NERVOUS SYSTEM , NEUROTRANSMITTERS. AND SYNAPSES.pdf

PRESBYTERIAN UNIVERSITY COLLEGE DEPARTMENT OF PHYSICIAN ASSISTANTSHIP  SYNAPSE & NEUROTRANSMITTERS By Daniel Nayembil  OUTLINE INTRODUCTION SYNAPSE Definition OF SYNAPSE Structural Components Of A Synapse Synaptic Neuro...

PRESBYTERIAN UNIVERSITY COLLEGE DEPARTMENT OF PHYSICIAN ASSISTANTSHIP  SYNAPSE & NEUROTRANSMITTERS By Daniel Nayembil  OUTLINE INTRODUCTION SYNAPSE Definition OF SYNAPSE Structural Components Of A Synapse Synaptic Neurons Types Of Synapses Classification Of Synapse (Electrical Synapse &Chemical Synapse) Formation Of Synapse Mechanisms Of Synaptic Transmissions Action Potential The Neuromuscular Junction NEUROTRANSMITTERS THE NERVOUS 3 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Introduction i. Axons generally conduct nerve impulses from the cell body to the end axon terminals. ii. At the axon terminal, they can affect the second cell in 2 ways; ✓Stimulate (excite) it or ✓Inhibit it THE NERVOUS SYSTEM_SYNAPSE&NEUROTRANSMITTERS 4  Introduction The effector cell in the CNS is usually a neuron The effector cell in the PNS may either be; ✓a neuron ✓effector cells in a muscle or gland at myoneural or neuromuscular junctions THE NERVOUS 5 SYSTEM_SYNAPSE&NEUROTRANSMITTERS SYNAPSE  Definition A synapse is defined as a functional communication btn 2 neurons or a neuron and another cell. Basically, a Gap btn the axon and the dendrite THE NERVOUS 7 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  SYNAPSE Functional connection between a neuron and another neuron or effector cell. Transmission is in one direction only. Axon of first (presynaptic) to second (postsynaptic) neuron. THE NERVOUS 8 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  SYNAPSE Synaptic transmission is through a chemical gated channel. Neurotransmitters are released across this gap Presynaptic terminal (bouton) releases the neurotransmitter (NT). THE NERVOUS 9 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Structural Components of a synapse i. Presynaptic neuron ii. Synaptic cleft iii. Postsynaptic neuron THE NERVOUS 10 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  SYNAPTIC NEURONS 1. Presynaptic neurons :transmit nerve impulses toward a synapse. 2. Postsynaptic neurons :conduct nerve impulses away from the synapse. Axons may establish synaptic contacts with any portion of the surface of another neuron (except those regions that are myelinated). THE NERVOUS 14-11 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 12 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Types of synapses: Structural based on contacts i. Axodendritic synapse ii. Axosomatic synapse iii. Axoaxonic synapse THE NERVOUS 14-13 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Formation of synapse Between two neurons, synapses can form between: an axon and a dendrite (axodendritic) an axon and an axon (axoaxonic) an axon and a cell body (axosomatic) THE NERVOUS 14 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Functional types of synapses 1. Electrical synapses (Use Gap junctions) 2. Chemical synapses –Use neurotransmitters across a synaptic cleft THE NERVOUS 14-15 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  MECHANISMS OF SYNAPTIC TRANSMISSIONS A gap (about 20nm) Where the nerve impulse passes from one cell to the next The electrical signal (the action potential) ➔ a chemical signal to cross the gap between the cells The neurotransmitter crosses this gap by diffusion This creates a small delay THE NERVOUS 16 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  1. Electrical Synapses i. This is movement of electrical impulses through gap junctions. ii. Electrical synapses are not very common in mammals. iii. In humans, these synapses occur primarily between smooth muscle cells where quick, uniform innervation is essential. THE NERVOUS 14-17 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Electrical Synapses iv. Electrical synapses are also located in the myocardium v. Gap junctions are potential spaces btn 2 mmb of cells through which ions and molecules can pass from one cell to the other. THE NERVOUS 18 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Electrical Synapses vi. They are typically present in the myocardium and smooth muscles vii. allowing excitatory, rhythmic contractions of groups of muscle cells. vii. The presence of gap junctions is not explicit in the brain, which may allow bidirectional transmission of impulses, in contrast with chemical synapse which