edexcel a level biology SNAB topic 8

define neurotransmitter

chemical substance, comes from vesicle that binds to presynaptic neurone, diffuses across synaptic gap, affects postsynaptic membrane (initiates action potential/depolarises)

suggest how SSRIs help reduce depression

not reabsorbed, can diffuse across synapse, high level, continues to bind to receptors in postsynaptic neurone

explain the decrease in muscle contraction (habituation)

• decreases

• calcium channels less responsive in sens. neurone

• less ca2+ taken up

• less neurotransmitter released

• fewer impulses from motor neurone/to muscle

suggest how a repeated stimulus could result in less response

• high freq. of impulses

• calcium ion channels less responsive in sens. neurone

• ca2+ ions taken up less

• neurotransmitter depleted, fewer across synapse

• post synaptic membrane not depolarised

• impulses do not reach muscle

Benefits of habituation

• avoids waster time/effort/resources (e.g. O2, energy)

• stimulus non-threatening, harmless e.g. wave action

• More receptive to important stimuli, allows maximum feeding effort

Explain how fMRI scans help us understand brain functions.

• brain activity in real time

• detects oxygen uptake/blood flow

• active areas of brain receives more blood

• oxyhaemoglobin absorbs fewer radio waves

• active areas lighter in scan, other: darker

• brain activity falls with habituation

Define habituation

• type of learning

• reduced response to repeated harmless stimulus

Carry out investigation on time taken for animal to lose habituation

• use habituated animal

• leave for diff. intervals w/o stimulus

• provide stimulus

• measure time taken for muscle contraction

• repeat at each time interval

Effect of no. of neurones on learning

• more synapses

• more neural connections

• greater learning capacity

A myelinated axon conducts impulses faster than a non-myelinated axon.Explain this difference.

• Schwann cells cover axon in myelinated neurone

• myelin insulates/prevents ion movement, hydrophobic

• action potential only @ nodes of ranvier

• saltatory conduction, action potential 'jumps'

• between myelin layers of Schwann cells

• nerve impulse does not travel whole length, local currents

Sodium and potassium ions can only cross the axon membrane through proteins. Explain why.

• cannot pass through phospholipid bilayer

• hydrophilic/charged/not lipid soluble

Explain what causes transmission at a synapse to occur in only one direction.

• vesicle containing neurotransmitter only in presynaptic neurone

• receptors only on postsynaptic neurone

• neurotransmitter diffuses down conc. grad.

Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.

• action potential arrives

• depolarisation occurs

• calcium ions enter synaptic knob

• presynaptic vesicles bind to membrane

• acetylcholine released

• bind to receptors on postsynaptic membrane

Explain how GABA inhibits transmission of nerve impulses by postsynaptic neurones.

• causes chloride ions to enter postsynaptic neurone

• inside becomes more negatively charged / hyper-polarised

• stimulation does not reach threshold level

• action potential not produced

• reduced effect of Na ions entering

Describe the process of synaptic transmission

• Action potential arrives at axon terminal Voltage gated Ca2+ channels open Ca2+ enters presynaptic neuron Ca2+ signals to neurotransmitter vesicles, breaks down actin preventing movement Vesicles move to membrane + fuse Neurotransmitters released via exocytosis Neurotransmitter bind to receptors on postsynaptic neuron Binding allows ligand-gated Na+ channel to open, Na+ enters neuron via facilitated diffusion

Explain how a resting potential is maintained in a neurone.

• sodium transported out via active transport

• potassium transported out via facilitated diffusion

• Na not permeable, cannot diffuse back in

• 3 Na out for every 2 K in

• inside more negative than outside (-70mv)

How does a CT scan help with diagnosis?

• nature of abnormality (via density) e.g. bleeding,

• location of abnormality, relates to functions

• extent / size of abnormality

• likely problems for treatment e.g. accessibility for surgery

Explain why the abnormalities in two different brains could cause different symptoms.

• found in diff. regions

• diff parts of brain have diff functions

• symptoms depend on region affected

• one brain could have more abnormalities

• diff. types of abnormalities have diff. associated symptoms eg. tumour vs hemorrhaging

Explain how the structure of the cell surface membrane of a motor neurone is related to the conduction of a nerve impulse along its axon.

• phospholipid bilayer

• hydrophobic properties prevent ion movement

• voltage gated na+ channel present

• allows for movement of na+ during depolarisation

• K+ ion channel present, allows K+ to diffuse

• sodium-potassium pump, exports na+, imports k+

• maintains resting potential

• nodes of ranvier involved, salutatory conduction

Eugenol decreases sodium ion concentration. Eugenol can be used to reduce pain. Suggest an explanation for how eugenol affects the movement of calcium ions and reduces pain.

