3.4.3 Nerve Impulses and Synaptic Transmission

Question 1

State the function of sensory neurones

Answer 1

Carriers nerve impulses from receptor towards intermediate neurones within CNS

Question 2

State the function of motor neurones

Answer 2

Carries nerve impulses away from the CNS towards an effector

Question 3

State the function of intermediate neurones

Answer 3

Connect sensory to motor neurones

Question 4

Where are intermediate neurones found?

Answer 4

Within spinal cord

(have numerous short dendrites)

Question 5

Describe the structure and function of the axon

Answer 5

Single long fibre that carriers nerve impulses

Question 6

Describe the structure and function of dendrites

Answer 6

Small extensions of cell body which carry impulses toward cell body

Question 7

Describe the structure and function of the cell body

Answer 7

• Contains a nucleus and large number of endoplasmic reticulum
• Produces neurotransmitters

Question 8

Describe the structure of the myelin sheath

Answer 8

• Covers the axon
• Made up of membranes of Schwann cells
• Membranes are rich in the lipid myelin

Question 9

Describe the structure and function of Schwann cells

Answer 9

Surround and wrap around axon, providing protection and electrical insulation

Question 10

Describe the structure of Nodes of Ranvier

Answer 10

• Small gaps between adjacent Schwann cell
• Sodium ion channels are concentrated at the nodes

Question 11

When is the sympathetic system activated?

Answer 11

In times of stress

Question 12

What is the sympathetic system responsible for?

Answer 12

For increasing heart rate and ventilation and pupil dilation

Question 13

When is the parasympathetic system active?

Answer 13

Most active in relaxed states

Question 14

What is the parasympathetic system responsible for?

Answer 14

Responsible for decreasing heart rate and ventilation rate and pupil constriction

Question 15

What does the parasympathetic system enable?

Answer 15

Enables everyday tasks to be completed (digest food, fight infections, etc.)

Question 16

Describe and explain the charge of a neurone’s resting state

Answer 16

• In neurone’s resting state, outside of membrane is positively charged compared to inside
  
  • ∵ more positive ions outside cell than inside
  • (resting potential = about -70 mv)​
• ∴ membrane = polarised
  
  • Difference in charge across it

Question 17

State how resting potential is created and maintained

Answer 17

By sodium-potassium pumps & potassium ion channels

Question 18

Describe how the resting potential is created and maintained by sodium-potassium pumps and potassium ion channels

Answer 18

1. Sodium-potassium pump uses active transport to move 3 Na⁺ out of neurone for every 2 K⁺ ions moved in
   
   1. ATP needed to do this
2. Membrane isn’t permeable to Na⁺ = can’t diffuse back
3. Creates sodium ion electrochemical gradient
   
   1. ∵ more Na+ outside cell than inside
4. Membrane is permeable to K⁺ = diffuse back out though K⁺ channels, down their concentration gradient
   
   1. facilitated diffusion
5. Makes outside of cell positively charged compared to inside

Question 19

Neurone cell membranes become ________ when they’re stimulated

Answer 19

Depolarised

Question 20

If a stimulus is big enough, it triggers rapid change in ___

Answer 20

p.d.

Question 21

Name the 5 stages of how an action potential occurs i.e. how neurone cell membranes become depolarised when they’re stimulated

Answer 21

1. Stimulus
2. Depolarisation
3. Repolarisation
4. Hyperpolarisation
5. Resting potential

Question 22

Action Potentials

1. Describe the stage stimulus

Answer 22

• Stimulus excites neurones cell membrane = Na⁺ channels to open, making membrane more permeable to Na⁺
• Na⁺ then diffuse (down electrochemical gradient) into neurone, making it less negative

Question 23

Action Potentials

2. Describe the stage depolarisation

Answer 23

Once threshold has been met (around -55mv), more Na⁺ channels open = more Na⁺ to diffuse in rapidly

Question 24

Action Potentials

3. Describe the stage repolarisation

Answer 24

1. (At around +30mV) Na⁺ channels close and the K⁺ channels open
   
   • (Na⁺ channels have to close or membrane will remain depolarised)
2. Membrane is now more permeable to K⁺
3. = K⁺ diffuse out of neurone down K⁺ conc. gradient
4. Gets membrane back to its resting potential

Question 25

Action Potentials 4. Describe the stage hyperpolarisation

Answer 25

1. K⁺ channels are slow to close so there’s a slight overshoot where too many K⁺ diffuse out of neurone 2. Causes p.d. to become more negative than resting potential

Question 26

Action Potentials 5. Describe the stage resting potential

Answer 26

1. Ion channels are reset 2. Sodium-potassium pump returns membrane to its resting potential 3. & maintains until membrane's excited by another stimulus

Question 27

Explain why after an action potential, the neurone cell membrane can't be excited again straight away

Answer 27

* ∵ ion channels are recovering & can't be made to open * Na⁺ channels are closed during repolarisation and K⁺ are closed during hyperpolarisation

Question 28

Describe how an action potential moves along a neurone

Answer 28

1. When action potential occurs, some Na⁺ that enter neurone diffuse sideways 2. Causes Na⁺ channels in next region of neurone to open and Na⁺ ions diffuse into that part 3. Causes wave of depolarisation to travel along neurone 4. Wave move away from parts of membrane in refractory period ∵ these parts can't fire an action potential

Question 29

What is the refractory period?

