Nervous 3

• Synapses are intercellular junctions (gap between neurons). Presynaptic cell transmits AP and postnynaptic cell receives it
• Three structures make up a chemical synapse

  • synaptic knob
  • synaptic cleft
  • plasma membrane of a postsynaptic neuron
• Chemical synapses have a chemical cleft between two cells. The synaptic knob contains synaptic vesicles with neurotransmitters
• Steps in the transmission of AP

  1. When the AP arrives, the voltage gated Ca 2+ channels open
  2. Calcium enters the synaptic knob
  3. Release of neurotransmitters by exocytosis (vesicles) into synaptic cleft
  4. Neurotransmitters float across and bind to receptors
  5. AP in the postsynaptic neuron is initiated
• After synaptic transmission, the signal must be stopped by removing neurotransmitters from the synaptic cleft:

  • immediately transported back into the presynaptic neuron (reuptake process)
  • enzyme degradation (breakdown)
  • diffuse out of cleft and into glial cells
• Neurotransmitters can be classified into 2 groups depending on type of neurotransmitter and type of postsynaptic cell

  • excitatory: depolarization and new AP
  • inhibitory: hyperpolarization (harder to generate new AP)
• There are around 100 known or suspected neurotransmitters
• Acetylcholine (Ach): standard excitatory neurotransmitter of skeletal muscle
• Glutomate: major excitatory neurotransmitter in the vertebrate CNS
• GABA: important inhibitory neurotransmitter
• Dopamine: neurotransmitter involved in emotional responses, addictive behaviors and pleasurable experiences
• Serotonin: neurotransmitter involved in sensory perception, temperature regulation, control of mood and sleep
• Epinephrine (adrenaline) and norepinephrine are responsible for the fight or flight response
• ACh steps

  1. Crosses the synapse between a motor neuron and a muscle fiber
  2. Binds to a receptor in the postsynaptic membrane
  3. Stimulates muscle contraction
• AChE (acetylcholinesterase) degrades ACh and causes muscle relaxation
• Valium: Anti-anxiety drug that enhances the action of GABA
• Prozac: anti-depression medication that acts as serotonin reuptake inhibitor (keep serotonin in cleft longer)
• Cocaine: produces euphoria and pleasure by blocking reuptake of dopamine
• MDMA (ecstacy): causes release of serotonin, dopamine and norepinephrine
• Habituation: prolonged exposure to a stimulus may cause cells to lose the ability to respond to it. The cell decreases the number of receptors because there is an abundance of neurotransmitters
• Drug addiction to cocaine

  • binds dopamine transporters and prevents the reuptake of dopamine
  • dopamine survives longer in the synapse and fires pleasure pathways more and more
  • prolonged exposure triggers the limbic system neurons to reduce receptor numbers
  • cocaine user is now addicted
• AP can be blocked by inhibitory neurotransmitters but also by external factors
• Neurotoxins: natural or synthetic compounds that interact with ion channels. they are NOT wanted in the body
• Tetrodotoxin (neurotoxin) from puffer fish: blocks Na+ voltage gated channels = no depolarization. This results in paralysis or death
• Dendrotoxin (neurotoxin) from black mamba snake: blocks voltage-gated K+ channels = no repolarization. This results in paralysis and death