Nervous Communication Flashcards
1
Q
What are the 2 main divisions in the nervous system?
A
- central nervous system.
- peripheral nervous system.
2
Q
The CNS is a major division of the nervous system. What does it include?
A
The brain and spinal cord.
3
Q
The PNS is a major division in the nervous system. What does it include?
A
It is made up of pairs of nerves that originate from either the brain or spinal cord.
4
Q
The PNS is a division of the nervous system. What can this further be divided into?
A
- sensory neurones. These carry nerve impulses from receptors to the CNS.
- motor neurones. These carry nerve signals away from the CNS to effectors.
5
Q
The motor nervous system (from the PNS) can be further subdivided into what?
A
- the voluntary nervous system. This carries nerve impulses to body muscles under voluntary (conscious) control.
- the autonomic nervous system. This carries nerve impulses to glands, smooth muscles and cardiac muscles and isn’t under conscious control; so is involuntary.
6
Q
What is a spinal cord?
A
A column of nervous tissue that runs along the back and lies inside the vertebral column for protection.
Emerging at intervals along the spinal cord are pairs of nerves.
7
Q
Define ‘reflex’.
A
An involuntary response to a sensory stimulus.
8
Q
Define reflex arc.
A
The pathway of neurones involved in a reflex.
Involves 3 neurones.
9
Q
Outline the reflex arc.
A
- The stimulus - heat from a candle.
- Receptor - temperature receptors generate nerve impulse in the sensory neurone.
- Sensory neurone - passes nerve impulses to spinal cord.
- Coordinator - passes impulses along spinal cord.
- Motor neurone - carries nerve impulses from spinal cord to muscle.
- Effector - muscle, which is stimulated to contract.
- Response - pull hand away from candle.
10
Q
What is the importance of reflex arcs?
A
- the absence of any decision making processes means the action is rapid.
- protect the body from harm. They are effective from birth and don’t need to be learnt.
- fast, because the neurones pathway is short with very few synapses where neurones communicate with each other (synapses are the slowest link in a neurone pathway). This is important in withdrawal reflexes.
- involuntary, so not need the decision making powers of the brain, thus leaving it to carry out more complex resources. In this way, the brain is not overloaded with situations in which the response is always the same.
11
Q
Outline the features of sensory receptors (including the pacinian corpuscle).
A
- it’s specific to a single type of stimulus.
- produces a generator potential by acting as a transducer.
Receptors in the nervous system convert (transduce) the energy of the stimulus into a generator potential (aka a nerve impulse).
12
Q
What is a transducer.
A
A transducer converts the change in the form of energy by the stimulus into a nerve impulse that can be understood by the body.
13
Q
What are pacinian corpuscles?
A
Sensory receptors that respond to mechanical stimuli eg pressure. They occur deep in the skin.
14
Q
How does the Pacinian Corpuscle work?
A
- In its resting state, the stretch mediated sodium channels of the membrane around the corpuscle are too narrow to allow sodium ions to pass along them.
- When pressure is applied to the corpuscle, it becomes deformed and the membrane around its neurone become stretched.
- This stretching widens the sodium channels and sodium ions can diffuse into the membrane.
- This influx of sodium ions causes depolarisation, thereby creating a generator potential.
- This generator potential creates an action potential (nerve impulse) that passes along the neurone and then the CNS.
15
Q
Where are the light receptors in mammals found?
A
In the retina.
16
Q
What are the 2 main types of light receptor?
A
Rod cells and cone cells.
17
Q
Both rod and cone cells act as ______________?
How do they act as this?
A
Transducers by conserving light energy into the electrical energy of a nerve impulse.
18
Q
Outline the features of rod cells.
A
- rod shaped
- sensitive to low level light
- give poor visual acuity
- more found at the periphery of the retina. Absent at the fovea.
- greater number of them than cone cells.
19
Q
Why are images only seen in black and white in rod cells?
A
Because rod cells cannot distinguish between different wavelengths of light.
20
Q
Why are rod cells sensitive to dim light?
A
Because many rods join one neurone (retinal convergence), so many weak generator potentials combine to reach the threshold and trigger an action potential.
There is enough energy in this dim light to trigger a generator potential which breaks down rhodopsin.
