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.
Boys normally enter puberty between ages 9-14 as evident by initiation of testicular growth. The actual pubertal growth spurt happens during mid-late puberty in boys. Puberty is complete within 2-4.5 years from the start depending on the boy. The peak growth velocity can be anywhere from 1-3 years after signs of initial testicular enlargement. Statistically, the average age for this is 12-14 yo.