Stimuli/response

• Cones have high visual acuity because each cone is connected to a single neurone. They send separate sets of impulses (action potentials) to the brain
• Rods have high visual sensitivity because many rods connected to a single neurone. Generator potentials combine to reach the threshold and trigger an action potential (spatial summation)
• The resting potential (-70mV) is maintained across a neurone due to the membrane being more permeable to potassium ions and less permeable to sodium ions. Sodium ions are actively transported out/ potassium ions in (sodium potassium pump)
• In the Pacinian corpuscle pressure causes membrane/lamellae to become deformed/stretched. Sodium ion channels in the membrane open and sodium ions move in. Greater pressure, more channels open/ sodium ions enter
• The membrane potential is the same whether a medium or heavy pressure is applied if the threshold has been reached. This causes maximal response (all or nothing principal)
• If the myelin sheath has been damaged there will be less/no saltatory conduction (impulse unable to jump from node to node), and depolarisation will occur over the length of the axon (slower)
• The simple reflex arc consists of three neurones; a sensory, relay and a motor neurone.
• Advantages of simple reflex arcs:

  • rapid
  • protect against damage to body tissues
  • do not have to be learnt
  • help escape predators
• The speed of transmission of impulses is faster along a myelinated axon as myelination provides (electrical) insulation. In myelinated saltatory conduction (depolarisation at nodes of ranvier). In non-myelinated depolarisation occurs along whole length of axon
• Transmission across a cholinergic synapse

  1. Depolarisation of presynaptic membrane
  2. Calcium channels open/calcium ions enter (synaptic knob)
  3. Calcium ions cause synaptic vesicles move to/fuse with presynaptic membrane and release ach/neurotransmitter
  4. Acetylcholine/neurotransmitter diffuses across (synaptic cleft)
  5. Acetylcholine attaches to receptors on the postsynaptic membrane
  6. Sodium ions enter (postsynaptic neurone) leading to depolarisation
• Dopamine (similar to acetylcholine) stimulates the production of nerve impulses in postsynaptic neurones by:

  • dopamine diffuses across synapse
  • attaches to receptors on postsynaptic membrane
  • stimulates the entry of sodium ions and depolarisation/action potential
• Some drugs (agonists) are similar in shape to the neurotransmitters so they mimic their action at the receptors. This means more receptors are activated
• Some drugs (antagonists) block the receptors so they cant be activated by neurotransmitters. This mean fewer receptors (if any) can be activated
• Some drugs inhibit the enzyme that breaks down neurotransmitters. This means there are more neurotransmitters in the synaptic cleft to bind to receptors and they're there for longer
• Excitatory neurotransmitters depolarise the postsynaptic membrane, making it fire action potentials is the threshold is reached. e.g. acetylcholine.
• Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane (make the potential difference more negative), preventing it form firing an action potential.
• Exercise causes an increase in heart rate.

  • Chemoreceptors detect the rise in carbon dioxide (decrease in pH).
  • send impulses to medulla
  • More impulses sent to SAN
  • By sympathetic nervous system (chemo)
• The cardiac muscle is 'myogenic'- it can contract without recieving signals from nerves
• Control of the regualr heart beat

  1. The SAN sends out regular waves of electrical activity to the atrial walls casuing both atria to contract at the same time
  2. These waves of electircal acticity are transferred from the SAN to the atrioventricular node(AVN)
  3. Then from the AVN to the Bundle of His
  4. Bundle of His splits into finer muscle fibres called Purkyne tissue
  5. Purkyne tissue carries electrical acitivity to the muscular walls of the ventricles, causing them to cotract simultaneoulsy
• Sympathetic pathway -for fight or flightUsed to increase stimulus
• Parasympathetic- calms body down 'rest/digest'Used to increase stimulus
• Nervous system communitcation is

  • localised-neurotransmitters secreted directly onto target cells
  • short lived- neurotransmitters are quickly removed once theyve done their job
  • Rapid
• The refractory period acts as a time delay between one action potential and the next. This means that

  • action potentials dont overlap, but pass along as discrete (seprate) impulses
  • theres a limit at which the nerve impulses can be transmitted
  • action potentials are unidirectional
• Action potential graph

  1. Stimulus- excites neurone cell membrane causing sodium ions channels to open...sodium ions diffue into neurone down Na+ electrochemical gradient (inside less negative)
  2. Depolarisation0 id potential difference reaches threshold, more sodium ion channels open... more sodium diffuses in
  3. Repolarisation- PD(+30mv) sodium ions channels close/ potassium ion channels open
• Summation- where the effect of neurotransmitter released from many neurones (or one thats stimulated alot in a short period) is added together
• Spatial summation is where many neurones connect to one neurone. The small amount of neurotransmitter released from each neurone can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential.
• Temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone, this makes the action potential more likely because more neurotransmitter is released into the synaptic cleft
• Neuromuscular junctions are slightly different to cholinergic synapses

  • The postsynaptic membrane has lots of folds that form clefts. These clefts store the enzyme that breaks down ACh (acetylcholinerase)
  • Postsynaptic membrane has more receptors than other synapses
  • ACh is always excitatory at a neuromuscular junction. When a motor neurone fires an action potential, it normally triggers a response in a muscle cell
• Nerve impulses are unidirectional because receptors are located only on the postsynaptic membrane
• when a wave of electrical activity reaches the AVN there is a short delay before a new wave leaves the AVN, this is to allow the atria to empty completely before the ventricles contract.
• Colour vision (using the fovea) is caused by photoreceptors being sensitive to different wavelengths of light
• . Both taxis and tropisms are a result of directional stimulus'

  . Taxis describes movement (of mobile/motile mammals) whereas tropism describes growth
• Depolarisation (in action potential)- if the potential difference reaches the threshold, MORE sodium ion channels open...more sodium ions diffuse rapidly into the neurone
• The release of ACh at an excitatory synapse depolarises the postsynaptic membrane- causing sodium ion channels to open.. sodium ions move in.... making post membrane potential less negative (reducing membrane potential)
• Transmission at a synapse is unidirectional as;

  • receptors are only found on the postsynaptic membrane
  • neurotransmitters only produced/released from presynaptic membrane
• In synaptic transmission calcium ions enter the presynaptic neurone/membrane (caused by the nerve impulse... Ca2+ channels open). The entry of Ca2+ causes fusion of vesicles with presynaptic membrane/causes exocytosis/release of transmitter.