21
Q
Why do rod cells have a low visual acuity?
A
Because many rods join the same neurone (retinal convergence). Therefore, only a single impulse is generated, regardless of how many neurones are stimulated (as they all link to a single bipolar cell).
This means that the brain cannot distinguish between the separate sources of light that stimulated them.
22
Q
What is needed to create a generator potential in rod cells?
A
The break down of rhodopsin. There is enough energy in low intensity light do this - explaining why rod cells are sensitive to low intensity light.
23
Q
Outline the features of cone cells.
A
- cone shaped
- concentrated at the fovea. fewer at the periphery of the retina
- give good visual acuity
- insensitive to low intensity light
- three types (each responding to a different wavelength of light)
- less of them than rod cells
24
Q
Why are cone cells sensitive to high intensity light, but not low intensity light?
A
Because one cone cell joins one neurone, so it takes more light to reach the threshold and trigger an action potential.
25
How are cone cells and rod cells different in the intensity of light?
Unlike rod cells, individual cone cells are connected to an individual neurone.
Therefore, the stimulation of a number of rod cells cannot be combined to help exceed the threshold value and is create a generator potential.
As a result, cone cells only respond to high intensity light an rod cells respond to low intensity light.
26
Cone cells are found on the fovea. Explain why.
Because light is focused by the lens onto the fovea - therefore receiving the highest intensity of light.
As a result, rod cells are not found here, but cone cells are.
27
Why are rod cells found at the edge of the retina?
Because light intensity is at its lowest here.
28
How does light create an electrical impulse?
1. Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments.
2. Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.
3. A generator potential is created. If it reaches threshold, a nerve impulse is sent along a bipolar neurone.
4. Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
29
What does the Autonomic Nervous System do?
The ANS controls the involuntary (subconscious) activities of internal muscles and glands. It has two divisions:
- sympathetic nervous system
- parasympathetic nervous system
30
In the autonomic nervous system, there is a division called the sympathetic nervous system. What is this?
This stimulates effectors, so speeds up any activity.
| It helps us to cope with stressful situations and prepares us for activity (fight or flight response).
31
In the autonomic nervous system, there is a division called the parasympathetic nervous system. What is this?
In general, this inhibits effectors, so slows down any activity.
It controls activities under normal resting conditions.
It is concerned with conserving energy and replenishing the body's reserves.
32
How is the autonomic nervous system antagonistic?
Because the sympathetic and the parasympathetic nervous systems normally oppose one another.
Eg if one system contracts a muscle, the other relaxes it.
33
The muscle of the heart is __________.
Myogenic.
This means it's contraction is initiated from within the muscle itself, rather than by nervous impulses from outside.
34
Where is the SAN found?
Within the wall of the right atrium of the heart.
35
Why is the SAN often called a pacemaker?
Because the SAN has a basic rhythm of stimulation that determines the beat of a heart.
36
Outline the the stages of the basic heart rate.
1. A wave of electrical excitation spreads out from the SAN across the right and left atria, causing them to contract.
2. A layer of non-conductive collagen tissue prevents the wave from crossing into the ventricle.
3. Instead, these waves are transferred to the atrioventricular node (AVN).
4. After a short delay, the AVN conveys a wave of electrical excitation onto the bundle of His.
5. This bundle of His conducts the wave through the atrioventricular septum to the apex, where the bundle branches into smaller fibres of Purkyne tissue.
6. The wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards.
37
What is 'Purkyne tissue'?
A series of specialised muscle fibres which collectively makes up the bundle of His.
38
What does the 'medulla oblongata' do?
Controls the changes of heart rate.
This has two centres:
- a centre that increases HR (linked to the SAN by the SNS)
- a centre that decreases HR (linked to SAN by the PNS)
39
In the context of the heart, describe chemoreceptors.
Found in the wall of the carotid arteries. They are sensitive to changes in the pH of the blood that result in changes in CO2 concentration. (In solution, CO2 forms an acid, thus lowering the pH).
40
Outline the stages of the heart when there is a higher concentration of CO2 in the blood (eg by exercise).
1. Blood = higher conc. of CO2, so pH is lowered.
2. The chemoreceptors in the wall of the aorta detect this, so more impulses are sent to the medulla oblongata.
3. This sends the impulses via the SNS. The sympathetic neurones secrete noradrenaline, which binds to receptors on the SAN.
