NEUROPHYSIOLOGY OF NEURAL SIGNALS Flashcards
Membrane potential
The electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell.
What are the 2 basic electric potentials that axons have?
- Resting membrane potential: The membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials.
- Action potential: The brief electrical impulse that provides the basis for conduction of information along an axon.
Action potential is the same all the time no big no small it’s a ALL OR NONE
the resting potential
When we insert our electrode into the axon, we get a straight line
(usually –60,-70)
How the membrane potential can change?
a. Hyperpolarization: An increase in the membrane potential of a cell, relative to the normal resting potential. drive it further away from zero)
Larger the hyperpolarization stimuli the larger the response the farther down it’ll go
b. Depolarization: Reduction (toward zero) of the membrane potential of a cell from its normal resting potential.Only depolarization causes action potential
Threshold of excitation
: The value of the membrane potential that must be reached to produce an action potential.
change that has to occur in order to trigger an action potential)
Once the response reached the threshold a series of events causes an action potential (very strong depolarizing spike that not only drives response toward 0 but overshoots it)(changing the inside and outside cell charge temporarily)
What are the values of Resting potential, Threshold ≈Action Potential
- Resting potential ≈ -70 mV
- Threshold ≈ -55 mV
Action Potential ≈ +40 mV and Hyperpolarization ≈ -90 mV
Takes approximately 2 msec!
Diffusion gradient
Movement of molecules from a region of high
concentration to regions of low concentration.
(e.g. sugar dissolving in water)
Electrostatic gradient
Molecules can carry charge (ions)
Cations (+ charge)
Anions (- charge)
Move towards areas of unlike charge
-THERE IS A GRADIENT BC OPPOSITES ATTRACT-
Dynamic equilibrium
Gradients can balance one another.
Intracellular fluid
The fluid contained within a cell
Extracellular fluid
Body fluids located outside the cell.
membrane equilibrium
certain ions inside and outside the cell
dynamic equilibrium
dynamic equilibrium maintained by sodium-potassium pump.
- Mechanical pump that exchanges sodium for K+, brings sodium out K= back inside
- Ex of an active transport- against concentration gradient
- electrostatic pressure and diffusion work to get Na+ inside the cell.
the sodium-potassium pump pushes it outside again to maintain equilibrium.
What happens inside of the cell? membrane equ’
The ones labeled A- are large proteins that can’t leave the cell and contribute to making inside the cell more negative
K+ diffuses from inside to outside (high to low concentration) the electrostatic pressure pushes it inside because K is + charged (the 2 forces are reaching equ bc they are opposing each other) , K+ can generally free flow in and out bc the 2 pressures are in equ
What happens outside of the cell? membrane equ’
Cl is negative so the electrostatic pressure pushes it outside and the diffusion gradient is pushing it inside bc there’s more outside than inside (dynamic equ= the 2 forces are working against each other keeping things balanced)
Na; more outside then inside so diffusion pushes it inside and electrostatic pressure is also pushing it inside the cell bc sodium is + so its being attracted to the more negative inside
But sodium can’t pass thru the lipid layer and it needs to wait for a sodium channel to open, which is the start of an action potential
Describe the sodium-potassium pump
- A- ions & K+ ions have higher concentration inside axon relative to outside…
- Cl- ions & Na+ ions are more concentrated outside the axon
- Na+ channels are ordinarily closed to prevent entry of Na+
- Na+/K+ pump exchanges 3 Na+ for 2 K+. The high concentration of extracellular Na+ is due to this pump. 10x as much Na+ is outside the cell as inside, contributing to the membrane’s RP of -70
- K+ is free to ENTER & LEAVE the cell but Na+ CANNOT reenter once pumped out
Describe how an action potential occurs
As soon as you make membrane potential more positive (cross threshold), sodium channels open sodium rushes in (everything is trying to push sodium into cell), large spike of action potential is a result of sodium being positive, temporarily making inside of the cell more + than outside, once you reach peak there is a refractory period when Na channels temporarily close, and K+ channel opens and rushes out which drive inside of the cell more negative and get to hyperpolarization, when K+ channels close sodium potassium pump starts exchanging to restore resting membrane potential (AT REST INSIDE IS MORE – BUT DURING ACTION POTENTIAL OUTSIDE IS MORE-)
- Every neuron has a resting charge or resting potential (-70 mv)
- Maintained by sodium potassium pump- continually pumping Na+ out and K+ in
- When an ion channel opens Na+ rushes into the cell and K+ goes out changing the potential
- With enough stimulation of this kind the resting potential passes a threshold (-55 mv) and the cell fires
- Thise reverses the polarity of the cell for a brief period- known as the cells action potential
- This is all generated at the axon hillock
- Sodium channels only open briefly and then cannot open for some period of time (absolute refractory period)
What do pufferfish contain?
