An intracellular recording from a spinal motor neuron shows a transient 5 mV depolarization following stimulation of one Ia afferent fiber. Which statement about this recorded event is correct?

• A It is all-or-none and propagates without decrement along the soma-dendritic membrane
• B It is generated by chloride influx through ligand-gated channels
• C It has a fixed duration identical to the presynaptic action potential
• D It is a graded potential whose amplitude depends on the amount of transmitter released ✓

Explanation

The recorded event is an excitatory postsynaptic potential, a graded local depolarization produced by cation influx through transmitter-gated channels. Its amplitude varies with the quantity of transmitter released and it decays passively with distance from the synaptic site. It is subthreshold and must summate spatially or temporally to bring the axon hillock to firing level. Chloride influx would produce an inhibitory postsynaptic potential, a hyperpolarization, not this depolarization.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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