Synaptic Transmission

Synaptic Transmission

The process of information transfer at a synapse

Who named the synapse?

Charles Sherrington in 1897

In what way do synapses transmit information

in one direction from pre to post synaptic neuron

Who demonstrated chemical synapses

Otto Loewi in 1912

Who demonstrated electrical synapses

Furshpan and Potter in 1959

Which type of synapse is more common

chemical

Electrical Synapses

Allow ionic currents to flow from cytoplasm to cytoplasm

Are electrical synapses uni or bi direction

bidirectional

What is the structure of electrical synapses

Gap junction consists of channels (6 connexon) that connect two cells together “electrically coupled”

What is the postsynaptic effect of electrical synapses

so tiny that they alone can’t rigger an action potential, but can help synchronize neuronal activity

Chemical Synapses

Releases neurotransmitter from vesicle that binds to postsynaptic receptors

Are chemical synapses uni or bi directional

unidirectional

What is the structure of chemical synapses

presynaptic cell that release neurotransmitters —> Synaptic cleft (or space inbetween the two cells) —> postsynaptic cell with receptors

What is the postsynaptic effect of chemical synapses

EPSP or IPSP, and the strengths depend on the transmitter released and the receptor available but can trigger AP’s or can be amplified by g-proteins

Synaptic Vesicles

contains neurotransimtter

secretory granules

dense, core vesicles that have soluble proteins (neuropeptides)

active zone

site of neurotransmitter release

postsynaptic density

site of receptors

Axoaxonic

Axon to Axon

Axosomatic

Axon to Soma

Axondendritic

Axon to dendrite

Gary’s Type I Synapse

Asymmetrical, excitatory - postsynaptic side is more thicker

Gary’s Type II Synapse

Symmetrical, inhibitory, similar sized membranes

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and skeletal muscle at the motor end-plan, neurons also synapse onto glands, smooth muscles and heart

Steps of Synaptic Transmission (6 steps)

1. Neurotransmitter is synthesized and loaded into vesicles

2. Depolarization of the axon terminal opens Ca2+ channels

3. Ca2+ rushes into the cell

4. Ca2+ changes the conformation of SNARE proteins causing docked vesicles to fuse to the membrane to release transmitter (exocytosis)

5. Transmitter diffuses across the cleft and binds postsynaptic receptors

6. Transmitter is removed by diffusion, reuptake into presynaptic terminal, or enzymatic degradation

Ionotropic Receptors

Transmitter-gated ion channels

binding results in opening of the channel directly to produce fast EPSP or IPSP

Metabotropic receptors

G-Protein-Coupled receptors (GPCRs)

Binding activated G-Proteins and Effector systems; slower, longer lasting and more widespread effects\

Autoreceptors

Presynaptic receptors sensitive to neurotransmitter release by the same terminal. Acts like AUTOregulation of the cell, often inhibiting further release and synthesis in some cases of the neurotransmitter

EPSP

Excitatory postsynaptic potential

Depolarization typically caused by opening channels permeable to Na+

IPSP

Inhibitory Postsynaptic Potential

Hyperpolarization typically caused by opening channels permeable to Cl-

Synaptic Integration

The process by which multiple synaptic potentials combine within one postsynaptic neuron. Most CNS neurons receive thousands of synaptic inputs

EX: Opening a single transmitter-gated ion channel

Quantal Analysis

Method of comparing the amplitudes of miniature and evoked postsynaptic potentials

Useful in determining how many vesicles release neurotransmitter during normal synaptic potential

How many vesicles are released in NMJ and what is the response?

200 vesicles, EPSP of 40mV or more

More reliable as motor neurons make muscle fibers fire

How many vesicles are released in CNS Synapse and what is the response?

About one vesicle, EPSP of few tenths and one mV. Many inputs must combine

How do dendrites affect synaptic integration

Dendrites are like long cables and depolarization falls off exponentially with increasing distance. Longer cables need stronger depolarization

Dendritic Length Constant

Greek letter = lamda

How far can the depolarization spread down a dendrite

True of false; The larger the length constant, the more likely EPSPS generated at distant synapses will depolarize the postsynaptic membrane?

True

What does length constant depend on

Internal Resistance and Membrane Resistance

True or False; All dendrite do NOT have ion channels

False

Excitable Dendrites

Dendrites that have voltage-gated sodium, calcium, and potassium channels that act as amplifiers of EPSPs, but can’t produce action potentials

Dendritic Sodium Channels

Some carry electrical signals in the opposite direction from soma to dendrite to signal to a dendrite that an action potential was triggered
may be helpful in learning and memory

Shunting Inhibition

Synapse inhibits the current flow from soma to axon hillock by opening Cl- channels to hyperpolarize the cell before the current reaches the terminal

Modulation

Synaptic transmission that changes the effectiveness of EPSPs produced by other synapses

EX: NE released at Beta receptors close K+ channels, increasing membrane resistance and the length constant so the neuron becomes more responsive to excitatory inputs.