15 - Nervous Coordination and Muscles

Question 1

motor neurones consist of

Answer 1

uses cell body, dendrons, dendrites, axon, Schwann cells, myelin sheath, nodes of Ranvier

Question 2

dendrons

Answer 2

extensions of the cell body which divide into branched fibres called dendrites

Question 3

dendrites

Answer 3

carry nerve impulses towards the cell body

Question 4

axon

Answer 4

single long fibre that carries nerve impulses away from the cell body

Question 5

Schwann cells

Answer 5

surround the axon, protecting it and providing electrical insulation

Question 6

myelin sheath

Answer 6

forms a covering to the axon and is made of the membranes of the Schwann cells

Question 7

nodes of Ranvier

Answer 7

constrictions between adjacent Schwann cells where there is no myelin sheath

Question 8

sensory neurones

Answer 8

transmit nerve impulses from a receptor to an intermediate or motor neurone

Question 9

motor neurone

Answer 9

transmit nerve impulses from an intermediate to an effective

Question 10

maintaining a resting potential

Answer 10

sodium ions are actively transported OUT the axon by the sodium-potassium pump
potassium ions are actively transported INTO the axon by the sodium potassium pump
active transport of sodium ions is greater than that of potassium ions
creates an electrochemical gradient
polarises axon and gives a resting potential of -65mV

Question 11

depolarisation of the axon

Answer 11

1. at resting potential some potassium voltage gated channels are open and the sodium voltage gated channels are closed
2. energy of the stimulus causes some sodium voltage gated channels to open
3. sodium ions diffuse into the axon through these channels, along the electrochemical gradient
4. causes reversal in the potential difference across the membrane
5. an action potential of +40mV is established

Question 12

saltatory conduction

Answer 12

occurs in myelinated neurones
myelin sheath is an electrical insulator, prevents action potentials occurring
action potentials can occur at nodes of Ranvier, action potentials can effectively jump to adjacent nodes
leads to faster transmission of action potential

Question 13

greater the diameter of the axon…

Answer 13

faster the speed of conductance (less leakage of ions from large axon)

Question 14

all or nothing principle

Answer 14

stimuli have to reach a certain threshold value in order to trigger an action potential

Question 15

refractory period

Answer 15

when an inward movement of sodium ions is prevented because sodium voltage gated channels are closed
impossible for further action potential to be generated

Question 16

purpose of refractory period

Answer 16

ensures action potentials are propagated in one direction only
produces discrete impulses - separates action potentials from one another
limits the number of action potentials that can pass along an axon in a given time

Question 17

unidirectionality

Answer 17

synapses can only pass neurotransmitters in one direction

Question 18

why summation is required

Answer 18

low frequency action potentials often lead to the release of insufficient concentrations of neurotransmitters
cannot trigger an action potential in the postsynaptic neurone
summation is used to trigger an action potential in the postsynaptic neurone in these circumstances

Question 19

spatial summation

Answer 19

a number of different presynaptic neurones together release enough neurotransmitter to meet the threshold value

Question 20

temporal summation

Answer 20

a single presynaptic neurone releases neurotransmitter many times over a short period to meet the threshold

Question 21

inhibitory synapses

Answer 21

1. the presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
2. chloride ion protein channels open and chloride ions move into the postsynaptic neurone by facilitated diffusion
3. nearby potassium channels open and potassium ions move out the postsynaptic neurone
4. the combined effect of negative chloride ions moving in and positive potassium ions moving out makes membrane potential more negative
5. membrane is hyperpolarised - less likely that an action potential will be created

Question 22

transmission across a synapse

Answer 22

1. An action potential arrives at the end of the presynaptic neurone and causes calcium ion protein channels to open
2. Calcium ions enter the synaptic knob by facilitated diffusion
3. The influx of calcium ions into the presynaptic neurone causes the synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
4. Acetylcholine molecules diffuse across the synaptic cleft quickly due to short diffusion path
5. Acetylcholine binds to the receptors sites on sodium ion protein channels in the postsynaptic membrane causing the sodium ion protein channels to open
6. This allows sodium ions to diffuse in rapidly along a concentration gradient
7. Influx of sodium ions generates an action potential in the postsynaptic neurone

Question 23

separate cells in a muscle are…

Answer 23

fused together into muscle fibres

the fibres share nuclei and sarcoplasm

Question 24

protein filaments in myofibril

Answer 24

actin - thinner, two strands twisted together

myosin - thicker, long rod-shaped tails with bulbous heads

Question 25

myofibril

Answer 25

tiny muscle fibres
alternating light (isotropic) and dark coloured (anisotropic) bands
A bands appear dark because the actin and myosin filaments are overlapping

Question 26

H zone and Z line

Answer 26

H zone = light coloured region at centre of each A band
Z line = line at centre of each I band

distance between adjacent Z lines indicates the sarcomere

Question 27

slow twitch fibres

Answer 27

contract more slowly and less powerfully than fast twitch
contract over long period
adapted for endurance and aerobic respiration (large store of myoglobin, and rich blood supply)

Question 28

fast twitch fibres

Answer 28

contract more rapidly and with more power
contract over shorter period
thicker and more numerous myosin filament
high concentration of myoglobin as well as high concentration of enzymes used in anaerobic respiration (phosphocreatine)

Question 29

phosphocreatine

Answer 29

rapidly generates ATP from ADP in anaerobic conditions

Question 30

transmission at a neuromuscular junction

Answer 30

1. nerve impulse is received at the neuromuscular junction
2. synaptic vesicles fuse with presynaptic membrane and release acetylcholine
3. acetylcholine diffuses to the postsynaptic membrane (membrane to the muscle fibre)
4. this alters its permeability to sodium ions which enter rapidly, depolarises the membrane

Question 31

when a muscle contracts, the following changes occur to a sarcomere:

Answer 31

I bands become narrower
Z lines move closer together
H zones become narrower

Question 32

muscle relaxation

Answer 32

1. when nervous stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP
2. the reabsorption of the calcium ions allows tropomyosin to block the actin filament
3. myosin heads are now unable to bind to actin filaments so contraction ceases
4. the muscle relaxes

Question 33

muscle contraction

Answer 33

1. calcium ions diffuse into myofibrils from sarcoplasmic reticulum
2. calcium ions cause movement of tropomyosin on actin
3. this movement causes exposure of the binding sites on the actin
4. myosin heads attach to binding sites on actin
5. hydrolysis of ATP causes myosin heads to bend
6. this pulls the actin molecules
7. attachment of a new ATP molecule to each myosin head causes myosin heads to detach

Question 34

repolarisation of the membrane

Answer 34

1. when an action potential of around +40mV is reached, the sodium voltage gated channels close and the potassium voltage gated channels start to open
2. potassium ions diffuse out the axon, starting the repolarisation of the axon
3. electrical gradient is temporarily overshot (hyperpolarisation)
4. potassium voltage gated channels close and the sodium voltage gated channels open, causing sodium ions to exit and the resting potential is reestablished