Sensory 2 Flashcards
Types of synaptic potentials
1) Postsynaptic potentials
2) Presynaptic potentials
Types of postsynaptic potentials
1) EPSP
2) IPSP
3) GPSP
Types of presynaptic potentials
1) Presynaptic Inhibition
2) Presynaptic facilitation
What is EPSP?
- A local state of partial depolarization of the postsynaptic membrane.
- Produced as a result of binding of an exitatory neurotransmittrer with its receptors.
Ionic basis of EPSP
- Opening of ligand-gated Na+ (or Ca++) channels.
- Closure of Cl- channels.
EPSP on neuron potential
It makes the inside of the cell membrane less negative, thereby more excitable as it is now nearer to the firing level.
Maximum of EPSP after…
1 - 5 msec.
Types of Summation
1) Time summation
2) Space summatiom
Time Summation
One presynaptic neuron is stimulated repetitively.
Space Summation
Several presynaptic neurons are stimulated simultaneously.
What is IPSP?
- A local state of partial hyperpolarization of the post synaptic membrane.
- Produced as a result of binding of an inhibitory neurotansmitter with its receptors.
Ionic basis of IPSP
- Opening of ligand-gated Cl- channels.
- Closure of Na+ (or Ca++) channels.
IPSP on neuron potential
It makes the inside of the cell membrane more negative, thereby less excitable as it is now away from the firing level.
Maximum of IPSP after…
1 - 2 msec.
What is GPSP ?
It is the summation of all EPSPs & IPSPs occurring at the same time.
Outcomes of GPSP
- A balance of excitatory & inhibitory inputs, so the PSMP remains close to the RMP.
- If Excitatory inputs slightly > Inhibitory inputs, Excitatory state (facilitation).
- If Excitatory inputs much > Inhibitory inputs, threshold is reached an AP will be generated.
- If Inhibitory inputs > Excitatory inputs, Inhibitory state (inhibition).
Postsynaptic Potentails
- Do not obey all or none law.
- Graded.
- No absolute refractory period.
- Summated.
- Not blocked by anesthesia.
- Not propagated.
- 20 msec.
Action Potentials
- Obey all or none law.
- Can not be graded.
- Absolute refractory period.
- Can not be summated.
- Blocked by anesthesia.
- Propagated.
- 1 msec.
Lowest threshold potentail is at…
Axon Hillox
Presynaptic Inhibition
- 3rd Inhibitory neuron.
- Axo-axonic synapse.
- Excitatory Presynaptic neuron.
- Inhibitory Neurotransmitter e.g. GABA.
- Ionic basis: Openning of Cl- or K+ channels.
- Hyperpolarization ==> reduced Ca++ influx ==> reduced release of neurotransmitter ==> reduced transmission.
Presynaptic Facilitation
- 3rd Excitatory neuron.
- Axo-axonic synapse.
- Excitatory Presynaptic neuron.
- Excitatory Neurotransmitter e.g. 5-HT.
- Increases cAMPin presynaptic terminals ==> Closure of K+ channels
- Prolongated Depolarization ==> more Ca++ influx ==> more release of neurotransmitter ==> more transmission.
Characters of Synaptic transmission
1) Unidirectional
2) Synaptic delay
3) Fatigue
4) Synaptic plasticity
Unidirectionality
One way only.
From Presynaptic to Postsynaptic neuron.
Synaptic Delay
The time taken by an impulse to be conducted through the synapse.
The Synaptic Delay represents …
1) The release of the chemical transmitter.
2) Its union with the receptors.
3) Opening of ionic channels.
4) Efflux of ions and building up of the postsynaptic potential to threshold value by summation.
Usage of Synaptic Delay
Calculation of Number of synapses in any reflex arc.
Fatigue
Decreased rate of impulses discharged from POSTsynaptic neuron after repetitive stimulation of PREsynaptic neuron.
Causes of Fatigue state
1) Exhaustion of synaptic vesicles in the presynaptic terminals by repetitive stimulation.
2) Inactivation of postsynaptic receptors.
Importance of Synaptic Fatigue
It stablizes the nervous system e.g. stopping discharge from overexcited areas in CNS during epileptic seizures.
Synaptic Plasticity
The ability to change or remodulate the function according to the demand placed on the synapse. So the synaptic transmission can be strengthened or weakened for short or long duration based on past experience.
These changes can be Presynaptic or Postsynaptic.
Importance of Synaptic Plasticity
They represent forms of learning and memory.
Forms of Synaptic Plasticity
1) Post-tetanic Potentiation.
2) Habituation.
3) Sensitization.
4) Long-term Potentiation.
5) Long-term Depression.
Post-tetanic Potentiation (PTP) Definition
- Continuous discharge from POSTsynaptic neuron after application of BRIEF TETANIZING train of impulses to a PREsynaptic neuron.
- It may last for few seconds to minutes.
Ionic basis of PTP
Accumulation of Ca++ in the presynaptic neuron after repetitve stimulation ==> Continuous release of neuritransmitter & Enhanced EPSPs until Ca++ pump can remove it.
Importance of PTP
Basic mechanism of IMMEDIATE memory.
Habituation Definition
Gradual loss of response to REPEATED BENIGN or NEUTRAL stimuli.
Ionic basis of Habituation
Gradual inactivation of Ca++ channels in the PREsynaptic terminals ==> decreased intracellular Ca++ ==> decreased release of neurotransmitter ==> transmission becomes weaker on repeated stimulation.
Importance of Habituation
Adaptation to benign stimuli e.g. noises.
Sensitization Definition
Augmented response of Postsynaptic neuron to mild stimuli after habituated stimulus is paired with a noxious stimulus.
Ionic basis of Sensitization
Presynaptic Facilitation:
A noxious stimulus activates a 3rd excitatory neuron secreting Serotonin ==> cAMP ==> Closure of K + channels ==> prolonged postsynaptic depolarization ==> prolonged Ca++ influx ==> excess neurotransmitter release ==> enhanced synaptic transmission.
Importance of Sensitization
1) The basis of SHORT-term memory.
2) Long-term memory ++ protein synthesis & growth of pre & post synaptic neurons and their connections.
Long-Term Potentiation (LTP)
- Persistent strengthening of the synaptic connections after repetitve stimulation.
- Resembles PTP but much more prolonged and can last for days or weeks.
- Various parts but specifically HIPPOCAMPUS.
Ionic basis of LTP
Glutamate stimulates NMDA & AMPA receptors ==> Depolarization of the postsynaptic neuron ==> Relieves the Mg++ block in the NMDA receptors ==> Ca++ & Na+ influx ==> Insertion of additional AMPA receptors into the postsynaptic membrane ==> the postsynaptic neuron is more likely to fire in response to presynaptic stimulation.
Long-Term Depression (LTD)
- Persistent weakening of the synaptic connections after its stimulation.
- The opposite of LTP but in the same fibers particularly in the hippocampus.
Ionic basis of LTD
CA++ ions that enter the through NMDA receptors initiate a different signaling cascade ==> removal of the AMPA receptors ==> decreasing the synaptic response.
