nervous transmission

resting potential

potential difference when a cell is at rest

what happens when a stimulus is detected

cell membrane becomes more permeable

• allows more ions to be actively transported out of the cell

generator potential

change in potential difference due to stimulus

when is action potential stimulated?

when generator potential reaches threshold level

importance of mitochondria in synaptic transmission

mitchondria provides ATP for AT of ions

when something is below the threshold

sub-threshold

action potential

electrical impulses along a neurone

explain why myelinated axons conduct impulses faster than unmyelinated neurones

• myelination provides electrical insulation

• saltatory conduction:

• impulse ‘jumps’ from one node of ranvier to another

how is resting potential established?

• sodium-potassium pump actively transports 3Na+ out and 2K+ in against conc. gradient

• this causes electrochemical gradient b/c higher concentration of K+ inside and higher concentration of Na+ outside

• membrane is more permeable to K+ and less permeable to Na+

  • Na+ actively transported out by Na+ carrier proteins

  • K+ actively transported in

• inside is negative compared to outside

A scientist investigated the effect of inhibitors on neurones. She added a respiratory inhibitor to a neurone and the resting potential of the neurones changed from -70mV to 0mV. Explain why this happened.

• no ATP produced

• no AT

  • Na-K pump inhibited

• so electrochemical gradient not maintained

• facilitated diffusion of ions causes change of resting potential to 0 mV

generation of an action potential

1. stimulus

   • Na+ channels open

   • membrane becomes more permeable to Na+

   • Na+ diffuse into cells down electrochemical gradient

2. depolarisation

   • if threshold potential is reached: action potential is generated

   • more Na+ channels open

   • more Na+ diffuses in rapidly

3. repolarisation

   • Na+ channels close and K+ channels open

   • K+ diffuses out of axon

4. hyperpolarisation

   • K+ channels are slow to close so theres a slight overshoot as too many K+ ions move out

5. resting potential restored by sodium/potassium pump

what would happen if the sodium ion channels remained opened all the time?

• the neurones would remain depolarised

• Na+ channels open and Na+ continues to enter

• so action potentials produced continuously

action potential graph

refractory period

time needed to restore the axon to resting potential when no further action potential can be generated (because Na+ channels are closed)

explain the importance of the refractory period

1. ensures unidirectional impulse

2. ensures discrete impulses

3. limits frequency of impulse transmission

what is the ‘all or nothing’ principle?

• for an action potential to be produced, depolarisation must exceed threshold potential

• all action potentials have the same magnitude/peak at the same potential

• once the threshold has been reached, the membrane potential remains the same

name and explain the factors that affect the speed of conductance

1. myelination

   • myelination provides electrical insulation

   • depolarisation at nodes of ranvier only: saltatory conduction

     • impulse jumps from one node of ranvier to another

   • if there’s no myelination, then more depolarisation occurs over the length of the membrane of the axon

     • so action potentials travel more slowly

2. axon diameter

   • larger diameter means less resistance to flow of ions in cytoplasm

3. temperature

   • increases rate of diffusion of ions bc theres more kinetic energy

   • slower diffusion of Na+

   • however too high of a temperature can cause enzymes & membrane proteins to denature

suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance

student’s t-test

appropriate units for the max frequency of impulse conduction

Hz

how can organisms detect the strength of a stimulus?

measured by frequency of action potentials

identify A,B and C

• A: vesicle

• B: neurotransmitter

• C: synaptic cleft

describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse

• depolarisation of presynaptic membrane occurs and causes Ca2+ channel proteins to open

• Ca2+ ions enter synaptic knob by facilitated diffusion

• Ca2+ causes synaptic vesicles to fuse with presynaptic membrane and release acetylcholine

• acetylcholine diffuses across synaptic cleft and attaches to receptors on postsynaptic membrane

• stimulates entry of Na+ into postsynaptic neurone (Na+ enters postsynaptic neurone), making the membrane potential less negative and leading to depolarisation on postsynaptic membrane

• if above threshold, then action potential produced

what happens at an inhibitory synapse?

• neurotransmitter binds to and opens Cl- channels on postsynaptic membrane

• triggers K+ channels to open

• Cl- moves in and K+ moves out via facilitated diffusion

• p.d becomes more negative: hyperpolarisation

• inside of post-synaptic neurone becomes more negative

• more Na+ required to reach threshold for depolarisation

what would happen if sodium ion channel remained open all the time

• neurones remain depolarised

• so no action potentials produced

define summation and name the 2 types

neurotransmitter from several sub-threshold impulses accumulate to generate an action potential

no summation at neuromuscular junctions

• temporal summation

  • one presynaptic neurone releases neurotransmitter several times in quick succession

• spatial summation

  • many presynaptic neurones share one postsynaptic neurone and release neurotransmitter

explain how channel proteins on presynaptic neurones are involved in reflex responses

• allows calcium ions in at the end of the presynaptic neurone

  • causing the release of a neurotransmitter

how might drugs increase synaptic transmission?

• inhibit AChE

• mimic shape of neurotransmitter

how might drugs decrease synaptic transmission

• inhibit release of neurotransmitter

• decrease permeability of postsynaptic membrane to ions

• hyperpolarise postsynaptic membrane

• low influx of Na+

• so no depolarisation

how do synapses ensure that nerve impulses only travel towards the muscle fibre

• neurotransmitters only made/stored in/released from pre-synaptic neurone

• neuroreceptors only on post-synaptic membrane

differences between a cholinergic synapse and a neuromuscular junction

cholinergic synapse	neuromuscular junction
neurone to neurone	neurone to muscle
action potential in neurone	no action potential in muscle
summation in neurone	no summation in muscle
neurone respone can be inhibitory	muscle response always excitatory