3.8 the nervous system Flashcards
1
Q
what is the nervous system responsible for?
A
- detecting changes within the internal or external environment (a stimulus)
- processing that information and initiating a repsonse
2
Q
what is the order of stimulus response?
A
- stimulus -> detector -> coordinator -> effector -> response
3
Q
what is a stimulus?
A
- a change in the environment
4
Q
what is a detector?
A
- contains cells which can detect stimuli
- e.g visible light by the retina, sound by the inner ear, temperature by the dermis of the skin
- it converts energy from one form e.g light, into an electrical impulse
5
Q
what is the coordinator?
A
- the central nervous system consisting of the brain and spinal cord
- it coordinates the response
6
Q
what is an effector?
A
- an effector brings about a response
- it is either a muscle or a gland
7
Q
what is the response?
A
- the response is the change in the organism
8
Q
in humans, what does the nervous system consist of?
A
- the central nervous system (CNS)
- the peripheral nervous system
9
Q
what is the peripheral nervous system made up of?
A
- somatic nervous system
- autonomic nervous system
pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS
10
Q
what does the somatic nervous system consist of?
A
- pairs of nerves that originate from the brain and spinal cord
- containing both sensory and motor neurones
- voluntary actions
11
Q
what does the autonomic nervous system do?
A
- controls involuntary actions
- e.g digestion and control of heartbeat
12
Q
what are the 3 types of neurones in humans?
A
- sensory neurones
- relay (or connector) neurones
- motor neurones
13
Q
what do sensory neurones do?
A
- carry impulses from receptors to the CNS
- via the dorsal root
14
Q
what do relay neurones do?
A
- are within the CNS (spinal cord)
- receive impulses from sensory or other relay neurones and transmit them onto motor neurones
15
Q
what do motor neurones do?
A
- transmit impulses from the CNS to effectors (muscles or glands)
- via the ventral root
16
Q
what does a motor neurone contain?
A
- cell body (centron)
- dendrites
- axon
- myelin sheath
- schwann cell
- nodes of Ranvier
- axon endings/terminals
- synaptic end bulbs
17
Q
what is the function of the cell body of a motor neurone?
A
- contains a granular cytoplasm with ribosomes for protein synthesis
- DNA is present within a nucleus and acts as the site for transcription
18
Q
what is the function of the axon of a motor neurone?
A
- carries the impulse away from the cell body
19
Q
what is the function of the myelin sheath of a motor neurone?
A
- surrounds the axon (and dendron in sensory neurones)
- providing electrical insulation resulting in faster impulse transmission
20
Q
what is the function of the schwann cell of a motor neurone?
A
- surround the axon (and dendron in sensory neurones)
- and forms the myelin sheath
21
Q
what is the function of the nodes of ranvier of a motor neurone?
A
- gaps in the myelin sheath between schwann cells are approx 1µm wide where the axon membrane is exposed
- they allow faster nerve impulse conduction
22
Q
what is the function of the axon terminals/endings of a motor neurone?
A
- secrete a neurotransmitter which results in depolarisation of the adjacent neurones
(are branched endings of an axon that approach the muscle fibre)
23
Q
what is the function of the synaptic end bulbs of a motor neurone?
A
- swelling found at the end of an axon where the neurotransmitter is synthesised
- contains synaptic vesicles filled with neurotransmitters
24
Q
what are reflexes?
A
- rapid, automatic responses to stimuli that could prove harmful to the body
- so are protective in nature
25
- in a reflex arc, a stimulus is detected by the _____ and passed to the ______ along a sensory neurone
- the impulse is then relayed directly to a motor neurone and its effector by a relay neurone
- receptor
- CNS
the response is rapid and involves the contraction of a muscle or release of a hormone (in most cases a reflex involves the spinal cord, but some reflexes e.g pupil reflex, will involve the brain as it is the closest part of the CNS)
26
do simple animals e.g Cnidarians like Hydra possess a nervous system?
no
- but they have a simplified nervous system called a nerve net
27
what do nerve nets consist of?
