The Nervous System

1. The Nervous System Cell Structure

2. Cells of the Central Nervous System • Receiver and Sender cells from sense organs • Specialized “helper” cells • Neurons: About 10 Billion • Specialized nerve cells in CNS and PNS •   many different specializations

3. Two basic kinds of cells in periphery and sense organs: • Receptor cells: •   embedded in sense organs • specialized to receive stimulation from environment and send to the brain • Effector cells • specialized to contract muscles and stimulate  glandular secretions • acted upon by nerves and neurons

4. Cells of the Nervous System • Glial cells: about 100 billion • Account for 90% of cells in adult human brain • Help hold neurons together, assist in neurotransmission • Provide supports for the nervous system • In periphery are rather rigid: • E.g., Schwann cells • In CNS: are soft and squishy: • E.g., Oligodenroglia cells

6. Parts of a Neuron • Soma: • cell body containing nucleus, • regulates life functions (metabolism) • Dendrites: • branched projections from soma • receive transmissions from   other neurons • Axon: •   long projection extending from soma •   transmits information to next neuron •   axon hillock: beginning of axon, important in transmission • End brush: •   branched portion of axon •   contains synaptic bulbs

7. Parts of a Neuron • Synaptic bulbs or terminal buttons: •   bulbous parts at end of end brush •   close contact with next neuron's dendrites •   contain neurotransmitters • Synapse or synaptic cleft •   space between neurons •   space between one neuron's synaptic bulb/other's dendrites • Myelin sheath: •   insulates neurons •   made of either glial cells or Schwann cells •   allows synaptic transmission to occur by jumping down axon •   exposed area between sheath = Nodes of Ranvier

10. Importance of Glial Cells • Critical for neurotransmission • Help neurons • Hold neurons in place • May store some neurotransmitter • May alter neurotransmission • Multiple Sclerosis • Allergic to own myelin sheath • Immune system attacks and destroys • Lose motor skills, cognitive function, even death • “waxes and wans” in that myelin will often regenerate if damage is not too severe

12. SYNAPTIC TRANSMISSION: HOW THE NEURON WORKS

13.  Resting potential • Beginning of the firing cycle: Resting potential action potential  refractory period- resting potiental •  Neurons sit at base level- or RESTING POTENTIAL • inside of axon is negatively charged: • more K+ ; some A- • outside of axon is positively charged: • More Na+; some Cl- • axon has voltage of -70mV at resting potential

16. Action Potential • Dendrites receive incoming neurotransmitter • Chemical fits in “lock” on dendrite • Alters the shape of the cell wall • Allows changes in cell wall that will change voltage inside the neuron •    if incoming message sufficient in strength- causes an ACTION POTENTIAL

17. Action Potential • All or None: Either voltage change is sufficient to stimulate action potential, or not • Voltage will change from -70 to +40 mV and back again • This “depolarization” begins at the axon hillock •    inside of axon becomes negative •    NaCl goes out of axon •    outside of axon becomes positive: K+ goes in •    result is voltage change as switch occurs •   depolarization moves down axon in wavelike form

18. Refractory Period • Neuron repolarizes (moves back toward resting potential) during refractory period • Resetting to normal • Ion pump kicks into action at end of action potential • Pumps ions • K+ in • Na+ out • Over does it a bit: cell ends up just below resting potential • Until returns to resting potential, very difficult, if not impossible, for cell to fire

19. Again: Three Steps for firing • Resting potential: voltage is about -70mV • Dendrites receive incoming signals • If sufficient, cell goes into firing mode • Action potential • Voltage changes from -70mV to +40mV • Ions exchange places • Occurs rapidly down axon • Only in places where myelin sheath doesn’t cover: Nodes of Ranvier • Refractory Period: • below resting or lower than -70mV • Cell recovers from firing • Brief time period when difficulty for it to fire again. • Back to Resting potential.

25. Why an action potential? • Allows release of neurotransmitter • Neurotransmitter is chemical • Several specific kinds- each act on certain neurons • Most neurons respond to and release one kind of neurotransmitter • Neurotransmitter stored in synaptic vesicles • Pressure of action potential pushes these to end of terminal button • There they attach to cell wall and release into the synapse

28. Action in the Synapse • Neurotransmitter is released into the synapse • Floats across synapse to next neuron’s dendrite • This “next dendrite” is post-synaptic   • Neurotransmitter is attracted to the POST-synaptic side: •   receptor sites on the next neurons dendrites •   attach if can find right spot •   if sufficient neurotransmitter is received, will cause an action potential   on next neuron

30. Neurons are efficient • Not all neurotransmitter is attached to post-synaptic receptor sites • Extra must be destroyed • Enzymes in synapse attack and destroy • Reuptake occurs: Neuron takes it back up and recycles it for later use

32. Neurotransmitters Chemical communicators

33. Two basic kinds of Neurotransmitters • Excitatory: • create Excitatory postsynaptic potentials: EPSP's • stimulate or push neuron towards an action potential • effect is merely to produce action potential- no behavioral effect as yet • Inhibitory: • Create Inhibitory postsynaptic potentials: IPSP's • Reduce probability that neuron will show an action potential • Effect is merely to lessen likelihood of an action potential- again not   talking about behavioral effects just yet! • Some neurotransmitters are both inhibitory and excitatory, depending upon  situation and location

35. Many different ways of manipulating neurotransmitters • Alter rate of synthesis: more or less NT • Alter storage rate: again, more or less NT • Leaky vesicles • Alter release: more or less release • Alter reuptake: more or less • SSRI’s • Alter deactivation by enzymes: MAO inhibitors • Block or mimic receptor site attachment • Block and prevent attachment to receptors • Mimic the NT at the receptor site

39. Function of Specific Neurotransmitters

40. Acetylcholine or ACh • Location • primarily in brain, spinal cord • target organs of autonomic nervous  system • Indicated effects: • excitation or inhibition of target organs • essential in movement of muscles • important in learning and memory • Too much: muscle contractions- e.g. atropine poisoning • Too little: paralysis: curarae and botulism toxin

41. Norepinephrine or NE • Called epinephrine in peripheral nervous system • Chemically extremely similar to Dopamine • Located in • brain, spinal cord • certain target organs • Effects: primarily excitatory • Too much: overarousal, mania • Too little: underarousal, depression

42. Dopamine or DA • Location: • primarily in brain • frontal lobe, limbic system, substania nigra • Indicated effects: •   inhibitory: reduces chances of action potential •   involved in voluntary movement, emotional   arousal •   reward learning and motivation to get reward •  Too little: Parkinson's disease •  Too much: schizophrenia •  Amphetamines mimic this neurotransmitter

43. Serotonin or 5HT • Located in brain and spinal cord • Both inhibition and excitation •   important in depression, sleep and emotional arousal •   very similar to NE and DA • Too little is linked to depression and sleep disorders • Many antidepressants are specific to this NT • SSRI’s • Block reuptake of 5HT in the synapse

44. Others • GABA • Histamine • Endorphins • Neuromodulators and neurohormones