Lecture 9

Question 1

name and describe two different types of synapses

Answer 1

electrical synapse - a gap junction where current flows directly from one cell to the other
chemical synapse - electrical signals in the presynaptic cell are converted to a chemical signal for transmission to the post synaptic cell

Question 2

what is a synapse

Answer 2

• the region where the neuron meets its target cell (neuron to neuron, neuron to muscle cell)

Question 3

what is a gap junction

Answer 3

• made of connexin proteins allow for very rapid communication and synchronization of activity within a network of cells

Question 4

describe the steps involved in chemical neurotransmission at the synapse

Answer 4

1. action potential arrives at axon terminal and depolarizes it
2. the depolarization opens voltage gated Ca2+ channels and Ca2+ enters the cell
3. calcium entry triggers exocytosis of synaptic vesicle contents (SNAREs allow for fusion of vesicle with membrane)
4. neurotransmitter diffuses across the synaptic cleft and binds with receptors on the postsynaptic cell
5. neurotransmitter binding initiates a response in the postsynaptic cell

Question 5

describe the steps of termination of the stimulus

Answer 5

• neurotransmitter action terminates when the chemicals are broken down, taken up into cells, or diffuse away from the synapse
  1. neurotransmitters can be returned to axon terminals for reuse or transported into glial cells
  2. enzymes inactivate neurotransmitters
  3. neurotransmitters can diffuse out of the synaptic cleft

Question 6

what is acetylcholinesterase

Answer 6

• enzymatic breakdown of acetylcholine

Question 7

what is a neurotransmitter

Answer 7

1. the chemical must be found in the presynaptic neuron
2. the chemical must be released in response to presynaptic depolarization
3. the chemical must act on specific receptors on the postsynaptic neuron to cause a rapid effect
4. after release the chemical signal must be terminated (can be via reuptake, enzymatic degradation, glial cell uptake, diffusion)
5. application of the chemical directly to the postsynaptic membrane should have the same effect as when it is released by a neuron

Question 8

what are some examples of neurotransmitters

Answer 8

• Acetylcholine
• Amines– Dopamine, Norepinephrine, Epinephrine, Serotonin, Histamine
• Amino Acids– Glutamate, Aspartate, GABA, Glycine
• Purines– ATP, AMP, Adenosine
• Gases– NO
• Peptides– Substance P, Opioids, others
• Lipid

Question 9

what are the main excitatory neurotransmitters

Answer 9

Glutamate- Primary excitatory neurotransmitter in the brain (learning, memory, plasticity).
Aspartate -Excitatory, but less common than glutamate.
Acetylcholine (ACh) - Excitatory at neuromuscular junctions; also important in learning/memory.
Norepinephrine (NE) - Arousal, attention, stress response; mainly excitatory.
Epinephrine (Epi)- Fight-or-flight hormone; mostly excitatory in the body.
Dopamine (DA) - Can be excitatory (reward, motivation, movement), but also modulatory depending on receptor.
Serotonin (5-HT) - Mostly excitatory (mood, sleep, appetite), but can also inhibit depending on receptor type.
Histamine - Excitatory; promotes wakefulness and arousal.

Question 10

what are the main inhibitory neurotransmitters

Answer 10

GABA - Primary inhibitory neurotransmitter in the brain; calming, reduces neuronal firing.
Glycine - Major inhibitory neurotransmitter in spinal cord and brainstem.
Adenosine - Inhibitory neuromodulator; promotes sleep, blocked by caffeine.
Endorphins/Opioids (peptides) - Inhibitory on pain pathways; reduce pain perception, induce euphoria.
Endocannabinoids (lipids) - Inhibitory modulators; regulate appetite, mood, memory, pain.

Question 11

what determines the amount of neurotransmitter released

Answer 11

• frequency and number of AP determines how many NT released
• stimulus intensity is coded by the frequency of AP firing and NT released
• weak stimulus releases little neurotransmitter
• strong stimulus causes more action potentials and releases more neurotransmitter

Question 12

describe post synaptic responses (synapses between neurons in the CNS)

Answer 12

• in the CNS there are a large number of neurotransmitters, excitatory and inhibitory
• the action of excitation or inhibition is not just dependent on the transmitter molecule but also on the type of post synaptic receptor
• EPSP = depolarization
• IPSP = hyperpolarization
• both are graded potentials

Question 13

what is ionotropic

Answer 13

• fast response
• mediated by receptor channels

Question 13

what is metabotropic

Answer 13

• slow response
• mediated by GPCRs
• neuromodulators create slow synaptic potentials and long term effects

Question 14

what are the different types of post synaptic responses

Answer 14

• neurotransmitters create rapid, short acting fast synaptic potentials
• directly binds to chemically gated ion channels
• ion channels open
• more Na+ in = EPSP = excitatory depolarization
• more K+ out or Cl- in = IPSP = inhibitory hyperpolarization
• neuromodulators create slow synaptic potentials and long term effects (GPCRs)
• activated second messengers pathways alters open state of ion channels
• ion channels close
• less Na+ in = IPSP
• less K+ out = EPSP

Question 15

what is divergence

Answer 15

• one presynaptic neuron branches to pass info to many downstream neurons

Question 16

what is convergence

Answer 16

• many presynaptic neurons send input to a smaller number of postsynaptic neurons

Question 17

what is global presynaptic inhibition

Answer 17

• all targets of the postsynaptic neuron are inhibited equally
  1. excitatory and inhibitory presynaptic neurons fire
  2. summed signal in postsynaptic neuron is below threshold
  3. no action potential at trigger zone
  4. no response occurs in any target cell

Question 18

what is selective presynaptic inhibition

Answer 18

• an inhibitory neuron synapses on one collateral of the presynaptic neuron and selectively inhibits one target
  1. an excitatory neuron fires
  2. an action potential is generated
  3. an inhibitory neuron fires, blocking neurotransmitter release at one synapse

Question 19

what are synaptic diseases

Answer 19

• synaptic transmission is particularly vulnerable step in nervous signaling (many things can go wrong)
• problems at the synapse are often good targets for drug therapy
• receptors are accessible to extracellular fluid

Question 20

what is an example of a synaptic disease

Answer 20

myasthenia gravis - attacks neuromuscular junction between somatic motor neurons and skeletal muscles
- symptoms are muscle weakness and fatigue
- caused by autoimmune mediated decrease in acetylcholine receptors
- treated with anti-cholinesterase to increase lifetime of acetylcholine in the synapse