L2 electric brain Flashcards
Name the atoms. (H, O, C, N, P, Na, K, Ca, Cl, Mg)
Hydrogen, Oxygen, Carbon, Nitrogen, Phosphorus, Sodium, Potassium, Calcium, Chlorine, Magnesium
Recipe for neurons.
Water (H2O), Salt (NaCl), Fat, Protein
Neurons are filled with… and bathed in…
intracellular fluid (cytosol, water); extracellular fluid (water) p.s. both fluids are salty water
Why water molecules stick together?
Electrostatic attraction and hydrogen bonds, p.s. water is a polarized molecule
What push the charged ions?
Brownian motion and diffusion
Which side has higher chance to be pushed to?
Left than right
Average net current will be close to…? Unless…
Zero. Unless the movement of ions are organised
What are cell membranes made of?
Water-impermeable sheets of phospholipids (fat)
Can electricity flow in and out of cell membranes?
No
What is the basis of electrical currents in and around nerve cells?
The movement of electrically charged ions that are dissolved in water
When can electric currents flow through the membranes?
Through pores (channels)
What are proteins?
Chains of amino acids
How many amino acids are used to make all proteins?
20
Types of amino acids.
Fat soluble (lipophilic), water soluble (hydrophilic), charged and uncharged
Binding amino acids can form… (short chain, long chain)
Peptides (short chain), proteins (long chains)
What kinds of useful building blocks are formed when proteins fold?
Trans-membrane channels, enzymes, structural proteins
What do enzymes do?
Catalyse all sorts of biochemical reactions within the cell
What do trans-membrane channels do?
Regulate the movement of ions and other substances through the cell membrane
What do receptors do?
Sense the presence/ absence of certain substances in the fluids outside the cell membrane
What do structural proteins do?
Act like scaffolding and determine the cells shape
What do sodium-potassium pump (k+ leakage channels) do?
Actively transports Na+ out of the cell and K+ into the cell
What powers the sodium-potassium pump?
Energy provided by ATP
What powers the ion exchangers?
Concentration gradients
Name 2 ion exchangers.
Na+ (to in) - Ca++ (to out) exchanger
Bicarbonate (to in) - Cl- (to out) exchanger
Which ion has the highest concentration inside a neuron?
K+
Which ion has the lowest concentration inside a neuron?
Ca++
Which ion has the highest concentration outside a neuron?
Na+
Which ion has the lowest concentration outside a neuron?
Ca++
Na+ & Cl- concentrations are … (higher/lower)… (outside/inside) the neuron.
Higher, outside
Which 2 concentrations are higher inside the neuron?
K+ & A-
Neurons keep what concentrations very low?
Intracellular Ca++
Neurons are in …
electrochemical equilibrium
What is the resting membrane potential? (voltage)
-70mV
If the membrane is 10 nanometers (bilionths of a meter) thick, the electric fields strength in the membrane is..,?
ca 7 million volts/ meter
Describe electrochemical equilibrium.
Ions diffuse through selective channels in a membrane.
Their partners of opposite charge are left behind. An electrical gradient is set up across the membrane. Further diffusion is opposed by the electrical gradient.
Strong electrostatic forces will lead to…
a redistribution of a modest amount of ions can give rise to sizeable potentials (voltages)
The amount of charge (number of ions) that needs to be moved to set up a particular voltage depends on…
The membrane capacitance C=Q/V
Electrochemical equilibrium is reached after…
Tiny changes in relative ion concentrations
Which equation predicts the equilibrium potential?
Nernst Equation
What is equilibrium potential?
Voltage large enough to stop net charge movement by diffusion (no net charge)
Which equation is the extension of Nernst Equation? And what does it consider?
Goldman Equation. Considers several ion species, and ‘weights’ the contribution of each ion by its respective permeability.
Whether the internal concentration appears in the numerator/ denominator depends on?
The sign of the charge of the ion
Gated ion channels open transiently when…
Certain stimuli or chemical signals are present
When gated channels open, voltage across the membrane will change to…
Reflect the new permeabilities as predicted by the Goldman equation
What can be encoded as a change in membrane potential?
Presence of physical/ chemical signals which are capable of opening the gated channels
Why ions flow easily along the membrane but not across it?
Membrane resistance is higher than that of intracellular and extracellular fluid
What needs to be done to change the potential on a distant patch of membrane?
Enough current has to flow to discharge the membrane capacitance at that point
What will happen when some of the current leak through the membrane?
Passively conducted signals decay after relatively short distances (small space constant)
What is the difference between channel leakage and leakage?
Capacitative leakage doesn’t require channels for leakage
What does the voltage gated ion channels do when the membrane is at rest?