4. HR increases. CO2 levels back to normal.
3. This centre increases the frequency of impulses via the SNS to the SAN. This, in turn, increases the rate of production of electrical waves by the SAN - thus increasing HR.
41
Outline the stages in the heart when there are low CO2 levels.
1. Chemoreceptors detect higher pH levels.
2. Nervous impulses are sent to the medulla oblongata.
3. This sends impulses along parasympathetic neurones.
4. These secrete acetylcholine, which binds to records on the SAN.
5. This causes the HR to decrease. CO2 concentration levels return.
42
How is pressure controlled in the heart?
- when blood pressure is higher than usual.
Then pressure receptors transmit more nervous impulses to the centre in the medulla oblongata.
This centre sends impulses via the PARASYMPATHETIC system the the SAN - leading to a decrease in HR.
- when blood pressure is lower than normal.
Then pressure receptors transmit more nervous impulses to the centre in the medulla oblongata.
This centre sends impulses via the SYMPATHETIC nervous system to the SAN - leading to an increase in HR.
43
What are the 2 main forms of coordinations in animals?
- the nervous system
| - the hormonal system
44
What does the nervous system do?
Uses nerve cells to pass electrical impulses along their length. They stimulate their target cells by secreting neurotransmitters directly onto them. This results in rapid communication between specific parts of an organism.
The responses are short lived and localised.
45
What does the hormonal system do?
Produces 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 concentration of hormones that stimulate them.
Thus results in slower, less specific forms between parts of an organism.
46
What is the main difference between the nervous system and the hormonal system?
The responses in the nervous system are often short lived, and restricted to localised region of the body.
The effect is usually temporary.
Transmission is by the neurones.
The responses in the hormonal system are slow, long-lasting and wide spread.
The effect may be permanent.
Transmission is in the blood system.
47
What is a neurone (nerve cell)?
Specialised cells which are adapted to rapidly carry out nerve impulses (electrochemical changes) from one area of the body to another.
48
What is in a mammalian motor neurone?
- cell body
- dendron
- axon
- Schwann cells
- myelin sheath
- nodes of Ranvier
49
What do Schwann cells do?
Makes up a mammalian neurone, Schwann cells surround the axon, protecting it and providing electrical insulation.
They also carry out phagocytosis.
50
What is an axon?
A single, long fibre that carries nerve impulses away from the cell body.
51
What are dendrons?
Extensions of the cell body, which subdivide into dendrites, that carry nerve impulses towards the cell body.
52
What is a myelin sheath?
This forms a covering to the axon and is made of the membranes of the Schwann cells.
These membranes are rich in a lipid called myelin.
53
What are nodes of Ranvier?
Constrictions between adjacent Schwann cells where there's no myelin sheath.
54
What do sensory neurones do?
Transmit nerve impulses from the receptor to a motor neurone
55
What does a relay neurone do?
Transmits nerve impulses between neurones.
56
What do motor neurones do?
Transmits nerve impulses from a relay neurone to an effector (eg muscle / gland).
57
In a neurone's resting state, the outside of the membrane is _______________ __________ compared to the inside.
This is because there are more ___________ _____ outside the cell than inside the cell.
Positively charged.
Positive ions.
58
How can sodium ions move out of the neurone but not back in?
Because the membrane isn't permeable.
59
Basically what happens at the potassium ion channel?
The potassium ion channels allow facilitated diffusion of potassium ions out (but not into) the neurone, down their concentration gradient.
60
Basically what happens at the sodium-potassium pump?
These pumps use active transport to move 3 sodium ions out of the neurone for every 2 potassium ions moved in.
ATP is needed to do this.
61
Why is there an electrochemical gradient in an axon membrane?
Because the outward movement of Na+ ions is greater than the inward movement of K- ions.
As a result, there are more sodium ions in the tissue surrounding the axon than in the cytoplasm, and more K- ions in the cytoplasm than the surrounding fluid.
62
How is an action potential caused?
When a stimulus of sufficient size is detected by a receptor in the nervous system, its energy causes a temporary reversal of the changes either side of this part of the membrane.
