nervous coordination

• Topic 68-Nervous Coordination
• Neurones


  In they meet meet when
• Ah, on to the good sta Nosad at the read K-com
• Neurone Cell Membranes


  Are Polarised at Rest
• Outside of the membrane

  Is positively charged compared to the inside
• Membrane is polarised


  There's a difference in charge (called a potential difference or voltage across the membrane)
• Resting potential


  Is about -70 mV
• Sodium-potassium pumps


  Move sodium ions out of the cell and potassium ions into the cell
• Membrane is permeable to sodium in


  But not out, so sodium can't diffuse back in
• This creates


  A sodium ion gradient across the membrane
• Sodium-potassium pumps


  Also move potassium ions into the neuron, but the membrane is permeable to potassium so they diffuse back out
• This makes


  The outside of the cell positively charged compared to the inside
• Neurone Cell Membranes


  Become Depolarised when They're Stimulated
• Stimulation triggers


  1. Sodium ion channels to open

  2. Rapid change in potential difference

  3. Sequence of events known as an action potential
• Changes in potential difference during an action potential
• Stimulation


  Excites the neurone cell membrane, causing sodium ion channels to open
• Membrane becomes more permeable to sodium


  Sodium ions diffuse into the neurone down the sodium ion electrochemical gradient
• This makes


  The inside of the neurone less negative
• Depolarisation


  The potential difference reaches the threshold (around -55 mV), more sodium ion channels open, more sodium ions diffuse rapidly into the neurone
• Repolarisation


  At a potential difference of around 30 mV the sodium ion channels close and potassium ion channels open, the membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient, this starts to get the membrane back to its resting potential
• Hyperpolarisation


  Potassium ion channels are slow to close so there's a slight undershoot where the potential becomes more negative than the resting potential (less than -70 mV) as too many potassium ions diffuse out of the cell
• Resting potential


  The ion channels are reset, the sodium-potassium pumps restore the resting potential and maintain it until the membrane is stimulated again
• Refractory period


  After an action potential, the neurone cell membrane can't be excited again straight away, because the ion channels are recovering and can't be made to open - sodium ion channels are closed during polarisation and potassium ion channels are closed during hyperpolarisation
• Refractory period


  Acts as a time delay between one action potential and the next, so action potentials don't overlap but travel as discrete separate impulses, limiting the frequency at which nerve impulses can be transmitted
• Action potentials


  Are unidirectional, they only travel in one direction
• All-or-nothing nature of action potentials

  Once the threshold is reached, an action potential will always fire with the same change in voltage no matter how big the stimulus is, but a bigger stimulus will cause them to fire more frequently
• If the threshold isn't reached, an action potential won't fire
• PA


  y = √x + 1
• L


  4 = 2x+1
• Meet when ea
• Three Factors Affect the Speed of Conduction of Action Potentials

  • Myelination
  • Axon diameter
  • Temperature
• Myelination


  • Some neurones are myelinated
  • The myelin sheath is an electrical insulator
  • In the peripheral nervous system, the sheath is made of a type of cell called a Schwann cell
  • Between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier. Sodium ion channels are concentrated at the nodes
  • In a myelinated neurone, depolarisation only happens at the nodes of Ramsier (where sodium ions can get through the membrane)
  • The neurone's cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps from node to node
  • This is called saltatory conduction and it's really fast
• Non-myelinated neurone


  • The impulse travels as a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane)
  • This is slower than saltatory conduction (although it's still pretty quick)
• Axon diameter


  • Action potentials are conducted quicker along axons with bigger diameters because there's less resistance to the flow of ions than in the cytoplasm of a smaller axon
  • With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
• Temperature


  • The speed of conduction increases as the temperature increases too, because ions diffuse faster
  • The speed only increases up to around 40 °C though-after that the proteins begin to denature and the speed decreases
• What is meant by the 'all-or-nothing nature of action potentials?
• What is the function of the refractory period?
• Synapse


  A Junction Between a Neurone and the Next Cell
• Synapse


  • The gap between the cell at a synapse is called the synaptic cleft
  • The presynaptic neurone is the one before the synapse
  • The synaptic knob contains synaptic vesicles
  • When an action potential reaches the end of a neurone, neurotransmitters are released into the synaptic cleft
  • The neurotransmitters bind to specific receptors on the postsynaptic membrane
  • When neurotransmitters bind to receptors they might trigger an action potential in a neurone, cause muscle contraction in a muscle cell, or cause a hormone to be secreted from a gland
  • Synaptic transmission is unidirectional - the impulse can only travel in one direction
• Acetylcholine (ACh) Transmits the Nerve Impulse Across a Cholinergic Synapse

  1. An action potential arrives at the synaptic knob of the presynaptic neurone

  2. The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open

  3. Calcium ions diffuse into the synaptic knob

  4. The influx of calcium ions into the knob causes the synaptic vesicles to fuse with the presynaptic membrane

  5. The vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft

  6. ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane

  7. This causes sodium ion channels in the postsynaptic neurone to open

  8. The influx of sodium ions into the postsynaptic membrane causes depolarisation

  9. An action potential on the postsynaptic membrane is generated if the threshold is reached

  10. ACh is removed from the synaptic cleft so the response doesn't keep happening. It's broken down by an enzyme called acetylcholinesterase (AChE) and the products are re-absorbed