• less ca+ enters presynaptic sensory neurone

• ca+ ion channel closed

• fewer vesicles fuse w/ presynaptic membrane

• less neurotransmitter released via exocytosis

• less binds to postsynaptic membrane receptors

• na+ ion channel does not open

• action potential less likely, threshold not achieved

• pain not felt, impulse not sent to CNS

With reference to the structures of dopamine and L-dopa, suggest why the drug L-dopa is effective in the treatment of Parkinson's disease

• parkinson's caused by low dopamine

• l-dopa structure similar to dopamine

• binds to dopamine receptor

• initiates action potential

• l-dopa can be converted to dopamine

• l-dopa can cross the blood-brain barrier

Explain why pd. of membrane goes from -70mv to 30mv

• permeability of membrane to Na+ increases

• voltage gated Na+ ion channels open

• Na+ move into neurone

• via diffusion, down conc. gradient

• stimulates more Na+ channels to open

• positive feedback

• pd positive, excess Na+ inside neurone

Explain what happens to the visual cortex when the right eye of this kitten is covered for the first seven weeks after birth.

• lack of stimulus = fewer impulses to visual cortex

• less neurotransmitter released

• synapses destroyed/weakened

• neurones for right eye lost

• neurones for left eye develop

• ocular columns larger in left eye

Describe how the release of this neurotransmitter generates action potentials in the postsynaptic neurone.

• neurotransmitter diffuses across cleft

• binds to receptors on post-synaptic membrane

• ligand gated Na+ ion channel opened

• Na+ diffuses into neurone, increased permeability

• positive feedback, more Na+ channels open

• summation of postsynaptic potential to form action potential

Describe how low serotonin levels in an individual can affect the transmission of impulses in their brain

• less neurotransmitter

• fewer depolarisations in postsynaptic membrane

• no action potential, threshold not met

Describe how the use of MDMA could affect the transmission of impulses in the brain

• releases/prevents reuptake of serotonin/dopamine

• blocks presynaptic receptors, bind to postsynaptic receptors

• more action potentials produced

Individuals who use MDMA may develop the symptoms of depression. Explain how the use of MDMA could result in the development of these symptoms

• results in depletion of serotonin

• postsynaptic membrane becomes less responsive to serotonin

• lack of serotonin associated with depression

Contraction of the muscle in the withdrawal response is stimulated by nerve impulses. These nerve impulses can be detected using electrodes. Explain the electrical changes in an axon that allow these nerve impulses to be detected

• pd across axon changing

• depolarised= -ve to +ve

• increased permeability to Na+

• voltage gated Na+ ion channels open, cause depolarisation

• repolarised: +ve to -ve

• K+ ion channels open, K+ move into axon

Describe the role of visual stimulation on the development of the visual cortex during the critical period

• ocular dominance columns form in visual cortex

• neurones form synapses with these columns

• stimuli needed to strengthen connections

• stimulation during critical window needed to form effective connections in visual cortex

Describe the role of the dendrites in a neurone.

• form connections with other neurones

• integrate/receive impulses from other neurones

• involved in summation

• initiate action potential in cell body/axon

Exp lain the advantages of using MRI scanning to identify tumours compared to using CT scanning.

• higher resolution, more detail seen

• no x-ray, safer, no risk of mutation

• can be used more often

' The length of a plant stem depends on the amount of red light or far red light they receive'.

Design an investigation the student could use to test this hypothesis.

• plants grown in pots containing same soil / pH / minerals / water as these factors can affect growth

• one group under lamp emitting red light and one group under a lamp emitting far red light

• keep temperature the same in both as enzymes involved in growth

• multiple plants, reliability

• cloned plants of same species, genes can affect growth

Explain how SSRIs help to maintain a feeling of well-being

• bind to reuptake proteins

• serotonin levels remain high in synapse

• serotonin binds to postsynaptic membrane receptors

• depolarisation of postsynaptic membrane

• action potentials initiated

No myelination

• nodes are sit of clusters of sodium gated channel proteins

• open/close when impulse arrives

• depolarisation @ nodes

Describe and explain the sequence of events that occurs at the synapse, after a neurotransmitter has been released.

• diffuses across gap

• binds to postsynaptic membrane receptors

• ligand gated Na+ ion channels open = depolarisation

• if threshold met, action potential

• temporal/spatial summation

• allows coordination/one way flow of info

• integration in postsynaptic cell

• neurotransmitter broken down by enzyme

• make receptors available again

• products reabsorbed through presynaptic membrane/resynthesised to neurotransmitter

myelinated sensory vs non-myelinated motor neurone

• dendron longer

• dendron myelinated

• axon shorter

• cell body towards middle

• no motor end plate

Suggest what happens in the rod cells during this two hours of darkness.