63
When does depolarisation occur?
Because the channels in the axon membrane change shape, and hence open or close, depending on the voltage across the membrane.
64
Outline the changes in potential difference during an action potential.
1. The energy of the stimulus opens sodium voltage gated channels in the axon membrane to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone (down the electrochemical gradient).
This makes the inside of the neurone more positive.
2. If the potential differences reaches threshold, more sodium ions channels open so Na+ enters.
3. The sodium ion channels close and potassium ion channels open. So, as the membrane is more permeable to K- ions, yet more K- ions diffuse out, starting repolarisation of the axon.
4. This causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual as lots of K- ions leave (hyperpolarisation).
5. The sodium potassium ions pump returns back to its resting potential until excited by another stimulus.
65
Outline the passage of an action potential along an unmyelinated axon.
1. At resting potential, the axon membrane is polarised.
2. A stimulus causes an influx of sodium ions, causing an action potential, and the membrane is depolarised.
3. This influx causes sodium ions in the next region of the neurone to open, so sodium ions diffuse into that part.
4. This causes a wave of depolarisation to travel along the neurone.
5. The wave moves away from areas of the membrane in the refractory period as these sorts can't fire action potentials.
66
Nerve impulses are described as the ________ __ _______ principle.
All or nothing.
67
Why are nerve impulses described as all or nothing responses?
If the threshold isn't reached, there will be no action potential - therefore no impulse generated.
However once the threshold is reached, an action potential will always fire with the same charge in voltage (no matter the size of the stimulus) - so a nerve impulse will travel.
68
How can an organism perceive the size of a stimulus?
- by the number of impulses passing in a given time. (Larger stimulus = more impulses in a given time).
- by having neurones with different 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).
69
What are the 3 factors that affect the speed of conduction for action potentials?
- the myelin sheath
- the diameter of the axon
- temperature
70
Outline the myelin sheath as a factor affecting the conduction at which an action potential travels.
- the myelin sheath acts as an electrical insulator; preventing an action potential forming in the part of the axon covered in myelin, and forcing them action potential to jump from one node of Ranvier to another (saltatory conduction). This increases the speed of conduction.
71
Why is an action potential slower in a non-myelinated axon?
Because in a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane).
This is slower than saltatory conduction.
72
Outline the diameter of the axon as a factor affecting the speed at which an action potential travels.
The greater the diameter of an axon, the faster the speed of conductance.
This is due to less leakage of ions from a large axon (as leakages make membrane potentials harder to maintain).
73
Outline temperature as a factor affecting the speed at which an action potential travels.
Temperature affects the rate of diffusion of ions, so therefore the higher the temperature, the faster the nerve impulse.
Also, the energy from active transport comes from respiration. And respiration (like the sodium potassium pump) is controlled by enzymes - which function more rapidly up to a point. Above a certain temperature, enzymes and the plasma membrane proteins are denatured and the plasma membrane proteins are denatured and impulses fail to be conducted at all.
74
What is the refractory period?
Once an action potential has been created in any region of the axon, there is a period afterwards when inward movement of sodium ions is prevented because the sodium-potassium ion gates are closed.
During this time it's impossible for an action potential to be created.
74
What's the difference between a weak stimulus that generates an action potential and a very strong stimulus that creates a stimulus?
The weak stimulus exceeds the threshold, so an action potential is created. However, in a very strong stimulus, the threshold still exceeded depolarisation but action potentials are the same size - just more frequent.
74
What are the three purposes of the refractory period?
- it ensures the action potentials are unidirectional.
- it produces discrete (separate) impulses.
- it limits the number of action potentials.
75
The refractory period limits the number of action potentials. Why?
Because they're separated from one another, this limits the number of action potentials that can pass along an axon in a given time - thus limiting the strength of the stimulus that can be detected.
76
The refractory period ensures the action potentials are unidirectional. Why?
Action potentials only pass from an active region to a resting region.
This is because action potentials cannot be propagated in a region that is refractory, meaning they can only move in a forward direction.
77
The refractory period ensures that action potentials are discrete (separate). Why?
Due to the refractory period, a new action potential can't be formed immediately behind the 1st one.
This ensures that action potentials are separated from one another.
