Neurotransmission - electrical signalling in neurons Flashcards
how do nerves make muscles move
frogs
Lucia and Luigi Galvani, 1781: electricity makes frogs legs muscles contract
- they decided that ‘animal electricity’ was present in the nerve
how do nerves make muscles move
humans
Giovanni Aldini (Galvani’s nephew), 1802: Electricity makes criminals’ corpses twitch
what is electricity
- Electrical currents are flows of charged particles (here electrons).
- Like charges repel, opposite charges attract.
- Currents only flow through materials that conduct electricity.
- Voltage is a measure of how much potential there is for charge to move –how much stored electrical energy (like water pressure).
electricity - Ohm’s law
multiply
current = potential x conductance
electricty - Ohm’s law
divide
current = potential / resistance
measuremnt for current and potential
current = charge per second/Amps
potential = volts
Conduction in nerves different to in wires
- Hermann von Helmholtz (1849) – 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.
how fast is nerve conduction
30-40 m/s
current flow do an axon
- known as action potential
1. current flowing across the membrane in one place
2. this flow across the membrane happening at adjacent bits of the axon
how do cells signal electrically?
- Movement of ions
- Electrically charged particle
- E.g. sodium chloride = Na+ and Cl-
- Different sizes
- Some ion flux (flow) happens at rest – this sets the neuron up to beready to send an electrical signal
- Some ion flux happens during signalling
- First, let’s consider the situation at rest – the resting membrane potential…
cell membrane
- cells are surrounded by a lipid membrane
- ## water soluble things can’t pass through
concentration gradient
process of action potention 1
-
Outside
- Na+
- Cl-
- bit of Ca2+
-
Inside
- Proteins (-ve)
- K+
ion channels [holes in the membrane]
process of action potention 2
- ions can’t get across membrane without channels
- leak potassium channels → always open
potassium conc gradient
process of action potention 3
- potassium channel allows some potassium ions to leave the cell
- positively charged ions left cell
electrical gradient
process of action potention 4
- now the inside is negative relative to the outside
- stops more potassium leaving the cell
- concentration gradient that encourages potassium to leave the cell, but an [negative] electrical gradient inside the cell encourage potassium back in to balance out potassium leaving
electrochemical gradient
process of action potention 5
- inside the cell now has a positive electric charge
- outside has negatve chage
equilibrium potentials
- potential across membrane at which there is no net flow of an ion
- equilibrium potential (E) dictated by concentration difference and ion charge
ion process of equilibrium potentials
- EK+ = -80 mV → potassium attracted to positively charged space
- E[Na+] = +62 mV → sodium encouraged into cell by both concentration and electrical gradient, so more sodium pulled into cell → eventually inside cell is positive so sodium is becoming repelled
- E[Cl-] = -65mV → negative Cl repelled by negative inside cell
membrane potential
ion transfer
- set by electrochemical gradient and permeability of membrane to different ions
- if membrane only permeable to potassium, Em = EK+ = -80mV
- but Em ≈ -70mV
- membrane slightly permeable to sodium too [ENa+ =+62mV]
- the resting membrane potential of neurons is near to the equilibrium potential for potassium
- this is because at rest the membrane is more permeable to potassium than any other ion [more K+ channels are open]
membrane potential
how it chnages due to altering the permeability
- set by electrochemical gradient and permeability of membrane to different ions
- if permeability of the membrane to an ion increases, the membrane potential will love towards the equilibrium potential for that ion
- at rest, high K+ permeability, and low permeability to Na+, so Vm [membrane potential] is near but not quite Ek+ = -80mV
- the membrane potential can change by altering the permeability of the membrane to different ions
maintaining ion gradiesnt
Na+/K+ ATPase [pump]
- pumps 3 Na out of cell and 2 K into cell
- outside cell positive, inside cell negative
electrical signals
- can be measured with a voltmeter
- resting membrane potential is negative
- action potential - wave of transient depolarisation that travels down the axon
- fast [compared to chemical signals]
what is the resting action potential inside a membrane
- 70mV
changing membrane permeability- ion channels
- holes in the membrane that allow ions to enter and leave the cell
- are selective for different ions
- can be open all the time [eg K+ leak channels that set the resting membrane potential]
- others are opened by different stimuli → eg a change in voltage, binding specific molecules
- ions flow down electrical gradients
the action potential is generated by opening and closing ion channels
- Wave of transient depolarisation of the cell’s membrane
- conveys a fast signal from one place to another in the body.
- is generated by changes in membrane permeability due to opening and closing of voltage gated ion channels.
what is action potential
- is a self-regenerating electrical wave
- is a transient change (~1ms) inmembrane potential.
- occurs only if a threshold membrane potential is achieved in the axon initial segment (axon hillock), which transiently opens voltage gated sodium channels.
the action potential events
- Threshold potential reached
- Depolarisation due to opening ofsodium channels
- Repolarisation due to inactivation ofsodium channels and opening ofvoltage-gated potassium channels
- Hyperpolarisation as voltage-gatedpotassium channels are still open.
- Sodium channels released from inactivation (can fire AP again)
- relatively few ions move during an action potential
- barely changes the concentration gradients [as long as keep pumping ions back]
what is the absolute refractory period
all sodium channels inactivated = absolute refractory period. enforces one way transmission
what is the relative refractory period
some sodium channels inactivity = relative refractory period. only very strong stimuli can re-open the sodium channels and generate an action potential
action potentials are all or nothing
For action potentials to be generated the local depolarisation must reach a threshold point, which is the voltage at which sodium channels starts to open
this results in a positive feedback:
1. increase in Na+ current
2. depolarise membrane poential
3. open Na+channels
4. repeat
5.
