Synapses

Question 1

Synapse Characteristics

Answer 1

– functional contact between 2 neurons that allow them to communicate with each other
o Presynaptic – neuron that is sending information
o Postsynaptic – neuron that receives information
o 2 General Classes: Electrical and Chemical Synapses

Question 2

Electrical Synapses

Answer 2

o Minority
o Function: synchronize electrical activity among populations of neurons
o Form a narrow gap junction between pre- and postsynaptic cells
 Contain precisely aligned paired channels (connexon) that form pores to the junction
• Each connexon is made up of 6 connexin proteins; each connexin protein has 4 transmembrane spanning domains
• Much larger pores than in ion channels
• Allow free diffusion of ions and large molecules (such as ATP)
o Permit direct, passive flow of electrical current
o ONLY excitatory and do not make long lasting changes in the electrical properties of postsynaptic cells
o Induced response < original signal
o Fast, bidirectional impulse transmission (no synaptic delay)

Question 3

Chemical Synapses

Answer 3

o Majority of synapses
o Pass information directionally from a presynaptic cell to a postsynaptic cell
o Excitatory or inhibitory and may change electrical properties of postsynaptic neuron
o Variable size of response induced
o Slow, unidirectional impulse transmission (synaptic delay)
o Two junctions
 Synaptic junctions – site of transmission; active zone; postsynaptic density
 Puncta adherentia junctions – mechanical adhesion site; where pre- and post-synaptic neurons physically make contact via transmembrane spanning proteins

Question 4

Chemical Synapse Presynaptic Components

Answer 4

– synaptic vesicles, exocytosis, large dense core vesicles, endocytic organelles, smooth ER, mitochondria, presynaptic dense grid

Question 5

Synaptic Vesicles

Answer 5

• Store small NONpeptide NTs that are recycled
• Regulated by Ca+ channel
• Fast, spatially precise signaling
 3 Pools
• Readily releasable pool
o Docked to the cell membrane and 1st vesicles to be released on stimulation
o Small amount and quickly exhausted
• Recycling Pool
o Proximal to the neuronal membrane but not docked to it
o Tend to be recycled at moderate stimulation
o Larger than readily releasable pool, but takes longer to become mobilized
• Reserved Pool – majority
o Farthest away from membrane
o May not be released under normal conditions
o Mobilized by intense stimulation and or if other two pools are exhausted

Question 6

Large Dense Core Vesicles

Answer 6

 Store neuropeptides, growth factors, hormones, amines
 Synthesized in cell body/transported
 NOT recycled
 Variable numbers, usually fewer than synaptic vesicles
 Located distant from active zone
 Trains of action potentials to release
 Slow, diffuse, neuromodulatory effects

Question 7

Exocytosis

Answer 7

 SNARE – SNAP Receptor – allow synaptic vesicles to be released
• SNAP – soluble NSF attachment protein
o NSF – N-ethylmaleimide sensitive fusion protein
• V-SNARE – synaptobrevin & synaptotagmin – attached to vesicle
• T-SNARE – syntaxin & SNAP25 – attached to axon terminal
 Synaptobrevin interacts with t-SNARE to anchor the synaptic vesicle near the presynaptic membrane
 Action potential opens a voltage gated Ca+  Ca+ allows syanptotagmin to interact with t-SNARE and synaptobrevin  brings synaptic vesicle closer to presynaptic membrane allowing fusion and release of NT

Question 8

Endocytic Organelles

Answer 8

 Clathrin coated pits & vesicles interact with dynamin for removal from membrane
• Get sorted for retrograde transport, degradation, or bulk endocytosis
 Vacuoles, cisternae, tubulovesicular elements, multivesicular bodies
 Segregated from active zones & synaptic vesicle cluster
 Few at rest, abundant after strong stimulation-possible role in vesicle recycling as well

Question 9

Smooth ER

Answer 9

 Interconnected network of tubules & cisternae extending throughout neuron
 Regulated Ca+ store
 Enhances efficiency of depolarization-secretion coupling
 Regulates release for large dense core vesicles, but NOT synaptic vesicle release

Question 10

Mitochondria

Answer 10

supply ATP for many steps of the vesicle cycle

Question 11

Presynaptic Dense Grid

Answer 11

 Refer to the electron dense patches of proteins associated with the active zone
 Comprised of proteins involved in the docking, fusion & recycling of synaptic vesicles
 Play a fundamental role in defining neurotransmitter release sites

Question 12

Chemical Synapse: Synaptic Cleft

Answer 12

o Fluid-filled space separating the presynaptic & postsynaptic neurons
o Prevents nerve impulses directly passing from one neuron to the next
o Transmission across the synaptic cleft:
 Is a chemical event
 Involves release, diffusion, and binding of NTs
 Ensures unidirectional communication between neurons
o EXCEPTION: metabotropic glutamate receptor MGLUR5 – resides on presynaptic neuron
 Glutamate binds to ionotropic receptor on postsynaptic neuron AND to this receptor
 Possibly modulates the effect on the presynaptic neuron

Question 13

Chemical Synapse: Postsynaptic Components

Answer 13

o Postsynaptic Density (PSD)
 Composed of protein scaffolds around receptor-signaling microdomains
 Their positioning is optimized for linking receptors to their second messengers 7 molecular signaling pathways
o Polyribosomes & SER
o Endosomal vesicles involved in endocytosis and membrane recycling
o Absence of mitochondria

Question 14

Dendritic Spines

Answer 14

– many synapses in the brain involve small protrusion from dendritic branches
o Form biochemical compartments by limiting diffusion of chemical signals with a narrow neck
 Synaptic Ca+ signals can be limited to the spine
o Small active zone and limited calcium signals allows it to have very defined role
o Diversity in size and shape and classified by shape (thin, stubby, mushroom, cup)
o Strong correlation between the size of spine and the strength of the synapse
o Small spines – plasticity and development
o Large spines – memory and learning
o Abnormal spine density or shape is associated with many nervous system disorders

Question 15

Types of Synapses

Answer 15

o Axosomatic – inhibitory
o Axodendritic – excitatory
o Axoaxonic – inhibitory

Question 16

Common Morphological Types of Synapses in CNS

Answer 16

o Gray Type I – glutamatergic/excitatory
 Contact dendrites
o Gray Type II – GABA-ergic/inhibitory
 Contact the cell body