Lecture 6 - Synapses: Synaptic Transmission, EPSP, IPSP, EPP

What are the two major types of synapses

Chemical Synapses and Electrical Synapses

Presynaptic Neuron

This is the first neuron, or communicating neuron

Postsynaptic Neuron

This is the second neuron or cell that receives the information from the first neuron

Examples of CNS Synapses

• Axodendritic

• Axosomatic

• Axoaxonic

• Dendritic (This is electrical)

• Neuromuscular Junction (Chemical, and is found in the PNS)

Electrical Synapse of the CNS

The Channel in which this occurs is called a connexon (usually two)

• Each Connexon has 6 subunits called Connexins

• Each of these subunits has 4 transmembrane domains

This is the flow of ions from cytoplasm to cytoplasm

Ex; Dendrodendritic Synapse

These are a very fast form of transmission

It involves a process called synaptic integrations in which serves PSPs occurring (either temporally or spatially) will excite a neuron (Causing a AP)

The two connexons have how many subunits

6 subunits called connexins which each have 4 transmembrane domains

Information on Gap Junctions

They are a family of transmembrane channels that form matching and communicating channels between apposing cells

They Facilitate the coupling of electrical signals from Cell to Cell, resulting in a synchronous behavior

Types of Chemical Synapses by Functions

Gray’s Type 1

• These are asymmetrical

• They are excitatory

• Their vesicles are very circular

Gray’s Type 2

• These are symmetrical

• They are inhibitory

• Their Vesicles are very flat

Presynaptic Elements of Chemical Synapses

• They Contain vesicles that hold the neurotransmitters

• The release of these neurotransmitters is Ca2+-dependent secretory process

• This Area is very very abundant in Mitochondria

Steps of Chemical Synaptic Communication

1. Action Potential Arrives at the Axon Terminal

2. Voltage Gated Ca2+ Channels are activated

3. This causes a Ca2+ Influx

4. Synaptic Vesicles docked at the presynaptic membrane have Ca2+ sensors which cause the vesicles to rupture

5. This rupturing and secretion of Neurotransmitters is called exocytosis

6. neurotransmitters will now bind to receptors on the post-synaptic membrane, where the chemical signal is now converted to an electrical signal (PSPs)

7. Neurotransmitters are now enzymatically degraded or recycled through reuptake

Postsynaptic Elements of Chemical Synapse

For Rapid Transmission

• Neurotransmitters gated Ion Channels (Ionotropic Receptors)

  • Nicotinic ACH receptors (Na+ and K+ Permeable) - End Plate Potential at NMJ

  • Glutamate Receptors (AMPA & NMDA) - EPSP

  • GABA Receptors - IPSP

For Slow Rapid Transmission

• Involves postsynaptic receptors, linked to G Protein

• Muscarinic ACH (IPSP)

What is the Criteria used for identifying Neurotransmitters SENSMM

• The substance must be synthesized in neurons and enzymes must be present

• Must be released in Sufficient quantity to elicit a response

• Mechanisms for removal of NT

• Mimic Actions of the Endogenously released NT when administered exogenously

SENSMM

Synthesized in neurons by enzymes

Sufficient quantity to elicit response

Mechanisms for removal

Mimic endogenous when administered exogenously

Classification of Neurotransmitters

Amino Acids

• Excitatory - Glutamate, Aspartate

• Inhibitory - GABA, Glycine

Amines

• ACH

• Catecholamines - Dopamine, Norepinephrine, Epinephrine

• Indoleamine - Serotonin (5-HT)

• Imidazole Amine - Histamine

• Purines - ATP, Adenosines

Neuropeptides

• Opioid Peptides - Beta-Endorphin, Methionine-Enkephalin, Leucine-Enkephalin, endomorphin, Nociceptin

• Substance P - Pain

• Somatostatin

• Neuropeptide Y

Gaseous NTs

• Nitric Oxide

• Carbon Monoxide

Neurotransmitter are released by

Exocytosis

Vesicle membrane are recycled

Endocytosis

Reuptake

Neurotransmitters re-enter the presynaptic axon terminal through transporters

Uptake by glial cells (Astrocytes)

Enzymatic destruction

Via Proteins inside the presynaptic terminal cytosol or synaptic cleft

Excitatory Postsynaptic Potential

This occurs through the transient postsynaptic membrane depolarization by presynaptic release of excitatory neurotransmitters

Occurs via Na+ Ionotropic Receptors

Inhibitory Postsynaptic Potential

Transient postsynaptic hyperpolarization of the postsynaptic membrane caused by the presynaptic release of inhibitory neurotransmitters

Occurs via efflux of K+ by Ionotropic Receptors

End Plate Potential

End plate potential is a localized depolarization of the muscle fiber membrane at the neuromuscular junction. It occurs when acetylcholine (ACh) released from motor neurons binds to receptors on the muscle cell membrane, leading to an influx of sodium ions. This depolarization can trigger an action potential if it reaches a certain threshold, resulting in muscle contraction. EPPs are crucial for the communication between nerves and muscles.

Quantum

This is a indivisible neurotransmitter releasing Unit

Quantal Analysis

Used to determine the number of vesicles that are released during neurotransmission

NMJ

• Has about 200 Synaptic Vesicles

• EPSP of 40mV or More

CNS Synapse

• Single Vesicle

• EPSP of a few tenths of an MV

Summation of PSPs

Spatial Summation - EPSPS or IPSPS generated simultaneously in different spaces

Temporal Summation - EPSPs or IPSPs generated in rapid succession

Ionotropic Receptors

Ligand-Gated

5 subunits & each subunit had 4 transmembrane

Metabotropic Receptor

G Protein & 2nd Messenger

1 subunit & has 7 transmembrane domains

Glutamate Receptors

NMDA - Sodium Receptors

AMPA - Ca2+ Receptors

Tetrameric - 4 subunits

Neuropharmacology

Receptor Antagonists - inhibitors of neurotransmitter receptors (nACH R. Curare)

Receptor Agonists - Mimic the actions of naturally occurring neurotransmitters (nACH R. Nicotine Agonist)