Week 10-Brain Plasticity

Question 1

What is brain plasticity?

Answer 1

The ability of the brain to CHANGE throughout life.

Question 2

What is Neurogenesis?

Answer 2

Brains ability to create new neurons

Question 3

How does the brain reorganise to support (the developing child) development?

Answer 3

Synaptic Pruning and reorganisation.

Childhood is a sensitive period for brain plasticity

Question 4

Synaptic Pruning-What happens in the developing brain?

Answer 4

• Synapses are rapidly formed in the first few years of life. (Newborn-2years). The number of synapses in the brain peaks in the first few years of life.
• As the child develops- synapses are removed (synaptic pruning in brain). The synapse number reduces from age 3 onwards.

Question 5

Synaptic Pruning-What happens in the developing brain?

Answer 5

• Synapses are rapidly formed in the first few years of life. (Newborn-2years). The number of synapses in the brain peaks in the first few years of life.
• As the child develops- synapses are removed (synaptic pruning in the brain). The synapse number reduces from age 3 onwards. (Age 3 to adolescence)

Question 6

Why is synaptic pruning important in brain development?

Answer 6

• Synaptic density peaks shortly after birth
• Loss of synapses during childhood/adolescence (‘pruning’)

-Only important and useful connections will be kept => efficient brain
-Input and “use” of regions is what guides pruning
..the developing brain’s structure is shaped by experience

Question 7

Why is synaptic pruning important in brain development?

What guides this pruning?

Answer 7

• Synaptic density peaks shortly after birth
• Loss of synapses during childhood/adolescence (‘pruning’)
• Only important and useful connections will be kept => efficient brain
• The DEVELOPING BRAIN STRUCTURE guides this pruning. ..the developing brain’s structure is shaped by input and experience that child receives.
• Input and “use” of regions is what guides pruning

Question 8

What is different about Synaptic pruning in blind people?

Answer 8

The visual cortex is thicker in blind people due to a lack of visual stimuli (it cannot prune out ineffective neurons).

Question 9

What causes some synapses to be maintained and strengthened and others to be lost in the synaptic pruning process? -Hebbian Modification

Answer 9

Hebbian Modification:

• Neurons which fire together wire together
• Neurons which fire out of synch lose their link

Growth of dendrites in tissue, connections are rapidly formed but also lost. Synapses get strengthened but also pruned out.
Synaptic rearrangement occurs as a consequence of neural activity and synaptic transmission

Question 10

What is HEMISPHERICTOMY?

Answer 10

• In childhood, the brain is highly plastic (responsive to input and experience)
• Demonstrated by hemispherectomy (severe epilepsy having half of the brain removed) patients. (LOOK AT GUIDED LEARNING)

-Early life removal of an entire brain hemisphere = surprisingly little loss of function. You can survive with half of the brain in childhood.

Question 11

Brain Plasticity in Childhood

Answer 11

• During brain development, the brain is very malleable = heightened plasticity
• During brain development, experiences (input) have a large impact on brain development, guiding synaptic pruning and rearrangement = ‘Sensitive period’
• The brain is highly sensitive to inputs during childhood- these causes changes in brain structure and connectivity.

Question 12

What is Homunculus?

Answer 12

A representation of the body parts onto the motor cortex. Basic blueprint in all individuals.

• Individual differences in the organisation of the homunculus
• Homunculus might be able to adapt depending on use; plasticity within the motor cortex

Question 13

Studying sensory disruption in the motor cortex-Finger Representations in Congenital Syndactylie in children

Answer 13

Children born with fingers joined together and corrected (separated)by surgery.
-Looking at homunculus before and after surgery. Motor cortex reorganises based on use.

