Nervous Coordination And Muscles Flashcards
(109 cards)
1
Q
Cons To Specialised Cells?
A
Cons:
- as species have evolved, certain cells have lost the ability to perform certain functions, therefore, they rely on other cells to do these other functions.
- all the cells must be able to function and be coordinated to perform efficiently.
In nervous coordination and muscles, we look at how these coordinations work.
2
Q
Forms Of Coordination In Animals?
A
There are two main ways in which animals coordinate:
- the hormonal system,
- the nervous system.
3
Q
The Nervous System?
A
The nervous system uses electrical impulses along nerve cells.
These electrical impulses stimulate target cells by secreting chemicals, known as neurotransmitters directly onto the target cells.
This results in rapid communication between parts of the organism.
The responses in the nervous system are short-lived and occur in a localised region of the body.
An example of nervous system of coordination:
- Reflex action,
- Withdrawal of hand from an unpleasant stimulus,
- For obvious reasons, this response is rapid and short-lived.
4
Q
The Hormonal System?
A
The hormonal system produces chemicals (hormones) that are transported in 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 stimulates them.
This results in a less-specific and slower communication between the parts of organism.
The responses are often long-lived and widespread.
An example of hormonal system:
- control of blood glucose concentration, which produces a slower response but is longer lasting and more widespread.
5
Q
Simplified Hormonal System List?
A
- Communication is by chemicals called hormones,
- Transmission is by blood system,
- Transmission is slow,
- Hormones travel all over the body but only target receptors can respond to their specific hormones,
- Response is widespread,
- Response is slow,
- Response is often long-lasting,
- Effect might be irreversible and permanent.
6
Q
Simplified Nervous System List?
A
- Communication is by nerve impulses,
- Transmission is by neurones,
- Transmission is very rapid,
- Nerve impulses travel to specific places in the body,
- Response is rapid,
- Response is often short-lived,
- Response is localised,
- Effect is usually temporary and reversible.
7
Q
Neurones?
A
Neurones (nerve cells) are adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to the other.
8
Q
Mammalian Motor Neurone Properties?
A
The mammalian neurone contains a:
- cell body,
- dendron,
- axon,
- Schwann cells,
- Myelin sheath,
- Nodes of Ranvier.
9
Q
Cell Body?
A
Cell body: includes all of the usual cellular organelles, including a nucleus and large amounts of rough endoplasmic reticulum. This is due to the production of proteins and neurotransmitters.
10
Q
Dendrons?
A
Dendrons: extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses toward the cell body.
11
Q
Axon?
A
Axon: single long fibre that carries nerve impulses away from the cell body.
12
Q
Schwann Cells?
A
Schwann Cells: surround the axon and protect it.
They provide electrical insulation.
They also carry out phagocytosis (the removal of cell debris) and play a part in nerve regeneration.
Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.
13
Q
Phagocytosis?
A
The removal of cell debris.
14
Q
Myelin Sheath?
A
Myelin Sheath: forms a covering to the axon and is made up of membranes of Schwann cells.
These membranes are rich of liquid known as myelin.
Neurones with a myelin sheath are called myelinated neurones.
15
Q
Nodes Of Ranvier?
A
Nodes Of Ranvier: constrictions between adjacent Schwann cells where there is no myelin sheath.
The constructions are 2-3um long and occur every 1-3mm in humans.
Sodium ion channels are conc at nodes.
This means depolarisation only happens at node (when sodium ions can get through membrane).
The neurones cytoplasm conducts enough electrical chant he to depolarise the next node, so the impulse jumps. This is called Saltatory conduction - fast.
16
Q
Sensory Neurones?
A
Transmit nerve impulses from a receptor to an intermediate or motor neurone.
They have one dendron that is often very long.
The dendron carries the impulse toward the cell body and then an axon carries the impulse away from the cell body.
Cell body in middle, in dorsal root ganglion.
Impulse travels from the nerve endings (dendron) at the skin to the axon.
Myelinated.
Impulse travels from the dendrites and cell body to the axon.
17
Q
Motor Neurones?
A
Transmit nerve impulses from an intermediate or relay neurone to an effector.
Effector examples: gland or muscle.
Motor neurones have a long axon and then lots of short dendrites.
Usually attached to some sort of muscle tissue.
Cell body at same side as dendrites.
Myelinated.
18
Q
Intermediate Neurones?
A
Also known as relay neurones.
Transmit impulses between neurones, for example, from sensory to motor neurones.
They have numerous short processes.
Cell body in middle.
Not myelinated.
