Intro to Physiology 7: Synaptic Transmission

what are synapses

• junction between a neuron and another cell

  • electrical activity or presynaptic neuron influences post synaptic neuron

what kind of synaptic integration is this

• convergence

  • inputs from many cells affects a single post synaptic cell

what kind of synaptic integration is this

• divergence

  • one cell affects multiple pathways

what are electrical synapses/ gap junctions

• presynaptic and postsynaptic plasma membranes joined by gap junctions for fast communication

what are connexons

• the protein channels of junctions

what are the functions of gap junctions

• allow small molecules to move directly from one cell to another

  • ions and secondary messengers

where can gap junctions be found and why

• cardiac and smooth muscle

  • synchronization of electrical activity in mass

what is a functional electrical syncytium

• network of separate cells that behave electromagnetically as a single, coordinated unit

what are some things that can close gap junctions

• voltage

• ion concentration

• pH changes

faults in gap junctions in cardiac muscle can result in…

• dysrhythmia

What are synaptic clefts

• the space between pre- and post-synaptic cells

  • no direct contact

how do chemical synapses release message

• depolarization of presynaptic axonal terminal releases chemical neurotransmitter

  • binds to and activate receptors on post-synaptic cell membrane

    • opens/closes ligand-gated ion channels leading to membrane potential change in postsynaptic neuron

what kind of receptor is this

• ionotropic receptor

  • when neurotransmitter binds, ion channel opens up

  • immediate influx of ions

what kind of receptor is this

• metabotropic receptors

  • G-protein coupled receptor activates interneural signal transduction cascade leading to cationic channels opening and depolarization

What is the biggest difference between ionotropic and metabotropic receptors

• ionotropic

  • direct

  • fast

• metabotropic

  • indirect

  • not fast

where are neurotransmitters stored

• stored in special compartments called synaptic vesicles

  • in presynaptic axon terminal

what are the steps in chemical synaptic transmission

• arrival of action potential depolarizes membrane potential

• depolarization opens voltage gated Ca+ channels in presynaptic membrane

• Ca+ moves INTO the axon terminal of presynaptic neuron

• releases of neurotransmitter by exocytosis into synaptic cleft

  • vesicle and cell membrane fuse together via SNARE proteins

• neurotransmitter release and diffuse

• neurotransmitter binds to post-synaptic receptor

• removal of neurotransmitter

how do neurotransmitters get removed

• reuptake

• degradation

• diffusion

what do SNARE proteins need?

need calcium influx

• calcium binds to synaptotagmin

describe how tetanus toxin impacts neurons

• toxin chews up SNARE complex in inhibitory neurons for motor neurons

  • more muscle contraction

describe how botulism toxin impacts neurons

• toxin attacks pre-synaptic SNARE in excitatory neurons

  • muscle weakness

describe excitatory postsynaptic potentials

• local depolarization caused by opening Na+ channels

  • brings membrane potential closer to threshold

  • more positive

describe inhibitory postsynaptic potentials

• local hyperpolarization caused by opening Cl- or K+ channels in postsynaptic membrane

  • more negative

what is synaptic integration

• the summed membrane potential changes of all excitatory and inhibitory synaptic inputs

  • needed to reach threshold potential for AP

(in most cells, the amplitude of postsynaptic potentials are too small to cause anything on their own)

what is temporal summation

• time

• occurs when input signal from the same presynaptic cell is repeatedly rapidly

what is spatial summation

• location

• occurs when input signal is coming from two or more postsynaptic potentials spreading from different locations

what are trigger zones in neurons

• regions where the threshhold for action potential firing is lowest

  • high density of Na+ channels

where are some possible locations of trigger zones

• axon hillock

• first node of ranvier near the axon terminal of sensory afferent neurons

what is synaptic strength

• the effectiveness of a given synapse on the membrane potential of the post-synaptic neuron

what is synaptic strength determined by in postsynaptic cells

• frequency of the action potential of the postsynaptic neuron

what is synaptic strength determined by in presynaptic cells

• axo-axonal synapses

  • regulate the amount of transmitter released

what is presynaptic facilitation/inhbition

• excitatory/inhibitory axo-axonic synapses cause presynaptic axons to release more or less neurotransmitter

what will happen if there is a presynaptic facilitation of an excitatory synapse?

• more release of excitatory transmitters

  • postsynaptic cell more likely to meet threshold

what will happen if there is a presynaptic facilitation of an inhibitory synapse

• more release of an inhibitory transmitter

  • postsynaptic cell less likely to meet treshold

what will happen if there is a presynaptic inhibition of an inhibitory synapse

• less release of an inhibitory transmitter

  • postsynaptic cell more likely to meet treshold

how do autoreceptors impact synaptic strength

• presynaptic receptors that respond to the neurotransmitters released into the terminal

  • use negative feedback

    • reduces the amount of transmitter that is released from the trial

what are the three regulatory factors of presynaptic strength

• availability of neurotransmitter

• auto receptor

• presynaptic facilitation/inhibition

what are the three regulatory factors of postynaptic strength

• types and number of available receptors

• temporal and spatial receptors