5.1: synapses

summary

  • the connections between nerve cells or nerve and muscle cells are known as synapses, of which there are two kinds: electrical and chemical

  • chemical synapses make use of NT and receptors to carry a signal from one cell to another

  • excitatory synapses

    • involve ligand gated cation channels, that open upon the binding of NT (e.g. glutamate or ach)

    • to raise the memb potential in the postsynaptic cell

  • inhibitory synapses

    • involve ligand gated anion channels, that open upon the binding of NT (e.g. glycine or GABA)

    • to lower the membrane potential iin the postsynaptic cell

synaptic transmission

synapse: connection between neurons or between neuron and muscle cell

“war of soups and sparks”: are messages passed between neurons by an electrical or chemical signal?

  • the revenge of electrical transmission

  • mostly chemical

  • but can be electrical

electrical synapse

two cell membranes that "stick" together

  • less common

  • direct current flow from one cell to another through gap junction channels (connexins)

  • fast

  • allow bidirectional transfer (goes both ways)

  • allow synchonicity

  • found commonly in the heart


what will happen to the membrane potential of a cell (initially at rest) if we open non-selective ion channels (lets any ion go through) ?

  • at rest -70

  • the membrane potential will go up, shift towards 0mv

  • not much K or Cl will move

  • like opening a Na channel, Na influx

chemical synapse

  • most synapses

  • rely on neurotransmitter release and receptor activation

  • postsynaptic receptor can gate an ion channel directly or indirectly

how to make chemical synapse quicker (speed up the process)?

  • make less, e.g. only two chemical synapses from brain to spine to limbs

  • long axons

anatomy of a synapse

recycling of vesicles and neurotransmitter

how many different kinds of protein are required for the chemical synapse to function? proteins at the synapse

  • channels

    • Ca: Cav

    • Na: Nav

    • K: Kv

    • ligand gated channel: AchR

  • vesicles

    • fusion proteins

    • budding proteins. e.g. clathrin

  • transporters

    • NaK pump

    • Ca pump/secondary active transporter

    • neutrotransmitter transporters in: plasma memb, vesicle

  • enzymes

    • acetyl choline esterase

    • reform avetyl choline

botox: botulinum toxin (“sausage poison”)

  • the vesicle won’t release neurotransmitters and not fuse

  • unable to contract nerves and muscles

  • lasts about 3 months

  • causes non funcitonal fusion proteins


how’s it used cosmetically?

  • very low, non fatal dose

  • stops muscle contracting locally

excitatory synapse

  • neurotransmitter release is quantised

  • cation channels

  • Na flows in post synaptic cell

  • depolarises post synaptic cell

  • ACh

    • main excitatory NT at neuromuscular junction w skeletal muscle

  • amino acid NT

    • main excitatory NT at nerve

    • nerve synapse

inhibitory synapse

  • anion selective ion channels

  • hyperpolarise post synaptic cell

  • Cl flows in