Grey Matter

Neurones

  • Neurone - single cell, nerve bundle of axons of many neurones surrounded by a protective covering 
    • Cell body - contain cell organelles such as a nucleus 
    • Dendrites - conducting impulses towards the cell body
    • Axons - transmitting impulses away from the cell body 
  • Sensory neurone - carries impulses from sensory receptors to the brain 
    • Before the CNS
    • Dendrites synapse with receptor cells, axons synapse with relay neurones in the CNS
  • Motor (efferent) neurone - carries impulses from the CNS to effectors
    • Cell body and dendrites in CNS
    • Dendrites synapse with other neurones in the CNS
    • Axons synapse with effector cells (muscles, glands) 
  • Relay neurones - carries information from one part of the CNS to another - dendrites and axons synapse with other neurones in the CNS
  • Myelin sheath - made up of shwann cells (lipids) - increases the speed of conduction along neurones - acts as an electrical insulator preventing any flow of ions accross the membrane 
    • Node of raniver - the only place where depolarisation can occur - impulse jumps from one node to the next via salutatory conduction 
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Nervous systems

Nervous system - communication within the body via nervous tissue and the spinal cord 

  • Central nervous system (CNS) - consists of the brain and spinal cord (base of the brain through the neural arches of the vertebra as far as the lumbar vertebra - relay neurones 
  • Peripheral nervous system (PNS) - sensory and motor neurones 
    • Somatic nervous system - voluntary, stimulates skeletal muscle 
    • Autonomic nervous system - involuntary, stimulates smooth muscle
      • Sympathetic nervous system - fight or flight response
      • Parasympathetic nervous system - prepares bodt for 'rest and digest' 
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Reflex arc - pupil reflex

Image result for pupil reflex a level biology (http://ib.bioninja.com.au/_Media/pupil-reflex_med.jpeg)

sensory neurones in optic nerve hit by high light levels - coordinating neurones in the midbrain, relay neurones - parasympathetic neurone - motor neurone -  radical muscles relax and circular muscles contract - constricts the pupil - reducing the amount of light entering the eye 

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Action Potential

Resting potential (-70mV) 

  • Na+/K+ pump creates a concentration gradient across the membrane 
    • 3Na+ ions out/2K+ ions in
    • Low K+ and high Na+ concentration outside the cell, high K, low Na+ inside
  • K+ diffuse out of the cell down the K+ concentration gradient, making the outside of the membrane positive and the inside negative creating a potential difference 
  • The potential difference will pull K+ back into the cell via the electrochemical gradient 
  • At -70 mV the two gradients counteract each other and there is no net movement of K+

Active Potential 

1. Depolarisation (+40mV) 

  • Stimulation of neurone causes some depolarisation (change in the potential difference)
  • Depolarisation opens some voltage-dependant sodium channels - sodium flows into the axon increasing depolarisation and causing more gates to open wants a certain threshold is reached, the opening of more gates causes further depolarisation - positive feedback loop 
    • All-or-nothing principle 
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Action Potential (2)

2. Repolarisation

  • After about 0.5ms the voltage-dependent Na+ channels spontaneously close and the permeability of the membrane to sodium returns to its low level
  • Volatage-dependant K+ channels open due to depolarisation of the membrane 
  • Potassium ions move out of the axon along the electrochemical gradient 
  • Causing the inside of the cell to become more negative than the outside 

3 Restoring resting potential (Refractory Period) 

  • The membrane is now highly permeable to potassium ions and more ions move out than occurs at resting potential - hyperpolarisation (-90mV) 
  • Voltage-dependent K+ channels close 
  • K+ diffuses into the axon to recreate resting potential 
    • A new action potential cannot be generated in the same section of the membrane for about 5 milliseconds until resting potential has been restored
    • Ensuring that impulses only travel in one direction 
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Synapses

Role of synapses

  • Control of nerve pathways allowing flexibility of response 
  • Coordination of response; integration of information from different neurones

Types of synapses 

  • Excitatory synapse - make the post-synaptic membrane more permeable to sodium ions 
    • Spatial summation - impulses from different synapses, and usually from different neurones
    • Temporal summation - several impulses arrive at the synapse having travelled along a single neurone - combined release of neurotransmitter generates an action potential in the postsynaptic membranes 
    • Several impulses arriving within a short space of time produce sufficient depolarisation to produce an action potential in the  postsynaptic cell
  • Inhibitory synapse - make it less likely that an action potential will occur in the postsynpatic cell
    • neutrotransmitters from these cells open channels for chloride ions and potassium ions in the postsynaptic membrane 
    • Chloride ions wil move into the cell carrying a negative charge and potassium ions will move out of the cell carrying a postive charge - causing hyperpolarisation 
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Intensity of stimulus

