Neurons & Neural Communication Flashcards
2 types of cells in nervous system
- neurons
- glia
3 types of glial cells
- oligodendrocytes
- astroglia
- microglia
function of oligodendrocytes
- myelinate/insulate MANY neurons in CNS -> create myelin sheath
- similar function to Schwann cells (mylinate ONE neuron each in PNS)
function of astroglia
- aka: astrocytes
- ensheath tripartite synapse and ensure ionic balance within synapse
- play a role in neuronal communication
function of microglia
act as immune cells in CNS
tripartite synapse
the proximity of presynaptic membrane, postsynaptic membrane, and surrounding glia and the way these 3 synaptic components produce activity at the synapse
major parts and functions of neurons
- nucleus: contains genetic material
- cell body: surrounds nucleus and other organelles inside neuron
- dendrites: receive signals and carry them to cell body
- axon: carries signals away from cell body (has synaptic bouton/pre-synaptic membrane at end of branch)
types of synapses
- axosecretory: axon terminal secretes directly into bloodstream
- axioaxonic: axon terminal secretes into another axon
- axiodendritic: axon terminal ends on dendrite spine
- axoextracellular: axon with no connection secretes into extracellular fluid
- axosomatic: axon terminal ends on soma
- axosynaptic: axon terminal ends on another axon terminal
resting membrane potential
- To measure: put intracellular electrode in neuron and extracellular electrode into extracellular fluid, measure difference
- Healthy neuron has RMP (or membrane voltage) of between –60 and –80 mV (~70 mV)
- at rest, more sodium ions outside neuron and more potassium ions inside neuron
generation and conduction of post-synaptic potentials (PSPs)
- occur when a neutrotransmitter molecule binds to a post-synaptic receptor, creating one of two localized effects (EPSP or IPSP)
- transmission of PSPs graded (varies in size, NOT all-or-none), rapid, and decremental (decreases over time) -> travels like an electrical signal along an uninsulated wire
Excitatory post-synaptic potential (EPSP)
- 1 effect of an NT binding to post-synaptic receptor
- Depolarizes the membrane (ie. Decrease membrane potential from -70 to -67 -> gets closer to 0)
- Increases likelihood that postsynaptic neuron will fire an action potential (AP)
Inhibitory post-synaptic potential (IPSP)
- 1 effect of an NT binding to post-synaptic receptor
- Hyperpolarizes the membrane (ie. Increase the membrane from -70 to -72 -> gets further from 0)
- Decreases likelihood that postsynaptic neuron will fire an action potential (AP)
summation of PSPs
- both EPSPs and IPSPs sum spatially and temporally
- spatial summation: when PSPs are released from multiple synapses, combining to produce a greater effect (either greater EPSP or IPSP, or cancel out 1 EPSP and 1 IPSP)
- temporal summation: when multiple PSPs are released from one synapse in rapid succession, combining to produce a greater effect (either greater EPSP or IPSP)
What is an Action Potential (AP), and how is it generated?
- massive momentary reversal of the membrane potential (e.g., from -70 to +55 mV)
- not graded (“all or none”: either happens or doesn’t), not decremental, less rapid than PSPs
- occurs if the sum of the EPSPs and IPSPs is enough to depolarize membrane at axon initial segment above its threshold of excitation (ex. -65mV)
Ionic basis of Action Potentials
- AP generation and conduction are both the result of voltage-activated ion channels
- when membrane is depolarized (due to PSPs), sodium channels open, driving excitation (lots of sodium outside cell; wants to move in)
3 phases of an Action Potential
- Rising Phase: sodium channels open, causing potassium channels to open too
- Repolarization: sodium channels inactivate
- Hyperpolarization: potassium channels close, leading to return to resting potential and a refractory period
2 types of refractory periods
- Absolute refractory period: no other AP could happen; causes conduction to only travel 1 way; during rising phase
- Relative refractory period: another AP could happen, but the buildup to it would be slower; during hyperpolarization
3 stages of sodium channels
- closed (before rising phase)
- open (during rising phase)
- inactivated (repolarization begins)
subthreshold vs. suprathreshold stimulation of an axon
- subthreshold: excitatory potential produced, but not enough to elicit AP
- suprathreshold: excitatory potential produced that exceeds threshold and produces AP
conduction of Action Potentials in myelinated vs. unmyelinated neurons
- myelinated: saltatory conduction -> jumping from one Node of Ranvier to the next -> faster conduction
- unmyelinated: continuous conduction -> no jumping -> slower
Multiple Sclerosis
- Disorder that progressively damages myelin
- 55-75K in Canada (3 new/day)
- Canadians have one of the highest rates of multiple sclerosis in the world
classic view of neurotransmission
- based on work done at one synapse: the neuromuscular junction (NMJ)
- What was true for that synapse was once (incorrectly) generalized to all nervous system synapses
5 ways in which classic view of neurotransmission was incorrect
- each cell has a single output & releases a single NT
- NTs deactivated by enzymes
- NTs produce 1 of either EPSPs or IPSPs
- each NT has single receptor
correction to classic view of neurotransmission: each cell has single output
- true for NMJ, but it’s an exception
- most cells receive input from many cells
correction to classic view of neurotransmission: NTs deactivated by enzymes
- true for NMJ -> enzyme deactivates acetylcholine
- rare elsewhere -> reuptake is major mechanism for deactivation of NTs
correction to classic view of neurotransmission: NTs produce 1 of either EPSPs or IPSPs
- true at NMJ -> acetylcholine produces EPSPs
- whether NT produces EPSP or IPSP later shown to be function of receptor type, not NT itself
correction to classic view of neurotransmission: each NT has single receptor
- Acetylcholine receptors bind nicotine better than muscarine, and vice versa for others
- but nicotinic and muscarinic receptors are ionotropic (form ion channel pore) and metabotropic (directly linked to ion channel) receptors, respectively
correction to classic view of neurotransmission: each cell releases a single NT
- true in NMJ
- “coexistence” of different transmitters found in many cells; neurotransmission a complicated and heterogenous process
major types of NTs
- amino acids (ex. glutamate, GABA)
- monoamines (ex. dopamine, epinephrine, norepinephrine, serotonin)
- acetylcholine
- neuropeptides (large molecule NTs… others above are small molecule)
