Topic 16 - Nervous Coordination and Muscles

Question 1

Structure of a neurone: cell body

Answer 1

Associated with production of proteins and neurotransmitters.

Question 2

Structure of a neurone: dendrite

Answer 2

Carries nerve impulses towards the cell body.

Question 3

Structure of a neurone: axon

Answer 3

Single long fibre that carries nerve impulses away from the cell body.S

Question 4

Structure of a neurone: myelin sheath

Answer 4

Covers the axon, made up of membranes from Schwann cells.

Question 5

Structure of a neurone: Nodes of Ranvier

Answer 5

Constrictions between adjacent Schwann cells where there is no myelin sheath.

Question 6

Why is it unlikely that the polypeptides and proteins reach the synaptic knobs by a process of simple diffusion?

Answer 6

Myelin sheath of Schwann cells is lipid soluble.

Question 7

How is the resting potential established?

Answer 7

1) Three sodium ions are actively pumped out of axon, and two potassium ions are actively pumped into axon by sodium potassium pump.
2) Forms an electrochemical gradient.
3) Sodium ions try to diffuse back in, potassium ions try to diffuse out axon.
4) Many voltage-gated Na+ channels closed, most of voltage-gated K+ channels open.
5) Potassium ions diffuse out of axon faster than sodium ions can diffuse in, via facilitated diffusion.
6) Tissue fluid becomes positively polarised, cytoplasm negatively polarised.
7) Very positive charge in tissue fluid, membrane in state of polarisation.
8) Equilibrium is achieved, no net movement of ions.
9) Electrical gradient becomes balanced, resting potential is established.

Question 8

What is an action potential?

Answer 8

1) At resting potential, voltage gated Na+ channels are shut.
2) When a stimulus occurs, a threshold value is reached, causing some of the Na+ channels to open.
3) Na+ diffuse from tissue fluid through the voltage-gated Na+ channels into cytoplasm of axon down electrochemical gradient. More Na+ channels open.
4) Depolarisation.
5) Action potential (+40mV) is established, voltage-gated Na+ channels close, voltage-gated K+ channels open.
6) Electrical gradient is reversed, more open.
7) More K+ diffuse into tissue fluid.
8) Hyperpolarisation
9) Voltage-gated K+ channels close, Na+/K+ pump moves 3 Na+ out and 2 K+ in.
10) Resting potential (-70mV) re-established, axon repolarised.

Question 9

The passage of an action potential in unmyelinated neurones

Answer 9

1) Resting potential - axon membrane polarised.
2) Initiation of 1st action potential.
3) Stimulation of next action potential.
4) Repolarisation of the axon.
5) Returning to resting potential.

Question 10

The passage of an action potential in myelinated neurones

Answer 10

Myelin sheath acts as an electrical insulator.
No action potentials can occur in area of myelination, only at Nodes of Ranvier.
‘Jump’ from node to node along nerve fibre - saltatory conduction

Question 11

What is the all-or-nothing principle?

Answer 11

If below threshold = no action potential.
If above threshold = rapid opening of Na+ channels –> action potential.

All action potentials are same size.

Question 12

What is the refractory period?

Answer 12

Once an action potential has been created, period afterwards when further inward movement of sodium ions prevented because sodium voltage-gated channels are closed.

No further action potentials can be generated.

Question 13

What is the purpose of the refractory period?

Answer 13

1) Ensuring that action potentials are only propagated in one direction.
2) Ensures action potentials are discrete impulses.
3) Limits number of action potentials in a given time as there must be gaps between them.

Question 14

What is a synapse?

Answer 14

The place where the axon of one neurone connects with the dendrite of another/with an effector.

Important in linking different neurones and therefore coordinating activities.

Question 15

How does information pass from one neurone to the next across a cholinergic synapse?

Answer 15

1) Impulses cause calcium ion channels to open, calcium ions diffuse into synaptic knob.
2) Vesicles move towards AND fuse with presynaptic membrane.
3) Acetylcholine is released via exocytosis.
4) Acetylcholine diffuses across synaptic cleft.
5) Acetylcholine binds with receptors on Na+ channels in postsynaptic membrane.
6) Na+ channels open, Na+ diffuse into postsynaptic membrane.
7) Depolarisation of postsynaptic membrane.
8) If above threshold, action potential produced.H

Question 16

How does the synapse ‘reset’ ready for the next impulse?

Answer 16

1) Calcium ions removed from synaptic knob by active transport, using ATP from mitochondria.
2) Acetylcholine removed from receptors from gated Na+ channel proteins by acetylcholinesterase. Catalyses hydrolysis of acetylcholine to acetate and choline, which diffuses back into synaptic knob.
3) Acetyl and choline combined to synthesise acetylcholine, so it can be used in vesicles again.

Question 17

What is temporal summation?

