NERVOUS SYSTEM

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The nervous system

  • as humans we can respond to changes in the enviroment to increase our chance of survival
  • nervous control has 3 parts:
  • 1) STIMLULIdetect changes in the external or internal environment
  • 2) RECEPTOR - detect stimuli, they are often part of the sense organs, they have protein on their surface that actually do the detecting
  • 3) CO-ORDINATOR - the name given to all the neurones that connect the sensory and motor systems
  • 4) EFFECTORS - cells that effect the response, muscles and glands
  • 5) RESPONSE - these aid survival and include all movement, behaviours and secretions from glands
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Nerve Cells

  • consists of a CELL BODY
  • several DENDRONS coming of the cell body that take nerve impulses TOWARDS the cell body
  • a single, long AXON that carries the nerve impulse AWAY from the cell body
  • 1 nerve is made up of thousands of neurones
  • nerve impulses are passed across a SYNAPSE and the numerous DENDRITES provide a large surface area for the attatchment to other nuerones
  • many neurones axons have SCHWANN CELLS which are wrapped around it many times to form a thick lipid layer called the MYELIN SHEATH  (this provides physical protection and electrical insulation)
  • there are gaps in the sheaht called the NODES OF RANVIER
  • humans have 3 types of neurone:
  • 1) SENSORY - have long dendrons and transmit nerve impulses from receptors to the CNS
  • 2) MOTOR - have long axons and transmit nerve impulses from the CNS to effectors
  • 3) INTERMEDIATE - much smaller, with many dendrites they make up the CNS and connect sensory and motor neurones
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Nerve impulses - resting potential

  • in a neurones resting state the outside of the axon is positivly charged in comparison to the inside
  • the voltage inside the axon in this state is around -70mV
  • it is created and maintained by SODIUM-POTASSIUM PUMPS and POTASSIUM ION CHANNELS
  • the sodium potassium pump move 3 Sodium Out and  2 Potassium In (SOPI)
  • this creates a concentration gradient of ions (electrochemical gradient) as there are more sodium outside the cell
  • the potassium ions diffuse out again through potassium ion channels which makes the outside even more positively charged
  • sodium ions channels remain closed
  • eventually the outside becomes so positive that no more potassium ions diffuse out
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Nerve impulse - action potential

  • 1) STIMULUS - exites the neurone cell membrane, causing sodium gated ion channels to open, the membrane becomes more permeable and sodium diffuses in, this makes the inside less negative
  • 2) DEPOLARISATION - if the potential difference reaches the threshold (-55mV) then more sodium channels open (positive feedback)
  • 3) REPOLARISATION - at a potential difference of around +30mV the sodium channels close and potassium voltage gated channels open, potassium diffuses out and begins to return the membrane back to its resting potential
  • 4) HYPERPOLARISATION - potassium gated channels are slow to close and so there is an 'overshoot' where the potential difference becomes less negative than at resting potential (less than -70mV)
  • 5) RESTING POTENTIAL - the channels reset and the sodium potassium pump returns the membrane to resting potential
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How are nerve impulses propagated

  • once an action potential begins it moves along the neurone automatically (like a mexican wave)
  • the reversal of the membrance potential is detected by surrounding voltage-gated ion channels which open
  • after a channnel has opened it needs a 'rest period' before it can open again, this is called the refractory period

the refractory period means:

  • the impulse is unidirectional
  • it is also non overlapping/discrete
  • the amount of impulses being limited

there is also something called the all or nothing principle:

  • there is a certain threshold value that triggers an action potential, below this there is no action potential, anything above the threshold will generate an action potential
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Speed of a nerve impulse

  • if a stimulus is weak then the frequency of action potentials will be low, if it is a strong stimulus it will cause a high frequency of action potentials

speed of an impulse is affected by 3 factors:

  • 1) TEMPERATURE - higher the temperature the faster the speed, so warm blooded animals have faster impulses than cold blooded ones
  • 2) AXON DIAMETER - larger the diameter the faster the speed - animals that live in cold temperatures have developed thick axons to speed up their responses e.g. squid
  • 3) MYELIN SHEATH - only vertebrates have myelin sheath, the voltage gated channels are only found at the nodes of ranvier and between these nodes the myelin acts as an electrical insulator, the action potential therefore jumps large distances from node to node (saltatory propogation) this is quicker than the impulse having to travel across the whole length of the neurone
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Synapses

