Topic 8 - Grey Matter Flashcards
1
Q
What is the role of receptor cells?
A
Detect stimuli in the internal and external environment
Send signals via nervous system or hormonal system to body’s coordination centres
2
Q
What is the role of effectors?
A
Parts of the body which respond
Muscles or glands
3
Q
What are the 2 parts of the human nervous system?
A
- Central Nervous system: brain and spinal cord
- Peripheral nervous system: all of the nerves in our body
4
Q
How is information sent through the nervous system?
A
In the form of electrical impulses that pass along nerve cells known as neurones
5
Q
What is the pathway that nerve impulses pass through the nervous system along?
A
Stimulus → receptor → sensory neurone → CNS → motor neurone → effector
6
Q
What are hormones?
A
Chemical substances produced by endocrine glands and carried by the blood - ‘chemical messengers’
7
Q
What is the role of hormones?
A
Chemical messengers which transmit information from one part of an organism to another, and bring about change by altering the activity of one or more specific target organs
8
Q
What is the pathway of hormone action?
A
Stimulus → receptor → hormone → effector
9
Q
How are the nervous and endocrine system different?
A
Nerves = electrical impulse, endocrine = chemical hormone
Nerves = fast transmission, endocrine = much slower
Nerves = shorter effect, endocrine = longer effect
10
Q
What is a neurone?
A
A specialised cell of the nervous system which carries electrical impulses around the body - a bundle of neurones is known as a nerve
11
Q
What features are found in all neurones?
A
- long fibre known as an axon
- cell body containing nucleus and other cellular structures
- end of axon = axon terminal with many nerve endings
12
Q
What is a myelin sheath, what is it made up of?
A
Fatty layer which insulates an axon.
Made up of specialised cells known as Schwann cells, which wrap themselves around the axon. There are uninsulated gaps between Schwann cells known as the nodes of Ranvier
13
Q
What are the 3 main types of neurones and their roles?
A
- sensory: carry impulses from receptors to the brain and spinal cord in the CNS
- relay: found entirely within the CNS and connect sensory and motor neurones
- motor: carry impulses from CNS to effector muscles or glands
14
Q
What is the structure of a motor neurone?
A
Large cell body at one end that lies within the spinal cord or brain
Many highly branched dendrites extending from cell body, providing many connections with axon terminals of other neurones
15
Q
What is the structure of relay neurones?
A
Short neurones with axons and highly branched dendrites
16
Q
What is the structure of sensory neurones?
A
A cell body that branches off in the middle of the axon and has no dendrites. Axon terminal attached to a receptor cell.
Section linking axon terminal with cell body = dendron
Section of neurone connecting cell body with CNS = axon
17
Q
What do the circular and radial muscles do to pupil size?
A
Circular muscles contract to CONSTRICT pupil
Radial muscles contract to DILATE pupil
Antagonistic pair
18
Q
What events happen to change eye muscles in the presence of bright light?
A
Bright light → light receptors in eyes → sensory neurone → CNS → motor neurone → circular muscles in iris (constrict pupil)
19
Q
Why does the pupil constrict in bright light?
A
Contraction of circular muscles
Limits amount of light entering eye to prevent damage to the retina
20
Q
What events happen to change eye muscles in dim light?
A
Low light → light receptors in eyes → sensory neurone → CNS → motor neurone → radial muscles in iris (dilate pupil)
21
Q
Why does the pupil dilate in dim light?
A
Contraction of radial muscles
Maximises amount of light entering the eye to improve vision
22
Q
What is resting potential?
A
The difference in charge between the inside and outside of the neurone due to different numbers on either side of the neurone membrane (at resting) = -70mV
23
Q
How is resting potential established and maintained?
A
- active transport of sodium and potassium ions
- difference in membrane permeability to sodium and potassium ions
24
Q
How does the sodium potassium pump work?
A
Carrier protein which uses ATP to actively transport 3 sodium ions out of the cell and 2 potassium ions into the cell. This creates a concentration gradient of ions.
25
How does difference in membrane permeability maintain the resting potential?
Neurone membrane is less permeable to sodium ions than potassium ions. Due to conc gradient, potassium ions diffuse out of the cell via potassium channels, whilst sodium ions diffuse back in via sodium channels but at a slower rate.
26
What is an action potential?
The potential electrical difference produced across the axon membrane when a neurone is stimulated
27
What are the sequence of events which occur when a neurone is stimulated?
