Signals of the knee-jerk reflex: Afferent signals

-The circuit of the knee-jerk reflex has monosynaptic and polysynaptic components

-The monosynaptic component consists of: Stretch receptor + extensor a motor neuron 

--At the trigger zone (initial segment of the stretch receptor axon; axon hillock of motor neurons), one or more action potentials (APs) are generated if the threshold (~ -50 mV) is reached, encoding the stretch amplitude into an AP number and frequency. 

Signals of the knee-jerk reflex: Efferent and interneuron signals

d. Glutamate binding to postsynaptic receptors opens the ligand-sensitive cation channels.

  -Ionic flux depolarizes the membrane potential, producing an EPSP

  -The EPSP amplituide is graded and depends on the amount of neurotransmitter release (therefore, also on the presynaptic AP frequency/number and Ca2+ influx)

f.  Thus, at the synapses, frequency-modulated APs are converted into an amplitude-modulated presynaptic Ca2+ signal and into an    

   amplitude-modulated post-synaptic potential.

How are signals propagated?

    -Capacitive current (Ic) discharges, later recharges, cm at each new signaling site

    -Myelin greatly reduces cm and therefore Ic by decreasing the transmembrane ionic attractions (see Below)

2) Membrane resistance (rm, in Ohms), depends on the ion channel density.

    -Ionic current (Ii) can only cross the membrane through the channels

3) Inside, ionic current is carried by internal ions through the axial resistance (ra, = cytoplasm), which is the third property.

    -Typically, ra is much smaller than rm, but ra of small processes such as axon terminals is significant

    -Outside, ionic current is carried by interstitial ions, which complete the circuit. This pathway typically offers negligible resistance.

At each site, Ic flows first, then Ii flows

1) Ic discharges the membrane capacitance

  -cm is determined by transmembrane ionic attraction

2) Ii changes VM

     -Flows through ra to new site, also through rm if open channels

3) Ic recharges the membrane capacitance and

4) Ii repolarizes VM

How passive membrane properties affect propagation?

- rm and cm  affect how fast signals rise and fall passively*

- rm  and ra affect how far signals propagate passively*

- ra and cm  affect conduction velocity of signals  

the time constant  

-t is the time it takes for the potential to reach 63% of the maximum signal amplitude or to decay by 63%, to 37% of the maximum.

-It is best to measure decay time. It is typically 1-20 msec, but it can be longer.

Length constant 

Length (space) constant  λ  =  √(rm /ra)

-λ is the distance at which a signal decays by 63%, to 37% of its initial peak amplitude.

  -With a longer λ, there is a higher probability of spatial summation (addition of signals generated at different sites), which    

   increases the probability that the signals will have an effect.