6.2 nervous coordination

Question 1

there are two main forms of coordination in animals - the nervous system and the hormonal system. give three differences between these two systems.

Answer 1

nervous system

• communication occurs via nerve cells.
• transmission occurs via neurones.
• the response produced is rapid and localised.

hormonal system

• communication occurs via hormones.
• transmission occurs via blood plasma.
• the response produced is slow and widespread.

Question 2

give an example of nervous coordination.

Answer 2

a reflex action, such as the withdrawal of a hand from a harmful stimulus.

Question 3

give an example of hormonal coordination.

Answer 3

the control of blood glucose concentration by hormones, such as insulin.

Question 4

neurones are specialised cells involved in nervous coordination. explain the role of a neurone in stimulating a nervous response.

Answer 4

neurones are specialised cells adapted to rapidly carry electrochemical changes, called nerve impulses, from one part of the body to another.

Question 5

give the three types of neurone present in the nervous system, including their functions.

Answer 5

sensory neurones - transmit nerve impulses from a receptor to a relay or motor neurone.

motor neurones - transmit nerve impulses from a relay neurone to an effector.

relay neurones - transmit impulses between neurones.

Question 6

describe the function of Schwann cells in the mammalian myelinated motor neurone.

Answer 6

Schwann cells surround the axon of the neurone, providing protection and electrical insulation.

Question 7

what is a nerve impulse?

Answer 7

• a temporary reversal of the electrical potential difference across the axon membrane.
• the reversal occurs between two states - the resting potential, and the action potential.

Question 8

the phospholipid bilayer of the axon plasma membrane prevents the diffusion of ions, such as sodium and potassium ions, across it. channel proteins span the phospholipid bilayer. explain the difference between ‘gated’ and ‘non-gated’ channel proteins.

Answer 8

• ‘gated’ channel proteins can be opened or closed, meaning that sodium and potassium ions can pass through them via facilitated diffusion at any one time, but not on other occasions.
• ‘non-gated’ channel proteins remain open all the time, so sodium and potassium ions can move unhindered through them via facilitated diffusion at any time.

Question 9

explain, in simple terms, the mechanism of the sodium-potassium pump, including the type of protein involved.

Answer 9

• some types of carrier protein can actively transport potassium ions into the axon and sodium ions out of the axon.
• this mechanism is known as the sodium-potassium pump.

Question 10

give the resting potential of the inside of a human axon.

Answer 10

65 mV.

Question 11

when the axon is at resting potential, it can be described as what?

Answer 11

polarised.

Question 12

during the establishment of a potential difference between the inside and outside of the axon, sodium ions move out of the axon whilst potassium ions move into the axon. explain why more sodium ions move out of the axon compared to the number of potassium ions that move into the axon.

Answer 12

• the active transport of sodium ions out of the axon is greater than that of potassium ions into the axon.
• as a result, three sodium ions move out of the axon for every two potassium ions that move in.

Question 13

explain how the movement of sodium and potassium ions within the axon creates an electrochemical gradient.

Answer 13

• the outward movement of sodium ions is greater than the inward movement of potassium ions.
• as a result, there are more sodium ions in the tissue fluid surrounding the axon, and more potassium ions within the cytoplasm of the axon.
• this creates an electrochemical gradient.

Question 14

state the change in charge of the inside of a human axon when a stimulus initiates an action potential.

Answer 14

if the stimulus is significant enough, the negative charge of -65 mV inside the membrane of the axon becomes a positive charge of +40 mV.

Question 15

explain why during an action potential, part of the axon membrane is described as ‘depolarised’.

Answer 15

• during an action potential, part of the axon membrane is described as ‘depolarised’.
• this depolarisation occurs because the ion channels in this part of the axon membrane change shape, and open or close, depending on the voltage across the membrane.

Question 16

explain how the diffusion of sodium ions into the axon membrane trigger a reversal in the potential difference across the membrane during an action potential.

