Physical Development in Infancy
Nervous System
Neurons are the cells in the nervous system.
Key parts of a neuron include:
Dendrites: Branch-like extensions that receive signals from other neurons and transmit them toward the cell body.
Cell Body (Soma): The main part of the neuron containing the nucleus, which processes information and maintains cell function.
Nucleus: Located within the soma; contains genetic material.
Axon: A long, tube-like structure that transmits electrical impulses away from the cell body to other neurons or muscles.
Axon terminals: Endings of the axon where signals are transmitted to the next neuron.
Myelin Sheath: A fatty covering around the axon that insulates and speeds up signal transmission.
Synapse: The gap between neurons where neurotransmitters are released to transmit signals to the next neuron.
Overall idea: Neurons communicate via electrical impulses along the axon and chemical signals (neurotransmitters) across synapses to connect neural networks that underlie infancy development.
The Nervous System (Dendrites, Soma, Axon, Myelin, Synapse)
Dendrites – receive signals from other neurons and transmit them toward the cell body.
Cell Body (Soma) – contains the nucleus; processes information and maintains cell function.
Axon – transmits electrical impulses away from the cell body to other neurons or muscles.
Myelin Sheath – fatty covering around the axon; insulates and speeds up signal transmission.
Synapse – the gap between neurons where neurotransmitters are released to transmit signals to the next neuron.
Neurotransmitters
Neurotransmitters are chemicals that transmit signals between neurons across synapses.
They are released, bind to receptors on another neuron, and can either continue the signal or inhibit it.
Common neurotransmitters:
Dopamine – reward, motivation, movement
Serotonin – mood, sleep, appetite
Acetylcholine – learning, memory, muscle action
GABA – main inhibitory transmitter (calms activity)
Glutamate – main excitatory transmitter
Synaptogenesis
The formation of new synapses between neurons.
Begins in early fetal development and continues throughout life, with rapid bursts during infancy and early childhood.
Increases neural connectivity, enabling learning, memory, and cognitive flexibility.
During infancy, a baby’s brain forms millions of new connections daily as they learn to process sensory information and interact with their environment.
Synaptic Pruning
The elimination of weaker, less-used synapses to make neural networks more efficient.
Peaks during childhood and adolescence; continues into early adulthood.
Strengthens frequently used neural pathways while removing unnecessary ones, improving cognitive efficiency.
Phonemic Discrimination
Early Sensitivity: Infants are born with the ability to perceive and differentiate phonemes from all languages, even those they are not exposed to.
Perceptual Narrowing: By around 6\text{-}12\text{ months}, infants stop showing sensitivity to phonemes not used in their native language.
This narrowing is due to synaptic pruning, where the brain strengthens connections relevant to the language(s) they hear.
Infants exposed to multiple languages maintain a broader phonemic discrimination ability for longer, aiding bilingual learning.
Plasticity
Experience-Dependent Plasticity: New synapses form in response to unique individual experiences (e.g., learning a new skill or language).
Plasticity is greatest during the first years of life; many developmentalists suggest that enriching environments during this time encourage healthy brain development and neural connections (e.g., talking, singing, playing).
While synaptogenesis is most intense in early development, plasticity continues throughout life, allowing for learning, recovery from brain injuries, and adaptation to new experiences.
Rapid Development
The average infant doubles in size over the first 5\text{-}m\text{ months} of life, weighing about 15\text{ pounds}.
By the age of 1\text{ year}, infants' weights have tripled from birth weight.
Brain Growth and Injury Risk
Baby brains greatly increase in size as neurons grow and become myelinated (myelin speeds transmission of nerve impulses).
The infant brain is highly sensitive to injury; shaken baby syndrome can cause blood vessel damage and ruptures in neural connections, leading to severe disability or death.
Fontanelle
The soft spot on a baby’s head; an area where the skull bones have not yet fused.
Allows for flexibility during birth and room for brain growth in early infancy.
Closes around 6\text{-}8\text{ weeks} after birth.
Sleep in Infancy
Newborns sleep 16\text{-}17 hours a day in cycles of about 2 hour spurts.
By the end of the first year, most infants sleep through the night.
Infants have more REM sleep (rapid eye movement) sleep, composing about half of total sleep at first and declining to about one third by 6\text{ months}.
Sleep practices are highly cultural.
Perspectives on Sleep: Bedsharing
Potential Benefits: Promotes bonding and attachment; easier nighttime feeding; some research suggests babies sleep more soundly when near a caregiver.
Cultural norms: In many cultures, co-sleeping is traditional and considered normal.
Room sharing (without bedsharing) is generally more recommended in many guidelines.
Risks: Increased SIDS risk due to softer sleep surfaces, pillows, and blankets; risk of parents accidentally rolling onto the baby.
Cry It Out? (Sleep Training)
Description: A sleep training approach where parents allow their baby to cry.
Based on idea that babies can adapt to sleeping without external soothing if given time.
Benefits: Promotes independent sleep; increases parental sleep.
