A2 Level Biology Unit 5 Edexcel Part 2

Revision cards for the 5th Unit of Biology A2 Level Edexcel.

HideShow resource information

Nervous system

The nervous system is divided into:

The central nervous system:

  • Brain
  • spinal cord

Peripheral nervous system:

  • sensory nerves - carrying sensory information from the receptors to the CNS
  • motor nerves - carrying the motor commands from the CNS to the effectors
1 of 71

Nervous system continued

The Peripheral nervous system is divided into:

Autonomic nervous system:

  • involuntary
  • stimulates smooth muscle, cardiac muscle and glands

Somatic nervous system:

  • voluntary
  • stimulates skeletal muscle

Autonomic nervous system is divided into:

Sympathetic nervous system:

  • prepares body for 'fight' or 'flight' responses 

Parasympathetic nervous system:

  • prepares body for 'rest and digest'
2 of 71


There are two types of main extensions from the cell body of a neurone:

  • dendrites that conduct impulses towards the cell body
  • the axon which transmits impulses away from the cell body

Motor neurone:

  • cell body is at the end of the neurone
  • its situated within the CNS
  • it conducts impulses from the CNS to effectors (muscles or glands)
  • they are also known as effector neurones

sensory neurones:

  • cell body is attached to the middle of the axon
  • they carry impulses from sensory cells to the CNS
3 of 71

Neurones continued

Relay neurone:

  • the cell body is in the middle of the axon
  • they are found mostly within the CNS
  • they can have a large number of connections with other nerve cells
  • they are also known as connector neurones and as interneurones

Myelin sheath:

  • is a fatty insulating layer around the axon
  • made of schwann cells wrapped around the axon
  • it effects how fast nerve impulses pass along the axon
4 of 71

Reflex arcs

nerve impulses follow routes or pathways through the nervous system. These pathways are called reflex arcs and are responsible for our reflexes.

An example is a reflex arc allowing withdrawal of the arm:

  • Receptors detect a stimulus and generate a nerve impulse
  • sensory neurones conduct a nerve impulse to the CNS along a sensory pathway
  • sensory neurones enter the spinal cord through the dorsal route
  • sensory neurone forms a synapse with a relay neurone
  • relay neurone forms a synapse with a motor neurone that leaves the spinal cord through the ventral route
  • motor neurone carries impulses to an effector which produces a response
  • in this case the bicep contracts to raise the arm away from the flame
5 of 71

The pupil reflex

How do the muscles of the iris respond to light?:

  • the iris controls the size of the pupil
  • it contains a pair of antagonistic muscles; radical and circular muscles
  • these are both controlled by the autonomic nervous system
  • the radical muscles are controlled by sympathetic reflex
  • the circular by parasympathetic reflex

Pupil constricted:

  • radial muscles relax
  • circular muscles contract

Pupil dilated:

  • radial muscles contract
  • circular muscles relax
6 of 71

Inside a resting axon

  • all cells have a potential difference across their surface membrane
  • the inside of the axon is more negative then he outside so the membrane is said to be polarised
  • the value of -70 mV is known as resting potential

Why is there a potential difference?:

  • the uneven distribution of ions across the cell surface membrane is achieves by the action of sodium-potassium pumps
  • they carry Na+ out of the cell
  • and carry K+ into the cell
  • these pumps act against the concentration gradients
  • and are driven by energy supplied by hydrolysis of ATP
  • the organic anions are large and stay within the cell
  • so chloride ions move out of the cell to help balance the charge
7 of 71

The resting potential

1. Na+/K+ pump creates concentration gradients across the membrane

2. K+ diffuse out of the cell down the K+ concentration gradient making the outside of the membrane positive and the inside negative

3. the electrical gradient will pull back K+ into the cell

4. at -70 mV potential difference the two gradients counteract each other and there is no net movement of K+

Why is the axon resting potential =70 mV?

there are two forces involved in the movement of the K+ ions:

  • the concentration gradient generated by the Na+/K+ pump
  • the electrical gradient due to the difference in charge on the two sides of the membrane resulting from K+ diffusion

the electrical gradient balances out the chemical gradient and there is no net movement of K+ so a steady state exists.

8 of 71

What happens when a nerve is stimulated?

- if an electrical current above the threshold level is applied to the membrane it causes a massive change in the potential difference

- the potential difference across the membrane is locally reversed

- this makes the inside of the axon positive and the outside negative

- this is known as depolarisation

- the potential difference becomes +40 mV for a very short time

- it then returns to its resting potential so more impulses can be conducted

- this is known as repolarisation

- the large change in voltage across the membrane is known as an action potential 

9 of 71

What causes an action potential?

