C2.2 Neural Signalling

Question 1

What does the nervous system consist of?

Answer 1

Nervous system consists of nerve cells called neurons. In humans there are approx. 85 billion neurons. Neurons help with internal communication by transmitting nerve impulses (an electrical signal).

Question 2

What are the components of the nervous systems, in hierarchical order?

Answer 2

Question 3

What is the role of a neuron?

Answer 3

Neurons transmit info along nerve fibers in form of electrical impulses. Electrical impulse is not like an electrical current that flows along wires. An impulse is a momentary reversal in electrical potential difference in membrane – a change in position of charged ions between inside & outside of membrane of nerve fibres.

Question 4

Annotate neuron.

Answer 4

Question 5

What is a neuron?

Answer 5

Neuron is basic functional unit of nervous system. Neurons have a cell body with cytoplasm & a nucleus, as well as a long narrow outgrowth called nerve fibres along which nerve impulses travel.

Question 6

What are the two types of nerve fibres?

Answer 6

• Dendrites (short branched nerve fibers e.g. those used to transmit impulses between neurons in one part of brain or spinal cord)
• Axons (very elongated fibers e.g. those that transmit impulses from tips of our toes/fingers to spinal cord)

Question 7

What is membrane potential?

Answer 7

If microelectrodes are placed inside & outside any living cell, a voltage across membrane will be detected. This voltage is usually between 10 and 100 mV; it is known as membrane potential.
This potential is due to an imbalance between net charge (negative or positive) of cytoplasm & fluid outside. Cytoplasm of cells is generally electrically negative compared with fluid outside.
For this reason, membrane potential is expressed as a negative value e.g. liver cells have a potential of -40 mV.

Question 8

How do neurons able to transmit information?

Answer 8

Nerves are able to transmit info rapidly from one part of your body to another. This info is passed along by electrical signals called nerve impulses. To record electrical activity of a nerve it is placed in an isotonic fluid bath. A reference microelectrode is placed in surrounding fluid. A recording microelectrode is inserted into cytoplasm of axon. Electrical disturbances are measured & displayed on oscilloscope. By convention outside of cell is given a value of zero, so inside has a negative relative charge ~ -70 millivolts (mV).

Question 9

What is a nerve impulse?

Answer 9

A nerve impulse is a result of a change in concentration of sodium (Na+) & potassium (K+) ions along the cell membrane.

Question 10

What are the two types of membrane potentials?

Answer 10

Depending on membrane potential (voltage), we can distinguish between a resting potential & an action potential.

Question 11

What is resting potential?

Answer 11

Resting potential: this is potential difference across a nerve cell membrane when it is not stimulated (transmitting an impulse). It is at approximately -70mV.

Question 12

What is action potential?

Answer 12

Action potential: This is sudden reversal (depolarisation) & restoration (repolarisation) of electrical potential across a plasma membrane as a nerve impulse passes along a neuron.

Question 13

How is membrane potential established?

Answer 13

All cells have a measurable membrane potential (voltage), which is established by imbalance of ions inside & outside of a cell. Membrane potential in neurons is – 70mV.

Question 14

Which three factors contribute to the establishment of negative membrane potential?

Answer 14

1. Sodium-potassium pumps
2. Leakage of ions back across membrane by simple diffusion
3. Negatively charged proteins inside nerve fibre.

Question 15

How does the sodium-potassium pump contribute to a negative membrane potential?

Answer 15

Sodium-potassium pumps in membrane transfer Na+ out of neuron & K+ into neuron at same time. This is active transport & requires ATP. Number of ions pumped is unequal – three Na+ are pumped out, two K+ ions are pumped in. This causes an imbalance of ions & conc gradients for both.

Question 16

What our to steps for the mechanism involved with the sodium-potassium pump?

Answer 16

Question 17

How does leakages of sodium and potassium ions contribute to a negative membrane potential?

