Fundamentals of the Nervous System and Nervous Tissue - Chapter 11 (Part 2) Synapses Nerve impulses pass from neuron to neuron at synapses Prevents nerve impulses from directly passing from one neuron to the next Ensures unidirectional communication between neurons

Types of Synapses Axodendritic –synapses between axon of one neuron and dendrite of another Axosomatic – synapses between the axon of one neuron and soma of another Axoaxonic (axon to axon) Electrical synapses are less common than chemical synapses Correspond to gap junctions found in other cell types Are important in the CNS in: Arousal from sleep Mental attention Emotions and memory Ion and water homeostasis Chemical Synapses Specialized for the release and reception of neurotransmitters Typically composed of two parts: Axonal terminal of presynaptic neuron, which contains synaptic vesicles Receptor region on the dendrite(s) or soma of the postsynaptic neuron Comparison

Synaptic Transmission As the nerve impulse enters the ______ _____ of the axon terminal it opens voltage gated channels in the __________________ membrane that allows _____to enter. The Ca++ promotes synaptic _________ containing a neurotransmitter to fuse with the axon membrane and release the _________________ into the _________ _____. The ________________ crosses the synapse and joins with the chemically gated protein _________ in the ____________________membrane which allows ____ to enter the cell causing the resting membrane to be ____________. snaptic knob presynaptic neuron Ca++ vesicles neurotransmitter synaptic cleft neurotransmitter receptor postsynaptic neuron Na+ depolarized

Post Synaptic Potentials – Graded Potentials Excitatory postsynaptic potentials (EPSPs) graded may or may not reach ____ ______ depolarizes membrane of postsynaptic neuron Na+ gates are opened. Na+ ions flow ____ the cell. Why? Internal cell membrane become ____ negative? This is called _____________. Will a current be likely to reach the axon hillock and become an action potential? axon hillock into less depolarization Inhibitory postsynaptic potentials (IPSPs) action potential of postsynaptic neuron becomes less likely Cl – or K+ channels are opened. Does the Cl- ions go into the cell or out? K+ into or out ? Does the internal cell become more or less negative? This is called ________________ Will current be likely to flow in the cell? yes hyperpolarization no

ACTION POTENTIAL GENERATION If the synaptic (local graded) potentials add up to _________ they will reach the _____ ______. The nerve impluse than continues down the axon as an ______ _______ Only the ____ can generate an action potential. all-or-nothing The action potential is generated along the axon membrane by opening voltage gated ____channels and Na+ diffuses in and the membrane is _____________ Soon after voltage sensitive __ channels open and K+ diffuses out, and the membrane is __________. This depolarization and repolarization occurs all the way down the axon with out diminishing in strength which is characterized as __-__-_______. unmyelinated axons conductions are slower because In myelinated axons the action potential jumps from _____ to _____ . This type of conduction is called ________ and is much faster then in ____________ axons. threshold axon hillock action potential axon Na+ depolarized K+ repolarized all-or-nothing node node saltatory unmyelinated

ON TO THE AXON HILLOCK (trigger zone) Depolarization causes a current to flow. The + charge flows along an electrical concentration ________ to the more - areas of the cell membrane toward the ____ ______ of the cell body. This is a _______ (local) potential meaning if it is not strong enough it will dissipate before it reaches the axon hillock. Graded potentials occur at the synapse in _________ and ___ ______. Threshold potentials Subthreshold potentials gradient axon hillock graded dendrites cell bodies Summation of EPSPs an IPSPs at the axon hillock More than one axon is stimulating a single neuron. (spatial summation) Two or 3 subthreshold EPSP's can add up to one _________ potential. A threshold EPSP and threshold IPSP stimulates a neuron simultaneously. Would an action potential result? A neuron sending repeated stimuli (temporal summation) at subthreshold to another neuron in rapid succession, may sum up to an ______ potential. threshold No, they would cancel each other. action

Meanwhile back at the synapse........ REPOLARIZATION Another nerve impulse may not be received by a postsynaptic neuron, until it goes back to the __________ state. polarized Refractory period Absolute -time when threshold stimulus does not start another action potential Relative -time when stronger threshold stimulus can start another action potential

Meanwhile back at the synapse........ REPOLARIZATION Neurotransmitter bound to a postsynaptic neuron producing a continuous postsynaptic effect must be removed from its receptor: Blocks reception of additional “messages” Removal of neurotransmitters occurs when they: Are degraded by enzymes Are reabsorbed by astrocytes or the presynaptic terminals Diffuse from the synaptic cleft Example - __________________ swiftly removes ACh from the receptors thus cutting off the influx of Na+ and eflux of ____. The ______ pump is activated to reestablish the ionic conditions. The membrane than returns to its ________ ________ potential of ____ mV. Membrane is ___________ to polarized. Acetylcholinesterase K+ Na+K+ resting membrane -70 repolarized

Control of Nerve Impulses at the Synapse How drugs or poisons control/or destroy at the synapse: Stop the chemical reactions that create neurotransmitters. Ecstasy -->effects serotonin and midbrain dopamine systems resulting in the progressive degeneration of nerve terminals. Interfer with vesicle storage or release of neurotransmitters opiates increase release of dopamine; THC decrease Interfers with deactivation of the neurotransmitter. - MAOI's inhibit the action of the enzyme Monoamine oxidase Bind to receptors in place of neurotransmitters - mimics (agonists) & blockers (antagonists) LSD ---replaces serotonin ---> hallucinates caffeine --->replaces adenosine --> facilitates Prevent neurotransmitters from returning to their sending neuron (reuptake) - Cocaine Paxil to control panic disorders Interfer with second messengers, the chemical and electrical changes that take place in a receiving neuron.

Postsynaptic Potentials & Neurotransmitters Inhibiting Postsynaptic Potentials (IPSP) Neurotransmitters that inhibit -Serotonin, GABA, endorphins, ACh, epinephrine Opiates (morphine) - inhibit the release of substance P Valium - enhances binding effect of GABA (agonist) Fluoxetine "Prozac“ blocks reuptake of serotonin Excitatory postsynaptic potentials (EPSP) Neurotransmitters -substance P, glutamate, dopamine, ACh , epinephrine LSD - competes with serotonin for receptor (antagonist) Amphetamines -enhance the release and decrease reuptake of dopamine caffeine - lowers the threshold at synapse (facilitation) nicotine - increase release of dopamine Effects of alcohol Increased turnover of norepinephrin and dopamine slow acetylcholine systems increased production of endorphins increased transmission in GABA systems Without alcohol With alcohol Chocolate Moods

Effect of ions on nerve impulse transmission Blocked Na+ channels Decrease of K+ in extracellular fluids causes hyperpolarization of RMP caused by starvation, alkalosis, kidney disease symptoms - muscle weakness, sluggish reflexes Increase or decrease threshold? Stimulus needs to be stronger or weaker? Increase of K+ in extracellular fluids causes depolarization of RMP nervousness, convulsions, paralysis Total depolarization would result in which of the symptoms?

References Web Sites on Nervous System Neuron Animations used in Class Graded Potentials Connecticut College Synapse Potential Connecticut College Explore the Brain and Spinal Cord(Great Explanations)