C2.2 - transmission of nerve impulses (6b)

Question 1

What is a neuron?

Answer 1

Neurons are cells within the nervous system that carry electrical impulses.

Question 2

What is the structure of a motor neuron?

Answer 2

• Cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres (dendrites and axons) of varying length projecting from it.
• Myelin sheath
• Axon terminal button

Question 3

What is the function of the nerve fibres?

Answer 3

Conduct electrical impulses

Question 4

What is the function of the axon terminal button?

Answer 4

Involved in synaptic transmission with dendrites of another neuron, or with muscle fibres at a neuromuscular junction.

Question 5

What are the similarities between axons and dendrites?

Answer 5

They are both nerve fibres that conduct electrical impulses.

Question 6

What are the differences between axons and dendrites?

Answer 6

• There is one axon per neuron, but multiple dendrites.
• Axons are longer than dendrites.
• Axons may be myelinated, but dendrites are not.

Question 7

What are nerve impulses?

Answer 7

Nerve impulses are action potentials that are propagated along nerve fibres.

Question 8

When do action potentials occur?

Answer 8

Action potentials occur when a region of the nerve fibre switches from a negative internal charge, to positive and back.
- This wave of depolarisation moves along the axon.

Question 9

How are action potentials formed?

Answer 9

Action potentials involve the movement of positively charged ions.
- Sodium and potassium ions move down the concentration gradients that were generated with the resting potential.
- The ions move through channel proteins, so this is an example of facilitated diffusion.

Question 10

Which type of transport and transmembrane protein is involved in nerve impulses?

Answer 10

Facilitated diffusion
- Ions move through channel proteins as charged molecules cannot pass through the hydrophobic core of the membrane.

Question 11

What is a membrane potential?

Answer 11

A membrane potential is the difference in electrical charge across a membrane

Question 12

What is meant by a ‘polarised membrane’?

Answer 12

When the amount of charge on either side of the membrane is different

Question 13

What causes membrane potential?

Answer 13

Membrane potential is caused by the electrochemical concentrations of ions
- There is a gradient of both charge and atoms

Question 14

What ion movement would cause an increase in membrane potential (depolarization)?

Answer 14

Movement of ions into the atom

Question 15

What is the resting potential?

Answer 15

When an action potential is not occurring, the membrane potential of the axons is about -70mV.

Question 16

How do ions form the resting potential?

Answer 16

This membrane potential results from generating concentration gradients of sodium and potassium ions.

Question 17

Which type of transport and transmembrane protein is involved in generation of resting potentials of neurons?

Answer 17

Active transport using pump proteins.

Question 18

What is the role of the sodium-potassium pump in generating the resting potential?

Answer 18

• Pumps 3 Na+ ions out of the axon for every 2 K+ ions in
• Pumping requires energy from ATP, as it involves active transport against the concentration gradients.
• The sodium potassium pumps causes a charge difference where the external surface of the plasma membrane is positive in charge, relative to the internal surface.

Question 19

How is a negative resting membrane potential generated?

Answer 19

• There is a net loss of positive charge with each cycle of the sodium-potassium pumps.
• Inside the axons are negative ions (chloride ions, charged proteins)

Question 20

What creates and maintains the resting potential?

Answer 20

Active transport

Question 21

What causes an action potential?

Answer 21

Passive transport

Question 22

What are synapses?

Answer 22

Junctions between neurons and receptor cells, effector cells or other neurons

Question 23

What are neurotransmitters?

Answer 23

Molecules that diffuse across the synaptic cleft, to transfer the impulse

Question 24

What is an example of an excitatory neurotransmitter?

Answer 24

Acetylcholine is an example of an excitatory neurotransmitter.
- This neurotransmitter exists in many types of synapses including neuromuscular junctions

Question 25

How are neurotransmitters released from a presynaptic membrane?

Answer 25

1. A nerve impulse arrives at the end of the axon, called the axon terminal/presynaptic button, causing depolarisation of the presynaptic membrane.
2. Depolarization stimulates calcium ion channels to open, causing uptake of calcium ions into the presynaptic button by diffusion.
3. Calcium ions acts as a signalling chemical inside the neuron, causing vesicles filled with neurotransmitter to move to the plasma membrane and fuse with it.
4. Neurotransmitter molecules are released by exocytosis, into the synaptic cleft.

Question 26

How is an impulse (excitatory potential) generated in a postsynaptic neuron?

Answer 26

1. Diffusion of neurotransmitters (e.g, acetylcholine) across the synaptic cleft.
2. Binding to transmembrane receptors in the postsynaptic membrane.
3. Receptors’ ion channels open, allowing sodium ions into the postsynaptic neuron.
4. If enough sodium ions enter the postsynaptic neuron, an action potential is generated and propagated along the nerve fibre.
5. The acetylcholine detaches from the receptors and are broken down in the synaptic cleft by the enzyme acetylcholinesterase, forming acetyl and choline.
6. Acetyl and choline are taken up by the presynaptic neuron, and acetylcholine is reassembled and packaged into vesicles.

Question 27

Why can’t impulses be transmitted from the postsynaptic to the presynaptic neurons?

Answer 27

• The dendrites on the postsynaptic membrane do not contain vesicles of neurotransmitter.
• The presynaptic membrane does not contain the receptors to bind the neurotransmitter molecules.

Question 28

What are 2 factors that affects the speed of nerve impulses?

Answer 28

• Myelination
• Axon diameter