Nerves

Question 1

What are the three divisions of the brainstem?

Answer 1

Midbrain
Pons
Medulla oblongata

Question 2

What is the function of the hypothalamus?

Answer 2

Integrates nervous and hormonal systems

Question 3

What is the difference between afferent and efferent neurons?

Answer 3

Afferent neurons are sensory. They detect changes in the environment. They travel from peripheral to central nervous system.
Efferent neurons are motor. They send a signal based on changes in the environment. They travel from central to peripheral nervous system.

Question 4

Name the three main types of neuron.

Answer 4

Afferent, efferent and interneuron

Question 5

What are the three main types of glia?

Answer 5

Microglia, astroglia, oligodendrocytes

Question 6

What is the general value taken for the resting membrane potential of neurons?

Answer 6

-70meV

Question 7

How is the resting membrane potential of a neuron maintained?

Answer 7

The membrane is impermeable to ions.
Protein pump is used to pump potassium ions into and sodium ions out of the cell, on a 1:1 ratio
Ion channel which is potassium permeable, allows potassium to diffuse out of the cell, down the concentration gradient until the electric gradient inhibits further removal of potassium.
This allows the inside of the cell to be negative with respect to the outside.

Question 8

What is meant by depolarisation of a membrane?

Answer 8

Increase in membrane potential so that it becomes less negative, and so less polar.

Question 9

What is meant by overshoot of the membrane potential?

Answer 9

Increase in membrane potential to the point where the inside becomes positive with respect to the outside.

Question 10

What is meant by repolarisation of a membrane?

Answer 10

Return to the resting membrane potential of -70meV.

Question 11

What is meant by hyperpolarisation of a membrane?

Answer 11

The decrease in membrane potential beyond the resting membrane potential of -70meV.

Question 12

Give four examples of graded potentials.

Answer 12

1. Post synaptic potential
2. Endplate potential in the neuromuscular junction
3. Generator potentials at sensory receptors
4. Pacemaker potentials

Question 13

What is the role of graded potentials?

Answer 13

To determine when action potentials fire

Question 14

Why are graded potentials called ‘graded’?

Answer 14

The intensity of stimulus is reflected in the amplitude of potential difference produced
I.e. not all or nothing.

Question 15

Why are graded potentials said to be decremental?

Answer 15

Amplitude of potential decreases with increasing distance from point of initial depolarisation, due to leakage of voltage.
Therefore they are unsuitable for long distance transmissions.

Question 16

What are the main differences between action potentials and graded potentials?

Answer 16

• Graded potentials are ligand gated, action potentials are voltage gated
• Graded potentials are graded. Action potentials are ‘all or nothing’
• Graded potentials can be hyperpolarising (IPSP) or depolarising (EPSP). Action potentials are always depolarising.

Question 17

Describe the changes which occur to ion channels in the membrane of a cell with an action potential forming.

Answer 17

• Normally, only potassium ion channels are open, allowing potassium ions to diffuse out of the cell, maintaining the RMP of -70meV.
• Once a graded potential reaches threshold (-55meV), voltage dependant sodium channels open in the membrane.
• This allows sodium ions to flood it, making the cell more positive.
• This operates by a positive feedback mechanism, where the entry of sodium ions stimulates the opening of further sodium channels, allowing the entry of more sodium ions
• After a short time, voltage dependant potassium channels open and potassium diffuses out of cell, repolarising and the hyperpolarising it.

Question 18

By what two mechanisms can conduction velocity of an action potential be increased?

Answer 18

• Increasing diameter of axon, to decrease axial resistance

- Insulation of axon with myelin sheath

Question 19

From what cell type is a myelin sheath composed of?

Answer 19

Schwann cells in central nervous system

Oligodendrocytes in peripheral nervous system

Question 20

What is meant by a refractory period?

Answer 20

This is a period immediately after the firing of an action potential, during which a nerve is unresponsive to further stimulation.

Question 21

What is salutatory conduction?

Answer 21

This is the conduction of an action potential rapidly between one node of ranvier to the next due to the presence of a myelin sheath around the neuron.

Question 22

What is meant by synaptic delay?

Answer 22

This is the slight delay which occurs as the neurotransmitter diffuses across the synapse at a rate of 1m per second.

Question 23

Describe how the end-plate potential is formed in a muscle cell.

Answer 23

• Action potential arrives in motor neuron
• Stimulates the opening of voltage gated Ca2+ channels on the presynaptic membrane.
• Calcium ions diffuse into the cell, promoting vesicles containing neurotransmitter Acetylcholine to move forward and fuse with the presynaptic membrane.
• Acetylcholine released into the synaptic cleft.
• Diffuses across cleft and binds to receptors on the post-synaptic membrane (sarcolemma)
• Generate the opening of ligand gated ion channels on the sarcolemma.
• Ions diffuse into the cell and graded membrane potential forms.
• If graded membrane potential reaches threshold, action potential fires.

Question 24

How does tetrodotoxin affect NMJ activity?

Answer 24

Blocks sodium channels and so prevents formation of action potential in muscle cell.

Question 25

How does joro spider toxin affect NMJ activity?

Answer 25

Blocks Ca2+ channels and so prevents neurotransmitter release

Question 26

How does botulinum toxin affect NMJ activity?

Answer 26

Blocks neurotransmitter release

Question 27

How does curare affect NMJ activity?

Answer 27

Blocks Ach receptors and so inhibits end-plate potential formation

Question 28

How does acetylcholinesterse affect NMJ activity?

Answer 28

Blocks breakdown of Ach and so increases transmission at NMJ.