Structure of the nerve, impulses, speed of transmission

Question 1

What are schwann cells?

Answer 1

Specialised cells that surround the axon, providing insulation and protection in the form of the myelin that they produce. Carry out phagocytosis of nearby debris (from damaged cells) and involved in nerve regeneration

Question 2

what are myelin sheaths?

Answer 2

Myelin is a type of lipid produced in the membrane of Schwann cells. Provides axon insulation. Neurones with a myelinated sheath are called myelinated neurones.

Question 3

what are nodes of ranvier?

Answer 3

Regions between Schwann cells. Exposes axon. This is where action potentials can occur.

Question 4

what is the axon terminal?

Answer 4

End of a neurone that communicates with either another neurone or effector. Releases neurotransmitter.

Question 5

what are motor neurones?

Answer 5

Receives e. i. from relay neurone and transmits to an effector (muscle or gland)

Question 6

what are sensory neurones?

Answer 6

Transmits e. i. from receptor and transmits to a relay neurone. They have long dendrons and axons

Question 7

what are relay neurones?

Answer 7

Receive e. i. from sensory neurone and transmit to motor neurone.

Question 8

what does it mean to say that the axon is polarised?

Answer 8

-This means that the inside of the axon is MORE NEGATIVELY charged compared to the tissue fluid outside (chemical gradient).

Question 9

what are neurones like at rest?

Answer 9

Neurones at rest are polarised and are easy to transmit a nervous impulse. Their reading is -70mV

Question 10

How is resting potential maintained?

Answer 10

• BASICALLY THE SODIUM POTASSIUM PUMP WHICH USES ATP -Na/K actively pumps 3 Na out for every 2 K in so the tissue fluid OUTSIDE THE AXON has more Na(positive) and the inside has more K (negative) -It stays like this because whilst NA would naturally diffuse back down the gradient INTO the axon, most of the NA channels are CLOSED -K wants to go out the axon and this happens because most of the K channels are open.

Question 11

What is action potential?

Answer 11

-When the resting potential (neg inside, positive outside) is reversed. -The negative charge of -70mV becomes +40mV

Question 12

What happens to a membrane during action potential?

Answer 12

1.The energy of a stimulus causes some Na voltage gated channels to open This allows some Na to diffuse into the axon If the *threshold is reached – this leads to an even greater influx (postive feedback) the membrane depolarises and an AP occurs (+40mV) 2. Once ~ 40mV is reached Na channels close Resting potential must be restored – ‘repolarization’ so more potassium channels opening and potassium diffuses out of the axon 3. The little dip in the graph is the brief moment when the membrane is extra negative where it is hyperpolarised.

Question 13

What causes hyperpolarization during the refractory period?

Answer 13

Caused by an ‘over-shoot’ of K+ out of the axon It means this section of membrane, momentarily, cannot allow another Action Potential And so the Threshold harder to reach in this state

Question 14

What is the benefit of having this refractory period?

Answer 14

Creates a ‘pause’ effect so each AP is discrete and uni-directional (can’t go upstream/backwards)

Question 15

Why is there such a rapid response to a really strong stimuli?

Answer 15

When you Stimulate an axon there is a small influx of Na leads to a big increase in membrane permeability which Results in even more rapid Influx! So Allows a very rapid response to all stimuli

Question 16

Describe the structure of a myelinated neurone:

Answer 16

-Cell body which is made from protein and is the site for neurotransmitter production -Dendrites which carry action potentials to surrounding cells -myelin sheath which carries the nervous impulse along neuron -Schwann cells which don’t allow charged ions or the impulse to pass through

Question 17

Describe an action potential

Answer 17

-When an impulse is received form receptors, sodium ion channels open so Na+ ions enter the neuron causing depolarisation, so the charge becomes more positive. -If depolarisation reaches the threshold potential (-50mV) voltage gated sodium ion channels are activated leading to an even higher influx which causes the AP (+40mV). -Voltage-gated Na+ channels close whilst voltage gated K+ channels open so repolarisation occurs as K+ ions leave the neurone. -This causes hyperpolarisation so the voltage gated K+ ion channels close. -Sodium-potassium pump returns neuron back to its resting potential

Question 18

Action potentials are an All or Nothing response-w hat does this mean?

Answer 18

-Once threshold potential is reached each action potential depolarises the axon to the same voltage by voltage gated sodium ion channels.

Question 19

What are non-myelinated neurons?

Answer 19

Neurons without a myelin sheath

Question 20

What is the transmission of action potentials in non-myelinated neurons?

Answer 20

-Transmission acts like a “Mexican wave” because when depolarisation occurs, voltage-gated sodium channels open further down the axon. By the time depolarisation has spread, part of the neuron has already repolarised.

Question 21

What is the transmission of action potentials like in myelinated neurons?

Answer 21

-Action potentials only occur at the Nodes of Ranvier, so action potentials “jump” from node to node which is quicker and called SALTATORY CONDUCTION

Question 22

What factors affect the speed of transmission?

