Module 5: Neuronal communication

Question 1

Dendrites conduct impulse …
Axons conduct impulse …

Answer 1

Towards the cell body.
Away from the cell body.

Question 2

Explain why the cell body in a neurone is important.

Answer 2

In their cytoplasm, there are lots of ER and mitochondria that produce neurotransmitters.

Question 3

What is an effector?

Answer 3

Cells that bring about a response to a stimulus, such as cells in muscle or a gland like the pancreas.

Question 4

What is the myelin sheath?

Answer 4

Made up of Schwann cells wrapped around the axon several times and is an electrical insulator. Allows electrical impulses to jump between the nodes of Ranvier on the axon. Travels down the axon faster than unmyelinated neurone.

Question 5

What is an energy transducer? Give an example.

Answer 5

A cell that converts energy from one form to another. Sensory receptor cells respond to stimuli like light and convert this to nervous impulses (generator potential) in sensory neurones. E.g. rod cell in eye respond to light and produces a GP.

Question 6

Facts about the 3 neurones?

Answer 6

Sensory - carry impulses from sensory organs to CNS, 1 short dendron, cell body in middle, 1 short axon.
Relay - carry impulses within CNS and connect sensory and motor neurones, nonmyelinated sheath.
Motor - carry impulses from CNS to effectors, 1 long axon, short dendrites, cell body at the end.

Question 7

Thermoreceptors detect …
Photoreceptors detect …
Mechanoreceptors detect …
Proprioceptors detect …
Chemoreceptors detect …
Nociceptors detect …

Answer 7

Change in thermal energy e.g.. in tongue.
Change in light energy e.g. cone cell detects different light wavelengths in eye.
Change in kinetic energy e.g. Pacinian corpuscle detects pressure.
Stretch in muscles.
Change in chemical energy e.g. in nose.
Harmful stimuli.

Question 8

Define stimulus.

Answer 8

A change in an organism’s environment that causes a response.

Answer 9

Axon of a myelinated neurone is covered in myelin (1); myelin is an electrical
insulator (1); the sheath is formed by Schwann cells growing around the axon several times (1); there
are gaps in the myelin sheath known as nodes of Ranvier; electrical impulse moves in a series of ‘jumps’ from one node to the next/saltatory conduction; impulse transmitted much faster than along an unmyelinated axon
Myelin speeds up the transmission of nerve impulses.

Question 10

MS is an autoimmune disease so …

Answer 10

Immune system mistakenly attacks healthy body tissue, leading to damaged myelin sheath and then axons, so impulse cannot reach the CNS/brain.

Question 11

Explain how Pacinian corpuscles are a transducer. What is their structure like?

Answer 11

Sensory receptors that only detect mechanical pressure so are mechanoreceptors. E.g. convert mechanical energy like touch into an electrical impulse.
Found on the skin in fingers and soles of feet.
They contain a sensory nerve ending, which is wrapped with connective tissue called lamellae.

Question 12

Explain what happens when a Pacinian corpuscle is stimulated.

Answer 12

1. At resting potential, stretch-mediated Na channels in sensory neurone membrane are too narrow to allow Na to pass through. When pressure is applied, corpuscle changes shape and lamellae deform.
2. The stretched membrane causes the stretch-mediated Na channels to open. Na+ diffuse into cell down a conc gradient, so more positive inside. Membrane becomes depolarised.
3. This results in a generator potential as the pd in and out the membrane change. If it reaches threshold/becomes depolarised enough, it triggers an ACTION potential along the rest of the neurone to CNS.

Question 13

How do plants respond to stimuli?

Answer 13

Instead of producing nerve impulses, their receptor cells produce chemicals.

Question 14

What is pd in millivolts (mV) across the membrane when a neurone is polarised?

Answer 14

-70mV. This is at the RESTING POTENTIAL. Positively charged on outside and negative on the inside. Different charges mean there’s a pd/voltage across the membrane at its resting potential. It is maintained by sodium-potassium pumps and potassium ion channels in its membrane.

Question 15

What is the threshold potential in mV?

Answer 15

-55mV

Question 16

Pd across membrane when membrane of neurone is depolarised?

Answer 16

+30mV

Question 17

What is an action potential?

Answer 17

Brief electrical impulse that travels down the axon of a neurone.