is uni- directional. THE NERVOUS 19 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Electrical Synapses ix. Gap junctions are also noted in glial cells where they act as channels for passage of molecules btn cells. x. Impulse transmission in electrical synapse can be regenerated without interruption in adjacent cells. THE NERVOUS 20 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 21 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Electrical Synapse THE NERVOUS 22 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  2. CHEMICAL SYNAPSES i. Most numerous type of synapse ii. Facilitates interactions between ✓ Neurons and neurons ✓ neurons and effectors. (glands, muscles) THE NERVOUS 14-23 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES This is a unidirectional transmission of chemicals (neurotransmitters) from presynaptic axon terminal boutons through a synaptic cleft to the post synaptic cell. THE NERVOUS 24 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 25 SYSTEM_SYNAPSE&NEUROTRANSMITTERS CHEMICAL SYNAPSES THE NERVOUS 26 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES There is lots of variation in synapses Some are excitatory (Type I) Some are inhibitory (Type II) THE NERVOUS 27 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES The release of neurotransmitters is by Exocytosis where membrane of vesicles containing the neurotransmitter fuses with the membrane of the axon terminal bouton to discharge its content into the cleft. THE NERVOUS 28 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES iii. The no. of vesicles under going through the processes of Exocytosis in a chemical synapse is largely dependent on the frequency of action potential produced at the presynaptic axon terminals. iv. Action potential is generated by summation of graded potentials at a threshold potential. THE NERVOUS 29 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES v. At the threshold potential, the portion of the cell membrane stimulated changes its permeability allowing channels selective to Na+ to open for the passage of Na+ into the cell. vi. This movement is further facilitated by the relative negatively charged intracellular environment of the cell. THE NERVOUS 30 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES In the process, the membrane loses its electrical charge and becomes depolarized. As the Na+ move into the cell, K+ channels open to allow the mov’t of K+ outside the membrane , thereby making the outside env’t more positively charged. THE NERVOUS 31 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES viii. The membrane becomes reporalized at this state. The rapid sequence of deporalization & reporalization make up the ACTION POTENTIAL THE NERVOUS 32 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  1. Action potential arrives at terminal button Vesicle storing neurotransmitter Ca2+ channel Membrane receptor for neurotransmitter THE NERVOUS 33 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  3. Ca2+ stimulates vesicles to fuse 2. Depolarisation with membrane opens Ca2+ channels Ca2+ enters terminal button Ca2+ Ca2+ Ca2+ Ca2+ 4. Exocytosis of neurotransmitter It diffuses 20nm across the synaptic cleft THE NERVOUS 34 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  The Synaptic Cleft An action potential travels down an axon to the terminal buttons The membrane of a terminal button depolarises Voltage-gated Ca2+ channels to open Ca2+ ions flood into the terminal button © 2016 Paul Billiet ODWS THE NERVOUS 35 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Action Potential This stimulates hundreds of synaptic vesicles, packed with neurotransmitter, to fuse with the membrane of the terminal button Exocytosis The Ca2+ ions are then pumped out again. THE NERVOUS 36 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 37 SYSTEM_SYNAPSE&NEUROTRANSMITTERS 5. Neurotransmitter receptor sites on the postsynaptic membrane are ion channels. They open when the neurotransmitter binds 6. Localised depolarisation as ions leak in or out THE NERVOUS of membrane. SYSTEM_SYNAPSE&NEUROTRANSMITTERS 38  A new action potential If the localised depolarisations build up to the nerve cell threshold, a full action potential will be produced This will travel away, down the postsynaptic neuron © 2016 Paul Billiet ODWS THE NERVOUS 39 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Action Potential The action of the neurotransmitters stops: (i) as they dilute by diffusion in the synaptic