13- neuronal communication

Question 1

What are the roles of neurones?

Answer 1

They transmit electrical impulses rapidly around the body so that the organism can respond to changes in it’s internal and external environment. They work together to carry information detected by a sensory receptor to the effector, which in turn carries out the appropriate response.

Question 2

What is the cell body of a neurone?

Answer 2

It contains the nucleus surrounded by cytoplasm. Have large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters.

Question 3

What are dendrons?

Answer 3

They are short extensions which come off the cell body. These extensions divide into smaller branches called dendrites. They’re responsible for transmitting electrical impulses towards the cell body.

Question 4

what is an axon?

Answer 4

Singular elongated nerve fibres that transmit impulses AwAy from the cell body. these fibres can be very long. The fibre is cylindical in shape and consists of a very narrow reigon of cytoplasm surrounded by a plasma membrane

Question 5

What are the three types of neurones?

Answer 5

Sensory neurone
Relay neurone
Motor neurones

Question 6

What is a sensory neurone and what is it’s role?

Answer 6

These neurones transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain. They have one dendron which carries the impulse to the cell body, and one axon which carries the impulse away from the cell body.

Question 7

What are relay neurones?

Answer 7

They transmit impulses between neurones e.g. between sensory neurons and motor neurones. They have many short axons and dendrons

Question 8

What are motor neurones?

Answer 8

They transmit impulses from a relay neurone or sensory neurone to an effector e.g. a muscle or a gland. They have one long axon and many short dendrites.

Question 9

What pathway do most electrical impulses follow in the nervous response?

Answer 9

Receptor -> sensory neurone -> relay neurone -> motor neurone -> effector cell.

Question 10

What is a myelin sheath?

Answer 10

Schwann cells produce layers of membranes by growing around the axon many times. Each time they grow around the axon, a double layer of phospholipid bilayer is laid down. The myelin sheath acts as an insulating layer and allows the myelinated neurones to conduct the electrical impulse at a much faster rate than unmyelinated neurones.

Question 11

What is a node of Ranvier and how does it affect how the electrical impulse travels?

Answer 11

Between each adjacent Schwann cell there is a small gap. This is the node of Ranvier. It creates a gap in the myelin sheath. The electrical impulse will jump from one node to the next. This allows the impulse to be transmitted much faster.

Question 12

What are the features of sensory receptors?

Answer 12

• They are specific to a single type of stimulus
• they act as a transducer, they convert a stimulus into a nerve impulse.

Question 13

what are the 4 main types of sensory receptors present in an animal?

Answer 13

-Mechanoreceptor: pressure and movement: e.g. Pacinian corpuscle(detects pressure) in the skin
- Chemoreceptor: chemical receptor e.g. olfactory receptor(detects smell) in the nose
- Thermoreceptor: heat receptor e.g. end- bulbs of Krause in the tongue
-Photoreceptors: light receptor e.g. Cone cell (detects different light wavelengths) in eye.

Question 14

How do sensory receptors act as a transducer?

Answer 14

Sensory receptors detect a range of different stimuli. The receptor converts the stimulus into a nervous impulse called a generator potential.

Question 15

What are Pacinian corpuscles?

Answer 15

They are specific sensory receptors that detect mechanical pressure. They’re most abundant in the fingertips and soles of feet. The sensory neurone is located in the centre of the corpuscle, surrounded by layers of connective tissue. The layers are seperated by a layer of gel.

Question 16

What is a stretch mediated sodium ion channel?

Answer 16

It is a neurone ending in the Pacinian corpuscle. When these channels change shape e.g. stretch, their permeability to sodium changes.

Question 17

What are the steps of how the Pacinian corpuscle converts mechanical pressure into a nervous impulse?

Answer 17

1. In it’s normal state, the stretch mediated sodium ion channel in the sensory neurone’s membrane are too narrow to allow sodium ions to pass through them. The neurone of the Pacinian corpuscle has a resting potential.
2. When pressure is applied to the corpuscle, it changes shape. This causes the membrane surrounding it’s neurone to stretch.
3. When the membrane stretches, the sodium ion channels present widen. Sodium diffuses into the channels.
4. The influx of the Na+ ions changes the potential of the membrane- it becomes depolarised. This results in a generator potential.
5. In turn, the generator potential creates an action potential that passes along the sensory neurone.