Answer 16

• the energy of the stimulus causes some sodium-gated channels in the axon membrane to open.
• this causes sodium ions to diffuse into the axon through these channels along an electrochemical gradient.
• the positive charge of the sodium ions trigger a reversal in the potential difference across the axon membrane.

Question 17

describe the events that occur following the establishment of an action potential of around +40 mV, leading to the hyperpolarisation of the axon.

Answer 17

• once the action potential of around +40 mV has been established, the voltage gates on the sodium ion channels close, preventing a further influx of sodium ions.
• the voltage gates on the potassium ion channels open, reversing the electrochemical gradient that was preventing a further outward movement of potassium ions.
• as a result of this reversal, more potassium ions diffuse out, which causes the hyperpolarisation of the axon.

Question 18

explain how the outward diffusion of these potassium ions causes the axon to become hyperpolarised.

Answer 18

• the outward diffusion of these potassium ions causes a temporary overshoot of the electrochemical gradient.
• this causes the axon to become hyperpolarised, as the inside of the axon is more negatively charged, relative to the outside, than usual.

Question 19

explain how the resting potential of -65 mV is reestablished within the axon, following hyperpolarisation.

Answer 19

• the gated potassium ion channels close.
• the sodium-potassium pumps on the cell-surface membrane cause sodium ions to be pumped out of the axon, and potassium ions to be pumped in.
• this reestablishes the resting potential of -65 mV, and the axon is said to be ‘repolarised’.

Question 20

describe the difference in the processes used to transport ions during the action potential, as opposed to the resting potential.

Answer 20

• the movement of sodium ions into the axon during the action potential occurs through diffusion, a passive process that does not require an external source of energy.
• the movement of ions which maintain the resting potential is sustained by active transport, an active process which requires energy from ATP.

Question 21

explain why the action potential can be described as a ‘travelling wave of depolarisation’.

Answer 21

• as one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon.
• the action potential can therefore be described as a ‘travelling wave of depolarisation’.

Question 22

in myelinated axons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming across the length of the membrane. where do action potentials occur in a myelinated axon?

Answer 22

action potentials in a myelinated axon occur at the nodes of Ranvier.

Question 23

what is ‘saltatory conduction’?

Answer 23

saltatory conduction is the generation of action potentials along myelinated axons from one node of Ranvier to the next.

Question 24

explain why an action potential passes along the axon of a myelinated neurone at a faster rate than an unmyelinated neurone.

Answer 24

• in a unmyelinated neurone, the events of depolarisation take place across the entire length of the axon membrane, which takes time.
• in a myelinated neurone, an action potential can only occur at the the nodes of Ranvier.
• because an action potential across a myelinated axon can only occur at certain points along the membrane, transmission is more rapid.

Question 25

give three factors which affect the speed at which an action potential travels.

Answer 25

• the myelin sheath.
• the diameter of the axon.
• temperature.

Question 26

explain how the diameter of the axon can affect the speed at which an action potential travels.

Answer 26

• the greater the diameter of an axon, the faster the speed at which an action potential travels.
• this is due to a decrease in the leakage of ions from a large axon; leakage makes membrane potentials more difficult to maintain.

Question 27

what is the ‘threshold’ value of a stimulus?

Answer 27

the threshold value of a stimulus indicates whether the energy transferred by the stimulus to the receptors is sufficient enough to generate an action potential.

Question 28

explain why nerve impulses are described as an ‘all-or-nothing’ response.

Answer 28

• the threshold value of a stimulus triggers an action potential.
• any stimulus that falls below the threshold value, regardless of the strength of the stimulus, will fail to generate an action potential, and therefore a nerve impulse.
• any stimulus which surpasses the threshold value will succeed in generating an action potential, and the travel of a nerve impulse.
• this is described as the ‘all-or-nothing’ principle of a nervous response.

Question 29

all action potentials are of the same size, therefore the strength of a stimulus cannot be detected by the size of the action potentials. give two ways in which an organism can perceive the strength of a stimulus.

Answer 29

• by the number of impulses passing in a given time - the stronger the stimulus, the more impulses that are generated in a given time.
• by having different neurone with different threshold values - the brain interprets the number and type of neurones that pass impulses as a result of a given stimulus, which determines the strength of the stimulus.