Question 14

Experiment-whether playing a musical instrument can change/ reorganise motor cortex(homunculus)- USE DEPENDENT PLASTICITY

Answer 14

-Tactile stimulation of the digits of the left hand: 
In musicians (compared to controls) the location of brain activity was shifted and the strength of response was greater

• There was a correlation between how long they’d been playing the instrument (experience) and the size of the change
• Right-hand stimulation: no significant differences

Evidence for expansion of the cortex(homunculus) controlling left hand, due to use.
CORTEX CAN BE REORGANISED DEPENDING ON USE

Question 15

Beyond Motor Cortex-What evidence is there for plasticity and other brain regions
(Blind people)

Answer 15

The auditory cortex contains a representation of sound frequencies (tonotopic map)

-In blind persons, the visual cortex is activated during Braille reading by touch

Suggests a change in function - visual cortex being used for reading

‘Cross-modal’ plasticity touch can replace vision in the visual cortex of blind individuals?

WHEN A PARTICULAR SENSORY INPUT IS LOST, A BRAIN REGION COMPLETELY CHANGES ITS FUNCTION TO IT.

Question 16

Cross-Modal Plasticity: Use of transcranial magnetic stimulation

Answer 16

• Using transcranial magnetic stimulation to DISRUPTS visual cortex function.
• Causes Braille-reading ERRORS and distorts the touch perceptions of blind individuals – no effects in controls asked to touch-read normal letters
• Confirms that visual cortex is being used for TOUCH perception in blind individuals.

Question 17

Brain Plasticity in Adulthood

Answer 17

Motor cortex can reorganise to support motor skills (e.g. in musicians)

And studies in blind individuals show plasticity in other brain regions too:
Expansion of auditory cortex but also cross-modal plasticity (visual/touch)

Plasticity = better performance (functional consequences)

Question 18

PLASTICITY MECHANISMS

What are the mechanisms behind this plasticity?

Answer 18

• Motor learning can provide useful insights
• Motor learning is associated with the REORGANIZATION of motor cortex

This can occur rapidly, through practising a movement
Occurs in the adult (as well as childhood) brain – shows plasticity is maintained throughout the lifespan

Question 19

HOW DO WE STUDY PLASTICITY IN THE ADULT BRAIN?-Brain Plasticity mechanisms in adulthood

Answer 19

• TMS(Transcranial Magnetic Stimulation)
• LTP (Long Term Potentiation)
• Plasticity occurs in adulthood as well as childhood: The brain retains the ability for plasticity throughout the life span
• The reorganisation of cortex occurs in response to experience (use-dependent plasticity) and in response to injury (e.g. stroke - injury-induced plasticity)
• Plasticity has functional consequences (improved performance- e.g.musicians expansion of representation repeated movement)
• Synaptic plasticity (e.g. LTP) is important for the reorganisation.

Question 20

Neurogenesis: As adults… can we still grow new neurons?

Answer 20

-Draganski et al. (2004): juggling training (complex motor training) in adult humans for 3 months
= increased brain volume in hippocampus

• Effects still present 3 month after end of training. Long term change in brain structure.
• Suggests adult HIPPOCAMPUS can grow new neurons, to support learning from experience

Question 21

Neurogenesis from learning-London taxi drivers

Answer 21

The study in London Taxi drivers showed enlarged hippocampus in more experienced drivers

-Suggests that learning can promote neurogenesis

Compared to experience-matched Bus drivers, London taxi drivers have larger posterior hippocampus (equivalent to rat dorsal hippocampus). The more experience the drivers have the larger their posterior hippocampus (not so for bus drivers).

Question 22

NEUROGENESIS – from exam revision?

Brain scan study in medical students

Answer 22

• MRI structural scan 3 months before medical exam, day after the exam, and 3 months later
• Hippocampus volume increased between the 1st, 2nd, and 3rd time points
• Strong evidence for LEARNING-INDUCED plasticity in young adulthood
• Timescale consistent with neurogenesis, since full maturation of new neurons requires several weeks to months

Question 23

NeuroGENESIS – from EXERCISE?

