6 Nervous coordination and muscles

Question 1

nervous system

Answer 1

uses nerve cells to pass electrical impulses along their length
they stimulate their target cells by secreting chemicals, known as neurotransmitters, directly on to them
this results in rapid communication between specific parts of an organism
the responses produced are often short-lived and restricted to a localised region of the body

Question 2

hormonal system

Answer 2

produces chemicals (hormones) that are transported in the blood plasma to their target cells
the target cells have specific receptors on their cell surface membranes and the change in the conc of hormones stimulates them
this results in a slower, less specific form of communication between parts of an organism
the responses are often long-lasting and widespread

Question 3

neurones

Answer 3

nerve cells
specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another

Question 4

what is a mammalian motor neurone made up of?

Answer 4

cell body
dendrons
axon
schwann cells
myelin sheath
nodes of Ranvier

Question 5

neurone cell body

Answer 5

contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum
this is associated with the production of proteins and neurotransmitters

Question 6

neurone dendrons

Answer 6

extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body

Question 7

neurone axon

Answer 7

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

Question 8

neurone schwann cells

Answer 8

surrounds the axon, protecting it and providing electrical insulation
they also carry out phagocytosis (removal of cell debris) and play a part in nerve regeneration
schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it

Question 9

neurone myelin sheath

Answer 9

forms a covering to the axon and is made up of the membranes of schwann cells
these membranes are rich in a lipid known as myelin
neurones with a myelin sheath are called myelinated neurones

Question 10

neurone nodes of Ranvier

Answer 10

constrictions between adjacent schwann cells where there is no myelin sheath
the constrictions are 2-3 micrometres long and occur every 1-3mm in humans

Question 11

sensory neurones function

Answer 11

transmit nerve impulses from a receptor to an intermediate or motor neurone
they have one dendron that is often very long
it carries the impulse towards the cell body and one axon that carries it away from the cell body

Question 12

motor neurones function

Answer 12

transmit nerve impulses from an intermediate or relay neurone to an effector, such as a gland or a muscle
motor neurones have a long axon and many short dendrites

Question 13

intermediate or relay neurones function

Answer 13

transmit impulses between neurones, for example, from sensory to motor neurones
they have numerous short processes

Question 14

resting potential

Answer 14

inside of an axon is negatively charged relative to the outside
this is known as the resting potential and is usually 65mV in humans
axon is said to be polarised
-Na+ actively transported out of the axon by the sodium-potassium pump
-K+ are actively transported into the axon by the sodium-potassium pumps
-the active transport of Na+ is greater than that of K+, so 3 Na+ move out for every 2 K+ that move in
-although both ions are positive, the outward movement of Na+ is greater than the inward movement of K+. as a result, there are more Na+ in the tissue fluid surrounding the axon than in the cytoplasm, and more K+ in the cytoplasm than in the tissue fluid, thus creating an electrochemical gradient
-the Na+ begin to diffuse back naturally into the axon while the K+ begin to diffuse back out of the axon
-however, most of the gates in the channels that allow the K+ to move through are open, while most of the gates in the channels that allow the Na+ to move through are closed

Question 15

action potential

Answer 15

when a stimulus of a sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the charges either side of this part of the axon membrane
if the stimulus is great enough, the negative charge of -65mV inside the membrane becomes a positive charge of of around +40mV
this is known as the action potential, and in this condition this part of the axon membrane is said to be depolarised
this depolarisation occurs because the channels in the axon membrane change shape, and hence open or close, depending on the voltage across the membrane.
they are therefore called voltage-gated channels.

Question 16

action potential sequence

Answer 16

1. at resting potential some K voltage-gated channels are open but the Na v-g channels are closed
2. the energy of the stimulus causes some Na v-g channels in the axon membrane to open and therefore Na+ diffuse into the axon through these channels along their electrochemical gradient. being positively charged, they trigger a reversal in the potential diff across the membrane
3. as the Na+ diffuse into the axon, so more Na channels open, causing an even greater influx of Na+ by diffusion
4. once the action potential of around +40mV has been established, the voltage gates on the Na+ channels close and the voltage gates on the K+ channels begin to open
5. with some K v-g channels now open, the electrical gradient that was preventing further outward movement of K+ is now reversed, causing more K+ channels to open. this means that yet more K+ diffuse out, starting repolarisation of the axon
6. the outward diffusion of these K+ causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual(=hyperpolarisation). the closable gates on the K+ channels now close and the activities of the s-p pumps once again cause Na+ to be pumped out and K+ in. the resting potential of -65mV is re-established and the axon is said to be repolarised

