Axons

Axons

1. Axon Hillock

   • Summation
     • Temporal
       • High-frequency stimulation by one presynaptic neuron
     • Spacial
       • Simultaneous stimulation by multiple presynaptic neurons
     • IPSP: Inhibitory Post Synaptic Potentials
       • Moves membrane potential further from threshold
     • EPSP: Excitatory Post Synaptic Potentials
       • Moves membrane potential closer to threshold
     • Process of adding together multiple electrical impulses at the axon hillock to initiate an AP
   • Action potentials
     • Resting membrane potential = 70mV
       • 1. Depolarization
         • Occurs when stimulus causes membrane potential to reach -55mV (threshold), which triggers Na+ influx
           • -55 mV → +30 mV
       • 2. Repolarization
         • Occurs when Na+ channels deactivate and K+ channels open, causing K+ efflux
           • +30 mV → -70mV
       • 3. Hyperpolarization
         • Occurs due to continued K+ efflux, membrane potential overshoots resting membrane potential
           • -70 mV → -85 mV
       • 4. Recovery
         • Occurs when voltage gated channels close and Na+/K+ pump reestablishes resting membrane potential
           • -85 mV → -70 mV
     • Ion channels
       • Leakage
         • Channels that are always open
       • Gated
         • Ligand-gated
           • Channels that open when a ligand binds to them
         • Voltage-gated
           • Channels that are closed at -70 mV and open by depolarization
             • Na+: open at -55 mV
             • K+: open at +30
     • All-or-None Law
       • When stimulus intensity increases, action potentials increase in frequency and not in amplitude.. AP will occur if threshold is reached.

2. Axon Terminals

   • Synapse
     • Chemical
       • Process:
         • 1. Action potential reaches axon terminal
           • 2. Calcium influx due to opened voltage-gated channels
             • 3. Calcium binds to synaptotagmin
               • 4. Vesicles with NT docked to membrane by SNARE
                 • 5. Vesicle fuses with membrane
                   • 6. NT released
       • Neurotransmitters
         • Acetylcholine
           • ACh reuptake
             • Acetylcholinesterase inactivates ACh into acetate and choline; reused in presynaptic cell
           • Muscarinic
             • G-protein coupled receptors that can be both excitatory when closed or inhibitory when open
             • Locations: Smooth & Cardiac muscle, Glands, & CNS
           • Nicotinic
             • Excitatory ligand-gated receptors
               • Locations: Autonomic ganglia, Neuromuscular junctions, & Some CNS pathways
         • Agonists
           • Drugs that stimulate a receptor
           • Ex, Nicotine & Muscarine
         • Antagonists
           • Drugs that inhibit a receptor
           • Ex, Curare & Atropine
         • Monoamines, amino acids, neuropeptides, endocannabinoids, gases, & purines
       • Receptor Channels
         • Ligand-gated
           • NT binding directly opens channel
         • G-protein coupled
           • NT binding indirectly opens channel, uses G-proteins
       • Transfer across synaptic cleft
     • Electrical
       • Gap Junctions
         • Connexin proteins that allow for ions and molecules to pass between cells
       • Direct cell-cell ion transfer
       • Found in smooth & cardiac muscle, between some brain neurons & glial cells
     • Gap between Axon terminals and dendrites of another neuron
   • Axon endings AKA terminal boutons

3. Myelin sheath

   • Oligodendrocytes
     • Form myelin sheaths around CNS axons
   • Schwann cells
     • Form myelin sheaths around peripheral axons
   • Nodes of Ranvier
     • Gaps in myelin that allow for increased conduction speed
     • ↓
     • ↓
   • Insulating layer surrounding axons

4. Nerve Impulse Conduction

   • Refractory period
     • Absolute
       • Voltage gated Na+ channels are temporarily inactivated; neuron incapable of firing another AP
     • Relative
       • Neuron capable of firing a second AP with a stronger stimulus due to some recovered Na+ channels & K+ efflux; occurs during hyperpolarization
   • Saltatory conduction
     • Allow Na+ and K+ to skip from node of Ranvier to node of Ranvier, speeding the speed of the action potential
   • Axon diameter
     • Reduces resistance, speeding up the speed of the action potential
   • AP at one region serves as depolarization stimulus for the next region

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