1. Depolarisation

  • when a neurone is stimulated some depolarisation occurs
  • this change in the potential difference changes the shape of the Na+ gate
  • this opens some of the voltage-dependent sodium ion channels
  • as the Na+ ions flow in depolarisation increases triggering more gates to open
  • the opening of more gates increases depolarisation further
  • this is an example of positive feedback
  • there is a higher concentration of Na+ ions outside of the axon
  • so Na+ ions flow rapidly inwards through the open voltage-dependent Na+ channels
  • causing a build-up of positive charges inside
  • this reverses the polarity of the membrane
  • the potential difference across the membrane reaches +40 mV
10 of 71

What causes an action potential? continued

2. Repolarisation

  • the voltage-dependent Na+ channels close
  • and the permeability of the membrane to Na+ ions decreases
  • voltage-dependent K+ channels open due to the depolarisation of the membrane
  • because of this K+ channels open due to the depolarisation of the membrane
  • K+ ions move out of the axon down the electrochemical gradient because they are attracted by the negative charge outside of the cell
  • as K+ ions move out of the cell the inside of the cell becomes more negative than the outside

3. Restoring the resting potential

  • the membrane is now very permeable to K+ ions and more ions move out
  • making the potential difference more negative than the normal resting potential, this is called hyperpolarisation of the membrane
  • the resting potential is back  by closing K+ channels and opening Na+
11 of 71

How is the impulse passed along an axon?

- when a neurone is stimulated it triggers a sequence of action potentials along the length of the axon

- at resting potential there is a positive charge on the outside with high Na+ concentration and a negative charge inside with a high concentration of K+ ions

- when stimulated voltage-dependent Na+ ions open and Na+ ions flow into the axon depolarising the membrane

- the Na+ ions move to the area in the membrane where change is causing the electrical charge to change

- the change in potential difference in the membrane next to the first action potential causes a second action potential

- at the site of the first action potential the Na+ channels close and the K+ channels open

- K+ ions leave the axon repolarising the membrane and it becomes hyperpolarised

- a third action potential is started by the second one

at the site of the first action potential K+ ions diffuse back into the axon restoring resting potential

12 of 71

Speed of conduction

- the speed of nervous conduction is in part determined by the diameter of the axon

- in general the wider the diameter the faster the impulse travels

- the myelin sheath acts as an electrical insulator along the axon

- it prevents any flow of ions across the membrane

- gaps known as nodes of Ranvier occur in the myelin sheath at regular intervals

- in these gaps is the only place where depolarisation can occur

- the impulse jumps from gap to gap making depolarisaion quicker

- this is called saltatory conduction

13 of 71

How does a nervous impulse pass between cells

- where two neurones meet is known as a synapse

- the cells do not touch there is a gap known as a synaptic celft

- the synaptic cleft separates the presynaptic membrane which the impulse arrives at from the postsynaptic membrane of the other cell

- a nerve impulse cannot jump across the gap

- in the cytoplasm at the end of the presynaptic neurone there are synaptic vesicles

- these synaptic vesicles contain a chemical called a neurotransmitter

14 of 71

How does the synapse transmit an impulse?

1. An action potential arrives at the presynaptic neurone

2. the membrane depolarises. Ca2+ ion channels open and Ca2+ ions enter the neurone

3. Ca2+ ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane

4. Neurotransmitter is released into the synaptic cleft

5. Neurotransmitter binds with the receptors on the postsynaptic membrane. Cation channels open. Na+ ions flow through the channels.

6. membrane depolarises and initiates an action potential

7. when released the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down), or it can diffuse away and be broken down

15 of 71

Nerve impulses

There are three stages leading to the nerve impulse passing along the postsynaptic neurone:

  • neurotransmitter release
  • stimulation of the postsynaptic membrane
  • inactivation of the neurotransmitter

Neurotransmitter release:

  • the presynaptic membrane is depolarised by an action potential, channels in the membrane open increasing permeability to Ca2+ ions
  • Ca2+ ion concentration is greater outside the cell so they diffuse into the cell
  • the increased Ca2+ concentration causes synaptic vesicles containing acetylcholine to fuse with the presynaptic membrane
  • this causes their contents to be released into the synaptic cleft by exocytosis
16 of 71

Nerve impulses continued

stimulation of the postsynaptic membrane:

  • embedded in the postsynaptic membrane are specific receptor proteins
  • these proteins have a binding site that have a complimentary shape to part of the acetylcholine molecule
  • the ecetylcholine molecule binds to the receptor changing the shape of the protein
  • this opens the cation channels and makes the membrane permeable to Na+ ions
  • the flow of Na+ ions across the postsynaptic membrane causes depolarisation
  • an action potential is produced

inactivation of the neurotransmitter:

  • an enzyme called acetylcholinesterase breaks down the acetylcholine so that it cannot bind to receptors
  • some of the products from the breakdown are then reabsorbed by the presynaptic membrane and are used again
17 of 71

Control and coordination

Synapses have two roles:

  • control of nerve pathways allowing flexibility of response
  • integration of information from different neurones allowing a coordinated response 

Two factors that affect the likelihood that a postsynaptic membrane will depolarise:

  • the type of synapse
  • the number of impulses received

- some synapses help stimulate an action potential 

- others are inhibitory and make it less likely that depolarisation will occur

- a postsynaptic cell has many inhibitory and excitatory synapses

- is an action potential is made or not depends on the balance of the synapses

18 of 71

Excitatory synapses

- Excitatory synapses make the postsynaptic membrane more permeable to Na+ ions

- more than one of these is needed to provide sufficient depolarisation each impulse adds to the effect of another this is called summation

There are two types of summation:

  • Spatial summation - 

- here the impulses are from different synapses, usually from different neurones.

