Neurotransmitters

Neurotransmitters

1. Vocab

   • Dendrites: receive synapses to contact other cells
   • Axons: conveys electrical impulses from dendrites to next neuron.
   • Myelin: fatty insulation
   • Nerve impulse: signal used by neurons to transmit info

2. Membrane potential

   • Semi-permeable
   • Difference in electrical change between the inside and outside of a cell.
   • Resting potential of the cell is -70mV.
     • Polarized
   • Positively and negatively charged particles are called ions.
   • Na+ (Sod) = more outside the cell K+ (potas) = more inside neuron
   • Pressure on Na+ to enter resting neuron
     • Electrostatic pressure, more likely to move down concentration gradient, areas of high conc to low.
       • Sodium ion channels in resting neuron are closed, reducing flow of Na+ ions into cell.
         • Sodium- Potassium pump: Exchange of 3 sodium ions out of cell to two K+ ions inside

3. Postsynaptic Potentials

   • Neuron fires, diffuses across synapses, receptor opens ion channels
   • Molecules bind to postsynaptic receptors
     • Depolarization: reduces resting potential from -70mv to 67mv
       • Excitatory post-synaptic potentials: EPSP's
         • Increase the likelihood of neuron firing
         • Decremental: decrease in amplitude as travel along neuron
     • Hyperpolarization: increases to -72mv
       • Inhibitory post-synaptic potentials: IPSP's
         • Decrease likelihood that the neuron will fire

4. Action Potentials

   • Whether a neuron fires is due to the balance between the Excitatory and inhibitory signals reaching its axon.
   • Generated in the Axon Initial Segment
   • If sufficient to depolarise the membrane to a level of threshold of excitation (-65mV) AP is generated
     • 1 Millisecond reversal of membrane potential from -70mV to +50mV. (EPSP)
   • All or none responses: every AP same amplitude, full extent
     • Refractory period: 1-2ms, AP only travels in one direction. Second AP cannot occur
   • Occur in voltage-activated ion channels- open/close in response to changes in level of membrane potential. Na+ ions rush in..-70mV to +50mv.
   • Non-decremental: Does not grow weaker as travels along axonal membrane
   • Spatial Summation: occurs when several EPSP's arrive at the axon hillock simultaneously
     • Temporal Summation: postsynaptic potentials created at the same synapse in rapid succession are summed

5. Myelination Axons

   • Greatly speeds up axonal conduction
   • Myelin insulates axons of neurons from extracellular fluid.
   • Ions can pass through axonal membrane at the nodes of ranvier
   • Transmission of AP's in myelinated axons is called Saltatory Conduction
   • Many neurons do not have axons, no AP's. Conduction here are passive.
     • Hodgkin-Huxley Model- neural conduction.
   • Cerebral Neurons: Fire continually even with no input.

6. Synaptic Transmission

   • Synapses: Gaps between neurons, diffuse across.
   • Synaptic Clefts: Induce EPSP's and IPSP's
   • Axodendritic synapses: synapses of axon terminal buttons on dendrites
   • Axsomatic synapses: on somas, selectively influence one neuron
   • Exocytosis: Neurotransmitter release when stimulated by AP. Voltage activated calcium channels open and Ca+ ions enter.
     • Synaptic ventricles fuse with presynaptic clefts at synapse.

7. Repute and Enzymatic Degradation

   • Reuptake: neurotransmitter once released are drawn back into presynaptic buttons by transporter mechanisms.
   • Enzyme degradation: enzymes= proteins that stimulate or inhibit biochemical reactions without being affected by them.
     • Acetylcholine: neurotransmitters for which ED main mechanism of synaptic deactivation, broken down by the enzyme
   • Gap Junctions: narrow spaces between adjacent cells

8. Neurotransmitters:

   • Amino Acids: Fast acting, molecular building blocks of proteins e.g glutamate, GABA.
   • Monoamine: Released into extracellular fluid, dopamine serotonin.
   • Acetylocholine: Neurotransmitter at neuromuscular junctions, at synpases in SNS, broken down in synapse.
   • Basal Ganglia uses these.

9. Drug Usage

   • Drugs have 2 effects on synaptic transmission: Inhibition or facilitation.
   • GABA agonists: Anti-anxiety, convulsant, anaesthetic
   • L-DOPA: Anti parkinson, causes psychosis at high doses
   • Dopamine blockers: Anti-psychotic, causes parkinson symptoms at high doses

10. Visualising and stimulating human brain

    • Contrast X-Ray: used to localise vascular damage and indicate the location of a tumour.
    • Computer tomography: CT scan of horizontal section of the brain. 3.D
    • MRI: Magnetic resonance imaging, measurement of waves that hydorgen atoms amit when they are activated by radio- frequency waves in a magnetic field. Cleaner images than CT, high spatial resolution
    • PET Scan- brain activity as opposed to structure, radioactive injected into patients cortical artery, accumulates in active neutrons, PET scans image of the level of radioactivity in various parts of the brain.
    • fMRI: functional MRI, images representing the increase in oxygen flow in the blood to active areas of the brain. Oxygenated blood accumulates in active areas of the brain. Oxygenated blood has magnetic properties that influence radio frequency waves emitted by hydrogen atoms in an MRI.
      • Signal BOLD: blood-oxygen level dependent signal
        • 4 advantage over PET
          • No Injections
          • Structrual and functional info
          • Better spatial resolution
          • Produce 3-D images over entire brain
          • BUT poor temporal resolution and time of neural events
    • Physiological Activity
      • EEG: electrical activity of the brain
      • PSP's: alpha waves
      • MEG: Magnetic fields, neural activity changes
      • EMG: Muscle tension
      • ECG: Cardiovascular
      • EOG: Eye-activity