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Neurons – Electrochemical Communication
Electrochemical Communications communication between neurons Neurotransmitters movie
Preview of today’s lecture
Structure of a synapse
presynaptic membrane
postsynaptic membrane
synaptic cleft
Structure of a synapse
synaptic vesicles are produced by Golgi apparatus in the soma
OR
by recycled matter in the cisternae of the terminal button: Pinocytosis
Neurons – from electrical to chemical
• axon terminal contains synaptic vesicles
• vesicles release neurotransmitters across the synaptic cleft• the released neurotransmitter leads to post-synaptic potentials (hyperpolarization or depolarization) that alter the firing rate of the receiving neuron (decrease or increase)
Structure of the synapse
• as viewed under an electron microscope
Structure of the synapse
• synaptic vesicles fusing with the presynaptic membrane
Structure of the synapse
• synaptic vesicles fusing with the presynaptic membrane
Neurochemicals
• Neurotransmitters - chemical substance released from the end of a neuron during the propagation of a nerve impulse; it relays information from one neuron to another.
• Neuromodulators – secreted in larger amounts and diffuse further (composed of peptides)
• Hormones – produced in endocrine glands – released into extracellular fluid to be taken up by specific target cells
Binding
• only specific neurotransmitters will bind with specific receptor sites – like a key in a lock
• chemical that attaches to a binding site is a ligand
• neurotransmitters are naturally produced ligands
• neurotoxins are also ligands and various drugs have their effect in the same manner – artificially produced ligands (e.g., LSD)
Neurons – from electrical to chemical
Only specific neurotransmitters will bind with the post-synaptic membrane.
Binding sitesAxodendritic – synapse on the dendrite of the neuronAxosomatic – synapse on the somaAxoaxonic – synapse on the axon
Axodendritic Axosomatic Axoaxonic
Receptors
• neurotransmitter specific postsynaptic receptors
• open to allow ions to flow into the postsynaptic neuron
• two main types• ionotropic• metabotropic
Ionotropic receptors
• receptor site has its own ion channel
• contain sodium channels
• fast acting and short lasting
Metabotropic receptors
• indirect method• located nearby G-
proteins• G-proteins in turn
activate an ion channel
• slower to begin and longer lasting
Metabotropic receptors
• G-proteins can also activate second messengers – enzymes that in turn activate an ion channel
Excitatory or inhibitory post-synaptic potentials.
• the change in the electric charge can be more positive than the resting potential (excitatory) or more negative than the resting potential (inhibitory)
Inhibitory
• once neurotransmitters are bound to the post synaptic membrane the electrical charge is now altered in the receiving neuron
Excitatory or inhibitory post-synaptic potentials.
Post-synaptic potentials
• determined by the ion channel opened by the neurotransmitter and not the transmitter itself
• graded – the potential dissipates with distance traveled
• smaller in magnitude than action potentials
• action potentials are always excitatory – post-synaptic potentials can be either excitatory or inhibitory
Post-synaptic potentials
• excitatory PSP – typically related to sodium ion channels (rush of Na+ into the cell makes it more positively charged)
• inhibitory PSP typically related to potassium ion channels (extra K+ maintained inside cell by sodium-potassium pump leaks out making the cell more negatively charged)
• action of Cl– channels depends on the state of the receiving neuron – if depolarised Cl– will bring the cell back to a resting state
Terminating the PSP
• reuptake – rapid removal of neurotransmitter from the synaptic cleft
• SSRIs (selective seratonin reuptake inhibitors – e.g, Prozac) prolong the PSP by inhibiting reuptake
Summation of post-synaptic potentials.• whether the PSP leads to the excitation or inhibition of the
neuron depends on the combined effects of many PSPs
Neural integrationSpatial integration: equal excitatory and inhibitory input will cause no change
Neural integrationTemporal integration: ripples can combine to make bigger ripples
Autoreceptors
• autoreceptors respond to neurotransmitters they produce
• regulate synthesis and release of other transmitters
• metabotropic
• usually inhibitory – may control amount of neurotransmitter released
Other types of synapses
• axoaxonic – modulate the neurotransmitters in the presynaptic neuron
• gap junctions – electrical synapses – the synaptic cleft is much smaller – ions pass directly from one neuron to another
Why do you need to know all this?
• different disease processes involve different aspects of the basic electrochemical transmission of neural information
• Parkinson’s Disease – dopamine deficiency
• Multiple Sclerosis – affects the myelin sheath of white matter
• Epilepsy – abnormal electrical stimulation
• Alzheimer’s Disease – neurofibrillary tangles may affect the transport of neurotransmitters
Review Questions
1 ) NeuromodulatorsA) are rarely of a peptide form.B) are secreted from a neuron and only effect an adjacent neuron.C) are inevitably inhibitory.D) are secreted from neurons, but dispersed widely in the brain.E) are typically secreted in very small amounts compared to neurotransmitters.
2 ) Most ________ are secreted into the extracellular fluid from endocrine glands or tissues.A) neurotransmittersB) neuropeptidesC) modulatorsD) hormonesE) Pheromones
3 ) Large synaptic vesicles are produced in theA) soma.B) dendrites.C) terminal buttons.D) dendritic spines.E) neuroglia.
4 ) Which of the following is true of neurotransmitter function?A) Neurotransmitters diffuse widely in the brain to exert changes in metabolism.B) Neurotransmitters directly alter ion channels using a second-messenger chemical.C) Neurotransmitters are released into the synapse from the cistaerna.D) Neurotransmitters open ion channels in the postsynaptic membrane.E) Neurotransmitters alter ion channel activity for minutes.
5 ) Which of the following will produce an EPSP?A) opening a sodium channelB) closing a sodium channelC) opening a potassium channelD) opening a manganese channelE) closing a calcium channel
Nice review animation
Review Questions
For Next Time
Start reading Chapter 3
Structure of the Nervous System
Movie Behaving Brain