Neuronal Transmission at the NMJ

Question 1

Describe the major steps in conscious control of skeletal muscle.

Answer 1

• CNS stimulates a motor-neuron (emanates from SC).
• Action potential propagates down the axon and reaches the synapse.
• Transmission of action potential to surface of muscle cell via an intermediate ‘transmitter’.
• Action potential on surface of muscle cell triggers contraction.

Question 2

List the most numerous cellular components of the CNS.

Answer 2

• Mainly neurons and glial cells.
• The most numerous glial cells are:
  
  • Oligodendrocytes
  • Microglial cells
  • Astrocytes
  • Ependymal cells

Question 3

How do neurons encode and transmit information?

Answer 3

• As electrical activity
• Electrical charge can be conducted both passively and actively

Question 4

Describe the difference between passive and active conduction.

Answer 4

• Passive conduction is very lossy, for example, heat travelling down a rod.
• Active conduction involves the generation of an action potential by opening of ion channels.

Nb. Lossy = the dissipation of electrical or electromagnetic energy.

Question 5

How long does depolarisation last?

Answer 5

1/1000th of a second.

Question 6

What causes depolarisation?

Answer 6

Rapid influx of Na+

Question 7

What causes repolarisation?

Answer 7

Efflux of K+

Question 8

Describe the action of voltage-gated sodium channels.

Answer 8

1. At RMP, Na+ channels are closed - the activation gate is closed.
2. Depolarisation opens the activation gate and Na+ flows into the cell along its electrochemical gradient.
3. A delayed component of voltage dependent activation is the blocking of the channel by the inactivation gate (after ~0.5ms).
4. Repolarisation of the cell re-sets both gates to their equilibrium positions.

Question 9

Describe how an action potential is propagated.

Answer 9

1. Voltage-gated Na+ channels open and an action potential is generated - causes depolarisation of the membrane potential.
2. Passive current flows to the next voltage-gated Na+ channels - if they are too far apart, the signal cannot propagate passively, so there is an optimum distance between these channels.
3. The passive current opens the voltage-gated Na+ channels and another action potential is generated.

Question 10

What happens during the refractory period of a voltage-gated sodium channel?

Answer 10

• Voltage-gated Na+ channels are blocked by the inactivation gate.
• This restores the membrane potential to threshold.
• Action potentials cannot be generated during this time.

Question 11

What happens to a cell during the absolute refractory period?

Answer 11

• The cell cannot be stimulated to its threshold potential.
• All Na+ channels are closed.

Question 12

What happens to a cell during the relative refractory period?

Answer 12

• An action potential could be induced, but only by a stronger stimulus than normal.
• Some Na+ channels are open but more K+ channels are open than usual.
• Cell is still hyperpolarised.

Question 13

Describe the effect of the refractory period on the direction of an action potential.

Answer 13

• Voltage-gated Na+ channels open and allow positive sodium ions into the axoplasm.
• These ions set up a more positive potential which, when great enough, causes the next Na+ channel to open.
• The refractory nature of Na+ channels keeps the impulse moving in one direction.

Question 14

Why is the refractory membrane always behind the action potential in progress?

Answer 14

To prevent backward spread and propagation of the action potential, allowing the membrane behind to reset.

Question 15

What effect does resistance have on action potential propagation?

Answer 15

• Passive movement of charge along the axon is easier with less resistance.
• The larger the diameter of the axon, the lower the resistance.
• Larger axons have faster passive charge movement.
• Optimise diameter of axon to have lower resistance to passive flow.

Question 16

What effect does capacitance have on action potential propagation?

Answer 16

• The higher the capacitance, the harder it is for charge to cross over the membrane.
• The greater the surface area there is on an axon, the higher its capacity to store charge across its membrane.
• Greater opportunity to cross the membrane = increased capacitance.
• It is more difficult to induce depolarisation across the membrane when there is a large capacitance.

Question 17

Which cells myelinate axons in the central nervous system?

Answer 17

Oligodendrocytes

Question 18

Which cells myelinate axons in the peripheral nervous system?

Answer 18

Schwann cells

Question 19

What happens to action potentials at a node of ranvier?

Answer 19

Action potentials are regenerated at the nodes of ranvier through the opening of voltage-gated sodium channels and the flow of ions through these channels.

Note that the action potential only exists at the node of ranvier.

Question 20

What is synaptic transmission?

Answer 20

The transfer of signal from a neuron to a target cell.

Question 21

How is an electrical synapse bridged?

Answer 21

Via a gap junction.

Question 22

How is a chemical synapse bridged?

Answer 22

Via an ‘intermediate’ between the presynaptic and postsynaptic cell - a neurotransmitter.

Question 23

What happens in the presynaptic cell when the membrane depolarises?

Answer 23

Voltage-gated calcium channels open.

Question 24

Describe postsynaptic ionotropic receptors.

Answer 24

• Ion channel linked receptors.
• Excitatory or inhibitory effects on excitability of post-synaptic cell membrane depending on selectivity for ions.

Question 25

Describe postsynaptic metabotropic receptors.

Answer 25

• Enzyme linked receptors.
• May be excitatory or inhibitory on excitability of postsynaptic cell if they modify action of separate ion channels.
• May trigger other processes (i.e. contraction of smooth muscle).

Question 26

Which type of receptors are affected by acetylcholine at the neuromuscular junction?

To which category do these receptors belong?

Answer 26

Nicotinic acetylcholine receptors.

These are ionotropic receptors.

Question 27

Briefly describe what happens when an action potential reaches the neuromuscular junction.

Answer 27

• AP in motor-neuron triggers release of acetlycholine (ACh) from nerve endings.
• Synapse with muscle fibre is a very well-defined region (NMJ).
• ACh acts on nicotinic acetlycholine receptors which are ionotropic.
• Influx of sodium on target cell is excitatory.
• Sufficient quanta of ACh = sufficient postsynaptic excitation = generation of AP on muscle fibre.

Question 28

Describe spatial summation.

Answer 28

Multiple synapses from different neurons stimulating the same postsynaptic cell.

Question 29

Describe temporal summation.

Answer 29

The same synapse repeatedly stimulating the sam postsynaptic cell.