Central Nervous system

Question 1

What is a neuron?

Answer 1

Neurons are specialised cells that function to transmit electrical impulses within the nervous system

Question 2

What are the three basic components of a neuron?

Answer 2

Dendrites – Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals

Axon – An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors

Soma – A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival

Question 3

What are some nerves insulated by?

Answer 3

The myelin sheath which improves the conduction speed of electrical impulses along the axon, but require additional space and energy

Question 4

What is a membrane potential and how does it form?

Answer 4

Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane

The unequal distribution of ions on different sides of the membrane creates a charge difference called a membrane potential

Question 5

What is a resting potential and what is its magnitude?

Answer 5

A resting potential is the difference in charge across the membrane when a neuron is not firing

In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV)

Question 6

Describe the process of a chemical gradient forming a resting potential in a nerve

Answer 6

The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps

-The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)
-It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)
-This creates an electrochemical gradient whereby the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)
-The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)

Question 7

What is an action potential?

Answer 7

Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing

Action potentials occur in three main stages: depolarization, repolarization and a refractory period

Question 8

State the first (1st) process in the action potential and describe its process

Answer 8

Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge

-In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon
-As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium
-The influx of sodium causes the membrane potential to become more positive (depolarisation)

Question 9

State the second (2nd) process in the action potential and describe its process

Answer 9

Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge)

-Following an influx of sodium, potassium channels open within the membrane of the axon
-As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium
-The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarisation)

Question 10

State the third (3rd) and final process in the action potential and describe its process

Answer 10

The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again

-In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential)
-Following depolarisation (sodium influx) and repolarisation (potassium efflux), this ionic distribution is largely reversed
-Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump

Question 11

List the three stages of the action potential and state which channels are open at which stage as well a their charge

Answer 11

Depolarisation - Sodium channel open (influx of Na+) [+30mV]
Repolarisation - Potassium channel open (efflux of K+) [-80mV]
Refractory period - Sodium-Potassium channel open (infllux K+ efflux Na+0 [-70mV]

Question 12

What is a nerve impulse?

Answer 12

Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation

Question 13

How does depolarisation occur across the entire neuron?

Answer 13

-Depolarisation occurs when ion channels open and cause a change in membrane potential
-The ion channels that occupy the length of the axon are voltage-gated (open in response to changes in membrane potential)
-Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon
-This causes depolarisation to spread along the length of the axon as a unidirectional ‘wave’

Question 14

How are action potentials generated within an axon?

Answer 14

Action potentials are generated within the axon according to the all-or-none principle

-An action potential of the same magnitude will always occur provided a minimum electrical stimulus is generated
-This minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels
-If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire

Question 15

How do threshold potentials control wether a neuron fires?

Answer 15

Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation

If the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section

Question 16

How do we analyse Oscilloscopes?

Answer 16

Oscilloscopes are scientific instruments that are used to measure the membrane potential across a neuronal membrane

Data is displayed as a graph, with time (in milliseconds) on the X axis and membrane potential (in millivolts) on the Y axis

A typical action potential will last for roughly 3 – 5 milliseconds and contain 4 key stages:

Resting potential: Before the action potential occurs, the neuron should be in a state of rest (approx. –70 mV)
Depolarisation: A rising spike corresponds to the depolarisation of the membrane via sodium influx (up to roughly +30 mV)
Repolarisation: A falling spike corresponds to repolarisation via potassium efflux (undershoots to approx. –80 mV)
Refractory period: The oscilloscope trace returns to the level of the resting potential (due to the action of the Na+/K+ pump)

An action potential will only occur if the initial depolarisation exceeds a threshold potential of approximately –55 mV

Question 17

What is myelin composed of?

Answer 17

Myelin is a mixture of protein and phospholipids that is produced by glial cells (Schwann cells in PNS; oligodendrocytes in CNS)

Question 18

What is the main purpose of the myelin sheath?

Answer 18

The main purpose of the myelin sheath is to increase the speed of electrical transmissions via saltatory conduction

Question 19

Compare myelinated and non-myelinated nuerons

Answer 19

Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation

In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier
This results in an increase in the speed of electrical conduction by a factor of up to 100-fold

Question 20

State a benefit and limitation of myelination

Answer 20

-The advantage of myelination is that it improves the speed of electrical transmission via saltatory conduction

-The disadvantage of myelination is that it takes up significant space within an enclosed environment

Question 21

What is a synapse and how do electrical impulse act upon encountering a synapse?

Answer 21

Synapses are the physical gaps that separate neurons from other cells (other neurons and receptor or effector cells)

Neurons transmit information across synapses by converting the electrical signal into a chemical signal

Question 22

Describe the chemical transfer of electrical impulses across a synapse

Answer 22

1. When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels
2. Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitter) with the cell membrane
3. The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft
4. Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels
5. The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal
6. The neurotransmitters released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)

Question 23

What is a neurotransmitter and how does it work?

Answer 23

Neurotransmitters are chemical messengers released from neurons and function to transmit signals across the synaptic cleft

-Neurotransmitters are released in response to the depolarisation of the axon terminal of a presynaptic neuron
-Neurotransmitters bind to receptors on post-synaptic cells and can either trigger (excitatory) or prevent (inhibitory) a response

Question 24

Describe the release of acetylcholine into the synapse and what happens to it after its release

Answer 24

One example of a neurotransmitter used by both the central nervous system and peripheral nervous system is acetylcholine

It is commonly released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction
It is also commonly released within the autonomic nervous system to promote parasympathetic responses (‘rest and digest’)

Acetylcholine is created in the axon terminal by combining choline with an acetyl group (derived from mitochondrial Acetyl CoA)

Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse

Acetylcholine activates a post-synaptic cell by binding to one of two classes of specific receptor (nicotinic or muscarinic)

Acetylcholine must be continually removed from the synapse, as overstimulation can lead to fatal convulsions and paralysis

Acetylcholine is broken down into its two component parts by the synaptic enzyme acetylcholinesterase (AChE)

AChE is either released into the synapse from the presynaptic neuron or embedded on the membrane of the post-synaptic cell
The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine

Question 25

Describe the function of excitatory and inhibitory neurotransmitters

Answer 25

A nerve impulse is only initiated if a threshold potential is reached, so as to open the voltage-gated ion channels within the axon

Excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation by opening ligand-gated sodium or calcium channels
Inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation by opening ligand-gated potassium or chlorine channels

Question 26

How doo hyperpolarisation and depolarisation effect wether a neuron fires?

Answer 26

If overall there is more depolarisation than hyperpolarisation and a threshold potential is reached, the neuron will fire

If overall there is more hyperpolarisation than depolarisation and a threshold potential is not reached, the neuron will not fire

Question 27

State and describe the types of neurons

Answer 27

Sensory neurons transmit information from sensory receptors to the central nervous system (CNS)

Relay neurons (interneurons) transmit information within the CNS as part of the decision-making process

Motor neurons transmit information from the CNS to effectors (muscles or glands), in order to initiate a response