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.
Normal puberty begins between eight and 14 years of age in girls and between nine and 14 years of age in boys. Pubic hair distribution is used to stage puberty, along with breast size and contour in girls and testicular volume in boys. Some children experience constitutional sexual precocity, but precocity is likely to be pathologic if it occurs in very young children, if there is contrasexual development or if the sequence of normal pubertal milestones is disrupted.