Synaptic Function and Communication

Cell-to-Cell Communication

  • Fast vs. Slow Communication

    • Nervous System (Fast): Wired, neurotransmitters, milliseconds, rapid, precise, short-acting.

    • Endocrine System (Slow): Wireless, hormones, minutes/longer, longer duration, widespread effects.

The Synapse

  • Junctions mediating information transfer between neurons or neuron to effector cell.

  • Electrical Synapses:

    • Gap junctions, connect cytoplasm of adjacent neurons.

    • Allow ion/small molecule flow, rapid transmission, synchronized activity.

    • Cannot be easily modulated.

    • Common in heart/smooth muscle.

  • Chemical Synapses:

    • Utilize neurotransmitters, slower than electrical.

    • Six Steps of Transmission:

      1. Action Potential (AP) arrives at axon terminal.

      2. Voltage-gated Ca2+Ca^{2+} channels open, Ca2+Ca^{2+} enters terminal.

      3. Ca2+Ca^{2+} entry causes synaptic vesicles to release neurotransmitter (NT) via exocytosis.

      4. NT diffuses across synaptic cleft, binds to specific receptors (often chemically-gated ion channels) on postsynaptic membrane.

      5. NT binding opens ion channels, creating graded potentials (Excitatory Postsynaptic Potential (EPSPs) or Inhibitory Postsynaptic Potential (IPSPs)).

      6. NT effects terminated by reuptake, enzymatic degradation, or diffusion.

Graded Potentials (Postsynaptic Potentials)

  • Chemically-Gated Ion Channels: Post-synaptic membrane has chemically/receptor activated channels (no AP generated here).

  • Factors Affecting Graded Potential Size:

    • Higher impulse frequency leads to more NT released.

    • Larger amount of NT leads to larger graded potential.

    • Type of NT and specific postsynaptic receptor activated determine effect.

  • EPSP (Excitatory): Local depolarization, brings neuron closer to AP threshold, opens Na+\text{Na}^{+} and K+\text{K}^{+} channels simultaneously.

  • IPSP (Inhibitory): Local hyperpolarization, drives neuron away from AP threshold, opens K+\text{K}^{+} or Cl−\text{Cl}^{-} channels.

Neural Integration & Summation

  • Axon Hillock: Integrates all incoming EPSPs and IPSPs.

  • Summation of Graded Potentials: Determined by both time (temporal) and space (spatial).

    • Temporal Summation: Multiple stimuli from one presynaptic neuron close in time add together.

    • Spatial Summation: Simultaneous stimuli from different presynaptic neurons at different locations add together.

    • EPSPs and IPSPs can cancel each other out.

Neural Circuits (Neuronal Pools)

  • Functional groups of neurons processing specific information, forming the nervous system's functional unit.

  • Diverging Circuit: One input, many outputs (e.g., brain neuron to many motor neurons).

  • Converging Circuit: Many inputs, one output (e.g., different sensory stimuli eliciting same memory).

  • Reverberating Circuit: Signal travels in a chain, feeding back to previous neurons; controls rhythmic activity (e.g., breathing, sleep-wake cycle, short-term memory).

Neuromuscular Junction (NMJ)

  • Specialized synapse between motor neuron and skeletal muscle fiber.

  • Neurotransmitter: Acetylcholine (ACh).

  • Acetylcholinesterase (ACh-esterase): Enzyme that breaks down ACh in the synaptic cleft, terminating its effect.

Disorders and Toxins Affecting Synaptic Function

  • Myasthenia Gravis:

    • Autoimmune disease, reduction/destruction of ACh receptors at NMJ.

    • Symptoms: Muscle fatigue, progressive weakness (ptosis, diplopia, difficulty swallowing, respiratory problems).

    • Treatment: ACh-esterase inhibitors.

  • Curare:

    • Plant toxin, prevents ACh binding by occupying receptors at NMJ.

    • Causes skeletal muscle paralysis.

    • Treatment: ACh-esterase inhibitors.

  • Organophosphates:

    • Inactivate ACh-esterase, leading to high levels of ACh in the cleft.

    • Used in nerve gases; causes muscle contractions, convulsions, twitching.

    • Treatment: Atropine (ACh receptor blocker).

  • Botulism:

    • Bacterial toxin, prevents release of ACh from presynaptic terminal.

    • Causes muscle paralysis (dry mouth, difficulty swallowing, muscle weakness, respiratory failure).

    • Treatment: Ventilator, supportive care.