5.1 NEURONAL COMMUNICATION

Question 1

Nervous system overview.

Answer 1

Nervous system overview:
- neurones form nerves
- central nervous system and peripheral nervous system
- CNS detects and sends signals to PNS to act
- afferent to central point, efferent away from central point
- PNS = motor neurons (efferent), sensory neurones (afferent)
- reflex loop when spinal chord makes decision instead of the brain

Question 2

What is a stimulus?

Answer 2

Stimulus:
- any change in the environment, either internal or external, can generate a stimulus
- anything capable of exciting a sensory receptor cell can be defined as a ‘stimulus’
- examples = sound, light, heat, cold, odour, colour, touch, pressure

Question 3

What are receptors?

Answer 3

Receptors are structures which detect a stimulus and convert it into an action potential.
- energy transducers
= convert energy from one form to another
= are specific to the type of energy they convert
= all convert to electrical impulse = impulse

Question 4

What are the stimulus, receptors and types of energy for mechanoreceptors?

Answer 4

Mechanoreceptors:
receptor - Pacinian corpuscle, morkel cell
stimulus - pressure (kinetic energy), touch (kinetic energy)

Question 5

What are the stimulus, receptors and types of energy for chemoreceptors?

Answer 5

Chemoreceptors:
receptor - taste buds, olfactory tracts, nociceptors
stimulus - chemicals in saliva (chemical energy), chemicals in inhaled air (chemical energy), pain due to trauma (kinetic energy)

Question 6

What are the stimulus, receptors and types of energy for thermoreceptors?

Answer 6

Thermoreceptors:
receptor - cold receptors in epidermis, warm receptors in dermis
stimulus - cool temperature (heat energy), warm temperature (heat energy)

Question 7

What are the stimulus, receptors and types of energy for photoreceptors?

Answer 7

Photoreceptors:
receptor - retina
stimulus - light (light energy)

Question 8

What is the Pacinian corpuscle?

Answer 8

Pacinian corpuscle:
- found in skin dermis
- found in abundance in fingers and feet soles
- in joints to tell you direction changes

Question 9

How does the Pacinian corpuscle stretch when pressure is applied to it?

Answer 9

PC stretching:
- corpuscle goes out of shape; membranes stretch causing sodium ion channels to open wider
- sodium ions can now diffuse into the neuron creating an action potential if enough channels are widened
- the harder the corpuscle is pressured the more channels open

Question 10

What are the features of neurones?

Answer 10

Features of neurones:
- long
- voltage gated ion channels in plasma membranes
- sodium potassium pumps in the cells surface membrane
- have a myelin sheath
- cell body contains nucleus and produces neurotransmitters
- dendrites that connect to other neurones
- dendron goes to the cells body and axon goes away from cell body

Question 11

What is myelination?

Answer 11

Myelination:
- speeds up rate of action travels
- reduces number of channels that have to open for the signal to travel

Question 12

What is a neurotransmitter?

Answer 12

A neurotransmitter is a chemical involved in communication across a synapse between adjacent neurones or a neurone muscle cell.

Question 13

Why are cone and rod cells (found on the retina of the eye) referred to as energy transducers?

Answer 13

Energy transducers produce a generator potential. Rod and cone cells respond to light and produce a generator potential.

Question 14

Why are reasoning and conscious thought not necessary or desirable features of reflex behaviours?

Answer 14

Reasoning and conscious thought are not necessary or desirable features of reflex behaviours because it means the signal would be slower to go to the brain.

Question 15

What is the different between a spinal reflex and cranial reflex?

Answer 15

In a spinal reflex, the impulse is controlled by the spinal chord and takes place in the rest of the body (e.g knee jerk reflex). The cranial reflex is controlled by the brain and occurs in the head region (e.g pupil reflex).

Question 16

What is the difference between a monosynaptic and polysynaptic reflex arc?

Answer 16

A monosynaptic reflex involves only one CNS synapse (knee jerk reflex) whereas polysynaptic reflex involves two or more CNS synapses (e.g pain withdrawal reflex).

Question 17

Would a monosynaptic or polysynaptic reflex produce the most rapid response, given similar length sensory and motor pathways?