Question 30

define the term ‘refractory period’.

Answer 30

• once an action potential has been generated in a region of an axon, there is a period afterwards when an inward movement of sodium ions is prevented because the sodium voltage-gated channels close.
• during this time, known as the refractory period, further action potentials cannot be generated.

Question 31

give three reasons why this refractory period occurs after the initial generation of an action potential.

Answer 31

• to ensure that action potentials are propagated in one direction only.
• to separate successive action potentials from one another, thereby producing discrete impulses.
• to limit the number of action potentials that can pass along an axon over a given period of time, limiting the strength of stimulus that can be detected.

Question 32

what is a synapse?

Answer 32

• a synapse is the point at which one neurone communicates with another or with an effector.
• synapses link different neurones together, and therefore coordinate nervous activity.

Question 33

what do synapses use to transmit information from one neurone to another?

Answer 33

neurotransmitters.

Question 34

give the name of the neurone that releases neurotransmitters, including where neurotransmitters are stored.

Answer 34

the presynaptic neurone releases neurotransmitters, which are stored in the synaptic vesicles.

Question 35

give an example of a neurotransmitter.

Answer 35

acetylcholine.

Question 36

give the enzyme which hydrolyses acetylcholine, and describe where in the synapse the products of the reaction end up.

Answer 36

acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone.

Question 37

give the three features of synapses.

Answer 37

• unidirectionality.
• summation.
• inhibition.

Question 38

explain why transmission across a cholinergic synapse is described as ‘unidirectional’.

Answer 38

transmission across a cholinergic synapse is described as unidirectional because cholinergic synapses can only pass on information in one direction - from the presynaptic neurone to the postsynaptic neurone.

Question 39

describe the two methods of summation which result in the rapid buildup of neurotransmitters in a cholinergic synapse.

Answer 39

• spatial summation - where a number of different presynaptic neurones collectively release enough neurotransmitters to exceed the threshold value of the postsynaptic neurone, together triggering a new action potential.
• temporal summation - in which a single presynaptic neurone releases neurotransmitters several times over a short period.
• if the concentration of neurotransmitters exceeds the threshold value of the postsynaptic neurone, a new action potential is triggered.

Question 40

explain how the hyperpolarisation of an inhibitory synapse decreases the chances of a new action potential being generated in the postsynaptic membrane.

Answer 40

• the presynaptic neurone releases a type of neurotransmitter which binds to chloride ion channels on the postsynaptic neurone.
• the neurotransmitter causes the channels to open, and chloride ions move into the postsynaptic neurone via facilitated diffusion.
• the binding of the neurotransmitter causes the opening of nearby potassium ion channels, which in turn causes the movement of potassium ions out of the postsynaptic neurone and into the synapse.
• the combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out causes the membrane potential to increase to around -80 mV.
• as a result, the membrane is hyperpolarised, which decreases the chances of a new action potential being generated.

Question 41

give the two main ways in which drugs act on synapses.

Answer 41

• stimulating the nervous system by creating more action potentials in postsynaptic neurones.
• inhibiting the nervous system by creating fewer action potentials in postsynaptic neurones.

Question 42

give two ways in which a drug may stimulate the nervous system by creating more action potentials in postsynaptic neurones.

Answer 42

• by stimulating the release of more neurotransmitter from the presynaptic membrane.
• by inhibiting the enzyme which breaks down a neurotransmitter.

Question 43

give two ways in which a drug may inhibit the nervous system by creating fewer action potentials in postsynaptic neurones.

Answer 43

• by inhibiting the release of neurotransmitter from the presynaptic membrane.
• by blocking receptors on the sodium or potassium ion channels in postsynaptic neurones.

Question 44

give the three types of muscle present in vertebrates, including where in the body they are found.

Answer 44

• cardiac muscle - found exclusively in the heart.
• smooth muscles - found in the walls of blood vessels and the intestines.
• skeletal muscle - attached to the bones by ligaments, and make up the majority of body muscle.

