3Neurons cell body dendrites (input structure) receive inputs from other neuronsperform spatio-temporal integration of inputsrelay them to the cell bodyaxon (output structure)a fiber that carries messages (spikes) from the cell to dendrites of other neurons
5Synapse site of communication between two cells formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell
6bipolar.about.com/library Synapseaxon of presynapticneurondendrite ofpostsynapticneuronbipolar.about.com/library
7Synapse a synapse can be excitatory or inhibitory arrival of activity at an excitatory synapse depolarizes the local membrane potential of the postsynaptic cell and makes the cell more prone to firingarrival of activity at an inhibitory synapse hyperpolarizes the local membrane potential of the postsynaptic cell and makes it less prone to firingthe greater the synaptic strength, the greater the depolarization or hyperpolarization
9Neural Communication: 1 Communication within and between cells
10Transmission of information Information must be transmittedwithin each neuronand between neurons
11The Membrane The membrane surrounds the neuron. It is composed of lipid and protein.
12Resting potential of neuron = -70mV The Resting PotentialThere is an electrical charge across the membrane.This is the membrane potential.The resting potential (when the cell is not firing) is a 70mV difference between the inside and the outside.+outside++++-inside----Resting potential of neuron = -70mV
14Ions and the Resting Potential Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-).The resting potential exists because ions are concentrated on different sides of the membrane.Na+ and Cl- outside the cell.K+ and organic anions inside the cell.Cl-Na+Na+Cl-Na+Na+outsideinsideOrganic anions (-)K+Organic anions (-)K+Organic anions (-)
15Ions and the Resting Potential Ions are electrically-charged molecules e.g. sodium (Na+), potassium (K+), chloride (Cl-).The resting potential exists because ions are concentrated on different sides of the membrane.Na+ and Cl- outside the cell.K+ and organic anions inside the cell.Cl-Na+Na+Cl-Na+Na+outsideinsideOrganic anions (-)K+Organic anions (-)K+Organic anions (-)
16Maintaining the Resting Potential Na+ ions are actively transported (this uses energy) to maintain the resting potential.The sodium-potassium pump (a membrane protein) exchanges three Na+ ions for two K+ ions.Na+Na+Na+outsideinsideK+K+
17Neuronal firing: the action potential The action potential is a rapid depolarization of the membrane.It starts at the axon hillock and passes quickly along the axon.The membrane is quickly repolarized to allow subsequent firing.
22Action potentials: Repolarization Sodium ion channels close and become refractory.Depolarization triggers opening of voltage-gated potassium ion channels.K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane.Na+K++-
25Action potentials: Resuming the Resting Potential Potassium channels close.Repolarization resets sodium ion channels.Ions diffuse away from the area.Sodium-potassium transporter maintains polarization.The membrane is now ready to “fire” again.K+Na+K+K+Na+
26The Action Potential The action potential is “all-or-none”. It is always the same size.Either it is not triggered at all - e.g. too little depolarization, or the membrane is “refractory”;Or it is triggered completely.
27Course of the Action Potential The action potential begins with a partial depolarization (e.g. from firing of another neuron ) [A].When the excitation threshold is reached there is a sudden large depolarization [B].This is followed rapidly by repolarization [C] and a brief hyperpolarization [D].There is a refractory period immediately after the action potential where no depolarization can occur [E]Membrane potential (mV)[A][B][C][D]excitation thresholdTime (msec)-70+40123[E]
28Action Potential Local Currents depolarize adjacent channels causing depolarization and opening of adjacent Na channelsQuestion: Why doesn’t the action potential travel backward?
30Conduction of the action potential. Passive conduction will ensure that adjacent membrane depolarizes, so the action potential “travels” down the axon.But transmission by continuous action potentials is relatively slow and energy-consuming (Na+/K+ pump).A faster, more efficient mechanism has evolved: saltatory conduction.Myelination provides saltatory conduction.
31Myelination Most mammalian axons are myelinated. The myelin sheath is provided by oligodendrocytes and Schwann cells.Myelin is insulating, preventing passage of ions over the membrane.