Closed
What will happen to the voltage gated ion channels when the membrane depolarises to a certain threshold?
Open briefly (ca 0.5 ms)
Once the voltage gated ion channels open, what will happen next?
It will close rapidly again and remains inactivated (refractory) for another 0.5 ms
Describe the positive feedback process of action potentials.
Depolarisation to threshold opens a few Na+ channels, which allows further Na+ influx, causing further depolarisation, which spreads passively down the axon allowing further Na+ channels to open. This process continues until all voltage gated Na+ channels in the local patch of membrane have been through the open state and are inactivated (refractory).
What is the advantage of positive feedback process?
The feedback current injection allows action potentials to travel along axons for considerable distances without loss of signal. Fresh Na+ currents will make up for leakage
What is the disadvantage of positive feedback process?
Action potentials are ‘all or nothing’. They cannot transmit information by their amplitude, so graded voltage signals are no longer possible. Therefore, spike codes have to employ other coding strategies, relying purely on the rate or timing of action potentials.
Who published the first intracellular recording of an action potential?
Hodgkin and Huxley
Describe the phases of an action potential.
Stable resting potential
Rapid rising phase
Rapid falling phase
Prolonged undershoot (1 ms)
What needs to be noted about the overshoot?
The action potential is positive
Which is slower: K+ rectifier channels/ Na+ channels?
K+ rectifier channels
What is the role of K+ rectifier channels after hyper-polarisation (Undershoot)?
Speed up the re-polarisation of the membrane following the Na+ action potential
What is advantage of performed experiment on giant axons of squid?
The axons are large enough to allow axoplasm to be replaced by fluids of known ionic composition
If injected current doesn’t depolarise the membrane to threshold, what will happen?
No action potentials will be generated
When will action potentials be generated?
When the injected current depolarises beyond the threshold
What is the relationship of action potential firing rate and depolarising current?
Positively related. (increasing ap firing rate will increase depolarising current)
If small, sub-threshold currents are injected into a nerve cell, what will happen?
They don’t do anything, they just leak out
What is rate coding?
Measuring the number of spikes that occur during a set period of time
What is the relationship between strength of action potentials and action potential rate?
Positively related. (The stronger the currents, the faster the action potential rate)
How does ap work as a digital code?
An axon connected to a muscle stretch receptor signals the degree of stretch by the temporal pattern of action potential. Changes in stretch cause a change in the rate of action potentials
What is duration of each ap?
ca 1 ms, so they are all roughly of the same height
When the Na+ channels are deactivated in the refractory period, what will happen?
During the absolute refractory period no action potentials can be generated
How long does the refractory period lasts?
ca 1 ms
No neurons can fire at firing rate higher than…?
ca 1000 Hz
Why neurons rarely sustain firing rates of a few hundred Hz for any length of time?
Because most neurons show some degree of adaptation
What kind of firing mode is an exception in the brain?
Regular firing in response to a steady state current input
What will most neurons usually do?
Show adaptation (decrease in firing rate), some neurons fire bursts
Neurons change from regular to burst mode, or vice versa depend on…?
Animal’s state of arousal or attention
What affects the firing modes?
Effect of other voltage gated channels
What is the process of ‘bursting’?
Active processes
What do the neuron effects imply about the relationship between input current and output firing rate?
These two can differ from a simple, linear proportionality
What are myelin sheathes?
Specialised extensions of the cell membranes of glial cells (Schwann cells and oligodendrocytes), which tightly wrap around axons.
What is the use of myelin sheath?
Provide additional isolation for the axon and thereby allows action potentials to travel faster
What is the disease of myelin sheath around axons becomes degraded? and what does it do?
Multiple Sclerosis. Reduce conduction velocity, but as the disease progresses, action potential conduction will be slowed and becomes unreliable and finally fails completely. This will blur the vision and numbness to muscle weakness or severe paralysis.
Why the brain’s ‘white matter’ is white?
High myelin content
What partly explains the neurological immaturity of human infants?
Myelination of axons in the brain of human infants is incomplete at birth
Myelin is an invention of…
Vertebrate nervous systems
p.s. so small squid needs large axons
What do moving ions from dissolved salts carry in neurons?
Electrical currents
What can create voltages across the cell membrane?
Differences in ion concentrations between inside and outside of neurons
What does the opening and closing chemically or mechanically gated ion channels in cell membranes change?
The permeability which changes the membrane voltage.
p.s. this can encode information
What does current leakage means?
Neurons can’t conduct electrical signals passively over distances greater than 1mm
What does nerve cells use to send signals along potentially very long axons?
Action potentials which propagate through the help of voltage gated ion channels
Advantages of myelin sheaths on axon to ap propagation.
Faster and more reliable
