are the effects of neuron diameter

Answer 13

• increased neuron diameter causes
  
  • increased Cm
  • decreased Rm
• remember
  
  • time constant
  • both effects cancel, hence time constant unaffected
• but
  
  • space constant
  • diameter affects Rl more than Rm
  • therefore larger space constant
  • therefore faster conduction

Question 14

what is a squid giant axon

Answer 14

• large diameter neurons taken to the extreme
  
  • squids haven’t evolved myelin sheath so this was their solution
  • 800MM (versus <20MM in mammals)
  • used to contract mantle muscle to produce jet propulsion
  • useful for rapid escape response
• also useful for science
  
  • visible with the naked eye
  • can insert an electrode into the axon

Question 15

what is myelin and saltatory conduction

Answer 15

• 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)

Question 16

what are the effects of myelin and neuron diameter

Answer 16

• large diameter axons are faster for both myelinated and unmyelinated fibres
• but - unmyelinated fibres take up more space

Question 17

what is the difference between the types of neurons

Answer 17

• conduction velocity is a function of diameter and myelination
• motor neurons (alpha and gamma)
• sensory neurons (beta and delta)
• pain neurons (C fibres)
• pain fibres are small and unmyelinated - very slow
• alpha motor neurons are large and myelinated - very fast

Question 18

how can you stimulate the human peripheral nerve

Answer 18

• at rest, the inside of a cell is more negative than the outside of a cell. This occurs because there is a slightly greater number of negative charges than positive charges inside of the cell (intracellular space), and a slightly greater number of positive charges than negative charge outside the cell (extracellular space). Because of the electrical difference, the cell is said to be polarised - just like a magnet, one side is more positive and the other side is more negative. If the electrical gradient were suddenly reversed, the cell would be depolarised and we might see action potential
• as a result of stimulation, an action potential is sent in both directions along the length of the nerve, starting at the cathode. All of the negative charge from the extracellular space is attracted to the anode, leaving the outside of the cell excessively electrically positive related to the inside of the cell. This cell is thus hyper polarised under the anode, meaning that it is very, very difficult to activate

Question 19

how do you measure conduction velocity

Answer 19

• stimulate motor neuron at two sides
• measure latency of evoked responses
• calculate velocity, based upon time and distance

Question 20

what is the effect of demyelination

Answer 20

• occurs in
  
  • multiple sclerosis
  • guillan-barre syndrome
• reduced amplitude and increased latency