Impulses travel from the dendrites inwards to the cell body in middle of neurone. They all travel inward (almost like a inward spiral).
19
Q
What Is A Nerve Impulse?
A
Self-propagating wave of electrical activity that travels along the axon membrane.
It is a temporary reversal of the electrical potential difference across the axon membrane.
This reversal is between two states, the resting potential and the action potential.
20
Q
How Is Movement Of Ions Across A Membrane Controlled?
A
The movement of ions (e.g. Na2+ and K+) across the axon membrane is controlled in many ways:
- The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it.
- Proteins, such as channel proteins, span these phospholipid bilayers. These proteins have channels, called ion channels, which pass through them. These channels can open or close so that sodium or potassium ions can move through them via facilitated diffusion at any one time. Some of the channels are open all the time.
- Some carrier proteins actively transport potassium ions into the axon and sodium ions out of the axon. This mechanism can be called a sodium-potassium pump.
21
Q
Resting Potential?
A
As a result of the control of ions that can cross a membrane, the inside of an axon is negatively charged relative to the outside of the axon.
This is known as resting potential. This negative charge is usually around (minus) 50-90 millivolts (mV) but is usually around (minus) 65mV in humans.
In this condition, the axon is said to be polarised.
22
Q
Steps Of Resting Potential?
A
- Sodium ions are actively transported out of the axon by sodium-potassium pumps.
- Potassium ions are actively transported into the axon via sodium-potassium pumps.
- The active transport of sodium ions is greater than that of potassium ions. Three sodium ions move out for every two that move in.
- Although both sodium and potassium ions are positive, the outward movement of sodium ions is greater than the inward movement of potassium. Therefore, there are more sodium ions in the tissue fluid outside the axon than in the cytoplasm. And more potassium ions in the cytoplasm than in the tissue fluid outside the axon. This creates an electrochemical gradient.
- The sodium ions begin to diffuse back naturally into the axon while the potassium begins to diffuse back out of the axon.
- Most of the gates in the channels that allow the potassium ions to move through are open, whilst most of the gates in the changes that allow sodium ions to move through are closed.
23
Q
Action Potential?
A
When a stimulus of a sufficient size is detected by a receptor in the nervous system, it’s energy causes a temporary reversal of the charges either side of this part of the axon membrane.
If the stimulus is great enough, the negative charge of -65mV in the axon becomes +40mV.
This is known as action potential and the part of the axon membrane is said to be depolarised.
24
Q
Why Does Depolarisation Occur?
A
This depolarisation occurs because the channels in the axon membrane change shape, and hence, open or close.
This depends on the voltage across the membrane.
They are therefore called voltage-gates channels.
The sequence of events in the steps of action potential all occur on a specific point on the axon and not the whole axon.
25
Steps Of Action Potential?
At resting potential, some of the potassium voltage-gated channels are open but sodium voltage-gated channels are closed.
The energy of the stimulus causes some sodium voltage-gated channels to open. Therefore, sodium ions diffuse into the axon through these channels along their electrochemical gradient.
Because sodium ions are positively charged, they trigger a reversal in the potential difference across the membrane.
As more sodium ions diffuse across the membrane, more sodium ion channels open. This causes an even greater influx of sodium ions into the axon.
Once the action potential of around +40mV has been established, the voltage gates on the sodium ion channels close and voyage gates on the potassium ions channels begin to open.
With potassium voltage-gates channels now open, the electrical gradient that was previously preventing further outward movement of potassium ions is now reversed. This means more potassium ions now flow out of the axon. This causes repolarisation of the axon.
The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient, with the inside of the axon being more negative than usual (hyperpolarisation).
The closable gates on the potassium ions channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions to be pumped in.
The resting potential is then reastablisbed and the axon is reploarised.
26
Passive And Active Processes?
Action potential is maintained via diffusion whilst resting potential is maintained via active transport.
This means action potential is a passive process.
Resting potential is an active process.
27
The Size Of Action Potential?
Once an action potential has been created, it moves quickly along the axon.
The size of action potential remains the same from one side of the axon to the next.
As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the next region of the axon to become depolarised.
Action potential is therefore a travelling wave of depolarisation.
28
How Is An Impulse Propagated In An Unmyelinated Axon?
1. At resting potential, no nerve impulse is being travelled and so no impulse is travelling yet.
2. A stimulus causes a sudden influx of sodium ions and hence a reversal of charge on the axon membrane. This is the action potential and the membrane is depolarised.
3. The localised electrical currents established by the influx of sodium ions cause the opening of sodium voltage-gates channels a little further along the axon. The influx of sodium ions causes depolarisation.
