Neural Conduction and Synaptic Transmission Flashcards
Membrane potential…
…is the difference in electrical charge between the inside and outside of a cell
Recording membrane potential (2)
- Position tip of one electrode inside the neuron and tip of another electrode outside the neuron in extracellular fluid
- Tip must be fine enough to pierce neural membrane without severely damaging it
Microelectrodes are…
… intracellular electrodes
Resting membrane potential (2)
-70 milivolts (mV)
(Potential inside neuron is 70mV less than outside)
Polarized state
Why are resting neurons polarized? (2)
- Salts in neural tissue are separated into positive and negative ions
- Greater concentration of negative ions in extracellular fluid
Random motion (3)
- Results in even distribution of positive and negative ions
- Ions in neural tissue kept in constant motion
- More likely to move from higher to lower concentration gradients (toward equilibrium)
Electrostatic pressure (2)
- Results in even distribution of positive and negative ions
- Like ions repel one another, and therefore cause a more even dispersion of ions.
Types of ions which contribute to the resting potential (4)
Na, K, Cl, various negative ions
Outside a resting ion, there is a higher concentration of…
… Na & Cl
Inside a resting ion, there is a higher concentration of…
… K & negative protein ions (which are synthesized within neuron)
Differential permeability
- Passive property that results in uneven distribution of ions
- K and Cl pass readily through neural membrane
- Na passes with difficulty
- Negative protein ions do not pass at all
- Specialized pores called ion channels allow movement of specific ions
Sodium-Potassium pumps (2)
- Energy consuming mechanisms in the cell membrane
- Continually exchange 3 Na inside for 2 K outside
Transporters are…
… mechanisms in the membrane of a cell that actively transport ions/molecules across the membrane
Neurotransmitters bind to __________ and have 2 effects.
Postsynaptic receptors
- Depolarize receptive membrane by decreasing resting potential
- Hyperpolarize receptive membrane by increasing resting potential
Postsynaptic depolarizations… (2)
- are called excitatory postsynaptic potentials (EPSP’s)
- increase likelyhood that neuron will fire
Postsynaptic hyperpolarizations…
- are called inhibitory postsynaptic potentials (IPSP’s)
- decrease likelyhood that neuron will fire
Graded responses (3)
- excitatory postsynaptic potentials (EPSP’s)
- inhibitory postsynaptic potentials (IPSP’s)
The amplitudes of EPSP’s and IPSP’s are proportional to the signal intensities that elicit them.
Transmission of postsynaptic potentials have 2 important characteristics:
- Rapid (assumed instantaneous)
- EPSP and IPSP transmission is decremental (decrease in amplitude as they trave through neuron)
Threshold of excitation (3)
Sum of depolarizations and hyperpolarizations sufficient enough result in the production of an action potential at the axon hillock
Action potential (2)
- Massive, momentary reversal of membrane potential from -70mV to +50mV
- All or none responses
Integration is…
… the addition or combination of multiple signals into one signal (over space and time)
Spatial summation (3)
Local IPSP’s sum to form greater IPSP
Local EPSP’s sum to form greater EPSP
Local EPSP’s and IPSP’s sum to cancel each other out
Temporal summation (2)
Signals produced in rapid succession at same synapse form a greater signal over time since potentials outlast them
If a synapse is activated before the previous postsynaptic signal has dissipated, the second signal will be superimposed on the lingering signal produced by the first.
How can a subthreshold excitatory stimulus activate neuron firing?
If it is administered twice in rapid succession, temporal summation will allow neuron firing.
Location of a synapse determines the neuron’s potential to fire in 2 ways
- Synapses located near the axon trigger zone better influence firing since IPSP’s and EPSP’s are decremental
- Some neurons have specific mechanisms to apply dendritic signals even if located far from the cell body
Voltage-activated ion channels…
… open and close in response to changes in membrane potential
When membrane potential is reduced to threshold of excitation… (3 phases [3, 2, 1])
-
Rising phase
Na channel opens
Na ions rush in
K channels open
K ions flow out -
Repolarization
K ions continue to flow out
K channels gradually close as resting potential is reached -
Hyperpolarization
Since closing of K channel is gradual, membrane potential dips below resting potential
Absolute refractory period…
… is the 1 to 2 milisecond interval following firing of action potential where it is impossible to fire a second action potential
Relative refractory period (2)
- Period after firing of action potential where it is possible to fire a second action potential, but only with higher-than-normal levels of stimulation
* *(Period where higher stimulation required to fire second action potential)** - At end of period, required stimulation for firing of action potential returns to baseline
Refractory period causes two important characteristics of neural activity:
- Action potentials travel along axons in only one direction
- Rate of neural firing is related to intensity of stimulation
Unlike EPSP’s and IPSP’s, action potentials are… (2)
- nondecremental as they travel along axonal membrane
- conducted slower
Which neural signals are active, and which are passive?