  • an action potential cant be carried across the synaptic cleft so instead the impulse is carried by chemicals called nuerotransmitters
  • these are stored in the pre synaptic neurone in vesicles and attatch to neuroreceptors on the post synamtic membrane
  • the process is as follows:
  • 1) on the pre synaptic neurone there are calcium voltage gated channels, when an action potential arrives these open and calcium floods in
  • 2) these calcium ions cause the synaptic vesicles to fuse with the membrane and release their contents (the nuerotransmitter - acetyl choline)
  • 3) acetyl choline diffuses across the cleft
  • 4) it then binds to neuroreceptors on the post synaptic membrane causing the (sodium) ion channels to open
  • 5) this causes depolarisation of the post synaptic membrane which initiates an action potential
  • 6) the acetyl choline is broken down by the enzyme cholinesterase to form choline and ethanoic acid
  • 7) the breakdown products are absorbed by the pre-synaptic neurone and used to make more neurotransmitter with energy in the form of ATP
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Types of synapse

  • EXCITATORY ION CHANNEL SYNAPSES - the one described on the previous page, these have neuroreceptors that are na+ channels and cause depolarisation, making action potentials MORE likely, neurotransmitters involve acetyl choline and glutamate
  • INHIBITORY ION CHANNEL SYNPAPSES - these have neuronreceptors that are cl- channels and cause hyperpolarisation, making an action potential LESS likely, neurotransmitters involve glycine and GABA
  • NON-CHANNEL SYNAPSES - the neuroreceptor is an ezymes which when activated catalyses the production of a 'messenger chemical' which affects many of the cells functions, involved in long lasting responses e.g. memory and learning, neurotransmitters involve adrenalin, noradrenilin, dopamine, seratonin, endorphins, acetylcholine
  • NEUROMUSCULAR JUNCTIONS - synapses between motor neurones and muscle cells, they always use acetyle choline and are always excitatory
  • ELECTRICAL SYNAPSES - where the 2 cells actually touch so pass on action potentials directly, very fast but very rare, found only in the heart and eye
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Summation

  • low frequency action potentials often produce insufficient neurotransmitter to initiate a new action potential, however, they can be made to do so by a process called summation which causes a build up of nuerotransmitter in the synapse to reach the threshold:
  • 1) SPATIAL SUMMATION - a number of different pre synaptic neurones together release enough neurotransmitter to reach the threshold
  • 2) TEMPORAL SUMMATION - a single presynaptic neurone releases neurotransmitter several times over a short period of time and eventually the threshold is reached
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Drugs and synapses

  • all drugs (medical and recreational) affect the nervous system, especially synapses
  • they affect synapses in numerous ways:
  • 1) mimic the neurotransmitter - stimulate synapse - e.g. levodopa (cure for parkinson's as it mimics the neurotransmitter dopamine which they lack)
  • 2) stimulate the release of neurotrasmitter - stimulate synapse - e.g. cocaine, caffeine (stimulants)
  • 3) open a neuroreceptor channel - stimulate a synpase - e.g. alcohol, marijuana, salbutamol (tranquillisers as they open an inhibitory channel)
  • 4) block a neuroreceptor channel - inhibit a synpase - e.g. atrophine, curare, opioids such as heroine, dopamine, methadone (all inhibit pain receptors)
  • 5) inhibit the breakdown enzyme - stimulate a synapse - e.g. DDT (causes muscle spasms and death as neuromuscular junctions are always active)

AGONISTS - stimulate a synapse

ANTAGONISTS - inhibit a synapse

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Receptors in the skin

PACINIAN CORPUSCLE 

  • mechanoreceptor found in the skin and joints
  • they detects strong vibrations and pressure
  • they consists of a sensory neurone surrounded by a capsule of layers of flattened schwann cells and fluid called lamelle
  • they are situated deep in the skin so are only sensitive to intense pressures
  • pressure distorts the cell membrane of the neurone and opens mechanically gated sodium channels by changing their shape
  • sodium ions diffuse in causing depolarisation (a generator potential)
  • the stronger the pressure the more intense the generator potential and the more likely it is to reach the threshold and initiate an action potential
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Receptors in the eye