4. Behind this region of depolarisation, the sodium voltage-gated channels close and the potassium ones open. Potassium ions leave the axon along their electrochemical gradient. This explains how the depolarisation current moves along the axon.
5. The action potential (depolarisation) is propagated in the same way further along the axon. The outward movement of potassium ions eventually causes repolarisation.
6. Repolarisation of the axon allows sodium ions to be actively transported out. This returns the axon to resting potential. The axon is ready for a new stimulus and action potential to occur.
29
How Does An Impulse Travel Along A Myelinated Axon?
The fatty sheath of myelin acts as an electrical insulator.
This prevents the action potentials from forming.
At intervals of 1-3mm, there are breaks in this myelin called the Nodes of Ranvier.
Action potentials can occur at these points.
The localised circuits therefore arise between these adjacent nodes of Ranvier and jump between the nodes.
30
What Is Saltatory Conduction?
The process of the impulse jumping from node to node in a myelinated neurone.
Saltatory conduction is faster than a normal conduction (I.e in an unmyelinated neurone of the same diameter).
This conduction is faster because the process and events of depolarisation must occur all the way along an axon that is unmyelinated and this takes more time than the process of depolarisation occuring just at nodes.
31
What Does A Localised Current Look Like On A Myleinated Axon?
Look at page 356.
32
Factors Effecting Speed At Which An Action Potential Travels?
Once an action potential begins, it travels at quickly at the same size.
However, there are factors that could effect the speed at which the impulse (action potential) travels:
1. Myelin sheath,
2. The diameter of the axon,
3. Temperature.
33
What Actually Is A Nerve Impulse?
The transmission of an action potential along the axon of the neurone is the nerve impulse.
34
Speed That Action Potential Can Travel?
As little as 0.5ms-1 to as much as 120ms-1.
Factors affect the speed at which a nerve impulse travels.
35
Myelin Sheath As A Factor Effecting Nerve Impulse Speed?
The myelin sheath acts as an electrical insulator.
It prevents an action potential forming in a part of the axon covered by myelin sheath.
Myelin sheath makes the nerve impulse jump from one node of Ranvier to the next node of Ranvier (places with no myelin sheath). This is called Saltatory conduction.
This increases the speed of conductance from 30ms-1 (in an unmyelinated neurone) to 90ms-1 (in a myelinated neurone of the same diameter).
36
Diameter Of The Axon As A Factor Effecting Nerve Impulse Speed?
The greater the diameter of the axon, the faster the speed of conductance.
This is due to less leakage of ions from a large axon.
Leakage makes membrane potentials harder to maintain.
37
Temperature As A Factor Effecting Nerve Impulse Speed?
Temperature effects the rate at which ions diffuse.
Therefore, the higher the temperature, the faster the impulse travels along an axon.
This is because the energy for active transport comes from respiration. Respiration is controlled by enzymes and enzymes work more at a higher temperature (to a certain point).
So, the higher the temperature, the more respiration occurs and therefore, more energy is available for active transport.
Active transport is used in the conduction of an impulse along an axon FINISH THIS, DONT UNDRRTSNAD WHAT ACTIVE TRANSPORT IS NEEDED FOR
38
Temperature As A Factor Effecting Nerve Impulse Speed Especially In Animals?
If temperature is too high, plasma membranes and proteins (enzymes) are denatured.
This means a nerve impulse cannot travel along an axon at all.
Temperature is important for response time in cold-blooded animals (ectothermic) whose body temperatures vary in accordance with the environment.
Temperature also effects the speed and strength of muscle contractions.
39
All Or Nothing Principle?
Nerve impulses are described as all-or-nothing responses.
There is a certain level of stimulus called the threshold value, which triggers an action potential.
Below the thread hold value, no action potential can be generated. Therefore, no no impulse is generated.
Any stimulus that is below the threshold value will fail to generate an action potential.
Any stimulus above the threshold value will succeed in generating an action potential and so a nerve impulse will travel.
All action potentials are more or less the same size, and so the strength of the stimulus can not be detected by the size of the action potential.
The all or nothing principle acts as a filter, preventing minor stimuli from setting up nerve impulses and thus, preventing the brain from becoming overloaded with information.
40
How Can An Organism Perceive The Size Of A Stimulus?
This is achieved in two ways:
1. By the number of impulses passing in a given time. The larger the stimulus, the more impulses generated in a given time.
2. By having different 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.
41
The Refractory Period?
One an action potential has been created in a region of the axon, there is a period afterwards where inward movement sodium ions is prevented.
This is because the sodium voltage-gated channels are closed.
