The Neuroscience Center

1. Embryonic induction

 

Cell Division & Cell Differentiation  

 

• individual cells stop dividing

• take on specific characteristics ex. neurons or glial cells

• Differentiation orders things spatially

• Different neurons various locations (pf)

 

 

three layers emerge

1. endoderm

2. ectoderm

3. mesoderm

 

each cell contains 25,000 genes containing the entire sequence of DNA instructions for development

 

signaling molecules released by the mesoderm turn on certain genes and turn off others, triggering some ectoderm cells to become nerve tissue in a process called neural induction

 

Subsequent signaling interactions further refine the nerve tissue into the basic categories of neurons or glia (support cells), then into subclasses of each cell type. The remaining cells of the ectoderm, which have not received the signaling molecules diffusing from the mesoderm, become skin.

 

proximity of cells to the signaling molecules largely determines their fate; concentration of these molecules spreads out and weakens the farther it moves from its source

 

Embryonic Induction: The influence of one cell group (inducer) over a neighboring cell group (induced) during embryogenesis. Also known as induction.

 

Sonic Hedgehog, is secreted from mesodermal tissue lying beneath the developing spinal cord. (position-sensing mechanism for neurons).

 

The bottom part of the neural tube expresses SHH

diffuses away from FPlate 

affects cells on the dorsoventral axis according to their distance from the floor plate.

 

SHH concentration induces the expression of a gene:

Closest: specialized class of glia (g-g, n-g)

Farther: lower concentrations become motor neurons (n-m)

Even farther: interneurons (n-n)

 

 

 

 

 

2. Migration AKA Pattern Formation

 

• journey to the proper position in the brain

• begins 3-4 weeks after conception

• third week when the embryo = 2 connected sheets of dividing cells

 

1. ectoderm starts to thicken and build up along the middle 

2. cells continue to divide

3. a flat neural plate grows

4. followed by the formation of parallel ridges

5. similar to the creases in a paper airplane that rise across its surface

6. Days later, the ridges fold in toward each other and fuse to form a hollow neural tube

7. top of the tube thickens into three bulges that form the hindbrain, the midbrain, and the forebrain

8. week 7: first signs of eyes and the brain’s hemispheres appear

9. As neurons are produced, they move from the neural tube’s ventricular zone, or inner surface, to near the border of the marginal zone, or outer surface

 

After neurons stop dividing, they form an intermediate zone, where they gradually accumulate as the brain develops.

then migrate to their final destination— with the help of a variety of guidance mechanisms.

 

The most common guidance mechanism, accounting for about 90 percent of migration in humans, are:

 

1. glia- project radially from the intermediate zone to the cortex. provide a temporary scaffolding for ushering neurons to their destination.

2. inhibitory interneurons, small neurons with short pathways usually found in the central nervous system, migrate tangentially across the brain

 

Additional helpers for migration

1. principal axes of the nervous system

• embryo sets up a number of localised polarizing regions in the neural tube that secrete signal molecules. form a gradient of concentration with distance. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This process can be affected by different factors

 

1. External forces, such as alcohol, cocaine, or radiation, can prevent proper migration, resulting in misplacement of cells, which may lead to mental retardation or epilepsy

2. Mutations in genes that regulate migration have been shown to cause some rare genetic forms of retardation and epilepsy in humans

 

3. Specializing

 

1. reach their final location

2. must make the proper connections so ex. vision/hearing can emerge

 

Various regions of the early nervous system express different subsets of genes, presaging the emergence of brain areas - forebrain, midbrain and hindbrain - with distinct cellular architecture and function.

Induction, proliferation, and migration -->>> occur internally during fetal development

Next phases of brain development -->>>dependent on interactions with the environment

 

After birth and beyond, such activities as listening to a voice, responding to a toy, and even the reaction evoked by the temperature in the room lead to more connections among neurons

 

Neurons become interconnected through growth of:

 

1. dendrites; extensions of the cell body that receive signals from other neurons

2. axons; extensions from the neuron that can carry signals to other neurons.

 

Axons enable connections between neurons at considerable distances, sometimes at the opposite side of the brain, to develop. In the case of motor neurons, the axon may travel from the spinal cord all the way down to a foot muscle.

