6 - Development of Nervous System

Axis within Adult Nervous System

• Most of nervous system

  • rostral — up (towards head)

  • caudal — towards tail

  • ventral (towards front) — anterior

  • dorsal (towards back) — posterior

  • ventricle demarcates the rostrocaudal axis — till midbrain

• rostrocaudal axis of nervous system “neuraxis” bends approx 90 deg btn midbrain and forebrain

• in human forebrain

  • rostral — sits anterior

  • caudal — sits posterior

  • ventral — sits inferior

  • dorsal — sits superior

• remnant of embryonic brain flexure

Trilaminar Emrbyo (18 Days)

Folding of Neural Tube

• (1→2) neural folds fold downwards and push neural plate away from dorsal surface of embryo and start pushing it down inside

• (1→2) simultaneously — midline of neural plate buckles in opposite direction — forms v shape going upwards

• (2→3) next stage — neural folds reach midline — migrate inside and form neural crest cells — forms whole of the PNS

• (2→3) medial border of epidermis fuse at the midline — one continuous layer of skin — separates NS from outside world

• (2→3) lateral borders of neural plate meet in centre and form one continuous tube

• (3) neural tube forms whole of CNS

• (3) hollow in the centre — goes on to form ventricles

• dorsal = sensory

• ventral = motor

Differentiation of Neural Tube — 3 Vesicle Stage

1) prosencephalon (rostral end) — forms forebrain structures

2) mesencephalon — forms midbrain structures

3) rhombencephalon — forms hindbrain structures

4) caudal neural tube — develops into spinal cord

Differentiation of Neural Tube — 5 Vesicle Stage

1a) upper subdiv of prosencephalon — telencephalon — forms cortex — hollow forms lateral ventricles

1b) subdiv 2 of prosencephalon — diencephalon — hollow bit forms 3rd ventricle — c shaped structure is optic cup (neural structures associated w eye forms)

2) mesencephalon

3a) upper subdiv of rhombencephalon — metencephalon — forms pons and cerebellum

3b) lower subdiv of rhombencephalon — mylenecephalon — forms medulla

Embryonic Brain Flexures — 3 Vesicle Stage

1) bend between midbrain and forebrain — cephalic flexure

2) bend between spinal cord and brainstem — cervical flexure

Embryonic Brain Flexures — 5 Vesicle Stage

1) ceph flex — maintained in adult

2) cerv flex

3) pontine flexure — divides upper half of brain stem (metencephalon) from lower vesicle (myelenecephalon) — responsible for open medulla/fourth ventricle

Derivatives of Neural Tube — Summary Sheet

Derivatives of Neural Crest

• neural crest cells can migrate throughout the body and give rise to many types of cells e.g.

  • sensory neurons (DRG)

  • autonomic ganglia

  • enteric neurons

  • schwann cells

    

Neuroepithelial Stem Cells in Ventricular Zone + Microglia

• produce diff CNS cells at diff times which then migrate into position

• all neurons and most glia of CNS are generated from NSC

• exception — microglia

  • this lineage migrates in from the yolk sac during early dev and then differentiates in CNS

  • brains immune cells that play key roles in brain dev, plasticity and health

Shaping the Telencephalon — Weeks 6 to 12

• basal part of telencephalon thickens to form pre-cursor of basal ganglia

• diencephalon thickens to form thalamus and hypothalamus — separated by a sulcus

• by end of 3rd month — massive fibre bundles form the internal capsule which connects the telencephalon and diencephalon

• axons start to grow in and out of cortex into thalamus — create massive white matter tracts incl internal capsules

Expanding the Cortex

• up to 15 million neurons born per hour

• centre of cerebral hemispheres anchored to deep structures below (basal ganglia, region of future insula)

• anterior and posterior regions are free to move and fill available space

Formation of Temporal and Frontal Lobes

• bones of skull and attachment via insula constrain the shape of the rapidly expanding cortex

• frontal lobe expands to fill anterior region

• originally caudal regions get pushed ard by skull to lie inferior to insula — forming temporal lobe

• results in formation of great C-shaped cortical arc

• cavity of neural tube is drawn into C shape — forming lateral ventricles in the telencephalon

Brain Growth Continues After Neurogenesis Finishes?

• because

  • neurons grow — somata + dendrites

  • axons grow

  • synapses form (and are refined)

  • glia cells born

  • axons myelinated

• together — causes cortical sheet to grow a little thicker — huge increase surface area

• expanding cortical sheet gets pushed into hills (gyri) and valleys (sulci)

• enables developing brain to make best use of available volume

• compared to adult brain volume

  • birth — 25%

  • 2 years — 80%

  • 14 years — 100%

Rates of Maturation Differ Greatly Btn Brain Regions

• measures synaptic density

• overproduction and then synaptic pruning

Timeline of Human Brain Development