Development of the Nervous System Flashcards
Introduction - Development of the Nervous System
The brain undergoes considerable development through its lifetime. Process starts with a single fertilised egg, ends with a functional adult brain. The brain is plastic, continuously changing in response to its genetic programs and environment. It is altered both by experience and as the brain develops throughout time. Neurodevelopment refers to the development of the brain and is influenced by many factors. Developing neurons undergo 5 different phases in order to become the human brain we know of. Some of these happen in order, whereas others are more long-term or overlap.
- Induction of the neural plate
- Neural proliferation
- Migration and aggregation
- Axon growth and synapse formation
- Neuron death and synapse rearrangement
A fertilised egg is totipotent, meaning that is has the ability to become any type of body cell. However, 4 days after embryological development, newly created cells lose their totipotency and begin to specialise. At this stage, developing cells are pluripotent, they can give rise to many different types of body cell and have unlimited capacity for self-renewal and become any type of mature cell. As the embryo develops, new cells become increasingly specialised. New cells are multipotent, can only develop into one class of cells e.g. blood cells. Most developing cells eventually become unipotent, they can only develop into one type.
Describe the process of Induction of the Neural Plate (First stage in dev of NS)
- 3 weeks after conception, patch of tissue on dorsal surface of embyro becomes neural plate.
- NP = Tissue going to develop into nervous system
- Development induced by chemical signals from mesoderm layer
- As pulled together, broader areas fuse. Neural tube forms basis of CNS. Neural crest forms basis of PNS.
- Cells of neural plate often refered to as stem cells which are pluripotent and have unlimited capacity for self-renewal.
- Growing neural plate folds to form neural groove. Lips of this form the neural tube which becomes the cerebral ventricles and spinal canal.
Describe the process of neural proliferation (2nd stage in neural development)
- Tissue which develops into neural tube. recognisble as fluid filled tube. Cells of this increase in species specific ways which results in characteristics.
- 3 swellings appear at end of neural tube - become the forebrain, midbrain and hindbrain.
- Pattern controlled by chemical signals from organiser areas of the neural tube - floor plate and roof plate.
Describe the process of migration (Third stage of neurodevelopment)
- Cells migrate to the target location by moving through cells already formed.
- Two types of neural tube migration
- Radial Migration = Moving out by moving along radial glial cells in straight line to outer wall of tube
- Tangenetial Migration - Moving up, occurs at righ angle ro radial.
- Two types of migration for developing cells
- Somal transolation - Growth of extension in direction of migration
- Glia-mediated migration - Temporary network of radial glia cells appear in developing neural tube and move along radial glia network.
- Movement guided by environment
Neural crest migration
- Travels along further distance
- Guided by chemical signals
- Glial cellls involved in release of chemical signals
Describe the process of aggregation (third stage in neurodevelopment)
After migration, cells need to align themselves with other cells and form structures.
Both processes mediated by cell-adhesion molecules.
No synpases or dendrite yet, so have narrow gap junctions.
Describe the process of axon growth (fourth stage of neurodevelopment)
Once migration complete and structures formed, axons and dendrites begin to grow.
Sperry (1963) tested the chemoaffinity hypothesis which is the hypothesis that postsynaptic surfaces release specific chemical labels which attract target axons in both dev and regeneration.
Found out that axons capable of precise growth by rotating eyes of frogs.
- When rotated 180 without cutting optic nerve, frog misdirects tongue strikes by 180.
- Optic nerve cut and eye rotated 180 = first blind, once nerve regen frog misdirects strike by 180. Axons grow back to original synaptic sites.
- Series of chemical signals attract and repel axonal growth
- Pioneer growth cones = First travel to route and interact with guidance molecules
- Fasiculation - Tendency of developing axons to grow along paths established by preding neurons
Topographic Gradient Hypothesis of Axonal Migration
- Axon targets arrange same way on terminal surface as original
- Growing axons guided to their target by 2 intersecting chemical gradients
- Axons maintain spatial relationships with neighbours due to chemical gradients on target cell body
Describe the process of synapse formation (4th stage in neurodevelopment)
- Once reached site, need to establish pattern of synapses
- Need coordinated activity of 2 neurons to create synapse between
- Depends on prescence of glial cells
- High levels of cholestrol needed
Describe the process of neuron death & synapse rearrangement (5th stage in neurodevelopment)
More neurons are produced than required.