RODS AND CONES

  • RODS
  • distributed throughout the retina so used for peripheral vison
  • only one type - monochromatic vision
  • many rods connected to 1 bipolar cell - retinal convergence
  • HIGH SENSITIVITY - one stimulus per rod accumalates in one bipolar neurone, reaches generator potential threshold and causes an action potential
  • LOW ACUITY several rod cells to one nuerone so only one impulse is generated and sent to the brain, therefore cant distiguish between two sources of light
  • CONES
  • found mainly in the fovea so used to detect images in the centre of the retina
  • 3 types (red, green and blue) - colour vision
  • each cone connected to one neurone so no convergence
  • LOW SENSITIVITY- one stimulus per cone is not enough to generate an action potential - need bright light to work
  • HIGH ACUITY - 1 cone to 1 neurone so seperate impulses are generated and sent to the brain allowing it to distinguish between different points
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Muscle

three types of muscle:

  • 1) SKELETAL MUSCLE - always attatched to skeleton, under voluntary control via the motor neurones (divided into slow and fast muscle)
  • 2) CARDIAC MUSCLE - looks and works much like skeletal muscle but is not under voluntary control or attatched to skeleton
  • 3) SMOOTH MUSCLE - found in internal body organs e.g. arteries, aris, gut and uterus it's under voluntary control, usually forms a ring which tightens when it contracts
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Muscle structure

  • 1 muscle e.g. a bicep contains around a thousand MUSCLE FIBRES
  • each muscle fibre is a MUSCLE CELL
  • these giant cells have many NUCLEI as they where formed from the fusion of many smaller cells
  • their SACROPLASM is packed full of MYOFIBRILS which are bundles of PROTEIN FILAMENTS that cause contraction
  • they also have many MITOCHONDRIA to provide ATP for muscle contraction
  • each myofibril is made up of repeating DARK and LIGHT bands
  • 1 Z LINE to the next is called the SACROMERE
  • the myofibril is made of PARALLEL FILAMENTS, there are 2 kinds of filament: thick and thin and they are linked at intervals by cross bridges
  • the THICK filament is made of a protein called MYOSIN which is shaped a bit like a golf club with 2 heads
  • the THIN filament is a protein called ACTIN which is a globular molecule which forms a double helix - it contains TROPONIN and TROPOMYOSIN (involved in the control of muscle contraction)
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Changes to the sacromere during muscle contraction

the sacromere it made up of the following:

  • 1) I BAND - this is the band that appear lIght on the microscope, it contains only ACTIN
  • 2) A BAND - this appears dArk on the microscope and contain ACTIN AND MYOSIN
  • 3) H ZONE - this is the section in the middle that contains only MYOSIN

during muscle contraction the following happens:

  • 1) the I BAND SHORTENS
  • 2) the A BAND STAYS THE SAME
  • 3) the H ZONE SHORTENS
  • 4) the SACROMERE SHORTENS

this is due to the actin filaments sliding over the myosin

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Sliding filament theory/cross bridge cycle

the cycle is as follows:

  • 1) the action potential travels deep into the fibre by T-tubles that branch through out the sacroplasm
  • 2) the tubles are in contact with the sacroplasmic reticulum which has activily absorbed calcium ions from the sacroplasm
  • 3) the action potential opens the calcium ion channels on the sacroplasmic reticulum and they flood into the sacroplasm down a diffusion gradient
  • 4) the calcium ions bind to TROPONIN which causes TROPOMYOSIN to move away and uncover the myosin binding sites on actin
  • 5) the ADP molecule attatched to the myosin head means it is in a state to bind to actin and form a cross bridge
  • 6) once attatched the myosin head changes its angle and pulls the actin along, releasing ADP
  • 7) ATP attatches to myosin causing it to become detatched from actin
  • 8) calcium ions activate ATPase which hydrolyses ATP to ADP and Pi to provide energy to recock the myosin head
  • 9) the myosin once more with an attatched ADP molecule attatches itself further along the actin and the cycle is repeated as long as nervous stimulation continues
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Muscle relaxation

  • when the muscle is contracting tropmyosin uncovers the myosin binding sites (through use of calcium) so that cross bridges can form
  • when the muscle is relaxing tropomyosin blocks the myosin binding site (because no action potential is formed and thus no calcium channels open to provide calcium to move tropomyosin away) so cross bridges cannot form
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Energy for muscle contraction

AEROBIC SYSTEM -

  • most of the time when muscles are resting or moderatly active ATP is generated by aerobic respiration
  • the aerobic system creates large amounts of energy (38 moles) but contraction is fairly slow
  • the fuel for this system is either triglycerides or glycogen

ANEROBIC SYSTEM -

  • as rate of muscle contraction increases ATP starts to be used faster that it can be made my the aerobic system
  • muscles begin to break down glycogen anaerobically, which is quick but produces the painful biproduct of lactate
  • this provides enough energy to last 10 seconds - 3 minutes (2 moles)