Answer 17

When a single presynaptic neurone releases neurotransmitters many times in a short period of time.

Question 18

What is spatial summation?

Answer 18

When several presynaptic neurones together release enough neurotransmitters to exceed threshold value.

Question 19

What’s the point of summation?

Answer 19

• Avoids the nervous system being overloaded.
• Synapses act as ‘barriers’.
• Effect of stimulus can be magnified.

Question 20

How do inhibitory synapses work?

Answer 20

1) Presynaptic neurone releases neurotransmitter that opens K+ protein channels on postsynaptic neurone.
2) Greater potential difference across membrane.
3) Na+ diffusing in not enough for postsynaptic membrane to reach threshold.
OUTWARD DIFFUSION of K+ ‘CANCELS OUT’ INWARD DIFFUSION of Na+

4) Presynaptic neurone releases neurotransmitter that binds to Cl- protein channel on postsynaptic neurone.
5) Greater potential difference across membrane.
6) Na+ diffusing in not enough for postsynaptic membrane to reach threshold.
INWARD DIFFUSION of Cl- ‘CANCELS OUT’ INWARD DIFFUSION of Na+

Question 21

What are skeletal muscles?

Answer 21

Thick muscle fibres made from myofibrils. Strong.

Act in antagonistic pairs against an incompressible skeleton. Stimulated to contract by motor neurones.

Question 22

What are myofibrils?

Answer 22

Long, thin cylindrical structures made from 2 proteins.

Question 23

What is the sarcolemma?

Answer 23

Membrane that surrounds each muscle fibre.

Question 24

What is the sarcoplasmic reticulum?

Answer 24

Specialised network of tubules that store calcium ions.

Question 25

What’s the advantage of muscle fibres being multinucleated?

Answer 25

Can use DNA (mRNA) from nuclei for protein synthesis to add extra protein to the muscle, to be able to do protein synthesis along whole fibre.

Question 26

What is the distribution of neuromuscular junctions in a muscle, and the importance of this distribution?

Answer 26

• Point where a motor neurone meets a skeletal muscle, located all along the muscle.
• If there was just one junction, it would take time for a wave of contraction to travel across a muscle, and contraction would not be simultaneous = slow movement.

Question 27

What is a motor unit?

Answer 27

All muscle fibres supplied by single motor neurone and act together as a single functional unit.

Gives control over the force that the muscle exerts. If only slight force needed = only a few units stimulated.

Question 28

What are the events that lead to the depolarisation of sarcolemma at the neuromuscular junction?

Answer 28

1) When a nerve impulse is received at neuromuscular junction, synaptic vesicles fuse with presynaptic membrane and release acetylcholine.
2) Acetylcholine diffuses to postsynaptic membrane, altering its permeability to Na+.
3) This enters rapidly, depolarising the membrane.

Question 29

Structure and properties of slow twitch fibres

Answer 29

• Contract slower than FTF.
• Provide less powerful contractions, but over longer periods.
• Adapted to endurance work.
• Common in calf muscles in humans.
• Large store of myoglobin.
• Rich supply of blood vessels for oxygen and glucose for aerobic respiration.
• Numerous mitochondria for ATP.

Question 30

Structure and properties of fast twitch fibres

Answer 30

• Contract rapidly and produce powerful contractions for a short period.
• Adapted to intense exercise.
• Common in bicep muscles.
• Thick, more numerous myosin filaments.
• High conc. of glucose.
• High conc. of enzymes involved in anaerobic respiration - provides ATP rapidly.
• A store of phosphocreatine, rapidly generates ATP from ADP in anaerobic conditions for muscle contraction.

Question 31

What evidence supports the sliding filament theory?

Answer 31

• Sarcomere shortens.
• I band and H-zone become narrower.
• A band remains same size.

Question 32

The structure of myosin

Answer 32

• Heads protruded (globular proteins).
• Tails wrap around one another to form filaments (fibrous proteins).

Question 33

The structure of actin

Answer 33

• Made from long chains of globular proteins, coiled around one another in a helix.
• Troponin = site where calcium ions bind.
• Tropomyosin = long thin thread wound around actin (covers myosin-binding site).

Question 34

What are the events that take place in a muscle fibre which cause a muscle to contract?

Answer 34

1) Depolarisation of sarcolemma.
2) Action potential passed to T-tubule (if above threshold).
3) Calcium ions released from sarcoplasmic reticulum.
4) Calcium ions bind to troponin and cause tropomyosin to move.
5) Exposes myosin binding sites on actin.
6) Myosin head binds to actin.
7) Form actinmyosin cross bridge.
8) Myosin head power stroke.
9) Actin filament moves relative to myosin.
10) Hydrolysis of ATP causes re-cocking of myosin head.
11) Myosin head binds to actin further along actin.
12) Repeated for muscle contraction.