Chapter 13: Neuronal Communication

Question 1

Outline process of cell signalling.

Answer 1

1. Release of signalling molecules by exocytosis.
2. Glycoproteins have receptors.
3. Receptors are specific.
4. Shape of signalling molecule + receptor are complementary.
5. Attachment of cell signalling molecule causes change on cell surface membrane.
6. Cell surface membrane allows entry of some signalling molecules.

Question 2

What is the route an impulse takes within the neuronal pathways?

Answer 2

1. Stimulus.
2. Receptor.
3. Sensory neurone.
4. CNS.
5. Relay neurone.
6. Motor neurone.
7. Effector –> muscle or gland

Question 3

Structure of cell body?

Answer 3

• Nucleus.
• Cytoplasm –> large amounts of ER and mitochondria –> make neurotransmitters.
• Aerobic respiration to produce ATP + protein synthesis.

Question 4

Structure + function of sensory neurone?

Answer 4

• One axon –> carries impulse away from cell body.
• One dendron –> carries impulse to cell body.
• Transmits impulse from sensory receptor to relay neurone, motor neurone or brain.

Question 5

Structure + function of motor neurone?

Answer 5

• One long axon –> in peripheral nervous system.
• Many short dendrites.
• Cell body found in CNS.
• Transfers impulse from relay/sensory neurone to effector muscle/gland.

Question 6

Structure + function of relay neurone?

Answer 6

• Many small axons + dendrites.
• Found in CNS.
• Transfer impulses between neurones.

Question 7

Define resting potential.

Answer 7

• The p.d. across the membrane of the axon of the neurone at rest (normally -65mV).

Question 8

Define action potential.

Answer 8

• The change in p.d. across the membrane of the axon of the neurone when stimulated (normally +40mV).

Question 9

Describe role of sensory receptor.

Answer 9

• Specific to a single type of stimulus.
• Detect stimulus and convert it to a nervous impulse/Action potential.
• E.g. Pacinian corpuscle.

Question 10

Describe how the Pacinian corpuscle converts mechanical pressure into an action potential/nervous impulse.

Answer 10

1. In resting state –> stretch mediated sodium channels in the sensory neurone’s membrane are too narrow for Na+ ions to diffuse in.
2. The neurone of the Pacinian corpuscle (Pc) has a resting potential.
3. When pressure applied to Pc –> changes shape + stretches membrane of the sensory neurone, widening the stretch mediated sodium channels.
4. Sodium channels wide enough for sodium ions to diffuse into the sensory neurone.
5. Influx of Na+ ions increases p.d. –> depolarisation + creates a generator potential.
6. Generator potential creates an action potential that passes along sensory nerve.
7. AP then travels along neurones to the CNS.

Question 11

How is a resting potential created?

Answer 11

1. Sodium ions actively pumped out of axon + potassium ions actively pumped in by sodium-potassium pump.
2. For every 3 sodium ions pumped out 2 potassium ions are pumped in –> 3:2 ratio.
3. More sodium ions outside ions outside membrane than inside axon + more potassium ions inside axon than outside.
4. Sodium ions diffuse into axon down electrochemical gradient and potassium ions diffuse out.
5. Voltage gated sodium ion channels close –> prevents diffusion of sodium ions into axon BUT voltage gated potassium ion channels remain open –> K+ continues to diffuse out of axon by facilitated diffusion.
6. More positively charged ions outside axon than inside –> creates RP across membrane of -70mV with inside negative relative to outside.

Question 12

Creation of an action potential (AP).

Answer 12

1. Neurone has RP –> some voltage gated potassium ions open but all voltage gated sodium ions closed.
2. Energy of stimulus –> triggers some voltage gated sodium ion channels to open –> more permeable to Na+ allows diffusion of sodium ions into axon –> depolarisation
3. P.d. becomes more positive –> reaches threshold.
4. Change in charge causes even more voltage gated sodium ion channels to open –> even more Na+ ions diffuse into axon –> example of positive feedback.
5. When p.d. reaches +40mV –> voltage gated sodium ion channels close –> impermeable to Na+ and voltage gated potassium ion channels open –> more permeable to K+.
6. Allows K+ to diffuse out of axon down electrochemical gradient –> reduce charge –> inside more negative relative to outside.
7. Hyperpolarisation –> initially lots of K+ diffuse out of axon, making inside of axon more negative than the resting potential.
8. Voltage-gated potassium ion channels close –> preventing movement of K+
9. Sodium potassium pump pumps Na+ out of axon and K+ into axon so original resting potential reached –> now repolarised.

Question 13

Explain how an AP is propagated?

Answer 13

1. Initial stimulus causes change in sensory receptor –> triggers AP in sensory receptor –> first region of axon membrane depolarised.
2. Acts as the stimulus for the depolarisation of the next region.
3. Proceeds along the length of the axon forming a wave of depolarisation.
4. Once in the axon –> Na+ ions attracted to the negative charge ahead + concentration gradient to diffuse further along axon –> depolarises next region.
5. Region of axon membrane that has undergone depolarisation is now repolarised to reach its original RP.
6. Refractory period:
   - Period within which an AP cannot be excited again.
   - Closes all voltage-gated sodium ion channels –> prevent movement of sodium into axon.
   - Prevents propagation of AP forwards or backwards.
   - Makes sure APs do not overlap, are unidirectional and occur in discrete impulses –> limit frequency of impulses.