- sensory photoreceptors and touch receptors in the wall of the body and tentacles
- Ganglion cells provide connections between the neurones in several directions but they dont form a brain
- (consists of interconnected nerve cells with short extensions allowing a response to a limited number of stimuli)
28
compare a Cnidarian nerve net to a mammalian nervous system:
CNIDARIAN NERVE NET:
- 1 type of simple neurone
- unmyelinated
- short, branched neurones
- impulses transmitted in all directions
- slow impulse transmission
- respond to a limited number of stimuli
MAMMALIAN NERVOUS SYSTEM:
- 3 types of neurones (sensory, relay and motor)
- myelinated
- long, unbranched neurones
- impulses transmitted in one direction
- fast impulse transmission
29
resting potential of a neurone definition
- potential difference across a neurone membrane when a nerve impulse is not being transmitted
- is around -70mV
- the membrane is polarised
30
when a neurone is at rest i.e no impulses are being transmitted, it is said to be at ____ ___
resting potential
31
what is the charge across the axon membrane when at rest?
around -70 mV
32
action potential definition
- the temporary change in electrical potential across the membrane of an axon as a nerve impulse passes
33
action potential steps:
- at resting potential the charge across the membrane is -70mV
- energy of stimulus arriving causing Na+ voltage-gated channels to open and Na+ ions flood in down their concentration gradient, depolarising the neurone. now the charge across the membrane becomes more positive due to more positive charges inside
- as more Na+ ions enter, more gates open so even more Na+ ions rush in (positive feedback)
- when potential reaches +40mV the neurone is depolarised. Na+ gates close preventing further influx of Na+ ions. K+ gates then begin to open
- K+ ions flood out of the neurone down their concentration gradient lowering the positive (+) gradient across the membrane. as a result, further K+ channels open, resulting in even more K+ ions leaving the neurone. the neurone becomes repolarised
- too many K+ ions leave the neurone so the electrical gradient overshoots -70mV reaching around -80mV (which is called hyperpolarisation). to re-establish the resting potential (-70mV) K+ gates now close and the Na+/K+ pump re-establishes the resting potential
34
Saltatory propagation:
- in myelinated neurons, ions can only move across the membrane at the nodes of Ranvier where there is no myelin present, therefore local circuits are established over greater distances (between each node)
- depolatisation only occurs at the nodes and the action potential effectively ‘jumps’ from node to node, increasing the speed of impulse transmission
35
is it the impulse that ‘jumps’ from node to node?
NO
- it’s the action potential
36
what is the total refractory period?
- represents the period during which it is not normally possible to send another impulse
- lasts for approx 6ms
37
what is the absolute refractory period?
- the period during which it is NOT possible to send another impulse, irrespective of how big the stimulus is
38
what is the relative refractory period?
- the period during which it is possible to send another impulse, if the stimulus is big enough to overcome the threshold
39
what is the all or nothing law/rule?
- impulses either pass or do not, and they are always the same size
(- impulses will pass if a threshold value is exceeded (usually -55mV).
- a large stimulus will result in more impulses passing per second (increased frequency of action potentials) rather than a greater level of depolarisation)
40
will a large stimulus result in more impulses passing per second (increased frequency of action potentials) or a greater level of depolarisation?
more impulses passing per second (increased frequency of action potential)
41
what are some factors affecting the speed of impulse transmission?
- myelination
- diameter of axon
- temperature
42
how does myelination affect the speed of impulse transmission?
- saltatory conduction is faster than impulse transmission in unmyelinated neurones, as depolarisation only occurs at the nodes of Ranvier so the action potential effectively ‘jumps’ from node to node
- the rate of transmission varies from 1m/s in unmyelinated neurones to 100m/s in myelinated ones
43
how does diameter of axon affect the speed of impulse transmission?
- impulse transmission speed increases with axon diameter due to less leakage of ions from larger axons (due to a larger volume to surface area)
- (the wider the diameter of an axon the less resistance there is so the action potential travels faster if axon diameter is increased)
44
how does temperature affect the speed of impulse transmission?