tetrodotoxin – sodium channel blocker prevents action potentials and the person suffocates
Laws of conduction
all-or-none – once triggered an action potential can’t be stopped.
variable information, representing the strength of a response to a stimulus (or the strength of a command to act), is conveyed by firing rate.(strong stimulus= very fast firing rate)
How does an action potential propagate along the axon?
- Axon Hillock - where the action potential begins.
- Terminal Buttons - the end point for the action potential.
- Action potential flows toward the terminal.
(does not reverse direction because area where the action potential came from is still in refractory.)
Describe the conduction of action potentials in unmyelinated axons
Sodium channels are available to be open all the way down the axon
Action potential happening, Na comes in then refractory period neighboring area start to depolarize etc and move down the end feet
Describe the Conduction of action potentials in myelinated neurons
• faster, cheaper
- Nodes of Ranvier have sodium channels that can be opened but the axon that have myelin, the myelin covers those potential channel openings
- Na + channels open up generating an action potential at the nodes of Ranvier
- The action potential propagates passively through the myelinated axon and when it reaches another node another action potential is triggered and continues from node to node as fast at 150 m/s up to 15 times faster than an unmyelinated axon
Described as jumping from node of Ranvier to node of Ranvier
cable properties
signal degrades as it travels along the axon
A stimulus will weaken over time as it passively travels over the axon
Saltatory conduction
Conduction of action potentials by myelinated axons. The action potential appears to jump from one Node of Ranvier to the next.
rejuvenated at the nodes of Ranvier- signal is strong enough even though its passively making its way through the myelinated axon that it can rise above the threshold and trigger a new action potential
What are the advantages of myelin?
conservation of energy & speed of conduction
How are the nodes of ranvier spaced?
Spacing of the nodes is optimized according to axon length and diameter.
What causes demyelination? And what does it result in
Multiple Sclerosis- less white matter areas
degeneration of myelin sheaths (plaques) or breakdown of myelin bc of Phagocytosis – macrophages gone wild – immune system glitch or
Apoptosis – programmed cell death
What are consequences of demyelination?
affects transmission of nerve impulse (myelin is perfectly spaced so signals can travel from node of Ranvier to node of Ranvier and be regenerated but degeneration because of MS causes much more noise to the system, traveling like an unmyelinated axon but even more disturbing)
What are the symptoms of MS?
Different symptoms that depend on area affected
visual disturbances (particularly colour vision or tunnel vision)
nystagmus- (twitching of the eyes)
loss of sensation
loss of motor control
What is Multiple Sclerosis?
autoimmune disease of the CNS
synaptic cleft
between the presynaptic membrane and postsynaptic membrane
post-synaptic potentials
the released neurotransmitter leads to post-synaptic potentials hyperpolarization or depolarization, make it harder or easier for the neighboring neuron to reach threshold ) that alter the firing rate of the receiving neuron (decrease or increase chance of neuron firing).- can be excitatory or inhibitory
Neurotransmitters
chemical substance released from the end of a neuron during the propagation of a nerve impulse; it relays information from one neuron to another.
has to be released bc of an action potential happening)
Neuromodulators
secreted in larger amounts and diffuse further (composed of peptides).
Hormones
produced in endocrine glands – released into extracellular fluid to be taken up by specific target cells.’
How do neurotransmitters bind to receptors?
only specific neurotransmitters will bind with specific receptor sites – like a key in a lock.