cleft (ii) by hydrolysis through the action of enzymes there THE NERVOUS 40 SYSTEM_SYNAPSE&NEUROTRANSMITTERS ACTION POTENTIAL THE NERVOUS 41 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  The Neuromuscular Junction (a synapse) The motor end plate is the terminal button of a motor neuron that makes contact with a muscle cell The motor end plate releases the neurotransmitter acetylcholine that ultimately causes the muscle cell to contract © 2016 Paul Billiet ODWS THE NERVOUS 42 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Blocking synapses Poisons Psychotic drugs Other Medication Botox Neonicotinoid pesticides © 2016 Paul Billiet ODWS THE NERVOUS 43 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Blocking synapses Botox = a neurotoxin produced by a bacterium Chlostridium botuninum It causes food poisoning (botulism) The toxin blocks the release of acetyl choline neurotransmitter to muscles Causes muscle paralysis THE NERVOUS 44 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Blocking synapses Uses: ✓Cosmetically used to smooth lines and creases ✓Medically used to relax muscles that causing illnesses such as spasms and strabismus (cross-eyed). THE NERVOUS 45 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  CHEMICAL SYNAPSES Presynaptic membrane: – releases a signaling molecule called a neurotransmitter, such as acetylcholine (ACh). – Other types of neurons use other neurotransmitters. Postsynaptic membrane: – Contains receptors for neurotransmitters THE NERVOUS 46 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS Dfn: Neurotransmitters are hormones working over a very short distance in synapses to stimulate or inhibit action potential. Some Neurotransmitters are hormones that fn both at synapses and in the circulatory system E.g. Adrenalin (Epinephrin). THE NERVOUS 47 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS i. Released from vesicles in the plasma membrane of the presynaptic cell. ii. Then binds to receptor proteins on the plasma membrane of the postsynaptic cell. iii. A unidirectional flow of information and communication THE NERVOUS 14-48 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS Two factors influence the rate of conduction of the impulse: axon’s diameter presence (or absence) of a myelin sheath. THE NERVOUS 49 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Neuronal Pools (or Neuronal Circuits or Pathways) Dfn: A neural pool is a circuit of impulse transmission There are billions of interneurons within the CNS grouped into complex patterns called neuronal pools/neuronal circuits or pathways THE NERVOUS 14-50 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Neuronal Pools; cont Neuronal pools are defined based upon FUNCTION, not anatomy, THE NERVOUS 51 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Neuronal Circuits or Pathways) 4 types of circuits: i. Converging ii. Diverging iii. Reverberating iv. Parallel-after-discharge A pool may be localized, or distributed in several different regions of the CNS. THE NERVOUS 52 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 53 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Chemical Synapse Terminal bouton is separated from postsynaptic cell mmb by synaptic cleft. NTs are released from synaptic vesicles. Vesicles fuse with axon membrane and NT released by exocytosis. Amount of NTs released depends upon frequency of AP. THE NERVOUS 54 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Synaptic Transmission NT release is rapid because many vesicles form fusion-complexes at “docking site.” As AP travels down axon to bouton, VG Ca2+ channels open. – Ca2+ enters bouton down concentration gradient. – Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs. THE NERVOUS 55 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Synaptic Transmission The Ca2+ activates calmodulin, which activates protein kinase. Protein kinase phosphorylates synapsins. – Synapsins aid in the fusion of synaptic vesicles. THE NERVOUS 56 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Synaptic Transmission NTs are released and diffuse across synaptic cleft. NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. Chemically-regulated gated ion channels open. – EPSP: depolarization. – IPSP: hyperpolarization. Neurotransmitter inactivated to end transmission. THE NERVOUS 57 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Chemical Synapses EPSP (excitatory postsynaptic potential): – Depolarization. IPSP (inhibitory postsynaptic