During this time, it is impossible for a further action potential to be generated.
This is known as the refractory period.
The refractory period limits the strength of stimulus that can be detected.
42
The Refractory Period Serves 3 Purposes?
1. It ensures that action potentials are propagated in one direction only.
2. It produces discrete impulses.
3. It limits the number of action potentials.
43
How Does The Refractory Period Ensure Action Potentials Are Propagated In Only One Direction?
Action potentials can only pass from an active region to a resting region.
This is because action potentials can not be propagated in a region that is refractory, which means that they can only move in a forward direction.
This prevents action potentials from spreading out in both directions (which they would otherwise do).
44
How Does The Refractory Period Produce Discrete Impulses?
Due to the refractory period, a new action potential cannot be formed immediately behind the first one.
This ensures action potentials are separate from each-other.
45
How Does The Refractory Period Limit The Number Of Action Potentials?
As action potentials are separated from one another, this limits the number of action potentials that can pass along the axon in a given time, and thus limits the strength of stimulus that can be detected.
46
What Is A Synapse?
A synapse is the point where one neurone communicates with another or with an effector.
They are important in linking different neurones together and therefore coordinating activities.
47
Structure Of A Synapse?
Synapses transmit information, but not impulses, from one neurone to another via chemicals known as neurotransmitters.
Neurones are separated by a small gap, called the synaptic cleft. This synaptic cleft is 20-30nm wide.
The neurone that releases the neurotransmitter is called the presynaptic neurone.
The presynaptic neurone ends with a swollen (light-bulb shaped) axon called the synaptic knob.
The synaptic knob has many mitochondria and large amounts of endoplasmic reticulum. These are required in the manufacture of the neurotransmitter which takes place in the axon.
Calcium ion protein channels are on the membrane of the presynaptic neurone.
The neurotransmitter is stored in the synaptic vesicles within the synaptic knob.
Once the neurotransmitter is released from the vesicles, it diffuses across to the post synaptic neurone which posses specific receptor proteins on its membrane to receive it.
48
Name Of Neurotransmitter In Presynaptic Neurone?
Acetylcholine - stored in the synaptic vesicles in the synaptic knob.
49
Features Of Synapses?
Features of synapses:
- Undirectionality,
- Summation (Temporal summation and spatial summation),
- Inhibition.
50
Unidirectionality?
A feature of a synapse.
Synapses can only pass information in one direction - from the presynaptic neurone to the postsynaptic neurone.
In this way, synapses act like valves.
51
Summation?
Another feature of synapses and also, rod cells.
Low-frequency action potentials (weak stimulus action potentials) often lead to the release of an insufficient concentration of neurotransmitters to trigger a new action potential in the postsynaptic neuron.
They can, however, do so in a process called summation. This involves a rapid buildup of neurotransmitter in the synapse by one or two methods:
- spatial summation,
- temporal summation.
52
Spatial Summation?
This is a type of summation; a feature of a synapse.
Spatial summation Is when a number of different presynaptic neurons together release enough neurotransmitter to exceed the threshold value of the postsynaptic neuron.
Together they therefore try get a new action potential.
53
Temporal Summation?
This is a type of summation; a feature of a synapse.
Where a single presynaptic neuron releases neurotransmitter many times over a very short period. If the concentration of neurotransmitter exceeds the threshold value of the postsynaptic neuron, then a new action potential is triggered.
54
Inhibition?
A feature of synapses.
Some synapses make it less likely that a new action potential will be created on the postsynaptic neuron. These are known as inhibitory synapses.
55
How Do Inhibitory Synapses Operate?
They operate as follows:
- The presynaptic neuron releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neuron.
- The neurotransmitter causes the chloride ion protein channels to open.
- Chloride ions (Cl-) move into the postsynaptic neuron by facilitated diffusion.
- The binding of the neurotransmitter causes the opening of nearby potassium (K+) protein channels.
- Potassium ions move out of the postsynaptic neuron into the synapse.
- The combined effect of negatively charged chloride ions moving in and positively charged potassium ions moving out is to make the inside of the postsynaptic membrane more negative and the outside more positive.
- The membrane potential increases to as much as -80mV compared with the usual -65mV (at resting potential).
- 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.
56
Functions Of Synapses?
Synapses transmit information from one neurone to another.
They act as junctions because of this. This allows:
- A single impulse along a neurone to initiate new impulses in a number of different neurons 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 the nerve impulses from receptors reacting to different stimuli to contribute to a single response.
57
Basic Functioning Of Synapses?
(This card is just some things you should understand to know the further, more complicated, functioning of synapses - later flash cards).
