Synaptic Potentials Lecture Notes PDF

# Synaptic Potentials ## A) Post Synaptic Potentials ### 1) Excitatory Post Synaptic Potential (EPSP): - Partial depolarization of the post synaptic membrane - Reaches its maximum after 1 - 1.5 ms. - Lasts for (its duration) 2 - 5 ms. - **Ionic Basis:** Excitatory transmitter binds to its receptors...

# Synaptic Potentials ## A) Post Synaptic Potentials ### 1) Excitatory Post Synaptic Potential (EPSP): - Partial depolarization of the post synaptic membrane - Reaches its maximum after 1 - 1.5 ms. - Lasts for (its duration) 2 - 5 ms. - **Ionic Basis:** Excitatory transmitter binds to its receptors, opening of ligand gated Na+ (or Ca++) channels, influx of Na+ (or Ca++) small depolarization making the membrane nearer to the firing level (facilitated). - To reach the threshold value, EPSP must be summated either by: #### a. Temporal (time) summation: The presynaptic neuron is stimulated repetitively multiple, successive and very close in time EPSPs (up to 50 action ### 2) Inhibitory Post Synaptic Potential (IPSP): - Partial hyperpolarization of the post synaptic membrane - Reaches its maximum after 1.5 - 2 ms. - Lasts for (its duration) 3 ms. - **Ionic Basis:** Inhibitory transmitter binds to its receptors, opening of ligand gated Cl (or K+) channels, influx of Cl (or efflux of K+), small hyperpolarization making the membrane away from the firing level. - IPSP can be also produced by closure of Na+ or Ca++ channels. - IPSP is a local state and can be summated by temporal and spatial summation as in case of EPSP. ### 3) Grand Post Synaptic Potential (GPSP): - It is the sum of all EPSPs and IPSPs occurring at the same time in one post synaptic neuron. - There are 4 possible outcomes: - A balance between facilitation and inhibition post synaptic potential is not affected. - Facilitation: If EPSP > IPSP (the post synaptic membrane does not reach the firing level). - Action potential: If EPSP >>IPSP (the post synaptic membrane reaches the firing level). - Inhibition (hyperpolarization): If IPSP > EPSP. ## B) Presynaptic Potentials ### Third neuron acting on the presynaptic neuron ### 1) Presynaptic Inhibition: - The third neuron is an inhibitory neuron which releases an inhibitory transmitter leading to: - Closure of Ca++ channels in the presynaptic neuron or - Opening of K+ or Cl channels. - The result is reduced Ca++ entry in the presynaptic knob and - release of the ch. transmitter. - Presynaptic inhibition develops slowly & lasts for minutes to hours. This differs from post synaptic ### 2) Presynaptic Facilitation: - The third neuron is an excitatory neuron which releases an excitatory transmitter (serotonin). - Serotonin ++ cAMP in the presynaptic knob - phosphorylates a protein in the K+ channels and closes them, prevent repolarization (prolong depolarization). - Prolonged depolarization keeps Ca++ channels openec ++ release of the transmitter # Characters of Synaptic Transmission 1) **Forward Direction (unidirectional):** Impulses are conducted from pre to post synaptic neuron (in one way). 2) **Synaptic Delay:** The time taken by an impulse to be conducted through the synapse (0.5 m.sec.). - The number of synapses in a reflex arc = central delay / 0.5 - Number of interneurones = No. of synapses - 1 3) **Fatigue:** Decreased rate of discharge of impulses from the post synaptic neuron due to repetitive stimulation. - **Causes:** - Exhaustion of synaptic vesicles in the pre synaptic terminals due to repeated stimulation. - Inactivation of post synaptic receptors 4) **Synaptic Plasticity:** The ability to change the function of synapse according to the demand (the synaptic transmission can be ++ or - for short or long duration by repeated stimulation). ## Forms of Synaptic Plasticity ### a. Short Term Inhibition (Habituation): - Gradual loss of response to a benign stimulus when it is repeated for several times at intervals. - **Mechanism:** Gradual inactivation of Ca++ channels -- intracellular Ca++ -- release of neurotransmitter ### b. Short Term Facilitation #### Ⅰ. Post titanic potentiation (facilitation): - Brief tetanizing stimuli to a pre synaptic neuron -- continuous discharge from the post synaptic neuron (few seconds to minutes). - **Ionic Basis:** Accumulation of Ca++ in the pre synaptic neuron (due to weakness of Ca++ pump) -- continuous release of ch. transmitter. - The mechanism of immediate memory. #### II. Sensitization: - Prolonged augmented response of the post synaptic neuron due to application of a noxious stimulus accompanying the benign stimulus. - **Ionic Basis:** Pre synaptic facilitation. ## Action Potential vs Post Synaptic Potential Table | Feature | Action Potential | Post Synaptic Potential | |-----------------------|----------------------------|-----------------------------| | Obey all or non law | Yes | No | | Can not be graded | Yes | No | | Can not be summated | Yes | No | | Propagated | Yes | No | | Absolute Refractory Period | Yes | No | | Blocked by anaesthesia | Yes | No | | Duration | 1 m.sec. | 20 m.sec. |