Answer 17

Leakage of potassium & sodium ions also contributes to difference between Na+ & K+ conc gradients, increasing overall charge imbalance across membrane. Axon membrane has a higher (50x more) permeability of K+, which leaks out of cell (by diffusion) through its membrane channels faster than Na+leaks in through its Na+ protein channels.

Question 18

How does negatively charged proteins inside the cell contribute to a negative membrane potential?

Answer 18

Due to this unequal distribution of ions, & abundance of negatively charged proteins (organic ions) inside axon fibre, membrane potential is found to be at ca. -70mV.

Question 19

How is action potential measured?

Answer 19

Changes in membrane potential in axons during Action Potential can be measured using electrodes placed at one position along a nerve fibre. Results are displayed on an oscilloscope. Oscilloscopes are scientific instruments that are used to measure membrane potential across a neuronal membrane
Data is displayed as a graph, with time(in ms) on X axis & membrane potential (in mV) on Y axis. A typical action potential will last for roughly 3 – 5 ms & contain 4 key stages:
1. Resting potential:Before action potential occurs, neuron should be in a state of rest (approx. –70 mV)
2. Depolarisation:A rising spike corresponds to depolarisation of membrane via sodium influx (up to roughly +30 mV)
3. Repolarisation:A falling spike corresponds to repolarisation via potassium efflux (undershoots to approx. –80 mV)
4. Refractory period:Oscilloscope trace returns to level of resting potential (due to action of Na+/K+pump)
An action potential will only occur if initial depolarisation exceeds a threshold potential of approx –55 mV.

Question 20

How can the cell potential be measured?

Answer 20

Cell potential (i.e. voltage produced by movement of ions) can be measured using microelectrodes impaled into cells. A minimal amount of stimulus is needed to fire an action potential (threshold minimum must be reached). An oscilloscope image showing changes (in mV) can be obtained.

Question 21

How does can the change in potential be shown?

Answer 21

Change in potential difference in plasma membrane of a neurone can be shown using an oscilloscope which traces changes in voltage over time. Action potential is transported along axon fibre.

Question 22

What does the action potential do to the membrane potential?

Answer 22

Due to an unequal distribution of ions, & abundance of negatively charged proteins inside axon fibre, membrane potential is found to be at ca. -70mV during resting potential. An action potential temporarily depolarises membrane to a positive value.

Question 23

How does action potential start?

Answer 23

An action potential starts when Na^+ ions suddenly rush into neuron upon stimulation of voltage gated membrane channels & reverse membrane potential making it more positive.

Question 24

What are the two subsequent stages that action potential is composed of?

Answer 24

1. Depolarisation
2. Repolarisation

Question 25

What is depolarisation?

Answer 25

Depolarisation: Cell membrane's charge becomes positive. This is caused by positive sodium ions going into cell.

Question 26

What is repolarisation?

Answer 26

Repolarisation: Cell membrane's charge returns to negative. This is caused by positive potassium ions moving out of cell.

Question 27

What causes depolarisation?

Answer 27

A stimulus causes sodium ions (Na+) to flow into cytoplasm of axon, reversing polarity of axon. This makes membrane potential more positive (from -70mV to +40mV).

Question 28

What causes repolarisation?

Answer 28

Towards end of action potential flow of sodium ions stops & potassium channels open up. This causes flow of potassium ions (K+) out of axon, bringing membrane potential back down to -70mV.

Question 29

What is action potential initiated by?

Answer 29

Action potential is initiated through activation of voltage sensitive gates on ion channels which open when a threshold voltage across membrane is exceeded.

Question 30

What is the difference between resting potential and action potential?

Answer 30

At resting potential voltage gated channel is closed. Flow of ions can only occur through leak channels or sodium potassium pump. An action potential starts once threshold potential of axon membrane reaches -50 mV. This causes voltage gated channels to open up, allowing Na+ ions to enter cell.

Question 31

Explain how depolarization is brought about.