DEPOLARISATION
- small number of Na+ ion channels open and therefore sodium ions begin to move into the axon
- reduces potential difference, and if it reaches -55mV (threshold) more sodium ion channels open (voltage gated ones), leading to a further influx of sodium ions which reverses charge to around +30mV
REPOLARISATION
- 1ms after action potential generated, all voltage gated sodium channels shut
- voltage gated potassium channels open, allowing diffusion of potassium out of the axon
- causes axon to become negatively charged again (hyperpolarisation = membrane potential slightly more negative than resting potential)
RESTING
- voltage gated K+ channels shut and sodium potassium pump works to restore resting potential
28
What is the refractory period?
When the membrane is hyperpolarised, and so is unresponsive to stimulation (new action potential cannot be generated)
29
What is the significance of the refractory period?
- makes action potentials discrete events
- impulse can only travel in one direction
30
How is an action potential transmitted?
Depolarisation of membrane at site of first action potential causes sodium ions to diffuse along the cytoplasm into next section of axon - this depolarises the membrane in the new section, triggering a new action potential. This process then repeats on the length of the axon (WAVE OF DEPOLARISATION)
31
What is the all or nothing principle?
An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
32
What is the speed of conduction like in unmyelinated neurones?
Relatively slow as depolarisation has to occur along the whole membrane of the axon.
33
How does myelin insulation increase the speed of conduction in a neurone?
Sections of membrane surrounded by a myelin sheath prevent the diffusion of sodium and potassium ions, meaning depolarisation cannot occur.
Action potentials can only occur at nodes of Ranvier, so the action potential jumps from one node to the next - SALTATORY CONDUCTION
34
How can medications prevent impulse transmission?
They may bind to sodium channels and prevent them from opening, preventing an influx of sodium ions. This prevents membrane depolarisation and an action potential cannot be generated - no impulse sent to CNS
35
What are synapses?
Junctions between cells in the nervous system
36
How are nerve impulses transmitted across a synapse?
1. action potential arrives at end of presynaptic neurone, depolarising the membrane and causing voltage gated Ca2+ ion channels to open
2. calcium ions diffuse into synaptic knob, causing vesicles to move towards presynaptic membrane where they fuse and release neurotransmitters via exocytosis
3. neurotransmitters diffuse across the synaptic cleft and bind with receptor molecules on postsynaptic membrane
4. this causes sodium ions channels to open, generating an action potential on the post synaptic membrane
5. neurotransmitters are then broken down to prevent continued stimulation of postsynaptic neurone
37
What is acetylcholine, how is it broken down?
A common neurotransmitter
Broken down by the enzyme acetylcholinesterase
38
What are the roles of a synapse?
They enable:
- unidirectionality of impulse transmission
- divergence of nerve impulses
- amplification of nerve impulses by summation
39
Why do nerve impulses arriving at a synapse not always cause an impulse to be generated in the next neurone?
It may be insufficient to generate an action potential in the post synaptic membrane
- only a small amount of acetylcholine may be released, and therefore only a small number of sodium ion channels open and an insufficient number of sodium ions pass through membrane (threshold potential not reached)
40
What is spatial summation?
Multiple presynaptic neurons release a neurotransmitter (inputs from multiple neurons needed to generate an action potential)
41
What is temporal summation?
Where one presynaptic neuron releases neurotransmitters several times in quick succession
42
What is the structure of a synapse?
The presynaptic neurone ends in a synaptic knob (contains lots of mitochondria and endoplasmic reticulum as well as vesicles)
Synaptic cleft = gap between 2 neurons
The postsynaptic neurone has many complementary receptors to neurotransmitter
43
What is the difference between an inhibitory and excitatory synapse?
Signals sent across excitatory synapses increase the activity of the receiving neuron, while signals sent across inhibitory synapses reduce neuron activity.
44
What happens in an inhibitory synapse?
Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane, triggering K+ to open. Cl- in, K+ out - hyperpolarisation, meaning no action potential generated
45
How does light enter the eye?
Enters via the pupil and is focused onto a region of the retina called the fovea. Amount of light entering the eye is controlled by the muscles of the iris.
46
How is light focused in the eye?
Using the lens, shape of which is controlled by ciliary muscles attached to lens by suspensory ligaments
47
What are the 2 types of photoreceptors in the eye?
- rod cells
- cone cells
48
What are the differences between rods and cones?
