3.8 -the Nervous System Flashcards
1
Q
What does the nervous system do (broad 3 points)
A
1) detects changes or stimuli inside the body and in the environment
2) processes and stores information
3) initiates a response
2
Q
What is a stimulus?
A
A detectable change in the internal or external environment of an organism that produces a response in that organism
3
Q
What give an organism its senses?
A
Sensory receptors
4
Q
What are sensory receptors?
A
Transducers because they detect energy in one form and convert it into a electrical energy.
5
Q
What are electrical impulses also known as?
Where do they travel?
What do they do?
A
Called: nervous impulses
Travel: travel along neurones
Action: initiate a response in an effective which may be a muscle or gland
6
Q
If visible light is the stimulus what is the sensory receptor location? And sense?
A
Sensory receptor= Retina
Sense= sight
7
Q
If sound is the stimulus what is the sensory receptor location and sense?
A
Sensory receptor= inner ear
Sense= sound
8
Q
If pressure is the stimulus what is the sensory receptor location and what is the sense
A
Sensory receptor location= dermis of the skin
Sense= touch
9
Q
If temperature is the stimulus what is the sensory receptor location and sense?
A
Sensory receptor location= dermis of the skin
Sense= temperature
10
Q
If gravity is the stimulus where is the sensory receptor location and sense?
A
Sensory receptor location = middle ear
Sense= balance
11
Q
List where mammals may sense pressure?
A
Dermis of the skin, blood vessels, UV light, electric fields, changes in humidity, magnetic fields, tissue damage and osmolarity of fluids
12
Q
What is a nerve?
A
A bundle of neurones or nerve fibres
13
Q
What are the 2 main parts of the central nervous system
A
1) central nervous system (CNS)
2) peripheral nervous system (PNS)
14
Q
What does the CNS comprise of?
A
Comprises of the brain and the spinal cord
15
Q
What is the role of the nervous system?
A
The CNS processes information provided by a stimulus.
16
Q
What are the parts of the CNS surrounded by?
A
Surrounded by a tough protective membrane collectively called meninges
17
Q
Describe the structure of the spinal cord
A
- white matter contains nerve fibres surrounded by myelin which is fatty so looks white
- the grey matter has much less myelinated and is largely the nerve fibres of relay neurones and the cell bodies of relay and motor neurones.
18
Q
What is the peripheral nervous system (PNS) comprised of?
A
The somatic nervous system and the autonomic nervous system
19
Q
Describe the somatic nervous system
A
- Pairs of nerves that originate in the brain or spinal cord and their branches.
- nerves contain the fibres of sensory neurones which carry impulses from receptors to the CNS, and motor neurones which carry impulses away from the CNS to effectors
20
Q
Describe the autonomic nervous system
A
Provides unconscious control of the function of internal organs e.g heartbeat and digestion.
21
Q
How can the speed at which we detect a change in stimuli benefit an organism
A
The ability to detect and respond to stimuli increases an organisms chance of survival
22
Q
What is the name of the simplest type of nervous response to a stimulus?
A
A reflex arc
23
Q
What is a reflex arc?
Give an example of a common reflex arc.
A
The neural pathway taken by the nervous impulses of a reflex action.
Example: the withdrawal of a reflex e.g when you immediately withdraw your hand when placed on a hot object.
24
Q
What is a reflex action?
A
A rapid, automatic, involuntary response resulting from nervous impulse initiated by a stimulus. (Generally has a protective function.)
25
Give the elements of a reflex arc in order
1. Stimulus (e.g heat)
2. Receptor (temperature and pain receptors in the dermis of the skin)
3. Sensory neurone (sends impulse up the arm to the spinal cord)
4. CNS (relay neurone in spinal cord transmits an impulse from a sensory neurone to a motor neurone)
5. Motor neurone (sends impulse to an effective in this case a muscle)
6. Response (arm muscles contract and the hand is removed from heat source)
26
What are some reflex arcs missing, give an example
Reflex arcs of some reflex actions do not have a relay neurone e.g the knee jerk reflex. The sensory neurone synapses directly with the motor neurone
27
What is a nerve net?
The simplest type of nervous system, a diffuse network of cells that group into ganglia, but do not form a Brain.
28
What are the two types of cells in a nerve net?
- ganglion cells which provide connections in several directions
- sensory cells which detect stimuli e.g light sound touch or temperature
29
What is the model organism for studying nerve nets?
| Why?