Answer 31

Depolarisation starts with an electrical stimulus being carried along neuron fibre. This acts on voltage gated ion channels embedded within membrane. Na+ channel gates open, allowing a flow of Na+ ions following concentration gradient, into interior of cell. This makes membrane potential more positive inside compared to outside (ca. + 40 mV).

Question 32

Explain how repolarization is brought about.

Answer 32

Na+ gated channels close again, & voltage gated K+ channels now open, allowing a K+ ions to diffuse out of cell. This makes the inside of cell more negative again. Resting potential is going to be restored.

Question 33

What happens after initiation of action potential?

Answer 33

Action potential then progresses along the whole length of axon fibre.

Question 34

What is meant by the term "threshold potential"?

Answer 34

A stimulus must be at or above a minimum intensity, known as threshold of stimulation to initiate an action potential. Either depolarization is sufficient to fully reverse potential difference in cytoplasm (from –70 mV to +40 mV), or it is not.

Question 35

What are the steps to action potential?

Answer 35

1. Axon membrane is at a resting potential of -70mV and then rises to threshold potential of -50mV, either due to local currents or to binding of a neurotransmitter at a synapse. 2. Membrane depolarises due to voltage-gated Na+ channels opening & Na+ diffusing in. 3. Membrane repolarises due to voltage-gated K+ channels & K+ diffusing out. 4. Membrane returns to resting potential due to pumping of Na+ out & K+ in to axon. This rebuilds conc. Gradients of both types of ion so another AP can occur.

Question 36

Annotate diagram of action potential graph.

Answer 36

Question 37

What causes the movement of an impulse (action potential)?

Answer 37

Movement of an impulse in form of an action potential along an axon is due to diffusion of sodium ions at inside & outside of the axon fibre.

Question 38

Why does the movement of ions (NA^+ and K^+) in- and outside the axon cause small local current which effectively are responsible for propagating a stimulus?

Answer 38

Depolarization of a part of axon is caused by diffusion of Sodium ions into axon. This reduces conc outside & increases it inside. Sodium ions diffuse between diff conc inside & outside in opposite directions. Movements of ions inside & outside axons = local currents.

Question 39

What is the role of the local current?

Answer 39

Local currents reduce conc gradient in part of neuron that has not yet depolarized. This makes membrane potential rise from -70 to -50mV. Sodium channels in axon are voltage gated, which means they are triggered to open when threshold potential of -50 mV has been reached. Opening Sodium channels causes depolarisation. Local currents therefore cause a wave of depolarization followed by repolarization.

Question 40

What is the speed of signal transmission in humans? And why? what is speed of other animals in comparison?

Answer 40

Signal transmission in humans is at about 1m per second. This speed is largely due to small diameter (1µm). Some animals have nerve fibres with larger diameters. A bigger diameter reduces resistance & therefore increases conduction speed.

Question 41

Griant axons in squids have diameters of up to 500 µm. How does this help the squid as an adaptation to life?

Answer 41

Large diameter of axon reduces electrical resistance & allows faster propagation of nerve impulse. This is crucial for rapid responses, as electrical signals can travel more quickly along axon.

Question 42

What is myelin?

Answer 42

Myelin is a concentrically laminated membrane structure surrounding an axon around which lamellae (or cellular protrusions) repeat radically at a period of about 12nm. Myelin lamellae is formed by fusion of apposed inner leaflets of plasma membrane in glial cell, with no intervening cytoplasm.

Question 43

What type of cells make myelin in the Peripheral Nervous System?

Answer 43

Schwann cells make myelin in PNS

Question 44

Are all axons myelinated? Which ones are, which ones aren't?

Answer 44

No; they can be either myelinated or unmyelinated. Myelinated axon < 1-2 micro meters in diameter & unmyelinated > 2 micrometre. CNS axon <0.2 micrometres are myelinated.

Question 45

How does myelin insulation speed up signal transmission in nerve cells?