78
The action potential mixes along the neurone as a ______ ____ _________________.
Wave of depolarisation.
79
How does the action potential move along the neurone as a wave of depolarisation?
1. An action potential happens, and some of the sodium ions that enter the neurone diffuse sideways.
2. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
3. This causes a wave of depolarisation to travel along the neurone.
4. The wave moves away from the parts of the membrane in the refractory period (as these areas can't fire an action potential).
80
What is the synaptic cleft?
The gap that separates neurones.
81
What is the synaptic knob?
The axon of the presentation neurone ends in a swollen portion - the synaptic knob.
82
What is summation?
Summation involves a rapid build up of neurotransmitter in the synapse by either:
- spatial summation
- temporal summation
83
Outline spatial summation.
A number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone.
Together they therefore trigger a new action potential.
84
Outline temporal summation.
A single presynaptic neurone releases a neurotransmitter many times over a short period of time.
If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered.
85
What is an inhibitory synapse?
A synapse that makes it less likely for an action potential to be created on the postsynaptic neurone.
86
How does the inhibitory system operate?
1. The presynaptic neurone releases a neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone.
2. The neurotransmitter causes the chloride ion channels to open.
3. Cl- ions move out of the postsynaptic neurone by facilitated diffusion.
4. This binding causes the opening of nearby K+ channels.
5. K+ ions move out of the postsynaptic neurone into the synapse.
6. The combined effect of -vly charged chloride ions and +vly charged K+ ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive.
7. This is called hyper polarisation 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.
87
Synapses transmit information from one neurone to another. In doing so, they act as junctions allowing:
- a single impulse along one neurone to initiate new impulses in a number of different 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 different stimuli to contribute to a single response.
88
Where is a neurotransmitter stored? And how is it released?
The neurotransmitter is stored in the synaptic vesicles.
When an action potential reaches the synaptic knob, the membranes of these vesicles fuse with the pre-synaptic membrane to release the neurotransmitter.
89
What are excitatory synapses?
Synapses that produce new action potentials by:
1. An action potential reaching the synaptic knob, so the membranes of these vesicles fuse with the pre-synaptic membrane to release the neurotransmitter (previously stored in the presynaptic neurone).
2. When released, the neurotransmitter diffuses across the synaptic cleft to bind to specific receptor proteins (found only on post synaptic neurone).
3. The neurotransmitter binds with the receptor proteins - leading to a new action potential in postsynaptic neurone.
90
What's a cholinergic synapse?
A synapse in which the neurotransmitter is acetylcholine.
91
How is a nerve impulse transmitted across a cholinergic synapse?
1. An action potential arrives at the synaptic knob of presynaptic neurone.
2. This stimulates calcium ion channels to open and enter the synaptic knob by facilitated diffusion.
3. This influx of calcium ions causes the synaptic vesicles to fuse with the presynaptic membrane - releasing acetylcholine (ACh) into synaptic cleft (aka exocytosis).
4. ACh diffuses across synaptic cleft. Then binds to receptor sites of sodium ion protein channels in the postsynaptic membrane.
5. So sodium ion channels open, and Na+ diffuses along a conc. gradient.
6. This influx of sodium ions into the post synaptic membrane causes depolarisation. An action potential is generated on post neurone if threshold is reached.
7. ACh is removed from synaptic cleft so respond doesn't keep happening. It's broken down by AChE (enzyme) and the products are reabsirbed by the pre neurone and used to make more ACh.
92
What do excitatory neurotransmitters do?
They depolarise the post synaptic neurone making it fire an action potential if the threshold is reached.
93
What do inhibitory neurotransmitters do?
Inhibitory neurotransmitters hyperpolarise the post synaptic membrane (make the potential difference more negative) - preventing it from firing an action potential.
94
How can drugs affect the action of neurotransmitters?
- some drugs are the same shape so mimic their action at receptors (agonists).
- some drugs block receptors so they can't be activated by neurotransmitters (antagonists).
- some drugs inhibit the enzyme that breaks down the neurotransmitter. So there are more neurotransmitters in the synaptic cleft to bind to receptors, and they're there for longer.
- some drugs stimulate the release of neurotransmitter from the pre neurone do more receptors are activated.