Model organism: Hydra, in the phylum Cnidaria
Model organism because:
1)has a simple pattern
2)is easy to manipulate in experiments
3)regenerates rapidly e.g when replacing a lost tentacle
30
Where is a hydras nerve net located?
In its ectoderm, the outer of the two layers of cells of its body wall.
31
How does the nerve net benefit the hydra?
It allows the hydra to sense light, physical contact and chemicals, so in response it can contract, perform locomotion, hunt and feed. (Even without a brain it shows complex movements and behaviour).
32
One negative of the hydra nerve net
It can’t detect the direction of a stimulus, although a larger stimulus stimulates more cells and triggers a larger response.
33
Compare and contrast the nervous system in the Hydra and humans
Nervous system type: Hydra= nerve net, Humans= CNS
Number of cell types in nervous system: Hydra= 2, Human= many
Regeneration: Hydra= rapid, Human= very slow If at all
Myelin sheath: Hydra =absent, Human =present
Conduction speed: Hydra =slow (5 ms-1), Human =fast (up to 120ms-1)
Ability to regenerate neurones: Hydra =Present, Human =Absent
34
What is important to note about certain hormones?
Vertebrate hormones e.g steroids occur in the hydras nerve net. However, the nervous systems of hydra and vertebrates are so different that it is unlikely that the hormones have the same function
35
What are nerve cells/neurones?
Specialised cells adapted to rapidly carry nervous impulses from one part of the body to another
36
What are the three types of neurones?
1. Sensory neurone (carries impulses from the sense receptors or organs to the CNS)
2. Motor neurone (carries impulses from the CNS to the effector organs I.e muscles or glands)
3. Relay neurone (receives impulses from sensory neurone or other relay neurones and transmits then to motor neurones on other relay neurones
37
List the parts of the mammalian motor neurone
```
Cell body/centron
Cytoplasm
Nucleus
Nissl granules
Dendrite
Axon
Schwann cells
Myelin sheath
Nodes of ranvier
Synaptic end bulb
Axon ending/terminal
```
38
What is the function of the mammalian motor neurones cell body/centron
Contains a nucleus and granular cytoplasm
39
What is the function of the mammalian motor neurones cytoplasm
Granular; contains many ribosomes
40
What is the function of the mammalian motor neurones nucleus
Holds DNA
41
What is the function of the mammalian motor neurones Nissl granules
Cytoplasmic granules comprising ribosomes grouped in RER
42
What is the function of the mammalian motor neurones Dendrite
Thin fibre carrying impulses towards cell body, a cell body may have several dendrites
43
What is the function of the mammalian motor neurones axon
Thin fibre carrying impulses away Fromm cell body, a cell body has only one axon
44
What is the function of the mammalian motor neurones Schwann cells
-surround and support nerve fibres, in vertebrate embryos they wrap around the developing axons many times and withdraw their cytoplasm, leaving a multi-layered phospholipid myelin sheath
45
What is the function of the mammalian motor neurones myelin sheath
Electrical insulated, speeds up the transmission of impulses
46
What is the function of the mammalian motor neurones nodes of ranvier
1um gaps in myelin sheath where adjacent Schwann cells meet and where the axon membrane is exposed.
They allow impulses to be transmitted rapidly.
47
What is the function of the mammalian motor neurones synaptic end bulb
Swelling at the end of the axon. It’s where neurotransmitter is synthesised.
48
What is the function of the mammalian motor neurones axon ending/terminal
Secretes neurotransmitter, which transmits impulse to adjacent neurone.
49
What is a resting potential?
The potential difference across the membrane of a cell when no nervous impulses is being conducted
50
What is the potential difference across a cell membrane?
| Why do we use a negative
70 mV
| The membrane is more negative inside the cell and therefore the resting potential is -70mV
51
What can a cell membrane with a potential difference be called?
Polarised (because there’s a potential difference across the cell membrane)
52
What generally causes the resting potential?
The negative ions of large proteins, organic acids (pyruvate) and of organic phosphates (e.g ATP4-)
53
Describe the concentration of sodium and potassium ions on the inside of a axon cell compared to the outside
The inside of a cell has both higher concentration of K+ ions and Lower concentration of Na+ ions than the outside
54
How is the axon membrane made more permeable to K+ ions than Na+ ions?
Naturally K+ ions tend to diffuse out and Na+ diffuse in.
Some of the channels that allow K+ ions
to diffuse out are open.
Most of the channels that allow the Na+ ions to move in are closed.
K+ ions :: diffuse out faster than Na+ diffuse in.
55
What are sodium potassium exchange pumps and what is their role?