Rods: primarily located around outer retina, sensitive to light intensity (detect presence and brightness), only black and white images, low acuity
Cones: found grouped together in the fovea, sensitive to different wavelengths of light (can be red green or blue), colour images, high acuity
49
How are signals from photoreceptors sent to the brain?
Action potentials in photoreceptors transmitted via the optic nerve to the brain
50
What happens to rod cells in the dark?
- cation channels are open, sodium ions can therefore diffuse into the cell
- sodium ions pumped out of cell via Na+/K+ pump
- membrane therefore becomes slightly depolarised
- this causes Ca2+ to flow into the cell, causing the release of the neurotransmitter glutamate
- glutamate inhibits the generation of an action potential, no image is sent
51
What happens to rod cells in the light?
- light bleaches rhodopsin into retinal and opsin
- opsin triggers a series of reactions at the cell membrane which lead to the closing of the cation channels, preventing Na+ from flowing in
- sodium ions still pumped out by Na+/K+ pump, causes rod cell to become hyperpolarised
- stops releasing inhibitory neurotransmitter, an action potential can therefore be generated at the bipolar neurone and an impulse is sent to the optic nerve
52
What is phototropism?
A plant growth response to light
53
What is geotropism?
A plant growth response to gravity
54
What is the difference between positive and negative tropisms?
Positive tropism = growth towards a stimulus
Negative tropism = growth away from a stimulus
55
What are plant growth factors?
Sometimes referred to as plant hormones, these are chemical messengers which regulate cell growth in response to a directional stimulus
56
What is IAA?
Indoleacetic acid, a type of auxin
57
How does IAA bring about plant responses?
Brings about responses such as phototropism by altering the transcription of genes within plant cells. This then changes the expressions of genes that code for proteins involved with cell growth
58
How is IAA transported?
Moved from cell to cell by diffusion and active transport
Transport of IAA over longer distances occurs in the phloem
59
How does IAA in plant shoots cause phototropism?
In shoots, higher IAA = higher rate of cell elongation.
When light shines on a stem from one side, IAA is transported to the shaded side, establishing an IAA gradient. Higher concentration of auxin on the shaded side causes a faster rate of cell elongation, so the shoot bends towards the source of light.
60
How does IAA in plant roots cause geotropism?
In roots, higher IAA = lower rate of cell elongation.
IAA is transported towards the lower side of plant roots, where it inhibits cell elongation. Therefore the lower side grows at a slower rate than the upper side of the root, causing it to bend downwards.
61
What is phytochrome?
A pigment that is a type of photoreceptor in plants
62
What are the 2 forms of phytochrome?
PR - inactive form, absorbs light from red part of spectrum
PFR - active form, absorbs light from the far red part of the spectrum
63
How does PR convert into PFR and vice versa?
When PR absorbs red light, it is converted into PFR.
When PFR absorbs far red light, it is converted back into PR.
In the absence of red light, unstable PFR converts back into PR
64
How do levels of phytochrome change throughout the day?
During the day, levels of PFR rises as there is conversion of PR to PFR
During the night, levels of PR rise as PFR slowly converts back to PR
65
How do levels of PFR affect flowering in long and short day plants?
In long day plants, PFR is needed to stimulate flowering
In short day plants, PFR inhibits flowering
66
How does PFR change flowering in long day plants?
High levels of PFR activates expression of genes that stimulate flowering, active gene is transcribed and translated and the resulting protein causes flowers to be produced rather than stems and leaves
67
What is photoperiod?
The relative length of day and night, the environmental cue that determines time of flowering. The ratio of PFR to PR enables determination of day and night
68
What effect does length of the night have on PFR to PR ratio?
Long winter nights give ample time for PFR to convert back to PR. Summer nights may not be long enough to do so, so some PFR may still be present in the morning
69
What are the benefits of plants being able to respond to changes in day length?
Flowering can happen at the right times (e.g. when insects available or seeds germinate at the right time) as day length varies throughout the year due to seasonality.
70
What are the main regions in the brain?
Cerebral hemispheres, hypothalamus,
cerebellum and medulla oblongata
71
Where is the hypothalamus located, what is it's function?
Located below thalamus and above pituitary gland.
Main control centre for autonomic nervous system, monitors blood flowing through brain and in response releases hormones or stimulates neighbouring pituitary gland.
