Neural Development Flashcards
What are the 4 phases of neuromuscular development?
- Axonal growth
- Myogenesis
- Synaptogenesis (formation of NMJs)
- Synapse elimination
What happens during the axonal growth phase of neuromuscular development?
- Axons from motor neuron cell bodies grow out to innervate myotome region of somite
- Connections between muscles & nerves are specific
When does neural outgrowth occur?
Before primitive muscle masses contain any muscle fibres
What happens during the myogenesis phase of neuromuscular development?
- Muscle tissue develops from mesoderm when mesoblasts differentiate into myoblasts
- Skeletal muscles develop the length of the embryo fro small clusters of cells (somites)
What is the number of muscle fibres determined by during myogenesis?
Number of foetal myoblasts
What happens during the synpatogenesis phase of neuromuscular development (formation of NMJs)?
- ACh receptors sprout all over the surface of the muscle fibre
- As spinal nerves sprout & target individual myoblasts, motor end plates are formed
- Initially multiple synaptic connections form
- NMJ formation: electrical stimulation & stretch contributes to preventing further innervations
What happens during the synapse elimination phase of neuromuscular development?
Extra synapses eliminated through neuronal death & axon retraction
When does synapse elimination also occur?
After injury to a nerve or post BTX-A injections to treat spasticity
When does distinction into slow & fast twitch muscle fibres occur?
18-20 weeks in utero
In normal growth & development, what is increase in gross muscle size due to?
Hypertrophy (increase in muscle cell size), not hyperplasia (increase in cell number)
What happens during postnatal growth of the brain?
- Increase in size of neurons
- Increase in number of supporting cells (glia)
- Continued development of neural processes & synapses
- Continued myelination
When does maximum density of synapses occur?
- Rapid formation in first 12 months, max density at approx 6-12 months
- Natural pruning/attrition occurs after this (brain retains connections that are used most)
What forms the basis of pathway/synapse retention?
- Stimulation & experience (movement & sensory) enhances function
- Lack of this may lead to loss of unused connections)
Where does the greatest metabolic activity occur in the neonate, at 2-3 months and 6-12 months?
- Neonate: Sensory-motor cortex & brain stem
- 2-3 months: Visual & adjacent parietal cortex (development of visuospatial integrative function)
- 6-12 months: Frontal cortex (development of higher cortical function)
When does myelination occur?
- Must occur before neurons with long axons become fully functional
- Begins in 4th fetal month, most sheaths completed by end of 3rd year of life
- Occurs at different rates in each system
What is the consequence of myelination occurring at different rates in each system?
When neurons are damaged perinatally (time around birth), motor deficits may not be observed until the child is older (i.e. when the systems damaged would normally have become functional e.g. walking)
What do typical developmental milestones reflect?
Maturation of the nervous system & progressive myelination
How does motor control develop & what may it be impacted by?
- Head to toe
- Proximal to distal
- Gross to fine
- May be impacted by requirement of postural stability before the skill can be demonstrated
- E.g. newborns in supported seating show some aiming of arm movement towards slowly moving attractive object
At what stages can disturbances to the developing brain occur?
- Prenatal
- Perinatal
- Postnatal
What are the prenatal causes of brain injury?
- Genetics
- Fetal malformation during growth
- Maternal injection (rubella, CMV, herpes simplex)
- Toxins (alcohol, anti-epileptics, lead)
- Vascular problems, hypoxia, thrombotic episodes
- Metabolic (iodine deficiency, maternal thyroid disease)
What are the perinatal causes of brain injury?
- Problems during labour & delivery (e.g. obstructed labour, antepartum haemorrhage, cord prolapse)
- Neonatal problems (e.g. severe hypoglycaemia, untreated jaundice, neonatal shock, infection, intracerebral haemorrhage, hypoxia ischaemic encephalopathy)
What are the postnatal causes of brain injury?
- Infection e.g. meningitis
- Hypoxia e.g. drowning
- Trauma e.g. MVA, ABI, NAI
What are the basic mechanisms supporting brain plasticity in children?
- Persistence of neurogenesis
- Elimination of neurons through programmed cell death
- Postnatal proliferation & pruning of synapses until 16 years (toddler has 2 x more synapses than adult)
What happens following transient disruption of normal maturation processes in the developing brain?
- Intrinsic attempt to compensate for the injury
- Evidence for functional reorganisation available
- Exact mechanisms are poorly understood
