6.2 Nervous Coordination + Synapses

Question 1

Neurone vs nerve

Answer 1

A nerve is a bundle of neurones

Question 2

Where do motor neurones go from/to?

Answer 2

From CNS to effector

Question 3

Function of dendrites

Answer 3

Carrying action potentials/nerve impulses towards the body
Connecting to many other neurones and receiving impulses from them, forming a network for communication

Question 4

Structure & function of cell body

Answer 4

Lots of RER
Production of proteins & neurotransmitters
Contains typical organelles in animal cell

Question 5

Function of axon

Answer 5

Carries impulses away from cell body along the motor neurone

Question 6

What is the myelin sheath made of?

Answer 6

Lipid/Schwann cells

Question 7

Function of myelin sheath

Answer 7

Doesn’t allow charged ions to pass through (insulation!)
Speeds up impulse transmission

Question 8

Where are the nodes of Ranvier?

Answer 8

Small uninsulated sections of the axon (sections without the myelin sheath)

Question 9

What do nodes of Ranvier enable?

Answer 9

Saltatory conduction: the impulse jumps from one node to the next, which speeds up conduction of the impulse

Question 10

Function of Schwann cells

Answer 10

Protect the axon & provide electrical insulation
Phagocytosis (removal of cell debris)
Nerve regeneration

Question 11

How exactly do Schwann cells form the myelin sheath?

Answer 11

They wrap around the axon along its length

Question 12

What type of transport is associated with Na+ & K+?

Answer 12

Facilitated diffusion!

Question 13

Are the sodium-potassium pumps in the axon considered as co-transport? Why/why not?

Answer 13

NONONONO
Na+ & K+ are moving in DIFFERENT DIRECTIONS

Question 14

What is resting potential?

Answer 14

When there is no stimulus, so no impulse. There is a p.d. between the outside and inside of the axon.
The inside is more negative than the outside. ~-65 mV

Question 15

What two factors are responsible for establishing and maintaining the resting potential?

Answer 15

1. Active transport of Na+ & K+
2. Differential membrane permeability

Question 16

Factors responsible for establishing & maintaining resting potential: active transport

Answer 16

Sodium-potassium pumps (carrier proteins) use ATP to actively transport 3 Na+ OUT for every 2 K+ actively transported IN.
There is a larger [+ve ions] outside the axon than inside.
Movement of ions via Na-K pumps establishes an ELECTROCHEMICAL GRADIENT

Question 17

Factors responsible for establishing & maintaining resting potential: differential membrane permeability

Answer 17

Proteins channels MORE PERMEABLE TO K+ than Na+ (more K+ channels, some channels remain permanently open).
This means K+ can diffuse back down their conc gradient, out of the axon, at a FASTER RATE than Na+

Question 18

What is an action potential/what is it caused by?

Answer 18

When an impulse is passed along a neurone, p.d. across membrane briefly reversed (i.e. inside more +ve than outside).
This is due to the rapid movement of Na+ & K+ across the membrane of the axon.

Question 19

How is an action potential formed? (4)

Answer 19

1. Stimulus causes voltage-gated Na+ channels in the axon membrane to open
2. DEPOLARISATION: Na+ diffuse into the cell DOWN THE ELECTROCHEMICAL GRADIENT
3. Depolarisation triggers more channels to open: more Na+ enters, causes more depolarisation (positive feedback)
4. If the threshold potential is reached, an action potential of around +40 mV is generated

Question 20

How does hyperpolarisation happen?

Answer 20

1. Voltage-gated Na channels close, voltage-gated K channels open
2. K+ diffuses out of axon, so axon becomes more negative
3. Potential differences more negative than -70mV: hyperpolarised
4. Voltage-gated K channels close

Question 21

How does axon return to resting potential after hyperpolarisation?

Answer 21

Voltage-gated K channels close
Na+ and K+ restored back to original positions by Na-K pump

Question 22

Why does the membrane enter a refractory period?

Answer 22

Can’t be stimulated: Na channels are recovering & can’t be opened

Question 23

Why is the refractory period important (3 reasons)?

Answer 23

1. DISCRETE IMPULSES produced - action potential CAN’T be generated immediately after another, so each is separate
2. action potentials TRAVEL IN ONE DIRECTION: stops it spreading out in two directions, which would prevent a response
3. LIMITS # OF IMPULSES TRANSMITTED: prevents overreacting

Question 24

How does temperature affect speed of conductance/transmission along an axon?

Answer 24

Ions diffuse faster
Enzymes involved in respiration work faster: more ATP for active transport in Na/K pump

Question 25

How does axon diameter affect speed of conductance/transmission along an axon?

