1. Review: What decision is made by a neuron? At what location the decision is made? How the decision is made? Simple decision rule: if Em > threshold (-40mV)  fire an action potential

2. After action potential is initiated, it propagates to the axon terminal in all-or-none manner - - Inject current Measure Em Threshold = -40mV Em Peak = +30mV Resting Em= -80mV Inject current Let us simulate excitatory input by injecting positive ions (electrical current) into neuron

3. What happens at the contact between two neurons? 1. Synapse (Greek “syn”= together + Greek “hapto”= to clasp) 2. Presynaptic neuron 3. Postsynaptic neuron

4. Let’s start with something simple: The only monosynaptic reflex in our body Stretch reflex or Patellar reflex or knee-jerk reflex Why doctors check for this reflex? Decreased knee- jerk reflex is a clinical sign of demyelination of peripheral nerves as occurs in untreated syphilis infection In patients with cerebellar disease, the leg can oscillate up to 8 times

5. Dorsal root From Latin Dorsalis = Back

6. Ventral from Latin ventralis= belly, abdomen

7. SAME DAVE Sensory – Dorsal Afferent – Afferent Motor – Ventral Efferent - Efferent In both spine and the brain: the back (posterior) is always sensory the front (ventral) is always motor Ventral Dorsal

9. Sensor: Muscle spindle is a stretch receptor = the length sensor ΔLΔL Em Frequency (rate) of action potentials Stimulus = ΔL Fmax Conclusion: Information is coded in frequency of action potentials (firing frequency): the higher the frequency  the stronger quadriceps muscle would contract

10. We talk of a reflex when consciousness is not involved. A patient could be unconscious, but the reflex will work Other examples of a reflex? Crossed extensor reflex (a withdrawal reflex) after stepping on a nail Note that the firing of the motor neuron can be inhibited by the brain Reflexes

11. Types of synapses The problem: we need transmit signal from one cell to another In the stretch reflex the signal needs to be transmitted the stretch receptor axon terminal to a motor neuron From the motor neuron axon terminal to the muscle The first type of connection that evolution tried was an electrical contact: gap junction Ions and small molecules (up to 2nm) can diffuse without leaking to extracellular space A kind of local plumbing system

12. Most cell in the body are connected; except red blood cells, neurons and skeletal muscles. Gap junctions couple neighboring cells in the heart, smooth muscles, in glands, liver, epithelia; at some stages of early development they couple all cells in an embryo.

13. Electrical synapses also sometimes connect neurons Example: crayfish giant motor synapse, that mediates emergency tail flip – the escape reaction of crayfish. 4 giant axons of the ventral nerve cord make contact with axons of giant motor neurons in each segment  all abdominal flexor muscles become activated simultaneously and without delay

14. (b) Photomicrograph of cross section of ventral nerve cord in abdomen of crayfish made near the level of the third root (arrow in a); the largest axons near the top are giant fibers used in escape responses (c) Relationship between lateral giant and motor giant axons as they make electrical synaptic contact near the third root of an abdominal ganglion in the crayfish

15. A gap junction can be regulated by: 1. A number of small molecules 2. By insertion or removing gap junctions from the membrane

16. Electrical synapseChemical synapse Until 1920 scientists did not know how neurons communicated. Scientists formed two camps: they called themselves soups and sparks. Soups were in favor of chemical transmission and sparks were in favor of electrical communication. They were arguing for 50 years and before 1920 sparks seemed to be winning. The solution came to Otto Loewi in a dream in 1920…

17. Loewi’s Experiment Demonstrating Chemical Transmission Stimulated vagus nerve  slowed heart A Injected solution from heart A into heart B  Heart B rate slowed Stimulated sympathetic nerve  sped up heart A. Injected solution from heart A into heart B  Heart B rate sped up Loewi’s conclusion – Heart uses chemical messengers, not electrical signal to control heart rate. – Loewi got a Nobel prize in 1936 17 AB

18. Which synapse is electrical? chemical? Electron microscopy of synapses Electrical synapseChemical synapse

19. Why wasn’t evolution satisfied with electrical synapses? – Gap junctions synaptic transmission is fast – They can be regulated PROBLEM: Gap junctions are straight forward transducers of electricity: – Pre depolarized  Post is depolarized; – Post depolarized  Pre depolarized It is impossible to build a highly compartmentalized system with just electrical synapses Evolution favored more complex systems. It is easier to build a complex system with chemical synapses: – cells are electrically isolated; – chemicals (neurotransmitters) are used to transmit signal between cells. – Some neurotransmitters depolarize the postsynaptic cell; some hyperpolarize – Some neurotransmitters have only transient effect (1ms); others long-lasting effect (100ms) Electrical synapseChemical synapse

20. Electrical synapse Chemical synapse A B Action potential in cell A A B E m cell A E m cell B E m cell A E m cell B No delay delay Differences between chemical and electrical synapse

