synaptic and neuromuscular transmission Flashcards
neuron structural types
structural classification based on # of processes that extend from the cell body
what are multipolar neurons?
3 or more processes extending from cell body
** MAJOR TYPE IN CNS and EFFERENT PNS
what are bipolar neurons?
- only 2 processes
- retina, olfactory system
what are unipolar neurons?
- single, short processes from cell body, then branches into 2 or more
- peripheral processes associated with sensory perception
- primary afferent PNS
what are electrical synapses?
- current flows between cells via GAP JUNCTIONS
- in cardiac and some smooth muscle
- very fast
what are chemical synapse?
- gap between presynaptic and postsynaptic cells- synaptic cleft
1. AP in presynaptic cell causes voltage-gated Ca2+ channels to open
- Ca2+ influx causes release of neurotransmitter from presynaptic terminal
- neurotransmitter binds ligand-gated Na+ channel on postsynaptic cell, leading to depolarization (can be excitatory or inhibitory)
describe synaptic arrangements
neurons are arranged in circuits where input cells synapse on output cells
- output cell may or may not fire an action potential depending on amount of excitation provided by the input cells
one-to-one synapses
- neuromuscular junction
- single AP in motoneuron cases single AP in muscle fiber
one-to-many synapses
- found in some motoneurons of spinal cord
- spinal AP in motorneurons causes many APs in postsynaptic cells
many-to-one synapses
- many presynaptic cells converge on a postsynaptic cell
- common
- need convergence of multiple input neurons to cause an AP -> sum inputs
net= depolarization=
excite
net= hyperpolarization=
inhibit
common to use many-to-one arrangement
inputs are excitatory or inhibitory
excitatory postsynaptic potential (EPSPs)
- pre-synaptic neuron depolarizes post-synaptic neuron, brings neuron’s membrane potential to threshold opens Na and K channels
- Ach, NE, Epi, dopamine, glutamate, serotonin are excitatory neurotransmitters
inhibitory postsynaptic potentials (IPSPs)
- pre-synaptic neuron hyper polarizes post-synaptic neuron and membrane potential moves away from threshold, opens Cl channels
- GABA, glycine are inhibitory neurotransmitters
temporal summation
- two inputs arrive at postsynaptic cell in rapid succession
- effects are additive
spatial summation
- two or more inputs arrive at postsynaptic cell simultaneously
- both inputs could be excitatory -> depolarization
- one could be excitatory, one inhibitory -> cancels/nothing
synaptic fatigue
repeated stimulation yields a smaller than expected response
ionotropic receptors
- ligand-gated ion channels that usually open in response to binding of neurotransmitter
- usually located along dendrites or cell bodies and receive incoming information from other neurons
effects on post-synaptic neuron by neurotransmitters
- may be excited due to Na influx causing depolarization
- may be inhibited due to either K efflux or chloride influx causing hyperpolarization
- Ca influx?: may or may not cause depolarization, Ca usually associated with 2nd messenger system
What does Glutamate (AA) do?
- primary excitatory neurotransmitters in the CNS
what are the types of glutamate receptors?
- AMPA
- NMDA
what does AMPA do?
- allows both Na and K to cross membrane, but gradient drives Na is stronger -> net effect depolarization
what does NMDA do?
depends on glutamate binding AND voltage, when around RMP, Mg ions blocks binding site/movement of other ions- when glutamate binds AND depolarization, Mg block removed
Acetylcholine
- only neurotransmitter used at neuromuscular junction
- also used by autonomic nervous system (PSNS and SNS)
Glycine and GABA (both AA)
- both neurotransmitters are used in the CNS and both are chloride channels
what does the binding of Glycine and GABA cause?
inhibition since increase in Cl permeability is inhibitory
GABAa receptor associated with…
chloride
what are metabotropic receptors?
- located along dendrites or cell bodies, receive incoming information, slower than ionotropic receptors
- can use G-protein-coupled receptors (GPCR)
steps in GPCR?
- G-protein activated when neurotransmitter binds receptor
- G-protein alpha subunit binds GTP
- beta y subunit may activate an ion channel (K+ channel) -> direct G-protein gating
- alpha subunit may activate enzymes that promote second messenger synthesis
- enzymes: phospholipase C, adenylyl cyclase, DAG
- second messengers: IP3, CAMP
what neurotransmitters activate G-protein-coupled receptors?
- norepinephrine (NE)
- glutamate (metabotropic receptor)
- GABA (GABAb receptor)
what does norepinephrine (NE) do?
uses alpha- or beta- adrenergic receptors
beta-adrenergic GPCRs couple to a stimulatory G-protein subunit ->
initiates cAMP second messenger system
alpha-2 adrenergic receptors couple to an inhibitory G-protein and…
suppress the cAMP system
Alpha-1 adrenergic receptors couple to G-protein, Gq ->
activates the phospholipase C second messenger system
Glutamate (metabotropic receptor)
- acts on post-synaptic sites in CNS
- modulates cell excitability and synaptic transmission via second messenger pathways
3 groups of Glutamate receptors
- group 1: increase neuron excitability; activate phospholipase C pathway
- group 2 and 3: suppress neuron excitability, inhibit adenylyl cyclase pathway
GABA (GABAb receptor)
- found in CNS and autonomic division of peripheral nervous system
- GABA acting via G-proteins linked to K channels -> hyper polarize cell at end of AP
neurotransmitters based on chemical properties
- acetylcholine
- biogenic amines: norepinephrine, epinephrine, dopamine, serotonin (5-HT), histamine
- amino acids: glutamate, glycine, gamma-aminobutyric acid (GABA)
- neuropeptides
neuropeptides
synthesized and packaged in nerve cell body instead of axon terminal
neuromodulators
- may act on pre-synaptic cell to alter amount of neurotransmitter released
- may be co-secreted with neurotransmitter to alter response of post-synaptic cell
neurohormones
released from neurons into blood