  • Temporal summation -

- in this case several impulses arrive at a synapse having traveled along a single neurone one after the other.

- their combined release of neurotransmitter generates an action potential in the postsynaptic membrane.

19 of 71

Inhibitory synapses

- inhibitory synapses make it less liely that an action potential will result in the postsynaptic cell

- the neurotransmitters from these synapses opens channels for Cl- ions and K+ ions in the postsynaptic membrane

- these ions then move through the channels down their diffusion gradients.

-Cl- ions move into the cell carrying negative charge and K+ ions will move out carrying a positive charge

- this results in a greater potential difference across the membrane as the inside becomes more negative than usual (-90 mV)

- this makes depolarisaion less likely as more excitatory synapses will be needed to depolarise the membrane

20 of 71

Coordination in plants

- plants use chemicals to coordinate growth these chemicals can be called plant growth regulators or plant growth substances

- plants have phototropism (bending of plants towards a light source)

- there is a chemical in the tip that travels down the coleoptile (a plant) it was found that this chemical is an auxin called indoleacetic acid

- when the tip of a plant is removed and placed on some agar jelly and then placed back on top of the plant, it started to grow again showing the chemical had diffused through the agar jelly

-  auxins are synthesised in actively growing plant tissues (known as meristems) such as shoot tips etc

- they bind with receptors on the plasma membranes in the zone of shoot elongation

- by doing this the auxins produce second messenger signal molecules that bring about changes in gene expression

- an increased potentail difference across the membrane enhances uptake of ions into the cell

- this causes uptake of water by osmosis causing cell elongation

21 of 71


- stimuli are detected by receptor cells that send electrical impulses to the central nervous system.

- some types of receptor cells are grouped together into sense organs

Different types of receptors


  • stimulated - by chemicals
  • examples of role - taste,smell and regulation of chemical concentrations in the blood.


  • stimulated by - forces that stretch, compress or move the sensor
  • examples or role - balance, touch and healing
22 of 71

Receptors continued


  • stimulated by - light
  • examples of role - sight


  • stimulated by - heat or cold
  • examples of role - temperature control and awareness of changes in the surrounding temperature

- all of the receptors except for photoreceptors work in the same way

- at rest the cell surface membrane has a negative resting potential, stimulation of the receptor causes depolarisation

- when the depolarisation goes above the threshold level it triggers an action potential

- it is either relayed across the synapse using neurotransmitters or passed directly down the axon of the sensory nerve

23 of 71

The structure of the eye

Conjunctiva - protects the cornea

cornea - bends light

lens - focuses light on retina

iris - controls amount of light entering the eye

sclera - protective layer

blind spot - no light-sensitive cells where optic nerve leaves the eye

yellow spot (fovea) - most sensitive part of the retina located in the macula the central area of the retina

retina - contains light-sensitive cells

vitreous humour - transparent jelly

choroid - black layer prevents internal reflection of light

ciliary muscle - alters thickness of lens for focusing

24 of 71


- the human retina contains two types of photoreceptor cells sensitive to light, these are rods and cones

-cones allow colour vision in bright light

- rods only give black and white vision but work in dim light and bright light

- in the centre of the retina there are only cones but over the remainder of the retina rods outnumber cones

- the rods and cones synapse with bipolar neurone cells

- which in turn synapse with ganglion neurones

- whose axons together make up the optic nerve

- light hitting the retina has to pass through the layers of neurones before reaching the rods and cones

25 of 71

How does lioght stimulate photorecertor cells?

- in both rods and cones a photochemical pigment absorbs the light resulting in a chemical change

- in rods the molecule is a purplish migment called rhodopsin

- rods contain an outer and inner segment these contain the many layers of flattened vesicles

- the rhodopsin molecules are located in the membranes of these vesicles

here i drew a picture of the structure of rods and cones within the retina because i could not find an appropriate one from the internet.

26 of 71

In the rod cells: dark


- sodium ions flow into the outer segment through non-specific cation channels

- the sodium ions move down the concentration gradient into they inner segment where pumps transport them back out of the cell

- this movement of Na+ produces a slight depolarisation of the cell

- the potential difference across the membrane is about -40 mV

- this slight depolarisation triggers the release of a neurotransmitter thought to be glutamate from the rod cells

- the rods release this neurotransmitter continuously

- the neurotransmitter binds to the bipolar cell stopping it depolarising

27 of 71

In the rod cells: light

- when light falls on the rhodopsin molecule, it breaks down into retinal and opsin non-protein and protein components, the opsin activates a series of membrane-bound reactions

- these reactions end in hydrolysis of a molecule attached to the cation channel in the outer segment

- the breakdown of this molecule results in the closing of the cation channels

- the entry of Na+ into the rod decreases while the inner segment continues to pump Na+ out

- this makes the inside of the cell more negative and because of this it becomes hyperpolarised and the release of glutamate stops

- the lack of glutamate results in depolarisation of the bipolar cell with which the rod synapses

- the neurones that make up the optic nerve are also depolarised and respond by producing an action potential

28 of 71

Phytochromes - plant photoreceptors

- a phytochrome molecule consists of a protein component bonded to a non-protein light-absorbing pigment molecule.

