In order to understand psychology from a biological perspective,  we must understand the development of the nervous system, starting from conception.

Each of us began as a single-celled zygote which multiplied to become an embryo (day 10 to 8 weeks), and then a fetus. In order for us become “us” — and not a blob of cells — many things had to happen, including development of the nervous system.

The development of the nervous system begins during the embryonic stage, about 2 to 2.5 weeks post-conception, when the neural plate forms from the inner layer, called the ECTODERM. Here’s a quick review of embryonic cell layers:

Embryonic Cell Layers

• Ectoderm – The inner layer of the cell which forms the nervous system
• Mesoderm – The middle layer of the embryonic cell which becomes connective tissue, bone, muscles, etc.
• Endoderm – The outer later of cells which develops into bodily organs

In order for proper development to occur, 3 things must happen, starting with the formation of the NEURAL PLATE, a small path of tissue on the dorsal (back).

1) Cell Differentiation

Differentiation refers to the creation of different types of cells. So, for example, some cells will need to become muscle cells, and some will need to become glial cells, etc.

• TOTIPOTENT – During the earliest stages of embryonic development, most cells are totipotent, which means that they are able to develop into any type of cell.
• MULTIPOTENT – As the embryo develops, cell differentiation becomes more specified. For example, cells in the neural plate can only mature into nervous system cells. These cells are often referred to as embryonic stem cells.

2) Neuronal Migration & Aggregation

The second thing that must occur for proper development is neuronal migration – the movement of different types of cells to the proper location (migration). Once the cells are in the right location, they must align (aggregation). There are 2 ways this can happen:

• SOMAL TRANSLOCATION – During somal translocation, cells migrate to the appropriate location by developing extensions that look for cues to point in the right direction. As the cells move, the “tail” or extension they have disappears behind them. Many chemicals have been identified that guide cells in the right direction. The 2 most important:
  • Glycoproteins
  • Chemoattractants
• GLIAL MEDIATED MIGRATION – As the walls of the neural tube thicken, a temporary network of glial cells (RADIAL GLIAL CELLS) develops inside the tube. During glial medial migration, cells latch onto the framework and inch along like a little worm as it guides them in the right direction.

3) Formation of Neural Connections (Axon Growth & Synapse Formation)

Once the cells are properly aligned, the neural pathways must be “hooked up” correctly. So, in the next step of the process, axons and dendrites grow out from the cell’s GROWTH CONE (swelling on one side of the cell which extends and retracts in search of the right direction). Guided by NEUROTROPINS (NGF – Nerve Growth Factor), FILOPEDIA protrude out

AXON GROWTH – In the 1940s, Sperry cut the optic nerves of frogs, rotated their eyeballs 180 degrees, and waited for the axons to regenerate. When he dangled a lure behind the frogs, the struck forward, suggesting that their visual world had also rotated. The same was true whether or not the optic nerve was cut. This behavioral evidence suggests that each retinal cell had grown back to the same point where it was originally connected. This was confirmed in 2000.

• Sperry’s Chemoaffinity Hypothesis of Axonal Development – Each post synaptic surface in the nervous system releases a unique chemical label, which attracts a growing axon. However, this hypothesis does not account for the fact that some axons follow the same circuitous route to reach their target in every member of a species, rather than going directly from point A to point B.
  • Elaboration of hypothesis based on new research indicates that axon growth is influenced by a series of chemical signals along the route. These guidance molecules are called CHEMOATTRACTANTS. Others repel (CHEMOREPELLANTS). Other signals comes from adjacent growing axons. The PIONEER GROWTH CONES lead the way, and others follow in their path. FASCICULATION is the tendency of developing axons to grow along the paths established by preceding axons. When pioneer axons die before reaching their destination, subsequent axons of the same nerves tend to die also.
• Topographic Gradient Hypothesis – Axons growing from one topographic surface to another are guided to specific targets arranged on the terminal surface in the same way the axon’s cell bodies are arranged on the original cell surface (guided to destination by 2 intersecting gradients on original surface).

SYNAPTOGENESIS (synapse formation) – It takes the coordination of 2 axons to create a synapse between them. Recent discovery – Astrocytes (star-shaped glial cells  in the brain and spinal cord) are necessary for this process.

Neuronal Death

The death of neurons is a normal part of the development process, and operates on a “survival of the fittest” principle. We produce about 50% more neurons than we need, and those that die generally lose the competition for the chemicals (target sites) they need. The ones that don’t get hooked up properly or used, die.

• Implanting new chemicals (target sites) reduces neuron death
• Destroying neurons in a particular area before the period of cell death increases the survival rate of the remaining neurons
• Increasing the number of axons that initially innervate a target reduces the number that survive

NECROSIS – Passive cell death

APOTOSIS – Active cell death; this is the safer process because the internal structures of the cell are packaged in membranes before the cell breaks. The membranes attract scavenger cells that prevent inflammation. If the process is inhibited, cancer can develop.  If the process is overstimulated, neurogenerative diseases may develop.