Topic 8 - Grey Matter

Nervous system

CNS - brain and spinal chord (and optic nerve), protected by bone and meninges

PNS - all of the nerves in the body

Hormonal system

Hormones - chemical substances produced by endocrine gland carried in blood, bind to receptors on cell surface or in cytoplasm, alter gene expression

Slower in action than nerves, do not need instant reuslts

Longer effect - hormones need sot be broken down

Myleinated

Myelin sheath made of specialised Schwann cells, uninsulated gaps between Schwann cells known as nodes of rnaveri, electrical impulses jump from node to node

Faster transmission of nervous impulses

Neurones

Sensory - impulses from receptors to brain and spinal chord

Relay - in CNCS, connect sensory and motor

Motor - CNS to effector

Eye

Circular muscles to constrict the pupil

Radial muscles to dilate

Potentials?

Inside has a negative potential compared to outside, difference in charge is due to difference in different numbers of ions, diffferenc in charge = polarised

-70mV when resting

Sodium potassium pumps are present int he cell surface membranes of neurones, these us ATP to actively transport sodium out of the axon and potassium into the axon (their ions),

3 sodium out, 2 potassium in, creasing a concentration gradient

This means both will diffuse back across the membrane, facilities diffusion w channels

Membrane less epermeable to sodium ions so potassium inside can diffuse out a a faster rate, resulting in more positive ions ours, generating a negative charge inside hence the resting potential -70

Action potential - once resting potential is reached its said to be polarised, to initiate a nerve impulse it must be depolarised

-70 ——> +30

Some of the ion channels are voltage gated ad open and close in response to changes int eh electrical portenail across the membrane, closes when membrane is at rest, not all are voltage gated

When stimulated, a small number of sodium ion channels open and sodium ions begin to move into the axon down their concentration gradient, reduces potential difference across the axon as the inside becomes less negative

If reaches -55 known as threshold potential, more sodium ion channels open and further influx of sodium ions

Action potential only if threshold is reached

Once it gets to +30 it is said to be depolarisation and an action potential has been generated

Repolarisation:

About one second after, the voltage gated sodium channels shut, and the v g k channels open allowing the diffusion of potassium ions out of the axon down their conc gradient (gradient is still present bc sodium potassium pupils still working)

This movement causes the inside to be negatively charged again - repolarisation

There is a short period where the membrane is more negative than resting potential - hyper polarisation , known as the refractory period, the membrane is unresponsive to stimulation so a new action. Potential cannot be generate, making them discrete evens and meaning the impulse can only travel in one direction

Voltage gated k Chanel’s then close and sodium potassiumm pump works to restore resting potention and only once its been restored can it be stimulated again

Transmission of an action potential

Once generated, it can be propagated along the length of an axon, depolarisation causes sodium ions to diffuse along the berg of the axon , depolarising the membrane at the new section, triggering another action potential, repeating along the length of the axon (any diffusing back can’t trigger cos fo hyperpolaristaion), wave of depolarisation

All or nothing - past threshold potential and action potential generated

Preventing transmission - drugs may bind to sodium ion channels and prevent them from opening and prevent an influx of sodiu ions when axon is stimulated, prevents membrane depolarisation a dn action potential cannot occur

Myelination

Unmyelinated - speed of conduct is slow because depolarisation must occur across whole membrane of axon, by insulating, it speeds up action potnetnails travelling, action potential as only at nodes of ranver, sodium ions diffuse between - local currents, jumping from node to node - saltatory conductaion

Synapses

Synaptic cleft, presynaptic neurone and knob, post synaptic neurone and knob , diffusion fo chemicals - neurotransmitters (within vesicles in the synaptic knob)

When an impulse arrive at the presynaptic knob it becomes depolarised and causes voltages gated calcium channels to open

Calcium ions diffuse into the knob via the channels and cause the vesicles to move towards the presynaptic membrane where they fuse with it and stimulate release by exocytosis

Neurotransmitters diffuse across and bind to receptor molecules on the post synaptic membrane, this causes associated sodium channels to open, allowing sodium to diffuse into the post synaptic cell, if enough neurotransmitters bind, then an action potential is generated whcih travels down the axon of the post synaptic neurone, this depend not eh threshold potential duh, whcih in turn depends on the number fo action potentials arriving at the presynaptic knob, many action potentials means more neurotransmitter released, so more sodium channels will open, large influx of sodium ions increasing the likelihood of the threshold being reached

The neutrotransmitters are then broken down to prevent continued stimulation to the post synaptic netone

Synapses enable unidirectionalyity prof nerve impulses (n released on one side and receptors on other)

Divergence of nerve impulses - one neurone can connect to several neurones at. A synapse