Answer 17

A monosynaptic reflex would be faster as the spine is closer and the signal does not have to be processed in the brain. It also only has to cross one CNS synapse.

Question 18

What is the adaptive value of primitive reflexes in newborns?

Answer 18

Some primitive reflexes in newborns hold survival value (e.g rooting and sucking reflexes).

Question 19

Why are newborns tested for the presence of primitive reflexes?

Answer 19

Newborns are tested for primitive reflexes to ensure there are no issues with their sensory cells or receptor organs.

Question 20

Nerves and neurones.

Answer 20

Nerves and neurones:
- a neurone is a specialised nerve cell
- 10s of 1000s of neurones make a nerve
- neurones are organised into bundles surrounded by a perineurium

Question 21

What is resting potential?

Answer 21

Resting potential is the potential difference across the membrane of an axon of a neurone at rest (normally around -70mv).

Question 22

What is action potential?

Answer 22

Action potential is the change in the potential difference across the neurone membrane of the axon when stimulated.

Question 23

How is a resting potential set up?

Answer 23

Resting potential:
1. sodium ions are actively transported out of the axon whereas potassium ions are transported into the axons by a sodium potassium pump - this process requires ATP
2. however, their movement is not equal - for every three sodium ions that are pumped out, two potassium ions are pumped in
3. there are now more sodium ions outside the membrane than inside the axon, whereas there are more potassium ions inside the axon than outside the membrane
4. both ions begin to diffuse in opposite directions across the membrane are down their electrochemical gradients
5. however, most voltage-gated sodium ion channels are closed, whereas many potassium ions are open. therefore, there are more positively charged ions outside the axon
6. this creates a resting potential across the membrane of -70mv, with the inside of the axon negative relative to the outside

Question 24

What is an all-or-nothing response?

Answer 24

All-or-nothing response:
- when a receptor detects a stimulus, there needs to be enough energy to generate an action potential
- a stimulus that reaches the threshold potential will always generate an action potential
- if the threshold value is not reaches, there is no action potential generated
- the threshold value is when the membrane is depolarised to -50mv/-40mv
- no matter the size of the stimulus, the action potential will always be the same size
- a larger stimulus just increases the frequency at which the action potential is generated

Question 25

How is an action potential set up?

Answer 25

Action potential:
1. polarised, resting state -65mv/-70mv
- sodium potassium pump is working
- K+ ion channels open, Na+ channels closed
2. Na+ voltage gated channels open
- energy from stimulus opens Na+ voltage gated channels so membrane becomes more permeable to Na+ ions
3. threshold potential -40mv/-50mv
- if enough Na+ diffuse down electrochemical gradient, threshold potential is reached
4. depolarisation +40mv
- Na+ flood into axon which causes depolarisation
- Na+ causes more Na+ channels to open (positive feedback)
5. repolarisation
- at +40mv Na+ voltage gated channels close and K+ voltage gated channels open
- K+ ions flood out down electrochemical gradient via diffusion
6. hyperpolarisation -75mv
- potential difference overshoots so the pd is more negative on the inside compared to its resting state
7. resting state -65mv/-70mv
- K+ voltage gated channels shut
- Na+/K+ pump works again to restore resting potential

Question 26

How is an action potential set up (simplified)?

Answer 26

Action potential:
1. stimulus triggers some voltage gated sodium ion channels to open in membrane at end of axon
2. sodium ions diffuse into axon down their electrochemical gradient making the inside of the neurone less negative
3. more sodium ion channels open due to the change in charge. more sodium ions diffuse into the axon
4. potential difference reaches +40mv. voltage gated sodium ion channels close and potassium ion channels open
5. potassium ions diffuse out of the axon down their electrochemical gradient. the inside of the axon becomes more negative than outside
6. outflow of potassium ions from axon results in the inside of the axon becoming more negative (relative to outside) than its normal resting rate. voltage gated potassium ions close
7. sodium-potassium pump causes sodium ions to move out of the cell and potassium ions to move in. axon returns to its resting potential

Question 27

Why does the cell body of neurones contain large numbers of mitochondria and ER?