  • The frequence of impulses 
  • The number of neurones in the nerve that are conducting impulses 

Across the synapse

  • Amount of neurotransmitter reaching the postsynaptic membrane 
  • The number of functioning receptors in the postsynaptic membranes

Excitatory or Inhibitory synapse 

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Eyes

Photoreceptors - Synapse with bipolar neurone cells which synapse with ganglion cells whose axons make up the optic nerve - Cones - allow colour vision in bright light, concentrated in the fovea - Rods - give black and white vision and can work in dim light 

In the dark 

  • Na+ ions flow through non-specific cation channels - sodium ions move down the concentration gradient into the inner segment where pumps continuously transports them out of the cell 
  • Influx of Na+ produces a slight depolarisation (-40mV) - triggers the release of a neuotransmitter - which binds to the bipolar cell stopping it from depolarising

In the light 

  • Light falls on the rhodopsin molecule - breaks down into retinal and opsin - Opsin activates a cascade of reactions that result in the closing of the cation channels so Na+ can't enter
  • Na+ actively pumped out causing the membrane to be hyperpolarised 
  • No neurotransmitter is released - no inhibition - cation channels in the bipolar cell open, membrane becomes depolarised - generating an active potential
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Photoreceptors in plants

Phytochromes - plant photoreceptors 

  • Pr - phytochrome red (absorbs red light) 
  • Pfr - phytochrome far red (absorbs far red light) 
  • Photoreversible 
    • In the sunlight Pr is converted to Pfr and Pfr to Pr  - Pfr accumulates in the sun as Pr to Pfr dominates 
    • In the dark Pfr is converted to Pr 

Trigger germination 

  • Final flash of light determines whether germination occurs, red light triggers germination, far red light inhibits germination 
  • When exposed to ref light Pr is converted to Pfr, stimulating responses that lead to germination 

Photoperiods - the relative length of day and night determines  the time of flowering 

Greening - Light activates phytochrome - activates proteins in a signal pathway, activate transcription factors 

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Coordination in Plants and Humans

Plants - Auxins - plant growth substance  - IAA (indoleacetic acid) -  Phototropism - plants grow towards the light - auxin inhibited by light - increased concentration of the auxin on the shaded side increased cell elongation as a result the shoot grew towards the light

Humans

Nervous control 

  • Electrical transmission by nerve impulses and chemical transmissions at synapses
  • Fast acting - short term changes  e.g. muscle contraction 
  • Action potentials carried by neurones with connections to specific cells 
  • Response if often very local, such as a specific muscle or gland 

Hormonal control

  • Chemical transmission through the blood
  • Slower acting - con control long-term changes e.g. growth 
  • Blood carries hormones to all cells, but only target cells are able to respond
  • Response may be widespread 
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Function of the Brain

  • Frontal lobe - higher brain functions such as decision making, reasoning, planning and consciousness of emotions, ideas. primary motor cortex which connects directly to the spinal cord and brain stem - to carry out movements and store information about movements 
  • Parietal lobe - orientation, movement, sensation, calculation, some types of recognition and memory
  • 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, involved in memory 
  • Thalamus - routing all incoming sensory information to the correct part of the brain 
  • Hypothalamus - lies below the thalamus - thermoregulatory centre - monitors temperature and initates corrective action - homeostasis, controls sleep thirst and hunger,  and acts as an endocrine gland producing hormones such as antidiuretic hormone - connects directly to the pitutiary gland 
  • Hippocampus - long term memory
  • Medulla oblongata - regulates the autonomous nervous system 
  • Cerebellum - balance, coordinates movement, recieving information from the primary motor cortex
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Brain Scanning

  • CAT imaging - uses thousands of narrow-beam X-rays rotated around the patient to pass through the tissue from different angles - each beam is attenuated according to the density of the tissue in its path - X-rays are detected and used to produce an image of a spliced section of the brain on the computer scan 
    • Can only give frozen moment picures - structures of the brain rather than fucntions 
    • Used to detect brain disease and to monitor the tissues of the brain 
    • Limited resolution and X-rays can be harmful 
  • Magnetic Resonance Imaging (MRI) 
    • Magnetic field + magnetic field of the high frequency radio waves - causes the direction and frequency of spin of the hydrogen nuclei to change taking energy from the radio waves to do so 
    • When the waves are turned off, the hydrogen returns to their origional alignement and release the energy they absorbed 
    • This energy is detected and a signal is sent to the computer, which analyses an image on the screen - different tissues respond differently to the magnetic field from the radio waves and so produce contrasting signals and distinct regions on the image 
      • Diagnosis of tumours, strokes, brain injuries and infections 
      • Higher resolution 
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Brain Scanning (2)