Amplification fo nerves signals by summation, several presynaptic neurones (synaptic convergence) or may action potentials arriving at a post synaptic knob in quick succession

Detection of light

Light enters through pupil, focused on fovea of retina, amount of light entering is controlled by iris, light is focused using the lens the shape of which is controlled by ciliary muscles and suspensory ligaments

The fovea contains light and photorectosp

Photoreceptors:

Rod cells - outer retina, sensitive to light intensity, images only generated black and white

Cone cells - grouped together in fovea, sensitive to different wavelengths fo light and so can detect colour red/green/blue, imagines in colour duh

Action potentials generated are transmitted to the brain via the optic nerve whcih leaves the back of the eye in the blind spot (no photorecptors)

Photoreceptors and nerve impulses

Action potentials generated when stimulated by bright light (rod) or certain wavelengths fo light (cone)

Light sensitive pigments inside pot receptors are bleached when light falls on them

Rod cells - rhodopsin, when light hits it breaks apart into retinal and opsin, this bleaching results in a chemical change that results in the generation of a nerve impulse

Nerve impulses travel along a bipolar neurone and then converge to a single ganglion cell (convergence doesn’t happen from bipolar neurones of cone cells to ganglion cells) and then to optic nerve

Rod cells initiate action potentials in neighbouring bipolar cells when they are hyperpolarised, so in the dark: sodiu ions are actively pumped out generating a conc gradient, they diffuse back in, little difference in charge so depolarised, this released neurotransmitters, synapse, bipolar neurone but rather than initiating an action potential in bipolar neurone, th neurotransmitter inhibits the generation of an action potential, preventing a nerve impulse from being seen tot the optic nerv - inhibitory neurotransmitter

In light: light bleaches rhodopsin to from retinal and opsin, this causes the sodium ion channels to close preventing sodium from diffusing back in,pump out is still taking place, inside becomes more negative and reaches a hyperpolarised state, stops releasing the inhibitory neurotransmitter so the generation of an a p is no longer inhibited, a p generated in bipolar cell and an impulse is sent to the optic nerve

The effects of IAA

Tropism - directional growth response, photo = light, geo= gravity, can be positive (towards) or negative (away)

Plant growth relies on chemical messengers that are released in response to stimuli, produce dint eh growing parts of the plant and move to other tissues where they regulate cell growth in response to a directional stimulus e.g. auxin promotes cell elongation in shoots and inhibits growth in plant roots

Indoleacetic acid - IAA, a type of auxin, brings about plant responses by altering transcription fo genes inside plant cells, altering expression fi genes involved in growth, produced by cells in the growing part of the plant and is redistributed to others plant tissues, by diffusion or active transport, or if overlong distances in the phloem, the redistribution is affected by environmental stimuli leading to uneven plant growth

In plant shoots: conc of IAA determines the rate of cell elongation within the stem, if conc is not uniform, uneven growth can occur , when light shine son one side, IAA goes out shaded side, faster rate of cell elongation here so it bends towards the source of light

How - IAA causes proton pumping across cell membranes, which activates an enzyme which allows cellulose molecules or move further apart ?

In roots : geotropism, but HIGHER CONC = LOWER RATE OF CELL ELONGATION, IAA transported towards lower side, inhibits cell elongation here top grows at faster rate, root bends down

Phytochrome

Flowering in plants is controlled by the stimulus of night length: nights are shorter in spring and summer and longer in autumn a dn winter, some plants flower when nights are short and some flower when nights a re long

Nights ta certain length == altered gene expression

The length of night is determined by the plant due to different forms fo a pigment called phytochrome int he leaf

Phytochrome: Pr is the inactive form and absorbs light from eh red part of the spectrum, Pfr is the active form and absorbs light from the far red part of the spectrum

Absorption fo different wavelengths of light causes a reversible conversion between Pr and Pfr

When Pr absorbs light is is converted into Pfr and when Pfr absorbs light it is converted back not Pr, in the absence of red light the unstable Pfr gradually converts back into Pr

During the day levels of Pfr rise (sun contains red light wavelength so Pr is converted into Pfr)

During the light, red wave lights are not available so Pfr is converted back int Pr

Long day plants have high levels fo the active form (Pfr) which activates flowering

Flowering: days are long so Pr is converted to Pfrat a grater rate than Pfr is converted into Pr, active form is present at high levels Pfr activates expression fo genes that stimulate flowering the active gene is blah blah resulting protein causes flowers