- A chemical (the neurotransmitter) is made only in the presynaptic neuron and not in the postsynaptic neurone.
- The neurotransmitter is stored in synaptic assaults. When an action potential reaches the post synaptic knob, the membranes of these vesicles fuse with the presynaptic membrane to release the neurotransmitter.
- When released, the neurotransmitter defuses across the synaptic cleft to bind to specific receptor proteins which are only found on the postsynaptic neuron.
58
What Are Excitatory Synapses?
The neurotransmitter binds with the receptor proteins and this leads to a new action potential in the postsynaptic neuron. Synapses that produce new action potentials in this way are called excitatory synapses.
59
What Is A Cholinergic Synapse?
A cholinergic synapse is a synapse in which the neurotransmitter is a chemical called acetylcholine.
Acetylcholine is made up of two parts: acetyl and choline.
Acetyl is just another word for ethanoic acid.
Cholinergic synapses are common in vertebrates, where they occur in the central nervous system and at neuromuscular junction is (junctions between neurons and muscles).
(The process of transmission across a call and a check synapse is described in the series of diagrams in figure 2 in topic 15.6).
60
Types Of Neurotransmitters?
There are many different types of receptor on the postsynaptic neuron. There are also many different neurotransmitters.
Each receptor is a protein that binds specifically to a neurotransmitter because they have complimentary shapes.
Some of these neurotransmitters and receptors are excitatory, that is, they lead to a new action potential in the postsynaptic neurone.
Others are in inhibitory, that is, they make it less likely that a new action potential will be created in the postsynaptic neurone.
Overall, the action of a specific neurotransmitter depends on the specific receptor to watch it binds.
61
Why Do Drugs Effect Us (Brief Understanding)?
Our perception of the world is due to:
- stimuli being detected by receptors,
- and information being transferred to the brain as nerve impulses by neurones that connect via synapses.
Therefore, it is not surprising that the effects of many medical and recreational drugs or due to their actions on synapses.
62
Drugs Act On Synapses In Two Main Ways?
Drugs act on synapses in two main ways:
- They stimulate the nervous system by creating more action potentials in postsynaptic neurones.
- They inhibit the nervous system by creating for your action potentials in the postsynaptic neurones.
63
How Do Drugs Act On Synapses By Creating More Action Potentials?
Drugs act on synapses by stimulating the nervous system.
They stimulate the nervous system by creating more action potentials in postsynaptic neurones.
A drug may do this by:
- mimicking a neurotransmitter,
- stimulating the release of more neurotransmitter,
- or inhibiting the enzyme that breaks down the neurotransmitter.
The outcome is to enhance the bodies for responses to impulses passed along the postsynaptic neuron.
For example, if the neurone transmits impulses from sound receptors, a person will perceive the sound has been louder.
64
How Do Drugs Act On Synapses By Creating Fewer Action Potentials?
Drugs act on synapses in this way by inhibiting the nervous system.
They inhibit the nervous system by creating fewer action potentials in postsynaptic neuron.
A drug may do this by inhibiting the release of neurotransmitter or blocking receptors on sodium/potassium ion channels on the postsynaptic neuron.
The outcome is to reduce the impulses passed along the postsynaptic neuron.
In this case, if the neurons transmit impulses from sound receptors, a person will perceive the sound as being quieter.
65
Why Do Effects Of Drugs On The Synapse Depend On The Type Of Transmitter?
The effects of a drug on the synapse depends on the type of transmitter.
For example, a drug that inhibits the action of an excitatory neurotransmitter will reduce a particular effect. However, a drug that inhibits an inhibitory neurotransmitter will enhance a particular effect.
66
Endorphins?
Endorphins are neurotransmitters.
They are used by certain sensory nerve pathways, especially pain pathways.
Endorphins block the sensation of pain.
Drugs such as morphine and codeine bind to specific receptors in the brain used by endorphins and so mimic the effects of endorphins.
67
Serotonin?
Serotonin is a neurotransmitter.
Serotonin is involved in the regulation of sleep and certain emotional states.
Reduced activity of the neurones that release serotonin is thought to be one cause of clinical depression.
Prozac is an antidepressant drug that affects serotonin within synaptic clefts.
68
GABA?
GABA is a neurotransmitter.
GABA inhibits the formation of action potentials when it binds to the postsynaptic neurones.
Valium is a drug that enhances the binding of GABA to its receptors.
69
Transmission Across A Synapse?
Page 366 in textbook.
(Very detailed diagrams to learn).
70
What Are Muscles, And What Types Of Muscles Are There?