Question 14

Describe the process of saltatory conduction.

Answer 14

1. Myelinated axons transfer electrical impulses faster because depolarisation of the axon only occurs at the Nodes of Ranvier where there is no myelin present.
2. Here Na+ ions diffuse into axon via facilitated diffusion in protein channels in the membrane.
3. Longer localised circuits arise between adjacent nodes.
4. AP jumps from one node to another.
5. Doesn’t require ATP as there is no repolarisation or need for sodium potassium pump.

Question 15

Describe the synaptic cleft.

Answer 15

• Gap which separates axon of one neurone and dendron of the next.

Question 16

Describe the presynaptic neurone.

Answer 16

• Neurone along which the AP has arrived.

Question 17

Describe postsynaptic neurone.

Answer 17

• Neurone which receives the neurotransmitter.

Question 18

Describe the synaptic knob.

Answer 18

• Swollen end of the presynaptic neurone.
• Contains lots of mitochondria –> ATP to make neurotransmitters.
• Contains lots of ER –> manufacture neurotransmitters.

Question 19

Describe the synaptic vesicles.

Answer 19

• Contain neurotransmitter.

- Move towards + fuse with presynaptic membrane, releasing neurotransmitter into synaptic cleft by exocytosis.

Question 20

Describe the steps of synaptic transmission (muscle contraction in the neuromuscular junctions).

Answer 20

1. AP reaches end of presynaptic neurone.
2. Depolarisation of presynaptic membrane causes calcium ion channels to open.
3. Calcium ions diffuse into presynaptic knob.
4. Synaptic vesicles (containing neurotransmitters) –> fuse with presynaptic membrane.
5. Neurotransmitter released into synaptic cleft by exocytosis.
6. Neurotransmitter diffuse across synaptic cleft and bind to specific + complementary receptors on the sodium channels on postsynaptic membrane.
7. Causes sodium ion channels to open.
8. Sodium ions diffuse into the postsynaptic neurone.
9. Triggers AP + impulse propagated along postsynaptic neurone.

Question 21

What happen to acetylcholine (neurotransmitter) after AP has been triggered?

Answer 21

1. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) –> diffuse across synaptic cleft into presynaptic neurone.
2. Hydrolysis of acetylcholine prevents it from continuously generating APs in the postsynaptic neurone.
3. ATP –> from mitochondria –> recombines choline + ethanoic acid into acetylcholine which is stored in the synaptic vesicles for future use.
4. Sodium ion channels close due to the absence of acetylcholine in the binding sites.

Question 22

Describe difference between CNS and peripheral nervous system.

Answer 22

• CNS = brain + spinal cord.

- PNS = all the neurones that connect the CNS to the rest of the body –> motor + sensory neurone.

Question 23

Describe difference between autonomic and somatic control.

Answer 23

• Autonomic = involuntary.

- Somatic = voluntary.

Question 24

Difference between anterior + posterior pituitary gland.

Answer 24

• Anterior = produces reproductive + growth hormones.

- Posterior = produces hormones released by hypothalamus.

Question 25

How do reflex impulses increase survival?

Answer 25

• Innate –> present from birth + do not need to be learned.
• Involuntary responses –> decrease time taken to respond.
• Extremely fast –> only one or two synapses involved.

Question 26

Describe structure of skeletal muscle.

Answer 26

• Striated.
• Made up of bundles of muscle fibres –> tubular and multi-nucleated.
• Conscious control.
• Rapid contraction + short response.
• Each muscle fibre made up of myofibrils –> made up of actin + myosin.

Question 27

Describe structure of cardiac muscle.

Answer 27

• Myogenic.
• Fibres branched + uninucleated + striated.
• Allows heart to contract in regular rhythm.
• Intermediate contraction + intermediate length of response.

Question 28

Describe structure of involuntary muscle (Smooth Muscle)

Answer 28

• Fibres spindle shaped + uninucleated + non-striated.
• Slow contraction but long lasting response.
• E.g. blood vessels + digestive tract –> vasoconstriction/dilation + peristalsis.

Question 29

Describe the I (light) + A (dark) bands.

Answer 29

• I-band –> regions light as there is no overlap between thin actin + thick myosin filaments.
• A-band –> dark due to thick myosin + overlap between myosin + actin.

Question 30

Outline the Z-line.

Answer 30

• Line found at centre of each light band.

- Sarcomere –> distance between adjacent Z-lines –> shortens when muscle contract.

Question 31

Outline the H-zone.

Answer 31

• Light region found at the centre of the dark band.
• Only myosin filaments present.
• Muscle contract –> H-zone decreases

Question 32

Structure of myosin.