Answer 29

When ion channels are recovering and can't be opened

Question 30

What does the refractory period act as?

Answer 30

Acts as a time delay between 1 action potential and the next

Question 31

Name 3 things the refractory period ensures

Answer 31

* Action potentials don't overlap * But pass along discrete (separate) impulses * Limit to frequency of which nerve impulses can be transmitted * Action potentials are unidirectional (only travel in 1 direction)

Question 32

Describe the all-or-nothing nature of action potentials

Answer 32

1. Once threshold is reached, action potential will always fire with same change in voltage * No matter how big stimulus is 2. If threshold isn't reached, action potential won't fire

Question 33

What does a bigger stimulus cause?

Answer 33

Causes action potentials to fire more frequently DOESN'T = bigger action potential

Question 34

Name 3 factors that affect the speed of conduction of action potentials

Answer 34

* Temperature * Axon Diameter * Myelination

Question 35

Describe and explain how temperature affects the speed of conduction of action potentials

Answer 35

* Speed of conduction increases as temp. increase * ∵ ions diffuse faster * Speed only increases up to 40°C * After proteins, denature & speed decreases * Active transport is used for sodium-potassium pump & enzymes are used

Question 36

Describe how axon diameter affects the speed of conduction of action potentials

Answer 36

Bigger diameter = faster the conduction of action potentials

Question 37

Explain why action potentials are conducted quicker along axons with bigger diameters

Answer 37

* ∵ there's less resistance to flow of ions in cytoplasm of bigger axon * ∵ less leakage of ions from axon * With less resistance, depolarisation reaches other parts of neurone cell membrane quicker

Question 38

In myelinated neurone, where does depolarisation only occur?

Answer 38

At nodes of Ranvier

Question 39

Describe and explain how myelination increases the speed of conduction of action potentials

Answer 39

* Neurone's cytoplasm conducts enough electrical charge to depolarise the next node ∴ impulse jumps from node to node * Called saltatory conduction & it's very fast * In non-myelinated neurone, impulse travels as wave along whole length of axon membrane * (Get depolarisation along whole length of membrane) * Slower than saltatory conduction

Question 40

What is a synapse?

Answer 40

Junction between 2 neurones or between neurone and effector cell

Question 41

What is a synaptic cleft?

Answer 41

Tiny gap between cells at synapse

Question 42

What is the synaptic knob?

Answer 42

The presynaptic neurone that has a swelling

Question 43

Synaptic knob contains synaptic vesicles that are filled with \_\_\_\_\_\_

Answer 43

neurotransmitters

Question 44

Explain how synapses make sure impulses are unidirectional

Answer 44

* (Vesicles containing) neurotransmitter only in presynaptic membrane/neurone * Receptors are only on postsynaptic membranes

Question 45

Describe what happens briefly when an action potential reaches the end of a neurone

Answer 45

* Causes neurotransmitters to be released into synaptic cleft * Diffuse across to postsynaptic membrane & bind to specific receptors * Can trigger action potential & cause muscle contraction or hormone secretion

Question 46

Describe what happens in the synapse to stop a response from keep happening

Answer 46

* Neurotransmitters are removed from cleft * Taken back into presynaptic neurone or broken down by enzymes

Question 47

What is a cholinergic synapse?

Answer 47

Synapses that use acetylcholine (neurotransmitter - ACh)

Question 48

Describe how a nerve impulse is transmitted across a cholinergic synapse

Answer 48

1. Arrival of action potential (at synaptic knob) causes calcium ion channels to open and Ca²⁺ to enter synaptic knob 2. Influx of Ca²⁺ causes synaptic vesicles to fuse with presynaptic membrane * Releases ACh from vesicles into synaptic cleft (exocytosis) 3. ACh diffuse across synaptic cleft and bind to specific cholinergic receptors on postsynaptic membrane 1. Na⁺ channels in postsynaptic neurone to open 4. Influx of Na⁺ causes depolarisation & generates action potential in postsynaptic neurone (if threshold reached) 5. AChE (acetylcholinesterase) hydrolyses ACh into choline and ethanoic acid (acetyl) which diffuses back into presynaptic neurone 1. Removed so that its response doesn't keep happening 2. Sodium channels close 6. ATP is released by mitochondria to recombine choline and ethanoic acid to form ACh * Stored in synaptic vesicles for future use

Question 49

Neurotransmitters can be both _____ and \_\_\_\_\_\_

Answer 49

Neurotransmitters can be both _excitatory_ and _inhibitory_

Question 50

What are excitatory neurotransmitters?