Question 18

What is the resting potential of an axon?

Answer 18

The outside of the membrane is more positively charged than the outside of the axon. It is said to be polarised. It is normally about -70mV.

Question 19

Why is there a resting potential?

Answer 19

It’s a result of the movement of sodium and potassium ions across the axon membrane. The phospholipid bilayer prevents these ionns from diffusing across the membrane and, therefore, they have been transported via channel proteins. Some of these channels are gated- they must be opened to allow specific ions to pass through them. Other channels remain open all the time allowing sodium and potassium ions to diffuse through them.

Question 20

What events result in the creation of an action potential?

Answer 20

-Na+ ions actively transported out the axon, K+ ions actively transported into the axon by a sodium-potassium pump. For every 3Na+ that are pumped out, 2K+ ions are pumped in.
- more sodium ions outside the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the cytoplasm than outside the axon. Sodium ions diffuse back into axon down electrochemical gradient + potassium ions diffuses out of the axon.
- Most gated Na+ ion channels are closed, preventing the movement of sodium ions, whereas K+ channels are open, allowing K+ ions to diffuse out of the axon. Therefore, there are more positively charged ions outside the axon than inside. This creates the resting potential across the membrane of -70mV, with the inside being negative relative to the outside.

Question 21

What is depolarisation?

Answer 21

When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reverses the charges on the axon membrane. As a result, the potential difference across the membrane rapidly changes and becomes positively charged at approx. +40mV. This is depolarisation.

Question 22

What is repolarisation?

Answer 22

As the impulse passes, a change in pd from positive back to negative. The neurone returns to it’s resting potential.

Question 23

When does an action potential occur?

Answer 23

It occurs when protein channels in the axon membrane change shape as a result of the change of voltage across it’s membrane. The change in protein shape results in the channel opening or closing. These channels are known as voltage- gated ion channels.

Question 24

Explain the 6 step sequence of events that take place during an action potential.

Answer 24

1. neurone has resting potential- not transmitting an impulse. Some Na ion channels are open( mainly non voltage gated ones) but Na voltage gated channels are closed
2. energy of stimulus triggers some sodium voltage-gated channels to open- membrane more permeable to Na+. Na+ diffuses into axon down electrochemical gradient. Inside of neurone is less negative.
3. Change in charge causes more Na+ ion channels to open, more Na+ moves in to axon.(positive feedback)
4. when PD reaches +40mV, voltage-gated Na+ channels close + voltage-gated K+ channels open. Na+ can’t enter axon, membrane is now more permeable to K+
5. K+ diffuses out of axon down electrochemical gradient, reduces charge, inside of axon is more negatively charged than the outside
6. Lots of K+ ions diffuse out of axon, inside becomes more negative than resting rate(hyperpolarisation). Voltage-gates K+ channels now close. Na+K+ pump causes Na+ ions to move out of the cell and K+ ions to move in. The axon returns to it’s resting potential- repolarised.

Question 25

Explain the first step of propagating an action potential

Answer 25

At resting potential, the conc of Na+ ions outside the axon membrane is relatively high compared to the inside, whereas the conc of K+ ions is high inside the membrane relative to the outside. The overall conc of + ions is higher on the outside, making this positive compared to the inside. The axon membrane is polarised.

Question 26

Explain the 2nd step of propagating an action potential.

Answer 26

A stimulus causes a sudden influx of sodium ions and hence the reversal of charge on the axon membrane. This is the action potential and the membrane is depolarised.

Question 27

Explain the third step of propagating an action potential.

Answer 27

The localised electrical circuits established by the influx of Na+ ions cause the opening Na voltage gated channels a little further along the axon. The resulting influx of Na+ ions in this region causes depolarisation. Behind this new region of depolarisation, the Na voltage gated channels close and the K+ ones open. K+ ions begin to leave the axon along their electrochemical gradient.

Question 28

explain the 4th step of propagating an action potential.

Answer 28

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 it’s original charged state, (positive outside, negative inside), it has been repolarised.

