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.
While each male adolescent is different, the following are average ages when puberty changes may occur: Beginning of puberty: 9.5 to 14 years old. First pubertal change: enlargement of the testicles. Penis enlargement: begins approximately one year after the testicles begin enlarging. Appearance of pubic hair: 13.5 years old. Nocturnal emissions (or wet dreams): 14 years old. Hair under the arms and on the face, voice change, and acne: 15 years old. Girls also experience puberty as a sequence of events, but their pubertal changes usually begin before boys of the same age.