Answer 45

Myelinated nerve fibres, axons are mostly covered by myelin sheaths & transmembrane currents can only occur at nodes of Ranvier where axonal membrane is exposed. Myelin is rich in lipids meaning it acts as an insulator (high transverse resistance & a low electrical capacitance). At nodes, voltage-gated sodium channels are highly accumulated & are responsible for generation of action potential. Due to presence of insulating myelin sheath at internodes & voltage-gated sodium channels at nodes action potential in myelinated nerve jumps from one node to next.

Question 46

What type of disease cause a damage to the myelin sheath, and what are the consequences?

Answer 46

Multiple sclerosis (MS) which causes spread of demyelination in CNA lesions in entire brain. Over time, this leads to visual loss, cognitive dysfunction, motor weakness & pain. Loss in myelin causes remarkable nerve dysfunction because nerve conduction can be slowed or blocked.

Question 47

What is myelin made out of?

Answer 47

They're lipid-rich substance surrounding nerve fibre.

Question 48

What is myelin composed of? And what is speed of transmission?

Answer 48

This coating is provided by a series of Schwann cells with gaps between called nodes of Ranvier. In myelinated nerve fibres the transmission can be up to 100m/s.

Question 49

What is the effect of myelin sheath?

Answer 49

Presence of a myelin sheath surrounding axon fibre increases speed of transmission of action potential. Only at junctions in sheath (nodes of Ranvier) axon membrane exposed. Elsewhere along fibre, electrical resistance of myelin sheath prevents depolarization of nodes. This means action potentials actually ‘jump’ from node to node (this is called saltatory conduction, meaning ‘to leap’). This greatly speeds up rate of transmission.

Question 50

What are the different steps for axon with myelin and without?

Answer 50

Question 51

What is a synapse?

Answer 51

A synapse = link point between neurons.

Question 52

What parts is a synapse composed of?

Answer 52

A synapse consists of swollen tip (synaptic knob) of axon of one neuron (pre-synaptic neuron) & dendrite or cell body of another neuron (post-synaptic neuron).

Question 53

What is a synaptic clef?

Answer 53

At synapse, neurons are extremely close but they have no direct contact. Instead there is a tiny gap, called a synaptic cleft, about 20 nm wide.

Question 54

What are the three main types of synapses?

Answer 54

- Synapses between neurons, in both brain & spinal cord. - Synapses between neurons & muscle fibres (effectors). - Synapses between neurons & glands (effectors).

Question 55

What is considered to be a fourth type of synapse?

Answer 55

Another (4th type) of synapse can be found between neurons and sensory receptors of sense organ (e.g. sense organ of the skin)…

Question 56

How does action potential travel across a synapse?

Answer 56

An action potential cannot cross synaptic cleft between neurons – nerve impulse is carried by chemical transmitter substances called neurotransmitters.

Question 57

What is a neurotransmitter? And what is an example?

Answer 57

Chemical transmitter substances E.g. Acetylcholine

Question 58

Where is a neurotransmitter made and where is it stored?

Answer 58

These chemicals are made in Golgi apparatus of cell that is sending impulse (pre-synaptic neuron) & stored in vesicles at end of axon.

Question 59

What are the post synaptic receptors?

Answer 59

Postsynaptic membrane contains specialized receptors. These neuroreceptors are protein channels embedded in cell membrane of postsynaptic synapse. Receptors are chemical gated, allowing Na^+ to rush into neuron upon binding of neurotransmitter to binding site on neuroreceptor.

Question 60

How does neurotransmitters result in the generation of a new excitatory action potential?

Answer 60

1. A nerve impulse (action potential) reaches terminal end of pre-synaptic neuron. 2. Depolarisation causes voltage gated calcium channels to open. Ca2+ rushes into neuron. 3. Ca2+ causes synaptic vesicles containing a neurotransmitter to move to membrane & fuse. 4. Neurotransmitters, such as acetylcholine, that were stored in synaptic vesicle now diffuse (from high to low conc) rapidly across synaptic gap. 5. Neurotransmitter binds with neuroreceptor (protein channels) in post-synaptic membrane. 6. Proteins channels of receptors open upon binding, allowing influx of Na+ ions. An excitatory action potential is initiated, & nerve impulse is propagated along post-synaptic neuron. 7. Enzymes in synaptic gap then break down NT. Products of break down are re-absorbed by active transport (hence large number of mitochondria).

