lecture 4

• A synapse is the functional connection between a neuron and the cell it is signaling, typically a junction between presynaptic and postsynaptic neurons.
• Most synapses in the human nervous system are chemical synapses at which a chemical messenger transmits information one way across a space separating the two neurons.
• The production of IPSPs is called postsynaptic inhibition.
• GABA and glycine produce IPSPs.
• IPSPs dampen EPSPs, making it harder to reach threshold.
• Presynaptic inhibition occurs when one neuron synapses onto the axon or bouton of another neuron, inhibiting the release of its neurotransmitter.
• In an electrical synapse, two neurons are connected by gap junctions, which allow charge carrying ions to flow directly between the two cells in either direction.
• A neurotransmitter is a chemical that is released by a terminal button and has an excitatory or inhibitory effect on another neuron.
• A neurotransmitter carries the signal across a synapse.
• Neurotransmitters produce postsynaptic potentials, which are brief depolarizations or hyperpolarizations that increase or decrease the rate of firing of the axon of the postsynaptic neuron.
• Graded Potential: When ligand-gated ion channels open, the membrane potential changes depending on which ion channel is open.
• Opening Na+ or Ca2+ channels results in a graded depolarization called an excitatory postsynaptic potential (EPSP).
• Opening K+ or Cl− channels results in a graded hyperpolarization called inhibitory postsynaptic potential (IPSP).
• A postsynaptic potential (PSP) is the graded potential in the dendrites of a neuron that is receiving synapses from other cells.
• EPSPs and IPSPs counter each other.
• It depends on the specific ion channels that are activated in the cell membrane.
• Hyperpolarization in a postsynaptic potential is an inhibitory postsynaptic potential (IPSP) because it causes the membrane potential to move away from threshold.
• Some synapses excite, whereas others inhibit, the postsynaptic neuron.
• Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization.
• Depolarization in a postsynaptic potential is called an excitatory postsynaptic potential (EPSP) because it causes the membrane potential to move toward threshold.
• EPSPs and IPSPs are graded potentials.
• Spatial and temporal summation of EPSPs allows a depolarization of sufficient magnitude to cause the stimulation of action potentials in the postsynaptic neuron.
• IPSPs and EPSPs from different synaptic inputs can summate.
• Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus.
• Summation of EPSPs and IPSPs at the initial segment of the axon determines whether an action potential occurs.
• EPSPs move the membrane potential closer to threshold and may require EPSPs from several neurons to actually produce an action potential.
• EPSPs are graded in magnitude, have no threshold, cause depolarization, summate, and have no refractory period.
• IPSPs move the membrane potential farther from threshold.
• Postsynaptic potentials can be depolarizing or hyperpolarizing.