- impulse transmission speed increases with temperature because the rate of diffusion increases due to the increased kinetic energy of ions involved, but only in organisms which do not control their internal body temperature (some ectotherms)
- (diffusion happens faster)
45
what is the synapse?
- a 20nm gap between two nerve cells or a nerve cell and an effector
46
how is an impulse transmitted from one neurone to the other?
- by a neurotransmitter
- which diffuses across the synaptic cleft from the pre-synaptic membrane to receptors on the post-synaptic neurone, triggering depolarisation in the post-synaptic neurone
47
what is an example of a neurotransmitter?
- acetylcholine
- used bu the parasympathetic nervous system
48
what are some functions that synapses have?
- transmit information between neurones
- transmit information in one direction only
- act as junctions
- filter out low-level stimuli
- prevent over stimulation of neurone and fatigue
- new impulses can be initiated in several different neurones for multiple simultaneous responses
49
synaptic transmission steps:
- an impulse arrives at the pre-synaptic knob
- calcium channels open, causing calcium ions to diffuse rapidly into the pre-synaptic knob
- the vesicles containing the neurotransmitter acetylcholine migrate to and fuse with the pre-synaptic membrane
- contents of the vesicles are released into the synaptic cleft by exocytosis
- acetylcholine molecules diffuse across the cleft and bind to receptors on the post-synaptic membrane causing sodium channels to open
- Na+ ions rush into the post-synaptic neurone resulting in depolarisation of the post-synaptic membrane. an action potential is initiated
- (acetyl) cholinesterase splits the acetylcholine into ethanoic acid and choline, releasing them from the receptor, and sodium channels close. the products diffuse back across the cleft
- products are absorbed into the pre-synaptic knob
- ATP is used to reform acetylcholine in the pre-synaptic knob
50
what is the repeated depolarisation of the post-synaptic neurone/merging of impulses prevented by?
- the hydrolysis of acetylcholine/ role of cholinesterase
- reabsorption of neurotransmitters/ ethanoic acid and choline back into the synaptic knob via transport proteins
- active transport of calcium ions out of the pre-synaptic knob, which prevents further exocytosis of neurotransmitter
51
what happens if insufficient acetylcholine is released?
- not enough sodium channels open on the post-synaptic membrane to exceed the threshold potential (of roughly -55mV) so an action potential is not initiated
52
what are the 2 main types of effects drugs have?
- excitatory (stimulants or agonists) e.g caffeine, cocaine, amphetamine which result in more action potentials/release of neurotransmitters such as noradrenaline
- sedatives (inhibitory) like cannabis which result in fewer action potentials
53
what effect does organophosphorus have on the transmission of impulses?
- by inhibiting cholinesterase
- preventing the hydrolysis of acetylcholine in the postsynaptic neurone
- results in continuous stimulation of the neurone
54
why is nicotine addictive?
- many drugs e.g nicotine, mimic the action of neurotransmitters
- but unlike acetylcholine, nicotine is not removed by the action of cholinesterase
- over time, the body produces less acetylcholine, and the person becomes more reliant on nicotine for the normal functioning of the synapse
- nicotine also causes the increases release of dopamine in the brain, resulting in additiction
55
what does the central nervous system consist of?
- brain and spinal cord
56
what is the dorsal root?
- 1 of 2 roots that emerges from the spinal cord
- travels to the dorsal root ganglion
- sensory neurones enter the spinal cord via the dorsal root
57
what is the ventral root?
- 1 of 2 roots that emerge from the spinal cord
- motor neurones leave the spinal cord via the ventral root
58
outline a simple reflex arc:
stimulus —> receptor —> sensory neurone —> relay neurone (in CNS) —> motor neurone —> effector —> response
59
how does a reaction differ from a reflex?
