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In many instances, the number of axons innervating a given cell is dramatically reduced during neuronal circuit maturation. For example, early in the postnatal period, a single neuron in the lateral geniculate nucleus of the thalamus (LGN) receives inputs from over 20 retinal ganglion cells (RGC). In the following three weeks of visual system development, the number of inputs drops to just one to three. However, synaptic drive, the response of LGN neurons to RGC activity, increases.

Which of the following could account for this seemingly counterintuitive phenomenon?

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While the number of axons (inputs) each LGN neuron receives decreases, the actual number of synapses increases. In other words, each of the remaining axons establishes many more new synapses with the target cell than are lost during the same period.


The amplitude and velocity of propagation of action potentials in the optic nerve increases to compensate for the loss in the number of inputs LGN neurons receive from RGC.


Synaptic efficacy at the remaining synapses increases greatly, which more than offsets any loss in the number of inputs.


Eye opening occurs some time after the refinement in the number of inputs is complete. This greatly increases the activity of the RGC and synaptic drive in the LGN.

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