Question 29

Explain the last step in the propagation of an action potential.

Answer 29

Following repolarisation, the axon membrane returns to it’s resting potential in readiness for a new stimulus if it comes.

Question 30

What is the refractory period?

Answer 30

After an action potential there is a short period when the axon cannot be excited again, this is the refractory period. During this time, voltage gated Na ion channels remain closed, preventing the movement of sodium ions into the axon.

Question 31

Why is a refractory period important?

Answer 31

It is important as it prevents the propagation of an action potential backwards along the axon as well as forwards. It makes sure that action potentials are unidirectional. It also ensures that action potentials don’t overlap and occur as discrete impulses.

Question 32

Why do myelinated axons transfer electrical impulses much faster than non-myelinated axons?

Answer 32

Depolarisation of the axon membrane can only occur at the nodes of Ranvier where no myelin is present. Here Na+ ions can pass through the protein channels in the membrane. Longer localised circuits therefore arise between adjacent nodes. The action potential then ‘jumps’ from one node to the next. This is saltatory conduction. This is much faster than a wave of depolarisation across the whole length of the axon membrane.

Question 33

How does saltatory conduction work?

Answer 33

Every time the channels open and ions move, it takes time, so reducing the number of places where this happens speeds up the action potential transmission. LT saltatory conduction is also more efficient. Repolarisation uses ATP in the sodium pump, so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient.

Question 34

What other factors affect the speed at which an action potential travels other than myelination?

Answer 34

Axon diameter- the bigger the axon diameter, the faster the impulse is transmitted. This is because there is less resistance to the flow of the ions in the cytoplasm compared with those in a smaller axon.
temperature- The higher the temperature, the faster the nerve impulse. This is because ions diffuse faster at higher temperatures. However, this generally only occurs up to about 40 degees as higher temps cause the proteins e.g. sodium and potassium pump to become denatured.

Question 35

What is the all or nothing principle?

Answer 35

Refers to nerve impulses, a certain level of stimulus, the threshold value, always triggers a response. If this threshold is reached, an action potential will always be created. No matter how large the stimulus is, the same sized action potential will always be triggered. If the threshold isn’t reached, no action potential will be triggered.
The size of the stimulus however, does affect the number of action potentials that are generated in a given time. The larger the stimulus, the more frequently the action potentials are generated.

Question 36

What are the key structures in a synapse?

Answer 36

Synaptic cleft- the gap which separates the axon of one neurone from the dendrite of the next neurone.
presynaptic neurone- neurone along which the impulse has arrived
post synaptic neurone- neurone that receives the neurotransmitter
synaptic knob- the swollen end of the presynaptic neurone. It contains many mitochondria and a large amount of endoplasmic reticulum to allow it to manufacture neurotransmitters.
Synaptic vesicles- vesicles containing neurotransmitters. The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft.
Neurotransmitter receptors- receptor molecules which the neurotransmitter binds to in the postsynaptic membrane.

Question 37

What are the two types of neurotransmitter?

Answer 37

Excitatory- these neurotransmitters result in the depolarisation of the postsynaptic neurone. If the threshold is reached in the postsynaptic membrane, an action potential is triggered. Acetylcholine is an example of an excitatory neurotransmitter
Inhibitory- These result in the hyper polarisation of the post synaptic membrane. This prevents an action potential being triggered.

Question 38

What causes the transmission of an impulse across a synapse?

Answer 38

-The action potential reaches the end of the presynaptic neurone
- depolarisationn of the presynaptic membrane causes Ca+ ion channels to open
-Ca+ ions diffuse into the presynaptic knob
- This causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
-Neurotransmitter diffuses across the synaptic cleft and binds with it’s specific receptor molecule on the postsynaptic membrane
- This causes sodium ion channels to open
-Na+ ions diffuses into the post synaptic neurone
- This triggers an action potential and the impulse in propagated along the postsynaptic neurone.

