Neurons

• vitalism 


  • refuted by Helmholtz =
  • neural signal physical (electrical) not a vital force of nature specific to particular senses
• Golgi 


  invented method of staining neurons with silver nitrate
• Cajal 


  • used Golgi's method to produce detailed drawings of neural assemblies
  • discovered synapse & functional polarity of neurons - direction signals travel - 1 direction
• Golgi, Cajal & Neuron Doctrine


  • disagree over whether nervous system composed of individual units that interact with neighbours (Cajal) or continuous mass (Golgi)
  • Cajal = proved correct
  • To some extent gap junctions (electrical connections) support Golgi’s ideas
• Donald Hebb: The Organization of Behaviour (1949)


  • 1st comprehensive theory of how complex psychological phenomena, perceptions, emotions, thoughts, & memories, might be produced by brain activity
  • alternative to dominant paradigm - behaviourism
  • simple rules on how active cells could form ‘assemblies’ (work together + over time) to form elements of cognition
  • Sequences of assemblies (‘synfire chains’) could explain perception of temporal patterns; i.e. those that unfold in time
• Neurons

  • Cajal - neirons = basic, distinct, units of nervous system
  • cell body = nuclus, mitochondria, other structurers involved in protein synthesis & transport (e.g. neurotransmitters)
  • processes - dentrites & axon (myelin sheath - lipid (fat like) & protein)
  • terminal buttons
• Membrane potential & Ion exchange 


  • transmission in 1 direction - dendrites to terminals
  • dendrites & soma recieve input from previous nerurons
  • input changes potential (electrical charge) of neuron
  • resting potential = charge across neural membrane at rest - membrane polarized
  • charge caused by differences in ion concentrations & maintained by diffusive & electroststic pressures & mechanical process
• membrane potential & Ion exchange - permeability regulation 


  • membrane of neuron = low permeability to sodium ions = restricts entry into cell
  • sodium ions must be kept in greater concentration outside cell by low permeability
• Membrabe potential & ion exchange - sodium-potassium pump


  • actively transports sodium ions out of cell while simultaneously bringing potassium ions into cell
  • operates by exchanging 3 sodium ions for every 2 potassium ions
  • process helps maintain concentration gradient of sodium ions = ensuring higher concentration outside cell
• Depolarization & Action Potential

  • Excitation: When neuron receives input from other neurons = may become depolarized
  • If depolarization reaches threshold = action potential generated
  • Voltage-dependent sodium channels opened
  • sodium enters cell - potential increases
  • voltage dependent potassium chanes opened
  • sodium channels close
  • potassium leaves cell - returning potential to normal
  • over time sodium potassium pump restores concentrations
  • depolarization occurs locally & spreads down axon in an all-or-none fashion
• The Synapse – neural transmission

  • Neurotransmitter molecule binds with a receptor (in post synaptic neuron)
  • causes opening of ion channels & changes to polarization of postsynaptic membrane
  • effects of transmitter depend on ion channel open
  • –Na + channels - depolarization = Excitatory post-synaptic potential (EPSP)
  • –K + channels - hyperpolarization = Inhibitory post-synaptic potential (IPSP)
• Excitatory & Inhibitory Post synaptic potential

  sodium chanel open → depolarisation → EPSP

  = moves potential closer to firing threshold

  potassium channels open → hyperpolarization → IPSP

  = moves potential away from threshold
• Neural Processing

  • ion channels open transiently - PSPs decay over time
  • Seversl EPSPs necessary for depolarization to reach threshold
  • this allows inputs from many neurons to be added together = summated
• Neural processing - summation 


  • temporal summation - PSPs in close succession overlap & add together
  • spatial summation - simultaneous PSPs at different locations add
  • EPSPs & IPSPs can cancel each other out
• Neurotransmitters

  • 2 primary transmitters Glutamate (I effects) & GABA (E effects)
  • many modulatory transmitters - regulate activity of neurons
  • effect of post-synaptic neuron determined by receptors present, state of neuron, presence of other transmitter substances
  • = allows complex modulation of neural processing
• Neurotransmitters - Glutamate

  • brain's most common excitatory transmitter
  • increases membrane potential of postsynapic cell - brings closer to threshold
  • is an amino acid prodcued by neuron's metabolism
  • activates several types of receptors - NMDA, AMPA
  • AMPA receptor controls Na+ gate → EPSP
• Neurotransmitters: Glutamate p2

  • NMDA receptor - controls Na+ & Ca2+ gates
  • Ca2+ = involved in changes to AMPA receptors, producing Long Term Potentiation
  • NMDA receptor blocked by Mg+ ion
  • this removed when membrane depolarized
  • other binding sites have modulatory effects on NMDA receptors
• Neurotransmitters - GABA - Gamma-AminoButyric Acid

  • brain's most common inhibitory transmitter
  • decreases membrane potential of postsynaptic cell - further from threshold
  • prevents excessive excitation
  • Inhibitory interneurons increase flexibility of nervous system
  • many receptor sites - allowing drug action
• Neurotransmitters: dopamine

  • Dopaminergic projections from substantia nigra & ventral tegmental area modulate activity in striate, limbic, & cortical areas
  • effects can be excitaory, inhibitory or modulatory (long lasting effects) depending on receptor
• Dopamine - Substania nigra 


  • modulates input areas of basal ganglia - involved in action control
  • Degeneration of dopaminergic neurons = associated with Parkinson’s disease
  • Treatment often involves L-DOPA - precursor of dopamine = compensate for dopamine deficiency
• dopamine - Ventral Tegmental area 


  • involved in reward, learning, and changing behaviour to unexpected or highly salient stimuli
  • Stimuli associated with VTA activation, particularly nucleus accumbens = perceived as exciting or rewarding (mesolimbic system)