Nervous System Lesson 4: Synapses

• Anatomy of a synapse

  Synaptic vesicles

  Presynaptic membrane

  Synaptic cleft

  Post synaptic membrane

  Neurotransmitter

  Receptors (protein)
• Explain a synapse


  Where neurons communicate with each other:

  - one neuron sends a message to another neuron (target neuron)

  - action potential triggers presynaptic neuron to release neurotransmitters
• Explain neurotransmitters


  Molecules (chemicals) released from the pre synaptic cell that bind to receptors on post synaptic cell that transmit the nerve impulse across the synaptic cleft
• What happens at the region of a synapse?

  {regions} where the end of an exon of one neuron meets/connects with a cell body or dendrite of an adjacent neuron
• What is an axon bulb?

  Enlarged terminal knobs on ends of axon terminal branches
• What is the presynaptic membrane?

  Membrane of axon at synaptic ending

  - where the impulse is sent from
• What is the post synaptic membrane?

  Membrane of dendrite or the other side of the synapse

  - where the impulse is received
• What is the synaptic cleft?

  Space between presynaptic/postsynaptic membranes
• What are synaptic vesicles?


  Transport neurotransmitters (membrane bound sacs)

  - located near the surface of the synaptic endings
• What's the rule for neurotransmitters to fire neurons?

  The neuron will only fire if the amount of excitatory neurotransmitters that binds at post - synaptic membrane is efficient to overcome the amount of inhibitor neurotransmitters that are also binding

  - excitatory has to be enough for inhibitor
• Explain an excitatory neurotransmitter


  Make action potentials happen

  - activates cells, involved in learning, rewards and feeling good

  - ex: glutamate, acetylcholine, noradrenaline/norepinephrine, dopamine
• Explain in inhibitory neurotransmitter: prevent action potentials from happening


  - make sure brain doesn't send signals "too easily"

  - reduce unwanted brain excitability

  - help keep brains overall level of neural activity in check

  -fine tune neural processing, support sleep: inhibit wake - promoting brain regions

  Ex: GABA (gamma aminobutyric acid), glycine, serotonin
• Explain step 1 of synaptic transmission

  Action potential travels along axon and reaches the axon bulb:

  a) Ca flows into bulb through protein channels

  b) contractile proteins (cytoskeleton) are signalled to pull synaptic vesicles to inner surface of presynaptic membrane
• Explain step 2 of synaptic transmission

  Vesicles fuse with presynaptic membrane
• Explain step 3 of synaptic transmission

  Neurotransmitters released into synaptic cleft via exocytosis
• Explain step 4 os synaptic transmission

  Neurotransmitters diffuse across the synaptic cleft to bind to receptors on the post synaptic membrane
• Explain step 5 of synaptic transmission


  Binding of neurotransmitters to specific receptors on post-synaptic membrane (dendrites or cell body) will open the ion (Na, K) channels of that neuron
• What does step 5 result in?

  Results in an ion flux (movement): "all or none response" where voltage of post synaptic membrane changes
• What happens if enough ion channels open (excitatory synapse)?

  The membrane voltage moves closer to the "threshold voltage" required for an action potential
• What happens if not enough channels open (inhibitory system)?

  The membrane is hyper polarized and returns to resting potential
• Is transmission synapse one way?

  Yes. Transmission across a synapse is always one-way since only the ends of axons have synaptic vesicles that are able to release neurotransmitters to affect the potential of subsequent neurons
• Explain neurotransmitter deactivation


  We need to prevent neurotransmitters from continuously action on post synaptic membrane (excitatory = seizure, inhibitory = shuts down impulse)
• Explain the 3 ways of neurotransmitter deactivation

  1. Neurotransmitter absorbed by presynaptic membrane

  - ex: endocytosis of serotonin

  OR

  2. Neurotransmitter is broken down by enzymes

  - ex: acetylcholinesterase breaks down acetylcholine

  OR

  3. Both: in some instances there is a combination reabsorption and breaking down

  - ex: noradrenaline is broken down by monoamine oxidase after it is reabsorbed
• What are glial cells (synaptic support)

  - regulate neurotransmittter clearance from synaptic cleft

  - release gliotransmitters (ATP) which modulate synaptic function