21. Electrical synapse Chemical synapse A B Action potential in cell A A B E m cell A E m cell B Action potential in cell B E m cell A E m cell B E m cell A E m cell B E m cell A E m cell B Differences between chemical and electrical synapse

22. From now on we’ll be talking about chemical synapse only. Rules of the game at a chemical synapse 1.Excitatory synapse  always depolarization 2.Inhibitory synapse  always hyperpolarization EmEm EmEm

23. Excitatory synapse Action potentials in the presynaptic cell 1 are 100ms apart  no temporal summation Action potentials in the presynaptic cell 1 are 5ms apart  temporal summation of EPSP results in action potential in the postsynaptic cell Simultaneous action potentials in presynaptic cell 1 and 2  spatial summation of EPSP results in action potential in the postsynaptic cell Inhibitory postsynaptic potential (IPSP) reduces the probability of action potential in the postsynaptic cell

24. Summary 1.Postsynaptic potentials last longer than action potentials 2.Postsynaptic potentials are local potentials – they are not usually amplified 3.Postsynaptic potentials are graded potentials: the more APs arrived  the greater EPSP (as opposed to all-or-none APs) 4.Since not amplified postsynaptic potentials as decreasing in amplitude as one measures change in Em away from a synapse axon dendrite synapse Em AP in presynaptic cell 10mV 5mV 3mV1mV

26. Usually one finds a number of inhibitory synapses right at the axon hillock So that even if there a lot of excitatory inputs, inhibitor can decide whether to fire AP or not

27. The mechanism of neurotransmitter release Let’s look at neuromuscular junction The goal of the neuron: to depolarize muscle so that muscle fires an action potential Transmission is chemical  neurotransmitter has to be delivered to the surface of the muscle

28. Neurotransmitter is stored inside synaptic vesicles What triggers the release of vesicles?

29. Increase of Ca ++ concentration from 0.0001mM to 1mM in close vicinity to vesicles triggers exocytosis Neurotransmitter packed inside vesicles at high concentration is spilled on the postsynaptic cell

32. There are usually huge number of postsynaptic receptors These receptors can be coupled to ion channels or to other proteins that transmit the signal to the postsynaptic cell

33. POSTSYNAPTIC RECEPTOR: In the case of a neuromuscular junction (nmj), ACh receptor itself is a channel Following binding of two ACh molecules, the channel opens Sodium enters the muscle and ….what happens? Positive sodium depolarizes the muscle, making it more likely to fire an action potential.

34. Positive sodium depolarizes the muscle  membrane potential more positive  more sodium channels open  Even more sodium ions enter the cells  membrane potential even more positive  on so on == Positive feedback loop == explosion == gun powder – – – – – – – – – – – – + + + + + + + + + + + + + + + + -40mV ++ + + [K + ]=150mM [Na + ]=15mM + + + + + + + + - - - - - - - - - + + + + + + + + - - - - - - - - - + + + + + + + + - - - - - - - - - + + + + + + + + - - - - - - - - - + + + + + + + + - - - - - - - - - + + + + + + + + - - - - - - - - - [K + ]=4mM [Na + ]=145mM ? Na + + + + + ++ + + + + + + + + ++ + + + - - - - - - - - - - - - - -- - - - - - - - +30mV

41. The release of one vesicle usually is not enough to trigger AP in a muscle. Many vesicles and many release zones have to exocytose to result in muscle AP and a corresponding twitch. Each neuronal AP releases MANY vesicles, Each neuronal AP normally results in a muscle AP

42. If Ach is allowed to hang around ACh receptor for too long new signal will not get through. Therefore ACh has to be quickly removed from synaptic cleft Name several possible mechanisms?

43. ACh is broken down by ACh esterase (AChE) AChE is located in postjunctional folds (AChR are on the surface) ACh is broken into choline and acetate Choline is transported back into the neuron by choline transporter AChE Choline transporter Inside the axon terminal: choline + AcetylCoA  ACh + CoA ACh is then packaged into vesicles AChR

44. Electron microscope autoradiograph of the vertebrate nmj, showing localization of ACh receptors (black developed grains) at the top one-third of the postsynaptic junctional folds. AChE is at the bottom of postsynaptic junctional folds

45. Find two errors

48. Disease: Myasthenia gravis G. myo=muscle + asthenia=weakness Severe dropping of eyelids Patient cannot move his eyes to look to either side Severity varies within a single day, day to day…

49. Disease: Myasthenia gravis Autoimmune disease in which antibodies are produced against the nicotinic AChR Viral or bacterial antigens may share epitopes with the AChR This reduces the number of receptors, increases receptor turnover and degradation

50. Normally an AP in a motor axon releases enough vesicles to induce muscle AP. In fact the number of vesicles can be reduced by 25% before it fails to initiate AP In MG the number of AChR goes down and infoldings are reduced  more ACh is broken down by AChE Disease: Myasthenia gravis