- some drugs inhibits the release of neurotransmitters from the pre neurone do fewer receptors are activated.
95
What is a skeletal muscle?
Skeletal muscles make up the bulk of body muscle in vertebrates.
It is attached to bone and acts under voluntary, conscious control.
96
What are myofibrils? RWA?
Millions of tiny muscles that make up individual muscles.
Like how a rope is made up of millions of separate threads.
97
Microfibrils are mainly made up of 2 types of protein filament:
- actin
| - myosin
98
What's actin?
A protein filament that makes up myofibrils.
They're thinner and consist of 2 long strands twisted around one another to form a helical strand. Actin is a globular protein.
99
What's myosin?
Myosin is a protein filament that makes up myofibrils. It is made up of two types of protein (a fibrous protein (tail) and a globular protein (head)).
Myosin is thicker and consistent of long rod shaped tails with bulbous heads that project to one side.
100
Why do myofibrils appear striped?
Due to their alternating light coloured bands (I bands) and dark coloured bands (A bands).
101
Do 'I bands' appear dark or light? Why?
Light.
This is because the thick and thin filaments do not overlap in this region.
102
Do 'A bands' appear light or dark? Why?
Dark.
Because the thick and thin filaments overlap in this region.
103
At the centre of each 'I band' is...
A line called the Z line.
The distance between adjacent Z lines is a sarcomere.
104
At the centre of each 'A band' is...
A lighter coloured region called the 'H zone'.
105
In myofibrils when would the pattern of light and dark bands change?
When a muscle contracts, the sarcomeres (distance between adjacent Z-lines) shorten and the pattern of light and dark bands change.
106
What are the two types of muscle fibre?
Slow twitch and fast twitch.
107
Outline slow twitch muscle fibres.
These contract more slowly than fast twitch fibres.
And they provide less powerful contraction but over a longer period.
They're therefore adapted to endurance work eg running a marathon.
In humans, they're common in calf muscles.
108
How are slow twitch muscle fibres adapted to their role for aerobic respiration?
They have...
- a large store of myoglobin (a red molecules that stores oxygen)
- a rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
- numerous mitochondria to produce ATP
109
Outline fast twitch muscle fibres.
These contract more rapidly.
And produce powerful contractions but only for a short period.
They're therefore adapted to intense exercise.
Common in the biceps which do short bursts of intense activity.
110
How are fast twitch muscle fibres adapted to their role?
They have:
- a high concentration of glycogen.
- a high concentration of enzymes involved in aerobic respiration which provides ATP rapidly.
- thicker and more numerous myosin filaments.
- a store of phosphocreatine (a molecule that rapidly generates ATP from ADP in aerobic conditions - providing energy for muscle contraction).
-
111
What is a neuromuscular junction?
The point where a motor neurone meets a skeletal muscle fibre.
112
What happens to the acetylcholine once it has been broken down?
The ACh is broken down by acetylcholinesterase to ensure the muscle isn't overstimulated.
The resulting choline and ethanoic acid (acetyl) diffuse back into the pre neurone, where they're recombined to form ACh using energy provided by the mitochondria found there.
113
Name the similarities between a neuromuscular junction and a cholinergic synapse?
- both have neurotransmitters that are transported by diffusion
- both have receptors, that, upon binding with the neurotransmitter, cause an influx of sodium ions
- use the sodium-potassium ion pump to depolarise the axon
- use enzymes to breakdown the neurotransmitter
114
Skeletal muscles act in ____________ _______.
Antagonistic pairs.
115
Outline how skeletal muscles act in antagonistic pairs.
The muscle pairs pull in opposite directions, and when one is contracted, the other is relaxed.
116
What evidence does the sarcomere provide for the sliding filament mechanism?
In the sarcomere,
- the I-band becomes narrower
- the Z-lines move closer together (the sarcomere shortens)
- the H-zone becomes narrower
117
What evidence gobies against the sliding filament theory?
The A-bands remain the same width.
As the width of this band is determined by the length of the myosin filament, it suggests that the myosin filaments have not become shorter.
118
Myosin is made up of two types of protein. What are these?
- a fibrous protein arranged into filament made up of several 100 molecules (the tail).
- and a globular protein formed into 2 bulbous structures at one end (the head).