Question 45

give the main difference in the control of cardiac and smooth muscle, as opposed to skeletal muscle.

Answer 45

• cardiac and smooth muscle act under involuntary control.
• skeletal muscle acts under voluntary, conscious control.

Question 46

give the name of the muscle fibres which individual muscles are composed of.

Answer 46

myofibrils.

Question 47

myofibrils share nuclei and cytoplasm. give the name of this shared cytoplasm.

Answer 47

the sarcoplasm.

Question 48

muscles are composed of hundreds of muscle cells fused together to form muscle fibres. explain why muscles would not be able to carry out their function of contraction effectively if they were made up of individual cells joined end to end.

Answer 48

• if muscle was made up of individual cells joined end to end, it would not be able to perform its function of contraction very efficiently.
• this is because the junction between adjacent cells would be weaker than if the cells were fused together, reducing the overall strength of the muscle.

Question 49

describe the two types of protein filament which make up myofibrils.

Answer 49

• actin - thin filament, which consists of two strands woven around each other.
• myosin - thicker filament, which consists of long rod-shaped tails, with myosin heads that project to the side.

Question 50

give the protein found in muscle which forms a fibrous strand around the actin filament.

Answer 50

tropomyosin.

Question 51

explain why myofibrils appear ‘striped’ when viewed under a microscope.

Answer 51

• myofibrils appear striped due to their alternating light-coloured and dark-coloured bands.
• the lighter isotropic bands (I bands) appear lighter because the thick and thin muscle filaments do not overlap in this region.
• the darker anisotropic bands (A bands) appear darker because the thick and thin muscle filaments overlap in this region.

Question 52

give the name of the lighter-coloured region found at the centre of each A band.

Answer 52

the H-zone.

Question 53

at the centre of each I band is a line called the Z-line. the distance between adjacent Z-lines is called a what?

Answer 53

a sarcomere.

Question 54

explain what happens to these sarcomeres when a muscle contracts.

Answer 54

when a muscle contracts, these sarcomeres shorten, and the pattern of light and dark bands changes.

Question 55

give the two types of muscle fibre, and describe the main differences between them.

Answer 55

• fast-twitch fibres - contract rapidly and produce powerful contractions, but only for a short period of time.
• slow-twitch fibres - contract more slowly than fast-twitch fibres and provide less powerful contractions, but over a longer period of time.

Question 56

slow-twitch fibres are adapted to carry out aerobic respiration in order to fulfil their role of endurance activities, such as running a marathon. give three ways in which slow-twitch fibres are adapted to carry out aerobic respiration.

Answer 56

in order to carry out aerobic respiration, slow-twitch fibres contain:

• a large supply of the oxygen-storing molecule, myoglobin.
• a rich supply of blood vessels, to deliver oxygen and glucose to the muscle for aerobic respiration.
• numerous mitochondria to produce ATP.

Question 57

give three ways in which fast-twitch fibres are adapted to their role of short bursts of intense activity, such as weight lifting.

Answer 57

fast-twitch fibres have the following adaptations:

• thicker and more numerous myosin filaments.
• a high concentration of glycogen, which can be broken down into ATP during aerobic respiration.
• a high concentration of enzymes involved in anaerobic respiration.

Question 58

what is a neuromuscular junction?

Answer 58

a neuromuscular junction is the point at which a motor neurone meets a skeletal muscle fibre.

Question 59

explain how a nerve impulse received at the neuromuscular junction leads to the depolarisation of the postsynaptic membrane.

Answer 59

• when a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane, releasing acetylcholine.
• acetylcholine diffuses into the postsynaptic membrane, altering its permeability to sodium ions.
• these sodium ions now rapidly enter the postsynaptic membrane, leading to depolarisation.

Question 60

myasthenia gravis (MG) is an autoimmune disease, whereby antibodies produced by the immune system disrupt neuromuscular connections, leading to muscular fatigue. explain how MG affects the functioning of the neuromuscular junction.