They are trans-membrane proteins with ATPase activity that transport K+ (back in) and Na+ (back out) across the membrane against a concentration gradient by active transport.
They maintain the concentration and an uneven distribution of ions across the membrane.
56
How is the inside of the membrane more negative than the outside!
Na+ ions are pumped out faster than K+ ions are pumped in, approx 3 Na+ ions are pumped out for every 2 K+ ions pumped in (per ATP molecule that is hydrolysed)
57
Define action potential
The rapid rise and fall of the electrical potential across a nerve cell membrane as a nervous impulse passes
58
Define depolarisation.
A temporary reversal of potential across the membrane of a neurone such that the inside becomes less negative than the outside as an action potential is transmitted
59
What is an excitable cell?
A cell in which the potential across the membrane can be altered
60
What is a nervous impulse
The transmission of a change in potential along a nerve fibre associated with the movement of sodium ions.
61
How is the small voltage change detected during a nervous impulse?
Picked up through a pair of micro electrodes and fed into an oscilloscope.
One has to be inside an axon and another in a bathing solution for changes in potential to be detected. This produces an oscilloscope trace (graph) showing how the voltage changes across a membrane with time.
62
What can an oscilloscope trace be useful for?
Measures the magnitude and speed of transmission of the impulse and analyse the pattern of impulses generated in different parts of the nervous system/different situations.
63
What is a voltage gates Chanel?
A channel which opens or closes in responce to a particular voltage across the membrane.
64
Explain the fluctuations of potential difference during an action potential
1) the energy from a stimulus causes some of the voltage gated sodium channels in the axon membrane to open
2) the sudden increase in the permeability if the membrane to Na+ ions allows them to rapidly diffuse into the axon, down a concentration gradient
3) this causes the negative resting potential of -70mV inside the axon to rapidly become +40 mV (this is depolarisation)
4) once the potential inside the cell is +40 mV the sodium channels close preventing further influx
5) next potassium channels open and K+ diffuse out of the axon down concentration gradient, the cell becomes less positive inside so membrane is repolarised
6) more K+ diffuse out than Na+ diffuse in so potential inside axon becomes more negative than resting potential; it is hyperpolarised
7) the sodium potassium pump pumps K+ ions back in and Na+ ions back out, restoring the ion balance of the resting potential
65
How long does a action potential last in a:
1) typical neurone
2) skeletal muscle
3) cardiac muscle
1) typical neurone= 1 ms
2) skeletal muscle= 2-5 ms
3) cardiac muscle= 200-400 ms
66
Describe the passage of an action potential along a non-myelinated nerve fibre.
- Na+ ions diffuse into the axon and K+ ions diffuse out.
- The reversal of potential sets up local currents as Na+ ions move laterally through the axon
- they depolarise the adjacent section of the membrane
- this opens more voltage gated sodium channels so more sodium floods in depolarising the axon at this point
- sodium ions then diffuse further down the axon and in this way, a self perpetuating wave of depolarisation spreads along the axon
67
Describe the re-establishment of the resting potential as an action potential travels through the non-myelinated nerve fibre.
- at the sight of initial action potential, the sodium channels are inactivated and can’t open again until the resting potential has been re-established
- this means a new action potential can’t be initiated there
- this is the absolute refractory period which lasts 1 me to ensure action potential is not propagated back in the direction which it came.
- for the next 5-10 ms (hyper-polarisation stage) while the sodium potassium pumps are restoring the resting potential, if an impulse is strong enough a new action potential may pass. This is known as the relative refractory period.
68
At what speeds so nervous impulses travel when:
a) controlling muscle position
b) during a pain impulse
c) during a touch impulse
A) nervous impulse controlling muscle position= 120 ms-1
B) nervous pain impulse= 0.61 ms-1
C) nervous touch impulse= 76.2 ms-1
69
Compared the speed of travel in a myelinated vs non myelinated nerve fibre
Myelinated nerve fibre= up to 120 ms-1
| Non myelinated nerve fibre= 0.5 ms-1
70
What is the all or nothing rule?
A nervous impulse is either initiated (if the intensity of the stimulus is above a threshold value) generating an action potential, or not, and it is always the same size (+40 mV)
It remains the same size as its propagated along the axon and no energy is lost in transmission.
71
What happens to the action potential when there is an increase intensity of stimuli?
The increased intensity does not generate a greater action potential, it remains at +40 mV. Instead the frequency of the action potentials increase.
72
What is the purpose of the all or nothing law?
