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Synaptic transmission - Coggle Diagram

- - - - opening of voltage gated Ca channels and Ca influx according to electrochemical gradient
        
        Migration of synaptic vesicles to active zones
    - - triggers binding of snares
        
        v snare (synaptobrevin) on membrane of synaptic vesicles
        
        T snare (syntaxin) on membrane of presynaptic neuron
    - - between 2 snare proteins
        
        exocytosis of neurotransmitters
  - - - Less -ve potential
        
        Excitatory post-synaptic potential
      - More -ve potential
        
        Inhibitory post synaptic potential
- - - - decreased rate of impulses discharged in postsynaptic neuron after repetitive stimulation of presynaptic neuron
    - - Exhaustion of synaptic vesicles in presynaptic neuron
      - inactivation of postsynaptic receptors
    - - Prevents overexcitation in CNS such as during epileptic fits
  - - - Short-term plasticity
        
        Short term inhibition (habituation)
        
        Definition
        
        gradual loss of response due to repeated benign stimulus at intervals
        
        Ionic basis
        
        Inactivation of Ca channels leading to decreased release of transmitter
        
        Importance
        
        Adaptation to benign (unimportant) stimulus (noises)
        
        May reappear if stimulus is stopped and reapplied again
        
        Post tetanic potentiation
        
        Definition
        
        continuous response for a few seconds after stoppage of brief rapid tetanizing stimuli
        
        Ionic Basis
        
        Ca channels opened for a longer period => +++Ca in presynaptic terminal
        
        Continuous release of neurotransmitter
        
        Importance
        
        basic mechanism for immediate memory
        
        Sensitization
        
        Ionic basis
        
        Presynaptic Facilitation
        
        Third neuron releases serotonin ---> increased cAMP --> closure of K channels
        
        prolonged depolarization --> Ca channels opened for a prolonged period of time
        
        Exocytosis of transmitter for a few mins after stoppage of stimulus
        
        Importance
        
        Basic mechanism for short term memory
        
        If repeated, protein synthesis occurs ---> long term memory
        
        Definition
        
        Prolonged augmented response due to application of noxious stimuli coupled with a habituated stimulus to a presynaptic neuron
      - Long-term plasticity
        
        Long term potentiation
        
        Definition
        
        Rapidly developing persisting response due to rapid stimulation of presynaptic neuron in the hippocampus
        
        Ionic basis
        
        Presynaptic neuron releases glutamate
        
        Opening Na channels in AMPA receptors --> depolarization
        
        Depolarization removes the Mg block on NMDA receptors --> Ca influx through NMDA
        
        Ca-Calmodulin complex activates kinase 2
        
        Phosphorylation of AMPA receptors --> more Na entry
        
        +++AMPA receptors and their movement to the postsynaptic membrane
        
        Release of NO from postsynaptic neuron --> release of glutamate from presynaptic neuron
        
        New protein synthesis --> growth of pre and post synaptic neurons and their connections --> new collaterals
        
        Importance
        
        Long term memory
        
        Specific in hippocampus
        
        Long term depression
        
        Defintion
        
        Prolonged depression of the post synaptic neuron after slow stimulation of the presynaptic neuron
        
        Ionic Basis
        
        slow stimulation leads to slow increase of Ca levels
        
        activation of different pathway --> Calcineurin phosphatase 1
        
        Removal of AMPA receptors from the membrane --> decreased response
        
        Importance
        
        Present in same fibers as the Long term potentiation (hippocampus)
- - - - why?
        
        Most abdundant channels
- - - - formation of ATP for
        
        synthesis
        
        exocytosis
        
        of neurotransmitters
        
        active reuptake
    - - Small vessicles
        
        Granular
        
        catecholamines
        
        Clear
        
        rapid transmitters
        
        GABA
        
        Glutamate
        
        Glycine
        
        Acetylcholine
      - Large, granular vessicles
        
        Slow transmitters
  - - - Rich in Na and Cl
      - Poor in K+
  - - - Bind with released neurotransmitter
      - Ligand-gated channels for NA, K+, Cl-.....
    - - Serpentine, G-ptn coupled receptor
      - +++ Ca, IP3, DAG and ---/+++ cAMP
      - Open specific ion channels
      - Activation of intracellular enzymes
      - Activation of gene transcription --> formation of neuproteins --> metabolic/structural change
- - - - Alkalosis
        
