The Nervous System

The nervous system allows the body to quickly respond to stimuli by using electric signals that travel very quickly down cells called neurons. As with all control systems, the nervous system must be able to:

Detect stimuli with receptor cells and sensory neurons
Process that stimuli with networks of neurons in the brain
Respond appropriately with motor neurons delivering signals to muscles and glands

Neurons are very long cells that can send electric signals from one end to the other and then send out chemical signals to the next neurons.

Need for Speed

If you've ever been startled a movie scene, a haunted house, or someone who claims to be your friend, then you probably understand how quickly nerve signals travel in the body. The sight or sound of the "stimulus" has to enter your eyes or ears and then travel to the brain which then sends a signal down the spinal cord to the muscles of your body that contract in a way that makes you jump. Neurons are long cells that are made to move signals as quickly as possible.

Neurons have 3 basic parts:

Dendrites - extensions that receive signals from other cells.
Cell Body - contains a nucleus, mitochondria and other organelles needed to keep the cell working.
Axon - long extension that carries a signal away from the cell body to another cell.

Electrical signals travel through a neuron because of diffusion and special membrane membrane proteins that pump ions out and other proteins that let them back in.

Sodium pumps create a high concentration of positive sodium ions outside the cell when a signal is NOT being sent.
- This makes the inside of the cell relatively negative
Sodium gateway proteins open up and the sodium diffuses in when a signal is being sent.
- This makes the inside of the cell positive.
- When the inside is positive, nearby sodium gates will open causing the positive wave to move down the cell toward the end of the axon.

When the signal reaches the end of a neuron, a chemical signal (neurotransmitter) is sent to the next cell.

A positive signal at then end of an axon causes the release of neurotransmitters.
- They diffuse across the synapse (space between the axon of one cell and the dendrite of another)
Receptor proteins bond with the neurotransmitters and open sodium gates on the dendrite.
- Open sodium channels cause the signal to move through the next neuron.

Neurons in the brain have many dendrites that connect to many different axons of other cells. The combination of signals that it receives can cause it to send, or not send a signal on to another neurons. Thoughts are determined by patterns of neurons firing, not single cells