Answer 27

The cell body contains large numbers of mitochondria for aerobic respiration to generate ATP for active transport. It also contains large numbers of ER for the production of neurotransmitters.

Question 28

What is the role of a sensory neurone?

Answer 28

A sensory neurone transmits impulses from a sensory receptor cells to a relay neurone, motor neurone, or the brain.

Question 29

What is the function of myelin sheath?

Answer 29

The myelin sheath is an electrically insulating layer and allows neurones to conduct electrical impulse at a faster speed.

Question 30

What is the structure and function of the axon?

Answer 30

The axon is a single, elongated nerve fibre that carries impulse away from the cell body.

Question 31

What is propagation of action potentials?

Answer 31

Propagation of action potentials:
- a stimulus causes a sudden influx of sodium ions causing the depolarisation of the axon membrane
- the depolarisation acts as a stimulus for the next region of the axon, causing voltage-gated sodium ion channels to be opened - this region is now also depolarised
- as the next region becomes depolarised, the voltage gated sodium ion channels close and the voltage-gates potassium ion channels open. this region is now repolarised
- the action potential continues to be propagated along the axon and repolarised region of the axon membrane causes the forward undirectional propagation of the action potential
- following the repolarisation of the membrane, voltage-gated potassium ion channels close and the resting potential is maintained

Question 32

What is saltatory conduction?

Answer 32

Saltatory conduction is the method by which an electrical impulse skips from node to node down the full length of a myelinated neurone, speeding the arrival of the impulse at the nerve terminal.

Question 33

What is a synapse?

Answer 33

Synapse:
- junction between 2 neurones
- the action potential must be transmitted across the gap via neurotransmitters

Question 34

What are the types of neurotransmitter:

Answer 34

Types of neurotransmitter:
- excitatory = causes depolarisation of the postsynaptic membrane, so the action potential travels through the next neurone (e.g acetylcholine)
- inhibitory = result in the hyperpolarisation of the postsynaptic membrane. this prevents an action potential being triggered (e.g CABA)

Question 35

What is a cholinergic synapse?

Answer 35

Cholinergic synapse:
- synapse that uses acetylcholine as the neurotransmitter
- found in CNS of vertebrates and as neurotransmusclar junctions (synapses leading to muscle)

Question 36

What is the process of synapse transmission?

Answer 36

Synapse transmission:
1. action potential arrives at the presynaptic knob
2. action potential causes voltage gated Ca2+ channels open
3. in the presence of Ca2+ vesicles move to the pre-synaptic membrane and fuse
4. acetylcholine (ach) is released via exocytosis into the synaptic cleft
5. ach diffuses across the cleft
6. ach binds to specific receptor
7. the binding of ach to receptor opens Na+ channels
8. the influx of Na+ causes the post synaptic membrane to depolarise
9. enzyme acetylcholinesterase breaks down ach to choline and ethanoic acid
10. choline and ethanoic acid diffuse back across cleft to be recycled back into ach - ATP used (in cleft)

9+10 = recycling of neurotransmitters

Question 37

What is the role of a synapse?

Answer 37

Role of synapse:
- ensure synapses are unidirectional
- the neurotransmitter receptors are only present on the postsynaptic membrane and neurotransmitters are only released from the presynaptic membrane

Question 38

What is spatial summation?

Answer 38

Spatial summation:
- several presynaptic neurones connect to one post-synaptic neurone
- each releases neurotransmitters which build up to a high enough level in the synapse to trigger an action potential in the postsynaptic neurone

Question 39

What is temporal summation?

Answer 39

Temporal summation:
- occurs when a single presynaptic neurone releases neurotransmitters 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 40

What are stimulant drugs?

Answer 40

Stimulant drugs are drugs that create more action potentials in the postsynaptic membrane leading to an enhanced response (e.g caffeine, cocaine, ecstasy).

Question 41

What are inhibitory drugs?

Answer 41

Inhibitory drugs are drugs that create fewer action potentials in the postsynaptic neurones leading to a reduced response (e.g alcohol, currare).

Question 42

What effect does caffeine have on the nervous system?

