Neurons and action potentials

Question 1

State the order of the 4 phases of an action potential?

Answer 1

Resting state, depolarisation, repolarisation, hyperpolarisation.

Question 2

Which voltage-gated ion channel opens during phase two and phase three of an action potential?

Answer 2

Phase 2, sodium, phase 3, potassium.

Question 3

Identify the main features of nerve cells.

Answer 3

Soma (cell body), axon, dendrites, terminal boutons (branches), axon hillock, myelin sheath, nodes of Ranvier.

Question 4

Describe the function of an axon.

Answer 4

To carry electrical impulses that are the means of communication within the brain and between the brain and the rest of the body.

Question 5

Describe the function of dendrites.

Answer 5

They receive and process those messages from the axons and transmit electrical stimulation to the soma.

Question 6

Describe the function of the soma.

Answer 6

Containing the nucleus, it support the chemical processing of the neuron; the most important of which is the production of neurotransmitters. It also integrates the signals coming in from the dendrites.

Question 7

Describe the function of the axon hillock.

Answer 7

It acts as an administrator by summing up the total signals received, both inhibitory and excitatory signals. If it determines the total signal is above the threshold, an action potential will be released.

Question 8

Describe the function of a myelin sheath.

Answer 8

It is an insulator wrapping around the nerve, allowing electrical impulses to transmit quickly and efficiently along the nerve cells.

Question 9

Describe the function of nodes of Ranvier.

Answer 9

They are within gaps along the myelin sheath, to facilitate and increase the conduction, speed and voltage of the electrical impulses travelling down the nerve.

Question 10

Describe the function of terminal boutons (branches).

Answer 10

This is the most distal part of an axon, and is crucial for neural communication as it helps release neurotransmitters to form the pre-synaptic aspect of a chemical synapse.

Question 11

Explain the functional classification of sensory, motor and interneurons.

Answer 11

Sensory neurons (sometimes referred to as afferent neurons) are nerve cells that carry nerve impulses from sensory receptors towards the central nervous system and brain.

Motor neurons (also referred to as efferent neurons) are the nerve cells responsible for carrying signals away from the central nervous system towards muscles to cause movement/contraction.

Interneurons allow sensory and motor neurons to communicate with each other. Relay neurons connect various neurons within the brain and spinal cord and are easy to recognise due to their short axons.

Question 12

Describe the differences between graded and action potentials.

Answer 12

Graded potentials are a local, passive event, affecting over a short distances where as action potentials are active events, affecting over a long distance. Graded potentials can have a variable amplitude and can summate, whereas action potentials have a set amplitude (all or nothing), and cannot summate. Repolarisation will decay over time in graded potentials and is dependent on ion channels in action potentials.

Question 13

Describe the mechanisms involved in the 4 stages of an action potential.

Answer 13

1. Resting membrane potential = Sodium and potassium channels are closed. Na+ rush into the cell; K+ are concentrated inside the cell.
2. Depolarisation = Sodium voltage-gated channels open in response to a stimulus and sodium ions rush into the cell through diffusion.
3. Repolarisation = sodium channels close and potassium voltage-gated channels open and potassium ions rush out of the cell through diffusion.
4. Hyperpolarisation = Sodium and potassium channels close, an exchange pump moves excess sodium ions out of the cell and potassium ions back in bringing the potential back to resting/normal levels.

Question 14

Describe the refractory period.

Answer 14

It occurs after an action potential and generally lasts one millisecond. It is when sodium channels become inactive and can only travel in one direction, making subsequent action potentials harder to generate.

Question 15

Why do action potentials propagate in one direction along an axon?

Answer 15

Because the sodium channels have a refractory period following activation, during which they cannot open again. This ensures that the action potential is propagated in a specific direction along the axon.

Question 16

Outline the presynaptic and postsynaptic structures involved in chemical neurotransmission.

Answer 16

Presynaptic - At the terminal bouton, where microtubules, voltage gate calcium channels and vesicle transport allow the fusion of vesicles to the presynaptic membrane. This causes the vesicle to open, release neurotransmitters into the snyaptic cleft to be able to transport to the postsynaptic cell with the help of calcium ions.

Post synaptic - main structure is the neuromuscular junction, where the fusing vesicles release acetylcholine to the nAChR, a protein receptor within the skeletal muscle fibre that changes shape when activated, to allow positive calcium ions to flood into the cell, causing depolarisation and repolarisation.

Question 17

Outline the structure of an electrical synapse. How do electrical synapses differ functionally from chemical synapses.

Answer 17

An electrical synapse is a more direct connection between cells with connexins separating two cells. The major difference between a chemical and an electrical synapse is that in a chemical synapse, the nerve impulse is transmitted chemically via neurotransmitters, whereas in an electrical synapse, the nerve impulse is transmitted electrically via channel proteins.

Question 18

Describe the membrane events associated with triggering an action potential.

Answer 18

A stimulus large enough to allow Na+ channels to open and reach a value of -55mV. The membrane depolarises above the threshold voltage, and the influx of sodium ions initiates the action potential.