Muscles are effector organs that respond to nervous stimulation by contracting and so bring about movement.
There are three types of muscle in the body:
- Cardiac muscle,
- Smooth-muscle,
- Skeletal muscle.
71
Cardiac Muscle?
Cardiac muscle is found exclusively in the heart.
Neither smooth-muscle or cardiac muscle is under conscious control and we remain largely unaware of their contractions.
72
Smooth Muscle?
Smooth muscle is found in the walls of blood vessels and the gut.
Neither smooth-muscle or cardiac muscle is under conscious control and we remain largely unaware of their contractions.
73
Skeletal Muscle?
Skeletal muscle makes up the bulk of body muscle in vertebrates.
It is attached to bone and acts under voluntary, conscious control.
74
Structure Of Skeletal Muscle?
Individual muscles are made up of millions of tiny muscle fibres called myofibrils.
In themselves, they produce almost no force while collectively they can be extremely powerful.
Myofibrils are parallel to each other. This maximises their strength.
Myofibrils are arranged so they are grouped into strings, the strings are grouped into small ropes and small ropes are grouped into bigger ropes (literally like a rope). (Look at figure 1 on page 367).
This structure allows the muscle to contract efficiently.
The structure avoids adjacent cells. This means there is no point of weakness (adjacent cells joining) and so, the overall strength of the muscle is not reduced.
The separate cells are fused together into muscle fibres in this structure.
75
Organelles In Skeletal Muscle?
The separate cells are fused together into muscle fibres in the structure of skeletal muscle (like a rope).
These muscle fibres share nuclei and also cytoplasm.
We refer to this cytoplasm as sarcoplasm, which is mostly found around the circumference of the fibre.
Within the sarcoplasm is a large concentration of mitochondria and endoplasmic reticulum.
(Figure 1 on page 367 is important).
76
Microscopic Structure Of Skeletal Muscle?
Each muscle fibre is made up of myofibrils.
Myofibrils are made up mainly of two types of protein filament:
- Actin,
- Myosin.
(Figure 2 on page 268).
Another important protein found in muscle is tropomyosin. This forms a fibrous strand around the actin filament.
77
Actin?
Actin is a thinner protein filament involved in the building of myofibrils.
It consists of two strands twisted around one another.
(Actin molecules are shown on diagrams as being two chains of circular spheres in the book - fig.2 on page 368).
78
Myosin?
Myosin is a thicker protein filament involved in the making of myofibrils.
Myosin is shown as being two, straight chains twisted together like a rope called myosin tail and then a balloon shaped head on each strand called the myosin head - fig.2 on page 368.
Many miles in molecules and chains twist together to form a thick filament.
Myosin molecules are joined tail to tail in the thick filament Z-line.
79
The Dark Band Is The?
A-band.
80
The Light Band Is The?
I-Band.
81
What Does The Arrangement Of Sarcomeres Allow?
The arrangement of sarcomeres into a long line means that, when one sarcomere contracts are little, the line as a whole contract a lot.
In addition, having the lines of sarcomeres running parallel to each other means that all the force is generated into one direction.
82
How Do We Recognise Myofibrils On Microscope Due To Light?
Myofibrils appear striped due to the alternating light-coloured and dark-coloured bands.
The light bands are called I-bands. They appear lighter because the thick and thin filaments do not overlap in this region. The I-band contains the actin.
The dark bands are called a-bands. They appear darker because the thick and thin filaments overlapping this region. The a-bands usually contain myosin.
At the centre of each a-band Is a lighter-coloured region called the H-zone. At the centre of each I-band is a z-line. The distance between the adjacent Z-line is called a sarcomere.
83
What Happens To Sarcomere When A Muscle Contracts?
When a muscle contracts. These sarcomeres shorten and the pattern of light and dark bands change.
When one sarcomere contracts, the whole line contracts.
84
What Is A-Band An Acronym For?
Anisotropic band.
85
What Is I-Band An Acronym For?
Isotopic band.
86
Types Of Muscle Fibre?
There are two types of muscle fibre, the proportions of which vary from muscle to muscle and person-to-person.
The two types are:
- Slow-twitch fibres,
- Fast twitch fibres.
87
Slow-Twitch Fibres?
Slow-twitch fibres contract more slowly then fast twitch fibres.
They provide less powerful contractions but these contractions occur over a longer period.
They are therefore adapted to endurance work, such as running a marathon.
In humans, they are more common in muscles like a calf muscle, which must contract constantly to maintain the body in an upright position.
88
How Are Slow-Twitch Fibres Adapted?
They are suited to endurance work by being adapted for aerobic respiration.