Necrosis - Passive cell death
Apoptosis - Active cell death
- Safer than necrosis
- Structures pulled apart carefully
- Inflammation prevented
- Consequence of cancer
Triggered by..
- Some genetically programmed for early death
- Failure to obtain chemicals, competition
Promoted by neurotrophins which promote growth and survival.
Synapse Rearrangement
- Space is left when neurons die. Filled by axon terminals.
- Rearrangement of synaptic connections leading to increased selectivity of transmission
Explain how the human brain is not fully developed at birth
- Find out from animal models. Develops slowly which is unique.
Focus on the Prefrontal cortex
- Age-related changes in cog function.
- Role in working memory - Keeping releavnt info accesible for short periods of time while completing task.
- Planning and carrying out sequences of actions
- Inhibiting responses inappropriate for current context
- Following rules of social behaviour
- Infants don’t display certain cog functions e.g. Piaget.
- Diamond (1991) says because neural circuity of prefrontal cortex not developed yet. Synaptogenesis not maximal until early in 2nd year.
The brain increases in size due to
- Synaptogenesis. Differs between regions.
- Myelination of sensory and motor areas - Increases speed of axonal conduction.
- Increased dendritic branches - Pattern progresses from deeper to more superficial layers.
Name the types of experience affecting early development
Two types of experience
- Permissive = Necessary for info in genetic program to be manifiested
- Instructive = Contribute to direction of development
Explain how experience affects neuroplasticity in adults
The mature brain can still change and adapt. Changing the way humans think about selves and those with brain damage.
Hertzog et al. (2008) - Exercise may reduce or delay memory problems.
Muhlnickel et al. (1998) - Tinnitus produces reorganisation of primary auditory cortex.
Elbert et al. (1995) - Musicians who played with left hand have enlarged hand rep area in right somatosensory cortex.
Describe autism and attempts to identify its neural mechanisms
- Apparent before age of 3, hetereogenous disorder (impaired in some things, superior in others) with genetic basis
- Neural mechansims - Damage to some neural structures. Fusiform face area shows less fMRI activity in response to faces. Mirror neurons - role in understanding intentions of others.
Describe Williams Syndrome and attempts to identify its neural mechanisms
- People sociable and empathetic, Good language skills, low iq
- Serious cognitive deficits such as attentional problems and spatial abilities.
- Missing one of the copies of chromosome 7
- Thinner cortex and white matter. Thinning in orbiotofrontal cortex and boundary of pariertal and occipital cortex.
Discuss brain tumors as a cause of brain damage and the different types
Meningiomas Tumours – Grow between the meninges and 3 membranes covering the CNS.
- Are also encapsulated tumours – Grow within own membrane, but usually benign.
Infiltrating Tumours – Grow through surrounding tissue.
- Usually malignant – difficult to remove
Gliomas Tumours – Develop from glial cells and rapidly infiltrate.
Metastatic Tumours – Grow from being transported to the brain through the bloodstream.
Discuss strokes as a disorder of brain damage
Produce dead tissue area called infarct. Surrounded by dysfunctional area called penumba which is the goal of stroke treatment.
Two types
- Cerebral Hamemorrhage: Cerebral blood vessel ruptures and blood seeps in and destroys surrounding tissue
- Cerebral Ischemia: Disruption of blood supply to brain. 3 causes, thrombosis - plug blocks blood, embolism, plug carried and ecomes lodged, Arteriosclerosis, walls of blood vessels thicken and create blockage.
Glutamate important
- NDMA most involved
- NA+ and CA2 enter postsynaptic triggering release of glutatame, spreading toxin
