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 age at which boys begin puberty is very variable and differs from one boy to another. Any age between around 9 years of age and 16 is considered to be within the normal range for puberty to start although on average most boys begin puberty around 11 or 12 years of age. They generally begin to produce sperm around a year after puberty has begun.