Grey Matter
- Created by: CookieMonster121
- Created on: 01-06-18 13:55
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
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'
Reflex arc - pupil reflex
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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?
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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