Answer 42

Caffeine:
- competes with adenosine, stops adenosine binding (inhibitor)
- binding of caffeine to receptors causes more to form, meaning more caffeine is needed
- increases production of adrenaline

Question 43

What effect does cocaine have on the nervous system?

Answer 43

Cocaine:
- causes increase in dopamine
- stops dopamine returning to nerve cells (blocks dopamine transporters)
- causes build up of dopamine in system and brain = leads to lowered inhibitions, can lead to deficiencies as more energy is needed to synthesise more dopamine, blocks other signals

Question 44

What effect does ecstasy have on the nervous system?

Answer 44

Ecstasy:
- effects neurotransmitters (increases production of serotonin)
- causes body to destroy too much serotonin after too much is released = leads to low mood
- too much use can damage nerve cells, damage production process of serotonin and potentially cause brain damage

Question 45

What effect does alcohol have on the nervous system?

Answer 45

Alcohol:
- suppresses glutamine production, causes more GABA production
- inhibits excitory pathways = about anxiety etc (things to keep alive) = glutamine
- neurones don’t reach postsynaptic membrane
- limits thoughts, only allow certain pathways through that are not essential for life

Question 46

What effect does cannabis have on the nervous system?

Answer 46

Cannabis:
- prevents the reset of synapses = no new signals, hyperfocus on single idea
- triggers release of dopamine
- starts one action potential but delays others

Question 47

What is the effect of myelination on the rate of conduction of an action potential and how is this effect achieved?

Answer 47

Myelination increases the speed of conduction due to saltatory conduction. This is because the action potential can jump from one node to another and doesn’t have a wave of depolarisation travelling down the length of the axon membrane, resulting in faster conduction.

Question 48

What is the structure of the (mammalian) nervous system?

Answer 48

Mammalian nervous system:
- central nervous system (CNS) consists of the brain and spinal chord, relay neurones also included
- peripheral nervous system (PNS) consists of all the neurones that connect the CNS to the rest of the body

Question 49

What is the PNS?

Answer 49

PNS:
- somatic nervous system = under conscious control, when you voluntarily decide to do something, requires input from sense organs and carries impulses to the body’s muscles
- autonomic nervous system = works constantly under subconscious control, used when the body does something autonomically/involuntarily, requires input from internal receptors and carries impulses to glands, smooth muscle and cardiac muscle

Question 50

What is the autonomic nervous system?

Answer 50

Autonomic nervous system:
- sympathetic nervous system = generally used in an attempt to increase activity - fight or flight - neurotransmitter is noradrenaline
- parasympathetic nervous system = generally used in an attempt to decrease activity - rest and digest response - neurotransmitter is acetylcholine

Question 51

What is the organisation of autonomic nervous system?

Answer 51

Organisation of autonomic nervous system:
- two systems generally have antagonistic effects and have their own neurones in anatomically-distinct nerves
- sympathetic nerves pass down the spinal chord
- parasympathetic nerves are mostly outside of the spinal chord, bundled into the large vagus nerve
- most body organs are innervated by effector neurones from each system

Question 52

What is the vagus nerve?

Answer 52

The vagus nerves is one of our cranial nerves - pairs of nerves that connect the brain to different parts of the head, neck and trunk.

Question 53

What a ganglion?

Answer 53

A ganglion is a group of neuron cell bodies in the PNS, typically linked by synapses.

Question 54

What are the effects of the sympathetic and parasympathetic nervous systems on the heart?

Answer 54

Heart:
sympathetic - increase rate and force of contraction
parasympathetic - reduces rate and force of contraction

Question 55

What are the effects of the sympathetic and parasympathetic nervous systems on the eye?

Answer 55

Eye:
Pupil
- sympathetic - dilates
- parasympathetic - constricts
Ciliary muscles
- sympathetic - relax, makes the lens thinner for distant vision
- parasympathetic - contract, makes the lens thicker for near vision

Question 56

What are the effects of the sympathetic and parasympathetic nervous systems on the digestive system?

Answer 56

Digestive system:
Exocrine gland
- sympathetic - little or no effect
- parasympathetic - stimulates secretion
Sphincter muscles (anus)
- sympathetic - contraction
- parasympathetic - relaxation
Liver
- sympathetic - release of glucose to blood
- parasympathetic - small increase in glycogen production

Question 57

What are the effects of the sympathetic and parasympathetic nervous systems on the skin?

