The Nervous System

Feeling nervous? 

By: Conor Chinitz, Kelvin Chen, Nathan Bolton, Jack Clifford

What is the Nervous System?

• The Nervous System - a network of specialized cells responsible for organizing and transmitting electrochemical signals throughout the body (2).

  • Composed of excitable cells called neurons (nerve cells) 

 • Neurons "fire" electrical signals to send messages

          " I thought I fired you! Why are you still working? "           " Because I'm a neuron. "

"No matter how inherently lazy you feel, you are never really doing nothing..." —Kelvin

Neuron Structure

 

Source: mhhe.com

The Axon• Axons are output zones where messages are sent to other cells

1.Trigger Zone - junction between cell body and axon (2)2.Nodes of Ranvier and Myelin Sheaths - conduction and

insulation zones along axon, respectively (2)3.Axon Terminal - end of axon (2)

Summary of Major Structures

Structure Function

Soma Cell body; contains nucleus; processes signals

 Dendrites

Receives electrical signals from neighboring neurons and directs to soma (input)

Axons Transmits signals to neighboring neurons (output)

Myelin Sheath Bead-like lipid structures that wrap around axons to facilitate signalling

Axon:  "Let's connect.  "Dendrite:  "On Facebook?  "Axon:  "No! With each other!  " Sources: 2,3

Neurons at Rest

• In order for neurons to fire, they store electrical energy in the form of  Na+ and K+ ions (2,3)

• Neurons at rest maintain a constant charge (voltage difference)o Resting Potential - the inherent voltage difference across the

plasma membrane of an axon at rest (-70 millivolts) (2,3) • Voltage difference caused by imbalance of Na+ and K+ ions across

axon membrane (2,3) 

Ion Distribution in Neurons

• At "rest," ions are still constantly entering and leaving the membrane through transport proteins(2)

• Facilitated ion transport happens actively and passively: • Passive transport: Na+ and K+ seep through channel proteins • Active transport: sodium-potassium pumps use ATP to move

ions against concentration gradient (2 K+ enter: 3 Na+ exit) (2)

• Resting potential is a misnomer as the cell is not at rest but constantly working to maintain this ratio (2)

Ion distribution at rest

Source: www.wikieducator.org

Action Potential

• When the neuron is at rest, it has a negative charge (2)

• Action Potential occurs when stimulus overcomes the threshold (>-70mV) (2)o at action potential, there is a reversal of Na+ and K+

ions in membrane (2)o sudden ion influx results in a reversal of charge

(membrane becomes positive in respect to extracellular fluid) (2)

Neuron 1: "Girl, what happened to you?"Neuron 2: "Your stimulus carried me over my threshold!"

Ion Behavior during Action Potential

• Stimulus overcomes threshold of excitation is reached  and initiates action potential (stimulus > -70 mV)

 • Ion permeability changes:

• Voltage-gated Na+ channel open, allowing Na+ to rush in• axon interior is now positively charged in respect to

extracellular fluid (polarity reversal)• Voltage-gated K+ channel open, allowing K+ to rush out• axon interior is now back to being negatively charged in

respect to extracellular fluid 1. Sodium-Potassium pump then restores resting-potential

distribution of Na+ to K+

Source: 2

Source: library.thinkquest.org

Source: https://www.youtube.com/watch?v=U0NpTdge3aw&feature=player_embedded

Still Confused?

Myelin Sheath

• Insulates electrical signal traveling along the axono Prevents electrical signal from dispersing (2)

 • Enhances rate of action potential propagation (2)

       

 

source: www.biology-forums.com

Thank you!

We hope this presentation excited your neurons.

Bibliography

1)   Abedon, Stephen T. "Transport Across Membranes." Home.                        Ohio State University, 28 Mar. 1998. Web. 24 Oct. 2011.            

            <http://mansfield.osu.edu/~sabedon/biol1055.htm>.    2)   Starr, Cecie, and Ralph Taggart. Biology: The Unity and                         

            Diversity of Life. 5th ed. Belmont, Calif.: Wadsworth Pub. 

            Co., 1989. Print.    3)  Campbell, Neil, and Heyden Robin. Biology: Exploring Life. Florida: 

            Pearson Education, Inc., 2009. Print.

Further Exploration

• Interactive Program for Action Potential:o http://outreach.mcb.harvard.edu/animations/

actionpotential.swf

• Applications of Neurons:o http://www.wired.com/wiredscience/2009/04/

lasercontrolledhumans/o  http://www.ted.com/talks/lang/eng/ed_boyden.html