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.
The third stage of male puberty occurs around age 13 or 14, but frequently as early as 11 or late as 16. Pubic hair begins to grow darker and fuller, and the penis now begins to grow in length. The testicles continue growing, erections become commonplace and the boy gains in height at a rate of over 3 inches (7.6 centimeters) a year.