Neurotransmission - electrical signalling in neurons

Question 1

how do nerves make muscles move

frogs

Answer 1

Lucia and Luigi Galvani, 1781: electricity makes frogs legs muscles contract

• they decided that ‘animal electricity’ was present in the nerve

Question 2

how do nerves make muscles move

humans

Answer 2

Giovanni Aldini (Galvani’s nephew), 1802: Electricity makes criminals’ corpses twitch

Question 3

what is electricity

Answer 3

• 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).

Question 4

electricity - Ohm’s law

multiply

Answer 4

current = potential x conductance

Question 5

electricty - Ohm’s law

divide

Answer 5

current = potential / resistance

Question 6

measuremnt for current and potential

Answer 6

current = charge per second/Amps

potential = volts

Question 7

Conduction in nerves different to in wires

Answer 7

• 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.

Question 8

how fast is nerve conduction

Answer 8

30-40 m/s

Question 9

current flow do an axon

Answer 9

• 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

Question 10

how do cells signal electrically?

Answer 10

• 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…

Question 11

cell membrane

Answer 11

• cells are surrounded by a lipid membrane
• ## water soluble things can’t pass through

Question 12

concentration gradient

process of action potention 1

Answer 12

• Outside
  
  • Na+
  • Cl-
  • bit of Ca2+
• Inside
  
  • Proteins (-ve)
  • K+

Question 13

ion channels [holes in the membrane]

process of action potention 2

Answer 13

• ions can’t get across membrane without channels
• leak potassium channels → always open

Question 14

potassium conc gradient

process of action potention 3

Answer 14

• potassium channel allows some potassium ions to leave the cell
• positively charged ions left cell

Question 15

electrical gradient

process of action potention 4

Answer 15

• 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

Question 16

electrochemical gradient

process of action potention 5

Answer 16

• inside the cell now has a positive electric charge
• outside has negatve chage

Question 17

equilibrium potentials

Answer 17

• potential across membrane at which there is no net flow of an ion
• equilibrium potential (E) dictated by concentration difference and ion charge

Question 18

ion process of equilibrium potentials

Answer 18

1. EK+ = -80 mV → potassium attracted to positively charged space
2. 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
3. E[Cl-] = -65mV → negative Cl repelled by negative inside cell

Question 19

membrane potential

ion transfer

Answer 19

• 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]

Question 20

membrane potential

how it chnages due to altering the permeability

Answer 20

• 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

Question 21

maintaining ion gradiesnt

Answer 21

Na+/K+ ATPase [pump]
- pumps 3 Na out of cell and 2 K into cell
- outside cell positive, inside cell negative

Question 22

electrical signals

Answer 22

• 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]

Question 23

what is the resting action potential inside a membrane

Answer 23

• 70mV

Question 24

changing membrane permeability- ion channels

Answer 24

• 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

Question 25

the action potential is generated by opening and closing ion channels

Answer 25

- 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.

Question 26

what is action potential

Answer 26

- 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.

Question 27

the action potential events

Answer 27

1. Threshold potential reached 2. Depolarisation due to opening ofsodium channels 3. Repolarisation due to inactivation ofsodium channels and opening ofvoltage-gated potassium channels 4. Hyperpolarisation as voltage-gatedpotassium channels are still open. 5. Sodium channels released from inactivation (can fire AP again) ## Footnote - relatively few ions move during an action potential - barely changes the concentration gradients [as long as keep pumping ions back]

Question 28

what is the absolute refractory period

Answer 28

all sodium channels inactivated = absolute refractory period. enforces one way transmission

Question 29

what is the relative refractory period

Answer 29

some sodium channels inactivity = relative refractory period. only very strong stimuli can re-open the sodium channels and generate an action potential

Question 30

action potentials are all or nothing

Answer 30

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.