- the five phytochromes differ in their protein component

- the non-protein component exists in two forms which are different isomers:

  • Pr - phytochrome red; absorbs red light
  • Pfr - phytochrome far-red; absorbs far-red light

- these two isomers are photoreversible but plants synthesise phytochromes in the Pr form

- absorption of red light converts Pr into Pft, absorption of far red light converts Pfr back into Pr

- in sunlight Pr is converted into Pfr and Pfr is converted into Pr

- the former reaction dominates in sunlight because more red than far-red light is absorbed

- therefore Pfr accumulates in the light

- and in the dark any Pfr present is slowly converted to Pr

29 of 71

Phytochromes trigger germination

- phytochromes were discovered through germination experiments

- they used seeds that have thin seed coats and few food reserves

- ones that do not germinate in the dark and only germinate when close enough to the soil surface

- the findings they produced where that red light is effective at triggering germination, while far-red light seems to inhibit germination

-  when lettuce seeds are exposed to red light Pr is converted to Pfr stimulating responses that lead to germination

- when they are kept in the dark no Pr is converted to Pfr

- because of this the seeds do not germinate because there is no presence of Pfr

- when exposed to far-red light Pfr is converted back to Pr inhibiting germination

30 of 71

Photoperiods, flowering and phytochromes

- the photoperiod is the environmental cue that determines the of flowering

- the ratio of Pr to Pfr in a plant enables it to determine the length of day and night

- long nights give time for Pfr to convert back to Pr so that all phytochrome will be Pr

- summer nights may not be long enough though so some Pfr may still be present in the morning

- long-day plant:

  • only flower when day length exceeds a critical value
  • flower when the period of uninterrupted darkness is less them 12 hours
  • they need Pfr to stimulate flowering

- short-day plants:

  • tend to flower in spring or autumn when the period of uninterrupted darkness is greater than 12 hours
  • they need long hours of drakness in order to convert all Pfr present back into Pr
  • Pfr inhibits flowering in shot-day plants
31 of 71

Phytochrome and greening

- once a shoot has broken through soil into sunlight the plant unsergoes big changes in both its form and biochemistry

- these changes are called greening

- once in the light phytochromes promote the development of primary leaves, leaf unrolling and the production of pigments

- they can also inhibit certain processes such as elongation of internodes

How do phytochromes which processes on or off?

- exposure to light causes phytochrome molecules to change from one form to another bringing about a change in chape

- the phytochromes may then bind to proteins or disrupt the binding of a protein complex

- these signal proteins may act as transcription factors or activate transcription factors that bind to DNA to allow transcription of light regulated genes

- the transcription and translation of proteins result in the plants response to light                                                                                            

32 of 71

Plants detect other environmental clues


- more than a short distance under the soil surface light cannot be the cue for the shoot to grow upwards and the root to grow donwards

- the stimulus for this is gravity and the response ensures that developing shoots reach the light while roots grow in the soil

Touch and mechanical stress:

- some plants are sensitive to touch and mechanical stress

- it is thought that mechanical stimulus (such as rubbing the plant stem) activates signal molecules whose end result is the activation of genes that control growth

- some plants have leaves that move rapidly in response  to mechanical stimulation

- the mechanism is that when touched speciaslised cells lose potassium ions

- water follows by osmosis and the cells become flaccid so no longer support the leaf and keep it upright

33 of 71

The brain

The cerebral hemispheres:

- the top of the brain is called the cortex it is made of mainly nerve cell bodies, synapses and dendrites

- this outer layer of the brain is known as the grey matter

- the cortex is the largest region of the brain, and is divided into left and right cerebral hemispheres

- each hemisphere is composed of four regions called:

  •  frontal lobe
  • parietal lobe
  • occipital lobe
  • temporal lobe

- the two cerebral hemispheres are connected by a broad band of white matter (nerve axons) called the corpus callosum.

34 of 71

The brain continued

Frontal lobe:

- is concerned with the higher brain functions such as decision making, reasoning, planning and consciousness of emotions, it is also concerned with forming associations and ideas

- it includes the primary motor cortex which has neurones that connect directly to the spinal cord and brain stem and from there to the muscles

- it sends infomation to the body via the motor neurones to carry out movements

- the motor cortex also stores information about how to carry out different movements

Parietal lobe:

- concerned with orientation, movement, sensation, calculation, some types of recognition and memory

35 of 71

The brain continued

Occipital lobe (visual cortex):

-  concerned with processing information from the eyes, including vision, colour, shape recognition and perspective

Temporal lobe:

- concerned with processing auditory information, i.e. hearing, sound recognition and speech (left temporal lobe). it is also involved in memory.

And the cerebellum.

The structures lying directly below the corpus callosum are:

The thalamus - responsible for routing all the incoming sensory information to the correct part of the brain, via the axons of the white matter.

36 of 71

The brain continued

The hypothalamus - contains the thermoregulatory centre so it monitors such things as core body temperature and initiates action to put it back to normal.