119
What is tropomyosin?
Tropomyosin forms long thin threads that are wound around actin filaments.
120
What are the 3 types of muscle in the body?
Cardiac muscle, smooth muscle and skeletal muscle.
121
Outline muscle stimulation.
1. An action potential reaches many neuromuscular junctions simultaneously; causing CA2+ channels to open and Ca2+ to diffuse into synaptic knob.
2. The calcium ions cause the synaptic vesicles to fuse the with presynaptic membrane and release their ACh into the synaptic cleft.
3. ACh diffuses across the synaptic cleft and binds with receptors on the muscle cell membrane - causing it to depolarise.
122
What is the sliding filament theory of muscle contraction?
1. The action potential travels deep into the fibre through T-tubules to the sarcoplasmic reticulum.
2. This causes the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm.
3. Ca2+ binds to tropomyosin, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the action filament.
5. ADP molecules attached to the myosin heads means that they're in a state to bind to the actin filament and form a cross bridge.
6. Once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along doing so; releasing ADP.
7. An ATP molecule attaches to each myosin head, causing it to return to it's ordinal position.
8. The calcium ions activate ATPase (which hydrolyses ATP -> ADP). This hydrolysis provides the energy to return to its original position.
9. The myosin head, with attached ADP molecule, then reattached further along the actin filament and the process repeats (as long as the concentration of calcium ions in the myofibril remains high).
123
What is the trigger of muscle contraction?
An influx of calcium ions.
124
In muscle contraction, what prevents the myosin head from attaching to the binding site on the actin molecule?
Tropomyosin.
125
If tropomyosin prevents myosin heads from attaching to the actin molecule, how do they end up attaching?
Because an influx of Ca2+ from the endoplasmic reticulum causes the tropomyosin molecule to change shape, so pull away from hot binding sites on the actin molecule.
126
Outline the stages of muscle contraction.
1. An action potential from a motor neurone stimulates a muscle cell. It travels deep into the muscle fibre through t-tubules.
2. Ca2+ are released from the endoplasmic reticulum, and bind to tropomyosin. This causes tropomyosin molecules to change shape and so pull away from the binding sites on the actin molecule.
3. Myosin heads can now attach to the binding sites on the actin filament. This forms an actin-myosin cross bridge.
4. The head of myosin changes angle, moving the actin filament along as it does so. The ADP molecule is released.
5. An ATP molecule fixes to the myosin head, causing it to detach from the actin filament.
6. The hydrolysis of ATP to ADP by ATPase provides the energy for the myosin head to resume its normal position.
7. The head of myosin reattaches to a binding site further along the actin filament and the cycle is repeated.
127
In muscle contraction, how do calcium ions change the shape of tropomyosin.
The presence of calcium ions changes the environment of tropomyosin (a protein), leading to a change in its tertiary structure.
128
Outline muscle relaxation.
1. When nervous stimulation ceases, Ca2+ are actively transported back into the endoplasmic reticulum using the energy from the hydrolysis of ATP.
2. The reabsorption of Ca2+ allows tropomyosin to block the actin filaments again.
3. Myosin heads are now unable to bind to actin filaments and contraction caresses (muscle relaxes).
129
Where does the nerdy come from for muscle contraction?
The energy for muscle contraction is supplied by the hydrolysis of ATP to ADP and inorganic phosphate (Pi).
130
In muscle contraction, what is the energy needed for?
- the movement of myosin heads.
| - the reabsorption of calcium ions into the endoplasmic reticulum by active transport.
131
In muscle contraction, what does phosphocreatine do?
Phosphocreatine cannot supply energy directly to the muscle, so instead it regenerates ATP, which can.
Phosphocreatine is stored in muscles and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so reform ATP.
This store of phosphocreatine is replenished using phosphate from ATP when the muscle is relaxed.
132
How is the phosphocreatine store replenished?
Using phosphate from ATP when the muscle is relaxed.
133
How is oxygen produced in very active muscles?
In a v active muscle, the demand for ATP (and therefore oxygen) is greater than the rate at which blood can supply oxygen.
Therefore a means of rapidly generating ATP anaerobically is also required. This is partly achieved by using phosphocreatine andnoartly by more glycolysis.
