Neuron, Development of Brain

A single neuron is the basic functional unit of the nervous system. It is like a brick in a house. As house is made up of many bricks similarly brain is made of a billion neurons. As cement is needed to provide strength , similarly the neurons are strengthened by neuroglia cells.

A single neuron has has s body (which is also known as soma) with nucleus. Inside  the body there are Nissl granules. From the body arise many projections which carry impulses to the body of the neuron, these tiny projections are called Dendrites ( look like branches of a tree). From the3 body or soma of the neuron a long Axon emerges. The junction of the body and axon is called Axon hillock. Signal and material from the body travel through the axon. Most of the axons are myelinated , i.e., axons are usually covered by a sheath of myelin. In the peripheral nervous sytem myelin is formed by Schwann cells and in the brain it is formed by Oligodendrocytes ( glia cells). This myelin provides white colour and the area of the brain where large number of myelinated fibres are present is called white matter. There are gaps in the myelin sheath along the entire length of axon, these gaps are known as Nodes of Renveer, which allows fast movement of electrical impulses ( action potentials) along the axon. At distal end axon synapses with other neuron or muscle fibre.

This image is a simple depiction to show the development of the brain. 

The brain arises from a tube which is known as neural tube in an embryo. 

In the early embryonic stage of development there are three layers , endoderm, mesoderm and ectoderm. 

Ectoderm is the outer layer of the embryo. Some of the ectoderm cells which are present in the center become destined to form  neural ectoderm. These cells invaginate and convert into a tube which is now, known as Neural tube which is open at both the ends, also known as anterior and posterioral neuropore.

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After 3rd week of embryo development cranial opening of the neural tube which is also known as anterior neuropore , closes. Any abnormality of the closure of the this end may lead to congenital birth anomaly of the brain like anencephaly.

The posterior or caudal neuropore closes at the end of 4th week of embryonic development. Abnormality of the closure of posterior neuropore may lead to spina bifida, split cord malformation. This fact is very significant because when a female becomes aware that she is pregnant most of the neural tube is already developed . A flic acid or folate deficiency has been known to cause neural tube defects, that is why, folate supplementation is advocated in females who plan a pregnancy.

To repeat it again,  brain arises from a tube which is known as neural tube in an embryo. A tube is shown in the diagram mentioned above.

Then the anterior or cranial end of this tube is expanded into three parts which form the brain and caudal or elongated part forms the spinal cord. The three segments of the cranial part of the neural tube are destined to form three major part of the brain. Forebrain, midbrain and hindbrain.

Three cranial expansions are : Prosencephalon ( Forebrain), Mesencehalon (Midbrain) and Rhombencephalon ( Hindbrain). Remember nemonic for 3 sementation stage: PMR. Cavities inside these structures form the ventricles.

Later there are further subdivisions and it becomes of five segments.

Prosencephalon divides into two ;  Telencephalon and Diencephalon. Telencephalon later transforms into two cerebral hemispheres and its cavities become lateral ventricles. Diencephalon may be simply imagined to be like a box , the cavity iside becomes Third ventricle. he lateral walls of this box become 2 thalami, floor becomes hypothalamus, roof becomes epithalamus and part becomes subthalamus.

Mesencephalon changes into midbrain.

Rhombenecphalon subdivides into two subdivision; Metencephalon and Myelencephalon. From Metencephalon Pons and Cerebellum develop. Myelencephalon transforms into medulla oblongata. The distal part of the neural tube gives rise to spinal cord.