Plays an important role in homeostatic mechanisms: regulating body temp, osmoregulation, digestive activity, endocrine functions
72
Where is the cerebellum located, what is it's function?
Located above spinal cord at the back of the brain
Coordination centre for movement, posture and balance (does not initiate, just coordinates)
73
Where is the medulla oblongata located, what is it's function?
Located at the base of the brain
Contains co-ordination centres that control different functions (cardiac, respiratory, thermoregulatory centres). Unconscious and reflex actions
74
What is the structure of the cerebrum?
2 halves known as the cerebral hemispheres. Joined together by a band of nerve fibres known as corpus callosum.
Has a thin outer layer called the cerebral cortex, or 'grey matter', which consists of cell bodies of neurones. Beneath the cerebral cortex is the 'white matter' which consists of the myelinated axons of neurones
75
Which sides of the body do the cerebral hemispheres control?
Right hemisphere controls left side of body, left hemisphere controls right side of body
76
What is the role of the cerebrum?
Responsible for controlling all conscious activities (vision, hearing, speech, memory). Passes information from receptor cells through different areas in the brain, which then send signals to motor cells.
77
How is the cerebrum's structure related to it's function?
Has many folds which increases surface area so a greater number of neurones can be contained. This means more neurone connections and a greater ability for the brain to carry out more complex behaviours.
78
What is a CT scan, what are it's pros/cons?
CT = computerised tomography.
Produces cross section images of the brain using x-ray radiation.
Pros: shows physical structures of brain and visualisation of any tissue damage
Cons: doesn't directly show function (has to be linked to symptoms), higher levels of X ray radiation than normal scans (pregnant women or children cannot have one)
79
What is an MRI, what are the pros and cons?
MRI = magnetic resonance imaging
Uses a combination of magnetic field and radio waves to generate images through the body
Pros: produces higher resolution images than CT, useful for identifying areas of abnormal or damaged tissue, less risk than CT as no x ray
Cons: much more expensive than CT, doesn't directly show function, magnetic field can interfere with medical devices e.g. pacemakers
80
What is an fMRI, what are it's pros and cons?
fMRI = functional MRI
Similar to MRI but allows brain function to be studied in real time. Shows location of oxygenated blood in brain to show which brain regions are active at any one time
Pros = can investigate function, can be used in medical diagnosis
Cons = expensive
81
What is a PET scan, what are it's pros and cons?
PET = positron emission tomography
Uses radioactive tracers which collect in areas where there is increased blood flow, metabolism or neurotransmitter activity - then examined on scanner to indicate which regions of the brain are active/inactive
Pros = helps build understanding of specific diseases like Alzheimer's, neurologists can use to study structure and function of brain in real time
Cons = pregnant women cannot have them due to danger of radioactive tracers to baby, can give false results in diabetics
82
What is the visual cortex?
The region of the cerebral cortex in which visual information is processed
83
What is the critical period?
Time period within which the eyes must be visually stimulated in order for neurones in the visual cortex to be organised properly in the period of early development
84
What happens during the critical period?
Synapses that receive visual stimulation and pass on action potentials into the visual cortex are retained and strengthened. Synapses that do not receive nerve impulses are lost and cannot be reformed/
85
Which part of the visual cortex is genetically determined?
The formation of columns in the visual cortex (ocular dominance columns) which are there before the critical period for the development of vision
86
How was evidence gathered for the critical period of the development of the visual cortex?
Hubel and Wiesel study using animal models (depriving various animal models of vision in one eye to learn about visual cortex development)
87
What is the role of visual stimulation in the development of the visual cortex?
- ocular dominance columns develop in the visual cortex
- neurones form synapses with these
- stimuli/action potentials along neurones required to strengthen connections with ocular columns
- stimulation during critical period needed to form effective connections in visual cortex
88
What are ocular dominance columns?
Groups of neurones in the visual cortex which respond to input only from one eye or the other
89
How did Hubel and Wiesel investigate and prove the significance of the critical period?
Used kittens as animal models
- stitched one eye shut
- after 3 months, found kittens blind in eye that was stitched shut
- also studied activity of neurones in ocular dominance columns (sewed up eye = smaller than normal, open eye = larger than normal), concluded that ocular dominance columns can redistribute
Research repeated using adult cat models, found no blindness resulted from 3 month vision loss, no changes to columns. Replicated results in young and adult monkeys - demonstrating essential nature of light stimulation in critical period
90
What are the arguments for animal research?