Question 39

Explain the process of transmission across cholinergic synapses

Answer 39

1. Arrival of an action potential at the end of the presynaptic neurone causes Ca2+ ion channels to open and Ca2+ ions enter the synaptic knob.
2. The influx of Ca2+ ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into the synaptic cleft.
3. acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone. This causes the sodium ion channels to open, allowing Na+ to diffuse in rapidly along a conc gradient.
4. The influx of Na+ ions generates a new action potential in the post synaptic neurone.
5. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone. In addition to recycling the choline and the ethanoic acid, the breakdown of acetylcholine also precents it from continuously generating a new action potential in the postsynaptic neurone.
6. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites.

Question 40

What are the role of synapses?

Answer 40

• they ensure impulses are unidirectional . As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone.
• They can allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses. This results in a single stimulus creating a number of simultaneous responses.
• alternatively, a number of neurones may feed into the same synapse with a single postsynaptic neurone. This results in stimuli from different receptors interacting to produce a single result.

Question 41

What is summation?

Answer 41

Each stimulus from a presynaptic neurone causes the release of the same amount of neurotransmitter into the synapse. In some synapses, however, the amount of neurotransmitter from a single impulse isn’t enough to trigger an action potential in the postsynaptic neurone, as the threshold level isn’t reached. However if the amount of neurotransmitter builds up sufficiently to reach the threshold then this will trigger an action potential. This is known as summation.

Question 42

What are the 2 types of summation?

Answer 42

Spatial summation- this occurs when a number of presynaptic neurones connect to one postsynaptic neurone. Each releases neurotransmitter which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone.
temporal summation- This occurs when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient to trigger an action potential.

Question 43

what is a transducer?

Answer 43

It changes one form of energy to another

Question 44

What are the 2 structural systems that the mammalian nervous system is structured into?

Answer 44

Central nervous system- Consists of the spinal chord and the brain.
Peripheral nervous system- Consists of all the neurones that connect the CNS to the rest of the body. These are the sensory neurones that carry nerve impulses from the receptors to the CNS and the motor neurones that carry nerve impulses away from the CNS.

Question 45

What are the 2 functional systems that the mammalian nervous system is organised into?

Answer 45

Somatic nervous system- Under conscious control- is used when voluntarily decide to do something e.g. when you decide to move muscles in your arm. Carries impulses to the body’s muscles
Autonomic nervous system- Works constantly- under subconscious control and is used when the body does something automatically. Its involuntary. e.g. causing the heart to beat, or to digest food. Carries nerve impulses to glands, smooth muscle and cardiac muscle.

Question 46

What 2 systems is the autonomic nervous system split into?

Answer 46

Sympathetic- increases activity e.g. increase in HR. Fight or flight response. Neurotransmitter= noradrenaline
Parasympathetic- decreases activity e.g. decrease in HR. Rest and digest response. Neurotransmitter= acetylcholine.

Question 47

What is the benefit of having one central control centre for the whole body?

Answer 47

Coordination between billions of neurones involved is much faster than if control centres for different functions were distributed around the body.

Question 48

What are the main 5 areas of the brain and what are their roles?

Answer 48

• Cerebrum; controls voluntary actions such as learning, memory, personality and conscious thought.
• Cerebellum; controls unconscious functions such as posture, balance and non-voluntary movements.
• Medulla oblongata; used in autonomic control e.g. controls HR and BR.
• Hypothalamus; regulatory centre for temperature and water balance
• Pituitary gland- Stores and releases hormones that regulate many bodily functions.

Question 49

What are the main functions of the cerebrum?

Answer 49

Split into frontal lobe, parietal lobe, occipital lobe and temporal lobe.
Cerebrum receives sensory information, processes it and then sends impulses along motor neurone to effectors to form appropriate responses. Its responsible for coordinating all of the body’s voluntary responses as well as some involuntary ones.
Outer layer of the cerebral hemispheres is called the cerebral cortex- The most sophisticated processes such as reasoning and decision-making occur in the frontal and prefrontal lobe of the cerebral cortex.
Each sensory centre within the cerebral hemispheres receives information from receptor cells located in sense organs. e.g. to move skeletal muscles. The size of the motor area allocated is in proportion to the relative number of motor endings in it. Main region which controls movement is the primary motor cortex which is located at the back of the frontal lobe.

Question 50

What is the role of the base of the brain?