Answer 22

-Myelination- increases speed -Axon diameter- the wider the axon, the more SA for movement of ions and the faster the impulse -Temperature- high temperatures so more kinetic energy for ions to travel across the membrane so increase the speed of impulse.

Question 23

Describe how myelination effects the transmission of nerv eimpulses

Answer 23

-Schwann cells wrap around the axon of neurones to create a myelin sheath. -The myelin sheath acts as a electrical insulator because it is impermeable to ions (NA+ and K+) -Depolarisation and action potentials can’t occur at the myelinated parts of the axon and can only occur in the gaps between (the nodes of ranvier) -The nervous impulse jumps from one node to the next

Question 24

What is a synapse?

Answer 24

A junction between a relay neurone and motor neurone AND between a motor neurone and a muscle cell.

Question 25

What are the features of a synapse?

Answer 25

-Synaptic knob -Post-synaptic membrane have receptors complementary to the neurotransmitters released from the synaptic knob. -Neurotransmitters which bind to complementary shaped receptors on the post -Synaptic vesicles which contain the NT -Synaptic knob where the vesicles are found. Have a lot of mitochondria so can have a lot of energy for synthesis of NT. -Pre-synaptic neurone -Synaptic cleft

Question 26

What are neurotransmitters?

Answer 26

-Chemicals that allow an action potential to be transmitted across a synapse and can be excitatory or inhibitory.

Question 27

What are excitatory neurotransmitters?

Answer 27

-Excitatory neurotransmitters generate an action potential in the post-synaptic cell. -When the neurotransmitters bind to the receptors on the post-synaptic membrane, the membrane is depolarised. -E.g when acetylcholine binds to the receptors on the post-synaptic membrane in the CNS an AP is established.

Question 28

What are inhibitory neurotransmitters?

Answer 28

-Inhibitory neurotransmitters prevent an action potential from being generated in the post. -When the NT bind to receptors on the post, the membrane is hyperpolarised and so the threshold is made harder to reach -E.g when the neurotransmitter GABA binds to a chloride ion channel, this channel opens and there’s an influx of CL- at the post-synaptic neurone. Then K+ channels open and K+ ions move out which makes the neurone more NEGATIVE(-). So no AP is stimulated

Question 29

Why is it important to balance excitation and inhibition?

Answer 29

-Excitation-inhibition balance is critical for optimal brain function (homeostasis). -Dysregulation of homeostasis is thought to be linked to diseases such as autism and schizophrenia

Question 30

What is summation?

Answer 30

-The process where neurotransmitters from multiple neurones are summed together to produce a response. -There are 2 types- spatial and temporal

Question 31

What is spatial summation?

Answer 31

• Takes place when multiple presynaptic neurones from a junction with a single neurone
• Each presynaptic neurone releases neurotransmitters. Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane

. -Together the neurotransmitters can establish a generator potential that reaches the threshold value, and an action potential is generated.

Question 32

What is temporal summation?

Answer 32

-Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time. -More neurotransmitters is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane -Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

Question 33

Describe the transmission of cholinergic synapse?

Answer 33

• Action potential at synaptic knob so membrane depolarizes and calcium voltage-gated channels open and calcium ion enter via facsilitated diffusion.
• Vesciles with acetylcholine fuse with membrane via exocytosis and release acetylcholine into the synaptic cleft which diffuses across and binds to cholinergic receptors on the post-synaptic neurone.
• Receptors change shape and Na+ channels open and let Na+ ions into the post. There’ll be a new AP if the threshhold is reached.
• Acetylcholine esterase removes acetylcholine from the receptors so they close

–esterase then breaks the acetylcholine into choline and ethanoic acid to be reabsorbed by the the pre. and reformed in new vesicles.

Question 34

Why is the transmission of neurotransmitters unidirectional?

Answer 34

Neurotransmitter is only made on pre-side and specific receptors are only found on post side.

Question 35

What happens to neurotransmission when a drug is/ is not present?

Answer 35

-Without drugs, the cycle of release, break up and reuptake maintains a normal amount of NT in the synapse -On drugs it can cause neurotransmission to increase or decrease dramatically beyond its limits.

Question 36

How can drugs stimulate more APs?

Answer 36

• Mimicking a NT • Stimulating the release of MORE NT • Inhibiting the enzyme that breaks it down • Blocking re-uptake Example- antidepressants like Prozac or analgesics like Heroin and Morphine

Question 37

How can drugs inhibit and create fewer APs?

Answer 37

• Inhibit release of NT • Block receptors Example- Sedatives like Valium

Question 38

Summarize in one sentence what drugs basically d0?

Answer 38

-All drugs of abuse target the brain’s pleasure centre and dopamine pathways.

Question 39

What is epilepsy though to be caused by? And how would you treat these?

Answer 39

• Insufficient GABA? (inhibitory neurone) • Faulty GABA receptors? Treating epilepsy? Drugs that…. -Mimic GABA or -Encourage GABA binding or -Decrease its breakdown

Question 40

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Question 41

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Question 42

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Question 43

Answer 43