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 develop and mature at different rates. Even if your friends have started to mature before you, don't worry; it's totally natural and there is nothing to be concerned about. Although the average age at which boys start puberty is 12 to 13 it can start at anywhere from 9 right up until 16.Puberty lasts around 5-6 years on average for boys. Most boys have finished puberty by the time they are 18-20.There are many factors involved and remember everyone is different.emales generally start puberty from the age of 9 -14 years old. however puberty varies in every case. Typically, girls start noticing the changes that occur with puberty be â¦ tween the age of 9 and 11.