Synapes and dendrites

Neuron Communication

  • Action Potentials:

    • Occur only on axons and are crucial for neuron communication.
    • Triggered at the beginning of an axon and propagated down the length of the axon.

  • Synapses:

    • The junction between two neurons where communication occurs.
    • Can be classified into:
    • Electrical Synapses: Direct transmission of action potentials through gap junctions.
      • Rare in vertebrates.
    • Chemical Synapses:
      • Space (synaptic cleft) between the presynaptic (sending) and postsynaptic (receiving) cells.
      • Involvement of neurotransmitters.

Neurotransmitter Release

  • Neurotransmitters:

    • Chemicals released by the presynaptic cell into the synaptic cleft to communicate with the postsynaptic cell.
    • Examples include:
    • Dopamine and Serotonin: Various functions in the nervous system.
    • Acetylcholine: Involved in muscle control.

  • Release Mechanism:

    • Action potential triggers voltage-gated calcium channels to open at the axon terminal.
    • Calcium influx causes vesicles filled with neurotransmitters to fuse with the cell membrane and release contents into the synaptic cleft.
    • Summary Steps of Release:
      1. Action potential reaches the axon terminal.
      2. Voltage-gated calcium channels open.
      3. Calcium triggers vesicle fusion with the membrane.
      4. Neurotransmitters are released into the synaptic cleft.

Postsynaptic Effects

  • Postsynaptic Binding:

    • Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell.
    • This binding opens ion channels, impacting membrane potential.

  • Changes in Postsynaptic Potential:

    • Postsynaptic potentials (PSPs) can be either:
    • Excitatory Postsynaptic Potentials (EPSPs): Bring the membrane potential closer to threshold (e.g., sodium influx).
    • Inhibitory Postsynaptic Potentials (IPSPs): Move the membrane potential away from threshold (e.g., potassium efflux or chloride influx).
    • Net Change: Depends on the sum of all excitatory and inhibitory inputs across the neuron's dendrites and cell body (postsynaptic summation).

Action Potential Initiation

  • Threshold Potential:

    • An action potential is generated if the summation of PSPs reaches threshold (approximately -55 mV) at the axon hillock.
    • If not, no action potential occurs.

  • Electrochemical Dynamics:

    • Voltage-gated channels for sodium and potassium open only at the axon segment; vital for action potential propagation.

Sodium-Potassium Pump

  • Critical Role:
    • Maintains sodium and potassium gradients essential for generating action potentials.
    • Operates continuously, making neurons energetically costly tissue due to high ATP consumption.