It allows the action potential value to act as a filter, preventing minor stimuli from setting up nervous impulses so the brain is not overloaded with information.
73
Why was it thought that some dinosaurs could transmit nervous impulses more rapidly than modern reptiles?
There is evidence that some dinosaurs were warm blooded, unlike modern reptiles suggesting that they could transmit nervous impulses more rapidly.
74
What are the three factors affecting the speed of conduction if the nerve impulse?
1. Temperature
2. The diameter of the axon
3. Myelination
75
Describe how temperature affects the speed of conduction of the nerve impulse
Ions mii on be faster at higher temperatures than at lower temperatures as they have more kinetic energy.
This means that birds, mammals, the two warm-blooded taxa transmit nervous impulses more quickly and have faster responses than all other groups of animals
76
Describe how the diameter of the axon can affect the speed of conduction of the nervous impulse
The greater the diameter of the axon the greater it’s volume in relation to the area of the membrane. This means more sodium ions can flow through the axon so impulses travel faster.
77
Give an example of a species with wide diameter axons and why
Marine invertebrates such as the squid evolved in cold habitats with cold water have giant axons with a diameter of up to 1 mm
78
Describe how myelination speeds up the rate of conduction of a nerve impulse along an axon
Speeds up the rate of transmission by the insulating the axon.
79
Describe saltatory conduction
Transmission of nervous impulse along a myelinated axon, in which the action potential jumps from one node of Ranvier to the adjacent node.
80
Describe why saltatory conduction occurs
Myelinated nerve fibres only depolarise where resistance is low (in the node of ranvier)
The voltage gated ion channels only occur at the nodes of ranvier so this is where sodium ions enter
This causes the action potentiometer to jump from node to node along the axon
81
What makes saltatory conduction fast?
The nodes of ranvier are only 1mm apart so saltatory transmission is rapid
(Much faster than conduction along a non-myelinated axon)
82
What are neurones separated by?
Separated by synapses which send the nervous impulse between neurones in one direction only
83
What are the 2 classes of synapse?
The electrical synapse
| The chemical synapse
84
Describe the electrical synapse
A gap or junction which is 3nm across, small enough that an electrical impulse is transmitted directly from one neurone to the next
85
Describe a chemical synapse
A 20nm gap, too big for nervous impulses to jump.
The impulse is transmitted by a neurotransmitter, a chemical that diffuses across the synaptic cleft, from the pre-synaptic membrane of one neurone to the post synaptic membrane of an adjacent neurone where it is initiated.
86
Define the term neurotransmitter
A chemical secreted in response to an action potential which carries a chemical signal across a synapse, from one neurone to the next, where a new action potential is initiated
87
Describe the process of the synaptic transmission of an action potential
1) arrival of impulse at the synaptic cleft end bulb alters it’s membrane permeability
2) this opens voltage gated calcium channels
3) calcium ions diffuse into end bulb down concentration gradient
4) this causes the synaptic vesicles to move toward and fuse with the pre-synaptic membrane
5) this releases the neurotransmitter acetylcholine by exocytosis into the synaptic cleft
6) the neurotransmitter diffuses across the synaptic cleft and binds to a receptor (an intrinsic protein spanning the post synaptic membrane)
7) the proteins sub-units have two receptor sites and the two acetylcholine molecules show co-operative binding when they attach
8) this causes the receptor protein to change shape
9) this opens a channel and sodium ions diffuse in down the concentration gradient
10) this causes the post synaptic membrane to become depolarised and If the threshold potential is reached an action potential is initiated
88
Describe the role of ions during synaptic transmission
Calcium ions enter through the pre-synaptic membrane to release acetylcholine, sodium ions enter through the post synaptic membrane to initiate a new action potential
89
What would happen if acetylcholine were to remain in the synaptic cleft?
It would constantly imitate new impulses in the post synaptic membrane.
90
How is acetylcholine prevented from continuously initiating new impulses?
1) there is direct uptake of acetylcholine into the pre-synaptic neurone so none remains in the synaptic cleft to bind to the post synaptic receptors
2) active transport of calcium ions out of the synaptic end bulb so no more exocytosis of acetylcholine occurs
3) hydrolysis of acetylcholine. After release acetylcholine is quickly destroyed by acetylcholinesterase in the synaptic cleft. The products of hydrolysis are choline and ethanoic acid. They diffuse back across the synaptic cleft into the pre-synaptic neurone, and reform acetylcholine.
91
How many molecules of Acetylcholkne are hydrolysed each second?
