nervous system

• Resting membrane potential (RMP)


  The electrical potential difference that exists across the membrane of a cell in the resting state
• How RMP arises


  1. Membranes are selectively permeable to ions

  2. Leak channels remain open in resting state

  3. Movement of ions still occurs

  4. Varying concentration of ions inside and outside the cell creates an electrical difference

  5. Usually -70mV
• Factors contributing to RMP

  • Potassium leak channels allowing potassium to diffuse out of the cell, reducing positive ions inside
  • Sodium-potassium pump actively pumping out sodium and in potassium against their gradients
  • Overall membrane permeability to other ions like sodium and chloride is not very high at rest
• Action potential


  An electrical event which refers to a brief reversal of resting membrane potential by a rapid change in plasma membrane permeability
• Steps in action potential

  1. Stimulus received

  2. Voltage-gated sodium channels open, increasing sodium inside cell, depolarising it

  3. Sodium channels close, potassium channels open, allowing potassium out, decreasing membrane potential

  4. Additional potassium leaves, hyperpolarising the cell

  5. Potassium channels close, less potassium leaks out

  6. Cell returns to resting ion permeability and RMP
• Refractory period


  A period of resistance to stimulation (decreased excitability) that occurs in a small patch of membrane at one time and only lasts for a few milliseconds
• Types of refractory period

  • Absolute refractory period - caused by residual inactivation of sodium channels, no stimulus can trigger action potential
  • Relative refractory period - caused by increased opening of potassium channels, strong stimulus can overcome it
• Speed of action potential propagation


  Proportional to diameter of neuron and amount of myelin wrapped around it
• Role of myelin


  • Insulator, preventing leakage of ions across axon membrane, increasing speed of conduction
  • Physical barrier, protecting axon from damage and providing metabolic support
  • Allows action potential to jump from one node of Ranvier to another
• Effect of axon diameter

  • Larger diameter reduces resistance to flow of ions during propagation, increasing speed
• Synaptic transmission and integration

  How action potentials are passed on and then integrated by another neuron or neuron target
• Steps of synaptic transmission

  1. Action potential reaching the axon terminal and depolarisation (output zone)

  2. Triggers the opening of voltage-gated calcium channels along the trigger zone (the axon hillock) which allows calcium ions to rush in

  3. Increased calcium levels activates proteins which stimulate synaptic vesicles to fuse with the neuronal membrane and release neurotransmitters into the synaptic cleft

  4. Neurotransmitters bind to receptors in the target cell at the input zone (the dendrites or cell body) and open ligand-gated ion channels

  5. Change in membrane potential (excitatory postsynaptic potential EPSP)

  6. EPSP reaches a threshold (usually -50mV) and triggers an action potential in the axon (conducting zone) of the postsynaptic cell
• Excitatory postsynaptic potential (EPSP)

  Electrical excitement of a cell which originates in the dendrites and propagates passively causing a shift in the resting membrane potential towards the threshold for an action potential, leading to depolarisation
• Inhibitory postsynaptic potential (IPSP)


  Electrical excitement of a cell which causes a shift in the resting membrane potential away from the threshold for an action potential, leading to hyperpolarisation
• Integration


  1. Sum (summation) of the synaptic events (EPSP and IPSP) on an individual neuron which takes place in the brain

  2. Subthreshold, threshold potential is not reached, thus no summation occurs, and the neuron remains in its resting state

  3. Temporal (time) summation of EPSPs at the same synapse occurs when there are multiple EPSPs generated in rapid succession at the same synapse

  4. Spatial (location) summation of EPSPs occurs when there are multiple EPSPs generated simultaneously at different synapses on the same neuron

  5. Spatial summation of EPSP and IPSP occurs when both EPSPs and IPSPs are integrated spatially
• Leak channels


  Type of channel that is always open and participates in the generation of the resting membrane potential
• Voltage-gated channels


  Complex protein molecules that form a selective pore for particular ions in the membrane, and are only open based on electrical potential. Responsible for changes in the membrane potential and generation of action potentials. Can be specific to sodium, potassium, and calcium ions.
• Ligand-gated channels


  Responsible for the generation of EPSPs and IPSPs. Opened by extracellular ligands such as neurotransmitters or intracellular ligands such as ATP, calcium, and nucleotides cAMP and cGMP.
• Mechanically-gated channels


  Responsible for the generation of the receptor potential. Opened by a mechanical force in response to stretch or deformation of the plasma membrane.
• Neurons can be classified by

  • Number of processes
  • Axon length
  • Function
  • Type of neurotransmitter utilised
• Unipolar neuron


  • Dendrite and axon derive from same process
• Bipolar neuron


  • Dendrite and axon on either end
• Multipolar neuron


  • Two or more dendrites plus an axon
• Pseudo unipolar neuron


  • No dendrites (technically a branched axon)
• Projection neuron


  • Long axons
• Local circuit neuron


  • Short axons
• Sensory neuron


  • Carries sensory information
• Motor neuron


  • Carries motor information
• Interneuron


  • Circuits of branching neurons in the Central Nervous System
• Glutamatergic neuron


  • Uses glutamate as neurotransmitter
• Noradrenergic neuron


  • Uses noradrenaline as neurotransmitter
• Cholinergic neuron


  • Uses ACh as neurotransmitter
• Glial cells in the nervous system

  • Astrocytes
  • Oligodendrocytes
  • Schwann cells
  • Microglia
• Astrocytes


  • Fill most space in the Central Nervous System
  • Control chemical content of extracellular space
  • Support neuron metabolism
• Oligodendrocytes


  • Lay down lipid-rich myelin (70% lipid, 30% protein)
  • Provide electrical insulation around some axons
• Schwann cells


  • Lay down lipid-rich myelin (70% lipid, 30% protein)
  • Provide electrical insulation around some axons
• Microglia


  • Phagocytes - immune cells
  • Remove debris of degenerated or dead neurons and other glial cells
• Nervous system structure


  • Afferent neurons
  • Interneurons
  • Efferent neurons
• White matter


  • Bundles of axons surrounded by myelin
• Grey matter


  • Cell bodies, dendrites, and axon terminals without myelin