Lecture 3 - Neurophysiology: Neural Signals Flashcards
Membrane potential
The electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell.
Inside the cell is usually more ____ charged than outside the cell, especially at rest
(-)ly
Axons have two basic electrical potentials:
- Resting membrane potential
2. Action potential
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.
How do we know if it is a squid axon?
- huge
- visible by the naked eye
- often used in petri dishs
When we insert our electrode into the axon, we get…
a straight line — the resting potential
How the membrane potential can change:
- Hyperpolarization
- Depolarization
- Threshold of excitation
Hyperpolarization
An increase in the membrane potential of a cell, relative to the normal resting potential. (further from 0)
Depolarization
Reduction (toward zero) of the membrane potential of a cell from its normal resting potential.
Threshold of excitation
The value of the membrane potential that must be reached to produce an action potential.
Changing membrane potential is…
creating unrest!
Neurons – resting and action potentials.
- Resting potential ≈ -70 mV
- Threshold ≈ -55 mV
- Action Potential ≈ +40 mV and Hyperpolarization ≈ -90 mV
Takes approximately 2 msec!
How long does an AP take?
~ 2 msec
AP’s are…
All-or-none & the same everytime
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 (opposites attract)
Dynamic equilibrium
Gradients can balance one another.
Where are these ions?
- Intracellular fluid
- Extracellular fluid
Intracellular fluid
The fluid contained within a cell.
Extracellular fluid
Body fluids located outside the cell.
Membrane Equilibrium
when there’s certains ions inside & outside the cell
A– = protein K+ = potassium Na+ = sodium Cl– = chloride
What is happening inside of cell?
- anions cannot leave cell
K+ is achieving dynamic equilibrium as the force of diffusion and electrostatic pressure balance each other out
- normally can free float inside & outside the cell b/c those 2 pressures are working against each other forming an equil.
What is happening outside of cell?
- low concentration of K+
High concentration of:
- Cl- is achieving dynamic equilibrium as force of diffusion and electrostatic pressure balance each other out
- Na+ is being forced into the cell as force of diffusion and electrostatic pressure are both pushing that direction
- but Na+ has to wait for a Na+ channel to open & the opening of that channel is the basis & start of an A
Dynamic equilibrium
- dynamic equilibrium maintained by sodium-potassium pump. (imp. for cleaning up after an AP)
- electrostatic pressure and diffusion work to get Na+ inside the cell. (so you need a mechanical pump)
- the sodium-potassium pump pushes it outside again to maintain equilibrium.
Describe the sodium-potassium pump
- A- ions & K+ ions have HIGHER concentration INSIDE axon relative to outside…
- whereas 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 Action Potentials -70mV
- every neuron has a resting charge or RESTING POTENTIAL ~=-70mV
- maintained by SODIUM-POTASSIUM PUMP - continually pumping Na+ out & K+ in
- when an ion channel opens Na+ rushes into the cell & K+ goes out changing the potential
- with enough stimulation of this kind the resting potential passes a threshold (~= -55 mV) & the cell ‘FIRES’
- this reverses the polarity of the cell for a brief period - known as the cell’s ‘ACTION POTENTIAL’
- this is all generated at the AXON HILLOCK
- sodium channels only open briefly & then cannot open for some period of time (absolute refractory period)
The pufferfish as a neuroscientific weapon
contains TETRODOTOXIN – sodium channel blocker prevents action potentials
Fugu - Japanese preparation of a pufferfish that has to be prepared carefully
“About three o’clock in the morning we found ourselves seized with an extraordinary weakness and numbness all over our limbs. I had almost lost the sense of feeling; nor could I distinguish between light and heavy bodies of such as I had strength to move, a quart pot full of water and a feather being the same in my hand….”
Captain James Cook
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.
Increased firing rate…
produces AP’s faster
Axon Hillock
where the action potential begins.
Terminal Buttons
the end point for the action potential.
What direction do AP’s flow? & can AP’s reverse?
Action potential flows toward the terminal.
- does not reverse direction because area where the action potential came from is still in refractory.
