Nervous System

1. Nervous System Neurons & Synapse

2. Interesting Facts • There are more nerve cells in the human brain than there are stars in the Milky Way • There are 100 billion neurons in your brain alone • Neurons do not under go mitosis • Neurons are the largest cells in the human body – up to 3 feet long • The nervous system can transmit impulses as fast as 100 meters per second

3. Neurons (Nerve Cell) • Dendrites: Short processes that conduct an impulse towards the cell body. • Cell Body: Contains the nucleus. Maintains the cell. • Axon: Long process conducts an impulse away from the cell body.

6. 3 Types of Neurons • Sensory Neurons • Long dendrite, short axon • Takes sensory info to the cell body • Interneurons • Short dendrite and axon • Entirely in the spinal cord of CNS • Connects sensory neuron with the motor neuron • Motor Neurons • Short dendrite, long axon • Takes the impulse to the Effector (either muscle or gland)

7. Reflex Arc

9. Myelin Sheath • The long fibres of the neurons are covered with a fatty sheath call myelin sheath • This myelin sheath is composed of Schwann cells that wrap around the nerve fibre

11. Myelin Sheath • The myelin sheath has 2 functions: • It insulates the neurons from each other as they pass through the nerve • It helps to speed up the impulse

12. Nodes of Ranvier • The points between the Schwann cells are called nodes of Ranvier. They speed the impulse as it jumps from node to node.

13. Action Potential • Nerve impulses are electrical in nature • When a nerve impulse travels along a nerve fibre, there is actually a wave of ionic changes that occur • This creates a very small shift in the electrical nature of the fibre

14. Action Potential • To understand how an impulse is conducted, you must know what an axon looks like “at rest” • At Rest • Sodium : concentrated on the outside of the axon • Potassium & negative organic ions are concentrated on the inside of the axon (in the axoplasm) At rest, the membrane is not permeable to these ions and they cannot move in or out. Because of the distribution of the ions, the OUTSIDE of the neuron is slightly POSITIVE when compared to the inside.

15. Action Potential Depolarization • If a stimulus is strong enough the membrane is suddenly made permeable to sodium • The gates for sodium are opened, and the sodium ions flood to the inside of the axon.

16. Action Potential Repolarization • As the sodium rushes in this causes the potassium gates to open • The potassium floods to the outside • This causes the polarity to be restored but the ions are in the reverse positions • At the same time the sodium gates are closing

17. Action Potential Recovery Period • Once the polarity is restored both the gates close completely • Then the sodium potassium pump (with the help of ATP) actively pumps the sodium and potassium ions back to their original condition • This re-establishes the resting condition so the neuron can conduct another impulse

18. Recovery Na/K pump

19. All or None Response • As long as the threshold stimulus has been reached, there will be an impulse • Each impulse is equal to all other impulses • This is called the “all or none” response • A stronger stimulus does not mean a bigger impulse, rather it means a greater number of impulses (more nerves involved or a single nerve conducting a series of impulses) will give bigger results.

22. Synapse • When an impulse arrives at the end of an axon, it must make a connection to the next nerve cell, or to the muscle or gland • The axon of one nerve cell does not actually come in direct contact with the membrane of the receiving cell • There is a small space, termed the synaptic gap, which the impulse must cross

23. Presynaptic membrane Postsynaptic membrane

25. Synapse Video • https://www.youtube.com/watch?v=rWrnz-CiM7A

26. Synapse • Step 1: When an impulse arrives at the end of an axon, the sodium gates open and sodium flood into the axon bulb/terminal • Step 2: At the same time, the calcium gates open and calcium also moves into the axon bulb/terminal • Step 3: The calcium binds with contractile proteins attached to the vesicles and this causes them to contract, thus pulling the vesicles towards the pre-synaptic membrane

27. Synapse • Step 4: Exocytosis occurs as the neurotransmitters are spilled into the synaptic gap. They diffuse across the gap. • Step 5: Neurotransmitters bond with receptor sites on the post-synaptic membrane. • Step 6: When the neurotransmitters attach to the receptors, the voltage of the post-synaptic membrane changes to cause the sodium gates to open. This depolarizes the membrane.

28. Synapse • Step 7: • If the synapse is between an axon and dendrite, then the Action Potential will continue down the next neuron. • If the synapse is between an axon and a muscle cell, then the muscle will contract. • If the synapse is between an axon and a gland, then the gland will release a hormone.

29. Synapse • Step 8: The synaptic gap contains enzymes that will destroy the neurotransmitters, thus returning the synapse to its original condition prior to the arrival of the impulse • Step 9: They calcium ions are returned to the synaptic gap by active transport. The synaptic region is returned to its pre-impulse condition so that it may be used again.

30. Neurotransmitters • Acetylcholine: • This is responsible for promoting all responses in a relaxed state • It is also involved in controlling skeletal muscles • It is destroyed by the enzyme acetylcholinesterase • Noradrenalin • This is the excitatory transmitter (also known as norepinephrine) • It almost always increases the activity of the receiving cell/tissue/organ • It is involved in “fight or flight” situations (stress) • It is destroyed by the enzyme monoamine oxidase