Answer 50

Impulses from each side of the body cross- left hemisphere receives impulses from the RHS of the body and vise versa. e.g. inputs from the eye pass to the visual area in the occipital lobe. Impulses from the right side of the field of vision in each eye are sent to the visual cortex of the left hemisphere. This allows the brain to judge perspective and distance.

Question 51

What is the role of the cerebellum?

Answer 51

Concerned with the control of muscular movement, body posture and balance- it doesn’t initiate movement, but it coordinates it. This means that if it becomes damaged, a person can suffer from jerky and uncoordinated movements. It receives information from the organs of balance in the ears and info about the tone of muscles and tendons. It then relays this info to the areas of the cerebral cortex that are involved in motor control.

Question 52

What is the role of the medulla oblongata?

Answer 52

Contains many important regulatory centres of the autonomic nervous system. These control reflex activities such as swallowing and coughing.

Question 53

What are the roles of the hypothalamus?

Answer 53

Main controlling region for the autonomic nervous system. Has 2 centres, one for sympathetic and one for parasympathetic nervous systems. Its functions include;
- controlling complex patterns of behaviour such as feeding, sleeping and aggression
- monitoring the composition of blood plasma, such as the conc. of water and blood glucose
- producing hormones- its an endocrine gland

Question 54

What is the pituitary gland and what are it’s main roles?

Answer 54

Found at the base of the hypothalamus and is approximately the size of a pea. Controls most the glands in the body, divided into 2 sections;
- Anterior pituitary- produces 6 hormones including FSH, which is involved in reproduction, and growth hormones
- Posterior pituitary- Stores and releases hormones produced by the hypothalamus such as ADH, which is involved in urine production.

Question 55

What is the general reflex arc pathway?

Answer 55

Receptor- detects stimulus and creates an action potential in the sensory neurone.
Sensory neurone- carries impulse to spinal chord
Relay neurone- connects the sensory neurone to the motor neurone within the spinal chord or brain
Motor neurone- carries impulse to the effector to carry out the appropriate response.

Question 56

Explain the withdrawal reflex when you touch a hot candle.

Answer 56

1. Stimulus= heat from candle flame
2. thermoreceptor in skin detects heat
3. sensory neurone passes nerve impulses to spinal chord
4. Relay neurone passes impulses across the spinal chord
5. motor neurone passes impulses to the muscle
6. effector contracts
7. Response- hand is moved away quickly from flame.

Question 57

Why is the knee jerk reflex tested by doctors?

Answer 57

Its a spinal reflex, which means that the neutral circuit only goes up the spinal chord not the brain. The absence of this reflex may indicate nervous problems and multiple oscillations in the leg may be a sign of a cerebellar disease.

Question 58

What happens during the knee jerk reflex?

Answer 58

When the leg is tapped just below the patella, it stretches the patella tendon and acts as a stimulus. This stimulus indicates a reflex arc that causes the extensor muscles on the top of the thigh to contract. At the same time, a relay neurone inhibits the motor neurone of the flexor muscle, causing it to relax. This contraction, coordinates with the relaxation of the antagonistic flexor hamstring muscle, causes the leg to kick.

Question 59

What is the blinking reflex and when is it stimulated?

Answer 59

Its an involuntary blinking of the eyelids. It occurs when the cornea is stimulated e.g. being touched. It’s purpose is to keep the cornea safe from damage due to foreign bodies such as dust entering the eye. Can also occur when loud noises are heard.

Question 60

How does the blinking reflex occur?

Answer 60

When the cornea of the eye is irritated by a foreign body, the stimulus triggers an impulse along a sensory neurone. The impulse then passes through a relay neurone in the lower brain stem. Impulses are then sent along branches of the motor neurone to initiate a motor response to close the eyelids. The reflex initiates a consensual response- both eyes are closed in response to a stimulus.

Question 61

Why do doctors test the blinking reflex when examining an unconscious patient?

Answer 61

If this reflex is present, it indicates that the lower brainstem is functioning. This procedure is therefore used as a part of an assessment to determine whether or not a patient is brain dead- if the corneal reflex is present, a person cannot be diagnosed as brain dead.