32Saltatory ConductionMyelinated regions of axon are electrically insulated.Electrical charge moves along the axon rather than across the membrane.Action potentials occur only at unmyelinated regions: nodes of Ranvier.Myelin sheathNode of Ranvier
33Synaptic transmission Information is transmitted from the presynaptic neuron to the postsynaptic cell.Chemical neurotransmitters cross the synapse, from the terminal to the dendrite or soma.The synapse is very narrow, so transmission is fast.
34Structure of the synapse An action potential causes neurotransmitter release from the presynaptic membrane.Neurotransmitters diffuse across the synaptic cleft.They bind to receptors within the postsynaptic membrane, altering the membrane potential.terminalextracellular fluidsynaptic cleftpresynaptic membranedendritic spinepostsynaptic membrane
35Neurotransmitter release Synaptic vesicles, containing neurotransmitter, congregate at the presynaptic membrane.The action potential causes voltage-gated calcium (Ca2+) channels to open; Ca2+ ions flood in.vesiclesCa2+Ca2+Ca2+Ca2+Ca2+Ca2+Ca2+Ca2+
36Neurotransmitter release Ca2+ causes vesicle membrane to fuse with presynaptic membrane.Vesicle contents empty into cleft: exocytosis.Neurotransmitter diffuses across synaptic cleft.Ca2+
38Ionotropic receptors (ligand gated) Synaptic activity at ionotropic receptors is fast and brief (milliseconds).Acetylcholine (Ach) works in this way at nicotinic receptors.Neurotransmitter binding changes the receptor’s shape to open an ion channel directly.AChACh
43Postsynaptic potentials Depending on the type of ion channel which opens, the postsynaptic cell membrane becomes either depolarized or hyperpolarized.Ions will tend to follow the concentration gradient from high to low concentration, and the electrostatic gradient towards the opposite charge.
44Excitatory postsynaptic potentials (EPSPs) Opening of ion channels which leads to depolarization makes an action potential more likely, hence “excitatory PSPs”: EPSPs.Inside of post-synaptic cell becomes less negative.Na+ channels (NB remember the action potential)Ca2+ . (Also activates structural intracellular changes -> learning.)Na+Ca2++outsideinside-
45Inhibitory postsynaptic potentials (IPSPs) Opening of ion channels which leads to hyperpolarization makes an action potential less likely, hence “inhibitory PSPs”: IPSPs.Inside of post-synaptic cell becomes more negative.K+ (NB remember termination of the action potential)Cl- (if already depolarized)Cl-+outsideinsideK+-
47Requirements at the synapse For the synapse to work properly, six basic events need to happen:Production of the NeurotransmittersSynaptic vesicles (SV)Storage of NeurotransmittersSVRelease of NeurotransmittersBinding of NeurotransmittersLock and keyGeneration of a New Action PotentialRemoval of Neurotransmitters from the Synapsereuptake
48Integration of information PSPs are small. An individual EPSP will not produce enough depolarization to trigger an action potential.IPSPs will counteract the effect of EPSPs at the same neuron.Summation means the effect of many coincident IPSPs and EPSPs at one neuron.If there is sufficient depolarization at the axon hillock, an action potential will be triggered.axon hillock
49Three Nobel Prize Winners on Synaptic Transmission Arvid Carlsson discovered dopamine is a neurotransmitter.Carlsson also found lack of dopamine in the brain of Parkinson patients.Paul Greengard studied in detail how neurotransmitterscarry out their work in the neurons. Dopamine activated acertain protein (DARPP-32), which could change the function of many other proteins.Eric Kandel proved that learning and memory processesinvolve a change of form and function of the synapse,increasing its efficiency. This research was on a certainkind of snail, the Sea Slug (Aplysia). With its relatively lownumber of 20,000 neurons, this snail is suitable forneuron research.
50Neuronal firing: the action potential The action potential is a rapid depolarization of the membrane.It starts at the axon hillock and passes quickly along the axon.The membrane is quickly repolarized to allow subsequent firing.