Nervous System and Response to Stimuli

• Resting Potential:

  • when a neurone is not firing It is polarised as the charge is different inside and outside the membrane
  • this is the resting potential at -70mV
  • this is caused by the action of the sodium-potassium pumps
  • they actively transport sodium and potassium ions in and out of the neurone
  • for three sodium out there is two potassium in to keep the charge difference
• Action Potential step 1: Depolarisation

  • sodium ion channels open up which causes sodium ions to flood into the neurone via facilitated diffusion
  • this causes the potential difference to reach +30mV
• Action Potential step 2: Repolarisation

  • sodium ion channels close and potassium ion channels open
  • potassium ions move out of the neurone down their concentration gradient
  • this movement of positive ions out means there is a charge difference again - repolarised
• Action potential step 3: hyperpolarisation

  • the charge difference exceeds the resting potentials -70mV which make it become hyperpolarised
  • this is because the potassium ion channels are slow to close and too many potassium ions diffuse out
  • the sodium-potassium ion pump action restores the balance
• Post Action potential: refractory Period

  • the neurone cannot be stimulated and action potential cannot occur
  • this is because the ion channels are recovering
  • this is crucial as it ensures action potentials don't overlap
• Post Action potential: Wave of depolarisation

  • once an action potential occurs in one part of the neurone it stimulates another action potential in the adjacent part of the neurone creating a wave of depolarisation.
  • this occurs as the sodium ions diffuse into the neurone sideways
  • this causes voltage-gated ion channels in the next part of the neurone to open causing sodium ions to move
  • the wave moves away as the neurone just fired is in the refractory period
• All or nothing:

  when enough charge is lost and depolarisation exceeds -55mV an action potential occurs as it crossed the threshold potential and must cross this potential creating the 'all or nothing'
• Size of Stimulus:

  • all or nothing means the size of the stimulus doesn't matter as long as the threshold potential is reached
  • however the bigger the stimulus, the more action potentials are fired
• factors that affect speed of action potential:

  • myelination
  • axon diameter
  • temperature
• axon diameter:

  • action potentials travel quicker neurones with bigger axons as there is less resistance meaning the depolarisation travels faster
• temperature:

  • as temperature increases, the speed of the action potential increases because ions have more kinetic energy
  • Only up to 40 degrees due to the denaturing of channel and carrier proteins
• myelination:

  • some axons have the myelin sheath
  • this acts as on electrical insulator meaning ions can't move out of myelinated portions
  • gaps in the myelin sheath called nodes of Ranvier which ion channels are concentrated
  • action potentials only occur at this node which depolarises the next node causing impulse to jump from node to node
  • this is called saltatory conduction which is faster than non myelinated neurones
• synapses:

  is a gap between neurones - impulses cannot pass through this gap so neurotransmitters stimulate the action potential in the next neurone

  neurone before the synapse is the presynaptic neurone and the neurone after is the postsynaptic neurone. the space inbetween is called the synaptic cleft.
• Synaptic transmission: Action potential arrives at the presynaptic neurone, triggering the opening of voltage-gated calcium ion channels
• Calcium ions move to the synaptic knob and trigger the movement of vesicles containing neurotransmitters to the presynaptic membrane
• Vesicles fuse with the presynaptic membrane and release contents by exocytosis
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
• Binding of neurotransmitters to receptors on the postsynaptic membrane triggers the opening of sodium ion channels
• Sodium ions move into the postsynaptic membrane, causing depolarization and action potential
• Neurotransmitter is removed from the synaptic cleft
• Cholinergic Synapses:

  work In the same way as normal synapses but use the neurotransmitter acetylcholine. Acetylcholine is broken down by the enzyme acetylcholinesterase and the products are reabsorbed into the presynaptic neurone to resynthesise the neurotransmitter.
• excitatory and inhibitory Neurotransmitters:

  Neurotransmitters can be classed as excitatory if they trigger an action potential in the postsynaptic neuron or inhibitory if they prevent an action potential from happening. Some neurotransmitters are both excitatory and inhibitory, with their effect determined by where in the body they are acting
• Summation:

  small amounts of neurotransmitter build up to trigger an action potential in the postsynaptic neurone
• spatial summation:

  occurs when lots of presynaptic neurons converge on a single postsynaptic neuron. Although each of the presynaptic neurons are releasing small amounts of neurotransmitter the combined amount is enough to stimulate an impulse in the next neuron.
• Temporal Summation:

  is when a single neuron fires action potentials in quick succession, repeatedly releasing neurotransmitter into the synaptic cleft. This causes the amount of neurotransmitter in the synaptic cleft to increase, making an action potential in the postsynaptic neuron more likely.
• Neuromuscular Junctions:

  a synapse between and motor neurone and a muscle cell. they use the neurotransmitter acetylcholine. unlike typical synapses:

  • post synaptic membrane are folded into clefts to store acetylcholinesterase
  • acetylcholine acts as an excitatory neurotransmitter
  • postsynaptic neurone contains a higher number of receptors
• the effect of drugs on synapses:

  • some bind to receptors to trigger an action potential
  • some block receptors to prevent neurotransmitters activating it
  • some inhibit enzymes that break down the neurotransmitter
  • some trigger or inhibit the release of neurotransmitters
• How the Nervous system works:

  • detects a stimulus through receptors
  • when receptors detect stimuli they send a signal to the central nervous system creating an electrical impulse
  • the sensory neurone sends the impulse from the receptor
  • these go to coordination centres which signal effectors by releasing an impulse to the motor neurone
  • relay neurones transmit impulses between sensory and relay neurones
• when the nervous system responds unconsciously :

  This occurs when we need to respond immediately to a harmful stimuli in our environment. These unconscious responses are reflex actions and protect us through a coordinated response which bypasses the brain. The information is sent directly to the spinal cord, where the electrical impulse is passed from the sensory neuron to motor neuron via a relay neurone
• Tropism: tropisms are responses by an organism in a particular direction as a result of an external stimulus.
• phototropism: phototropism is a directional response to sunlight Plant shoots show positive phototropism and grow towards the sun. This maximises the amount of light they can absorb for photosynthesis. Roots show negative phototropism and grow away from the sun. This ensures that the roots bury themselves deeper within the soil, so they can absorb more water for photosynthesis.
• Geotropism: Geotropism is a directional response to gravity. Plant shoot show negative geotropism and grow away from the force of gravity. This ensures that plants grow upwards and means that more light is absorbed for photosynthesis. Roots show positive geotropism and grow towards the force of gravity. This ensures that roots bury themselves deeper within the soil, so they can anchor the plant and absorb more water for photosynthesis
• Growth Factors: used to respond to their environment

  • Auxins - promote cell elongation in shoots opposite in roots
  • Gibberellins - stimulate seed germination and flowering
  • abscisic acid - helps respond to environmental stress
  • cytokinins - cell division and differentiation
  • Ethene - flowering and fruit ripening
• Indoleacetic acid (IAA):

  Is a type of auxin which allows the plant to respond to light and gravity.

  • it enters the nucleus and binds to promoter regions of DNA
  • it then acts as a transcription factor to inhibit or activate genes that code for cell growth
  • transported in the phloem
• IAA in Shoots and Roots:

  • Shoots - IAA accumulates in the shaded area where it activates cell elongation genes. this makes cell walls looser and stretchier causing elongation which causes them to bend towards the sun
  • Roots - IAA accumulates in the shaded area but inhibits cell growth meaning more cells in the non shaded area causing it to bend away from the sun
• Taxes and Kinesis:

  small mobile organisms show simple responses to optimise environmental conditions
• Taxes:

  the organism moves towards or away from a directional stimulus
• Kinesis:

  movements in response to a non-directional stimulus
• Receptors:

  used to detect stimuli and pass this information to the CNS. Receptors can be whole cells or proteins which are found on the cell membrane. Each receptor is specific to a type of stimulus. When a receptor is not stimulated, there is a charge difference between the in and out of the membrane and is polarised. When the receptor detects a stimulus, the permeability of its cell membrane changes changing the charge difference across the membrane. If the potential difference change is large enough, it will cause an action potential.
• Types of receptors:

  • Chemoreceptors - detect chemicals
  • Thermoreceptors - detect heat
  • Mechanoreceptors - detect pressure
  • Photoreceptors - detect light