Neuronal Physiology Flashcards
Factors affecting ion flux across membranes
- Ion (membrane) conductance
- Ion gradient
Renal systems maintains?
Near constant internal environment, so ion gradient changes are minimal
Unexcitable membrane
Dendrite and cell body
- don’t contain voltage-gated channels
Excitable membrane
Axon hillock and axon
- contain voltage-gated channels
Axons can be what in length?
up to feet
When we open channels…
Generate ion flux which changes membrane potential
Action potential syn
Nerve impulse, spike
Axon syn
Nerve fiber
Axon terminal syn
Synaptic knob, terminal bouton, presynaptic terminal, axon knob
Cell body syn
Soma
Cell membrane/Plasma membrane syn
Axolemma
Two types of disrupting events
- Graded potentials
- Action potentials
Graded potentials
A change in membrane potential that varies in size
a. Magnitude of response is directly proportional to the magnitude of stimulus
b. Does not transmit over long distances (decremental conduction)
c. Effects can be summated, added together
- found in unexcitable membrane
- either ligand-gated or mechanically-gated channels
Graded potential is relative to?
Resting potential
Graded potentials decrease in?
Strength as they spread out from the point of origin due to current leak out of the cell
Additive effect
Na+ influx and Ca2+ influx
- calcium is similar to sodium as it depolarizes the cell and has a driving force into the cell
Action potential
Change in a membrane potential of excitable membrane
a. Action potential are “All-or-nothing”, magnitude is “fixed” after threshold
b. Capable of transmitting over long distances (> 1m)
c. Do not summate
- found in neurons & some are non-neural tissue (ex: muscle)
Threshold
The MINIMUM stimulus necessary to elicit a response (action potential)
An above threshold stimulus applied to the membrane can cause a change in?
Voltage, resulting in the opening of voltage-gated channels
ex: during the rising phase of an AP, gNa (mem. conductance for Na+) can increase 600x over resting values
Membranes also possess voltage-gated K+ channels which open in response to?
Changes in membrane potential, but are 10x slower than Na+ channels
- increased K+ conductance results in K+ efflux & repolarization
- K+ channels only close when membrane potential returns to normal (-70mV)
K+ has a huge driving force…
Outside of the cell
Refractory period
Time period after an action potential in which Na+ gates may be inactive and the K+ channels may be open so that a second AP is impossible
Voltage-gated Sodium Channel
- Closed but capable of opening at first
—at resting potential (-70mV)
2 Then open (activated)
—from threshold to peak potential (-50mv to +30mV) - Closed/locked and not capable of opening (inactivated)
—from peak to resting potential (30mV to -70mV)
Essentially takes on 3 shapes
- Na+ has a huge driving force into cell
Triggering event takes cell to?
Threshold
Voltage-gated Potassium Channel
- Initially closed
—At resting potential; delayed opening triggered at threshold; remains closed to peak potential (-70mV to 30mV) - Then opened
—From peak potential through after hyperpolarization (30mV to -80mV)
Rising phase
Caused by Na+ influx
- depolarizing
Falling phase
Caused by K+ efflux
- repolarizing
Na+ movement
Essentially going down the membrane
- works on patches, moves adjacently
- lots of action potentials happening in chunks of axon and moved along
Action potential recurring
Positive feedback loop
Because of refractory period…
action potential only moves one direction
For a depolarization to occur…
Na+ (or Ca2+) influx must exceed K+ efflux
- disproportionate ion flux
Rate of transmission is dependent on?
Axon diameter
- wider diameter has lower resistance
AP’s usually begin at?
Excitable tissue of the axon hillock
AP is unidirectional due to?
Refractory period
- however, leakage of charge occurs during transmission
Local current flow
Movement of AP from one region to neighboring region (moving down axon)
- limitations = slow
- all muscle cells use this
Myelin sheath
Insulator for axon
- multiple layers of cell mem
- wrap around & suck out cytoplasm
- membranes generally don’t let ions in
AP is open for?
4-5 msec
Neuroglial (glial) cells
Assist cells for neurons
- Schwann
- Oligodendrocytes
- Astrocytes
- Microglia
Schwann cells
Myelin forming cells of PNS
- help to prevent leakage of charge
Oligodendrocytes
Myelin forming cells of CNS
- help to prevent leakage of charge
- extends & wraps several cells (1:4 & 1:6)
Astrocytes
Star shaped cells that surround unmyelinated cells that give structural support
Microglia
????
- modified immune cells
- act as scavengers
Myelinated cells each have to be what?
Individually taken care of
- uses a lot of energy
Node of Ranvier
Section of unmyelinated axon membrane b/t two Schwann cells
- only space consisting of VG channels
Schwann cell nucleus
Pushed to outside of myelin sheath
Saltatory conduction
AP propagation by “jumping” from node to node in myelinated fibers
- moving along an axon
- normal nerve conduction in myelinated axons
Loss of myelination results in loss of?