- reaction is voluntary and coordinated by the brain
- reflex is non-voluntary and does not involve the brain
60
describe a simplified structure of a motor neurone:
- short dendrites carry impulses from CNS to cell body
- cell body found at one end of neurone
- long axon carries impulses from cell body to effectors
61
what is the function of dendrites?
- short, branched extensions of the cell body
- receive nerve impulses from other neurones
62
describe the cell body of a motor neurone:
- the region of the neurone that contains the organelles, notably the nucleus and rough endoplasmic reticulum
63
describe the additional features of a myelinated neurone:
- schwann cells : wrap around axon, involved in electrical insulation, phagocytosis, nerve regeneration
- myelin sheath: made from myelin-rich membranes of schwann cells
- nodes of ranvier: small gaps between neighbouring schwann cells where there is no myelin sheath
64
how is resting potential established?
- membrane more permeable to K+ than Na+
- sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
- organic phosphates and large protein anions remain in cytoplasm
- establishes electrochemical gradient: cell contents more negative than extracellular environment
65
name the stages of an action potential
- depolarisation
- repolarisation
- hyperpolarisation
- return to resting potential
66
what happens during depolarisation?
- stimulus causes a change in the voltage across an axon membrane, opening voltage-gates Na+ channels
- Na+ diffuse into the axon
- potential difference across membrane becomes more positive, membrane depolarises
67
what happens during repolarisation?
- membrane potential reaches +40mV
- voltage-gated Na+ channels close and voltage-gates K+ channels open
- facilitated diffusion of K+ ions out of cell down their electrochemical gradient
- potential difference across membrane becomes more negative, membrane repolarises
68
what happens during hyperpolarisation?
- ‘overshoot’ when K+ ions diffuse out
- potential difference becomes more negative than resting potential
- membrane hyperpolarises, preventing another impulse occuring
69
why is the refractory period important?
- ensures that action potentials can only be propagated in one direction
70
explain why myelinated axons conduct impulses faster than unmyelinated axons
- saltatory propagation:
- impulse ‘jumps’ from one node of ranvier to another (depolarisation cannot occur where myelin sheath acts as electrical insulator)
- therefore, impulses dont travel along the whole axon length
71
describe the structure of a synapse
- presynaptic neurone ends in synaptic knob
- synaptic knob contains a high concentration of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter
- synaptic cleft, 20-30nm gap
- postsynaptic neurone has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
72
what is the synaptic cleft?
- a small gap between neurones across which a nerve impulse is transmitted via neurotransmitters
73
describe synaptic transmission in the presynaptic neurone:
- wave of depolarisation travels down presynaptic neurone, causing voltage-gates Ca2+ channels to open
- Ca2+ cause vesicles of acetylcholine to move towards and fuse with presynaptic membrane
- exocytosis of neurotransmitter (e.g acetylcholine) into synaptic cleft
74
how do neurotransmitter cross the synaptic cleft?
via simple diffusion
75
describe synaptic transmission in the postsynaptic neurone:
- acetylcholine diffuses across synaptic cleft and binds to specific receptors on postsynaptic membrane
- ligand-gates Na+ channels open
- if influx of Na+ ions raises membrane to threshold potential, action potential is generated
76
what is the role of cholinesterase in synaptic transmission?
- hydrolyses acetylcholine in the postsynaptic neurone
- products diffuse back across the cleft
77
where are organophosphates commonly found?
- component of insecticides
78
a diagram shows a transverse section of the spinal cord and an associated reflex arc.
there is white and grey matter.
explain why there is a difference in colour in these two areas [1]
- grey matter contains the (darkly staining) cell bodies/nucleus of neurones
- whilst the white matter is mainly (axons/myelin)
79
what is the function of the myelin sheath in the conduction of an action potential? [2]
- insulates the axon
- allows saltatory conduction/impulse jumps from node to node
- so speeding up the transmission of the action potential / increase speed of conduction
80
multiple sclerosis is a progressive, degenerative disease of the nervous system in which the myelin sheath is destroyed. symptoms include muscle weakness and loss of vision.
suggest a possible medical treatment for multiple sclerosis [1]
- remyelinate the axon / stem cells / make the membrane add Na+ channels in bare areas / prevent further demyelination / immune (suppressants/inhibitory) drugs
81
what is an example of an effector?
