Nervous System

• Nervous System Cell

  • Neuron
  • Nerve cell
  • Structure fits function
  • Many entry points for signal
  • One path out
  • Transmits signal

  

• Fun Facts about Neurons

  • Most specialized cell in animals
  • Longest cell
  • Blue whale neuron
    • 10–30 meters long
  • Giraffe axon
    • 5 meters long
  • Human neuron
    • 1–2 meters long

• Transmission of a Signal

  • Start the signal
  • Trigger the signal
  • Propagate the signal
  • Re-set the system

• Transmission of a nerve signal

  • Neuron has a similar system
  • Protein channels are set up
  • Once the first one is opened, the rest open in succession
    • All or nothing response
  • A “wave” action travels along the neuron
  • Have to reset channel, so the neuron can react again

• Cells Surrounded by Charged Ions

  • Cells live in a sea of charged ions
  • Anions (negative)
    • More concentrated within a cell
    • Cl-, charged amino acids
  • Cations (positive)
    • More concentrated in the extracellular fluid
    • Na+

• Cells have Voltage

  • Opposite charges on opposite sides of the cell membrane
  • Membrane is polarized
    • Negative inside; positive outside
    • Charge gradient
    • Stored energy

• How Does a Nerve Impulse Travel

  • Stimulus: nerve is stimulated
  • Reaches threshold potential
    • Opens Na+ channels in cell membrane
    • Na+ ions diffuse into cell
  • Charges reverse at that point on neuron
    • Positive inside; negative outside
    • Cell becomes depolarized
  • Wave: nerve impulse travels down neurons
  • Change in charge opens next Na+ gates down the line
    • “Voltage-gated” channels
  • Na+ ions continue to diffuse into the cell
  • “Wave” move down neuron
    • Action potential
  • Reset: 2nd wave travels down neuron
  • K+ channels open
    • K+ channels open up more slowly than Na+ channels
  • K+ ions diffuse out of the cell
  • Charges reverse back at that point
    • Negative inside; positive outside
  • Combined waves travel down neuron
  • Wave of opening ion channels moves down neuron
  • Signal moves in one direction
    • Flow of K+ out of cells stops activation of Na+ channels in the wrong direction
  • Action potential propagates
  • Wave
    • Nerve impulse or action potential
  • Brain to fingertips in milliseconds

• Voltage-gated channels

  • Ion channels open and close in response to changes in charge across membrane
  • Na+ channels open quickly in response to depolarization and close slowly
  • K+ channels open slowly in response to depolarization and close slowly

• How does the nerve reset itself

  • After firing, a neuron has to reset itself
  • Na+ needs to move back out
  • K+ needs to move back in
  • Both are moving against concentration gradients
    • Need a pump
  • Sodium-Potassium Pump
  • Active transport protein in membrane
    • Requires ATP
  • 3 Na+ pumped out
  • 2 K+ pumped in
  • Resets charge across membrane

• Action Potential Graph

  

• Myelin Sheath

  • Axon coated with Schwann cells
  • Insulates axon
  • Speeds up signal
    • Signal hops from node to node
    • Saltatory conduction
  • 150 m/sec vs. 5 m/sec

• Chemical synapse

  • Events at synapse
  • Action potential depolarizes membrane
  • Opens Ca+ channels
  • Neurotransmitter vesicles fuse with membrane
  • Release neurotransmitter to synapse
  • Neurotransmitter binds with protein receptor
    • Ion-gated channels open
  • Neurotransmitter degraded or reabsorbed

• Nerve impulse in next neuron

  • Posy-synaptic neuron
  • Triggers nerve impulse in next nerve cell
    • Chemical signal opens ion-gated channels
    • Na+ diffuses into cell
    • K+ diffuses out of cell
    • Switch back to voltage-gated channels

• Neurotransmitters

  • Acetylcholine
  • Transmit signal to skeletal muscle
  • Epinephrine (adrenaline) and norepinephrine
  • Fight or flight response
  • Dopamine
  • Widespread in brain
  • Affects sleep, mood, attention, and learning
  • Lack of dopamine in brain, associated with Parkinson’s disease
  • Excessive dopamine is linked with schizophrenia
  • Serotonin
  • Widespread in brain
  • Affects sleep, mood, attention, and learning
  • Weak point of nervous system
  • Any substance that affects neurotransmitters or mimics them affects nerve function
    • Gases
    • Nitrous oxide
    • Carbon monoxide
    • Mood altering drugs
    • Stimulants

Amphetamines

Caffeine

Nicotine

• Depressants

Quaalude

Barbiturates

• Hallucinogenic drugs
  • LSD
  • Peyote
  • SSRI
  • Prozac
  • Zoloft
  • Paxil
  • Poisons
  • Acetylcholinesterase
  • Enzyme which breaks down acetylcholine neurotransmitter
    • Acetylcholinesterase inhibitors are neurotoxins
    • Snake venom
    • Sarin
    • insecticides

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