Influence of early experiences on development

Structure of a neuron

A neuron is composed of a:

• cell nucleus

• dendrites

• a cell body

• axons

• neural impulses

• myelin sheath

• axon terminals.

The adult brain has something like 100 billion connected networks of neurons with about 7,000 synapses per neuron making for 100 to 500 trillion synapses in total. Therefore, the adult brain is composed of a very dense network of interconnected neurons.

Developmental time course of synaptogenesis and synaptic pruning

📈 Early development (0–15 months)

👉 Synaptogenesis

• Rapid growth of synapses

• Infants have more connections than adults (~5,000 trillion)

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📉 Childhood (~10–12 years)

👉 Synaptic pruning

• Large reduction in synapses

• Removes weaker/unused connections

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📉 Adulthood

👉 Gradual decline

• More efficient, specialized networks

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🎯 Big idea

👉 Brain starts overconnected → becomes efficient
👉 “Use it or lose it”

What is neural plasticity and what are its types?

Neural plasticity = ability of the brain to change structure or function based on experience

👉 Allows learning and adaptation

Morphology

👉 Changes in neuron structure

• dendritic growth (more branches)

• synaptic receptor changes

👉 = how neurons are shaped

Cell Number

👉 Changes in number of neurons

• neurogenesis (birth of neurons)

• cell death (apoptosis)

👉 Humans have limited neurogenesis → supports stable long-term memory

⚠ Fix this idea:
👉 It’s NOT simply “more neurogenesis = less cognition”

👉 It’s about a tradeoff:

• more neurogenesis → flexibility

• less neurogenesis → stability

Connectivity

👉 Changes in connections between neurons

• synapse formation

• synaptic pruning (elimination)

👉 = strengthening useful connections, removing unused ones

Plasticity at multiple levels of organization: Molecular level

🧪 Definition

👉 Changes in neurotransmitter receptors at the synapse in response to activity

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⚙ How it works

👉 More receptors → stronger effect of neurotransmitters → ↑ neural firing
👉 Fewer/blocked receptors → weaker effect → ↓ neural firing

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🎯 Function

👉 Strengthens or weakens synapses based on experience
👉 Supports learning and memory

Plasticity at multiple levels of organization: Endocrine regulation

🧠 Core idea

👉 Brain and hormones interact bidirectionally

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⚙ How it works

👉 Neurons can produce/release hormones
👉 Hormones can alter neural activity (firing rates)

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🧪 Example

👉 Pheromones can:

• accelerate puberty

• delay puberty

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🎯 Big idea

👉 Environmental input → hormonal changes → changes in brain development

Plasticity at multiple levels of organization: Sensory systems

🧠 Core idea

👉 Sensory systems change based on experience and input

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🔊 Types

👉 Visual (vision)
👉 Auditory (hearing)
👉 Somatosensory / tactile (touch)

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⚙ What changes

👉 Sensitivity to stimuli
👉 Organization of sensory maps in the brain

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🎯 Function

👉 Experience shapes how we:

• perceive the world

• process sensory information

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💡 Big idea

👉 “Use it shapes it”
👉 More stimulation → stronger processing

Plasticity at multiple timescales

🧠 Core idea

👉 Brain changes occur over different time scales

👉 Some changes happen:

• very quickly (seconds/minutes)

• others over development (months/years)

Plasticity at multiple timescales: Cats Example

👉 In the visual cortex, neurons are organized into columns that respond to line orientation and receive input from both the left and right eyes (ocular dominance columns).

👉 If one eye is blocked early in development during the critical period, the open eye’s input takes over all the neural space in the visual cortex.

👉 When the blocked eye is later opened, it is nonfunctional (functionally blind) because it never formed proper connections.

👉 This shows that visual input during an early critical period is required to organize the brain’s visual architecture.

👉 If input is missing during this window, the deficit is permanent.

👉 Therefore, some forms of plasticity (like vision/ocular dominance) occur only during a short critical period in early development.

Plasticity at multiple timescales: : Enriched environments vs. impoverished environments

👉 In studies with rats, those raised in enriched environments (more physical and social stimulation) had brains that were structurally different from rats raised in impoverished environments (isolation).

👉 Importantly, these experience-based changes occur in both young and adult rats.

👉 This shows that plasticity is not limited to early critical periods, but can occur throughout the lifespan.

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Plasticity at multiple timescales: : BiredsBirds

👉 In some birds, learning and brain growth occur only during specific seasons.

👉 Food-caching birds:

• learn locations of stored food

• this is linked to hippocampus growth

• happens in spring/summer, NOT fall/winter

👉 Birdsong learning:

• brain regions for song become plastic only during certain seasons

• birds can learn songs only during these windows

👉 These abilities usually occur early in life,
BUT if missed, they can still happen later during seasonal breeding periods

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🎯 Big idea

👉 Plasticity is limited to specific times of year, not just early development

Plasticity at multiple timescales; Language Learningtimescales:

👉 First language:

• must begin in the first few years of life

• reflects a critical period

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👉 Second language:

• native-like mastery possible up to ~age 6

• ability declines from ~6–16

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👉 Adulthood:

• learning is still possible

• but less efficient / rarely native-like

Plasticity at multiple timescales;Memory & Aging (Gradual Decline)

👉 Normal aging:

• learning and memory show a gradual decline

• many people can still learn into old age

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👉 Individual differences:

• some maintain strong memory longer

• others show earlier decline

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👉 Pathological aging:

• early-onset Alzheimer’s/dementia → decline begins much earlier (e.g., 40s)

• predicts more severe deficits over time

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🎯 Big idea

👉 Plasticity in memory persists across life but declines gradually, with sharper declines in disease

Human eyeblinking

👉 Eye-blink conditioning = learning a simple reflex

Example:

• A tone plays 🔊

• Then air puffs your eye 💨

• After a while → you blink when you hear the tone

👉 That’s learning.

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🧠 What the study found

👉 Younger adults (around 20):

• learn this very easily

👉 As you age (20 → 40):

• you get worse at learning this simple association

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🎯 So what does it mean?

👉 Even VERY basic learning
(not just complex memory)

👉 gets worse with age

Olfactory System

👉 Olfactory receptor neurons (smell) are easily damaged

• can temporarily lose sense of smell

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👉 These neurons:

• die off

• then regenerate after a few weeks

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👉 This follows a genetically programmed cycle

• birth → death → regrowth (apoptosis + regeneration)

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🎯 Big idea

👉 Plasticity can involve continuous neuron replacement

👉 Helps:

• maintain function

• protect against damage