The Nervous System

• Short lived- neurotransmitters removed once they finish their job
• Localised- neurotransmitters are released directly onto cells
• Fast- impulses are fast, allowing quick reactions

Nervous system sends info as electrical impulses using neurones...

Types of Neurone:

• Sensory- take impulses from receptors to central nervous system (CNS)
• Relay- transmit impulses between sensory and motor neurones
• Motor- transmit impulses from CNA to effectors

Stimulus--> receptors-------------------> CNS----------------> Effectors--> Response

                                  Sensory neurone                  Motor Neurone

e.g friend waves> photoreceptors detect wave> CNS processes info> Muscle cells stimulated> contract so you wave back.

• Long lasting- aren't broken down by neurotransmitters.
• Widespread- transported all over body, wherever the target cells are.
• Slow- not released directly onto cells, have to travel in the blood.

Glands:

• secrete hormones e.g. pancreas secretes insulin.
• stimulated by change in substance conc. (e.g. hormone) or an impulse.

Hormones:

• Chemical messengers which travel in the bloodstream.
• Will only bind to specific receptors found on 'target cell' membranes.

Stimulus--> Receptor--> Hormone--> Effector--> Response

e.g. low blood glucose> Pancreas receptors> Pancreas releases glucagon> glucagon detected& glycogen converted to glucose> glucose released in blood.

• Receptors are specific to one kind of stimulus & they can be cells/proteins.
• Potential difference is the difference in voltage on the outside & inside of the cell. Resting potential is the potential difference at rest.
• A stimulus excites the receptor, allowing more ions in & out of the cell.
• The bigger the stimulus, the bigger the generator potential, and the more likely an action potential will be produced.

Pacinian Corpuscles:

• Mechanoreceptors- detect pressure, vibrations etc. and are found in skin
• Contain a sensory neurone ending wrapped in layers of lamellae
• Lamellae get squashed by pressure etc, and deform the stretch-mediated  sodium channels
• Sodium channels open, allowing Na+ ions into cell and creating a generator potential.
• If the generator potential reaches the threshold value, an action potential is produced.

Eyes:

• Light enters through pupil, iris muscles determine how much light enters
• Lens focuses light onto retina, where it's detected by photoreceptors (fovea is a part of retina which has the most photoreceptors).

Rod photoreceptors:

• Many rods to one neurone therefore...
• High sensitivity- work in dim light (many weak generator potentials combined= threshold value is easier to reach & fire an action potential)
• Low acuity- light from 2 close objects hit 3 rods, only fire 1 action potential

Cone photoreceptors:

• One cone joins to one neurone therefore...
• Low sensitivity- not in dim light (takes more light to reach threshold)
• High acuity, more detail- light hits 2 cones, & 2 action potentials are sent.

Types of Neurone:

• Sensory- transmits impulses from receptors to the CNS
• Relay- transmits impulses between sensory & motor neurones
• Motor- transmits impulses from CNS to effectors

Key Facts:

• 'Resting state' is when the neurone is more + charged outside than inside- membrane is polarised (i.e. there's a difference in voltage)
• Resting potential is about -70mV
• Sodium potassium pumps move sodium (x3) out of cells but they can't diffuse back in, and potassium ions (x2) move into cells.
• Potassium pumps move K+ ions out again via facilitated diffusion
• Hence outside is more + charged than inside

Processes:

• Stimulus causes action potentials to excite the membrane of a neurone, and sodium channels open.
• Sodium enters cell (membrane more permeable) so inside is less negative
• Depolarisation- if potential difference reaches threshold value (aprox. -55mV) more sodium channels open & more sodium ions diffuse into cell. Some Na+ ions diffuse sideways, causing a wave of depolarisation.
• Repolarisation- at potential difference of +30mV, sodium channels close & potassium channels open. K+ ions diffuse out of cell
• Hyperpolarisation- K+ channels close too slow, so potential difference is more negative than resting potential.
• Resting potential- sodium-potassium pump returns & maintains it (-70mV.)
• Refractory period- membrane can't be excited again straight away, as sodium & potassium channels are closed and recovering- also ensures the action potential only moves in one direction!
• If threshold value isn't reached, none of this would happen & no action potential would be produced!

Myelination:

Some neurones have a myelin sheath (myelinated neurones) which act as an electrical insulator. The myelin sheath is made from a Schwann cell.

• In between mylin there's bare patches of membrane from the neurone axon called Nodes of ranvier.
• Depolarisation can only take place at the Nodes of Ranvier, where Na+ ion channels are concentrated. (Ions can't get through myelin sheaths).
• The impulse 'jumps' from node to node via saltatory conduction.
• This is faster than in a non-myelinated neurone, as the impulse can jump over the myelin sheaths, rather than take the slower route all along the unmyelinated axon membrane.

Temperature:

• speed of conduction increases as temperature increases-ions diffuse faster

Synapses are the junctions between one neurone and another, or between a neurone and an effector cell. The gap in between is called the synaptic cleft.

• Presynaptic neurone has a synaptic knob with vesicles containing neurotransmitters
• Action potential reaches end of a neurone & causes neurotransmitters to be released.
• Neurotransmitters diffuse across synaptic cleft & bind to specific receptors on the postsynaptic membrane- can only move in one direction!
• Neurotransmitters are then removed e.g. broken down by enzymes so response eventually stops.

