A microtubule protofilament is formed by the: lateral association of only γ-tubulin subunits. head-to-tail association of only γ-tubulin subunits. lateral association of tubulin dimers. head-to-tail association of tubulin dimers.

head-to-tail association of tubulin dimers.

Explanation:

Microtubules are made up of a single type of globular protein, called tubulin. Tubulin is made up of two 55 kD polypeptides, α-tubulin and β-tubulin. Tubulin dimers are polymerized into microtubules, consisting of 13 linear protofilaments coupled around a hollow core. These protofilaments are composed of head-to-tail matrices of tubulin dimers, which are arranged in parallel.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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