Lecture 12 Synapses Flashcards
Types of Synapses
electrical
chemical
Electrical Synapses
gap junctions - direct electrical connection between cells
uncommon in the nervous system
Chemical Synapses
most common type in the nervous system
release a chemical neurotransmitter which binds to a receptor
-presynaptic cell, synaptic cleft, postsynaptic cell
Presynaptic cell
neuro
axon terminal (synaptic end bulb)
synaptic vesicles - contain neurotransmitter
Postsynaptic cell
neuron or muscle fiber
receptor proteins
Neuromuscular junction
synapse between a motor neuron and a skeletal muscle cell
Acetylcholine (ACh)
is the neurotransmitter
ACh binds to receptors on the postsynaptic membrane
ACh receptor is chemically-gated (ligand-gated) ion channel
opening of chemically-gated channels redults in a graded postsynaptic potential (PSP)
Synaptic Transmission at the Neuromuscular Junction 1-2
- Action potential arrives at the presynaptic axon terminal
- Voltage-gates calcium (Ca2+) channels open in the presynaptic membrane, allowing Ca2+ ions to flow into the presynaptic cell
Synaptic Transmission at NMJ 3-4
- Synaptic vesicles migrate to the presynaptic membrane, releasing ACh into the synaptic cleft
- ACh molecules diffuse across the synaptic cleft and bind to postsynaptic ACh receptors
Synaptic Transmission NMJ 5-7
- ACh binding to receptors opens chemically-gated ion channels in the postsynaptic membrane. These channels are premeable to Na+ and K+ ions
- Na+ ions flow into the postsynaptic cell, causing a graded depolarization of the postsynaptic membrane
7.ACh is rapidly broken down by acetylcholinesterase
ion channels close and membrane returns to resting state
Postsynaptic potential
graded potential in the postsynaptic cell membrane that results from binding of neurotransmitters to receptors (synaptic transmission)
Excitatory postsynaptic potential (EPSP)
depolarizes the postsynaptic membrane toward the threshold for an AP
can result from opening of Na+ channels or closing of K+ channels
increases the likelihood of an AP forming in the postsynaptic cell
Inhibitory postsynaptic potential (IPSP)
hyperpolarizes the postsynaptic membrane or holds it near the resting level
can result from opening of K+ channels or Cl- channels
decreases the likelihood of an AP forming in the postsynaptic cell
Neural integration
synaptic inputs
neural networks
summation
Synaptic inputs
neurons have multiple inputs from other neurons
EPSPs and IPSPs formed at the dendrites and cell body spread toward the trigger zone
APs are triggered at the axon hillock only when the membrane reaches threshold
Neural networks
connection pathways between groups of neurons
- divergent pathways
- convergent pathways
Summation
summation of many EPSPs and IPSPs determines if APs are formed in the postsynaptic cell
- spatial summation
- temporal summation
spatial summation
EPSPs from different synapses can add together or IPSPs can cancel out EPSPs
Temporal summation
EPSPs can add together if they occur close together in time
Cholinergic receptors
Bind to acetylcholine
- Nicotinic cholinergic receptor
- muscarinic cholinergic receptor
Nicotinic cholinergic receptor
ion channel receptor binding of ACh directly opens chemically gated channels, allowing Na+ to flow in fast response, direct always excitatory found in the NMJ and autonomic ganglia
Muscarinic cholinergic receptor
G protein coupled receptor (GPCR)
binding of ACh to receptor activates a G protein, activated by binding of GTP
activated G protein sub unit interacts with K+ ion channel, causing it to open or close
slower response, indirect
can be excitatory (e.g. in the gut) or inhibitory (e.g. in the heart)
Found in parasympathetic target cells: heart, GI tract
Adrenergic Receptors
bind to norepinephrine (NE) and epinephrine (Epi)
- alpha adrenergic receptors
- beta adrenergic receptors
Alpha adrenergic receptors
G protein coupled receptors
activate a 2nd messenger system
excitatory
location: vascular smooth muscle (vasoconstriction)
Beta adrenergic receptors
G protein coupled receptors
activate a 2nd messenger system (diff system than alpha)
excitatory or inhibitory
locations: cardiac muscle-excitatory, bronchial smooth muscle- inhibitory
Classes of neurotransmitters
acetylcholine
amines: norepinephrine (NE), dopamine (rewards & pleasure), serotonin (SSRI’s)
amino acids: glutamate (excitatory in CNS), Gamma-aminobutyric acid GABA (inhibitory in CNS), Glycine (inhibitory in CNS)
neuropeptides: Endogenous opioids
gaes: nitric oxide (NO)
Nicotinic cholinergic synapses
synaptic pharmacology examples
botulinum toxin - blocks ACh release (Botox)
Curare- blocks ACh receptors
nerve gas - inhibits ACh-esterase
Muscarininc cholinergic synapses
synaptic pharmacology examples
Atropine - blocks muscarinic ACh receptors
monoamine synapses
synaptic pharmacology examples
MAO inhibitors - block breakdown of NE, dopamine and serotonin (non-selective)
SSRIs - selective serotonin reuptake inhibitors which increases concentration of serotonin at synapses
clinical application - antidepressant drugs
