The Nerves

The body is made up of trillions of cells. Cells of the nervous system, called neurons, are specialized to carry "messages" through an electrochemical process. Just by itself, the human brain has about 1 trillion neurons.

Neurons (nerve cells) come in many different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide, while some of the biggest neurons have cell bodies that are 100 microns wide. (Remember that 1 micron is equal to one thousandth of a millimeter!!).

Neurons are similar to other cells in the body in some ways such as:

1. Neurons are surrounded by a cell membrane.
2. Neurons have a nucleus that contains genes.
3. Neurons contain cytoplasm, mitochondria and other "organelles".
4. Neurons carry out basic cellular processes like protein synthesis and energy production.

However, neurons differ from other cells in the body in that:

1. Neurons have specialized extensions called dendrites and axons.
2. Dendrites bring information to the cell body and axons take information away from the cell body.
3. Neurons communicate with each other through an electrochemical process.
4. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.

The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts.

Dendrites (see figure to left), receive information from another cell and transmit the message to the cell body. The axon conducts messages away from the cell body. Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.

Some axons are wrapped in a myelin sheath  (see figure to left), formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwann cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal.

Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second.

 

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