Physical Ageing

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Brain differences between infancy and adulthood


  • At birth, the infant brain resembles the adult brain. All 100-200 billion neurons present.
  • Main structures are present.


  • 25% of adult brain weight
  • At birth, 1/6 of the connections that are present in the adult brain
  • Less myelin (fatty sheath) than adult brain
  • Less dendritic spines
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How does the brain develop?

The brain grows faster than any other part of the body. 

  • By 24m, the brain in 75% of adult size and weight.
  • By 5 years, the brain is 90% of adult size and weight, whereas body size is only 30% compared to adults.

The brain undergoes 5 different processes during development:

1) Synaptogenesis

2) Synaptic Pruning

3) Myelinisation

4) Migration

5) Lateralisation

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What is synaptogenesis?

  • Synaptogenesis is the increase in the number and length of dendrites and terminal branches.
  • This increases the number of interconnections among cells.
  • By about 12 months, the infant brain and 2X and many synapses as the adult brain.
  • Over the first two years of life, networks of neurons become increasingly complex and interconnected - this supports more complex communication between neurons, and permits rapid growth of cognitive skills.
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What is synaptic pruning?

Synaptic pruning occurs when the neural network becomes too complicated.

  • It eliminates unnecessary/unused synaptic connections
  • For those that remain, the connections become stronger.
  • Allows established neurons to build more elaborate communication networdks with other neurons.
  • Both synaptogenesis and pruning occur in stages across the brain.
  • It is not uniform across the brain
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What is myelinisation?

  • The amount of fatty insulation myelin increases rapidly. 
  • Helps to speed up neuronal transmission. 
  • The process of myelinisation doesnt slow down until adolescence.
  • With the fatty sheath, there is a 340x difference in speed - action potentials occur 10x more compared to unmyelinated.

It is also gradual, and stage based:

  • Hippocampal myelinisation completed by preschool age - creates massive improvements in memory. 
  • Reticular formation myelinisation completed by 5 years - helps to increase concentration.
  • Myelinisation of corpus callosum starts by 1 year, increases between 3-6 years - facillitates transfer across both hemispheres. 
  • Frontal lobe myelinisation continuesright through to adulthood.
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What is migration and lateralisation?


  • Neurons reposition themselves with growth, becoming arranged by function.
  • Disruptions in neuronal migration have been linked to developmental disorders such as dyslexia and ASD.


  • Left hemisphere bias in newborn speech perception
  • By 4-5 months, left hemisphere bias for positive emotions and categorical perception
  • Right hemisphere shows early preference for global processing of facial patterns, negative emotions, colour categorisation.
  • However, lateralisation is limited - it changes with experience, individual differences and has cultural and gender differences.
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How does the brain develop during adolescence?

  • The brain produces oversupply of grey matter during adolesence. 
  • This is later pruned back at a rate of 1-2% per year.
  • Continued myelinisation of some areas (frontal/pre-frontal cortices) and pruning of others leads to increased complexity of neural networks and sophistication of thinking.
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How does this aid cognitive development?

  • Development of the pre-frontal cortex during adolescence during adolescence fuels more complex and sophisticated cognitive skills.
  • But, adolescent pre-frontal cortex is biologically immature = ability to inhibit impulses is not fully developed. The PFC does not develop fully until early 20's.
  • It is more vulnerable at this stage. 

Between adolescence and adulthood:

  • Functional and structural changes in the brain continue to emerge.
  • Activation of the brain becomes more fine-tuned to task - e.g. executive function tasks (increased recruitment of PFC and frontal cortices and decreased recruitment of sensory region).


  • Myelinisation continues well into adulthood, but decreases over middle age.
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What are the reflexes of infants?

  • Rooting reflex - disappears at 3 weeks. It is the neonates tendency to turn its' head towards things that touch its cheeks. Possible function (PF): Food intake.
  • Stepping reflex - disappears at 2m. Movement of legs when held upright with feet touching the floor. PF: Prepares infants for independent locomotion.
  • Swimming reflex - disappears at 4-6m. Infants tendency to paddle and kick in a sort of swimming motion when lying face down in a body of water.PF: Avoidance of danger
  • Moro reflex - disappears at 6m. Activated when support for the neck and head is suddenly removed. The arms of the infant are thrust outward and then appear to grasp onto something. PF: Similar to primates' protection from falling.
  • Babinski reflex - disappears at 8-12m. An infant fans out its toes in response to a stroke on the outside of its foot.
  • Startle reflex (remains in different form). An infant (in response to a sudden noise), flings out its arms, arches its back and spreads its fingers. PF: Protection.
  • Eye-blink - remains. Rapid shutting and opening of eye on exposure to direct light. PF: Protection of eye from direct light.
  • Sucking reflex - remains. Infant's tendency to **** at things that touch its lips. PF: Food intake.
  • Gag reflex - remains. An infants reflex to clear its throat. PF: Prevents choking.
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How are motor skills developed in infants?