Answer 57

Skin:
Sweat glands
- sympathetic - increases sweating
- parasympathetic - little effect, except to increase sweating on palms of hands
Erector muscles
-sympathetic - contract, making hairs stand on end
- parasympathetic - no effect
Arterioles
- sympathetic - vasoconstriction
- parasympathetic - no effect

Question 58

How are things detected to determine how the heart rate should be changed?

Answer 58

Detection:
- receptors detect changes in pressure and chemical levels in the blood to determine how the heart rate should be changed
- baroreceptors detect changes in the blood pressure and are present in aorta, vena cava and carotid arteries
- chemoreceptors detect changes in levels of particular chemicals in the blood and are present in the aorta, medulla and carotid arteries

Question 59

What is the medulla oblongata?

Answer 59

Medulla oblongata:
- part of the brain that controls heart rate
- linked to the SAN of the heart via the autonomic nervous system
- has 2 centres = one centre for increasing heart rate and the other centre of decreasing heart rate

Question 60

How does the sympathetic nervous system increase heart rate?

Answer 60

Increasing heart rate- sympathetic:
- increases heart rate by stimulating SAN
- also links to walls of ventricle where it increases the force of contraction
- works through the accelerator nerve

Question 61

How does the parasympathetic nervous system decrease the heart rate?

Answer 61

Decreasing heart rate- parasympathetic:
- decreases heart rate by releasing a neurotransmitter at the SAN
- works through the vagus nerve

Question 62

What are the reasons for increasing heart rate?

Answer 62

Reasons for increasing heart rate:
- blood pressure too low
- oxygen levels too low
- carbon dioxide too high
- pH too low

Question 63

What are the reasons for decreasing heart rate:

Answer 63

Reasons for decreasing heart rate:
- blood pressure too high
- oxygen levels too high
- carbon dioxide too low
- pH too high

Question 64

What influence do hormones have on the control of heart rate?

Answer 64

Influence of hormones:
- the adrenal medulla releases
= adrenaline - increases heart rate
= noradrenaline - works with adrenaline in times of stress to also increase heart rate
- both speed up heart rate by causing an increase in the frequency of impulses produced by the SAN

Question 65

What are the external structures of the brain?

Answer 65

External structures of the brain:
- protected by the skull
- surrounded by protective membranes called meninges = meningitis is an infection of the protective membranes that surround the brain and the spinal chord

Question 66

What is the role of part of the brain stem, the medulla oblongata?

Answer 66

Medulla oblongata:
- important role in autonomic nervous system
- controls activities such as ventilation, heart rate, swallowing, peristalsis and coughing

Question 67

What is the role of part of the brain stem, the hypothalamus?

Answer 67

The hypothalamus:
- main controlling region for the autonomic nervous system
- two centres = one for sympathetic nervous system and one for parasympathetic nervous system
- functions include
= complex behaviour patterns (e.g feeding, sleeping)
= monitoring blood plasma composition (e.g water, glucose levels)
= endocrine gland

Question 68

What is part of the brain stem, the pituitary gland?

Answer 68

Pituitary gland:
- found in base of hypothalamus
- controls many glands in the body
- divided into two sections
= anterior pituitary - produces six hormones including reproduction and growth hormones
= posterior pituitary - produces, stores and releases hormones produced by the hypothalamus (e.g ADH)

Question 69

What is part of the brain structure, the cerebellum?

Answer 69

The cerebellum:
- area of the brain concerned with muscle movement, body posture and balance
- does not initiate movement, but coordinates it
- if this part of the brain is damaged, a person will suffer from jerky and uncoordinated movement
- receives information and relays it to areas of the cerebral cortex involved in motor control

Question 70

What is part of the brain structure, the cerebrum?