Question 62

How can reflexes increase your chances of survival?

Answer 62

• being involuntary responses; the decision- making regions of the brain aren’t involved, therefore the brain is able to deal with more complex responses. It prevents the brain from being overloaded with situations in which the response is always the same.
• Not having to be learnt; they’re present at birth and therefore, provide immediate protection.
• Extremely fast; the reflex arc is extremely short. It normally only involves one or two synapses, which are the slowest part of nervous transmission.
• Many reflexes we would consider as everyday actions, such as those which keep us upright and those which control digestion.

Question 63

What are the 3 types of muscle?

Answer 63

• Skeletal muscle; cells responsible for movement e.g. biceps
• Cardiac muscle; found only in the heart, they’re myogenic, meaning they contract without the need for a nervous stimulus, causing the heart to beat in a regular rhythm.
• Involuntary muscle (smooth muscle); found within walls of hollow organs e.g. stomach and bladder. They’re also found in the walls of blood vessels and the digestive tract, where through peristalsis they move food along the gut.

Question 64

What are the features of skeletal muscle?

Answer 64

• Striated
• voluntary
• arranged regularly so muscle contracts in one direction
• rapid contraction speed
• short contraction length
• muscles showing cross striations are known as striated or striped muscles. Muscle fibres are tubular and multinucleated.

Question 65

What are the features of cardiac muscle?

Answer 65

• Specialised striated
• involuntary control
• cells branch and interconnect, resulting in simultaneous contraction
• contraction is immediate
• length of contraction is intermediate
• Cardiac muscle shows striations but they’re much fainter than those in skeletal muscle. Fibres are branched and uninucleated.

Question 66

What are the features of involuntary muscles?

Answer 66

• non-striated
• involuntary control
• no regular arrangement- different cells can contract in different directions
• slow contraction speed
• can remain contracted for a relatively long time
• muscles showing no cross striations- non-striated. Fibres are spindle shaped and uninucleated.

Question 67

What are muscle fibres?

Answer 67

Skeletal muscles are made up of bundles of muscle fibres, enclosed in a plasma membrane- sarcolemma.
Contain a number of nuclei + are longer than normal cells. Shared cytoplasm within a muscle fibre= sarcoplasm.
Parts of sarcolemma fold inwards- t tubules, to help spread electrical impulses throughout the sarcoplasm, therefore the whole of the fibre receives the impulse and contracts at the same time.
Have lots of mitochondria to provide ATP for muscle contraction.
Have a sarcoplasmic reticulum which extends throughout the muscle fibre and contains calcium ions required for muscle contraction.

Question 68

What are myofibrils?

Answer 68

Each muscle fibre contains many myofibrils. They’re long cylindrical organelles made of protein and specialised for contraction.
They’re lined up in parallel to provide maximum force when the all contract together.
Made of 2 protein filaments;
- Actin: the thinner filament, consists of 2 strands twisted around each other
- Myosin: thicker filament, consists of long rod-shaped fibres with bulbous heads that project to one side.

Question 69

Why do microfibrils have a striped appearance?

Answer 69

Light bands- appear light as they’re a region where the actin and myosin filaments don’t overlap. AKA i-bands
Dark bands- appear dark due to presence of thick myosin filaments. Edges are particularly dark as the myosin is overlapped with actin. AKA a- bands.
Z-line - line found at the centre of each light band. The distance between adjacent Z-lines is called a sarcomere. It is the functional unit of the myofibril. When a muscle contracts, the sarcomere shortens.
H-zone - Lighter coloured region found in the centre of each dark band. Only myosin filaments are present at this point. When the muscle contracts, the H- zone decreases.

Question 70

What is the sliding filament model?

Answer 70

During contraction, the myosin filaments pull the actin filaments inwards towards the centre of the sarcomere. This results in:
- Light band becoming narrower
- Z lines moving closer together, shortening the sarcomere.
- H-zone becoming narrower.
- Dark band stays the same as myosin filaments have not shortened, but now overlap the actin filaments by a greater amount.

Question 71

Explain the structure of myosin.

Answer 71

Myosin filaments have globular heads that are hinged which allows them to move back and forwards. On the head is a binding site for each of actin and ATP. The tails of several hundred myosin molecules are aligned together to form the myosin filament.