Brain structure and function

Brain and spinal chord - CNS

Medulla oblongata - involuntary actions, cardiac centre, respiratory centre

cerebellum - valence, muscle coordination a dn movement

Cerebrum - largest part of brain , divided into two halves known as the cerbreal hemispheres which are joined by teh corpus callosum, thin outer layer called grey matter whcih consists of cell bodies of neurones, highly folded to increase as and allow it to contain a greater number of neurones = complexity, beneath the cerebral cortex is th white matter whcih contains myleinated axons voluntary actions, conscious activities, thought, vision, speech, memory, problem solving

Hypothalamus - monitors blood as it flows through the brain and in response releases hormones or stimulates the neighbour in pituitary gland to release hormones, monitors blood temperature, monitors water balance for blood and releases ADH if blood is too concentrate, regulates digesting activity , controls endocrine funcations (causes the pituitary gland to release hormones) control centre that monitors blood and helps maintain homewostasis

Pituitary glad - regulation

Studying the brain

CT - cross section images using x ray radiation, denser regions absorb more x rays and show up lighter, shows physical structures and allows visualisation fo any tissue damage don’t show the functions fo the areas, not reccomended for pregnant patients or children due to the risks fo exposure to x ray radiation whcih is a higher level than in normal x ray, risks of damage are still low

MRI - combination of a magnetic field and radio waves to generate images, soft tissues seen clealtl, images ta jigehru reolution than those with CT, only useful for identifying areas of abnormal or damaged tissue, linking dames on a scan with visible symptoms on patient, useful fro tumour diagnosis, more expensive but no risks associated with x rays, can integer with metal medical devices such as insulin pump and pacemakers so they cannot have MRI scans

Functional MRI - uses magnetic field and radio waves but allows the brain to be studies in real time, shows the location of oxygenated blood therefore indicting which regions of the brain are active, measures ratio of oxygenated to deoxygenated haemoglobin, patients asked to carry out particular actions or answer questions and the changes in blood flow to regions of her brainchild an be assese, medical diagnosis e.g. cause of seizures or psychological research

PET - use radioactive tracers whcih collect in areas of increased blood flow, metabolism or neurotransmitter activity, introduced into the blood before scan e.g. radioactively labelled glucose to see areas of high metabolic activity, the scanner detects high radioactivity and so the movement of the tracer can be tracked, and the amount of the tracer inducts whether that part of the brain is active or not, real time

Development of the visual cortex

Region in the cerebral cortex in whcih visual information is processed, soon after birth the neurones of the visual cortex of baby mammals begins o from synapses allowing visua l information to be transferred thought and processed by the visual cortex, both eyes need to be visually stimulated fo eh neurones int eh visual cortex to be organised correctly during this period of early development, known as the critical period, synapses that pass on nerve impulses during this critical period are strengthened and become permanent parts of the visual cortex, synapses haha do not relieve nerve impulses are lost and cannot be reformed whcih can result in blindness in one or both eyes, evidence comes from study using animal models

Roel of animals in research

Ethics, similar Brian’s, Huber and Wiesel reasech into Brian’s visual systems, long term impact of depriving various animal models of vision in one eye and in turn learn about the development of the brains visual cortex (part of cerebral cortex where visual information is processed)

Kittens used, at birth they stitched one eye, after 3 months they were blind int eh eye that had been stitched,, studied activity fo eh neurone sin the visual cortexes using electrodes, they found the ocular dominance columns corresponding to the stitched eye were small than normal kittens but the ocular dominance columns for the open eye were larger

Ocular dominance columns are groups of neurones int eh virtual cortex that respond to input from one eye or the other right from right, left from left

They concluded that the ocular dominance columns for the blinded eye were bale to redistribute, switch dominance to the other eye

Research was then repeated using adult cat models, no blindness resulted from eh 3 month loss of vision and no changes in distructrion of the ocular dominance columns

Results replicated in young and adult monkey s

Demonstrating the essential nature of light stimulation to the visual cortex during the earl months of development

ETHICS

Habituation

Detecting and responding to stimuli requires energy so it so important that energy isn’t wasted on non-threatening stimuli

If a stimulus is repeated many times with no negative outcome then an animal will learn not to respond to it - habituation

Animals become habituated due to changes in the transmission fo nervous impulses, when habituation has taken place fewer calcium ions move into the presynaptic neurone so less neurotransmitter release and an action potential is les silently

Imbalances in Brian chemicals leading to diseases

Parkinson’s - affects coordination fo move to, caused by the loss of neurones in some parts of the brain, symptoms: tremor in specific body parts, slow movement, stiff and inflexible muscles, difficulties with balance, changes to speech, the lost neurones normally produce dopamine whcih sis involved in muscle control, less dopamine is realised into the synaptic cleft, less is bale to bind with receptors so fewer sodium channels not eh membrane are opened so the shod is not met, so depolarisation fo eh psotysnaptic neurone doesn’t occur so fewer action potentials whcih crates movements such as tremors and slow moebemt