Question 17

passage of an action potential along an unmyelinated axon

Answer 17

1. at resting potential the conc of Na+ outside the axon membrane is high relative to the inside, whereas that of the potassium ions is is high inside the membrane relative to outside. the overall conc of pos ions is greater on the outside, making this pos compared with inside. the axon membrane is polarised.
2. a stimulus causes a sudden influx of Na+ ions and hence a reversal of charge on the axon membrane. this is the action potential and the membrane is depolarised.
3. the localised electrical currents established by the influx of Na ions causes the opening of Na voltage gated channels a little further along the axon. the resting influx of Na ions in this region causes depolarisation. behind this new region of depolarisation, the Na v-g channels close and the K+ ones open . K ions begin to leave the axon along their electrochemical gradient. so, once initiated, the depolarisation moves along the membrane
4. the action potential (depolarisation) is propagated in the same way further along the axon. the outward movement of the K ions has continued to the extent that the axon membrane behind the action potential has returned to its original charged state (pos out neg in) it has been repolarised
5. repolarisation of the axon allows sodium ions to be actively transported out, once again returning the axon to its resting potential in readiness for a new stimulus if it comes

Question 18

passage of an action potential along a myelinated axon

Answer 18

in myelin axons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming
at intervals of 1-3mm there are breaks in this myelin insulation, called nodes of Ranvier
action potentials can occur at these points.
the localised circuits therefore arise between adjacent nodes of Ranvier and the action potentials in effect jump from node to node in a process called saltatory conduction.
as a result, an action potential passes along a myelinated neurone faster than along the axon of an unmyelinated neurone, the events of depolarisation have to take place all the way along an axon and this takes more time

Question 19

factors affecting the speed of an action potential

Answer 19

myelin sheath
diameter of axon
temperature

Question 20

how does axon diameter affect speed of action potential

Answer 20

the greater the diameter, the faster the speed as there is less leakage of ions from a large axon
leakage makes membrane potentials harder to maintain

Question 21

how does temp affect speed of action potential

Answer 21

affects rate of diffusion of ions and therefore the higher the temp the faster the nerve impulse
the energy for active transport comes from respiration
respiration, like the sodium-potassium pump, is controlled by enzymes
enzymes function more rapidly at higher temps up to a point
above a certain temp, enzymes and the plasma membrane proteins are denatured and impulses fail to be conducted at all

Question 22

all or nothing principle

Answer 22

nerve impulses are described as all or nothing responses
there is a certain level of stimulus, called the threshold value, which triggers an action potential.
below the threshold value, no action potential, and therefore no impulse, is generated.
any stimulus, of whatever strength, that is below the TV will fail to generate an action potential.
any stimulus above the TV will succeed in generating an AP and so a nerve impulse will travel.
all APs are more or less the same size, and so the strength of the stimulus cannot be detected by the size of the APs.

Question 23

how can an organism perceive the size of a stimulus?

Answer 23

• by the number of impulses passing in a given time. the larger the stimulus, the more impulses that are generated in a given time
• by having diff neurones with diff threshold values. the brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size.

Question 24

refractory period

Answer 24

once an action potential has been created in any region of an axon, there is a period afterwards when inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed
during this time it is impossible for a further action potential to be generated

Question 25

purpose of refractory period

Answer 25

• it ensures that action potentials are propagated in one direction only
• it produces discrete impulses
• it limits the number of action potentials

Question 26

structure of the synapse

Answer 26

synapses transmit information (not impulses) from one neurone to another by means of chemicals known as neurotransmitters
neurones are separated by a small gap, called the synaptic cleft
the neurone that releases the neurotransmitter is called the presynaptic neurone
the axon of this neurone ends in a swollen portion known as the synaptic knob. this possesses many mitochondria and large amounts of endoplasmic reticulum. these are required in the manufacture of the neurotransmitter which takes place in the axon.
the neurotransmitter is stored in the synaptic vesicles.
once the neurotransmitter is released from the vesicles it diffuses across to the postsynaptic neurone, which possesses specific receptor proteins on its membrane to receive it

Question 27

features of synapses

Answer 27

unidirectionality

summation

Question 28

unidirectionality of synapse

Answer 28

synapses can only pass information in one direction
from the presynaptic neurone to the postsynaptic neurone
in this way, synapses act like valves

Question 29

summation of synapse

Answer 29

low frequency action potentials often lead to the release of insufficient concs of neurotransmitter to trigger a new action potential in the postsynaptic neurone
they can, however, do so in a process called summation
this entails a rapid build up of neurotransmitter in the synapse by one of two methods:
-spatial summation
-temporal summation

