Nervous co-ordination

• Resting membrane potential


  In a neurone's resting state (when it's not being stimulated), the outside of the membrane is positively charged compared to the inside
• There are more positive ions outside the cell than inside, so the membrane is polarised — there's a difference in charge (called a potential difference or voltage) across it
• Resting potential


  The voltage across the membrane when it's at rest, about -70 mV (millivolts)
• Movement of sodium and potassium ions

  1. Sodium-potassium pumps use active transport to move three sodium ions (Na*) out of the neurone for every two potassium ions (K*) moved in

  2. Potassium ion channels allow facilitated diffusion of potassium ions (K+) out of the neurone, down their concentration gradient
• The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn't permeable to sodium ions, so they can't diffuse back in
• This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside
• The sodium-potassium pumps also move potassium ions in to the neurone
• When the cell's at rest, most potassium ion channels are open, so some potassium ions diffuse back out through potassium ion channels
• Even though positive ions are moving in and out of the cell, in total more positive ions move out of the cell than enter, making the outside of the cell positively charged compared to the inside
• Action potential

  Rapid change in potential difference that occurs when a neurone is stimulated, causing the cell membrane to become depolarised
• Sequence of events in an action potential

  1. Stimulus

  2. Depolarisation

  3. Repolarisation

  4. Hyperpolarisation

  5. Resting potential
• Stimulus


  Excites the neurone cell membrane, causing sodium ion channels to open. Sodium ions diffuse into the neurone down the sodium ion electrochemical gradient, making the inside of the neurone less negative.
• Depolarisation


  If the potential difference reaches the threshold (around -55 mV), more sodium ion channels open. More sodium ions diffuse 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, starting to get the membrane back to its resting potential.
• Hyperpolarisation


  Potassium ion channels are slow to close so there's a slight 'overshoot' where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (i.e. less than -70 mV).
• Resting potential


  The ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting potential until the membrane's excited by another stimulus.
• Refractory period


  After an action potential, the neurone cell membrane can't be excited again straight away because the ion channels are recovering and they can't be made to open
• Refractory period


  1. Sodium ion channels are closed during repolarisation

  2. Potassium ion channels are closed during hyperpolarisation
• Refractory period


  • Acts as a time delay between one action potential and the next
  • Makes sure that action potentials don't overlap but pass along as discrete (separate) impulses
  • Means there's a limit to the frequency at which the nerve impulses can be transmitted
  • Means action potentials are unidirectional (they only travel in one direction)
• Waves of depolarisation


  1. Sodium ions that enter the neurone diffuse sideways

  2. Causes sodium ion channels in the next region of the neurone to open

  3. Sodium ions diffuse into that part

  4. Causes a wave of depolarisation to travel along the neurone
• Wave of depolarisation


  Moves away from the parts of the membrane in the refractory period because these parts can't fire an action potential
• All-or-nothing principle

  • Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is
  • If the threshold isn't reached, an action potential won't fire
• Bigger stimulus


  Won't cause a bigger action potential but it will cause them to fire more frequently
• Speed of conduction


  The speed at which action potentials are conducted
• Factors affecting speed of conduction of action potentials

  • Myelination
  • Axon diameter
  • Temperature
• Myelination

  • Some neurones, including many motor neurones, are myelinated - they have a myelin sheath
  • The myelin sheath is an electrical insulator
  • In the peripheral nervous system, the sheath is made of Schwann cells
  • Between the Schwann cells are nodes of Ranvier
  • Sodium ion channels are concentrated at the nodes of Ranvier
  • In a myelinated neurone, depolarisation only happens at the nodes of Ranvier
  • The neurone's cytoplasm conducts enough electrical charge to depolarise the next node
  • 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
  • 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
• As temperature increases


  The speed of conduction increases
• The speed only increases up to around 40 °C though - after that the proteins begin to denature and the speed decreases
• Synapse


  The junction between a neurone and another neuron, or between a neurone and an effector cell, e.g. a muscle or gland cell
• Synaptic cleft


  The tiny gap between the cells at a synapse
• Presynaptic neurone

  The neurone before the synapse
• Synaptic knob


  A swelling at the end of the presynaptic neurone that contains synaptic vesicles filled with neurotransmitters
• Neurotransmitters


  Chemicals contained in the synaptic vesicles
• Effect of an action potential

  1. Action potential reaches the end of a neurone

  2. Neurotransmitters released into the synaptic cleft

  3. Neurotransmitters diffuse across to the postsynaptic membrane

  4. Neurotransmitters bind to specific receptors

  5. Receptors might trigger an action potential, cause muscle contraction, or cause hormone secretion
• Synapses


  • Make sure impulses are unidirectional - the impulse can only travel in one direction
  • Neurotransmitters are removed from the cleft so the response doesn't keep happening
• Acetylcholine (ACh)


  A neurotransmitter that binds to cholinergic receptors
• Cholinergic synapses


  Synapses that use acetylcholine as the neurotransmitter
• Transmission across a cholinergic synapse

  1. Action potential arrives at the synaptic knob of the presynaptic neurone

  2. Action potential stimulates voltage-gated calcium ion channels to open

  3. Calcium ions diffuse into the synaptic knob

  4. Calcium ions are pumped out afterwards by active transport