(a muscle or a gland)
- e.g endocrine gland
- endocrine responses are slower but longer lived than nervous responses
- endocrine responses involve hormones being released into the blood stream
- the hormones circulate around the body in the blood and affect target organs
82
- the spinal cord runs down the middle of the vertebrate
- it is surrounded by three membranes collectively called the meninges
- the outside of the spinal cord is made of ‘white matter’ - this contains myelinated neurons
- down the middle of the spinal cord is a central canal filled with cerebro-spinal fluid
- on each side of the spinal cord the spinal nerves exit and enter
- on the dorsal side (the back) is where the sensory neurones enter (this is called the dorsal root); it is characterised by a swelling called the dorsal root ganglion
- the cell bodies of the sensory neurons are found in the dorsal root ganglion
- on the ventral side (front) the motor neurones are found in the ventral root
- motor neurons have their cell bodies in the grey matter of the spinal cord
- sensory and motor neurons are connected by relay neurons in the grey matter
- relay neurones have synapses to sensory and motor neurons
83
what is the spinal cord surrounded by?
- three membranes collectively called the meninges
84
what is the outside of the spinal cord made of?
- white matter
85
what does white matter contain?
- myelinated neurones
86
what is found closer to the center of the spinal cord?
- grey matter
87
what does grey matter contain?
- many nuclei, cell bodies and non-myelinated neurones
88
what is found down the middle of the spinal cord?
- central canal
- filled with cerebro-spinal fluid
89
on each side of the spinal cord the spinal nerves enter and exit
90
where do the sensory neurones enter?
- on the dorsal side (the back)
- called the dorsal root
91
what is the dorsal root characterised by?
- a swelling called the dorsal root ganglion
92
what is found in the dorsal root ganglion?
- the cell bodies of the sensory neurones
93
on what side are motor neurones found?
- on the ventral side (front)
- are found in the ventral root
94
where do motor neurones have their cell bodies?
in the grey matter of the spinal cord
95
what are sensory and motor neurones connected by?
- relay neurones in the grey matter
- (relay neurones have synapses to sensory and motor neurones)
96
how can a nerve impulse be measured?
- using micro-electrodes placed in the axoplasm and in the fluid outside
- a potential difference is recorded and displayed graphically on an oscilloscope
97
a sodium potassium pump uses ATP to actively transport three Na+ out of the neurone and two K+ into the neurone
- potassium ion channels are leaky and potassium ions pass out of the axoplasm by facilitated diffusion
- the sodium ion channels are closed so sodium ions remain outside of the neurone
- large proteins and organic phosphates in the cytoplasm are negatively charged
98
what does saltatory conduction result in?
- an increased speed of transmission of action potentials
99
- another advantage of saltatory conduction is that the sodium potassium pumps only operate at the nodes, so less ATP is used in myelinated neurones
- the voltage-gated channels are only found at the nodes of Ranvier so only these regions are depolarised
100
why is a lot of mitochondria found in the synaptic knob?
- calcium ions stimulate the vesicles of neurotransmitters to move to the presynaptic membrane and fuse to it
- the neurotransmitters are released into the synaptic cleft by exocytosis
- which requires ATP
(the neurotransmitters than diffuse over the synaptic cleft)
101
what are some ways in which antagonists may work?
- antagonists (sedative drugs) affect synapses by preventing post-synaptic depolarisations
may work by =
- preventing calcium ion channels opening/Ca2+ being released
- preventing exocytosis
- blocking receptors
- causing hyperpolarisation of post-synaptic membrane (so its harder to reach threshold)
- raising threshold
- changes shape of neurotransmitters
102
what are some ways in which agonists may work?
