6.2.1 nervous impulses

structure of a myelinated motor neurone

describe resting potential

inside of axon has a negative charge relative to outside (more positive ions outside)

explain how a resting potential is established across the axon membrane in a neurone

Na⁺ / K⁺ pump actively transports 3 Na⁺ out of axon and 2 K⁺ into axon

creating an electrochemical gradient - higher K conc. inside and higher Na conc outside

different membrane permeability:

• more permeable to K - move out by facilitated diffusion

• less permeable to Na

stimulus

Na channels open - membrane permeability to Na increases

Na diffuse into axon down electrochemical gradient (causing depolarisation)

depolarisation

if threshold potential reached, an action potential is generated

as more voltage-gated Na channels open

so more Na diffuse rapidly

repolarisation

voltage gated Na channels close

voltage gated K channels open - K diffuse out of axon

hyperpolarisation

K channels slow to close so there is a slight overshoot - too many K diffuse out

resting potential

restored by Na/K pump

explain how chnages in mebrane permeability lead to depolarisation and the generation of an action potential

graph showing action potential

describe all or nothing principle

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

action potential produced are always same size at same potential

• bigger stimuli instead increase frequency of action potential

explain how passage along a non-myelinated axon results in nerve impulses

• action potential passes a wave of depolarisation

• influx of Na inone region increases permeability of adjoining region to Na by causing voltage gated Na channels to open so adjoining region depolarises

explain how the passage of an action potential along a myelinated axon results in nerve impulses

• myelination provides electrical imsulation

• depolarisation of axon at node of Ranvier only

• resultin gin saltatory conduction (local current circuits)

• no need for depolarisation along whole length of axon

suggest how damage to myelin sheath can lead to slow responses

• less saltatory conduction - depolarisation occurs along whole of axon , so nerve impulses take longer to reach neuromuscular junction, delay in msucle contraction

• ions may pass to other neurones, causing wrong msucle fribres to contract

describe nature of refractory period

time taken to restore axon to resting potential when no futher action potential can be generated

as Na channels are closed

explain the importance of the refractory period

ensures discrete impulses are produced (action potentials don’t overlap)

limits frequencies of impulse transmisiion at a certain intensity

• hgiher intensity stimulus causes higher frequency of action potential

• but only up to certain inetensity

ensures action potentials travel in one direction

in the second half of the refractory period an action potential can be produced but requires a greater srimulatin

factors that affect speed of conduction

myelination

axon diameter

temperature

myelination

depolarisaation at nodes of ranvier only - saltatory conductiomn

impulse doesnt travel whole length of axon

axon diameter

bigger diameter means less resistance to flow of ions in cytoplasm

temperature

increases rate of diffusion of Na and K as more kinetic energy

proteins/enzymes could denature at a certain temperature