Answer 25

Wider diameter = higher speed Less leakage of ions so action potential travels faster

Question 26

How does having a myelin sheath affect speed of conductance/transmission along an axon?

Answer 26

Myelin sheath is insulating, so ions can't diffuse in/out, so they jump to regions without insulation (SALTATORY CONDUCTION) Action potential jumps from node to node, so faster b/c doesn't have to generate it along the entire length

Question 27

What is the all or nothing principle?

Answer 27

If depolarisation doesn't exceed the threshold potential (-55 mV), no action potential is produced. Any stimulus that triggers depolarisation to -55 mV ALWAYS PEAKS AT THE SAME MAXIMUM VOLTAGE

Question 28

Effect of bigger stimuli on action potentials

Answer 28

Bigger stimuli increase the FREQUENCY of action potentials!! ALL ACTION POTENTIALS ARE THE SAME SIZE!!!!

Question 29

What organelles are inside the pre-synaptic neurone?

Answer 29

Mitochondria Smooth endoplasmic reticulum Vesicles containing neurotransmitters

Question 30

Describe synapse transmission (7)

Answer 30

1. Action potential arrives at SYNAPTIC KNOB 2. Depolarisation of synaptic knob leads to VOLTAGE-GATED Ca channels to open, so Ca2+ diffuses into synaptic knob via FACILITATED DIFFUSION 3. Vesicles with neurotransmitters move towards & FUSE with presynaptic membrane 4. Vesicles release neurotransmitters into SYNAPTIC CLEFT 5. Neurotransmitters diffuse & bind to receptors on Na+ channels in post-synaptic membrane 6. Na+ channels open, Na+ diffuses in 7. If enough Na+ moves into post-synaptic neurone to exceed threshold potential, depolarisation occurs

Question 31

How does the neurotransmitter go back into the presynaptic neurone?

Answer 31

Enzymes hydrolyse the neurotransmitter, which diffuse back across the synaptic cleft into the presynaptic neurone. This prevents neurotransmitters from continuously generating new action potentials, so THE TRANSFER OF INFORMATION IS DISCRETE. ATP combines hydrolysed components of neurotransmitters back together & into vesicles

Question 32

Named example of a neurotransmitter and how it is restored back to the neurotransmitter?

Answer 32

Acetylcholine Acetylcholinesterase hydrolyses it into acetyl & choline

Question 33

How do synapses ensure that action potentials are unidirectional?

Answer 33

1. Neurotransmitters are only released from the presynaptic neurone 2. Only receptors for neurotransmitters in postsynaptic neurone, only voltage-gated Ca channels in presynaptic neurone

Question 34

What is summation, and what are the two types?

Answer 34

The rapid buildup of neurotransmitters to help generate an action potential by two methods: spatial or temporal summation

Question 35

Why is summation needed?

Answer 35

Some action potentials are too low frequency, so they don't release neurotransmitters of a sufficient concentration to generate an action potential

Question 36

Describe neurone arrangement of spatial summation

Answer 36

2 presynaptic, 1 postsynaptic

Question 37

How does spatial summation work?

Answer 37

An action potential is only triggered (i.e. threshold potential is only exceeded at the postsynaptic neurone) if neurotransmitters are released from two presynaptic neurones instead of one

Question 38

Describe neurone arrangement of temporal summation

Answer 38

1 presynaptic, 1 postsynaptic

Question 39

How does temporal summation work?

Answer 39

One neurone release neurotransmitters at a high frequency (several action potentials in quick succession) in order to exceed threshold value

Question 40

How do inhibitory synapses work?

Answer 40

1. Neurotransmitters diffuse across synaptic cleft: bind to receptors on post-synaptic membrane 2. This causes CHLORIDE ION CHANNELS to open, so Cl- moves into postsynaptic neurone 3. K+ channels also open, so K+ moves OUT of the postsynaptic neurone 4. Inside of postsynaptic neurone more negative relative to outside: HYPERPOLARISATION This means an action potential is less likely to be triggered in the postsynaptic neurone

Question 41

How do some drugs stimulate the nervous system

Answer 41

Creating more action potentials in the post-synaptic neurone 1. Mimicking neurotransmitter 2. Stimulating release for more neurotransmitter 3. Inhibiting breakdown of neurotransmitter

Question 42

How do some drugs inhibit the nervous system?

Answer 42

Creating fewer action potentials in postsynaptic neurone 1. Inhibiting release of neurotransmitter 2. Blocking receptors on post-synaptic membrane