- it also acts as a endocrine gland which secretes hormones such as antidiuretic hormone

- it connects directly to the pituitary gland, which in turn secretes other hormones

The hippocampus: - is involved in laying down long-term memory

The basal ganglia - is a collection of neurones that lie deep within each hemisphere

- they are responsible for selecting and initiating stored programmes for movement.

37 of 71

The cerebellum and brain stem

- the brain stem is situated at the top of the spinal column and  it extends from the midbrain to the medulla oblongata

Corpus callosum -

  • white matter made mainly of axons and it has white myelin sheaths
  • it provides connections between the cortex and the brain and the structures below
  • it also forms connections between the two hemispheres of the cortex


38 of 71

The cerebellum and brain stem continued


  • responsible for balance
  • coordinates movement as it is being carried out, receiving information from the primary motor cortex, muscles and joints.
  • constantly checks whether the motor programme being used is the correct one, e.g. referring to incoming information about posture and external circumstances 


  • relays information to the cerebral hemispheres, including auditory information to the temporal lobe, and visual information to the occipital lobe

Medulla oblongata:

  • regulates those body processes that we do not consciously control, such as heat rate, breathing, and blood pressure.
39 of 71

The effects of strokes

- brain damage caused by a stroke can cause problems with speaking, understanding speech, reading and writing

- lesions in small cortical area in the in the left frontal lobe were responsible for deficits in language production

- some patients can recover some abilities after a stroke  showing the potential of neurones to change in structure and function

- this change is known as neural plasticity

- the structure of the brain remains flexible even in later life and can respond to changes in the environment

- brain structure and functioning is affected by both nature and nurture

40 of 71

Brain imaging

CT scans: (Computerised Axial Tomography)

- use narrow-beam X-rays rotated around the patient to pass through the tissue from different angles

- each narrow beam is reduced in strength depending on the density of the tissue in its path

- the x-rays are detected and are used to produce an image of a thin slice of the brain on a computer screen in which the different sort tissues can be distinguished

- they only give frozen pictures

they look at the structures in the brain and can detect brain disease and monitor the tissue of the brain over the course of an illness

- they do not use harmful x-rays

41 of 71

Brain imaging

Magnetic resonance imaging (MRI):

- uses a magnetic field and radio waves to detect soft tissues

- when placed in a magnetic field the nuclei of atoms line up with the direction of the magnetic fiels

- in an MRI scanner the magnetic field runs down the centre of the tube in which the patient lies

- another magnetic field is superimposed on this which comes from the magnetic component of high frequency radio waves

- the combined fields cause the direction and frequency of spin of the hydrogen nuclei to change taking energy from the radio waves

- when the radio waves are turned off the hydrogen nuclei return to their original alignment and release the energy they absorbed

- this energy is detected and a signal is sent to a computer which analyses it to produce an image

- it is used in the diagnosis of tumors, strokes, brain injuries and infections of the brain and spine

42 of 71

Brain imaging

Functional magnetic resonance imaging (fMRI):

- fMRI is used to look at the functions of the different areas of the brain by following the uptake of oxygen in active brain areas

-  it works because the deoxyhaemoglobin absorbs the radio wave signal where as oxyhaemoglobin does not

- increased neural activity requires an increased demand for oxygen and because of this increase in blood flow

- there is a large increase in oxyhaemoglobin levels in the enhanced blood flow so less signal is absorbed

- the less radio signal there is absorbed the higher the level of activity in a particular area

- active areas of the brain 'light up'

43 of 71

From the eye to the brain

- the axons of the ganglion cells that make up the optic nerve pass out of the eye and extend to several parts of the brain

- it extends to part of the thalamus

- and then impulses are then sent along other neurones to the primary visual cortex where the information is then processed

- before reaching the thalamus some of the neurones in each optic nerve branch off to the midbrain

- here they connect to motor neurones involved in controlling the pupil reflex and movement of the eye

- audio signals also arrive at the midbrain so we can quickly turn our eyes in the direction of a visual or auditory stimulus

44 of 71

Visual development

- the human nervous system begins to develop after conception

- by the 21st day the neural tube has formed the front part of the neural tube goes on to develop into the brain where the rest of it develops into the spinal cord

- there is no large increase in the number of brain cells after birth but there is a large increase in brain size

- this increase in brain size is due to the elongation of axons, myelination and the development of synapses

- once neurones have stopped dividing the immature neurones migrate to their final position and start to wire themselves

- axons lengthen and synapse with the cell bodies of other neurones

- neurones must make the correct connections in order for a function such as vision to work properly

45 of 71

Axon growth

- axons of the neurons from the retina grow to the thalamus where they form synapses with neurones in the thalamus

- axons from these thalamus neurons grow towards the visual cortex in the occipital lobe

- the visual cortex is made of columns of cells, axons from the thalamus synapse within these columns of cells

- columns of cells receive stimulation from the same area of the retina in the left and right eye

- it used to be thought that these column of cells in the visual cortex were formed during a critical period for visual development, it is now found not to be the case

- periods of time during postnatal development have been identified when the nervous system must obtain specific experiences to develop properly

- these are known as critical periods, critical windows or sensitive periods

46 of 71

Evidence for a critical period in visual developme

Medical observations:

- a young boy who had a eye infection had his eyes bandages for two weeks and when it was removed he had permanently impaired vision

- some people are born with cataracts, which is a clouding of the lens which affects the amount of light entering the retina

- they can have permanent impairment of their ability to perceive shape of form, including difficulties in face recognition

- but elderly people who develop cataracts in later life and have them for several years have normal vision after they are removed


- research is conducted on just a few types of animals so a lot of information is available about them they are known as animal models

- most animal models are easy to obtain, easy to breed, have short life cycles and a small adult size

- for example mice are used extensively in the study of cancer and disease

47 of 71

Studies of newborn animals

- in one study one group of newborn monkeys were raised in the dark for 3 to six months and another exposed to light but not to patterns

- when they were returned to the normal world both groups had difficulty with object discrimination and pattern recognition

Hubel and Wiesel:

- they raised monkeys from birth to six months depriving them of any light stimulus in one eye, this is known as monocular deprivation

- after 6 months the eye was exposed to light and it was clear that the monkey was blind in the light-deprived eye

- retinal cells in the deprived eye did respond to the light stimuli but the cells in the visual cortex did not respond to any visual input from the deprived eye

- deprivation in adults had no effect

48 of 71

What happens during the critical period?

In the visual cortex:

- overlapping columns in the visual cortex are present at birth

- in a normal adult the critical period produces the distinctive pattern of columns for the left and right eye

- columns that receive input from a light-deprived eye become much narrower


- columns with axons from the light-deprived eye are narrower than those receiving light stimulation

- dendrites and synapses from the light-stimulated eye take up more territory in the visual cortex

- this suggests that light stimulation is needed for the refinement of the columns and full development of the visual cortex

- axons compete for target cells in the visual cortex

- every time a neurone fires onto a target cell the synapses of another neurone sharing the target cell are weakened and they release less neurotransmitter

49 of 71

Making sense of what we see

- neurones in the visual cortex are alve to respond to the information from the retina

- individual neurones in the columns of cells respond in different ways to the information from the retina and to different characteristics of the object being viewed

- some neurones called simple cells respond to bard of light

- others called complex cells respond to edges, slits or bars of light that move, others to the angle of the edge and others to contours, movement or orientation

- visual perception involves knowledge and experience as the brain interprets the sensory information received from the retina

50 of 71

Depth perception

Close objects:

- for close objects we depend on the presence of cells in the visual cortex that obtain information from both eyes at once

- the visual field is seen from two different angles

- the cells in the visual cortex let us compare the view from one eye with that from the other

- this is called stereoscopic vision and allows relative position of objects to be perceived

Distant objects:

- for far objects the images on our two retinas are very similar, so visual cues and past experiences are used with interpreting the images

- overlaps of objects and changes of colour also help in judging depth

- for example when a car drives away we perceive it as moving further away not getting smaller

51 of 71

Cross-cultural studies

- people from different cultures may not share the same beliefs and they may show different behaviours

Carpentered world hypothesis:

- those who live in a world dominated by straight lines and right angles perceive depth cues very differently from those who live in a 'circular culture'

- when surrounded by buildings with right angle corners unconsciously from an early age tend to interpret images with acute and obtuse angles as right angles

- people who live in 'circular culture' with few straight lines or right angle corners

- they have have little experience of interpreting acute and obtuse angles on the retina as representations of right angles

- studies show them to be rarely fooled by optical illusions

52 of 71

Studies with newborn babies

The visual cliff:

- babies are encouraged to crawl across a table made of glass or perspex, below which is a visual cliff

- patterns placed below the glass create the appearance of a steep drop

- if the perception of depth is innate then babies shout be aware of the drop even if they have not previously experienced this stimulus themselves

- young babies were reluctant to crawl over the 'cliff' even when their mothers encouraged them

- the experiment was repeated with animals that can walk as soon as they are born (e.g. chicks)

- they too refused to cross the cliff

53 of 71

Learning and memory

- the nervous system changes  with changes occurring in our network of neurones, often by the modification of synapses

- it also changes when changes in the synapses that underpin learning and memory changes

- memory is located in different parts of the cortex with different sites for short and long-term memory

- different types of memory are controlled by different parts of the brain

How memories are stored:

- in the brain every neurone connects with many other neurones to make up a complex network

  • the pattern of connections
  • the strength of synapses
54 of 71

Sea slugs and habituation

- there are no fundamental differences between the nerve cells and synapses of humans and animals such as sea slugs

- but sea slugs have less neurones so their neurobiology is much simpler than that of humans

- sea slugs also have large accessible neurones so those involved in particular behaviors can be identified

- sea slugs behaviour can be modified by learning and the effects on neurones and synapses studied

- the sea slug breathes through a gill located in a cavity on the upper side of its body

- water is expelled through a siphon tube at one end on the cavity

- if the siphon is touched the gill is withdrawn into the cavity, this is a protective reflex action

55 of 71

Sea slugs and habituation continued

- sea slugs are habituated to waves, habituation is a type of learning

- so the gill withdraws when siphon is stimulated but after a few minutes of repeated stimulation the siphon no longer withdraws