Answer 32

• Globular heads –> binding sites of actin + ATP.
• Heads –> hinged –> allow them to move backwards + forwards.
• Tails wrap around myosin molecules to form the filament.

Question 33

Structure of actin.

Answer 33

• Many actin-myosin binding sites.

- Binding sites blocked by tropomyosin.

Question 34

How does muscle contraction occur in the sarcoplasm?

Answer 34

1. Tropomyosin prevents binding of myosin to actin-myosin binding sites on actin.
2. When AP reaches sarcoplasmic reticulum –> Ca2+ ion channels open:
   - Ca2+ ions diffuse into sarcoplasm down conc. gradient + bind to troponin –> pull tropomyosin out of the binding site.
3. Myosin heads bind to actin to form actin-myosin cross bridge.
4. Myosin heads flex –> pulling actin filament along + ADP released from myosin head.
5. ATP –> binds to myosin head –> detaches from actin filament
6. Ca2+ ions –> activate ATPase of myosin –> hydrolyses ATP into ADP and phosphate –> returns myosin to its original position on the filament + releases energy.
7. Head of myosin reattaches to binding site further along the filament + process repeats –> continuous flexing –> continuous shortening of sarcomere –> muscle contraction.

Question 35

How is high levels of creatine phosphate in muscle cells advantageous for athletes during exercise?

Answer 35

• O2 cannot be replaced as fast as it is produced.
• Aerobic respiration not enough to meet demands/anaerobic respiration needed.
• Creatine phosphate –> reserve supply of phosphate.
• The more creatine phosphate –> the more ADP can be phosphorylated.
• Muscles perform at maximum rates for longer.

Question 36

Why does a lack of ATP prevent muscles relaxing.

Answer 36

• ATP needed to break myosin-actin cross bridges.
• No ATP available –> myosin remains bonded to actin.
• Filaments remain in contracted state.
• Filaments can’t slide back to original position.

Question 37

What happens when length of sarcomere increases?

Answer 37

• Overlap between actin + myosin decreases.
• Fewer cross-bridges form during contraction.
• Reduced power stroke.

Question 38

What happens when length of sarcomere decreases?

Answer 38

• Reduced sliding of filament.

- Reduced contraction of muscle.

Question 39

Function of cerebrum?

Answer 39

• Controls voluntary actions –> learning, memory, personality and conscious thought.

Question 40

Function of cerebellum?

Answer 40

• Controls unconscious functions –> posture, balance + non-voluntary movement.

Question 41

Function of medulla oblongata?

Answer 41

• Used in autonomic control –> heart rate + breathing rate.

Question 42

Function of hypothalamus?

Answer 42

• Regulatory centre for temp + water balance.

Question 43

Function of pituitary gland?

Answer 43

• Stores + releases hormones that regulate many body functions.

Question 44

Outline ways in which structures of sensory neurone + motor neurone are similar.

Answer 44

• Both have myelin sheath covered by Schwann cells.
• Both have sodium-potassium pump.
• Both have voltage-gated channels.
• Both have dendrites and an axon.
• Both have cell body.

Question 45

Outline role of synapses in nervous system.

Answer 45

• Allows neurones to communicate/cell signalling.
• Ensures transmission between neurones is unidirectional.
• Allows impulses to be passed from one neurone to many neurones.
• Allows impulses to be passed from many neurones to one neurone.
• Only stimulation that is strong enough will be passed on –> filters out low level stimuli.
• Prevents fatigue/over-stimulation.
• Allows many low level stimuli to be amplified.
• Presence of inhibitory + stimulatory synapses allows neurones to follow a specific path.
• Permits memory/learning/decision making.

Question 46

Why is conduction of action potential faster in myelinated neurone than non-myelinated neurone?

Answer 46

• Depolarisation only occurs where Na+ channels are present.
• Myelinated have longer sections with no Na+ channels present.
• Ion movement can only take place at nodes of Ranvier.
• Longer localised circuits.
• Saltatory conduction.

Question 47

Difference in structure between motor and sensory neurone?

Answer 47

Motor:

• Cell body in CNS.
• Cell body at end of neurone.
• Dendrites connect directly to cell body.
• Longer axon.
• Dendron absent.
• Ends at motor end plate.

Sensory:

• Cell body in PNS.
• Cell body in middle of neurone.
• Dendrites at the ends of axon/dendron –> do not connect directly to cell body.
• Shorter axon.
• Dendron present.
• Starts at sensory receptor.

Question 48

What is the function of the myelin sheath/Schwann cells?

Answer 48

• Produces myelin.
• Prevents depolarisation –> movement of ions into/out of neurone.
• Saltatory conduction.
• Electrical insulation.
• Speeds up conduction of impulse/AP.
• AP/local circuits/depolarisation only occur at nodes of Ranvier.

Question 49

How do synapses allow transmission to be unidirectional?

Answer 49

• Only presynaptic neurone releases acetylcholine.
• Only presynaptic membrane has Ca2+ ion channels.
• Only postsynaptic membrane has acetylcholine receptors.
• Acetylcholine broken down at postsynaptic membrane.