Development of motor skills can be related back to brain development and myelination of neurons in areas of the brain related to balance and coordination (e.g. motor cortex, cerebellum). They also rely on practice and experience.

  • 1) 3.2 months: rolling over
  • 2) 3.3 months - grasping rattle
  • 3) 5.9 months - sitting without support
  • 4) 7.2 months - standing while holding on
  • 5) 8.2 months - grasping with thumb and finger
  • 6) 11.5 months - standing alone well
  • 7) 12.3 months - walking well
  • 8) 14.8 months - building tower of two cubes
  • 9) 16.6 months - walking up steps
  • 10) 23.8 months - jumping in place
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Are there any gender differences in motor dev?

  • Gender differences become more pronounced during middle childhood (e.g. throwing skills better in boys than girls at 8 years old), but equal participation in activities evens them out.
  • More societal than physical.
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What is physical dev like in adulthood?

Most basic physical maturation is complete by early adulthood. 

  • Early to mid adulthood brings the first real negative consequences of development - e.g. senescence (naturally occurring ageing) and environmental/lifestyle choices/illness.
  • Gomperts Law - Death rates for contemporary humans double every 8 years (e.g. a 38yr old would be 2x as more likely to die as a 30 year old). Additionally, men are 2x as likely to die as women.
  • Most signs of early ageing are cosmetic - hair, change of skin elasticity, loss of height.
  • There are also internal indicators too - the brain becomes lighter and smaller, resp and dig systems decline, muscle fibres decrease and hormonal levels decrease.
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How do reaction times change as we age?

1) Middle adulthood: Negligible change - can be compensated for by being more careful and practicing the skill.

2) Older adults (65+): Significant decline - Peripheral slowing hypothesis: Overall processing speed declines in peripheral nervous system. Generalized slowing hypothesis: Processing in all parts of the nervous system, including the brain, is less efficient.

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Why do our bodies grow old?

  • The maximum lifespan for the human species appears to be around 120 years. 
  • Over recent years, life expectancy has risen, but lifespan hasn't changed at all.
  • There are two basic kinds of theory of ageing - chance (external events) and fate (result of an inernal necessity).
  • No single theory of ageing appears to explain all the complex processes that occur in cells and body systems.
  • The question remains whether or not its possible to intervene to correct damage to the ageing body or modify the genetic program.
  • Compression morbidity theory claims that we should aim for healthy old age, followed by a rapid decline and death.
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What is wear and tear theory?

Wear and tear theory expresses the idea that ageing is the result of chance (i.e. we're unlucky enough to be exposed to the outside environment). Its a good explanation for some aspects of ageing, such as the fact that our joints and bones become damaged over time as an outcome of living.

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What is somatic mutation theory?

Somatic mutation theory states that cells can be damaged by radiation, and as a result, mutate or experience genetic changes. A more modern and sophisticated version of the wear and tear theory, but has little science behind it.

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What is error accumulation theory?

Decremental changes of senescene are the result of chance or random changes that downgrade the genetic code. Over time, small errors in genetic coding occur and eventually make the later copies unreadable. Another variation of wear and tear, updated for the genetic age.

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What is accumulative waste theory?

The accumulation of waste products eventually interferes with cell metabolism and leads to death. But, research has found that, although waste products do accumulate, there is little evidence of harm to the organism.

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What are the other bio theories?

  • Autoimmune theory - the system may eventually become defective and no longer distinguish the body's own tissues from foreign tissues.
  • Ageing clock theory - Ageing is programmed into our bodies like a clock ticking away from conception. It is perhaps centred around the endocrine system (hormones). Myelin-based theory falls into this broad category.
  • Celluar theory - Ageing ultimately results from the progressive weakening of capacity for cell division, perhaps through exhaustion of the genetic material.
  • Cross-linkage theory: Bodily changes during ageing result from the accumulatio of cross-linking compounds in the body's collagen, which gradually becomes stiff. Connective tissue loses elasticity, resulting in physical signs of ageing. Piling up of harmful molecules impairs cell function.
  • Free - radicals - Damage created by free radicals (molecules that appear as a byproduct of oxygen metabolism in cells) eventually gives rise to the symptoms we see as ageing. The body produces anti-oxidants - substances that prevent damage to cells. But, animal studies on anti-oxidants show minimal effect on ageing.
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Is ageing inevitable?

There is only one environmental intervention that has scientifically been shown to be connected to longevity in mammals: reducing food intake.

  • When caloric intake is reduced by 40% calories than normal, age-related deterioration slows down, and age-related diseases are diminished. This equates to 1400 calories/day.
  • This is possibly because caloric reduction slows the metabolism - biological clock slow down?
  • Consistent with many theories - dna- based, free-radical based, immune-system based, immune system based. 
  • But there is little human evidence.
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