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Physiology of Synapses in the CNS- L3. Faisal I. Mohammed, MD, PhD. Objectives. Students should be able to: Define synapse and list the types of synapse Describe the mechanism of neurotransmitter release List the major types of neurotransmitters (NT)

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physiology of synapses in the cns l3

Physiology of Synapses in the CNS- L3

Faisal I. Mohammed, MD, PhD

University of Jordan

objectives
Objectives

Students should be able to:

  • Define synapse and list the types of synapse
  • Describe the mechanism of neurotransmitter release
  • List the major types of neurotransmitters (NT)
  • Compare the small molecules NT and Neuropeptides
  • Describe the resting membrane potential and Nernst Equation
  • Determine the how EPSP, IPSP and Presynaptic inhibition develops
  • Describe summation of EPSP and IPSP
  • Describe the characteristics of synapse (Fatigue and Delay)

University of Jordan

slide3

Neurotransmitters

University of Jordan

comparison between small molecules and neuropeptides neurotramsmitters nt
Comparison between Small Molecules and Neuropeptides Neurotramsmitters (NT)
  • Small molecules NT are rapidly acting as compared to slowly acting neuropepides
  • Small molecules NT are have short lived action compared to prolonged time of action for neuropeptides
  • Small molecules NT are excreted in larger amounts compared to smaller quantities of neuropeptide
  • Small molecules NT vesicles are recycled but neuropeptide ones are not
  • Neuropeptides are co-secreted with small molecules NT
  • Neuropeptides are synthesized at the soma while small molecules could be formed at the presynaptic terminals

University of Jordan

removal of neurotransmitter
Removal of Neurotransmitter
  • Diffusion
    • move down concentration gradient
  • Enzymatic degradation
    • Acetylcholinesterase for (Ach), peptidases for neuropeptides
  • Uptake by neurons or glia cells
    • neurotransmitter transporters
    • Prozac = serotonin reuptake inhibitor

University of Jordan

electrical events during neuronal excitation
Electrical Events During Neuronal Excitation
  • Resting membrane potential ~ -65 mV
    • Results from the distribution of ions across the neuronal membrane:
    • Na+ gradient from outside to inside
    • K+ gradient from inside to outside
    • Cl- gradient from outside to inside
  • The movement of the ions down the concentration gradients causes neuronal activation.

University of Jordan

nernst potential
Nernst Potential
  • Potential that exactly opposes the movement of an ion across the neuronal membrane.
  • Nernst Equation:
    • Electro Motive Force (EMF) (mV)

EMF (mV)

    • The sign is (-) for a positive ion and (+) for a negative ion.

University of Jordan

slide10

A membrane potential of 61mV

would be required to prevent the

influx of sodium

Dendrite

X

Na+: 142 mEq/L

14 mEq/L

K+ : 4.5 mEq/L

120 mEq/L

Axon

Cl- : 107 mEq/L

8 mEq/L

61mV

University of Jordan

slide11

Dendrite

A membrane potential of -87mV

would be required to prevent the

efflux of potassium

Na+: 142 mEq/L

14 mEq/L

X

K+ : 4.5 mEq/L

120 mEq/L

Axon

Cl- : 107 mEq/L

8 mEq/L

-87mV

University of Jordan

slide12

Dendrite

A membrane potential of -69mV

would be required to prevent

the influx of chloride

Na+: 142 mEq/L

14 mEq/L

K+ : 4.5 mEq/L

120 mEq/L

Axon

X

Cl- : 107 mEq/L

8 mEq/L

-69mV

University of Jordan

postsynaptic potentials
Postsynaptic Potentials
  • The Excitatory Postsynaptic Potential (EPSP)
    • Na+ ions rush to inside of membrane through ionophores opened by transmitter.
    • Rapid influx of positively charged Na+ ions neutralizes part of negativity of the resting membrane potential.
    • The increase in voltage above the normal resting potential (to a less negative value ~ -45mV) is the excitatory postsynaptic potential.

University of Jordan

excitory and inhibitory postsynaptic potentials
Excitory and Inhibitory Postsynaptic Potentials
  • The effect of a neurotransmitter can be either excitatory or inhibitory
    • a depolarizing postsynaptic potential is called an EPSP
      • it results from the opening of ligand-gated Na+ channels
      • the postsynaptic cell is more likely to reach threshold
    • an inhibitory postsynaptic potential is called an IPSP
      • it results from the opening of ligand-gated Cl- or K+ channels
      • it causes the postsynaptic cell to become more negative or hyperpolarized
      • the postsynaptic cell is less likely to reach threshold

University of Jordan

slide16

EPSP, increased permeability

to sodium causes membrane

potential to be less negative

Dendrite

Na+: 142 mEq/L

14 mEq/L

K+ : 4.5 mEq/L

120 mEq/L

Axon

Cl- : 107 mEq/L

8 mEq/L

-45mV

University of Jordan

postsynaptic potentials1
Postsynaptic Potentials
  • The Inhibitory Postsynaptic Potential (IPSP)
      • Inhibitory synapses open K+ or Cl- channels and causes hyperpolarization of the neuron.
      • Positively charged K+ ions moving to exterior make membrane potential more negative than normal.
      • Negatively charged Cl- ions moving to interior make membrane potential more negative than usual.
      • Increase in negativity beyond the normal resting membrane potential level is the inhibitory postsynaptic potential (IPSP).

University of Jordan

slide18

IPSP, increased permeability to

potassium and chloride causes

membrane potential to be more negative

Dendrite

Na+: 142 mEq/L

14 mEq/L

K+ : 4.5 mEq/L

120 mEq/L

Axon

Cl- : 107 mEq/L

8 mEq/L

-90mV

University of Jordan

slide19

Inhibition by short circuit: an

increase in chloride permeability

will balance the depolarization

caused by an influx of sodium

Dendrite

Na+: 142 mEq/L

14 mEq/L

K+: 4.5 mEq/L

120 mEq/L

Axon

Cl-: 107 mEq/L

8 mEq/L

-65mV

University of Jordan

thank you
Thank You

University of Jordan

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