Question 18

Explain what happens to sodium and potassium ions across a neurone cell membrane at the resting potential.

Answer 18

1. Sodium-potassium pumps (by active transport) move 3 sodium ions out of the neurone for every 2 K+ ions moved in. ATP needed to do this.
2. When the cell is at rest, most K+ channels are open, so they allow facilitated diffusion of K+ out of the neurone, down their conc gradient. Therefore the membrane is permeable to K+, so some diffuse back through the potassium ion channels.
3. The sodium ion channels are closed at rest. So the membrane isn’t permeable to sodium so they can’t diffuse back in. This creates a Na+ electrochemical gradient as there’s more positive Na+ outside the cell than inside.

Question 19

A stronger stimulus means …

Answer 19

More frequent action potentials are generated. Change in voltage is the same.

Question 20

At resting potential which channels are open?

Answer 20

Sodium/potassium pump is always active, some K+ ion channels are open, Na+ channel CLOSED.
So sodium/potassium pump and potassium ion channels create and maintain resting potential, but not Na+ ion channels.

Question 21

What is an action potential?

Answer 21

Rapid change in voltage across AXON cell membrane that send an electrical impulse in the body. Shifts from negative to positive.
Action potentials don’t overlap and are unidirectional as they have a refractory period - ion channels are recovering and can’t be made to open. No more Na+ can diffuse into neurone to fire another action potential.

Question 22

Explain what happens in a change in pd action potential graph.

Answer 22

1. Resting potential.
2. At the threshold of -55mV, stimulus triggers some voltage gated Na+ channels to open, so membrane more permeable to Na+. Na+ diffuse down electrochemical gradient into axon, making the inside less negative.
3. DEPOLARISATION - more voltage gated Na+ channels open (positive feedback).
4. REPOLARISATION - when peak pd reaches +30mV, voltage gated Na+ channels close and voltage gated K+ channels open. Membrane now more permeable to K+, so more K+ diffuse out of axon down the electrochemical gradient. (negative feedback) Inside of axon now becomes more negative.
5. HYPERPOLARISATION - K+ channels are slow to close so lots of K+ ions diffuse out of axon. Inside axon becomes more negative than resting potential.
6. REPOLARISED - Na+ channels are closed (so no more movement by facilitated diffusion), some K+ is opened and closed. The sodium/potassium pump causes Na+ to move out and K+ in, so the membrane returns to resting potential. Until membrane is excited by another stimulus.

Question 23

Explain what the refractory period is.

Answer 23

Time delay between one action potential and the next, occurs immediately after an action potential. Ion channels are recovering and can’t be made to open. Na+ channels are closed (so sodium can’t go into axon) during repolarisation. It makes sure action potentials don’t overlap and travel in one direction. It prevents another AP from being generated.
During this phase, Na+ channels are closed and K+ channels are open, sodium/potassium pump continues to work to ensure ions are correctly redistributed and resting potential is restored before another AP is generated.

Question 24

Absolute refractory period.
Relative refractory period.

Answer 24

Na+ channels are inactivated - ensures AP’S are unidirectional.

Na+ channels can open again if the stimulus is strong enough.

Question 25

Explain what happens during a wave of depolarisation.

Answer 25

When an action potential happens, some Na+ that has entered the neurone diffuse sideways. This causes voltage gated Na+ channels in the next region to open and Na+ diffuse into that part. The electrical impulse propagates along the neurone.

Question 26

Saltatory conduction and myelinated neurones.

Answer 26

Myelinated - some neurones have a myelin sheath surrounding the AXON that is an electrical insulator. Depolarisation only happens at nodes of Ranvier so much quicker conduction than non-myelinated.
Saltatory conduction is where the action potential jumps from node to node (local current flow) so myelin sheath speeds up electrical impulse. The neurones cytoplasm conducts enough electrical charge to depolarise the next node. rather than propagating continuously along the entire length of the axon.
Saltatory conduction requires less energy because the action potential is regenerated only at the nodes of Ranvier, rather than along the entire length of the axon. This makes saltatory conduction more energy-efficient compared to continuous conduction.
Sodium ions can only get through membrane at nodes of Ranvier - due to Na+ diffusion along axon’s cytoplasm.

Question 27

Myelinated neurones are only present in …

Answer 27

Sensory and motor neurones in PERIPHERAL NERVOUS SYSTEM. The sodium ion channels are concentrated at the nodes of Ranvier!
Relay neurone doesn’t as it’s close to the brain.