25,000 molecules of acetylcholine are hydrolysed each second, so acetylcholine is quickly removed from the synaptic cleft as ethanoic acid and choline
92
Why do neurones only transmit impulses in one direction?
1) repolarisation happens behind an action potential and so depolarisation could not happen at that point
2) synaptic vesicles only occur at the end bulb of the pre-synaptic neurone
3) neurotransmitter receptors only occur on the post synaptic membrane.
93
Describe some general properties of synapses
1) transmit information between neurones
2) pass impulses in one direction generating precision in the nervous system
3) act as junctions
4) protect the response system from overstimulation because the impulse is always the same size whatever the stimulus
5) filter out low level stimuli (action potential is only initiated when threshold value is met)
94
Membrane potential be built to reach the threshold value?
1) Temporal summation where depolarisation builds up I’ve time to reach threshold where an action potential is initiated
2) spatial summation where sever pre-synaptic neurones synapse with the same post synaptic neurone and all contribute to the growing depolarisation which generates an action pontential when large enough
95
Define the term drug
A drug is a molecule that has a physiological effect on the body when ingested, inhaled, absorbed or injected.
96
Describe some molecules that are neurotransmitters
Acetylcholine
GABA
Monoamines e.g dopamine, serotonin
Neuropeptides e.g endorphins
97
Describe the 2 most commonwealth drug groups and their effect on the nervous system
1)sedatives- example= alcohol
They inhibit the nervous system by creating fewer action potentials in post-synaptic neurones
2)Stimulants- example= amphetamines
They stimulate the nervous system by allowing more action potentials in post-synaptic neurones
98
Why is alcohol a sedative?
It suppresses activity in the areas of the brain that contribute to self-control
99
List 2 ways a drug may act on the nervous system
1) they may mimic the action of neurotransmitters
| 2) they may inhibit the breakdown of neurotransmitters
100
Describe how a drug may mimic the action of neurotransmitters
They may have the same shape and bind to the post-synaptic neurone in the same way.
This would increase the frequency of action potentials.
101
Describe how nicotine can affect the action of acetylcholine
The positively charged nitrogen atom in nicotine and acetylcholine can both bind to the same receptor.
This means nicotine increases the frequency of impulses.
Unlike acetylcholine, nicotine is not removed by hydrolysis so it continues to imitate impulses.
The body may become habituated to nicotine, meaning the nervous system only functions if nicotine is present.
102
What happens is a person has become habituated to nicotine?
The nervous system would only function normally if nicotine is present (drug tolerance)
If it’s not present impulses are not transmitted normally and the unpleasant symptoms associated with withdrawal are experienced.
This can be made worse as another effect of nicotine is the release of neurotransmitter dopamine in the brain, generating pleasurable sensations that they can no longer experience.
103
Describe how drugs which act by preventing the breakdown of neurotransmitter work
Organophosphates inhibit acetylcholinesterase.
In the presence of organophosphates acetylcholine is not hydrolysed and :: remains in the synaptic cleft, causing repeated firing of the post-synaptic neurone.
104
What are organophosphates and give some examples.
Organophosphates are esters of phosphoric acid.
They can be ingested, absorbed or inhaled.
They include: insecticides, herbicides and nerve gasses.
105
Describe how nerve gasses work.
They inhibit acetylcholinesterase at neurotransmitter junctions, generating repeated uncontrollable contractions of muscles.
Where this occurs in antagonistic muscle pairs, it can break bones
106
How are nerve gasses regulated?
The use of nerve gases as biological weapons has been banned by international treaties but they have nether the less been used.
107
Describe how psychoactive drugs work
They act primarily on the CNS
They affect different neurotransmitters or their receptors affecting the firing of neurones altering brain function, mood, consciousness and behaviour.
The changes may be pleasant (e.g euphoria or increased alertness) or may be abused creating damaging life threatening affects.
108
Give some examples of psychoactive drugs
```
Ritalin
Prozac
Paxil
And recreational drugs such as:
Nicotine
Alcohol
Cannabis
Cocaine
Amphetamines
Ecstasy
Heroin.
```
109
Describe how caffeine works as a drug
Caffeine is a stimulant which increases the metabolic rate in pre-synaptic cells. ATP production is increased allowing more neurotransmitter synthesis.
110
Why are mitochondrion featured in axon end bulbs?
Because energy is required to re-form neurotransmitter molecules e.g choline + Acetyl CoA —> CoA + acetylcholine.
Energy is also needed for their exocytosis
Mitochondria generate this energy through the synthesis of ATP