Question 61

What are neuroreceptors?

Answer 61

Neuroreceptors are protein channels embedded in cell membrane of postsynaptic synapse. Upon binding of neurotransmitter to binding side of receptor protein Na+ rushes into neuron.

Question 62

What is acetylcholine?

Answer 62

Acetylcholine is one of most common neurotransmitters & is used at many synapses, including neuromuscular junctions. It is an organic molecule composed of acetyl CoA & choline. It acts as a neurotransmitter for both, PNS & CNS. In PNS, it is main neurotransmitter in autonomic nervous system where it is responsible for activating muscles.

Question 63

What happens to avoid overstimulation at post synapse?

Answer 63

Transmitter substance on receptors is immediately inactivated by enzyme action to avoid overstimulation. Enzyme Acetylcholinesterase (AChE) hydrolyses ACh to choline & ethanoic/acetic acid, which are inactive as transmitters. As a consequence, ion channel of receptor protein closes, & resting potential in post-synaptic neuron is re-established.

Question 64

What is saltatory conduction?

Answer 64

Ion pumps & channels are clustered at nodes of Ranvier. An action potential is therefore generated only at these points, and from there propagated in “jumps” from node to node. This is called saltatory conduction.

Question 65

How does saltatory conduction effect the speed of signal transduction?

Answer 65

As a result of this, signal transduction occurs much faster (ca. 200m/s) than at unmyleinated axon fibers (2m/s).

Question 66

What are exogeneous chemicals?

Answer 66

Exogeneous chemicals are substances which enter the body from an outside source through ingestion, inhalation or absorption through the skin.

Question 67

What is the effect of neonicotinoids on bees? An what is an example of a neonicotinoid used?

Answer 67

Use of neonicotinoids as pesticides has resulted in effects on synaptic transmission in honey-bees. One of most commonly insecticides is imidacloprid, which is often applied on crop fields in order to fend off plant pests. Increasing effects these insecticides have on honey-bees have become more & more of a concern.

Question 68

What are the two types of receptors for Acetylcholine?

Answer 68

Acetylcholine binds to acetylcholine receptors. There are two types of acetylcholine receptors (AChR) on postsynaptic membrane & transmit its signal: muscarinic AChRs & nicotinic AChRs.

Question 69

How do the two types of acetylcholine receptors function differently?

Answer 69

These receptors are functionally different. Muscarinic type mediates a slow metabolic response, while nicotinic type mediate a fast synaptic transmission of neurotransmitter.

Question 70

What are neonicotinoids?

Answer 70

Neonicotinoids are a class of neuro-active insecticides chemically similar to nicotine & bind to nicotinic acetylcholine receptors of a cholinergic synapse in CNS of insects & prevent the binding of Acetylcholine. AChE cannot break down the pesticide.

Question 71

What is the effect of neonicotinoids?

Answer 71

Acetylcholinesterase can't break down neonicotinoids (ACh receptors agonists), leading to paralysis (due to blockage of Ach receptors.

Question 72

What symptoms do neonicotinoids in bees cause?

Answer 72

In insects the overstimulation at the synapses results in paralysis & death.

Question 73

Why are humans seemingly not affected by neonicotinoids contained in pesticides?

Answer 73

Neonicotinoids have mostly consequences for bees but are not toxic to humans and other mammals. This is mostly because insects have a much greater proportion of cholinergic synapses in the CNS, and the neonicotinoid also binds much stronger to acetylcholine receptors in insects.

Question 74

What is the effect of cocaine on the synaptic transmission?