Question 28

Describe the structure of myelin sheath.

Answer 28

Formed by Schwann cells grown around axon several times.

Question 29

Myelin sheath in CNS is formed from …

Answer 29

Cells called oligodendrocytes.

Question 30

Describe 2 factors that effect the speed of an action potential.

Answer 30

1. Bigger axon diameter - quicker conduction of action potentials as there’s less resistance to flow of ions in cytoplasm.
2. Higher temperature - ions diffuse faster so faster impulse transmitted. Higher than 40 degrees C denatures the proteins (like Na+/K+ pump) and speed decreases.

Question 31

What is a synapse?

Answer 31

The junction between 2 neurones. Impulses are transmitted across the synapse using neurotransmitters.

Question 32

What is the synaptic cleft?

Answer 32

Gap that separates the axon of one neurone from the dendrite is the next neurone.

Question 33

Presynaptic neurone is where …
Postsynaptic neurone is where …

Answer 33

Impulse has arrived.
Neurone that receives the neurotransmitter.

Question 34

What is the synaptic knob?

Answer 34

Swollen end of presynaptic neurone that contains mitochondria and ER to manufacture neurotransmitters.

Question 35

What is the role of synaptic vesicles?

Answer 35

They fuse with the presynaptic membrane and release their neurotransmitters into the synaptic cleft.

Question 36

What are the neurotransmitter receptors?

Answer 36

Neurotransmitters bind to these in the postsynaptic membrane.

Question 37

What are the two types of neurotransmitters and explain the difference.

Answer 37

Excitatory - result in depolarisation of postsynaptic neurone. If threshold is reached in postsynaptic membrane, action potential is triggered. E.g acetylcholine.

Inhibitory - result in hyperpolarisation of postsynaptic membrane. Prevents action potential being triggered. E.g GABA found in some synapses in the brain.

Question 38

Explain how an impulse is transmitted across a synapse.

Answer 38

1. AP arrives at end of presynaptic neurone.
2. Depolarisation of presynaptic membrane causes voltage gated calcium ion channels to open, so calcium ions diffuse into presynaptic knob.
3. This influx of Ca2+ causes the cytoskeleton to move the synaptic vesicles to fuse with presynaptic membrane. So acetylcholine is released by exocytosis.
4. Acetylcholine diffuse down their conc gradient across the synaptic cleft.
5. Neurotransmitter binds to complementary cholinergic receptors on the postsynaptic membrane, causing voltage gated Na+ channels to open.
6. This causes depolarisation of post-synaptic membrane which may initiate an action potential if the threshold is reached.

Question 39

What happens to the neurotransmitter/ACH that has been released into the synaptic cleft?

Answer 39

Acetylcholinesterase (ACHe) enzyme converts acetylcholine into choline and ethanoic acid, which diffuse back across synaptic cleft into presynaptic knob by endocytosis.
ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites.

Question 40

Give a benefit of recycling the neurotransmitter from the synaptic cleft.

Answer 40

The breakdown of acetylcholine prevents it from continuously generating an action potential in postsynaptic neurone.

Question 41

Where are cholinergic synapses found?

Answer 41

In CNS and at neuromuscular junctions (where motor neurone meets muscle).

Question 42

How do synapses ensure impulses are unidirectional?

Answer 42

Neurotransmitter receptors are only present on postsynaptic membrane.
Also only presynaptic neurones contain vesicles of acetylcholine.

Question 43

Difference between synapse convergence and divergence.

Answer 43

Divergence - one presynaptic neurone to several postsynaptic neurones, resulting in a stimulus creating a number of different responses.
Convergence - several presynaptic neurones to 1 post synaptic neurone. Stimuli from different receptors interact to produce a single result.

Question 44

What is temporal summation?

Answer 44

When a single presynaptic neurone release neurotransmitter as a result of high frequency of action potentials over a SHORT period. This builds up in the synapse until the quantity is sufficient to trigger an action potential.

Question 45

What is spatial summation?

Answer 45

When lots of presynaptic neurones connect to 1 postsynaptic neurone. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential.

Question 46

What is the ‘all or nothing’ law?

Answer 46

If stimulus isn’t strong enough, threshold isn’t reached so no AP.