Answer 70

Cerebrum:
- controls voluntary actions including learning, memory, personality and conscious thought
- receives sensory information, interprets it in relation to previous experiences and then sends impulses along motor neurons to produce an appropriate response
- controls voluntary and involuntary responses
- highly convoluted = large surface area and therefore, greater capacity for complex activity
- split into two hemispheres = the left and right cerebral hemispheres
- the left side controls the right side of the body an vice versa for the right side
- the outer 2-4mm layer of the cerebrum is called the cerebral cortex

Question 71

What is the temporal lobe?

Answer 71

The temporal lobe is concerned with processing auditory information (e.g hearing, sound, recognition of speech). also involved in memory

Question 72

What is the parietal lobe?

Answer 72

The parietal lobe is concerned with orientation, movement, sensation, calculation and types of recognition and memory

Question 73

What is the occipital lobe?

Answer 73

The occipital lobe is the visual cortex, concerned with processing information from the eyes, including vision, colour, shape and perspective

Question 74

What is the frontal lobe?

Answer 74

The frontal lobe is concerned with higher brain functions such as decision making, reasoning, planning an consciousness of emotions. it includes the motor cortex which stores information about how to carry out different movements

Question 75

What is the motor cortex?

Answer 75

The motor cortex is part of the frontal lobe of the cerebrum:
- region of the brain that controls all voluntary movement
- different parts of the motor cortex send out different motor neurones to different parts of the body
- the greater the region of each moveable part of the body in the motor cortex, the greater the ability of movement

Question 76

What is a reflex?

Answer 76

A reflex is a response that occurs without conscious thought (involuntary)

Question 77

How do reflexes increase the chance of survival?

Answer 77

Reflexes increase chance of survival:
- involuntary responses = decision making regions of the brain are not involved, therefore, the brain can deal with more complex tasks
- do not need to be learned = innate responses from brain
- extremely fast = reflex arc is very short (involves few synapses)
- many reflexes are not considered reflexes = but keeps us alive (e.g digestion, standing upright)

Question 78

What are the types of muscle?

Answer 78

Types of muscle:
skeletal - type of muscle cells required for movement
cardiac muscle - myogenic (contract without the need for nervous stimulus) cells found only in heart
involuntary muscle - muscle (smooth muscle) found in the walls of blood vessels, the digestive tract and walls of some organs

Question 79

What is involuntary smooth muscle?

Answer 79

Involuntary smooth muscle:
structure-
- individual cells with nucleus
- cells are 500um x 5um
- non-striated (non stripy)
action-
- contract slowly and steadily so tire slowly
- contraction initiated by action potentials from autonomic nervous system
- have sliding actin and myosin filaments (no myofibrils or sarcomeres)
location-
- walls of intestines and blood vessels
- inside of eye
- walls of stomach and bladder
- cervix of uterus

Question 80

What is cardiac muscle?

Answer 80

Cardiac muscle:
structure-
- atrial and ventricular
- excitory and conductive muscle fibres
- striated and have nucleus
- each cell has fibrils made of a sarcomere
- branch out and form connections with adjacent cells
- have intercalated discs separating fibres
- more mitochondria than skeletal muscles
action-
- contract like skeletal muscle, without tiring/fatiguing
- repetitive, regular contractions
- some fibres are myogenic but autonomic ns can speed up or slow down contractions

Question 81

What is skeletal muscle?

Answer 81

Skeletal muscle:
structure-
- specialised cells that form fibres
- multi-nucleated (syncitium) (multi nuclei per cell)
- muscle fibre = specialised cells formed of myofibres

Question 82

What are myofibrils?

Answer 82

Myofibrils:
- long, cylindrical organelles made of protein
- specialised for contraction
- made of two types of protein
= actin (thinner filament- two strands twisted together)
= myosin (thicker filament- rod shaped fibres with bulbous heads)

Question 83

What is myosin?

Answer 83

Myosin:
- fibrous protein
- thick filament
- each molecule has a tail and two heads
- each thick myofilament has many myosin molecules with heads on opposite sides

Question 84

What is actin?

Answer 84

Actin:
- globular protein
- thin filaments
- 2 strands of mainly F actin
= has A actin subunits
= tropomysoin (rod shaped protein) coiled around the F actin
- troponin complex is attached to each tropomyosin molecule
= troponin has 3 polypeptides
= one binds to actin, one to tropomysoin, and one to Ca2+ ions

Question 85

What is the appearance of myofibrils?