51. The weakness is reversed by drugs that inhibit AChE E.g. neostigmine block the breakdown of ACh by AChE and temporarily increases the levels of ACh at the neuromuscular junction

52. Garrett: Brain & Behavior 4e NeurotransmitterFunction Biogenic amines (contain NH 2 amino group) Acetylcholine (Ach)Neurotransmitter at the neuromuscular junction; in brain, involved in learning, etc. Dopamine (DA)Involved in reward circuits in the PFC. Abnormalities in DA system are implicated in schizophrenia and Parkinson’s disease. Cocaine blocks DA reuptake. Attention deficit disorder drug Ritalin also blocks DA reuptake. Amphetamine (Adderall) modulates DA as well as 5-HT and NE). Serotonin (5-HT)Involved in regulation mood, sleep and arousal, sexuality, aggression, body temperature. Abnormalities in 5-HT system are implicated in depression, obsessive-compulsive disorder. Psychedelic drugs like LSD activate 5-HT receptors as well as some DA receptors. Norepinephrine (NE)Released during stress. Neurotransmitter in the brain to increase arousal and attentiveness to events in the environment. ADD drug Strattera affects only NE. Epinephrine (Epi)A stress hormone related to norepinephrine. Also called Adrenalin. Histamine (His)Modulates sleep: antihistamines induce sleepiness (Dramamine) Amino Acids Glutamate (Glu)The major excitatory neurotransmitter in the brain and spinal cord. Gamma-aminobutyric acid (GABA) The major inhibitory neurotransmitter. Its receptors respond to alcohol and the class of tranquilizers called benzodiazepines. Deficiency in GABA or receptors is one cause of epilepsy. Glycine (Gly)Inhibitory neurotransmitter in the spinal cord and lower brain. Works by opening Chloride channels that hyperpolarizes the cell. The poison strychnine causes convulsions and death by blocking Gly binding site on the Chloride channels. Types of neurotransmitters (You are not responsible for learning different types of neurotransmitters, their function, and location)

53. NeurotransmitterFunction Neuropeptides Endorphins (over 85 different neuropeptides) Endogenous opioids (5 amino acid polypeptide) reduce pain and enhance reinforcement. Exogenous opioids: morphine, heroine. Substance PNeurotransmitter in neurons sensitive to pain. Neuropeptide YInitiates eating and produces metabolic shifts. Gases Nitric OxideViagra enhances male erections by increasing nitric oxide’s ability to relax blood vessels and produce penile engorgement. COcarbon monoxide Every molecule of neurotransmitter has a number of different target receptors, increasing the system complexity

54. Presynaptically Transmitter release facilitation Depression Calcium channel regulation Autoreceptors provide feedback mechanism AP duration (AP longer  more Ca++ in  more release Regulate the number of active zones per synapse Regulate the number of synapses between the cells Modify recycling of neurotransmitter Regulation of the synaptic strength Postsynaptically Modification of receptors via ligands Modify receptor subunit composition Change number of receptors

55. Regulate the number of synapses between the cells: grow dendritic spines

56. Sleep and memory Sleep is good for memory consolidation (conversion of memory into permanent memory). Memory improvement is most dramatic for procedural memory (riding a bike, skating, playing the piano). Even a short nap can really improve memory.

57. Poisons Botulinum toxin (from Latin “sausage”)  fusion protein are cut  vesicles are not released Most potent toxin on the planet. Medial lethal dose 1ng/1kg (compare to strychnine 1,000,000ng/kg) You only need 300gram to kill every person on earth

58. Botulinum toxin cuts Syntaxin, SNAP-25, and Synaptobrevin

59. Clostridium botulinum Canned food creates ideal anaerobic environment for the bacterium Clostridium botulinum  allows the spores to germinate, after which the bacteria can multiply and produce toxin  Do not eat food from cans that are swollen! The spores survive boiling at 100°C To kill spores canning companies use high pressure to raise temperature to 121°C for 3 min Bottox

60. Poisons Botulinum toxin (from Latin “sausage”)  fusion protein are cut  vesicles are not released Black widow spider toxin  vesicles are released without Ca++ Cobratoxin binds tightly to nAChR, preventing activation of nAChR Organophosphates (biological weapons): irreversible inhibitors of AChE  ACh is not broken down  desensitization of AChR AChE Choline transporter AChR Botulinum toxin Black widow spider toxin Cobratoxin Organophosphates Why all victims die?

61. Conclusions Most interesting staff – short-term memory – long-term memory – learning happens at the synapses.

62. Example: growth of neuronal connections Within the first day after plating one of the short processes of a hippocampal neuron becomes defined as the single axon of the cell that grows out very rapidly. Both rate and direction of outgrowth are influenced by the nature of the substrate the axon encounters. This movie follows the elongation of the axon on a uniform substrate over a period of 16 hours. Note how the axon elongates in spurts. Time lapse: 16.5 hours (one loop of movie)