Lecture 4: Neurotransmission 1 - Electrical Signalling in Neurons Flashcards
(18 cards)
Who and how was it discovered that nerves made muscles move?
Lucia and Luigi galvani found that electricity makes frogs’ legs muscles contract (they connected a wire to the nerves and a wire to the muscle) and they decided that ‘animal electricity’ was present in the nerve
Who and how were nerves found to move muscles in humans?
Giovanni Aldini (Galvani’s nephew) who found that electricity makes criminals’ corpses twitch.
What famous theory came from early learning about nerves and muscle movement?
Frankenstein by Mary Shelly
What are electrical currents
Electrical currents are flows of charged particles (electrons). The negatively charged electrons repel each other and move towards the positive pole. Like charges repel and opposite charges attract.
Current only flow through materials that conduct electricity. Flow will carry if there is this movement of electrons.
What is voltage?
A measure of how much potential there is for charge to move - how much stored electrical energy (like water pressure).
How can we quantify electricity/current
Using Ohm’s law
current (charge per second/amps) = potential (volts) x conductance (the higher the conductance the higher the current)
or
current = potential/resistance (the higher the resistance the lower the current)
Who and how was it found that conductance in nerves is different than in wires
Hermann von Helmholtz measured speed of nerve conduction by stimulating frog sciatic nerve and measuring time to constrict muscle.
Nerve conduction ~ 30-40 m/s, 1 million times slower than electricity flows down a wire.
So it was confirmed that conduction in nerves was not electrical current it was something different.
How does current flow down an axon in nerves and what is it called
Action potential - wave of positive charge moving into the membrane a.k.a. wave of transient depolarisation that travels down the axon.
Current flows across the membrane in one place and this flow across the membrane happens at adjacent bits of the axon.
Fast compared to chemical signals.
How do cells signal electrically
- Movement of ions
- Electrically charged particle
e.g. sodium chloride = Na+ and Cl-
all different sizes
How does a resting membrane potential work? (setting the neuron up to be ready for sending electrical signals)
Phospholipid bilayer in membrane where water soluble things can’t pass through. This means there are concentration gradients outside (Na+, Cl- and Ca2+) and inside the nerve cell (negative proteins and potassium). The membranes have ion channels that only let potassium ions through the membrane. Now the inside is more negative (difference of -70mV), potassium then wants to go back due to conc gradient in and therefore reach a balance in flow = equilibrium potential
Then sodium channel opens and sodium moves in creating a more positive environment repelling the positive sodium ions from coming in. Chloride then flows into the cell and eventually there is no net flow.
What is an equilibrium potential/electrochemical equilibrium and how is it dictated
Potential across membrane at which there is no net flow of an ion - dictated by concentration difference and ion charge. So it is positive on the outside and negative on the inside as an balanced equilibrium. This was most ions need an ion channel to cross the membrane. Electro gradient and concentration gradient are equal and opposites
The Ek+ = -80mV
ENa+ = +62mV
EcL- = -65mV
How is the membrane potential of a nerve cell set?
It’s set by electrochemical gradient and permeability of membrane to different ions.
If membrane only permeable to potassium the Em=Ek+ = -80
but Em~ -70mV if membrane is slightly permeable to sodium too Ena+ = +62mV as it increases the positivity of the inside from -80 to -70.
The resting membrane potential of neurons is near to the equilibrium potential for potassium.
How does the membrane make sure it has the right amount of positive and negative ions inside and out during rest?
They have a sodium-potassium pump Na+/K+ ATPase pump which maintains ion gradients how they should be for a nerve to work - it pumps 3 sodium ions out the cell and 2 potassium ions inside the cell. This pump requires ATP for energy and this is the most energy consuming process/protein in the brain (~20-40% energy use)
How are electrical signals measured
With a voltmeter
How do ion channels change membrane permeability
There are ‘holes’/channels that allow ions to enter and leave the cell which are selective for different ions through passive diffusion.
Some can be open all the time like K+ leak channels which set the resting membrane potential as potassium leaves the cell.
Others are opened by different stimuli e.g. a change in voltage, binding specific molecules.
Sodium channel opens and lets sodium in once the inside is more positive (from -70mV to -55mV) and then voltage gated potassium channels open and let potassium in once the inside is very positive (from -55mV to 30mV)
How is the action gradient generated
By opening and closing ion channels as there is a wave of depolarisation of the cell’s membrane which conveys a fast signal from one place to another in the body.
It is generated by changes in membrane permeability due to opening and closing of voltage gated ion channels.
It is self-generated and occurs if a threshold membrane potential is achieved in the axon initial segment (axon hillock), which transiently opens voltage gated sodium channels.
Describe the 3 main events of action potential
1) Threshold potential reached
2) Depolarisation due to opening of sodium channels.
3) Repolarisation due to inactivation of sodium channels and opening of voltage-gated potassium channels.
Define electrochemical gradient
At rest, there is an ionic imbalance where there are more positively charged ions (sodium) outside the cell and more potassium ions inside the cell (less positively charged ions inside). There is an electrical and a chemical imbalance.