Answer 60

• MG occurs when communication between the nerve and muscle is interrupted at the neuromuscular junction.
• when MG occurs, antibodies produced by the body’s immune system block, alter, or destroy the receptors for the binding of acetylcholine at the neuromuscular junction.
• as a result, acetylcholine cannot bind with the receptors on the postsynaptic membrane, preventing muscle contraction.

Question 61

give two similarities and two differences of a neuromuscular junction and a cholinergic synapse.

Answer 61

similarities

• both have neurotransmitters that are transported by diffusion.
• both use a sodium-potassium pump to repolarise the axon.

differences

• only motor neurones are involved in neuromuscular junctions, whereas motor, sensory, and intermediate neurones may be involved in a cholinergic synapse.
• the action potential ends at the neuromuscular junction, whereas in a cholinergic synapse, a new action potential may be stimulated along another neurone.

Question 62

give the name of the mechanism which brings about the contraction of the muscle fibre.

Answer 62

the sliding filament mechanism.

Question 63

give the changes which occur to a sarcomere when a muscle contracts.

Answer 63

• the I-band becomes narrower.
• the Z-lines move closer together, which causes the sarcomere to shorten.
• the H-zone becomes narrower.

Question 64

explain how the movement of myosin filaments during muscle contraction discounts the theory that muscle contraction is due to the filaments themselves shortening.

Answer 64

• the width of the A-band is determined by the length of the myosin filaments.
• when a muscle contracts, the A-band remains the same width, which shows that the myosin filaments do not shorten.
• this therefore discounts the theory that muscle contraction is due to the myosin filaments themselves shortening.

Question 65

give the three main proteins involved in the sliding filament mechanism of muscle contraction.

Answer 65

• myosin, which is made up of a fibrous protein and a globular protein.
• actin, a globular protein whose molecules are arranged to form a helical strand.
• tropomyosin, which forms long thin threads that are wound around actin filaments.

Question 66

explain how the globular protein heads of the myosin filaments form cross bridges with the actin filaments.

Answer 66

• the globular protein heads of the myosin filaments form cross bridges with the actin filaments by attaching themselves to binding sites on the actin filaments.
• these flex in unison, pulling the actin filaments along the myosin filaments.
• the myosin filaments then become detached and using ATP as a source of energy, return to their original angle, and reattach further along the actin filaments.

Question 67

give the three processes involved in the sliding filament mechanism of muscle contraction.

Answer 67

• stimulation.
• contraction.
• relaxation.

Question 68

describe the steps involved in the sliding filament mechanism of muscle contraction.

Answer 68

• tropomyosin prevents myosin from attaching to the binding site on the actin molecule.
• calcium ions released from the sarcoplasmic reticulum cause the tropomyosin molecule to change shape and pull away from the binding site of the actin molecule.
• the myosin head attaches to the binding site on the actin filament and changes angle, which causes the actin filament to move along, releasing a molecule of ADP.
• a molecule of ATP attaches to the myosin head, which causes it to detach from the actin filament.
• the hydrolysis of ATP to ADP and inorganic phosphate by ATPase provides the energy required for the myosin head to return to its normal position.
• the myosin head reattaches to a binding site further along the actin filament, and the cycle is repeated.

Question 69

muscle contraction requires considerable amounts of energy. this energy is supplied by the hydrolysis of ATP to ADP and inorganic phosphate. give two uses of the energy released from this process.

Answer 69

• for the movement of the myosin heads.
• for the reabsorption of calcium ions into the sarcoplasmic reticulum via active transport.

Question 70

give the chemical found in the muscle which rapidly generates ATP via anaerobic respiration.

Answer 70

phosphocreatine.

Question 71

explain how phosphocreatine regenerates ATP in order to supply energy to an anaerobically respiring muscle.

Answer 71

• phosphocreatine cannot supply energy directly to an anaerobically respiring muscle, so it instead regenerates ATP, which can.
• phosphocreatine is stored in the muscle, and acts as a reserve supply of phosphate, which is available immediately to combine with ADP, reforming ATP.