BRAIN DEVELOPMENT FROM INFANCY TO ADOLESCENCE

Question 1

Nervous system

Answer 1

The nervous system is not a static network of interconnected elements; rather, it is a plastic (changeable), living organ that grows and changes continuously in response to its genetic programs and its interactions with the environment

Question 2

Cells in the human nervous system

Answer 2

• Neurons are the basic functional units of the nervous system. They take in information from other neurons (reception), integrate those signals (conduction), and pass signals to other neurons (transmission)
• Glial cells nourish, protect, and physically support neurons and are thought to be particularly critical in brain development. One type of glial cell, the oligodendrocyte, covers the axons of neurons with myelin, a substance critical to the effective functioning of the brain

Question 3

Speed of propagation of the action potential

Answer 3

• Speed of propagation of the action potential is determined by:
  o Diameter of axon (bigger=faster)
  o Presence or absence of a myelin sheath

Question 4

Myelination

Answer 4

The myelin can block the potassium and sodium channels
Because the myelin is prohibiting the ion transfer, that ion transfer only happens in the gaps. How does that speed up the action potential?
Like a Mexican wave – individuals sitting down and standing up down propel it, it’s because each person waving triggers the next person along. That’s what’s happening in an axon

Question 5

Embryonic Development of the Nervous System

Answer 5

• At 18 days post conception, an embryo consists of 3 layers of cells: endoderm, mesoderm, and ectoderm

Question 6

The ectoderm of the embryo thickens to form the neural plate

Answer 6

o The cells in the ectoderm become specialised tissue that will borm the brain and spinal cord
o This sheet of specialised cells is called the neural plate

Question 7

Cross section of the developing neural tube

Answer 7

• The neural tube will grow to form the central nervous system
• The neural crests will develop into the peripheral nervous system

Question 8

Cross Section of the developing neural tube

Answer 8

This developing neural tube begins to form discrete enlargements of vesicles. These embryonic vesicles will develop into the major regions of the brain: the forebrain, midbrain and hindbrain.

Question 9

Development of the Nervous system: Neurogenesis

Answer 9

• Neurogenesis – the mitotic division of nonneuronal cells to produce neuroblasts

Question 10

Development of the Nervous system: Cell Migration

Answer 10

• Cell migration – the massive movements of nerve cells or their precursors to establish distinctive nerve cell populations (nuclei in the CNS, layers of the cerebral cortex etc)

Question 11

Development of the Nervous system: Differentiation

Answer 11

• Differentiation – of cells into distinctive types of neurons or glia

Question 12

Development of the Nervous system: Synaptogenesis

Answer 12

• Synaptogenesis – the establishment of synaptic connections as axons and dendrites grow

Question 13

Development of the Nervous system: Neuronal Cell Death

Answer 13

• Neuronal cell death – the selective death of many nerve cells

Question 14

Development of the Nervous system: Synapse Rearrangement

Answer 14

• Synapse rearrangement – the loss of some synapses and development of others to refine synaptic connections

Question 15

Neurogenesis

Answer 15

• The production of nerve cells is call neurogenesis
• The cells that will give rise to neurons begin as a single layer along the inner surface of the neural tube. These cells divide in a process called mitosis and gradually form a closely packed layer of cells
• Immature neurons produced via mitosis are called neuroblasts

Question 16

Cell migration

Answer 16

• Cells migrate to establish distinct nerve cell populations (nuclei in the CNS, layers of the cerebral cortext etc)
• Cells are still immature (no axons, dendrites)
• Cells migrate outwards, so innermost cell layer is the oldest

Question 17

Differentiation

Answer 17

• Once immature neurons have completed their migration and have reached the embryonic region where they become functional units of the nervous system, differentiation takes place
• Differentiation allows the cell to acquire the distinctive appearance of neurons characteristic of that particular region (i.e – cells become distinct types of neurons or glia as processes are formed)
• Two influences on differentiation
  o Intrinsic self-organisation
  • Cell-autonomous differentiation
  • Cell uses information contained within itself (e.g. the cell transcribes a particular subset of genes to make the particular protein it needs)
  o Neural environment
  • Induction-based differentiation
  • Cell uses information from neighbouring cells

Question 18

Synaptogenesis

Answer 18

• Establishment of synaptic connections
  o Growth of axons, dendrites and synapses
• Tips of axons and dendrites have growth cones
  o Filopodia (fine outgrowths) and Lamellipodia (sheetlike outgrowths) adhere to extracellular environment
  • Growth cone is pulled along, extending the axon or dendrite in that direction
• Growth pathway is guided by chemicals released from the target cell
  o Chemoattractants and chemorepellents act on specific growth cones
• Connections are affected by experience

Question 19

Neuronal Cell Death

Answer 19

• As strange as it may seem, neuronal cell death is a crucial phase of brain development, especially during prenatal stages
• The physiological process of eliminating differentiating neurons is very important for the correct design of regions and circuitry in the nervous system
• Selective death on many nerve cells
  o Naturally cocuring cell death (apoptosis)
  o Sculpting and refining process
• Between 20-80% of cells in a given region will die

Question 20

Many neurons die during normal early development (part 2)

Answer 20

• The basic principle of selective cell death is that neurons must establish synaptic contacts with other neurons (or neuromuscular plates with muscles) in order to survive: those immature neurons that do not establish functional connections are eliminated
• Cells compete not just for synaptic sites, but for a chemical that the target structure makes and releases. Such target-derived chemicals are called neurotrophic factors

Question 21

Synapse Rearrangement

Answer 21

• Finally, the process of synapse rearrangement allows the synaptic connections between the remaining cells to be reorganised
  o Active synapses are maintained, inactive synapses are retracted, and new synapses are also formed

Question 22

Development of the brain: Key influences: Intrinsic Factors

Answer 22

o Genes

Question 23

Development of the brain: Key influences: Extrinsic Factors

Answer 23

o Nutrients
o Drugs and toxins
o Cell-cell interactions

Question 24

Development of the brain: Key influences: Neural Activity

Answer 24

• Post-natal sensory input (experience)

Question 25

Structural Brain Changes: Cortical Changes

Answer 25

o Infancy and early childhood is characterised by a dramatic period of synaptogenesis, followed by an adaptive process of cell death and pruning. There is another notable surge of synapse growth just before puberty.
o The strengthening or elimination of synapses is dependent on environmental demands or experience; those that are more often used are strengthened and those that are rarely used are eliminated
- Overall, grey matter (neuronal cell bodies, dendrites, and glial cells) development follows an inverted U pattern of growth, first thickening in volume, peaking, and then thinning

Question 26

White matter changes (myelinated axons)

Answer 26

• White matter increases in a roughly linear pattern throughout childhood, adolescence, and into early adulthood
• However, different brain structures myelinate at different times: myelogenetic cycles
  o Sensorymotor pathways myelinate early
  o Regions mediating higher order functions myelinate late (e.g. the prefrontal cortex)

Question 27

Development of the Prefrontal Cortex

Answer 27

• Synaptic density – reaches adult levels in adolescence
• Myelination – thought to be complete in the early 20s
• Therefore, there is relatively ‘late’ maturation of the prefrontal cortex
  o What are the implications of this?
  • To answer this, we ned to know what the PFC does!

Question 28

Functions of the prefrontal cortex

Answer 28

• ‘executive’ functions
• working memory
• planning and organisation
• inhibitory control
• self-monitoring
• insight and judgement
• cognitive flexibility
• selective and sustained attention