This is in order to avoid a buildup of lactic acid, which would cause them to function less efficiently and prevent long-duration contraction.
Other adaptations include:
- A large store of myoglobin (a bright red molecule that stores oxygen, which accounts for the red colour of slow twitch fibres).
- A rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration.
- Numerous mitochondria to produce ATP.
89
Fast-Twitch Fibres?
Fast twitch fibres contract more rapidly and produce powerful contractions but only for a short period.
They are, therefore, adapted to intense exercise, such as weightlifting.
As a result, they are more common in muscles which need to do short bursts of intense activity, like the biceps muscle of the upper arm.
90
How Are Fast-Twitch Fibres Adapted?
Fast twitch fibres are adapted to their role by having:
- Thicker and more numerous myosin filaments.
- A high concentration of glycogen.
- A high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly.
- A store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provides energy for muscle contraction.
91
Neuromuscular Junctions?
A neuromuscular junction is the point where a motor neuron meet a skeletal muscle fibre.
There are many neuromuscular junctions along the muscle.
Rapid and coordinated muscle contraction is frequently essential for survival. This means many neuromuscular junction spread throughout the muscle is needed for survival.
This insures that contraction of the muscle is rapid and powerful when it is simultaneously stimulated by action potentials.
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What Is A Motor Unit?
All muscle fibres supplied by a single motor neurone act together as a single functional unit and are known as a motor unit.
This arrangement gives control over the force that the muscle exerts.
If only slight force is needed, only a few units are stimulated.
If a greater force is required, a large number of units are stimulated
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What Happens At The Neuromuscular Junction?
When a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release there acetylcholine.
The acetylcholine diffuses to the postsynaptic membrane (which is the membrane of the muscle fibre).
This alters the membranes permeability to sodium ions.
The sodium ions enter rapidly, depolarising the membrane.
(A description of how this leads to the contraction of the muscle is given on a later flashcard).
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What Happens To Excess Acetylcholine In The Neuromuscular Junction?
The acetylcholine is broken down by the acetylcholinesterase to ensure that the muscle is not over-stimulated.
The resulting choline and ethanolic acid (acetyl) diffuse back into the neuron, where they are re-combined to form acetylcholine using energy provided by the mitochondria found there.
(The structure of a neuromuscular junction is on pages 370).
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Similarities Between Neuromuscular Junction And Synapse?
Both a neuromuscular junction and a synapse:
- Have neurotransmitters that are transported by diffusion.
- Have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions.
- Use a sodium-potassium pump to repolarise the axon.
- Use enzymes to breakdown the neurotransmitter.
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Differences Between Neuromuscular Junction And Synapse?
Differences between the neuromuscular junction and synapse include:
- A neuromuscular junction is only excitatory whilst a cholinergic synapse cane be inhibitory and excitatory.
- A neuromuscular junction has only motor neurones whilst a synapse has a motor, sensory and intermediate neurons.
- A neuromuscular junction‘s action potential will end at the neuromuscular junction (it is the end of a neural pathway) whilst the synapse’s action potential may be produced along another neurone (the postsynaptic neurone).
- Acetylcholine binds to receptors on membrane of muscle fibre whilst acetylcholine binds to receptors on membrane of post-synaptic neurone.
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The Skeleton?
It is important to appreciate that skeletal muscle is attached to the human skeleton.
In humans, the skeleton is made up of bone which is incompressible. Therefore, if muscle exerts a force, via tendons the bone moves rather than the muscle changing shape. (The contraction of skeletal muscle will result in the movement of bones).
The different parts of the skeleton can be moved relative to one another around a series of pints called joints.
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Antagonistic Muscles?
The contraction of skeletal muscle will move a part of the skeleton, for example, a limb, in one direction but the same muscle cannot move it in the opposite direction.
Muscles cannot push they can only pull.
To move the lamb in the opposite direction requires a second muscle that works antagonistically to the first one. (I.e. in the opposite direction.)
In doing so, it stretches its partner muscle (which has relaxed) returning it to its original state, ready to contract again.
Skeletal muscles occur in antagonistic pairs.
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The Sliding Filament Mechanism?
(Figure 2 on page 371).
This process is repeated 100x per second, in a cycle.
The process has been split into:
- Stimulation,
- Contraction,
- Relaxation.
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Evidence For Sliding Filament Mechanism?
Myofibrils appear darker in colour where the actin and myosin filaments overlap and lighter where they do not.
If the sliding filament mechanism is correct, then there will be more overlap of actin and myosin in a contracted and muscle then in a relaxed one.
