Intrinsic Properties of Motor Neurons Flashcards
(19 cards)
What is the basic morphology of motor neurons?
- Large cells with large cell bodies
What effect does motor neuron size have on its electrical properties?
V = IR
As R depends on ion channels, and larger neurons have more of these they have less resistance. Smaller motor neurons have smaller resistance and therefore an input of smaller injection of current has a larger change in voltage
What receive most of the motor neuron inputs?
- Dendrites, which make up 97% of the motor neuron surface area
- Can extend some distance away from the soma
What are inward currents?
Mediated by sodium or calcium influx, depolarise the membrane potential moving it closer to the threshold for firing of action potentials
What are outward currents?
Mediated by potassium efflux, hyperpolarise the membrane moving it away from threshold
What is the process by which fast inactivating Na+ channels operate?
- Activate with depolarisation
- Then are quickly inactivated (h gate closing)
- Upon repolarisation deactivate (closing m gates)
- At resting potential inactivation is removed but deactivation still present so channel is closed
What is spike frequency adaptation?
- At a high frequency stimulation peaks are initially fast before becoming slower
- In muscles this is possibly as a large stimulation is needed initially to overcome inertia
How do fast inactivating sodium channels contribute towards spike frequency adaptation?
- There is time-dependence to the removal of sodium inactivation, if you trigger a spike after the first it will be smaller as inactivation has not been totally removed
- Using computer modelling and removing this factor produces consistent high frequency firing
What are the properties of high voltage activated calcium channels?
- Need a higher voltage to activate (above -20mV)
- Persistent (show no-deactivation)
What are low-voltage activated ca currents?
- Are deinactivated with hyperpolarisation and become activated at depolarisation below threshold
- Are transient (show inactivation)
What are delayed-rectifier type channels?
- Persistent K channels
- Show no inactivation
- Increase in outward current with depolarisation
What is the function of persistent K+ channels?
- Re-polarise the action potential
- Blokcing with application of TEA produces wider action potentials
What are transient K+ channels?
- Inactivating K+ channels only present at the beginning of depolarisation
- Can be found when subtracting current when depolarised from lower voltage (-80mV) compared to -40mV
- Produce latency to first spike in motorneuron
What are Ca2+ activated K+ channels?
- K+ channels which open upon increase of intracellular Ca2+
- Blocked by apamin
- Generated afterhyperpolarisation, therefore apamin application produces a much higher rate of firing
What are bistable motorneurons and what might be there utility?
- Motorneurons which exist in a state of no firing or persistent firing until the injection of a hyperpolarising stimulus
- Potentially useful for maintaining posture where neurons must be persistently active
What findings were found in lumbar spinal cords isolated from mice during the posture-development stage?
- Blocking L-type Ca2+ channels (previously considered to mediate this process) did not completely abolish palteau potentials in all samples
- Blocking INAP (persistent sodium currents) did abolish the plateau potentials, however did not abolish the slow afterdepolarisation which is what is believed to summate to produce pateau potentials
- Blocking ICAN (non-selective cation) currents by blocking transient receptor potential (TRP) V channels which when blocked increased afterdepolarisation amplitude, agonist to TRPV2 specifically increased persistent firinng
What was the hypothesis proposed by the studies codnducted using isolated lumbar spinal cord motor neurons from mice at the posture-development stage explaining persistent firing?
1) Brief depolarisation activates L-type calcium currents
2) Increased intracellular Ca2+ activated ICAN currents via TRPV2
3) Increased cations from channel will bring initial sADP to transient Na channel threshold promoting repetitive firing
4) sADP summate to produce plateau potentials which can only be abolished by the injection of a hyperpolarising stimulus
What are the different subtypes of high voltage activated calcium channels?
L-type (long-lasting) - found in neurons and muscles, especially in the heart
P/Q (Purkinje) - take 2s to inactivate, intially throught to be important for integrating synaptic inputs
N-type (neuronal) - found in pre-synaptic CNS synapses, more negative voltage dependence and faster inactivation than L-type
R-type (residual) - after blocking still remains open
Describe the sequence by which bursting is produced
- Ipsilateral stimulation causes opening of Na+ channels leading to depolarisation and activation of HVA N and P/Q channels
- Duration of this action potential determines Ca2+ entry
- Repolarisation is determined by transient K+ channels and delayed-rectifier type channels
- Transient v-gated K+ channels determine the fast component of the after hyperpolarisation and the slow component is regulated by the KCa channels and some sodium dependent K channels
- Low voltage activated Ca channels are de-inactivated by the hyperpolarisation and can therefore be activated at a depolarisation below threshold boosting any synaptic potential and ensuring that any inhibitory phase is followed by a rebound