- habituation allows animals to ignore unimportant stimuli

- this is so that limited sensory, attention and memory resources can be concentrated in more threatening or rewarding stimuli

How habituation is achieved:

- with repeated stimulation Ca2+ channels become less responsive so less Ca2+ crosses the presynaptic membrane

- therefore less neurotransmitter is released

- there is less depolarisation of the postsynaptic membrane so no action potential is triggered in the motor neurone

56 of 71


What is happening during sensitisation?:

- sensitisation is the opposite of habituation, it happens when an animal develops an enhanced response to a stimulus

-in sea slugs if a predator attacks it becomes sensitised to other changes in its environment and responds strongly to them

sensitisation: (a shock to the tail enhances the gill withdrawal due to the water jet)

  • impulse due to electric shock to tail
  • serotonin released
  • greater calcium ion uptake
  • impulse passes along sensory neurone
  • more neurotransmitter released
  • greater depolarisation
  • higher frequency of action potentials
  • enhanced gill withdrawal response
57 of 71

Ethics of using animals in medical research

The importance of consent:

Accepting that animals have rights we could only use animals that consented to participatein medical experiments, just like we only use humans if that give their consent.

Animal welfare rather than animal rights:

A widespread belief is that humans should treat animals as well as possible. No country in the European Union is allowed to use vertebrates in medical experiments is there are non-animal alternatives.

Animal suffering and experience of pleasure:

both the animal rights approach and the animal welfare apporach assume that animals can suffer and experience pleasures.

A utilitarian approach to the use of animals:

Utilitarianism is the belief that the right course of action is one that maximises the amount of overall happiness or pleasure in the world. A utilitarian framework allows certain animals to be used in medical experiments provided the overall expected benifits are greater than the overall expected harms.

58 of 71

Dopamine and Parkinson's disease

- dopamine is a neurotransmitter secreted by neurones, included many located in part of the midbrain

- these neurones normally release dopamine in the motor cortex

- Parkinson's patient motor cortexes receive little dopamine and there is a loss of control of muscular movements

- the main symptoms of the disease are:

  • stiffness of muscles
  • tremor of the muscles
  • slowness of movement
  • poor balance
  • walking problems
  • depression
  • difficulties with speech and breathing
59 of 71

Treatment for parkinsons disease

- slowing the loss of dopamine from the brain by using drugs such as selegiline. The drug inhibits the enzyme monoamine oxidase which breaks down dopamine in the brain.

- A drug called L-dopa can be given. Once in the brain L-dopa is converted into dopamine, increasing the concentration of dopamine.

- the use of dopamine agonists. They are drugs that activate the dopamine receptor directly, they bind to dopamine receptors at synapses and trigger action potentials.

- gene therapy can be used. genes for proteins that increase dopamine production and that promote the growth and survival of nerve cells are inserted into the brain. cell therapy in which the proteins themselves are injected is also being trialled.

- New surgical approaches are being trialled, some of which are generating encouraging results.

60 of 71


-  neurones that secrete serotonin are stimulated in the brain stem. a lack of serotonin has been linked to depression.

- Their axons extend to the cortex, the cerebellum and the spinal cord, targetinga a huge area of the brain.

- depression is a multifactorial condition; several genes my be involved but so my environmental factors.

- a gene called 5-HTT is known to influence our susceptibility to depression, people with the 'short' version of the 5-HTT gene are more likely to develop depression after a stressful life event.

- when someone is depressed, fewer never impulses than normal are transmitted abound the brain, which may cause low levels of neurotransmitters to be produced.

- serotonin binding sites are more numerous than normal when depressed to make up for the low levels of the molecule.

Drug treatment for depression:(SSRI and Prozac)

- the drugs inhibit the reuptake of serotonin from synaptic clefts

- this type of drug is called a Serotonin Reuptake Inhibitor (SSRI) so it blocks the only uptake of serotonin.

61 of 71

How drugs affect synaptic transmission

The effect of ecstasy:

- effects thinking, mood and memory and can also cause anxiety and altered perceptions. Its most desirable effect is that it provides feelings of emotional warmth and empathy.

- there are five different stages in synaptic transmission that can be affected by drugs:

  • neurotransmitter synthesis and storage
  • neurotransmitter release
  • neurotransmitter-receptor binding
  • neurotransmitter reuptake
  • neurotransmitter breakdown

- short-term effects include changes in behaviour and brain chemistry, sweating, dry mouth, increased heart rate, fatigue, muscle spasms and hypothermia.