potential): – Hyperpolarization THE NERVOUS 58 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Chemical Synapses THE NERVOUS 59 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS; e.g. i. Acetylcholine ii. Monoamines iii. Serotonin iv. Dopamine v. Norepinephrine vi. Aminoacids vii. Polypeptides viii.Nitric oxide THE NERVOUS 60 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  THE NERVOUS 61 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS The arrival of action potential causes vesicles to move to end of axon and discharge contents into the synaptic cleft NTs diffuse across cleft, and bind to receptors on other cell’s membrane, causing ion channels on that cell to open THE NERVOUS 62 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS destroyed by specific enzymes in cleft diffuse out of cleft, or are reabsorbed by cell THE NERVOUS 63 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  NEUROTRANSMITTERS depending of type of neurotransmitter and type of receptor, the response of postsynaptic neuron can be toward excitation or toward inhibition destroyed by specific enzymes in cleft, diffuse out of cleft, or are reabsorbed by cell THE NERVOUS 64 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  1. Acetylcholine (ACh) as NT ACh is both an excitatory and inhibitory NT, depending on organ involved. – Causes the opening of chemical gated ion channels. Nicotinic ACh receptors: – Found in autonomic ganglia and skeletal muscle fibers. Muscarinic ACh receptors: – Found in the plasma membrane of smooth and cardiac muscle cells, and in cells of particular glands. THE NERVOUS 65 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Ligand-Operated ACh Channels Most direct mechanism. Ion channel runs through receptor. – Receptor has 5 polypeptide subunits that enclose ion channel. – 2 subunits contain ACh binding sites. Channel opens when both sites bind to ACh. – Permits diffusion of Na+ into and K+ out of postsynaptic cell, where Inward flow of Na+ dominates. Produces EPSPs. THE NERVOUS 66 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Ligand-Operated ACh Channels THE NERVOUS 67 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  G Protein-Operated ACh Channel Only 1 subunit. Ion channels are separate proteins located away from the receptors. Binding of ACh activates alpha G-protein subunit. Alpha subunit dissociates. Alpha subunit or the beta-gamma complex diffuses through membrane until it binds to ion channel, opening it. THE NERVOUS 68 SYSTEM_SYNAPSE&NEUROTRANSMITTERS G Protein-Operated ACh Channel THE NERVOUS 69 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Acetylcholinesterase (AChE) Enzyme that inactivates ACh. – Present on postsynaptic membrane or immediately outside the membrane. Prevents continued stimulation. THE NERVOUS 70 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Acetylcholinesterase (AChE) THE NERVOUS 71 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  ACh in CNS Cholinergic neurons: – Use ACh as NT. – Axon bouton synapses with dendrites or cell body of another neuron. First VG channels are located at axon hillock. EPSPs spread by cable properties to initial segment of axon. Gradations in strength of EPSPs above threshold determine frequency of APs produced at axon hillock. THE NERVOUS 72 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  ACh in PNS Somatic motor neurons synapse with skeletal muscle fibers. – Release ACh from boutons. – Produces end-plate potential (EPSPs). Depolarization opens VG channels adjacent to end plate. THE NERVOUS 73 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  2. Monoamines as NT Monoamine NTs: – Epinephrine. – Norepinephrine. – Serotonin. – Dopamine. Released by exocytosis from presynaptic vesicles. Diffuse across the synaptic cleft. Interact with specific receptors in postsynaptic membrane. THE NERVOUS 74 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Inhibition of Monoamines as NT Reuptake of monoamines into presynaptic membrane. – Enzymatic degradation of monoamines in presynaptic membrane by MAO. Enzymatic degradation of catecholamines in postsynaptic membrane by COMT. THE NERVOUS 75 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Inhibition of Monoamines as NT THE NERVOUS 76 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  3. Serotonin as NT i. NT (derived from L-tryptophan) for neurons with cell bodies in raphe nuclei. ii. Regulation of mood, behavior, appetite, and cerebral circulation. iii. SSRIs (serotonin-specific