At the synapse, or the motor end plate, acetylcholine opens sodium channels causing sodium influx into the muscle fiber causing depolarization. This depoâ¦larization causes calcium released from the sarcoplasmic reticulum that initiates muscle contraction by activating sarcomeres.
On average, girls begin puberty around ages 10â11; boys around ages 11â12. Girls usually complete puberty around ages 15â17, while boys usually complete puberty around ages 16â17. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12â13; for males, it is the first ejaculation, which occurs on average at age 13.
Young males typically begin puberty between the ages of 9 and 16, and it can last anywhere from two to five years. The age of onset is not a reliable indicator of the duration of puberty or a person's final height, which depends on when the epiphyseal growth plates close. This usually occurs near the end of puberty. At this point, the bones stop lengthening and men reach their full height.