A.T. Robertson Commentary Acts 5:21

"And when they heard [this], they entered into the temple about daybreak, and taught. But the high priest came, and they that were with him, and called the council together, and all the senate of the children of Israel, and sent to the prison-house to have them brought." — Acts 5:21 (ASV)

About daybreak (υπο τον ορθρον). From ορνυμ, to stir up, to arouse, so the dawn (Luke 24:1; John 8:2). Old word, but in the N.T. only these three passages. "Under the dawn" or "about dawn." Sub lucem. The temple doors would be open for early worshippers and traffickers (John 2:14).

Taught (εδιδασκον). Imperfect active, began to teach.

The council (το συνεδριον). The Sanhedrin.

The senate (την γερουσιαν). From γερων, an old man, just as the Latin senatus is from

senex, old. Like the γεροντες in Homer and the Elder Statesmen in Japan. Apparently the senate of the people were also part of the Sanhedrin and the use of "and" (κα) is explanatory and adds this item in particular. Page thinks that this group of elders were not members of the Sanhedrin at all.

To the prison house (εις το δεσμωτηριον), another word for prison (τηρησις δημοσια in verse 18, η φυλακη in verse 19). See also verses 22,23,25. This from δεσμος, bond, and τηρεω, to keep, place where bound men are kept.