Question 72

Explain the structure of actin.

Answer 72

Actin filaments have binding sites for myosin heads; actin-myosin binding sites. These sites are often blocked by tropomyosin which is held in place by troponin.
When muscle is resting, actin-myosin sites are blocked by tropomyosin. Myosin heads can’t bind with actin, filaments can’t slide past each other.
When a muscle is stimulated to contract, myosin heads form bonds with actin filaments; actin-myosin cross-bridges. Myosin heads flex in unison, pulling actin filaments along the myosin filament. Myosin detaches from actin and reattaches further along the actin filament.

Question 73

How do contractions occur at a neuromuscular junction?

Answer 73

Muscle contraction is triggered when an action potential arrives at a neuromuscular junction, many junctions along the length of a muscle to ensure that all muscle fibres contract simultaneously.
All muscle fibres are supplied by a single motor neurone, a motor unit- the fibres act as a single unit. If a strong force is needed, a large number of motor units are stimulated, if a small force is required, only a small number are stimulated.
When an action potential reaches a neuromuscular junction, it stimulates Ca2+ ion channels to open. Ca2+ then diffuses from synapse into the synaptic knob, where they cause synaptic vesicles to fuse with the presynaptic membrane. Acetylcholine is released into synaptic cleft by exocytosis and diffuses across the synapse. Binds to receptors on the post synaptic membrane, opening up sodium channels and resulting in depolarisation.
Acetylcholine is broken down by acetylcholinesterase into choline and ethanoic acid- prevents muscle from being overstimulated. Choline and ethanoic acid diffuse back into neurone.

Question 74

Describe the process of muscle contraction in the sarcoplasm.

Answer 74

Depolarisation of sarcolemma travels deep into the muscle fibre by spreading through the T tubules. These are in contact with the sarcoplasmic reticulum which contains Ca2+ which it actively absorbs form the sarcoplasm.
When the action potential reaches the sarcoplasmic reticulum it stimulates Ca2+ ion channels to open. Ca2+ ions diffuse down their conc. gradient, flooding the sarcoplasm with Ca2+.
Ca2+ binds to troponin, changing its shape, pulling the tropomyosin, moving it away from the actin-myosin binding sites on actin filament. Heads are exposed, actin-myosin cross-bridge is formed.
Myosin head flexes, pulling actin filament along. The ADP molecule bound to myosin head is released, an ATP molecule can now bind to myosin head- head then detaches from the actin filament.
ATPase hydrolyses ATP to ADP and phosphate, releasing energy which the myosin head uses to return to it’s original position.
Cycle is then repeated, as long as the muscle remains stimulated.

Question 75

How is energy supplied during muscle contractions? Why is it needed?

Answer 75

Provided by the hydrolysis of ATP into ADP and Pi. The energy is required for the movement of myosin heads and to enable the sarcoplasmic reticulum to actively reabsorb Ca2+ from the sarcoplasm.

Question 76

What are the 3 main ways that ATP can be generated?

Answer 76

-Aerobic respiration
-Anaerobic respiration
-Creatine phosphate

Question 77

How does aerobic respiration produce ATP?

Answer 77

Most of the ATP used by muscle cells is regenerated from ADP during oxidative phosphorylation. It takes place inside the mitochondria which are plentiful in the molecule. It is used for long periods of low-intensity exercise.

Question 78

How does anaerobic respiration produce ATP?

Answer 78

In a very active muscle, oxygen is used up more quickly than the blood supply can replace it. ATP has to be generated anaerobically. It is made by glycolysis but, as no oxygen is present, the pyruvate which is also produced is converted into lactic acid. This can quickly build up in the muscles, resulting in muscle fatigue. It is used for short periods of high- intensity exercise.

Question 79

How does creatine phosphate produce ATP?

Answer 79

Creatine phosphate acts as a reserve supply of phosphate, which is available immediately to combine with ADP, but the store of phosphate is used up quickly. As a result, this is used for short bursts of vigorous exercise. When a muscle is relaxed, the creatine phosphate store is replenished using phosphate from ATP.