- long-term effects include changes in behaviour and brain structure

62 of 71

How drugs affect synaptic transmission

How ecstasy affects synapses:

- ecstasy increases the concentration of serotonin in the synaptic cleft

- it does this by binding to molecules in the presynaptic membrane that are responsible for transporting the serotonin back into the cytoplasm

- this prevents its removal from the synaptic cleft

- the drug may also cause the transporting molecules to work in reverse, further increasing the amount of serotonin outside the cell

- these higher levels of serotonin bring about the mood changes seen in users of the drug

- there is growing evidence of long-term effects including insomnia, depression and other psychological problems

63 of 71

Better treatments

- the deciphering of the base sequence in the human genome as part of the Human Genome Project (HGP) means we are now getting a better understanding of the way genes control our phenotype

- a genome is all the DNA of a organism (or species), including the genes that carry the information for making the proteins required by the organism (or species)

- these proteins help determine all the characteristics of the organism from individual biochemical pathways to its overall appearance

- In 1977 Fred Sanger the first DNA sequencing process

- DNA is used as a template to replicate a set of DNA fragments, each differing in lenth by one base

- the fragments are are separated according to size using gel electrophoresis and the base at the end of each fragment is identified 

- this allows the sequence of bases in the whole DNA chain to be determined

64 of 71

Human Genome Project

Issues with the Human Genome Project:

- testing for generic predisposition has many implications

- who should decide about the use of generic predisposition tests, and on whom should they be used?

- making and keeping records of individual genotypes raises acute problems of confidentiality

- many medical treatments made possible through the development of genetic technologies will initially be very expensive 

- restricted availability of many medical treatments will add considerably to the problems faced by the health services in deciding who is eligible for the treatments

65 of 71

Using genetically modified organisms to produce dr

-it has been possible to genetically modify non-human organisms to produce specific human proteins, such as the human growth hormone, insulin and collagen

- the artificial introduction of genetic material from another organism  through genetic modification produces a transgenic or genetically modified organism (GMO)

- genetic modification is also known as genetic engineering or genetic manipulation or recombinant DNA technology

Modifying organisms:

- the first success in genetic engineering was with bacteria

- bacteria contain simple DNA structures, plasmids, which can be transferred from one cell to another

- using restriction enzymes, the circular plasmid can be cut, and using another set of enzymes a piece of DNA from another species can be inserted in

66 of 71

Modifying organisms

- an example of how bacteria can be used for modifying organisms is to produce the human protein insulin

- a vaccine was then produced using this to protect against hepatitis B

The steps in using bacteria to produce human insulin:

  • a plasmid is extracted from a bacterial cell
  • the extracted plasmid is then cut with restriction enzyme
  • an isolated human gene is spliced into the plasmid
  • the modified plasmid is then put back into the bacterial cells
  • the cells then multiply in a fermenter
  • the bacterium then produces human insulin
  • the bacterial cells are destroyed
  • and the insulin protein is extracted and purified
67 of 71

Genetically modified plants

- for centuries farmers have picked out the hardest and most prolific plants from their crops and have saved the seeds from these plants for sowing the following year

- because of this crops have steadily improved, this is called artificial selection

- genetic engineers introduce new genes with alleles for desired characteristics into a plants DNA resulting in genetically modified plants

- Genes are inserted into plant cells. this can be done by:

  • a bacterium that infects many species of plant. When the bacteria invade the plant cells genes from plasmid DNA become incorporated into the chromosomes of the plant cells
  • Minute pellets that are converted with DNA carrying the desired genes are shot into the plant cells using a particle gun.
  • viruses are sometimes used. They infect cells by inserting their DNA or RNA. They can be used to transfer the new genes into the cell.
68 of 71

Genetically modified plants continued

- scientists need a method of screening to find out which plant cells actually have the new gene

- this is generally done by incorporating a gene for antibiotic resistance, often called a marker gene, along with the new desired gene

- the plant cells are then incubated with the antibiotic which kills of any unsuccessful cells that have not taken up the new genes

- the only cells to survive are the ones that have successfully incorporated the new genes and are resistant

- the genetically modified plant cells can then be cultured in agar with nutrients and plant growth substances to produce new plants (sucrose, amino acids, inorganic ions and plant growth substances)

- the plantlets are then separated and grown into full size plants to produce transgenic plants

69 of 71

Genetically modified animals

- ways in which genes can be inserted into animal cells:

  • injecting DNA directly into the nucleus of a fertilised egg
  • the egg is then implanted into a surrogate female
  • retroviruses have also been used to introduce new genes into fertilised eggs
  • this type of virus incorporates its DNA into the hosts DNA

- tracey was the first transgenic sheep, her DNA contained the human gene for the protein AAT

- AAT is normally made by our liver cells and inhibits the enzyme elastase. elastase is released from neutophils, the white blood cells that fight infection.

- protease digests damaged or aging lung cells, foreign particles and bacteria. ATT prevents elastase attacking normal tissue.

- the inherited disease A1AD mutates the gene coding for ATT. A lack of ATT can cause lung disease such as emphysema, as the elastase attacks normal lung tissue

70 of 71

Concerns about genetic modification


  • transfer of antibiotic-resistance genes to microbes
  • formation of harmful products by new genes 
  • transfer of viruses from animals to humans

Environmental concerns about GMOs are:

  • transfer of genes to non-GM plants (e.g. cross-pollination caused by wind or insects)
  • increased chemical use in crops 


  • ensure that outcrosses are not fertile and cannot proliferate
  • development of technology whereby the pollen does not contain the modified gene so it cannot be spread
71 of 71


Alex W



Very considerate of you to make and share these. Thanks




your a legend

joyce omatseye




Thanks :) really helpful for last minute revision :D


Just got this, and loving it so far!

Similar Biology resources:

See all Biology resources »See all resources »