Unit 2: Chemical Signaling in Nervous System

• axodendritic synapses are formed between a presynaptic axon and a postsynaptic dendrite
• axosomatic synapses form between an presynaptic axon and the soma of a cell
• axoaxonic synapses form between two axons.
• two types of CNS synaptic membrane:

  • gray’s type I: asymmetrical, usually excitatory
  • gray’s type II: symmetrical, usually inhibitory
• communication between neurons is both electrical and chemical
• gap junction

  • electrical communication occurs via gap junctions: cell-to-cell channels that allow for the direct exchange of ions and other solutes.
  • in vertebrates, these are made up of proteins called connexins.
  • invertebrates form similar structures with completely unrelated proteins called innexins.
• loewi's experiment:

  • Loewi removed two frog hearts, one with the parasympathetic vagus nerve still attached.
  • He stimulated the vagus nerve for fifteen minutes, causing the first heart to slow.
  • removed some of the fluid from the first heart and applied it to the second.
  • caused the second heart to beat more slowly.
  • concluded that the vagus nerve released a chemical (vagusstoff) that could be transferred from one heart to the other.
• vagusstoff:

  • Vagusstoff turned out to be the neurotransmitter acetylcholine (ACh).
  • ACh has a positive charge.
  • rapidly broken down after release by an enzyme called acetylcholinesterase (AChE).
• neuromuscular junction:

  • ACh is the neurotransmitter released by motor neurons that binds to receptors in muscle cells, causing them to depolarize and contract.
  • This special synapse is called the neuromuscular junction (NMJ).
  • The NMJ is big, easy to access (it’s peripheral), and fail-safe (every nerve stimulation causes a contraction).
• acetylcholine at NMJ:

  • Stimulation of motor neurons caused the appearance of acetylcholine in the perfused fluid.
  • No acetylcholine when muscle is denervated.
  • When postsynaptic response was blocked, acetylcholine was still released.
  • When conduction failed after repeated stimulations, acetylcholine was no longer released.
• Reciprocal inhibition: contraction of one muscle set accompanied by relaxation of antagonist muscle
• how inhibitory reflexes work:

  • contraction of the bicep causes the tricep to go flaccid.
  • a sensory neuron detects contraction in the bicep and sends a signal back to the spinal cord.
  • this signal excites the motor neuron that stimulates the bicep (a feed-forward loop).
  • sensory neuron also stimulates an inhibitory interneuron.
  • interneuron inhibits firing of the motor neuron that innervates the tricep muscle.
• curare:

  • curare binds to the Ach receptors at the NMJ, blocking transmission.
  • Fatt and Katz used curare to block some of their ACh receptors.
  • allowed for them to study the response of the unblocked ACh receptors without worrying about action potentials firing (when Na+ and K+ channels would make up the bulk of the signal) and muscle contraction disrupting their recordings.
  • by blocking part of the muscles response to ACh, they keep the muscle from going above the threshold voltage for an action potential.
• end-plate potentials:

  • discovered by fatt and katz
  • with some curare around, they didn’t get action potentials.
  • measured only the depolarization produced by the opening of ACh receptors. These are excitatory post synaptic potentials (EPSPs).
  • called end-plate potentials because they measured them from the motor endplate (the postsynaptic side of the NMJ).
• mini end-plate potentials:

  • if presynaptic neurons weren't stimulated, fatt and katz noticed small depolarizations (mini end plate potentials)
  • concluded that ACh was released in small packets (quanta) from presynaptic terminal rather than spilling out
  • quanta are like synaptic vesicles
• chemical synapses:

  • lots of vesicles containing neurotransmitter and some bigger dense-core vesicles that contain peptide neurotransmitters.
  • presynaptic side contains active zones where vesicles are docked for release
  • there are lots of mitochondria at the presynaptic terminal.
  • produce the ATP needed to load vesicles, power vesicle fusion, and restore the ionic gradients after an action potential (Na/K ATPase).
• chemical synapses:

  • Actin is present on the postsynaptic side and is important for remodeling the postsynaptic cell (a component of plasticity).
  • Microtubules on the presynaptic side act as highways for proteins and other materials transported down the axon from the cell body.
  • receptors for the neurotransmitter glutamate are visible on the post synaptic side.
• Ca+ role:

  • extracellular Ca2+ is required for neurotransmitter release.
  • in an experiment, motor neuron stimulated and measured the resulting depolarization of the muscle.
  • response of the muscle indicates that ACh is released from the motor during stimulation.
  • muscle only depolarized when the neuron was stimulated in the presence of Ca2+.
  • if Ca2+ wasn’t present during stimulation, there was no transmitter release.
• Ca2+ current mostly comes in during the downstroke and undershoot of the action potential.
• latrotoxin:

  • Latrotoxin is from the black widow spider.
  • causes the presynaptic terminal to dump all of its vesicles.
  • because the toxin inserts itself into the presynaptic membrane and creates a Ca2+-permeable pore.
• release of neurotransmitter by exocytosis:

