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.
Girls develop breasts and start their periods, and boys develop a deeper voice and start to look like men. The average age for girls to begin puberty is 11, while for boys the average age is 12. But thereâs no set timetable, so donât worry if your child reaches puberty before or after their friends. Itâs completely normal for puberty to begin at any point from the ages of 8 to 14. The process takes about four years overall.