- new drugs need to be tested on whole organism, not just groups of cells
- animals more acceptable alternative than testing on humans before we know drugs are safe
- animal research is only carried out when no other alternatives, governed by strict welfare regulations
- animal anatomy often similar to humans so results can be transferrable
91
What are the arguments against animal research?
- many believe animals should have same rights as humans when it comes to welfare and consent
- animals may be killed to gain tissue samples, despite regulations, many animals still suffer distress and pain
- animal and human anatomy and physiology is not identical, medications can have different effects
92
What is Parkinson's?
A brain disorder that affects the co-ordination of movement, caused by the loss of neurones in some parts of the brain
93
What are the symptoms of Parkinson's?
- tremors to specific parts of the body
- slow movement
- stiff and inflexible muscles
- difficulties with balance
- changes to speech
94
What is the effect of insufficient amounts of dopamine producing neurones for those with Parkinson's?
Dopamine is a neurotransmitter involved in muscle control
- less dopamine released into synaptic cleft, less able to bind with receptors on post synaptic membrane
- fewer sodium channels on membrane are opened, depolarisation of post synaptic neurone does not occur
- fewer action potentials, creating symptoms such as tremors and slow movement
95
What are the drugs that can be used to treat Parkinson's?
Dopamine agonists = bind to and activate dopamine receptors on postsynaptic membrane
Dopamine precursors = chemicals that can be converted into dopamine in the neurones (e.g. L dopa)
Enzyme inhibitors = inhibit activity of enzymes that would normally break down dopamine in synaptic cleft, raising levels of dopamine present in the brain (e.g. MAOB inhibitors)
96
What are the future possible treatments for Parkinson's?
Gene therapy = involve addition of genes to affected cells in brain to increase dopamine production or prevent destruction of dopamine producing cells
Stem cell therapy = stem cells used to replace lost dopamine producing cells in the brain
97
What are the possible causes of depression?
- low levels of neurotransmitter serotonin (transmit nerve impulses through areas of brain controlling mood)
- other brain chemicals linked to depression = noradrenaline and dopamine
98
What is the general action of antidepressants to work against depression?
Increase levels of relevant neurotransmitters in the brain
e.g. preventing uptake of serotonin at synapses to increase overall levels (selective serotonin reuptake inhibitors), inhibit enzymes breaking down neurotransmitters in synaptic cleft
99
How can drugs increase transmission of impulses at a synapse?
- causing more NT to be produced in synaptic knob
- causing more NT to be released at presynaptic membrane
- imitating effect of NT by binding to and activating receptors on postsynaptic membrane
- preventing breakdown of NT by enzymes
- preventing reuptake of NT by presynaptic cell
100
How can drugs decrease the transmission of impulses at a synapse?
- preventing production of NT in presynaptic knob
- preventing release of NT at presynaptic membrane
- enabling NT to gradually leak out of presynaptic knob so little left when action potential arrives
- binding to receptors on postsynaptic membrane to prevent NT from binding
101
What is MDMA, what effects does it have?
Recreational drug also known as ecstasy
- inhibits reuptake of serotonin into presynaptic neurone by binding to specific proteins enabling reuptake, increase serotonin levels in brain
- also triggers release of further serotonin from presynaptic neurones
Effects of more serotonin = extreme euphoria, enhanced touch and bodily sensations
102
What is L dopa, what is its effects?
A drug used to treat symptoms of Parkinson's
- has a very similar structure to dopamine, transported from blood to brain where it is converted into dopamine by enzyme dopa-decarboxylase
- this increases levels of dopamine in the brain
Effects = more dopamine means more nerve impulses transmitted in parts of brain controlling movement, giving patients better control over their movement
103
Why can dopamine not be given directly to those with Parkinson's?
Dopamine cannot cross the blood brain barrier
104
What is the Human Genome project?
A project that involved the sequencing of the entire human genome (all the DNA found in a human, involving genes on different chromosomes)
105
What were the aims of the Human Genome project?
- identify all genes in human genome
- to find the location of all the genes
- determine sequence of base pairs making up human DNA
- find functions of different genes
- publish results on a public database
106
How can the outcomes of genome sequencing projects be used
in the development of personalised medicine?
Develop targeted drugs to treat individuals with different genotypes
Genetic screening can allow preventative measures to be taken (e.g. adjusting lifestyle choices)
Individual genomes can be used to work out how they might respond to specific treatments
107
What are the social, moral and ethical issues with personalised medicine due to gene sequencing projects?