 

GC, specialized for moving through tissue

target reached=no longer able to voyage

Target cell selected with high precision

may have to cross over other growth cones heading for different places.

Along the path, guidance cues that attract (+) or repel (-) the growth cones help them find their way

 

Many molecules help guide growth cones, some molecules either:

 

1. lie on the cells that growth cones contact

2. are released from sources found near the growth cone; bear molecules that serve as receptors for the environmental cues

• binding of particular signals with receptors tells the growth cone whether to move forward, stop, recoil, or change direction. (proteins with names such as netrin, semaphorin, and ephrin)(families of molecules are used; at least fifteen semaphorins and at least nine ephrins)

 

1. axons reach their targets

2. form connections with other cells at synapses

3. the electrical signal of the sending axon is transmitted by chemical neurotransmitters to pt-syn

4. either provoke or prevent the generation of a new signal

 

Responsible for the astounding information-processing capacity of the brain:

 

1. regulation of transmission at synapses

2. integration of inputs from the thousands of synapses each neuron receives

 

 

Connectional specificity:  For processing to occur properly, the connections must be highly specific.

Specific sites on the neuron are dedicated to recieving and sending an action potential

 

Specificity arises from the mechanisms that guide each axon to its proper target area

Additional molecules mediate target recognition when the axon chooses the proper neuron.

They often also mediate the proper part of the target once the axon arrives at its destination.

 

 

Dendrites also are actively involved in the process of initiating contact with axons and recruiting proteins to the “postsynaptic” side of the synapse

 

The tiny portion of the axon that contacts the dendrite becomes specialized for the release of neurotransmitters, and the tiny portion of the dendrite that receives the contact becomes specialized to receive and respond to the signal.

 

also other ensure that the synapse can transmit signals quickly and effectively.

and other molecules coordinate the maturation of the synapse after it has formed so that it can accommodate the changes that occur as our bodies mature and our behavior changes.

 

A combination of signals also determines the type of neurotransmitters that a neuron will use to communicate with other cells. For some cells, such as motor neurons, the type of neurotransmitter is fixed, but for other neurons, it is not.

 

Just as genes turn on and off signals to regulate the development of specialized cells, a similar process leads to the production of specific neurotransmitters

 

Location of the synapse determines what gene will express and the chem msngers produced

 

Despite spatial arrangement & specific connectivity is established, wiring is subject to activity-dependent refinement, ex. pruning of axons and the death of neurons.

 

Critical Periods

 

CP: the developing nervous system must obtain certain critical experiences, such as sensory, movement, or emotional input, to mature properly.

 

characterized by:

1. high learning rates

2. enduring consequences for neuronal connectivity

 

After a critical period, connections diminish in number and are less subject to change, but the ones that remain are stronger, more reliable, and more precise. turn into the unique variety of sensory, motor, or cognitive “maps” that best reflect our world

 

The last step in the creation of an adult human brain, the frontal lobes, whose function includes judgment, insight, and impulse control, continues into the early 20s

 

Ex.1. a monkey raised from birth to 6 months of age with one eyelid closed permanently lost useful vision in that eye because of diminished use. Loss of vision is caused by the actual loss of functional connections between that eye and neurons in the visual cortex. This finding has led to earlier and better treatment for the eye disorders of congenital cataracts and “lazy eye” in children. Sharp vision influenced by patterned electrical activity in the retina & initial exuberant set of connections is sculpted during a critical period, after which the basic pattern of the visual system is complete, at around eight weeks of age in monkeys, perhaps a year in humans.

 

Ex.2. cochlear implants introduced in infancy are most effective in restoring hearing to the congenitally deaf.

 

Cognitive recovery greatest early in life:

1. social deprivation

2. brain damage

3. stroke 

 

Many people have observed that children can learn languages or develop musical ability (absolute pitch) with greater proficiency than adults.

 

Heightened activity in the critical period may, however, also contribute to an increased incidence of certain disorders in childhood, such as epilepsy. Fortunately, as brain activity subsides, many types of epilepsy fade away by adulthood.

 

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