6.2.1 Nerve impulses

Describe the structure of a myelinated motor neurone

• one long axon (carries impulse away from cell body)

• Many short dendrons (carries impulse towards cell body)

Describe resting potential

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

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

• Na⁺/K⁺ actively transports:

• → 3 Na⁺ out of axon and 2 K⁺ into axon

• creating an electrochemical gradient

• → higher K⁺ concentration inside and higher Na⁺ concentration outside

• Differential membrane permeability:

• → more permeable to K⁺ → move out by facilitated diffusion

• → less permeable to Na⁺ (closed channels)

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (stimulus)

• Na+ channels open; membrane permeability to Na+ increases

• Na+ diffuse into axon down electrochemical gradient (causing depolarisation)

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (depolarisation)

• if threshold potential is reached, an action potential is generated

• as more voltage-gated Na+ channels open (positive feedback effect)

• so more Na+ diffuse in rapidly

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (repolarisation)

• voltage-gated Na+ channels close

• voltage-gated K+ channels open; K+ diffuse out of axon

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (hyperpolarisation)

• K+ channels slow to close so there’s a slight overshoot - too many K+ diffuse out

Explain how changes in membrane permeability lead to depolarisation and the generation of an action potential (resting potential)

• restored by Na+/K+ pump

Draw/ label a graph showing an action potential

Describe the all-or-nothing principle

• if depolarisation exceeds threshold potential → action potential

• if depolarisation does not exceed threshold potential → no action potential

• action potentials produced are always same magnitude / size / peak at same potential

• → bigger stimuli instead increase frequency of action potentials

Explain how the passage of an action potential along non-myelinated axons result in nerve impulses:

• action potential passes as a wave of depolarisation

• influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage-gated channels to open so adjoining region depolarises

• depolarisation along whole length of axon

Explain how the passage of an action potential along myelinated axons result in nerve impulses:

• myelination provides electrical insulation

• depolarisation of an axon at nodes of Ranvier only

• resulting in saltatory conduction (local currents circuits)

• so there is no need for depolarisation along whole length of axon

Suggest how damage to the myelin sheath can lead to slow responses and / or jerky movement

• less / no saltatory conduction; depolarisation occurs along whole length of axon

• → so nerve impulses take longer to reach neuromuscular junction; delay in muscle contraction

• ions / depolarisation may pass / leak to other neurones

• → causing wrong muscle fibres to contract

Describe the nature of the refractory period

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

• as Na+ channels are closed / inactive / will not open

Explain the importance of the refractory period

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

• limits frequency of impulse transmission at certain intensity (prevents over reaction to stimulus)

• → higher intensity stimulus causes higher frequency of action potentials

• → but only up to certain intensity

• also ensures action potentials travel in one direction → can’t be propagated in a refractory region

In the second half of the refractory period an action potential can be produced but requires greater stimulation to reach threshold.

Describe the factors that affect the speed of conductance

Myelination:

• depolarisation at Nodes of Ranvier only → saltatory conduction

• impulse doesn’t travel / depolarise 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

• but proteins / enzymes could denature at a certain temperature