Drugs : dopamine agonists - produce the same effect as dopamine, dopamine precursors - chemicals that can be converted into dopamine e.g. L-dopa, enzyme inhibitors - MAOB inhibits inhibit the entity of MAOB which would normally break does ndopamien at the synaptic cleft, increasing dopamine levels in brain

Other treatments reaseach ongoing: gene therapy to either increase dopamine production or prevent destruction of dopamine producing cells, stem cell therapy to replace lost dopamine producing cells

Depression: low levels of neurotransmitter serotonin, serotonin transmits nerve impulses through areas of the brain that control mood, low levels increases episodes of depression, also noradrenaline and dopamine involved,

Antidepressants work by increasing the levels of the relevant neurotransmitters in the brain; SSRIs (selective serotonin reuptake inhibitors) prevent the uptake of serotonin at the synapses, increasing overall serotonin in the brain, TCAs increase levels of both serotonin a dn noradrenaline in the brain, MAOB inhibitors so the m=neurotransmuters arent broken down in the synaptic cleft

Action of drugs on synapses

Many drugs impact the nervous system bu altering the events that occur at the synapses

Increase transmission at synapse by:

• causing more neurotransmitter to be produced or more to be released

• Imitating the effect of the neurotransmitter by binding to and activating receptors ont eh postsynaptic membrane

• Parenting the breakdown fo neurotransmitters by enzymes

• Preventing the reuptake of neurotransmitter by the presynaptic cell

  Decrease transmission by:

• Preventing production of n in the presynaptic knob

• Preventing release

• Enabling n to gradually leak out fo there is little left when an action potential arrives and the n that leaks out is destroyed by enzymes

• Binding to the receptors ont eh post synaptic I knob to prevent neurotransmitters form binding

MDMA

Multiple n most notably serotonin: inhibits reuptake of serotonin into the presynaptic neurone by binding to specific proteins that enable serotonin reuptake located ont eh presynaptic membrane, therefore th amount of serotonin present int eh brain increased, also triggers releases of further serotonin from presynaptic neurones

L-dopa

Sued to that the symptoms of Parkinson’s, very similar structure to dopamine, transported form blood to brain where it is converted into dopamine in ca reaction catalysed by dopa-decarboxylation

Increases dopamine levels in brain (dopamine cannot be given directl as it cna cross the bbb), increased dopamine means more nerve impulses are transmitted in parts that control movement giving them better control and lessening symptoms

Personalised medicine

Development of target drugs to treat a variety fo diseases in individuals with different genotypes, tested on synthetic tissues cultured in lab and genetically identical

HGP sequencing of entire human genome stored in databases, genes that come for certain proteins can be frown and analysed whcih allows the development of drugs that tangent the specific proteins

And identification of risk, genetic screening for prevnatative measures , and the individuals genome cna be used to see how well they might respond to certain treatments

Increased reasceh costs - only wealthy people have access, insurance companies and employers discriminations, some may refuse personalised medicine,

Producing drugs using GMOs

Removing gene for desired thing from one and placing it in an other organism for it to be expressed

So the genetically engineered organism contains recombinant DNA and is a GMO

Restriction enzymes to remove

Many copies made using PCR

Copies inserted into plasmids which then transferrs the copies into microorganisms (plasmids are called DNA vectors)

The genetically modified microorganisms are grown in large fermenters containing nutrients enabling them to multiple to produce large quantities of the new protein

The protein can be isolated and purified before being packaged and distributed

Human insulin and human blood clotting factors are examples of medicinal proteins produced by genetically modified bacteria

Genetically modified plants - bacteria is used to infect plant

Genetically modified animals - gene for desired protein is injected into a zygote and implanted into uterus and then the portion can be purified

Risks and benefits of GMOs

Benefits

• crops higher yield

• Crops resistant to pests

• Cost effective to produce nexyesused in dintrual proceses]

• Disease treated increased of using animal proteins reducing the risk of allergic reactions

• Vaccines more accessible to people in rural areas

• Low cost supply of medications

  Risks

• - long term impact on health

• Pests may develop resistance

• Transmission or genetic material

• Monocultures whcih a re bad for biodiversity

• Moral objections to genetically modifying animals for the sole purpose of benefitting humans

Nature vs nurture int eh brain development

Difficult - animal experiments: study effects different environments have on brain development of same species, genetic modification to switch off certain genes so impact of genetic factors cna be studies, twin studies - genetically identical but they are raised in same environment, non identical used as control group genetically different but also same environment

So diff traits cos of nature, little diff traits environment

Cross cultural studies

Newborn studies

Brain damage studies