Answer 50

* Neurotransmitters that depolarise postsynaptic membrane * Making it fire action potential if threshold is reached

Question 51

State where is acetylcholine is excitatory

Answer 51

At cholinergic synapse in CNS

Question 52

State what (excitatory) acetylcholine does at the cholinergic synapses in CNS

Answer 52

Binds to cholinergic receptors = causes action potential in postsynaptic membrane and neuromuscular junctions

Question 53

What are inhibitory neurotransmitters?

Answer 53

* Hyperpolarise postsynaptic membrane (make pd more negative) * Preventing it from firing action potential

Question 54

State where is acetylcholine is inhibitory

Answer 54

At cholinergic synapse in heart

Question 55

State what (inhibitory) acetylcholine does at the cholinergic synapses in the heart

Answer 55

Binds to receptors = causes potassium ion channels to open on postsynaptic membrane = hyperpolarising it

Question 56

What is summation?

Answer 56

The effect of neurotransmitter released from many neurones is added together

Question 57

What does summation allow?

Answer 57

Allows synapses to accurately process info, finely turning the response

Question 58

Name 2 types of summation

Answer 58

* Temporal summation * Spatial summation

Question 59

What is temporal summation?

Answer 59

Where 2 or more nerve impulses arrive in quick succession from same presynaptic neurone

Question 60

Why does temporal summation make an action potential more likely?

Answer 60

∵ more neurotransmitters is released into synaptic cleft

Question 61

Describe how spatial summation results in an action potential

Answer 61

* Sometimes many neurones connect to 1 neurone * Small amount of neurotransmitter released from each neurone = enough altogether to reach threshold in postsynaptic neurone & trigger action potential

Question 62

What is a neuromuscular junction?

Answer 62

Is synapse between motor neurone and muscle cell

Question 63

What neurotransmitter is used in a neuromuscular junction?

Answer 63

* ACh * Binds to cholinergic receptors (called nicotinic cholinergic receptors) * Work same way as cholinergic synapse

Question 64

Name 3 differences between neuromuscular junctions and cholinergic synapses

Answer 64

* Postsynaptic membrane has lots of folds that form clefts * Clefts stores acetylcholinesterase (AChE) * Postsynaptic membrane has more receptors than other synapses * ACh is always excitatory at neuromuscular junction * When motor neurone fires action potential = triggers response in muscle cell * Isn't the case for synapse between 2 neurones

Question 65

Name or describe 5 types of drugs that affect synaptic transmission

Answer 65

* Agonists * Antagonists * Some inhibit enzymes that break down neurotransmitters * Some stimulate the release of neurotransmitter from presynaptic neurone so more receptors are activated or vice versa

Question 66

Drugs Affect Synaptic Transmission What are agonists?

Answer 66

* Have same shape as neurotransmitters = mimic their action at receptors * Means more receptors are activated * e.g. nicotine mimics ACh so binds to nicotine cholinergic receptors in brain

Question 67

Drugs Affect Synaptic Transmission What are antagonists?

Answer 67

* Block receptors so they can't be activated by neurotransmitters * Fewer receptors can be activated * e.g. curare blocks effects of ACh by blocking nicotinic cholinergic receptors at neuromuscular junctions * So muscle cells can't be stimulated = paralysed

Question 68

Drugs Affect Synaptic Transmission Describe the benefit of drugs that inhibit enzymes that break down neurotransmitters

Answer 68

* ∴ there's more neurotransmitters in synaptic cleft to bind to receptors and they're there for longer * e.g. nerve gases stop ACh from being broken down = loss of muscle control

Question 69

An action potential is generated at the cell body of the motor neurone. Explain how this action potential passes along the motor neurone to the neuromuscular junction. (3)

Answer 69

* Depolarisation of axon membrane/influx of Na⁺ establishes local currents * Change permeability to Na⁺/open Na⁺ gates of adjoining region * Adjoining region depolarises/influx of Na⁺

Question 70

In an investigation, the higher the concentration of sucrose in a rat's mouth, the higher the frequency of nerve impulses from each taste receptor to the brain. If rats are given very high concentrations of sucrose solution to drink, the refractory period makes it impossible for information about the differences in concentration to reach the brain. Explain why. (2)

Answer 70

1. (Refractory period) leads to separate nerve impulses * OR (Refractory period) limits frequency of nerve impulses 2. When maximum frequency reached, no further increase in information/all (higher) concentrations of sucrose seem the same

Question 71

The rate of ATP consumption in a non-myelinated neurone is greater than that of a myelinated neurone when conducting electrical impulses at the same frequency. Explain why. (2)

Answer 71

* Greater entry of sodium ions * Active transport