Concerns: Not suitable for newborns; can disrupt feeding schedules; may cause emotional distress and cortisol release.
Breastfeeding
The American Academy of Pediatrics recommends infants be fed breast milk for the first year.
Benefits of breast milk include easily digestible proteins, DHA, colostrum contains immunoglobulins, lipase and other enzymes.
In the United States, weaning off of breastfeeding or bottle-feeding usually starts around 3\text{-}4\text{ months}; solids start around 6\text{ months}—1–2 tablespoons of solid food per day; by 9\text{ months}, 2–3 solid snacks per day.
Extended Breastfeeding (2+ Years)
Mongolia: Socially acceptable to breastfeed well into toddlerhood (often 3\text{-}4\text{ years}).
Kenya (Kikuyu & Luo Tribes): Breastfeeding seen as bonding ritual and a way to strengthen immunity.
Rural India: Breastfeeding for 2+\text{ years} common, gradual weaning via Ayurveda-based diets.
Some Scandinavian countries: Generous maternity leave (up to 1\text{ year} paid) supports longer breastfeeding.
Earlier Weaning (6–12 months)
Countries/areas mentioned: United States, Japan, China, urban areas in India, Canada.
Why Families May Not Breastfeed
May not have options; maternal health issues; insufficient milk supply; postpartum depression (PPD);
Chronic illnesses (e.g., HIV, cancer, tuberculosis) may require medications unsafe for breastfeeding
inverted or severely cracked nipples
infant health conditions like lactose intolerance or metabolic disorders (e.g., galactosemia) requiring formula
premature infants in NICUs may need specialized formula or feeding tubes
work and maternity leave constraints
short leave and lack of lactation spaces can hinder pumping.
Formula
Definition: Infant formula is a scientifically developed milk substitute that provides essential nutrients for newborns and infants who are not breastfed or are partially breastfed. It mimics breast milk but lacks antibodies.
Pros: Convenient; easier to track intake; no dietary or medicinal restrictions for the mother; longer feeding gaps because formula digests more slowly.
Cons: Lacks immune antibodies; more expensive; higher risk of digestive issues for some babies; requires clean water and proper preparation.
Malnutrition
Lack of proper nutrients and a balanced diet can lead to delayed growth and lower IQ scores later in life.
Malnutrition due to food insecurity is greatest in developing nations.
Undernutrition can occur even if children are not severely malnourished.
Common deficiencies: fatty acids, iron, calcium, and vitamin D.
Benefit Programs
WIC (Women, Infants, and Children Program):
Free formula and baby food for infants up to 12 months
food vouchers for pregnant and postpartum mothers
breastfeeding support (lactation consultants, free breast pumps).
SNAP (Supplemental Nutrition Assistance Program)
Infant Obesity
Definition: Obesity is a weight greater than 20\% above the average for a given height.
There is no conclusive link between infant obesity and obesity in adolescence.
Overfeeding during infancy can predispose a person to being overweight; emphasis on appropriate, varied nutrition rather than weight alone.
Gross Motor Skills
By 6\text{ months}: sit upright unsupported.
By 8\text{-}10\text{ months}: crawl.
By 1\text{ year}: most infants walk unsupported.
Development is supported by myelination, strengthening of muscles, and head-to-body proportion.
Fine Motor Skills
By 3\text{ months}: coordinate movement of limbs.
By 4\text{ months}: begin reaching.
By 2\text{ years}: can drink from a cup without spilling.
Pincer grasp (thumb to index finger) improves greatly with age, enabling precise motor control.
Cultural Variation in Motor Skill Development
Cultural factors influence timing of motor milestones.
Aché people (Paraguay): infants kept in direct physical contact with the mother for safety; slower motor skill development.
Kipsigis people (Kenya): encourage standing and walking earlier than North American infants.
Visual Perception
Visual Cliff Test: assesses depth perception and fear of heights in infants.
Face Preference: infants show preferences for certain facial configurations.
Face Configurations: Upright faces are preferred; scrambled and inverted face configurations yield different looking-time responses.
Key findings (illustrative values):
Upright face preference shows significant looking-time advantage with p-values like p<0.03 or p<0.001 in certain configurations.
Top-Heavy vs Bottom-Heavy configurations influence preference; data indicate some configurations elicit stronger responses across ages.
Visual Perception: Summary Data
Infants show preferences for faces and certain configurations, suggesting early social processing abilities and sensitivity to social cues.
Auditory Perception
Early localization: Infants have poorer sound localization due to having a smaller head than adults, improving to adult levels over the first year.
Language discrimination: Infants can distinguish their native language from other languages; they can also distinguish their mother’s voice from other women’s voices.
Promoting Sensory Stimulation (Infant Development)
Practical strategies to promote sensory development:
Carry babies in different positions to vary proprioceptive input.
Let infants explore their environment to encourage curiosity and tactile experience.
Let babies touch and play with their food to foster oral-motor development and sensory exploration.
Provide multimodal toys that combine sight, touch, and sound to integrate multiple sensory modalities.