Development of the brain
- Proliferation - production of new glia cells and neurons in the brain primarily occuring early in life.
- Migration - movement of the new neurons and glia to their eventual locations.
- Differentiation - forming of the axon and dendrite that gives the neuron it's distinctive shape.
- Myelination - process by which glia produce the fatty sheath that covers the axons of some neurons.
- Synaptogenesis - Formation of the synapses between neurons.
Development of the brain: Evidence
- Originally thought new neurons were not formed after initial development. Research has indicated otherwise - receptors for the olfactory system.
- Olfactory system - System used for the sense of smell.
- New olfactory receptors also continually replace dying ones. Olfactory receptors are undifferentiated cells found in the interior of the brain that generate daughter cells which can transform into glia or neurons.
- Development in the brain also occurs in other brain regions
- Songbirds have a steady replacement of new neurons in the singing area of the brain.
- Stem cells also differentiate into new neurons in the adult hippocampus to facillitate learning.
- The brain also has the ability to reorganize itself in response to experience.
1) Axons and dendrites continue to modify their connections throughout their lifetime.
2) Dendrites occasionally grow new spines - this indicates new connections. It is also found as a function of physical activity.
3) Declines in the cerebral cortex in old age much less in those that are physically active
- Neurons also become more finely tuned and responsive to experiences - Blind people often have enhanced tactile senses and increased verbal skills. The occipital lobe normally dedicated to processing visual information adapts to also process tactile and verbal information.
- Extensive practice of a skill changes the brain in a way that improves the ability for that skill. For example:
1) MRI studies reveal the temporal lobe of professional musicians in the right hemisphere is 30% larger than non musicians. Thicker grey matter in part of brain responsible for hand control and vision of professional keyboard players.
- Brain practicing - skill reorganizes the brain to maximise peformance. Training effects bigger when skill begins early.
- Example: Focal hand dystonia or 'musicians cramp'
- 1) Reorganisation of the brain goes too far.
- 2) The fingers of musicians who practice extensively become clumsy, fatigue easily and make involuntary movements.
- 3) This condition is a result of extensive reoorganisation of the sensory thalamus and cortex so that touch responses to one finger overlap those of another.
Brain damage: Causes and Recovery
Recovery after brain damage
- Some behavioural recovery after brain damage.
- Mechanisms of recovery include those similar to the mechanisms of brain development such as new branching of the axons and dendrites.
Causes of brain damage
- Brain tumours - mass of cells growing independently
- Cerebrovascular disorders - strokes are sudden onsent cerebrovascular disorders.
- Closed head injuries - head is not penetrated, blow to the head. Contusions - internal haemorraghing, results from brain slapping against the skull.
- Infections of the brain - Bacterial: Meningitis, Syphillis. Viral infections: Rabies and Aids.
- Neurotoxins - heavy metals, mercury and lead.
- Genetic disorders - Down's syndrome, effecting development mentally and physcially.
- Epilepsy - Characterized by seizures, cause chronic brain dysfunction, 1% of the population are affected by this.
- Parkinson's disorder - A movement disorder, 5% of population, males > females. Dopamin, allievated L-dopa. Synthesized dopamine.
- Multiple Sclerosis - Progressive disorder, attacks myelin of axons in the CNS. Autoimmune disorder.
- Alzheimer's disease - Most common cause of dementia. 10% over 65 and 35% over 85.
- Huntington's disorder - Single dominant gene.
Plasticity after brain damage
- Damage to the nervous system may trigger four neuroplastic responses:
4) Recovery of function
- Anterograde degeneration is the degeneration of the distal segment - the segment of a cut axon between the cut and the synaptic terminals.
- Retrograde degeneration is the degeneration of the proximal segment - the segment of a cut axon between the cut and the cell body.
- Nerve damage without severing the Schwann cell sheaths of the individual axons regenerate to their correct targets.
- When nerve damaged Schwann cell sheaths are slightly separated, individual axons often regenerate incorrectly and reach incorrect targets.
- When nerve damaged and Schwann cell sheaths are widely separated, there is typically no functional regeneration.
The two stage model of neural reorganisation:
1) Strengthening of existing connections through release from inhibition.
2) Establishment of new connections by collateral sprouting.
- Recovery of function - Motor recovery in stroke patients correlated with motor cortex reorganisation (Lipert et al, 2000)
- The area of the somatosensory cortex that originally recieved input from the damaged body part now recieves input from areas of the body normally mapped onto adjacent areas of somatosensory cortex.
- Dr Ramachandran found a way to relieve the pain of phantom limb by moving the phantom limb.
- Knowing feedback is important in movement - a special feedback apparatus was used.
- This was a box divided into two by a vertical mirror.