U3 Nervous system: Transmission of a nerve impulse

Question 1

What are the five steps of nerve impulse conduction?

Answer 1

1. Neuron’s membrane is polarised
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
5. Return to norma

Question 2

What happens during step 1 (neuron’s membrane is polarised) of nerve impulse conduction?

Answer 2

• Neuron is not stimulated and is inactive
• Neuron is sitting with no impulse to transmit, so its membrane is polarised
• Electrical change on outside of membrane is positive (excess sodium ions)
• Electrical change on inside of membrane is negative (excess potassium ions)
• Neuron is fast its resting membrane potential (-70mV)

Question 3

What happens during step 2 (depolarisation) of nerve impulse conduction?

Answer 3

• A stimulus reaches a resting neuron
• Gated ion channels of the neuron’s membrane open to allow sodium ions on the outside of the membrane to go in the cell
• All of none phenomenon occurs:
• If stimulus doesn’t exceed threshold level and cause all gates to open: no results
• After more positive ions go inside the membrane the inside becomes positive and polarisation is removed and the threshold (-55mV) is reached
• More gated ion channels open to allow more sodium inside the cell, causing complete depolarisation of the neuron and action potential is created
• The neuron continues to open sodium channels along the membrane

Question 4

What happens during step 4 (repolarisation) of nerve impulse conduction?

Answer 4

• Gated ion channels open to let potassium move outside the membrane to restore the membrane’s electrical balance
• After the potassium gates open, the sodium gates close to allow the membrane to repolarise
• The electrical balance is the opposite of the initial polarised membrane that had sodium on the outside and potassium on the inside

Question 5

What happens during step 5 (hyperpolarisation) of nerve impulse conduction?

Answer 5

• As the neuron has more potassium on the outside than sodium on the inside, the membrane potential drops lower than the resting potential and is therefore hyperpolarised
• After the impulse has travelled through the neuron, the action potential is over and the cell memrbne returns to normal (the resting potential)

Question 6

Define nerve impulse

Answer 6

An electrochemical change that travels along the membrane of a nerve cell

Question 7

How does a nerve impulse occur?

Answer 7

• A single action potential occurs in one section of a membrane
• It triggers an action potential in the adjacent membrane
• This process continues along the length of the neuron

Question 8

How does a nerve impulse occur along an unmyelinated fibre?

Answer 8

• Depolarisation of one area of the membrane causes movement of sodium ions into adjacent areas
• The movement stimulates the voltage gated sodium ion channels in the next part of the membrane to open, which initiates an action poetnail in that area
• The process repeats along the whole length of the membrane away from the point of stimulation
• The nerve impulse is prevented from going backwards along the nerve fibre by the refractory period, which stops an action potential being generated at that point on the fibre

Question 9

How does a nerve impulse occur along a mylinated fibre?

Answer 9

• The myelin sheath insulates the nerve fibre from the extracellular fluid
• This doesn’t occur at the nodes of Ranvier as the myelin sheath is absent there
• Where the nerve fibre is surrounded by myelin, ions can’t flow between the inside and outside of the membrane and action potential can’t form
• Instead, the action potential jumps from one node of Ranvier to the next (called saltatory conduction)
• Saltatory conduction means the nerve impulse travels much faster along myelinated fibres than unmyelinated ones

Question 10

How does transmission across a synapse occur?

Answer 10

1. Nerve impulse reaches an axon terminal
2. Voltage gated calcium ion channels are activated, allowing calcium ions to enter from the extracellular fluid to the intracellular fluid
3. The calcium causes the synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitter by exocytosis
4. The neurotransmitter diffuse across the gap and attach to specific protein receptors on the post-synaptic membrane
5. This stimulates ligand gated protein channels to open, allowing sodium ions to rush in and initiate an action potential in the post-synaptic membrane
6. The nerve impulse eneters the post synaptic neuron