Answer 85

Myofibrils appearance:
- have alternating light (isotopic bands or I-bands) and dark bands (anisotropic bands or A-bands) which then give a striped appearance
- the sarcomere is the distance between each Z-line (the region found at the centre each light band) and each one is a functional unit of a muscle fibre
- in the light bands, actin and myosin do not overlap
- in the dark bands, actin and myosin do overlap

Question 86

What is the process of muscle contraction initiation at a neuromuscular junction? (motor neurone to muscle fibre)

Answer 86

Muscle contraction initiation:
1. an action potential arrives at the neuromuscular junction
2. calcium ion channels are then stimulated to open - calcium ions diffuse into the presynaptic neuron
3. the influx of calcium ions stimulates synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft via exocytosis
4. acetylcholine diffuses across the cleft and binds to receptors on the post synaptic membrane (the sarcolema)
5. sodium ion channels subsequently open, allowing depolarisation to occur
6. acetylcholine is then broken down into choline and ethanoic acid (by acetylcholinesterase) to prevent overstimulation of muscle

Question 87

What is the process of muscle contraction?

Answer 87

Muscle contraction:
1. depolarisation
- depolarisation travels deep into muscle fibre by spreading through T-tubules (transverse tubules)
- these are in contact with the sarcoplasmic reticulum which contains stored calcium ions
2. release of calcium ions
- action potential reaches sarcoplasmic reticulum
- stimulates calcium ions to be released into sarcoplasm
3. removal of inhibitor
- calcium ions bind to troponin, causing it to change shape
- as it changes shape, it moves the tropomyosin that was blocking the actin-myosin binding sites
4. myosin binding
- binding sites now exposed
- myosin binds to actin filament, forming an actin-myosin cross-bridge
5. pulling the actin
- once attached to the actin, the myosin flexes, pulling the actin filaments along
- ADP bound to myosin head is released
6. detachment
- ATP molecule now binds to myosin head, causing the myosin to detach from actin
7. reset to rebind
- calcium ions in sarcoplasm activate ATPase which hydrolyses ATP to ADP
- this releases the energy that allows the myosin head to return to its original position
8. reattach, repeat
- myosin head can now attach itself to a binding site further along the actin and the cycle is repeated, as long as the muscle remains stimulate

Question 88

How is energy supplied for muscle contraction?

Answer 88

Energy supplied for muscle contraction:
- hydrolysis of ATP (to ADP and Pi) provides energy for the detachment of myosin head from actin bonding sites and also for the active transport of calcium ions from the sarcoplasm back to the SR
- normally a ‘relaxing’ muscle cell only contains enough ATP for around 2 seconds of intensive contraction
- mitochondria can continue to generate more ATP through the aerobic respiration of glucose, but this is a relatively slow process as the supply of oxygen limits the rate

Question 89

How is energy supplied for muscle contraction by myoglobin?

Answer 89

Myoglobin:
- a protein molecule found in muscle that makes muscle appear red in colour
- has a higher affinity for oxygen than hb. at very low partial pressures, oxygen is transferred to it, from hb, so it rapidly becomes 95-100% saturated inside the muscle cell
- acts as an additional oxygen store within muscle cells, which is used when the oxygen supply via hb is limited

Question 90

How is energy supplied for muscle contraction during anaerobic respiration?

Answer 90

Anaerobic respiration:
- can continue to generate ATP after the oxygen supply from hb and myoglobin has run out, but this process produces lactic acid which is toxic
- cannot continue indefinitely

Question 91

How is energy supplied for muscle contraction by creatine phosphate?

Answer 91

Creatine phosphate:
- muscle cells also contain a chemical called creatine phosphate (in sarcoplasm)
- when broken down it can rapidly regenerate ATP from ADP by transferring a phosphate ion
- this reaction is catalysed by an enzyme called creatine phosphotransferase
- ADP + creatine phosphate = ATP + creatine
- the amount of phosphocreatine in a muscle cell is limited, but enough is present to enable a muscle to keep contracting until respiration catches up with demand