        ++excitability and transmission => convulsions (liver failure, epilepsy, and hyperventilation)
      - Acidosis
        
        --excitability and transmission => coma (diabetic coma)
    - - hypoxia leads to ++acids (anaerobic metabolism) -->--transmission
      - Interuption of cerebral blood flow for 3-5 secs causes unconsciousness
      - Prolonged ischemia (minutes) as in cerebral thrombosis leads to brain damage
    - - glucose is the preferable fuel of the brain
    - - ++excitability (guards of Na channels not present) ---> tetanic convulsions
  - - - Caffeine, theobromine, theophylline
        
        depolarize postsynaptic membrane
      - Strychnine
        
        blocking the action of inhibitory neurotransmitter
    - - analgesics and anastheitcs
        
        Hyperpolarization
        
        --synthesis, release, or reuptake of neurotransmitter
  - - - prevents release of GABA (inhibitory neurotransmitter)
      - characterized by muscle spasm, lock jaw, and asphyxia
    - - block ACh release in neuromuscular junction
      - leads to flaccid paralysis
      - Medical uses
        
        muscle spasms of lower esophageal sphincter (achalsia)
        
        remove wrinkles (facial muscles)
- - - - Definition
        
        Local state of partial depolarization of post-synaptic neuron
      - Duration
        
        reaches maximum after 10-15 msec
      - EPSP must be summated (40-50 times) to reach threshold value
        
        Spatial summation
        
        summation of simultaneous EPSPs from multiple presynaptic neurons
        
        Up to 40 knobs needed for AP
        
        Temporal summation
        
        summation of multiple successive very close EPSP originating from a single presynaptic neuron
        
        Amount of released neurotransmitter and magnitude of the EPSP is proportional to frequency of presynaptic impulses
        
        50 EPSPs needed for AP
      - Ionic Basis
        
        release of excitatory neurotransmitter and binding to its receptors on postsynaptic membrane
        
        Opening of ligand-gated Na or Ca channels or Closing of Cl channels
        
        Influx of positive ions due to electrochemical gradient
        
        leading to partial depolarization ---> nearer to firing level (membrane is said to be facilitated ie: weaker stimulus is needed than at rest)
    - - Definition
        
        local state of partial hyperpolarization of the postsynaptic neuron
      - Duration
        
        Reaches maximum after 15-20 msec
      - Ionic Basis
        
        Release of inhibitory neurotransmitter and binding to its receptor
        
        opening of ligand-gated Cl or K channels
        
        Influx of Cl or efflux of K according to electrochemical gradient leading to hyperpolarization
        
        NOTEs
        
        IPSP can be produced by closure of Na and Ca channels too
        
        IPSP is a local state that can be summated by temporal and spatial summation as well
    - - definition
        
        Sum of all ISPS and ESPS in one neuron at one time
      - possible outcomes
        
        EPSP=IPSP
        
        postsynaptic membrane potential close to firing level
        
        EPSP>IPSP
        
        facilitated if threshold not reached
        
        may reach threshold and AP is produced
        
        IPٍSP>EPSP
        
        hyperpolarized
      - Micturition response
        
        Sensory signals from UB receptors
        
        Faciliatory and inhibitory signals from the brain stem
        
        Voluntary control from the cerebral cortex
  - - - A third inhibitory neuron synapses with presynaptic neuron axoaxonically (axon terminal to axon terminal)
      - Release of GABA leading to opening of K and Cl channels ---> K efflux and Cl influx
      - Hyperpolarization of axon terminal ---> closure of voltage gated Ca channels
      - decreased calcium levels lead to decreased neurotransmitter released and inhibition of transmission
      - NOTE: presynaptic inhibition develops slowly and lasts mins to hours while postsynaptic inhibition develops rapidly and lasts for a few seconds
    - - A third excitatory neuron synapses with presynaptic neuron axoaxonically (axon terminal to axon terminal)
      - release of serotonin --> ++cAMP in presynaptic terminal
      - Phosphorylation and closing of K channels ---> prolonged depolarization
      - Ca channels opened for a longer time --> ++Ca ==> exocytosis