AP signal
- causes multiple sclerosis (MS)
Large axons
Offer less resistance to current flow, but occupy space
- 0.8 mm diameter (pencil lead size)
Small myelinated axons
Conduct AP’s as rapidly as large unmyelinated axons
Largest cells in our body are
A-alpha
- part of our somatic nervous sys
Somatic nervous system goes to?
Skeletal muscles
Myelin comes at a cost, but…
Is also a benefit as it is faster
- size & myelin both have effects
Synapse
Anatomically specialized junction b/t 2 cells, where the electrical activity of one cell influences the excitability of another
Two types of synapses
- Electrical (ions)
- Chemical (messenger): neurotransmitter flow b/t a neuron & an adjacent cell
Gap junctions are synapses found in?
Cardiac
Once an AP reaches the end of an axon, how is the signal transmitted to adjacent cells?
- AP travels down axon to terminal bouton
- Depolarization open voltage-gated calcium channels leading to calcium influx
- Calcium influences cytoskeleton & causes fusion of neurotransmitters filled vesicles w/ plasma membrane
- Neurotransmitter (NT) released into synaptic cleft * diffuse across to postsynaptic mem
- NT binds to receptors on postsynaptic mem leading to graded potential
— dealing w/ unexcitable membrane (ligand-gated channels)—> open a channel (NTs)
Fast postsynaptic graded potentials
a. Excitatory Post-Synaptic Potential (EPSP)
b. Inhibitory Post-Synaptic Potential (IPSP)
Excitatory Post-Synaptic Potential (EPSP)
Depolarizing graded potential in postsynaptic neuron
- Na+ pr Ca2+ influx
- Does not necessarily meet threshold
Inhibitory Post-Synaptic Potential (IPSP)
inhibits depolarization in postsynaptic neuron
- makes it hard to depolarize postsynaptic neuron
–hyperpolarization from K+ efflux
–stabilization by Cl- channels opening
Grand Post-Synaptic Potential (GPSP)
- The cumulative change in mem potential
- All EPSPs + IPSPs
Two methods to enhance GPSP to initiate an AP
- Temporal summation
- Spatial summation
Temporal summation
Summation of 2 stimuli from the SAME neuron that follow one another in time
Spatial summation
Summation of graded potentials from several sources
- also close in time, NOT from same neruon
Common neurotransmitters
- Acetylcholine (Ach)
- Biogenic Amines
- AA
- Misc.
Acetylcholine (Ach)
In a class by itself
- An ester & choline
- most abundant NT
Biogenic Amines
AA derivatives
a. Primary catecholamines: Epi, NorEpi, Dopamine (both a hormone & NT, based on the way it travels)
b. Others:
—serotonin is made from tryptophan
—histamine is made from histidine
AA
a. Glutamate: excitatory
b. Asparate
c. GABA
d. Glycine
Misc
a. Neuropeptides (substance P, opioid pep.)
b. Purines (AMP & ATP)
c. Gases (NO & CO)
d. Lipids (Eicosanoids)
All post-synaptic (in NS) work either by:
- Ligand-gated ion channels
- GPCR
Postsynaptic receptors
- Cholinergic
- Adrenergic
Cholinergic
Respond to Ach
a. nicotinic
b. muscarinic
Nicotinic
Receptor acts as Na+ (and K+) channel
- monovalent cationic channel (+1)
- biggest effect is Na+ influx = depolarization = excitation
- found on skeletal muscle (somatic NS)
- found in autonomic PNS
- found in CNS
Muscarinic
Utilizes G-protein & 2nd messenger sys
- mult. subtypes of receptor
- found in autonomic PNS
- found in CNS
(NOT in skeletal muscle)
Adrenergic
Respond to Epi/NE
a. Alpha 1,2
b. Beta 1,2
Alpha 1,2
Utilize 2nd messenger sys. (PL-C & cAMP)
Beta 1,2
Utilize cAMP 2nd messenger sys
Enzymes that degrade NTs
- Ach degraded by: Acetylcholinesterase (Ache)
- Epi/NE degraded by: monoamino oxidase (MAO) & catechol-o-methyltransferase (CoMT)
Enzymes are outside of?
postsynaptic cell
No summation
Two graded potentials will not cause an action potential if they are far apart in time
Summation causing action potential
If two subthreshold potentials arrive at the trigger zone within a short period of time, they may sum and create an action potential
Convergence
Neurons are constantly being influenced/impacted by other neurons
Divergence
Same messenger, but can have diff responses depending on diff receptors
- Our axon can split
- Not limited to one or two
- Colateral = AP goes down both axons
