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Synapse: special site where neuron communicates with another cell

Presynaptic neuron Sends message

Postsynaptic cell Receives message Chemical synapses: signal transmitted across a gap by neurotransmitters Most common type of synapse between neurons Only type of synapse between neurons & non-neurons Presynaptic/postsynaptic cells separated by SYNAPTIC CLEFT Whether action potential travels to postsynaptic cell, depends on: AMOUNT of neurotransmitter released SENSITIVITY of postsynaptic cell (receptors) NEUROMUSCULAR JUNCTION Synapse between neuron & skeletal muscle cell NEUROGLANDULAR JUNCTION Synapse between neuron & gland cell Chemicals in SYNAPTIC VESICLES in AXON TERMINAL of presynaptic cell Released into synaptic cleft (triggered by arrival of AP) Affect RECEPTORS of postsynaptic membrane Broken down by enzymes Reabsorbed & reassembled by axon terminal Chemicals in SYNAPTIC VESICLES in AXON TERMINAL of presynaptic cell Released into synaptic cleft (triggered by arrival of AP) Affect RECEPTORS of postsynaptic membrane Broken down by enzymes Reabsorbed & reassembled by axon terminal Chemicals in SYNAPTIC VESICLES in AXON TERMINAL of presynaptic cell Released into synaptic cleft (triggered by arrival of AP) Affect RECEPTORS of postsynaptic membrane Broken down by enzymes Reabsorbed & reassembled by axon terminal Release ACETYLCHOLINE (ACh) at: ALL neuromuscular junctions (skeletal muscle fibers) MANY synapses in CNS ALL neuron-to-neuron synapses in PNS ALL neuromuscular & neuroglandular junctions in PARASYMPATHETIC DIVISION OF ANS Action potential arrives at axon terminal, depolarizes axon membrane Calcium ions enter axon terminal, trigger release of ACh ACh binds to receptors on postsynaptic membrane. Sodium ions enter, depolarizing cell, possibly generating action potential. ACh removed from synaptic cleft by acetylcholinesterase (AChE) AChE breaks ACh down into acetate & choline Choline reabsorbed into axon terminal, acetate diffuses away (re-uptake) Happens during periods of intense stimulation Neurotransmitter not recycled fast enough to meet demands Response of synapse weakens until ACh is replenished Classes of neurotransmitters: EXCITATORY NEUROTRANSMITTERS Cause depolarization of postsynaptic membranes Promote action potentials INHIBITORY NEUROTRANSMITTERS Cause hyperpolarization of postsynaptic membranes Suppress action potentials Classes of neurotransmitters: EXCITATORY NEUROTRANSMITTERS Cause depolarization of postsynaptic membranes Promote action potentials INHIBITORY NEUROTRANSMITTERS Cause hyperpolarization of postsynaptic membranes Suppress action potentials Effect of neurotransmitter on postsynaptic membrane: Depends on properties of receptor NOT on the nature of the neurotransmitter NOREPINEPHRINE (NE) – also called “noradrenaline” Released by adrenergic synapses Excitatory, depolarizing effect Widely distributed in brain & portions of autonomic nervous system DOPAMINE CNS neurotransmitter (inhibitory or excitatory) Inhibitory: prevents overstimulation of muscles Excitatory: cocaine inhibits re-uptake of dopamine More dopamine remains in synapse (“high”) GAMMA-AMINOBUTYRIC ACID (GABA) Primary inhibitory neurotransmitter in CNS Reduces excitability across neurons Can reduce anxiety, some drugs enhance this SEROTONIN Excitatory or inhibitory neurotransmitter in CNS Affects attention & emotional states Some drugs (Zoloft, Prozac) inhibit re-uptake of serotonin More serotonin remains in synapse (inhibitory) Integration of various stimuli into single message Occurs in POSTSYNAPTIC CELL At the simplest level (individual neurons): Many dendrites receive neurotransmitter messages SIMULTANEOUSLY Some excitatory, some inhibitory NET EFFECT at axon hillock determines if action potential is produced Potentials in the postsynaptic cell Graded potentials develop in response to neurotransmitters

Types of postsynaptic potentials: EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP) Graded DEPOLARIZATION of postsynaptic membrane INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP) Graded HYPERPOLARIZATION of postsynaptic membrane Neuron receiving mostly IPSPs is INHIBITED from producing AP Membrane is HYPERPOLARIZED (more negative) Stimulation needed to reach threshold is INCREASED To trigger an action potential: One EPSP is not enough (0.5 mV) EPSPs (& IPSPs) combine through SUMMATION Temporal summation Spatial summation Rapid, repeated stimuli at a single synapse

Frequency of “fire” determines magnitude of response

Many refills of bucket will eventually fill bathtub Simultaneous stimuli arrive at multiple synapses (lots of neurotransmitters) Many channels open, postsynaptic neuron gets closer to threshold Many buckets dumping into bathtub at same time Neuron becomes facilitated as EPSPs accumulate: Membrane potentials rise closer to threshold (more positive) Facilitated = easier to generate an action potential Neuron becomes inhibited as IPSPs accumulate: Membrane potentials move further from threshold (more negative) Inhibited = harder to generate an action potential If EPSP greater than IPSP Net change is positive (depolarization) Action potential generated if axon hillock depolarizes (threshold = -60 to -55 mV) If IPSP greater than EPSP Net change is negative (hyperpolarization) No action potential, cell is inhibited If EPSP = IPSP No net change No action potential, membrane stays at resting membrane potential (-70 mV) Information relayed in the form of action potentials Magnitude of sensory stimulation/motor response proportional to frequency of AP Neurotransmitters released at a synapse may be excitatory or inhibitory Effect on axon hillock = net effect of arriving stimuli Response of postsynaptic neuron can be altered by: Activity under way at other synapses on the postsynaptic cell Modification of rate of neurotransmitter release through facilitation or inhibition