Gap Junction:

Characteristics

Synaptic Junction:

Characteristics

Gap Junction:

Structure

3.5 nm wide.

Each cell contributes a connexon, which is made up of six connexin subunits.

Ion channel has 1.5 nm pore diameter.

Gap Junction:

Physiology

Low-resistance, high-conductance ion channels.

Current flow depolarizes postsynaptic cell.

Gap Junction:

Properties (6)

1) No synaptic delay.

2) Two or more coupled neurons.

3) All-or-none.

4) Transmit developmental and regulatory signals. 5) No inhibitory mechanisms.

6) No long-term modulation mechanisms.

Chemical Transmission:

Characteristics

Mediated by release and diffusion a chemical messenger.

Morhphological specialization of cell membranes.

1) Synaptic vesicles.

2) Active zones (dense projections).

3) Autoreceptors

1) Neurotransmitter receptors.

2) Postsynaptic densities.

CNS Synapse Types:

Gray's Type I

Round vesicles with wide synaptic cleft.

Prominent postsynaptic densities -> asymmetric synapse.

Often excitatory.

CNS Synapse Types:

Gray's Type II

Flat or pleomorphic vesicles with a narrower synaptic cleft.

Less prominent post-synaptic densities.

Often inhibitory.

CNS Synapse Types:

Monaminergic

Dense-core vesicles with wide synaptic cleft.

Axonal varicosities.

No synaptic densities.

CNS Synapse Types:

Peptidergic

Neuromuscular Synapse:

Properties (5)

1) Presynaptic dense bars.

2) Ca2+ channels.

3) Primary and secondary clefts

4) ACh receptors in active zones

5) AChesterase and basal lamina in clefts

Chemical Transmission:

Steps (9)

(also see figures in notes)

1) Neurotransmitter synthesis.

2) Docking at active zone.

3) Vesicle interaction with membrane.

4) Vesicle-membrane fusion.

5) Neurotransmitter release (fission).

6) Diffusion.

7) Transmitter binding to receptor.

8) Post-synaptic response.

9) Transmitter termination.

Chemical Transmission:

Steps

1) Neurotransmitter Synthesis

Chemical Transmission:

Steps

2) Docking

Dephosphorylated synapsin I anchors vesicles to cytoskeleton.

Once phosphorylated by CAM kinase II, synapsin I releases vesicles, which are then propelled toward the membrane by Rab3A.

Chemical Transmission: S

teps

3)Interaction

Hypothesis 1: v-SNAREs bind t-SNAREs to form a fusion-mediating complex. SNAREs interact with SNAP which interacts with NSF. NSF unravels the SNARE complex for recycling.

Hypothesis II: NSF and SNAP prime vesicles for fusion and release.

SNAP REceptor proteins.

v-SNARE= vesicular synaptobrevin (VAMP= vesicle-associated membrane protein).

t-SNARE= target plasma membrane (ex: syntaxin and SNAP-25).

Chemical Transmission:

Steps

4) Pre-Synaptic Fusion

Chemical Transmission:

Steps

5) Release (Fission)

Chemical Transmission:

Steps

6) Diffusion

Chemical Transmission:

Steps

7) Binding to Receptors

Chemical Transmission:

Steps

8) Post-synaptic Response

Chemical Transmission:

Steps

9) Termination (4)

1) Diffusion.

2) Hydrolytic destruction.

3) Autoreceptor binding and activation, which initiates negative feedback loop.

4) Presynaptic reuptake.

Neurotransmitter Receptors:

Properties

Membrane-spanning with specific external recognition sites.

Direct or indirect (via 2nd messenger system) modulation of ion channel gating.

Fast.

Receptor is part of the channel (ionotropic).

Slow (several steps).

G-protein cascade-mediated involving cAMP or cGMP (metabotropic).

Chemical Transmission:

Properties (5)

1) Amplification - many cells can be activated.

2) Plasticity of response - alter # and type of receptors.

3)Synaptic type and transmitter diversity - plurichemical transmission= co-release of multiple transmitters.

4) Neuromodulation.

5) Synaptic delay.

Gaseous Transmission:

Properties

Synthesis and diffusion of gaseous messengers.

No unique specializations have been identified.

Gaseous Transmission:

Steps (5)

1) Nitric oxide synthase (NOS) activation

2) NO Binding

3) Guanulate cyclase activation

4) Ion channel gating

5) Inactivation and removal of NO

Gaseous Transmission:

Steps

1) NOS Activation for Neuro-Neuronal Synapse (4)

1) Neurotransmitter Glutamate binds to NMDA receptor.

2) Ca2+ enters via NMDA receptor and binds with calmodulin, which activates NOS

3) Arg -> NO + Citrulline

4) NO acts inside of cell or diffuses and acts there

Gaseous Transmission:

Steps

1) NOS Activation for Neuro-Effector Synapse (3)

1) AP enters varicosity

2) Ca2+ activates NOS, NO produced

3) NO diffuses to smooth muscle cells

Gaseous Transmission:

Steps

2) Binding

Gaseous Transmission:

Steps

3) Guanylate Cyclase Activation

Gaseous Transmission:

Steps

4) Ion Channel Gating

Gaseous Transmission:

Steps

5) Inactivation and Removal

Gaseous Transmission:

 Biochemistry and Function (6)

1) NOS pathway.

2) Co-localized and co-released with other transmitters.

3) Ca2+ binding proteins are critical in pathway.

4) Involves hemoprotein binding.

5) Effective at directed (neuro-neuronal) and non-directed (neuro-effector) synapses.

6) Alters ion channel gating via second messenger cascade.