The Nervous System

1. The Nervous System

2. Communication Center • Central Nervous System (CNS): system of nerves, the spinal cord, and the brain that receives signals from environment and sends out responses to those signals • Neuron: nerve cell • Dendrites: fan like branches that receive impulses • Cell body: main area of cell • Axon: long extension that sends impulses on to other neurons or body cells • Myelin Sheath: nerve cells (Schwann cells) that wrap around the axon to increase impulse speed • Nodes of Ranvier: spaces between myelin Schwann cells

3. Types of Neurons • Sensory neurons: receive signals from the environment and send impulses to the spinal cord and brain • Intermediate neurons (Interneurons): nerves that make up the brain and spinal cord; process impulses and send response impulses to motor neurons • Motor neurons: react to impulses from brain and spinal cord; activate glands, muscles, etc..

4. They Mighty Reflex • I throw a ball at Tom. What nerves first take in the input? • Sensory nerves in eyes • The input to the brain quickly calculates distance, speed, angle, etc… to conclude if the ball will hit him. What nerve cells to this? • Intermediate neurons • What is Tom’s reaction? What caused this? • Muscles move body away from ball; Motor neurons (effector neurons) • If input is strong enough, Tom doesn’t even need his brain!

5. They Mighty Reflex • Where are intermediate neurons located? • Brain AND Spinal Cord • Reflex action: • When input is significantly higher/lower than set point, spinal cord sends response before brain even gets input • Cuts only fractions of a second off but can save your life • Reflex arc: • Sensory impulse travels to intermediates in spine, set point comparison causes response on effector neurons • Explains why we can feel temp, text, etc… BEFORE we feel pain (Brain is too slow)

6. How Neurons Send Impulses • Ion channels allow the inward flow of K+ and limit the flow of Na+ • Concentration difference between Na+/K+ is made greater by Na+/K+ Pump • Membrane is polarized: • Inside has negative charge • Outside has positive charge • About -65-70mV difference (resting potential) • Impulse is sent as swift of K+ and Na+ across the membrane change the polarity of the cell (action potential)

7. How Action Potential Happen • Action potentials controlled by movement of Na+ into the cell and K+ out • Stimulus breaks threshold potential; Na+channels open and Na+ flow in • + ions change membrane potential to +40mV; Na+ channels close • K+ channels open and K+ flow out • Leaving K+ drop membrane potential to – • Once under resting potential (about -80mV), K+ channels close • In refectory period Na+/K+ pump must reset resting potential

8. The Action Potential • Action potential change in membrane potential that is strong enough to cause depolarization • Normally 10-20 mV above the resting potential • The stable resting potential will keep the neuron slowly depolarizing till a threshold potential is hit • All-or-Nothing Principle • Then in about 1 msec the cell’s potential will rise to about +40mV and then drop to -80mV (hyperpolarized) • Refractory period section of axon cannot be restimulated; keeps signals moving in one direction

9. Na+/K+ Pump • Concentration gradient of Na+ and K+ most be large so the flow of ions is fast • Na+/K+ Pump uses ATP to pull in K+ and push out Na+ 1 ATP= 2 K+ in; 3 Na+ out • Also helps to repolarize membrane after impulse

10. Sending an Impulse • Sending an impulse starts with an action potential • Stimulus must be strong enough to start action potential; pass threshold • Na+ channels in the nerve membrane open up; Na+ rush into cell down a concentration gradient • Inside cell changes from –65mV to +40mV • Shift causes other Na+ channels to openand signal moves like a wave down the axon • Signal strength is only influenced by frequency of impulses

11. Sending an Impulse • After impulse, the K+ channels open and K+ rush out, changing the inside of the cell from +40mV to –65mV • Repolarizing (outside +; inside -) the area of nerve must happen to send another signal • Signals can “jump” down axon by traveling to pockets between myelin sheaths

12. Faster and Faster We Go • Stimulating each area of the axon is not fastest method • Increase the diameter of the axon and the speed can increase (25m/sec), but this results in a selective limit on size (largest is 1.7mm) • Saltatory conduction fast propagation on thin axons through insulation by myelin sheathes • Ions cannot flow in areas covered by sheathes • Na+ can’t leave fast enough after stimulus, so the diffuse to next open section (nodes of Ranvier) • 3mm axon can fire at 130m/sec • Unmyelinated at 130/sec would have to be 300mm

13. The Big Five • Most animals posses 5 types of sensory receptors: • Mechanoreceptors detect pressure and body movement; ears • Pacinian corpuscle: free nerve end capsulated by collagen fibers; very sensitive • Photoreceptors detect light; eyes • Chemoreceptors detect specific chemicals; taste buds • Thermoreceptors detect temperature; skin • Nociceptors detect damage; everywhere but the brain

14. Neuron to Neuron Signals • Nerve cells don’t touch; signal “wave” has to be passed from cell to cell • Synaptic cleft: small space (10-20 nm) between neurons • Synapse: synaptic cleft plus the pre and postsynaptic neuron • Neurotransmitter: chemical signals that diffuse from the axon of the presynaptic neuron to the dendrite of the postsynaptic neuron; triggered by Ca2+ channels

15. Sending Neurotransmitters • Over 40 types of Neurotransmitters: • Acetylcholine common transmitter found through the body • Dopamine found only in the mind • Impulse travels down axon to synaptic blub • Ca2+ channels open; Ca2+ rushes in • Premade vesicles of ACh fuse with presynaptic blub and release Ach into synapse

16. Receiving Neurotransmitters 4) AChreceptor proteins in the postsynaptic neuron take in ACh and temporarily binds to it 5) Bind of receptor site causes Na+ channels to open and depolarization of postsynapse neuron • What happens if ACh is not regulated? • Nerve continues to send impluses • ACh is quickly broken down by acetylcholinesterase and reabsorbed into the presynaptic neuron to be reused

17. Why Do We Need Synapse? • Do synapse increase impulse speed? • No! Synapse actually slow down the transmission of information a lot • Why do we need synapses? • Help keep one-way transmission of information • Enhancement impulse from one sensory nerve can spread to more nerves to increase strength (frequency) • Inhibition synapse can stop transmission of information • Learning enhancement, inhibition, and building connections in how complex thought is controlled

18. Messing With Synapse • By allowing more points for regulate and control impulses, we also create places where we can have moremalfunctions • Drugs, alcohol, genetics…can all effect neural pathways • Nicotine drug in cigarettes; activates ACh receptors in postsynaptic neurons BUT is not easily broken down; causes dopamine spike • Botulinum Toxin Stops ACh release; can be deadly • Used to relax muscles of face and take away wrinkles • Insecticides and Nerve Gas can cause permanent release or inhibition of neurotransmitters; body over active or shuts down