In a contracted muscle, the following changes occurs to a sarcomere:
- The I-band becomes narrower.
- The Z-lines move closer together or, in other words, the sarcomere shortens.
- The H zone becomes narrower.
- The A-band (which shows the length of myosin filaments) remains the same width which proves that the length of the myosin filaments has not become shorter and so they must overlap.
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Proteins Involved In Sliding Filament Mechanism?
Three main proteins involved in the sliding filament mechanism:
- Myosin (is made up of two types of protein: a fibrous protein arranged into a filament made up of several hundred molecules (the tail) and a globular protein formed into two bulbous structures at one end (the head)).
- Actin (a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand.
- Tropomyosin forms long thin threads that are wound around actin filaments.
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Muscle Stimulation?
The first step in the sliding filament mechanism of muscle contraction.
1. An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open.
2. Calcium ions defuse into the synaptic knob.
3. The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft.
4. Acetylcholine defuses across the synaptic cleft and binds with receptors on the muscle cell-surface membrane, causing it to depolarise.
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Muscle Contraction?
The second step in the sliding filament mechanism of muscle contraction.
(FIGURE 4 ON PAGE 374 - might make this process more clear. A lot of info on this page).
1. The action potential travels deep into the fibre through a system of tubules (T-tubules).
2. The tubules are in contact with the endoplasmic reticulum of the muscle (sarcoplasmic reticulum) which has actively transported calcium ions from the cytoplasm of the muscle leading to very low calcium ion (Ca2+) concentration in cytoplasm.
3. The action potential opens the calcium ion protein channels on the endoplasmic reticulum and calcium ions diffused into the muscle cytoplasm (sarcoplasm) down a concentration gradient.
4. The calcium ions bind to the troponin on the tropomyosin which causes the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away.
5. ADP molecules attached to the myosin heads mean they are 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 as they do so and releasing a molecule of ADP.
7. An ATP molecule attaches to each myosin head, causing it to become detached from the actin filament.
8. The calcium ions then activate the enzyme ATPase, which hydrolyses the ATP to ADP. The hydrolysis of ATP to ADP provides the energy for the myosin head to return to its original position.
9. The myosin head, once more with an unattached ADP molecule, then reattaches itself further along the actin filament and the cycle is repeated as long as the concentration of calcium ions in the myofibril remains high.
10. As the myosin molecules are joined tail to tail into oppositely facing sets, the movement of one set of myosin heads is in the opposite direction to the other set. This means that actin filaments to which they are attached also move in opposite directions.
11. The movement of actin filaments in opposite directions pulls them towards each other, shortening the distance between the two adjacent Z-lines. This process takes place repeatedly and simultaneously throughout a muscle. This process shortens the muscle and brings about movement of that part of the body.
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What Are T-Tubules?
Tubules that are extensions of the cell-surface membrane and branch out throughout the cytoplasm of the muscles.
The tubules are in contact with the endoplasmic reticulum of the muscle (sarcoplasmic reticulum)
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Muscle Relaxation?
The third step of the sliding filament mechanism of muscle contraction.
1. When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic reticulum using energy from the hydrolysis of ATP.
2. This reabsorption of the calcium ions allows tropomyosin to block the actin filament again.
3. Myosin heads are now unable to bind to actin filaments and the muscle relaxes.
4. In this state force from antagonistic muscles can pull actin filaments out from between myosin (to a point).
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Energy Supply During Muscle Contraction?
Muscle contraction requires energy. This energy is supplied by the hydrolysis of ATP to ADP and inorganic phosphate (Pi).
Most ATP is re-generated from ATP during the respiration of pyruvate in the mitochondria (mitochondria are particularly plentiful in the muscle).
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What Is Energy Needed For In Muscle Contraction?
The energy released is needed for:
- the movement of the myosin heads,
- the reabsorption of calcium ions into the endoplasmic reticulum by active transport.
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Why Might ATP Be Generated Anaerobically?
In a very active muscle, the demand for ATP is high.
Therefore, the demand for oxygen is high.
The demand for oxygen is greater than the rate at which the blood can supply oxygen.
Therefore, a means of rapidly generating ATP anaerobically is also required.
This is achieved using:
- a chemical called phosphocreatine
- and partly by more glycolysis.
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How Is ATP Generated Anaerobically?
Phosphocreatine cannot supply energy directly to the muscle, so instead it generates regenerate ATP.
ATP CAN directly supple energy to the muscle.
Phosphocreatine is stored in muscle.
Phosphocreatine acts as a reserve supply of phosphate. It is available immediately to combine with ADP and so reform ATP.
The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed.