- increased research costs means only wealthier people have access to personalised medicine
- insurance companies/employers may use personalised medical data to unfairly discriminate against certain individuals
- knowing that only medication available may not work can cause psychological stress
108
What is genetic engineering?
The modification of genomes/DNA via the addition of genetic material from another organism
109
What is a GMO?
Genetically modified organism, which contains recombinant DNA
110
What is the role of enzymes in genetic modification?
Restriction endonucleases used to cut genes out of desired DNA, which is then amplified by PCR.
Enzymes also open the plasmid, and the same endonuclease is used to produce 'sticky ends' meaning H bonds can form between bases.
Ligase joins gene to plasmid, phosphodiester bonds form
111
How is genetic modification carried out?
- desired gene identified and cut out using restriction endonucleases
- gene is then inserted into a plasmid (sticky ends - use same endonuclease, ligase to join)
- plasmid then put into host cells e.g. via gene gun
112
What are some of the risks associated with GMOs?
- gene transfer to other species possible
- consequences of this transfer include antibiotic/pesticide resistance
- long term effects of consuming/implications for health
- risk with use of viral vectors
- creates monocultures, bad for biodiversity
- seeds very expensive, only accessible to developed world
113
What are the benefits of GMOs?
- can produce vaccines in genetically modified plant tissues
- higher crop yields
- crops resistant to pests
- low cost supply of human medication
114
How can drugs be produced from genetically modified plants?
Bacteria with plasmid and desired gene introduced into plant DNA (via infection or gene gun)
Transfer of gene into plant cell nucleus, so each cell of plant contains a copy of gene coding for desired protein
Protein can be purified from plant tissue e.g. for medical purposes
115
How can drugs be produced from genetically modified animals?
Gene for desired protein injected into zygote nucleus, implanted into uterus of surrogate animal where it develops into adult animal.
Every cell contains gene coding for desired protein, protein can be purified
116
What is the difference between nature and nurture?
Nature = genes provide blueprint for all behaviour, which is caused by innate characteristics you are born with
Nurture = behaviour determined by an individual's environment
117
Why is it difficult to determine whether nature or nurture has the greatest influence on brain development?
Genes and environment interact from birth, difficult to separate impacts of each
118
What are the methods for investigating the effects of nature and nurture?
- animal experiments
- twin studies
- cross-cultural studies
- newborn studies
- brain damage studies
119
What are twin studies, how do they investigate nature v nurture?
Identical twins = genetically identical, so when raised in different environments, differences in brain development are due to NURTURE
Non identical twins (often used as control group) = not genetically identical, so when raised in same environment, differences in brain development are due to NATURE
Traits more common in identical twins = genetics
Traits with little difference = environment
120
What are cross cultural studies, how do they investigate nature v nurture?
Different cultures provide different environmental influences on brain development of children
- difference in brain development due to nurture, whereas similarities will be due to nature
121
What are brain damage studies, how do they investigate nature v nurture?
Brain damage in children allows study of effects of brain damage on development
- compare characteristics in those with and without brain damage: e.g. if those with brain damage show development of the characteristic, due to nurture, if not = nature
122
What are newborn studies, how do they investigate nature v nurture?
The environment outside the womb will not affect brain development, therefore the level of brain development in babies is due to nature. However, the development of skills like speaking shows nurture plays a role
123
What are animal studies, how do they investigate nature v nurture?
Can study impact of different environments on brain development of individuals of same species. Differences = nurture as they have same genes
Genetic modification can be used to switch off function of certain genes, differences in brain development would be due to nature
124
What is habituation?
A reduced response to a repeated stimulus. Form of learning
125
How do animals become habituated?
Due to changes in the transmission of nerve impulses from one neurone to the next
Fewer Ca2+ ions move into presynaptic membrane on arrival of an impulse. As a result, less neurotransmitter is released and an action potential is less likely to be generated in the postsynaptic membrane.
126
Why does less calcium ions moving into the pre synaptic membrane cause habituation to occur?
- less neurotransmitter released
- fewer neurotransmitter molecules bind to postsynaptic receptors
- fewer sodium ion channels open, so fewer sodium ions move into the axon
- threshold potential is not reached, no action potential generated (less depolarisation)
- nerve impulse doesn't reach effector organ so the animal does not respond to stimuli
