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