Question 30

spatial summation

Answer 30

in which a number of diff presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone. together they therefore trigger a new action potential

Question 31

temporal summation

Answer 31

in which a single presynaptic neurone releases neurotransmitter many times over a very short period
if the conc of NT exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered

Question 32

inhibitory synapses

Answer 32

synapses that make it less likely that a new action potential will be created on the postsynaptic neurone

Question 33

inhibition- synapses

Answer 33

• the presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
• the neurotransmitter causes the chloride protein channels to open
• chloride ions move into the postsynaptic neurone by facilitated diffusion
• the binding of the NT causes the opening of nearby potassium protein channels
• potassium ions move out of the postsynaptic neurone into the synapse
• the combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive.
• the membrane potential increases to as much as -80mV compared to the usual -65mV at resting potential.
• this is called hyperpolarisation and makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one

Question 34

functions of synapses

Answer 34

transmit info from one neurone to another
they act as junctions allowing:
-a single impulse along one neurone to initiate new impulses in a number of diff neurones at a synapse. this allows a single stimulus to create a number of simultaneous responses
-a number of impulses to be combined at a synapse. this allows nerve impulses from receptors reacting to diff stimuli to contribute to a single response

Question 35

cholinergic snapse

Answer 35

one in which the neurotransmitter is a chemical called acetylcholine

Question 36

mechanism of transmission across a cholinergic synapse

Answer 36

1. the arrival of an AP at the end of the presynaptic neurone causes calcium ion protein channels to open and calcium ions enter the synaptic knob by facilitated diffusion.
2. the influx of Ca2+ into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
3. acetylcholine molecules diffuse across the cleft quickly as the diffusion pathway is short. AC then binds to receptor sites on the sodium ion protein channels in the membrane of the postsynaptic neurone. this causes the sodium ion protein channels to open, allowing sodium ions to diffuse in rapidly along a conc gradient.
4. the influx of sodium ions generates a new action potential in the postsynaptic neurone
5. acetylcholinesterase hydrolyses AC into choline and ethanoic acid, which diffuse back across the synaptic cleft into the PreS neurone. in addition to recycling the choline and ethanoic acid, the rapid breakdown of AC also prevents it from continuously generating a new action potential in the postS neurone, and so leads to discrete transfer of info across synapses.
6. ATP released by mitochondria is used to recombine choline and ethanoic acid into AC. this is stored in synaptic vesicles for future use. sodium ion protein channels close in the absence of AC in the receptor sites.

Question 37

cardiac muscle

Answer 37

found exclusively in the heart

Question 38

smooth muscle

Answer 38

found in the walls of blood vessels and the gut

Question 39

skeletal muscle

Answer 39

makes up the bulk of body muscle in vertebrates

is attached to bone and acts under voluntary, conscious control

Question 40

skeletal muscle structure

Answer 40

Protein fibres called myofibrils run through these cells increasing their strength.
Myofibrils are made from thick and thin filaments which overlap in places to give a banded appearance.
Within these the thick filaments are made of myosin and the thin filaments are made of actin.
Two actin molecules are twisted together to make the filament.
The myosin has heads which can attach to specific binding sites on the actin when the muscle is contracting.

Question 41

actin

Answer 41

a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand

Question 42

mysosin

Answer 42

made up of two types of protein

• fibrous protein arranged into a filament made up of several hundred molecules (the tail)
• a globular protein formed into two bulbous structures at one end (the head)

Question 43

tropomyosin

Answer 43

forms long thin threads that are wound around actin filaments

Question 44

I band

Answer 44

The area adjacent to the Z-line, where actin myofilaments are not superimposed by myosin myofilaments.

Question 45

A band

Answer 45

The length of a myosin myofilament within a sarcomere.

Question 46

H zone

Answer 46

The area where myosin myofilaments are not superimposed by actin myofilaments.

Question 47

Z line

Answer 47

Neighbouring, parallel lines that define a sarcomere.

