Synaptic Transmission

Question 1

neurons

Answer 1

• specialized to rapidly send and receive information to maintain homeostasis and adapt to change
• dendrites, cell soma, axon with myelin, presynaptic terminals

Question 2

dendrites

Answer 2

• tapered processes arising from the cell body that greatly increases the receptive surface
• contain receptors for transmitters and voltage gated ion channels that can amplify the graded synaptic signal
• primary area for receiving and integrating complex information from thousands of synapses
• arbors can be simple or complex

Question 3

cell soma

Answer 3

• surrounds nucleus and contains the ER and golgi
• performs housekeeping functions-portein synthesis, degradation, processing
• membrane contains receptors that bind chemical transmitters released by afferent neurons

Question 4

axon

Answer 4

• single thin process arising from the cell body at the axon hillock
• all or none APs to the terminals after integrating transmitter mediated bioelectrical changes received in the dendrites and cell body
• some can be a meter long, containing 1000 times cyto-called axoplasm as a cell body
• many axons surrounded by myelin sheath

Question 5

presynaptic terminals

Answer 5

• specialized structures that concert electrical signals propagated down the axon into chemical signals (APs to neurotransmitter)
• neurotransmitters are released from presynaptic vesicles and transmitted to target cell at synapse
• post synaptic potentials are small graded changes

Question 6

axoplams

Answer 6

• contains parallel arrays of MTs and neurofilaments that provide structural stability and a means to transport materials back and forth between the cell body and presynaptic terminals
• fast transport both forward (kinesin) and backward (dyenin)
• cancer patients receiving MT disrupters may have peripheral neuropathies due to gradual vesicle depletion

Question 7

electrical synapses

Answer 7

• speed and synchrony
• bidirectional
• gap junctions and connexins
• connexons are hemi channels combine to form gap junction
• current flow changes the postsynaptic membrane potential, which can lead to generation of AP
• pores are much bigger-unselective diffusion of ions and ATP/metabolites
• less common in nervous system- regulation of breathing

Question 8

chemical synapses

Answer 8

• directionality, amplification, excitation and inhibition, plasticity, integration in space and time
• neurotransmitters in synaptic vesicles- clear and small with small molecules or large and dense with peptides

Question 9

criteria for chemical neurotransmitters

Answer 9

1. present in presynaptic terminal
2. released in a voltage and calcium dependent manner (calcium high outside cells
3. specific receptors present in post synaptic target cell
4. means to inactivate the transmitter

Question 10

chemical synapse

Answer 10

1. neurotransmitter molecule sysnthesized and packaged in vesicles
2. an action potential arrives and depolarizes the terminal membrane of the pre-synaptic terminal
3. voltage gated Ca channels open and Ca comes in
4. rise in Ca triggers fusion of synaptic vesicles with the presynaptic membrane
5. transmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic cell
6. bound receptors activate the postsynaptic cells- causes IPSP or EPSP
7. neurotransmitter breaks down, is taken up, or diffuses away
   - extreme activation all the time can lead to de-sensitization

Question 11

synapses

Answer 11

• presynaptic terminals have mitochondria and synaptic vesicles
• subset of vesicles docked at active zones- preferentially released
• post synaptic target cell has electron dense area with many receptors- post synaptic density
• glucose used in brain for pre-synaptic terminals
• fMRI can measure regional blood flow associated with changes in local cerebral glucose metabolism

Question 12

exocytosis of vesicles

Answer 12

• vesicle has synaptotagmin and synaptobrevin (vSNARE)
• membrane has SNAP 25, syntaxin and n-sec 1 on syntaxin (t SNAREs)
• n-sec dissociates and syntaxin and SNAP 25 form complex
• synaptobrevin winds around the complex
• draws vesicle in
• synaptotagmin interacts with calcium to fuse vesicle
• complex dissociated with alpha SNAP and NSF (ATPase)
• botulinum cleaves SNAP 25 or synaptobrevin- no EPSP

Question 13

chemical transmitters and time courses

Answer 13

• 1 msec to 10 days

- Ach (nicotinic) is fast, peptides are slow

Question 14

ionotropic

Answer 14

-contain an ion channel as part of their structure and binding triggers a rapid response

Question 15

metabotropic

Answer 15

-linked to G proteins that transduce a slower signal

Question 16

Ach

Answer 16

• common in periphery and CNS
• choline and acetate
• rapid with nAchR
• slower is mAchR
• AchE terminates
• also pump that puts it back into presynaptic

Question 17

post synaptic potentials

Answer 17

• conductance changes due to ion channel openings, leads to ionic flow through the channels that lead to changes in membrane potential
• excitatory makes the membrane potential more likely to fire AP
• inhibitory makes it less likely to fire AP

Question 18

glutamate

Answer 18

• major excitatory neurotransmitter and binds to ionotropic and metabotropic receptors
• AMPA receptors are selectively activated by AMPA and mediate a fast EPSP (ionotrppic) with Na and K
• NMDA activated by NMDA and mediate a slower EPSP via Na, K and Ca- only open if membrane is pretty depolarized because they are blocked in a voltage dependent manner by Mg (also ionotropic)
• metabotropic linked to G protein and slower

Question 19

GABA

Answer 19

• major inhibitory transmitter
• binds to ionotropic and metabotropic receptors
• hyperpolarizing responses- IPSP
• GABA A receptor channels mediate fast IPSP by Cl ions down gradient
• pentobarbital elicits larger IPSP when GABA is present because it increases channel open time/ single channel current
• GABA B receptors are slower because G linked

Question 20

neuronal integration

Answer 20

• brains contain 100 billion neurons, and each one integrates info from numerous synapses
• EPSP can be only a fraction of a milivolt, below threshold
• neurons innervated by thousands of synapses so summation occurs

Question 21

morphology of synapses

Answer 21

• AP in one neuron transmitted to another at synapse
• terminal regions of axons branch and form synaptic boutons
• myelination lost when bouton in close contact with dendrite or other cell body
• vesicles from the synaptic bouton (end of the axon) fuse with presynaptic membrane

Question 22

distance and potential

Answer 22

-potential changes decrease with increased distance from the stimulus

Question 23

passive events

Answer 23

• dendrites don’t transmit APs because not many Na channels and thresholds high
• small depol or hyperpol are passive events
• spread only a few mm and become smaller at greater distances from stim
• temporal and spatial effects-cable theory-capacitance, resistance, and longitudinal cytoplasmic resistance
• dendrites and long and membranes are thin and leaky to electric current
• before EPSPs reach soma, potential is lost- decremental conduction

Question 24

temporal summation

Answer 24

• firing of two APs quickly enough to reach threshold at axonal hillock
• membrane has longer time constant (holds potential longer)

Question 25

spatial summation

Answer 25

• firing of APs from multiple places with enough of the same effect to reach threshold
• need bigger space constant-holds EPSP for longer distance

Question 26

temporal and spatial summation

Answer 26

• determine the probability that and AP with fire
• neurons integrate lots of information
• axon hillock has high density of voltage gated Na channels