• opsin unbinds from rod cell surface membrane

• trans retinal to cis retinal

• rhodopsin reformed

• permeability of membrane to Na+ increases

• less hyperpolarisation

• more neurotransmitter released

Describe how the nervous system controls the pupil reflex in a mammal in response to bright light

• light hits photoreceptors in retina

• impulses sent to visual cortex via sensory neurone

• automatic response

• impulse sent along parasympathetic nerve

• via motor neurone

• circular muscle constricts

• radial muscle relaxes

• pupil contracts

pros of animal testing

• medical treatment, greater understanding of human body

• utilitarian philosophy

• reduced chances of harm when testing on people

nervous vs hormonal coordination similarities + diffs

similarities:

• hormones + neurotransmitters both chemicals

• both use receptors

diffs: nervous:

• electrical

• fast transmission

• short duration hormonal:

• chemical

• slow transmission

• long duration

Explain why the lack of dopamine leads to the symptoms of Parkinson's disease.

• non-functioning of synapses

• in motor neurones

• in motor cortex of cerebellum

Explain how nerve impulses are conducted along a myelinated axon after an action potential has been initiated.

1. Reference to local currents;

2. Depolarisation of {adjacent / next} section of axon;

3. Sodium channels open / sodium ions move in (to axon);

4. More sodium ions move in so more sodium channels open / positive feedback idea;

5. Correct reference to +40 mV / {becomes positive / less negative}inside the axon;

6. Potassium channels open / potassium ions move out (of the axon);

7. Membrane repolarises;

8. Wave of depolarisation / depolarisation - repolarisation process repeats itself along the axon / eq;

9. Refractory period / hyperpolarised / more negative than resting potential;

10. Idea of impulse moves in one direction only;

11. Myelin sheath speeds up conduction; 12.Current jumps between {nodes of Ranvier / gaps in the myelin sheath} / saltatory effect;

Explain why plant b did not flower after being left for less than the critical period in darkness

• not enough time (in the dark)

• Pfr levels remain too high

• once Pfr below a certain level (flowering happens)

• flowering stimulated (by fall in Pfr)

The protein coded for by the tom-1 gene is involved in neurotransmitter release from a sensory neurone. Suggest how a mutation in this gene could produce the results observed for the tom-1 mutant strain. (increased habituation)

• reduced activity of ca2+ channnel

• less neurotransmitter released

• from sensory neurone

• fewer action potentials

• of motor neurone

Suggest how this change in the activity of the sodium channels can result in an increase in the electrical activity in the brain of a person with epilepsy.

• more Na+ ion channels open

• membrane depolarises

• reaches threshold, generates action potential

• in a large no. of neurones

Compare this response of a shoot to light with hormonal coordination in animals

• both chemical

• both move away from production site

• one in blood, one diffuses

• slower in plants

• animal hormones do not just affect growth

• different stimuli, humans don't react to light

Suggest why the pupil appears black.

pigment in the back of the eye absorbs light

The pupil increases in diameter in dim light. Explain how neurones enable this response to occur.

• depolarisation

• bipolar neurone

• sensory neurone

• optic nerve

• motor neurone

• radial muscle contracts

Tropicamide is a drug used in eye drops. Tropicamide has an effect on the diameter of the pupil in the eye. This makes it easier for the doctor to examine the retina or lens in the eye of a patient. Suggest how tropicamide in the eye drops makes it easier to examine the retina.

• affects nervous system of retina

• radial muscles contract

• prevents pupil constriction

• pupil dilates

• all of retina visible

Describe how PET scans can be used to investigate brain structure

• uses radioactive tracers

• detects emissions of positrons/production of gamma rays

• provides 3D image

Describe the role of sodium ions in the functioning of a mammalian rod cell

• sodium ions pumped out of rod cell

• light condition: do not move back into rod cell, hyperpolarised

• dark condition: move back into rod cell, depolarised

Explain why stems of weary lettuce do not respond to gravity

• cells in stem do not detect changes in gravity

• no auxin produced

• no stimulation of cell elongation on side of stem facing downwards

Cells in the tip of the oat coleoptile release IAA Explain how the IAA affects the growth of the coleoptile

• IAA diffuses from the tip of the coleoptile

• IAA can be taken up by cells in zone of elongation

• causes cells to elongate

• lowers pH in cellulose cell wall

• causes the coleoptile to grow towards the light, positive phototropism

Nicotine binds to acetylcholine receptors Explain how nicotine causes an action potential in the post-synaptic neurone that releases noradrenaline

• similar shape to acetylcholine, excitatory effect

• NA+ channels open, increased permeability

• causes depolarisation of post-synaptic membrane, Na+ move into neurone

• depolarisation reaches threshold

Explain how an increase in calcium ion uptake leads to release of noradrenaline

• calcium ions cause vesicles to release noradrenaline through exocytosis

Explain how the action of the drugs used to treat schizophrenia may lead some patients to experience symptoms similar to those of Parkinson's disease

• Parkinson's symptoms arise due to low dopamine levels

• transmission of nerve impulses disrupted causing lack of control of muscle movements

• schizophrenia drugs lower dopamine levels

• less dopamine in synaptic knob

• less dopamine available to bind to receptors

• no/little change in permeability of postsynaptic cell

• fewer sodium ions enter postsynaptic cell

• less depolarisation of postsynaptic cell