  • Synaptic vesicles dock with the presynaptic membrane using proteins called SNAREs.
  • There are vesicle (V) SNARES and target (T) SNAREs.
  • Once the vesicles dock, they can fuse with the presynaptic plasma membrane upon Ca2+ entry.
  • After fusion, the vesicle is retrieved and recycled from the presynaptic membrane.
• • synaptobrevin (VAMP) is the V-SNARE.
  • SNAP-25 and syntaxin are T-SNAREs.
  • synaptotagmin is the Ca2+ sensor that triggers fusion.
• • Botulinum toxins cleave several parts of the SNARE complex (synaptobrevin, SNAP-25, and syntaxin).
  • Tetanus toxin cleaves synaptobrevin.
  • Both prevent vesicles from being released.
  • Botulinum toxin produces paralysis because it prevents transmission at the NMJ.
  • Tetanus toxin produces uncontrolled muscle contraction because it prevents transmission from neurons in the spinal cord that inhibit motor neurons.
  • Overactivity in motor neurons due to loss of inhibition causes muscle contraction.
• tetanus:

  • Muscles contract uncontrollably.
  • Tetanus toxin has a similar mechanism of action to botulinum toxin
• synaptic vesicle recycling:

  • Once vesicles fuse with the presynaptic membrane, they must be retrieved and recycled.
  • majority of recycling happens via clathrin-coated pits.
  • clathrin forms a cage around the new vesicle as it buds off the presynaptic terminal, stabilizing it.
  • once the vesicle buds off, it loses its clathrin cage an is ready to be reloaded with transmitter and released.
• representative neurotransmitters:

  • Glutamate (the major excitatory neurotransmitter in the central nervous system)
  • GABA, and glycine (both of which are inhibitory transmitters)
  • Norepinephrine is an example of a monoamine neurotransmitter (along with epinephrine, dopamine, serotonin).
  • Acetylcholine is sort of in a category all its own.
  • several neurotransmitters are small proteins called peptides.
  • Molecules like ATP and histamine are also released as neurotransmitters.
• neurotransmitter synthesis and storage:

  • neurotransmitters are synthesized at the presynaptic terminal.
  • specialized enzymes are involved in making neurotransmitters.
  • Peptide neurotransmitters are short proteins.
  • DNA encoding the peptides resides in the nucleus. An mRNA copy of the peptide transmitter gene leaves the nucleus.
  • Peptides are synthesized by ribosomes at the rough ER
  • peptides are then transported through the Golgi
  • Eventually, peptide laden vesicles bud off from the Golgi and are transported to presynaptic terminals by motor proteins traveling along microtubules.
• amino acid neurotransmitters:

  • Glycine and glutamate are used to make proteins.
  • GABA is synthesized from glutamate by glutamic acid decarboxylase, which removes a carboxyl group from glutamate.
• neurotransmitters getting into vesicles:

  • Neurotransmitters are loaded into vesicles using transport proteins.
  • Most of these transport proteins use a proton (H+) gradient to concentrate neurotransmitters (pumping a H+ out with every neurotransmitter molecule pumped in).
  • proton gradient is set up by vesicular ATPases that hydrolyze ATP and use that energy to pump protons into the vesicle.
• acetylcholine synthesis and breakdown:

  • Acetylcholine is synthesized from Acetyl CoA and choline by the enzyme choline acetyltransferase.
  • This enzyme is a specific marker for cholinergic neurons.
  • ACh is broken down rapidly in the extracellular synaptic cleft by acetylcholinesterase (AChE) to acetate and choline.
• life cycle of ACh:

  • ACh is synthesized at the presynaptic terminal and loaded into vesicles using proton-dependent transporters.
  • After it is released, it can act on postsynaptic receptors.
  • It is quickly broken down to AChE into choline and acetate.
  • The choline is taken back up into the presynaptic terminal to make more ACh.
• monoamine neurotransmitters:

  • serotonin (synthesized from tryptophan)
  • catecholamines (dopamine, norepinephrine, epinephrine), which are synthesized from the amino acid tyrosine.
• catecholamine synthesis

  
• serotonin synthesis

  
• generating an EPSP (excitatory post-synaptic potential):

  • EPSPs are produced by the opening of excitatory neurotransmitter-gated ion channels.
  • Transmitter binding opens a pore that is permeable to Na+ and K+ (reversal potential around 0 mV). This depolarizes the presynaptic terminal, producing an EPSP.
  • At negative membrane potentials, the major ion movement will be Na+ coming into the cell.
• generating IPSP (inhibitory post-synaptic potential)

  • produced in response to GABA and glycine
  • made by neurotransmitter-gated chloride channels.
  • Opening these channels brings the membrane potential closer to the reversal potential for chloride (around -65 mV in a typical neuron)
• EPSP Summation:

  • unlike NMJ, not every stimulation of a produces a contraction at a synpase
  • presynaptic stimulation might result in the release of a single vesicle at a synapse
  • making a postsynaptic action potential requires more work
• spatial summation: multiple EPSPs added together produce a bigger depolarization when they arrive at the same time
• temporal summation: epsp arrive in rapid succession
• EPSPs decay with distance from site of stimulation