Answer 23

• Physical exercise can trigger new neurons to be formed.
• Exercise can INCREASE HIPPOCAMPAL VOLUME IN HUMANS
• EXCERCISE IN LATER LIFE to maintain hippocampal volume (aerobic training group showed a 2% increase in hippocampal volume vs control stretching group showed shrinkage in hippocampus)

Studies in rodents:
Laboratory rodents given access to a running wheel run as much as 5–8 km a night
This stimulates hippocampal neurogenesis in rodents of all ages. Linked to improved learning (water maze task) in older animals. Learning and memory functional consequences.

Question 24

HIPPOCAMPAL VOLUME ACROSS THE HUMAN LIFESPAN

Answer 24

• Although hippocampus tends to SHRINK as we get OLDER…
• Variability in hippocampus size and variability but EXERCISE HELPS MAINTAIN THIS
• Hippocampal neurogenesis could explain why some individuals in their 70s have similar hippocampal volumes to individuals aged 20-30
• Better memory in later life

Question 25

Neurogenesis from learning-London taxi drivers

Answer 25

The study in London Taxi drivers showed enlarged hippocampus in more experienced drivers (navigation and memory). LEARNING and EXPERIENCE trigger growth in the hippocampus. -Suggests that learning can promote neurogenesis Compared to experience-matched Bus drivers, London taxi drivers have larger posterior hippocampus (equivalent to rat dorsal hippocampus). The more experience the drivers have the larger their posterior hippocampus (not so for bus drivers).

Question 26

Plasticity in Later life

Answer 26

- Factors such as exercise help preserve cognitive performance in later life-People who are physically active in later life, show less decline in cognitive performance as they get older - The brain can also compensate for the effects of ageing in other ways, to try and maintain good function.

Question 27

The 'Normal' Ageing brain- What 3 things happen to the brain structure when we age?

Answer 27

-In normal ageing, changes in brain structure take place- 1) Neurochemical and synaptic changes 2) Volume loss: on average, the brain shrinks 5% to 10% between the ages of 20 and 90 3) Deterioration of white matter fibres(bundles of axons that connect brain regions) The human adult brain stays plastic even in later life – and can counteract/compensate for these changes

Question 28

THE AGEING BRAIN – fMRI EVIDENCE

Answer 28

- Some older individuals maintain excellent cognition, suggesting potential for the brain to adapt - ‘Cognitive reserve’ models suggest the brain has a ‘buffer’ that can adapt and compensate for ageing How? -fMRI studies show the brain activates more brain regions as we get older.

Question 29

THE AGEING BRAIN – BILATERALITY

Answer 29

- fMRI studies have shown greater bilateralisation of activity with ageing: - Older adults additionally recruit the other hemisphere to do a task. HELP COMPENSATE REDUCTIONS IN AGEING BRAIN RECRUITING MORE BRAIN REGIONS. - Led to ‘Hemispheric asymmetry reduction in older adults

Question 30

THE AGEING BRAIN – COMPENSATION?

Answer 30

-To test this, transcranial magnetic stimulation (TMS) can be used to temporarily disrupt a brain region applied to the left or right prefrontal cortex (PFC) during the recognition memory task N=66, <45 and >50 years -In subjects aged <45 , disrupting right (but not left) PFC affected performance - But in subjects aged >50, disrupting EITHER right or left PFC affected performance - Suggests that older adults are using BOTH hemispheres to perform the task – so it’s a compensatory mechanism

Question 31

Brain Plasticity from Childhood to adulthood

Answer 31

-In childhood, experience drives synaptic pruning, which leads to a more efficient brain In adulthood, plasticity is maintained: -Experience causes reorganisation of relevant cortical areas Plasticity also allows the brain to respond to injury - In adulthood, neurogenesis in the hippocampus can occur. Experience, learning, and physical exercise promotes this and can help maintain cognitive function with ageing - In later life, plasticity is also seen as additional brain regions are used to compensate for effects of ageing. (adapt to changes of ageing) BRAIN IS HIGHLY ADAPTIVE AND PLASTIC STRUCTURE.