Only specific neurotransmitters will bind with the post-synaptic membrane.(will open a channel or keep it close, depolarize or hyperpolarize the stimulus)
ligand
chemical that attaches to a binding site
neurotransmitters are naturally produced ligands
neurotoxins are also ligands and various drugs have their effect in the same manner – artificially produced ligands (e.g., LSD).
Axodendritic
synapse on the dendrite of the neuron
Axosomatic
on the soma
-terminal button is connecting to some
Axoaxonic
on the axon
- terminal button forming a connection with the terminal button of another neuron
Receptors
neurotransmitter specific postsynaptic receptors
open to allow ions to flow into the postsynaptic neuron
What are the 2 main types of receptors?
ionotropic
Metabotropic
Ionotropic receptors
receptor site has its own ion channel.
contain sodium channels.
fast acting and short lasting.
Metabotropic receptors
- indirect method.
- molecules bind with receptors
- located nearby G-proteins.
- G-proteins in turn activate an ion channel.
- slower to begin and longer lasting.
- G-proteins can also activate second messengers – enzymes that in turn activate an ion channel.
Excitatory or inhibitory post-synaptic potentials
once neurotransmitters are bound to the post synaptic membrane the electrical charge is now altered in the receiving neuron.
the change in the electric charge can be more positive than the resting potential (excitatory) or more negative than the resting potential (inhibitory).
Describe the characteristics of a post-synaptic potentials
determined by the ion channel opened by the neurotransmitter and not the transmitter itself.
graded – the potential dissipates with distance traveled. (changes in membrane potential that vary in size, as opposed to being all-or-none, the variable-strength signals that can be transmitted over short distances whereas action potentials are large depolarizations that can be transmitted over long distances.)
smaller in magnitude than action potentials.
action potentials are always excitatory – post-synaptic potentials can be either excitatory or inhibitory
excitatory PSP
typically related to sodium ion channels (rush of Na+ into the cell makes it more positively charged).
inhibitory PSP
typically related to potassium ion channels (extra K+ maintained inside cell by sodium-potassium pump leaks out making the cell more negatively charged).
- cl channels open making it more negative
action of Cl– channels depends on the state of the receiving neuron
–if depolarised Cl– will bring the cell back to a resting state
reuptake
rapid removal of neurotransmitter from the synaptic cleft.
SSRIs
(selective seratonin reuptake inhibitors – e.g, Prozac) prolong the PSP by inhibiting reuptake.
Summation of post- synaptic potentials
whether the PSP leads to the excitation or inhibition of the neuron depends on the combined effects of many PSPs.
Spatial integration
: equal excitatory and inhibitory input will cause no change
Temporal integration
ripples can combine to make bigger ripples
both subthreshold EPSPs occurred at the same time, however, they could sum, or add up, to bring the membrane potential to threshold.
Autoreceptors
respond to neurotransmitters they produce.
regulate synthesis and release of other transmitters.
metabotropic
usually inhibitory – may control amount of neurotransmitter released
Why do you need to know all this?
different disease processes involve different aspects of the basic electrochemical transmission of neural information.
Parkinson’s Disease – dopamine deficiency.
Multiple Sclerosis – affects the myelin sheath of white matter.
Epilepsy – abnormal electrical stimulation.
Alzheimer’s Disease – neurofibrillary tangles may affect the transport of neurotransmitters.
Describe the direction of travel of AP
a. In response to a signal, the soma end of the axon becomes depolarized
b. The depolarization spreads down the axon. Meanwhile, the first part of the membrane repolarizes. Because Na+ channels are inactivated & additional K+ channels have opened the membrane cannot depolarize again
c. The AP continues to travel down the axon
What would an image of a patients brain with Multiple Sclerosis look like?
less white matter areas
Name 3 neurochemicals
- Neurotransmitters
- Hormones
- Neuromodulators
Describe how the signal goes from from electrical to chemical
- electrical charge causes calcium channels to open on the terminal button which causes calcium to come into the cell which causes the vesicles filled with neurotransmitters to bind to the presynaptic membrane and release the neurotransmitter into the cleft
- axon terminal contains synaptic vesicles
- Vesicles release neurotransmitters across synaptic cleft
- Neurotransmitter binds to receptors on post synaptic membrane- basis for - neural
- the released neurotransmitter leads to post-synaptic potentials