Question 48

sliding filament mechanism

Answer 48

actin and myosin filaments slide past one another during muscle contraction
the bulbous heads of the myosin filaments form cross-bridges with the actin filaments
they do this by attaching themselves to binding sites on the actin filaments and then flexing in unison, pulling the actin filaments along the myosin filaments
they then become detached and, using ATP as a source of energy, return to their original angle and re-attach themselves further along the actin filaments

Question 49

muscle stimulation- sliding filament mechanism

Answer 49

• an action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob
• the calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetlycholine into the synaptic cleft
• acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell-surface membrane, causing it to depolarise

Question 50

muscle contraction- sliding filament mechanism

Answer 50

• the action potential travels deep into the fibre through a system of tubules that are extensions of the cell-surface membrane and branch throughout the cytoplasm of the muscle
• the tubules are in contact with the endoplasmic reticulum of the muscle which has actively transported calcium ions from the cytoplasm of the muscle leading to very low Ca2+ conc in cytoplasm
• the AP opens the calcium ion protein channels on the ER and Ca ions diffuse into the muscle cytoplasm down a conc gradient
• the Ca ions cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away
• ADP molecules attached to the myosin heads mean they are in a state to bind to the actin filament and form a cross-bridge
• once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along as they do so and releasing a molecule of ADP
• an ATP molecule attaches to each myosin head, causing it to become detached from the actin filament
• the Ca ions then activate the enzyme ATPase, which hydrolyses the ATP to ADP. the hydrolysis provides the energy for the mysosin head to return to its original position
• the myosin head, once more with an attached ADP molecule, then reattaches itself further along the actin filament and the cycle is repeated as long as the conc of Ca ions in the myofibril remains high
• as the myosin molecules are joined tail to tail in two oppositely facing sets, the movement of one set of myosin heads is in the opposite direction to the other. meaning that actin filaments to which they are attache also move in opposite directions
• the movement of actin filaments in opp direcs pulls them towards each other, shortening the distance between the two adjacent Z lines

Question 51

muscle relaxation- sliding filament mechanism

Answer 51

• when nervous stimulation ceases, Ca ions are actively transported back into the ER using energy from the hydrolysis of ATP
• this reabsorption of the Ca ions allows tropomyosin to block the actin filament again
• myosin heads are now unable to bind to actin filaments and contraction ceases. the muscle relaxes

Question 52

energy supply during muscle contraction

Answer 52

energy needed fir muscle contraction is supplied by the hydrolysis of ATP to ADP and inorganic phosphate
the energy released is needed for:
-movement of myosin heads
-the reabsorption of calcium ions into the ER by active transport

Question 53

types of muscle fibre

Answer 53

slow-twitch fibres

fast-twitch fibres

Question 54

slow-twitch fibres

Answer 54

contract slowly and provide less powerful contractions but over a longer period
they are adapted to endurance work, e.g. running a marathon
more common in muscles like calf muscles, which must contract constantly to maintain the body in an upright position
suited for this role by being adapted for aerobic respiration in order to avoid a build-up of lactic acid

Question 55

adaptations of slow-twitch fibres

Answer 55

• having a large store of myoglobin ( a bright red molecule that stores oxygen, which accounts for the red colour of slow-twitch fibres)
• a rich supply of blood vessels to deliver O2 and glucose for aerobic respiration
• numerous mitochondria to produce ATP

Question 56

fast-twitch fibres

Answer 56

contract more rapidly and produce powerful contractions but only for a short period
are adapted to intense exercise, e.g. weightlifting
more common in muscles which need to do short bursts of intense activity, like the biceps muscle

Question 57

adaptations of fast-twitch fibres

Answer 57

• thicker and more numerous myosin filaments
• higher conc of glycogen
• higher conc of enzymes involved in anaerobic respiration which provides ATP rapidly
• a store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction

Question 58

neuromuscular junction

Answer 58

point where a motor neurone meets a skeletal muscle fibre. many junctions along the muscle.
all muscle fibres supplied by a single motor neurone act together as a single functional unit and are known as a motor unit.
this arrangement gives control over the force that the muscle exerts

Question 59

nerve impulse at neuromuscular junction process

Answer 59

when a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine
the acetylcholine diffuses to the postsynaptic membrane, altering its permeability to sodium ions, which enter rapidly, depolarising the membrane
the acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not over-stimulated
the resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form acetylcholine using energy provided by the mitochondria there

Question 60

evidence for the sliding filament mechanism

Answer 60

myofibrils appear darker in colour where the actin and myosin filaments overlap and lighter where they don’t
if the sliding filament mechanism is correct, then there will be more overlap of actin and myosin in a contracted muscle than in a relaxed one
when a muscle contracts, the following changes occur to a sarcomere:
-the I-band becomes narrower
-the Z-lines move closer together, the sarcomere shortens
-the H-zone becomes narrower
The A-band remains the same width

Question 61

phosphocreatine

Answer 61

chemical that regenerates ATP, in order to supply energy to the muscle
it is stored in muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so reform ATP
the phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed

Question 62

Muscles act in antagonistic pairs

Answer 62

In an antagonistic pair, one muscle contracts when the other muscle relaxes.