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
Conduction of action potentials in unmyelinated axons
- have Na+ channels available to be open all the way down an axon
- you’ll get an AP happening, you’ll get Na+ coming into the cell in the localized area that’ll go into refractory period
- you’ll get a neighbouring area start to get depol., Na+ will come rushing in & will go into refractory period
- as so on as you move you’re way down towards the endfeet
Conduction of action potentials in myelinated neurons
- faster, cheaper
- Na+ channels open, generating an AP
- Depolarization spreads within the axon very rapidly, like electricity through a wire
- The AP is triggered at the new node &…
- continues from node to node as fast as 150 m/s, up to 15 times faster than in unmyelinated axons
Cable properties
signal degrades as it travels along 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
Myelin confers two main advantages:
conservation of energy & speed of conduction
Action potentials GENERATED (or re-generated) at the _________
Nodes of Ranvier
Spacing of the nodes is optimized according…
to axon length and diameter.
What is a consequence of demyelination?
Multiple Sclerosis
What are possible causes of M.S./consequences of demyelination?
- degeneration of myelin sheaths (plaques). (mutliple abnormal white areas that interfers with myelin)
- PHAGOCYTOSIS – macrophages gone wild – immune system glitch
- APOPTOSIS – programmed cell death
In patients with M.S, white matter looks LESS white. Why?
b/c of degradation of myelin
M.S. affects…
transmission of nerve
What are the different symptoms that depend on area affected, for M.S.?
- visual disturbances (particularly colour vision)
- nystagmus (twitching of the eyes - eyes moving from side to side)
- loss of sensation
- loss of motor control
- normally end up wheelchair bound
- sometimes ends up with tunnel vision
Multiple Sclerosis definition
autoimmune disease of the CNS (brain & spinal cord)
What is included in the structure of a synapse?
- presynaptic membrane
- postsynaptic membrane
- synaptic cleft
Describe neurons from electrical to chemical
- vesicles release NEUROTRANSMITTERS across the SYNAPTIC CLEFT
- the released neurotransmitter leads to POST-SYNAPTIC POTENTIALS (HYPERPOLARIZATION or DEPOLARIZATION) that alter the firing rate of the receiving neuron (DECREASE or INCREASE)
- axon terminal contains SYNAPTIC VESICLES
When you touch something hot…
SN –> interneurons in CNS –> MN (skeletal muscles in arm causing them to contract & pull arm away)
Synaptic vesicles fusing with the…
presynaptic membrane
What are the 3 neurochemicals?
- Neurotransmitters
- Neuromodulators
- Hormones
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.
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.
Binding
- only specific neurotransmitters will bind with specific receptor sites – like a key in a lock.
- chemical that attaches to a binding site is a LIGAND.
- 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).
Only _____ neurotransmitters will bind with the post-synaptic membrane. & explain how.
SPECIFIC
- transmitter binds to the binding site
- the pore opens, allowing the influx or efflux of ions
What are the 3 binding sites?
- Axodendritic
- Axosomatic
- Axoaxonic
Axodendritic
synapse on the dendrite of the neuron
Axosomatic
on the soma
Axoaxonic
on the axon
Receptors
- neurotransmitter specific postsynaptic receptors
- open to allow ions to flow into the postsynaptic neuron
What are 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.
- 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).
Post-synaptic potentials
- determined by the ion channel opened by the neurotransmitter and not the transmitter itself.
- graded – the potential dissipates with distance traveled.
- 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).
Action of Cl– channels depends on…
- if depolarised…
- the state of the receiving neuron
- Cl– will bring the cell back to a resting state.
Describe Terminating the PSP
- 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.
What are the 2 kinds of Neural integration?
- Spatial integration
- Temporal integration
Spatial integration
equal excitatory and inhibitory input will cause no change
- inhibition counteracts excitation; no action
Temporal integration
ripples can combine to make bigger ripples
Autoreceptors
- 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
- Multiple Sclerosis
- Epilepsy
- Alzheimer’s Disease
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.
For a neuron at rest, the interior of the cell
is negatively charged relative to the outside.
The ________is defined as the difference in electrical charge between the inside and the outside of an undisturbed axon membrane
C) resting potential
Movement of the axon membrane potential from -70 mV to -90 mV would be termed a(n)
D) hyperpolarization
Neuromodulators
D) are secreted from neurons, but dispersed widely in the brain.
Most ________ are secreted into the extracellular fluid from endocrine glands or tissues.
D) hormones
Which of the following will produce an EPSP?
A) opening a sodium channel
