5. Neural Networks - An introduction

Neural networks are the basic communicating and processing channels for brain. Just like any other pictorial representation, neural network is also represented by dots, connection & junctions but in real, a neural network is the network of individual cells!

When we realize that a neuron is just a cell, we would also realize how tiny they are and their huge population in a human brain. A human brain weighs, approximately, 1500g! Just like any other cells, neuron's size and shape vary depends on it's location in brain. It size varies from 4 micron which has nucleus of size 3 microns in diameter up to 100 micron in diameter that has bigger nucleus. Just imagine how many neurons could possible be there in a 1500g weighing brain! Now, lets explore them more.

Neuron cells:
Basically our brain and the spinal cord are made up of many neurons. Neurons are cells that send and receive electrochemical signals to and from the brain and nervous system. There are about 100 billion neurons in the brain. Though their size says that they are tiny, those cells are not necessarily short. Their length varies from a fraction of an inch to several feet. Neurons transmit the nerve signals to and from the brain at up to 200 mph.

The neuron consists of a cell body (or Soma) with branching dendrites (signal receivers) and a projection called an axon, which conduct the nerve signal. At the other end of the axon, the axon terminals transmit the electrochemical signal across a synapse (the gap between the axon terminal and receiving cell). Below picture illustrates the structure of a neuron.

Axon:
The axon, a long extension of a nerve cell, and take information away from the cell body. Bundles of axons are known as nerves or, within the CNS (central nervous system), as nerve tracts or pathways. Dendrites bring information to the cell body.

Myelin sheaths:
Myelin coats and insulates the axon (except for periodic breaks called nodes of Ranvier), increasing transmission speed along the axon. Myelin is manufactured by Schwann's cells, and consists of 70-80% lipids (fat) and 20-30% protein.

Soma or cytoplasm of a neuron:
The body of a neuron is called as Soma. It contains the neuron's nucleus (with DNA and typical nuclear organelles). Dendrites branch from the cell body and receive messages. A typical neuron has about 1,000 to 10,000 synapses (that is, it communicates with 1,000-10,000 other neurons, muscle cells, glands, etc.).

Types of neurons:
There are different types of neurons. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are found in different parts of the body. Unlike most other cells, neurons cannot regrow after damage (except neurons from the hippocampus). There are three types of neurons.
    * Sensory neurons or Bipolar neurons carry messages from the body's sense receptors (eyes, ears, etc.) to the CNS. These neurons have two processes. Sensory neuron account for 0.9% of all neurons. (Examples are retinal cells, olfactory epithelium cells.)
    * Motoneurons or Multipolar neurons carry signals from the CNS to the muscles and glands. These neurons have many processes originating from the cell body. Motoneurons account for 9% of all neurons. (Examples are spinal motor neurons, pyramidal neurons, Purkinje cells.)
    * Interneurons or Pseudopolare (Spelling) cells form all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord; one with either the skin or muscle. These neurons have two processes. (Examples are dorsal root ganglia cells.)

Glial cells:
If such active neurons are too busy at their work, who is feeding and managing them? Like brain that manages and coordinates other organs, there must be somebody else to take care of neurons! In fact, along with neurons there are Glial cells. There are many more Glial cells; they provide support functions for the neurons, and are far more numerous than neurons in count.

Glial cells make up 90 percent of the brain's cells. Glial cells are nerve cells that don't carry nerve impulses. The various glial (meaning "glue") cells perform many important functions, including: digestion of parts of dead neurons, manufacturing myelin for neurons, providing physical and nutritional support for neurons, and more. Types of glial cells include Schwann's Cells, Satellite Cells, Microglia, Oligodendroglia, and Astroglia. There is another type of brain cell called 'Neuroglia' (meaning "nerve glue"). These cells guide neurons during fetal development.

I hope that this gives a strong idea of neurons and their network. In my next posts, lets explore how these neurons process signals and patterns to interact with the external world!