Functional magnetic resonance imaging fMRI - used to look at the functions of different parts of the brains by following the uptake of oxygen in active brain areas

  • Deoxyhaemoglobin absorbs the radio waves, oxyhaemoglobin does not - increased neural activity in a brain area results in increased demand for oxygen and blood flow 
  • So there is a large increase in oxyhaemoglobin in the enhanced blood flow, so less signal is absorbed - fMRI - can produce 4 images per second

Positron Emission Tomography (PET) scans - detailed images that allow the structure and functioning of tissue and organs to be evaluated - heart disease, cancer 

  • Isotopes with short half lives C-11, N-13, - incorperated into compounds such as glucose or water or into molecules that bind to receptors - radiotracers
  • As the radiotracer decays it emits positrons - when a tissue is active there is increased energy use - increased blood flow, increased glucose, more radiotracer atoms will be present in that area 
  • Positron collides with an electron, two gamma rays emitted - picked up by detectors
  • Expensive and can only be done once or twice a year for safety reasons 
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Critical period and visual development

  • Visual development - rate of brain growth 250,000 neurones per minute to reach a total of 100,000 neurones, after birth increase in size due to elongation of axons, myelination and the development of synapses
  • Axon growth - ordered arangement of growth - axons from thalamus neurones grow towards the visual cortex in the occipital lobe 
  • Visual cortex - column of cells, proven in staining techniques and by usig electrical stimulation 
  • Critical window - specific periods of development in a young to enable proper development of a nervous system 
  • In monkeys at birth there is a great deal of overlap between the territories of different axons, in adults there is less even though the mass of the brain is greater - after light deprivation the columns of axons in the light-deprived eye are more narrow, than those receiving light stimulation 
    • There is a lack of visual stimulation in one eye 
    • Axons from the visually deprived eye do not pass impulses to cells in the visual cortex
    • Axons from the non-deprived eye pass impulses to cells in the visual cortex
    • Inactive synpases are eliminated
    • Synapses made by active axons are strengthened 
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Experiments on visual development

  • Crowley and Katz working with ferrrets - injected labelled tracers to show that the columns were formed before the critical period for vision, also seen in monkeys - suggesting genetic determination rather than environmental stimulation 
  • Hubel and Weisel - testing kittens for the effects of monocular deprivation at different stages of development - deprivation of under three weeks and after three months had no effect on long-term vision -deprivation at four weeks had a major effect even if the eye was closed for merely a few hours, saw the same thing in monkeys 

Ethics 

  • Importance of consent - annimals cannot give consent, do they have rights? 
  • Animal welfare rather than animal rights - no country in the European Union is allowed to use verterbrates in medical experiments if there is a non-animal alternative
  • Animal suffering and pleasure -  if the animals do not feel pain/cannot suffer then experimenting on them doesn't increase their suffering e.g. insects and spiders
  • Utilitarianism 
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Habituation

Memories created in two ways - the pattern of connections, the strength of synapses 

Sea slugs and habituation 

  • Only 20,000 neurones - those involved in particular behaviours can be identified 
  • Habituation - gives animals the ability to ignore unimportant repetitive stimuli so that limited sensory, attention and memory resources can be concentrated on more threatening or rewarding stimuli 
  • If the sea slugs siphon is touched, the gill is withdrawn into the cavity - protective reflex action 
  • After several minutes of repeated stimulation the gill is no longer withdrawn 
  • With repeated stimulation Ca2+ channels become less responsive so less Ca2+ cross the presynaptic membrane
  • So less neurotransmitter is released 
  • There is less depolarisation of the postsynaptic membrane so no action potential is triggered in the motor neurone 
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Parkinson's

Parkinson's Disease

  • Dopamine - neurotransmitter - in people with Parkinson's dopamine-secreting neurones in the basal ganglia die, which normally release dopamine into the motor cortex
  • loss of control of muscle movements - stiffness of muscles, tremor, slowness of movement, poor balance, walking problems 
  • Depression, difficulties with speech and breathing 
  • Treatment
    • Slow loss of dopamine - selegiline inhibts monoamine oxidase (MOA) which is the enzyme responsible for the break down of dopamine 
    • Treat the symptoms - L-dopa - a precursor to the manufacture of dopamine can cross the blood brain barrier, L-dopa is converted to dopamine in the brain 
    • Dopamine agonists - activate the dopamine receptor directly and mimic the role of dopamine in the brain 
    • Gene therapy 
    • Deep-brain stimulation 
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Depression

  • Multifactorial condition; genetic link, environmental factors trigger events
  • Serotonin is an important mood regulator - neurones that secrete seratonin are situated in the brain stem - lack of serotonin linked to depression
  • Dopamine and noradrenaline, reduced serotonin levels most common 
  • When someone is depressed fewer nerve impulses than normal are transmitted around the brain; low levels of neurotransmitters, abnormalities in pathways involving serotonin
  • Treatment - Selective Serotonin Reuptake Inhibitor (SSRI) - prevents the reuptake of serotonin into the synaptic cleft e.g. Prozac 