reuptake inhibitors): – Inhibit reuptake and destruction of serotonin, prolonging the action of NT. – Used as an antidepressant. Reduces appetite, treatment for anxiety, treatment for migraine headaches. THE NERVOUS 77 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  4. Dopamine an NT NT for neurons with cell bodies in midbrain. Axons project into: – Nigrostriatal dopamine system: Nuerons in substantia nigra send fibers to corpus straitum. Initiation of skeletal muscle movement. Parkinson’s disease: degeneration of neurons in substantia nigra. THE NERVOUS 78 SYSTEM_SYNAPSE&NEUROTRANSMITTERS – Mesolimbic dopamine system: Neurons originate in midbrain, send axons to limbic system. Involved in behavior and reward. Addictive drugs(Promote activity in nucleus accumbens) THE NERVOUS 79 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  5. Norepinephrine (NE) as NT NT in both PNS and CNS. PNS: – Smooth muscles, cardiac muscle and glands. Increase in blood pressure, constriction of arteries. CNS: – General behavior. THE NERVOUS 80 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  6. Amino Acids as NT i. Glutamic acid and aspartic acid: – Major excitatory NTs in CNS. ii. Glutamic acid: – NMDA receptor involved in memory storage. iii. Glycine: – Inhibitory, produces IPSPs. – Opening of Cl- channels in postsynaptic membrane. Hyperpolarization. – Helps control skeletal movements. THE NERVOUS 81 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Amino Acids as NT GABA (gamma-aminobutyric acid): – Most prevalent NT in brain. – Inhibitory, produces IPSPs. Hyperpolarizes postsynaptic membrane. –Motor functions in cerebellum. THE NERVOUS 82 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  7. Polypeptides as NT Cholecystokinin (CCK): – Promote satiety following meals. – Major NT in sensations of pain. Synaptic plasticity (neuromodulating effects): – Neurons can release classical NT or the polypeptide NT. THE NERVOUS 83 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Polypeptides as NT Endogenous opiods: – Brain produces its own analgesic endogenous morphine-like compounds, blocking the release of substance P. – Beta-endorphin, enkephalins, dynorphin. Neuropeptide Y: – Most abundant neuropeptide in brain. – Inhibits glutamate in hippocampus. – Powerful stimulator of appetite. THE NERVOUS 84 SYSTEM_SYNAPSE&NEUROTRANSMITTERS NO: – Exerts its effects by stimulation of cGMP. – Macrophages release NO to helps kill bacteria. – Involved in memory and learning. – Smooth muscle relaxation. THE NERVOUS 85 SYSTEM_SYNAPSE&NEUROTRANSMITTERS 8. Endogenous Cannabinoids, Carbon Monoxide a. Endocannabinoids: – Bind to the same receptor as THC. – Act as analgesics. – Function as retrograde NT. b. Carbon monoxide: – Stimulate production of cGMP within neurons. – Promotes odor adaptation in olfactory neurons. – May be involved in neuroendocrine regulation in hypothalamus. THE NERVOUS 86 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  EPSP No threshold. Decreases resting membrane potential. – Closer to threshold. Graded in magnitude. Have no refractory period. Can summate. THE NERVOUS 87 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  EPSP THE NERVOUS 88 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Synaptic Integration EPSPs can summate, producing AP. – Spatial summation: Numerous boutons converge on a single postsynaptic neuron (distance). – Temporal summation: Successive waves of neurotransmitter release (time). THE NERVOUS 89 SYSTEM_SYNAPSE&NEUROTRANSMITTERS Synaptic Integration THE NERVOUS 90 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Long-Term Potentiation May favor transmission along frequently used neural pathways. Neuron is stimulated at high frequency, enhancing excitability of synapse. – Improves efficacy of synaptic transmission. Neural pathways in hippocampus use glutamate, which activates NMDA receptors. – Involved in memory and learning. THE NERVOUS 91 SYSTEM_SYNAPSE&NEUROTRANSMITTERS  Synaptic Inhibition Presynaptic inhibition: – Amount of excitatory NT released is decreased by effects of second neuron, whose axon makes synapses with first neuron’s axon. Postsynaptic inhibition (IPSPs): – No threshold. – Hyperpolarize postsynaptic membrane. – Increase membrane potential. – Can summate. – No refractory SYSTEM_SYNAPSE&NEUROTRANSMITTERS periodTHE. NERVOUS 92 Synaptic Inhibition THE NERVOUS 93 SYSTEM_SYNAPSE&NEUROTRANSMITTERS END OF PRESENTATION THANK YOU QUESTIONS THE NERVOUS 94 SYSTEM_SYNAPSE&NEUROTRANSMITTERS