PHOSPHOCREATINE SYSTEM -

  • very fast, one step reaction that provides ATP for up to 10 seconds of work (1 mole)
  • phosphocreatine is a short term energy store in the muscle cells
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Slow and fast twitch muscles

  • SLOW-TWITCH (type 1)
  • adapted for aerobic respiration and therefore prolonged activity
  • slow contraction speed limited by rate of oxygen supply
  • no lactate produced so not susceptible to muscle fatigue
  • contain many mitochondria (to form ATP) and myoglobin (which is similar to heamaglobin and helps store oxygen in muscles)
  • supply of glycogen
  • rich supply of blood vessels
  • FAST-TWITCH (type 2)
  • adapted for anaerobic respiration and therefore short bursts of activity
  • fast contraction speed, not limited by oxygen supply
  • lactate production leads to low PH and muscle fatigue
  • store of phosphocreatine to generate ATP from ADP quickly in anaerobic conditions
  • less myoglobin and mitochondria
  • high concentration of anaerobic enzymes
  • thicker and more numerous myosin filaments
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Animal responses

TAXES

  • DIRECTIONAL response to a DIRECTIONAL stimulus
  • can be positive (towards the stimulus) or negative (away from the stimulus)
  • common stimuli include: light (phototaxis), gravity (geotaxis), chemicals (chemotaxis), movement (rheotaxis)

KINESES

  • response to a CHANGING stimulus by changing the amount of MOVEMENT
  • can change the rate of movement or the rate of turning
  • the response is NOT directional
  • used when the stimulus doesnt have one extreme or another
  • the end result is to keep the animal in favourable conditions
  • woodlice are a common example (they dry out in dry conditions as they breath through gills)

ALL these responses are INVOLUNTARY and AID SURVIVAL

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Reflexes

  • a reflex is a specific response to a specific stimulus, all animals have them and they are ESSENTIAL for survival
  • 5 key features:
  • 1) FEW NEURONES - usually 3
  • 2) IMMEDIATE - very fast as there are few neurones and therefore few synapses (the slower link of a response)
  • 3) INVOLUNTARY - no choice or though is involved
  • 4) INVARIABLE - a given reflex to a given stimulus is always the same
  • 5) INNATE - genetically programmed, not learnt
  • humans have reflex arcs to protect out bodies from damadge e.g. withdrawing our hand from a sharp object
  • only 3 neurones involved: sensory, intermediate and motor
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Organisation of the human nervous system

  • human nervous system is split into 2 parts:
  • central nervous system (CNS) - brain and spinal cord
  • peripheral nervous system (PNS) - sensory and motor neurones
  • the pheripheral nervous system is split into two parts:
  • motor neurones - messages to effectors
  • sensory neurones - messages from receptors
  • the motor neurones are split into two:
  • autonomic nervous system - involuntary responses to smooth muscle and glands
  • voluntary nervous system - to skeletal muscle
  • autonomic nervous system is split into two:
  • sympathetic nervous system - 'fight or flight' speeds up impulses (noradrenelin)
  • parasympathetic nervous system - resting, slows down impulses (acetylcholine)

KEY PARTS OF THE BRAIN INVOLVED IN INVOLUNTARY RESPONSES:

  • hypothalamus (homeostasis)
  • pituitary gland (LDH and FSH: menstrual cycle)
  • medulla oblongata (controls heart rate)
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Control of heart rate

  • within the MEDULLA OBLONGATA there are two centres:
  • a centre that INCREASES heart rate which is linked to the SA node by the sympathetic nervous system
  • a centre that DECREASES heart rate which is linked to the SA node by the parasympathetic nervous system
  • which of these is stimulated depends on information recieved from the following receptors: baroreceptors (pressure) and chemoreceptors (chemicals)
  • when exercising there is an increased level of co2 in the blood which lowers PH and stimulates chemoreceptors
  • these send a message to the medulla oblongata to increase the frequency of nerve impulses, these are the sent down the sypathetic nerve to the SA node and heart rate is increased
  • increased bloodflow leads to more co2 being removed by the lungs which increases PH
  • this is detected by the chemoreceptors and a message is sent to the medulla oblongata to reduce impulses, the message is then sent down the parasympathetic nerve to the SA node and HR is retured to normal
  • in terms of baroreceptors, they reduce the heart rate if pressure increases and increase it if it decreases
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