Biological 2

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Glial Cells

  • Non-neuronal - do not carry messages
  • Supportive role in holding neurons in place, supplying nutrients and oxygen, providing insulation
  • Destroy pathogens
  • Remove dead neurons
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The Neuron

  • Neuron - the basic unit of structure and function in the Nervous System, in both CNS and PNS
  • 100-150 billion neurons, each with different functional capabilities
  • Each enuron can be connected to 1000 or more other neurons - estimated 100 trillion connections
  • Sensory neurons - respons to external stimuli and transmit sensory info to CNS
  • Motor neurons - carry messages from CNS to muscles and glands
  • Interneurons - Transmit messages to other neurons (located in the CNS)
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How do Neurons Communicate?

  • Primary function - transmit info
  • Generating a signal within the neuron, called electrical transmittion
  • Passing the message from one neuron to another, called chemical transmission
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Electrical Transmission

  • Mebrane defines the intracellular and extracellular compartments
  • If we place an electrode inside and outside of the membrane, we would see a difference in electrical charge
  • At rest, the inside of the neuron would be more negative than the outside
  • Due to the positively and negatively charged particles (ions) contained in the intracellular and extracellular fluids
  • Resting membrane potential is -70mv and the neuron is polarised
  • Movement of ions is limited by membrane and special channels
  • Membrane has selective permeability - some can pass and some cant
  • Concentration gradient- ion movement from high to low concentration
  • Electrostatic forces - act on the ions ie. opposites attract and like signs repel
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Sodium-Potassium Pump

  • Voltage dependent and fully operational at RMP
  • If the membrane potential becomes less negative the SPP braks down - neuron is stimulated
  • Na+ rush into the cell and reverse the normal polarity
  • Inside of the cell becomes positive
  • Active neuron is depolarised
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Action Potential

  • All or nothing event - either axon fires or it doesnt
  • After an action potential occurs, there is a brief refractory period when the neuron cannot be stimulated again
  • Then axon returns to a resting state
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Electrical to Chemical Transmission

  • Once the electrical signal has reached the axon terminals, neurons communicate with eachother by means of chemical signalling
  • Chemical transmission is vital for our understanding of how drugs work and influence behaviour
  • Communication occurs at the synapse, where a neuron comes into close contact with the surface of the cell body (Axo-somatic)
  • Can also be dendrties (axo-dendritic) or the terminals of another cell (Axo-axonic)
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Neurotransmitters

  • Neurotransmitter molecules bind to receptors on the membrane of the postsynaptic cell and change the electrical characteristics of that cell, known as postsynaptic potentials
  • Changes can be excitatory or inhibitory
  • Excitatory NTs make it more likely that the postsynaptic neuron will fire - causes postsynaptic neurons sodium channels to open, triggering action potential (depolarisation)
  • Inhibitory NTs make is less likely that the postsynaptic neuron will fire - causes positive potassium ions to flow from the neuron, making it more negative (hyperpolarisation)
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Synapse - Ending Chemical Transmission

  • Autoreceptors - on presynaptic membrane prevent further release of neurotransmitter
  • Degradation - neurotransmitter is broken down and deactivated by enzymes
  • Re-Uptake - neurotransmitter is taken back up by the pre-synaptic terminal and the effect of the neurotransmitter is diminished
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What are Neurotransmitters?

  • Formed within a neuron
  • Released when action potential occurs
  • Can alter activity of postsynaptic cell
  • Deactivated by reuptake or broken down by enzymes
  • Neuromodulators - diffuse away from the synapse to target further away cells
  • Neurohormones - can travel further throughout the body
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Criteria to be a Neurotransmitter

  • Substance must be present in nerve terminals
  • Precursors and enzymes must be present
  • Nerve stimulation must cause its release
  • Exogenous application should mimic effect of nerve stimmulation
  • Specific receptors for the substance must be present
  • Blocking release of the substance should prevent the presynaptic neuron from altering the activity of the postsynaptic cell
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Key Neurotransmitters

  • Norepinephrine - increase heart rate, cause contraction of smooth muscles of blood vessels
  • Acetylcholine - Excitatory effects, control of muscle
  • Serotonin - appetite, mood and sleep
  • Dopamine - movement, reward, addiction 
  • Glutamate - Most abundant
  • GABA- Inhibitory, stops neurons firing uncontrollably
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Endorphins

  • Neuropeptides (Endogenous Opiates)
  • Released from terminal buttons, cell body and axons
  • Diffuse widely
  • Inhibit pain sensitivity
  • Associated with wellbeing and relaxation
  • Released in primates during grooming
  • Released during sex, laughter, massage and exercise
  • Laughter associated with comedy increased pain tolerance for actors and audience
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Drugs and Brain

  • Influence the action of neurotransmitters at the synapse
  • Drugs have an affinity for a receptor
  • Some drugs facilitate transmission - agonists
  • Inhibit transmission - antagonists
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Serotonin

  • Monoamine - indoleamine - neurons containing it are serotonergic
  • Synthesised from tryptophan, obtained from grains, meat and dairy
  • Implicated in mood, appetite and sleep
  • Low in depression
  • Agonist drugs mimic serotonin
  • Increase in tryptophan increases serotonin
  • Reserpine interferes with serotonin storage
  • MDMA increases serotonin
  • SSRIs (prozac) inhibit reuptake of serotonin
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MDMA

  • Stimulant
  • Increases release of dopamine and serotonin 
  • Associated with euphoria, relaxation and closeness
  • Heavy users show loss of neurons containing serotonin
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Depression

  • Emotional: sadness, numbing
  • Cognitive: Negative self view, guilt, blame, hopelessness
  • Motivational: trouble getting started, physical inertia, difficulty making decisions
  • Somatic: loss of appetite, sleep disturbance, fatigue, loss of libido, hypochondriasis
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Biochemical Theories of Depression

  • Monoamine - depression is the result of a deficit in NTs Noradrenaline and Serotonin
  • Based on observed side effects of antihypertensive drug reserpine
  • Caused depression in normal patients
  • Later found drug interfered with storage of NA and 5-HT in synaptic vesicles
  • Artificial depletion of 5-HT and NA also induces depressive symptomatology
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Antidepressants

  • Tricyclics
  • Block transporter proteins that reabsorb serotonin, dopamine and neuropinepherine into presynaptic neuron
  • Prolong presence of the NTs
  • Selective Serotonin Reuptake Inhibitors
  • Prozac
  • Prevent reuptake of serotonin
  • Monoamine Oxidase Inhibitors
  • Block enzyme monoamine oxidase- processes serotonin
  • More serotonin available
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