CNS and PNS

1. brain complexity

- There are some difference between nervous system of invertebrates and vertebrates. In neuroscience field, using model organism for research. Two main model organism is C.elegans and Drosophlia. They have fewer number of neurons. C.elegans has 302 neurons on average. Drosophlia has hundred thousand of neurons. But Vertebrates, they have more neurons in their nervous system. For example, In human, we have 85 millions of neurons. Normally, high evolutionary organisms have more neurons than low evolutionary organisms.Percent of glia also different. In C.elegans and Drosophila, Glia make up about 15% of their nervous system. In human, percent of glia is estimated range from 50 to 90% of cells. This difference imply greater glial numbers were essential for achieving increased brain complexity.

 

2. Action potential

Depolarization phase – Na+

Repolarization phase – K+

Hyperpolarization phase – K+ channel remain open after resting potential. Nerve impulse 방향 유지가능하게 해줌

 

3. myelination

•Nonmyelinated axons

  - Velocity of the AP depends on diameter of axon

  - low evolutionary axon

•myelinated axons

  - Saltatory conduction

  - velocity of conduction  :  increase 50~100 folds

my thinking : Complex Brain in Vertebrates (because of Limiting bony structures, Longer axonal lengths in CNS and PNS, Pack many more axons in a given space so need Many small-diameter axons, To reduce the effective capacitance, and To increase the effective membrane resistance

 

4. glial cell

- Non-neuronal cells that support neurons and its signal transduction

- To surround neurons and hold them in place

- To supply nutrients and oxygen to neurons

- To insulate one neuron from another

- To destroy pathogens and remove dead neurons

- Human glial cells divided into 2 large categories, PNS and CNS. Satellite cells and schwann cells are in PNS, oligodendrocytes, astrocyte, microglia and ependymal cells are in CNS.

- CNS

-> astrocyte, sometimes called as astroglia, this cell is star-shaped and located in brain and spinal cord. It connect to capillary, forming BBB(Brain Blood Barrier), molecule transport, absorbing Ca2+, H2O for maintain homeostasis of CNS

-> microglia account for 10 to 15% of all cells found in brain. Microglia is resident macrophage cell, so they act as the first and main form of active immune defense. They remove useless component and protect neurons from microorganisms.

-> ependymal cell. They forming thin epithelial lining of the ventricular system. And they involved in the production of cerebrospinal fluid(CSF) and neuroregeneration

-> oligodendrocyte. It forms myelin sheath in CNS and do the same function done by schwann cells in PNS.

- PNS

-> schwann cells form myelin sheath and relate to signal transduction.

-> satellite cell cover the muscle fibers and neuronal cell bodies. It supply nutrients and transport O2 and CO2 in PNS.

5. neural stem cell

- Neural stem cells (NSCs) are self-renewing, multipotent cells that generate the neurons and glia of the nervous system of all animals during embryonic development.

- NSCs are stimulated to begin differentiation via exogenous cues from the microenvironment, or stem cell niche. This capability of the NSCs to replace lost or damaged neural cells is called neurogenesis.

Q. How the body can distinguish CNS and PNS?

- CNS에선 oligodendrocyte 가 myelination 을 하고, PNS 에선 schwann cell 이 myelination을 한다. 각각 조금 다른형태의 myelination 을 하지만, develop 과정에서든, adult stage 에서든 그들의 기원은 같다. 그렇다면 우리의 몸 혹은 태아 단계에선 어떻게 CNS 와 PNS 를 구분하고, 그에 따른 differentiation 을 할까?

=> CNS의 neural density 가 PNS에서의 neural density 가 높다. 따라서 단위 부피 안에서 짧은 시간동안 발생하는 neuron activation 에 의한 전기 발생량이 차이가 날 것이고, 그것을 통해 구별하지 않을까 싶다. 실제 우리 brain imaging 기술 중 하나인 Magnetoencephalography 기술은 미세한 전기에 의해 발생된 자기장을 통해 imaging 을 한다. 그 기술을 변형시켜, 수정란이나 실제 mice 에게 해가 되지 않을 정도의 양의 미세전류를 특정 stage 마다 흘려줌으로써 변화를 관찰한다. 나의 hypothesis 가 맞다면, 미세전류를 많이 흘려줄 경우, CNS 의 발달이 PNS 와 비교했을 때, 월등 할 것이다.