Answer 74

- Cocaine acts at synapses that use dopamine as a. neurotransmitter. - It blocks receptors on dopamine re-uptake pumps, which therefore causes it to remain in the synaptic cleft. - Therefore, dopamine builds up in the synaptic gap. - Increases post-synaptic transmission & continuous excitement.

Question 75

What is the effect of cocaine on the body?

Answer 75

- Dopamine is a pleasure NT - Causes enhanced feelings of pleasure & euphoria - Increases energy & alertness - Highly addictive - Associated with depression (body reduces its own production of dopamine).

Question 76

What can happen to pre-synaptic neurons when neurotransmitter binds?

Answer 76

Pre-synaptic neurons can either excite or inhibit post-synaptic transmissions. This depends on which neurotransmitter is used, and which receptor they bind to.

Question 77

What are examples of excitatory neurotransmitters?

Answer 77

- Acetylcholine - Glutamate - Dopamine

Question 78

What are the consequences of excitatory synapses?

Answer 78

Increased influx of Na+ ions into postsynaptic membrane, membrane more positive, easier depolarization.

Question 79

What are examples of inhibitory neurotransmitters?

Answer 79

GABA Dopamine

Question 80

What are the consequences of inhibitory synapses?

Answer 80

Increased influx of Cl- ions into postsynaptic membrane & hyperpolarization. Membrane is more negative – more difficult to depolarize, impulse inhibited.

Question 81

What is alcohol an example of?

Answer 81

Alcohol (like THC or Benzodiazepines) is an example of a sedative substance which affects inhibitory neurotransmitters receptors.

Question 82

How does alcohol have an inhibitory effect? And what are the impacts on our body?

Answer 82

Alcohol binds to glutamate receptors in the brain & enhances inhibitory effects of neurotransmitter GABA which hyperpolarizes postsynaptic neurone. It also helps to increase release of dopamine. Alcohol particularly interacts with areas of brain involved in decision making, memory formation & impulse control. It impairs reaction times & muscle coordination.

Question 83

What decides whether an impulse is passed between nuerones?

Answer 83

Usually, postsynaptic neurons have many synaptic junctions with presynaptic ones. Thing that decides if an impulse is passed on to create further neural activity  is overall summation of excitatory & inhibitory input to post synaptic membrane.

Question 84

What is the effect of each input called?

Answer 84

Effect of each input from a pre-synaptic neuron is summative – & if summative effect reaches threshold, an AP is propagated in axon of post-synaptic neuron.

Question 85

What happen if both excitatory and inhibitory signals are below threshold?

Answer 85

If sum of inhibitory & excitatory signals are below threshold, no action potential will be triggered.

Question 86

What are pain receptors?

Answer 86

Pain receptors in form of free nerve endings in skin & other parts of body perceive stimuli such as a bee’s sting, heat or or puncturing of skin with a needle. Nerve endings are receptors of sensory neurons & they are associated with channels for positively charged ions.

Question 87

What happens when ion channels at pain receptors are activated?

Answer 87

When ion channels are activated & threshold potential is reached, a nerve impulse is passed through sensory neuron to spinal column & from there it is transduced to cerebral cortex in brain where pain is sensed & interpreted.

Question 88

What stimuli can act on pain receptor channel proteins?

Answer 88

Many different types of stimuli can act on the receptor channel protein to cause positive ions to move into the axon and trigger an action potential. E.g. temp, acid, specific chemical, mechanical/physical sensation

Question 89

What stimulates pain receptor channel protein when eating chilli?

Answer 89

Out of a group of many membrane proteins, one channel protein responds to both, temperature & capsaicin (a chemical compound found in chilli peppers).

Question 90

What does to be conscious mean?

Answer 90

To be conscious of something means to be aware of it. Exact way of how consciousness is generated is not entirely clear yet. However, it has been agreed on that consciousness emerges from interaction of individual neurons in brain.

Question 91

What is consciousness an example of?

Answer 91

Consciousness is an example of an emergent property (properties which arise through interactions among smaller parts that alone don't exhibit properties). It is a complex interaction between individual parts of the body.