- agonists (excitatory drugs) cause more post-synaptic depolarisation
may work by =
- inhibiting the enzyme (like acetylcholinesterase) that break down neurotransmitter
- mimicking the effect of neurotransmitters by binding to the receptors
- stimulate release of transmitters/causing more exocytosis
- reduce threshold for excitation of post synaptic membranes
103
describe how the resting potential is maintained in the neurone [3]
- sodium/potassium pumps
- ATP/active transport
- 3Na+ out 2K+ in
- organic anions, -ve charged molecules/proteins
- potential difference across membrane -70mV
- membrane leakage, more permeable to K+ than Na+
104
suggest how cardiac muscle fibres would respond when the voltage across the membrane rises?
contraction
105
the cardiac muscle fibres used to obtain an oscilloscope trace were obtained from a frog’s heart. consider the ethics of killing a frog to obtain cardiac muscle fibres by giving one argument in favour and one against [2]
- frog has right to life / (suffering/pain/distress/harm) of frog
- benefits to MEDICINE/HEALTH OF HEART research
106
an action potential is illustrated on a graph. has y axis of membrane potential and x axis of time. what are the units?
- membrane potential = mV / milliVolts
- time = milliseconds
107
explain why a stimulus failed to initiate an action potential [2]
- failed to reach threshold potential
- all or nothing response
- didnt reach -55mV
- so (too few) sodium gates opened / not enough depolarisation
108
when a person touches a hot object with their finger they will move away from the heat. state the advantage to humans of having this type of response to an external stimulus [1]
- protective / prevents (damage/injury/harm)
109
acetyl cholinesterase is one of the many enzymes needed for the functioning of the nervous system of vertebrates and insects. certain chemical classes of pesticides, such as organophosphates, work against pests by inhibiting cholinesterase. while the effects of cholinersterase-inhibiting products are intended for insect pests, these chemicals can also be toxic to humans. in severe cases of exposure to organophosphates, symptoms include uncontrollable muscular tremors, breathing difficulty and possible death.
explain the toxic effects of the organophosphates [5]
- acetylcholine is normally hydrolysed/broken down by acetyl cholinesterase
- organophosphates prevent the breakdown of acetylcholine
- so acetylcholine remains bound to receptors on post-synaptic membrane
- so (synaptic transmission/action potentials) continue to be generated
- sustained contraction of muscle
- uncontrolled contractions of (diaphragm/intercostal muscles) interferes with breathing
110
give two reasons that would prevent a nerve impulse travelling in the opposite direction [2]
- refractory period / hyperpolarisation
- synaptic vesicles only on presynaptic membrane side of synapse
111
what is a reflex action? [2]
- (rapid) reaction / response to a stimulus
- automatic / involuntary / brain not involved
112
explain why a myelinated axon uses less ATP to transmit a nerve impulse than a non-myelinated axon of the same diameter [2]
- ATP is required for active transport / ref Na/K pumps
- Na+ ions (actively) moved out only at nodes in myelinated
- Na+ ions (actively) moved out along whole length of axon in non-myelinated
113
name the main class of biological molecule found in myelin [1]
lipid/fat
114
suggest two symptoms which could occur in someone who has demyelinated neurones [2]
- person would feel tired/exhaustion/feel weaker
- lack of feeling
- (longer/sower) reaction times
- paralysis/no response/loss of muscle control
115
lidocaine is a local anaesthetic used by dentists to numb pain. it is believed that it blocks the sodium ion channels in a neurone membrane.
explain how lidocaine acts as a local anaesthetic [4]
- (sodium IONS/Na+) can no longer enter/fewer sodium ions enter (the axon)
- threshold is not reached
- there is no action potential/no depolarisation
- along SENSORY neurone (to brain)
116
multiple sclerosis is caused by the immune system destroying the myelin sheath of neurones. explain why this condition leads to a slowing down of the transmission of a nerve impulse [3]
- myelin sheath electrical insulation
- ion exchange only at nodes of Ranvier / depolarisation only at nodes
- action potential jumps from one node of Ranvier to next
- saltatory conduction
- if no myelin sheath local circuits
- saltatory conduction much faster than local circuits
