BIO 2.5

• Resting membrane potential


  • -70 millivolts
• Neuron at rest


  • Not transmitting any signal
• Membrane potential


  Charge difference or voltage across a membrane
• In a resting neuron, there are more potassium ions inside the cell while there are more sodium ions outside the cell
• Sodium potassium pump


  1. Transports three potassium ions into the cell and two sodium ions out of the cell

  2. Uses energy to transport ions against their concentration gradient
• Potassium ion leak channels


  • Allow the diffusion of potassium out of the neuron
  • Contribute to the negative resting membrane potential
• Negative ions or anions


  • Present inside the neuron
  • Cannot pass through the neuron's plasma membrane
  • Cause an imbalance in the distribution of charges inside and outside a neuron
• Stimulus


  Brings a change in the resting potential of a neuron
• Gated ion channels


  Ion channels that open or close in response to a stimulus
• Sodium-gated ion channels open


  • More sodium enters the cell
  • Membrane potential becomes more positive (depolarization)
• Potassium-gated ion channels open


  • More potassium leaves the cell
  • Membrane potential becomes more negative (hyperpolarization)
• 5 phases of an action potential are:

  1. Resting state

  2. Depolarization

  3. Rising phase

  4. Falling phase

  5. Undershoot
• Threshold reached


  • More voltage-gated sodium ion channels activated and open
  • Further influx of sodium ions into the cell
  • Membrane potential becomes more positive (rising phase)
• Membrane potential reaches positive value


  • Voltage-gated sodium ion channels become inactivated
  • Voltage-gated potassium ion channels open
  • Potassium ions exit the cell
  • Membrane potential returns to resting potential (falling phase)
• Undershoot


  • Some voltage-gated potassium ion channels remain open
  • Membrane becomes more negative than resting potential
• Nerve impulse transmission


  1. Action potential in one area leads to depolarization of neighboring areas

  2. Action potential generated in neighboring areas

  3. Action potential seems to spread along the axon
• Myelin sheath


  • Insulates the axons of mammals
  • Speeds up the transmission/conduction of impulse
• Nodes of Ranvier


  • Gaps in the myelin sheath
  • Voltage-sodium gated ion channels located here
  • Depolarization and action potential generation limited to these nodes
  • Increases speed of nerve impulse transmission
• Saltatory conduction


  The "leaping" mechanism of the action potential from one node to the other
• Electrical synapse


  Electrical current generated by action potential flows from one neuron to another
• Chemical synapse


  • Depolarization of an axon triggers the release of neurotransmitters
  • Neurotransmitters diffuse into the next neuron and generate an action potential
• Neurotransmitters


  • Acetylcholine
  • Glutamate
  • GABA
  • Glycine
  • NO2
• Nerve impulse transmission relies on membrane potential - defined as the charge difference or voltage across a membrane
• A neuron which does not transmit any signal is said to be in its resting state and has a resting membrane potential of -70 mV
• Stimulus


  Brings a change in the resting membrane potential through the selective opening of various voltage-gated membrane ion channels
• Depolarization


  May lead to the generation of an action potential in an area of an axon
• Nerve impulse transmission


  Generation of action potential repeated along neighboring zones in the axon
• In myelinated axons, the generation of action potential occurs only in axonal areas which are unmyelinated
• This limits the amount of ion channels which need to open to transmit the signal, resulting in faster nerve impulse transmission
• Opening of the voltage-gated sodium ion channels


  1. Influx of sodium into the neuron

  2. Membrane potential becomes more positive

  3. Membrane potential reaches threshold of -50 mV
• Reaching threshold of -50 mV


  1. More voltage-gated sodium ion channels activated and opened

  2. Further influx of sodium ions into the cell

  3. Membrane potential becomes more positive

  4. Rising phase of action potential
• Membrane potential reaches positive value


  1. Voltage-gated sodium ion channels become inactivated

  2. Voltage-gated potassium ion channels open

  3. Potassium ions exit the cell

  4. Membrane potential returns to resting potential of -70 mV

  5. Falling phase of action potential
• Reaching resting potential of -70 mV


  1. Some voltage-gated potassium ion channels remain open

  2. Membrane becomes hyperpolarized and more negative

  3. Undershoot
• Voltage-gated potassium ion channels close


  Membrane potential returns to resting potential
• Action potential in one area of axon


  1. Depolarization of neighboring areas

  2. Generation of action potential in neighboring areas

  3. Nerve impulse transmission
• Myelin sheath in mammals


  • Speeds up transmission/conduction of impulse
• Myelin sheath


  Insulation around axons, produced by glial cells, mainly made of lipids which are poor conductors
• Nodes of Ranvier


  • Gaps in myelin sheath where voltage-gated sodium channels are located
  • Depolarization and action potential generation limited to these nodes
  • Increases speed of nerve impulse transmission
• Saltatory conduction


  Leaping of action potential from one node to the next
• Types of synapses


  • Electrical synapse
  • Chemical synapse