Exitatory Neurotransmitters:

• Depolarise postsynaptic membrane making it fire an action potential if threshold value is reached.
• Acetylcholine (ACh) is an exitatory neurotransmitter which binds to postsynaptic receptors causing an action potential to be fired.

Inhibitory Neurotransmitters:

• Hyperpolarise postsynaptic membrane, stopping it from firing action potentials.

Temporal Summation:

• 2 or more impulses sent in quick succession in the same neurone- more neurotransmitter is released therefore more chance of reaching threshold value & more chance of an action potential being fired.

Spatial Summation:

• many neurones linked to the same neurone/cell- combines amounts of neurotransmitter, making it more likely that the threshold value is reached & an action potential is fired- but can have no effect if a few of the neurones release inhibitory neurotransmitters instead of exitatory.

Drugs & the synapse:

• Some drugs (agonists) are the same shape as neurotransmitters and they act like neurotransmitters, i.e. they bind to receptors so more are activated.
• Others don't activate receptors (antagonists), they just block the receptors so the neurotransmitters can't bind to the receptors & activate them.
• Yet other drugs can inhibit the enzyme which normally breaks down the neurotransmitters, resulting in more receptors being activated for longer.
• Certain drugs can stimulate the release of neurotransmitter from the vesicles, so more receptors are activated.
• Other drugs can inhibit the release of neurotransmitter, so fewer receptors are activated.

• Skeletal muscle- large bundles of muscle fibres, the membrane of which is called the sarcolemma. (muscle cytoplasm= sarcoplasm)
• T tubules stick into sarcoplasm and help to spread impulses.
• Sarcoplasmic reticulum releases calcium ions needed for contraction.
• Lots of mitochondria to provide energy, have many nuclei (multinucleate).
• Muscle fibres contain microfibrils made up of proteins, for contraction.
• Mycrofibrils contain thick myosin filaments & thin actin filaments (dark A bands are both myosin & actin filaments, lighter I bands contain just actin)
• Ends of the sarcomere marked with Z line, middle of myosin= M line
• H zone is the area which only contains myosin filaments

Slow twitch- contract slowly, used for keeping posture (e.g. back muscles) & endurance activities (e.g. posture, marathons). Long time without getting tired (energy released slowly- anaerobic). Many mitochondria, rich in myoglobin- store alot of O2.

Fast twitch- contract quickly, used for fast movement e.g. sprinting, blinking. release energy aerobically in short bursts, tire quickly. Few mitochondria, not much myoglobin (so can't store much O2).

Sliding filament theory: actin & myosin filaments slide over each other:

• Myosin have hinged globular heads & binding sites for ATP & actin.
• Actin has binding sites for myosin heads (actin-myosin binding sites)
• Tropomyosin & troponin found in actin. Attatched together, help to move.
• Tropomyosin blocks binding sites in resting muscles, held by troponin.
• Action potential depolarises sarcolemma & T tubules releasing Ca2+ ions.
• Calcium ions bind to troponin- changes its shape. The troponin pulls away with tropomyosin, exposing the binding site & allowing the myosin head to bind to the actin filament (forming an actin-myosin cross bridge).
• Ca2+ ions also activate ATPase- breaks down ATP to provide energy.
• This energy moves myosin head, pulling actin filament along with it.
• Head reattatches to another binding site, sarcomere shortens- muscle contracted
• Cycle is repeated as long as Ca2+ ions are present & bound to troponin.
• Exitation stops- Ca2+ ions leave the binding sites on troponin, troponin returns to original shape & blocks actin-myosin binding sites again.
• Muscles not contracted- actin filaments slide back into their original position, lengthening the sarcomere.

Control of Heart Rate:

• SAN generates impulses which causes cardiac muscles to contract.
• Rate is controlled unconsciously by a part of the brain called the medulla.
• Baroreceptors detect pressure in the aorta & vena cava. Chemoreceptors detect chemical stimuli in the aorta, medulla & neck artery. Monitor O2, pH & CO2 levels.
• either acetylcholine/noradrenaline bind to receptors in the SAN

Taxis- directional movement e.g. woodlice moving away from light

Kinesis- non directional movement e.g. turning in favourable environment

Reflexes:

• receptors in skin detect heat- sensory neurone- relay neurone- motor neurone- effector- hand moves away to stop it being damaged.

Tropisms:

• Response to a directional stimulus: positive tropism=  growth towards stimulus, negative tropism= growth away from stimulus.
• Shoots are positively phototropic and negatively geotrophic.
• Roots are negatively phototropic and positively geotrophic.

Growth Factors:

• Chemicals produced in shoot tips/leaf which speed up/slow plant growth
• Auxins stimulate growth of shoots by elongation- become loose & stretchy
• However, high concentrations of auxins inhibit growth in roots.
• Indoleacetic Acid (IAA)- auxin produced in shoot tips, moves via diffusion & active transport over short distances, or phloem over long distances.
• Moves to shaded parts of shoot/root in phototropism, uneven growth.
• Moves to underside of shoots/roots in geotropism- uneven growth.