Development OF THE NERVOUS SYSTEM Development and Plasticity

DEVELOPMENT AND CHANGE IN THE NERVOUS SYSTEM The nervous system begins as a hollow tube that later becomes the brain and spinal cord. begins development when the surface of the embryo forms a groove. The edges of this groove curl upward until they meet, This makes a groove into a tube. About day 18 post-conception Development of the nervous system then proceeds in four distinct stages: cell proliferation migration circuit formation circuit pruning

Figure 3.21 Development of the neural tube Left: Photograph of the stages of neural tube development. Right: A drawing showing the tube’s location on the embryo’s surface.

proliferation and migration Cells that will become neurons divide and multiply at the rate of 250,000 new cells every minute. occurs in the ventricular zone Ventricular zone = the area surrounding the hollow tube that will later become the ventricles and the central canal. Migration: newly formed neurons migrate move from the ventricular zone outward to their final location. Aided by specialized radial glial cells.

Figure 3.22 A neuron migrates along glial scaffolding At left, immature neurons migrate from the inner layer where they were “born” to their destination between there and the outer layer. Right, a close up of one of the neurons climbing a radial glial cell scaffold.

Circuit formation Circuit formation: axons of developing neurons grow toward their target cells form functional connections. Growth cones: Form at tip of axons Allow axons to find their way form These sample the environment for directional cues. Chemical and molecular signposts attract or repel the advancing axon, coaxing it along the way until neuronal axons reach their final destinations. Push, pull, and hem neurons in from the sides Chemical & molecular forces guide neuron to intermediate stations Guide past inappropriate targets

Figure 3.23 Neurons with growth cones

circuit pruning Pruning: next stage of neural development involves elimination of excess neurons and synapses. Neurons that unsuccessful in finding a place on a target cell, or that arrive late, die. Second step of circuit pruning: refining organization nervous system refines its organization continues to correct errors by eliminating large numbers of excessive synapses. Synapses are strengthened or weakened depending on whether presynaptic and postsynaptic neurons fire together.

Importance of neurotrophins Neurotrophins = chemicals that enhance development and survival of neurons It is thought that the postsynaptic neuron sends feedback to the presynaptic terminals in the form via this chemical decreases plasticity (ability to be modified) of these synapses

Ontogeny recapitulates Phylogeny!

Anacephaly Anencephaly: absence of a major portion of the brain, skull and scalp These portions of brain fail to develop during embryonic period. cephalic disorder that results from a neural tube defect  occurs when the rostral (head) end of the neural tube fails to close, usually between the 23rd and 26th day following conception children born with this disorder usually only lack a telencephalon: the cerebral hemispheres remaining structure usually covered only by a thin layer of membrane skin, bone, meninges, etc. are all lacking infants with this disorder usually do not survive longer than a few hours or possibly days after their birth. In the U.S.: occurs in about 1 out of every 10,000 births.

DEVELOPMENT CHANGEs: Fetal Alcohol Syndrome Fetal alcohol syndrome (FAS) often produces mental retardation caused by the mother’s use of alcohol during a critical period of brain development. FAS brains are often small and malformed neurons are dislocated. Why? Damage occurs during migration many cortical neurons fail to line up in columns as they normally would The radial glial cells revert to more typical glial form prematurely. Other neurons continue migrating beyond the usual boundary of the cortex.

number of the neurons have migrated beyond the rest of the neurons. (a) In the normal brain the neurons (the dark spots) tend to line up vertically. ( b) In the alcohol-exposed brain the neurons are arranged randomly. Also, a number of the neurons have migrated beyond the rest of the neurons. B. Figure 3.24 Fetal alcohol syndrome in the mouse brain (a) In the normal brain the neurons (the dark spots) tend to line up vertically. (b) In the alcohol-exposed brain the neurons are arranged randomly. Also, a number of the neurons have migrated beyond the rest of the neurons.

experience is critical? Stimulation continues to shape synaptic construction and reconstruction throughout the individual’s life. Reorganization a shift in connections that changes the function of an area of the brain. Much of the change resulting from experience in the mature brain involves

Specific areas for specific functions? Yes and NO! Doctrine of specific nerve energies Johannes Müller’s States: each sensory projection area produces its own unique experiences regardless of the kind of stimulation it receives. Somewhat true This is why you “see stars” when your rollerblades shoot out from under you and the back of your head (where the visual cortex is located) hits the pavement. But functions are also shared and redundant

Fixing the broken brain? Why study development of nervous system? Gives us clues about how to repair the nervous system when it is damaged by injury, disease, or developmental error Tells about organization, growth, patterns. Regeneration: Growth of severed axons Occurs in amphibians Occurs in mammals’ peripheral nervous system. Myelin = critical importance: provides a guide tube for the sprouting end of a severed neuron to grow through extending axon guided to its destination as it during development But: typically only in peripheral nervous system.

neurogenesis Neurogenesis: the birth of new neurons Nervous system could repair itself is if it could grow new neurons In adult mammalian brain produces relatively few new neurons BUT: evidence for significant neurogenesis in two areas. hippocampus, near the lateral ventricles, supplying the olfactory bulb Both important for memory, so perhaps memory does “grow”.

How fix brain if can’t grow new neurons? Compensation Having one brain area take over the functions damaged in another area simplest neural recovery: uninjured tissue takes over functions of lost neurons. Presynaptic neurons sprout more terminals form additional synapses with their targets and postsynaptic neurons Also add more receptor cells. Can even get reorganization A more dramatic form of neural recovery Can involve major brain areas.

Evidence for regeneration? Ramón y Cajal declared: no regeneration in the central nervous system Today: data suggest may be several strategies for inducing self repair following damage. How? neuron growth enhancers counteracting forces that inhibit regrowth providing guide tubes or scaffolding for axons to follow Can already repair peripheral nervous system fairly well

How repair CNS? Alternative:? The most exciting research uses stem cells to replace injured neurons. Stem cells: undifferentiated cells that can develop into specialized cells such as neurons, muscle, or blood Obviously highly controversial. One method: Placing embryonic stem cells into an adult nervous system Encourages new neurons to differentiate into neurons appropriate to that area. But: work like fetal cells- not perfect Alternative:? Grow your own Use stem cells from other body parts

Figure 3.27 A stem cell Because they can develop into any type of cell, stem cells offer tremendous therapeutic possibilities.