Active: postsynaptic signals (IPSP’s and EPSP’s)
Passive: action potentials
Antidromic conduction
If electrical stimulation applied to terminal end of axon is strong enough, an action potential will be generated which will travel toward the cell body.
Orthodromix conduction
Action potential generated in the natural direction (cell body to terminal buttons)
Myelin is…
… fatty tissue which insulate axon (create myelin sheath)
Nodes of Ranvier are…
… the gaps between adjacent myelin segments
Saltatory conduction
transmission of action potential in myelinated axons
Axons with a larger diameter allow for…
… faster transmission of action potentials
Mammalian motor neurons… (2)
- Synapse on skeletal muscles
- Large and unmyelinated
Interneurons (3)
Neurons without axon or with very short axons
Many do not display action potentials
Conduction is usually passive and decremental
Axodendritic synapses
synapses of axon terminal buttons on dendrites
Axosomatic synapses
synapses of axon terminal buttons on somas (cell bodies)
Dendrodendritic synapses are…
… capable of transmission in both directions
Axoaxonic synapses can…
… mediate presynaptic facilitation and inhibition, which can selectively influence a particular synapse rather than entire presynaptic neuron
Directed synapses
Neurotransmitter release and reception are close in proximity
Nondirected synapses (“string of beads synapses”) (2)
Neurotransmitter release and reception are distanced
Neurotransmitters emitted from varicosities (bulges/swellings) along axon
Small neurotransmitters (3)
- Synthesized in cytoplasm of terminal button
- Packaged in synaptic vesicles by Golgi Complex
- Stored in clusters next to presynaptic membrane
Large neurotransmitters
- Neuropeptides (3 to 36 amino acids)
- Assembled by ribosomes, packaged by Golgi Complex, transported by microtubules to terminal buttons
Coexistence in neurons is…
… the presence of two different neutransmitters
Exocytosis is…
… the process of releasing neurotransmitters
Exocytosis mechanism (3)
- Action potential stimulated opening of Ca channels
- Ca enters and synaptic vesicles bind to membrane
- Vesicles empty contents into synaptic cleft
Neurotransmitters are considered ligands because…
… each one binds to a receptor protein
Receptor subtypes are…
… different types of receptions to which a specific neurotransmitter can bind
Ionotropic receptors are…
… receptors associated with ligand activated ion channels (induces immediate postsynaptic signal)
Metabotropic receptors are…
… receptors associated with signal proteins (inside cell) and G proteins (outside cell)
G protein importance (3)
- When a neurotransmitter binds to a metabotropic receptor, a subunit of G protein dissociates
- This subunit may bind to a nearby ion channel or
- may trigger synthesis of “second messenger” which may influence neuron activity and bind to DNA to influence genetic expression)
Autoreceptors (3)
metabotropic receptors that have two unconventional characteristics:
- Bind to own neuron’s neurotransmitter
- Monitors amount of neurotransmitters in synapse and controls release levels
Reuptake
Majority of neurotransmitters immediately drawn back into buttons by transporter mechanism
Enzymatic degredation
Breakdown of neurotransmitters via enzymatic activity
4 common amino-acid neurotransmitters
- Glutamine
- Aspartate
- Glycine
- Gamma-aminobutyric acid (GABA)
4 monoamine neurotransmitters
- Dopamine
- Epinephrine
- Norepinephrine
- Serotonin
3 catecholamines (monoamines) (4)
- Dopamine
- Epinephrine
- Norepinephrine
Synthesized from tyrosine
Acetylcholine (3)
- Small
- Acetyl + choline
- Enzyme: acetylcholinesterase
- Neurons that releace ACh are cholinergic
Soluble-gas neurotransmitters (3)
- e.g. nitric oxide and carbon monoxide
- passes through membranes since lipid soluble
- stimulate production of second messengers
Endocannabinoids
e.g. anandamine
What is the maximum velocity of conduction in human neurons?
60m/s
Indolamines
synthesized from tryptophan
e.g. serotonin
