Development of the Nervous System

1. Development of the Nervous System

2. Development of the CNS • The embryo begins as a flat disk with three layers of cells. • Endoderm forms the lining of the viscera (internal organs). • Mesoderm becomes the bones and muscles. • Ectoderm gives rise to the nervous system and skin. • Neural plate becomes the nervous system.

3. Neurulation • At about 3 weeks, a rostral to caudal groove forms in the neural plate. • The two sides of this groove fold together and fuse forming a neural tube. • The entire nervous system develops from the neural tube. • Part of the tube pinches off and becomes the neural crest – the neurons of the peripheral nervous system.

4. Differentiation • Structures become more elaborate and specialized during development. • The neural tube forms three vesicles. • Forebrain becomes the neocortex. • Midbrain becomes the tegmentum and tectum (substantia nigra, inferior/superior colliculi) • Hindbrain becomes the cerebellum, pons, medulla, and brain stem.

5. Other Names for the Parts • Telencephalon and diencephalon – forebrain. • Mesencephalon – midbrain • Rhombencephalon -- hindbrain

6. Ventricles • Ventricles are open, fluid-filled spaces within the brain. • Cerebrospinal fluid (CSF) • Ventricles correspond to the three parts and can be used as landmarks across species. • Cerebral aqueduct (narrow channel) identifies midbrain. • Ventricles are continuous and continue into spinal cord.

7. Neocortex • The terms cortex and neocortex are used interchangeably when referring to humans. • Only mammals have neocortex. • Neocortex is used for different things in different species. • Evolution has its greatest impact on the size and function of the neocortex. • Localization of functions to areas of the brain is referenced to Brodmann areas.

9. Names for Collections of Neurons • Gray matter – neuronal cell bodies. • Cortex – a thin sheet of neurons on the brain’s surface. • Nucleus – a mass of neurons deep in brain. • Substantia – related neurons with less distinct borders than a nucleus. • Locus – small, well-defined group of cells. • Ganglion – group of peripheral NS neurons.

10. Names for Collections of Axons • Nerve – bundle of axons in peripheral NS. • White matter – CNS axons. • Tract – axons with a common origin and destination. • Bundle – axons that run together for awhile. • Capsule – axons connecting brain stem with cortex. • Commisure – axons connecting hemispheres. • Lemniscus – a tract that runs through the brain like a ribbon.