Development across the Life Span 323MOTOR DEVELOPMENT: FROM CRAWLING TO A BLUR OF MOTION Infants manage a
tremendous amount of development in motor skills from birth to about 2 years of age.
Figure 8. 5 on the next page shows some of the major physical milestones of infancy.
When looking at the age ranges listed, remember that even these ranges are averages
based on large samples of infants. An infant may reach these milestones earlier or later
than the average and still be considered to be developing normally.
BRAIN DEVELOPMENT At birth, an infant’s brain consists of more than 100 billion
neurons. Rapid and extensive growth of these neurons occurs as the brain triples in
weight from birth to age 3 years, with much of the increase caused by growth of new
dendrites, axon terminals, and increasing numbers of synaptic connections (Nelson,
2011). Surprisingly, the development of the infant brain after birth involves a neces-
sary loss of neurons called synaptic pruning, as unused synaptic connections and nerve
cells are cleared away to make way for functioning connections and cells (Couperus &
Nelson, 2006; Graven & Browne, 2008; Kozberg et al., 2013; Zhan et al. 2014). This pro-
cess is similar to weeding your garden—you take out the weeds to make room for the
plants that you want.
BABY, CAN YOU SEE ME? BABY, CAN YOU HEAR ME? SENSORY DEVELOPMENT
I’ve heard that babies can’t see or hear very much at birth.
Is that true?Figure 8.4 Five Infant Reflexes
Shown here are (a) grasping reflex; (b) startle reflex (also known as the Moro reflex); (c) rooting reflex (when you touch a baby’s cheek, it will turn
toward your hand, open its mouth, and search for the nipple); (d) stepping reflex; and (e) sucking reflex. These infant reflexes can be used to check
the health of an infant’s nervous system. If a reflex is absent or abnormal, it may indicate brain damage or some other neurological problem.
a.d.b. c.e.Interactive