Ecstacy (MDMA) - emotional warmth and empathy, anxiety, altered perceptions and depression  - derivative of amphetamine - short term effect - changes in the behaviour and brain chemistry, long term - changes in behaviour and brain structure

  • MDMA increases the concentration of serotonin in the synaptic cleft by binding to the molecules in the presynaptic membrane responsible for bringing serotonin back into the cytoplasm 
  • Long term effects of insomnia, depression and other pschological problema - becauses the drug has stimulated so much serotonin the cells cannot synthesise enough to meet demand once it has gone 
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Human Genome Project

  • 1977 - Fred Snager invented the first DNA sequencing process 
  • 1990 Human Genome Project formed - USA and UK, rest of Europe and Japan joined in 1992 - rapid progress 
  • 2001 - the whole of the genome was sequenced with 99.99% accuracy 
  • 3,200,000,000 bases, 20,000 to 25,000 genes, average human gene 3000 bases 
  • Identification of new genes -  possible to located a candidate gene ( a gene that may cause a specific disease) on our DNA and screen this gene in affected individuals e.g. six genes have been identified as increasing susceptibility to Alzheimer's disease - ApoE4 
  • Identification of new drug targets - a drug target is a specific molecule that a drug intereact with to bring about its effect - it is now thought that as many as 3000 genes express proteins whose activites could be altered by medecines 
  • Personalised medicine - preventative and improved drug treatment - phamacogenomics

Ethical dilemmas 

  • Implications of testing for genetic dispositions e.g, for medical insurance
  • Who should decide about the use of genetic disposition tests
  • Confidentiality
  • Discrimination 
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Genetic modification/engineering

Transgenic or genetically modified organism (GMO) - produced by artificially introducing genetic material 

  • Plasmids - can be transferred from one cell to another - using restriction enzymes the circular plasmid can be cut and usinganother set of enzymes a piece of DNA from another species can be inserted into it. The plasmid is then inserted back into the bacteria, which are allowed to multply in a fermenter. The protein produced is extracted from the culture e.g. insulin 
  • Plants - process of plant breeding (aritificial selection) is slow - genetically engineered plants can be produced in months rather than years - potential to mass produce medecines and other chemicals cheaply and efficiently 
    • Bacterium that infects the plant can be used - when the bacteria invades the plant cells, plasmid DNA gets incorporated into its DNA - insert the desired gene into the plasmid, which then carries these genes into the plant DNA 
    • Minute pellets that are covered in DNA carrying the desired genes are shot into plant cells using a particle gun 
    • Viral infection 
  • Animals -  - injecting DNA directly into egg nucleus and implanted into a surrogate female 
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Concerns about genetic modification

  • Health - transfer of antibiotic-resistance genes into microbes, formation of harmful projects by new genes. Transfer of viruses from animals to humans 
  • Antibiotic resistance - GM plants contain not only the useful gene, but also a marker gene to select for the new plants - these are sometimes antibiotic-resistant genes - this could be transferred into the pathogenic microbes in the gut - as yet we have no evidence for this
  • Harmful products from new genes - difficult to extract and purify proteins from GMPs to the standard required for medical use - no reported cases of ill health resulting from the consumption of GMOs 0
  • Transder of viruses from animals to humans 
  • Choice 
  • Environmental issues - transfer of genes to non-target species, possible breeding of superweeds, the possibility that GN crops will lead to increased use of chemicals in agriculture 
    • Cross-pollination 
  • Who owns these new organisms? 
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Nature versus nurture

Cross-cultural studies - carpentered world hypothesis - those who live in a world dominated by straight line and right angles tend to see depth perception cues very differently to those who live in a 'circular culture'  - for instance the Zulu people of Africa are rarely fooled by the Muler-Lyer illusion 

Studies with new born babies - the visual cliff - if the perception of depth is innate then the babies should be aware of the drop even if they have not previously experienced this stimulus - it is only possible with babies who have already learned how to crawl - learned depth perception - it was therefore repeated with animals that could walk as soon as they are born; chicks, goat kids, and lambs, they too refused to cross the cliff 

Twin studies - can help determine whether a disease or a certain trait has a genetic cause. Identical twins produced by the division of the same egg, are virtually genetically identical - degree of similarity between the monozygotic twins is a measure of the influence of genes on that characteristic

Individuals with damaged brain areas - Broca's area -lesions in a small cortical area in the left frontal lobe were responsible for deficits in language production, 'face-blind' - facial recognition unit in the temporal lobe, 

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