Regeneration of Nerve Fibres and Neuronal Integration

Regeneration of Nerve Fibres

  • Schwann Cells
    • Form a regeneration tube in the Peripheral Nervous System (PNS).
    • Guide regeneration of cut axons in the PNS.
  • Oligodendrocytes
    • Myelinate fibers in the Central Nervous System (CNS).
    • Do not support regeneration of cut central axons; inhibit regeneration.
  • Future Possibility
    • Research aims to induce regeneration of damaged fibers.

Synapses and Neuronal Integration

  • Neuron Functions
    • A neuron may terminate on a muscle, gland, or another neuron.
    • A synapse is defined as the junction between two neurons, which is the primary means by which one neuron interacts with another directly.
    • When a neuron terminates on a muscle or a gland, it is said to innervate the structure.

Important Terms Related to Synapses

  • Presynaptic Neuron
    • Conducts action potentials toward the synapse.
  • Synaptic Knob
    • Contains synaptic vesicles that store neurotransmitters.
  • Synaptic Vesicles
    • Store neurotransmitters—chemicals that carry signals across a synapse.
  • Postsynaptic Neuron
    • The neuron whose action potentials are propagated away from the synapse.
  • Synaptic Cleft
    • The space between presynaptic and postsynaptic neurons.

Synapse Behaviour

  • Function of Neurotransmitters
    • Binding of neurotransmitters changes the membrane potential in the postsynaptic neuron.
  • Types of Synapses
    • Excitatory Synapses
    • Excite the postsynaptic neuron, making it more likely to fire an action potential.
    • Inhibitory Synapses
    • Inhibit the postsynaptic neuron, making it less likely to fire an action potential.

Receptor Combinations

  • Neurotransmitters vary from synapse to synapse, with specific patterns of release.
  • The same neurotransmitter is always released at a specific synapse.
  • Responses to a neurotransmitter-receptor combination at any given synapse are consistent and reproducible.
  • Some neurotransmitters consistently produce either excitatory or inhibitory effects, while others may have variable effects, resulting in
    • IPSPs (Inhibitory Postsynaptic Potentials)
    • EPSPs (Excitatory Postsynaptic Potentials)

Neurotransmitter Removal Mechanisms

Several mechanisms exist for removing neurotransmitters from the synaptic cleft:

  1. Diffusion
    • The neurotransmitter diffuses away from the synaptic cleft into surrounding areas.
  2. Enzymatic Inactivation
    • Specific enzymes in the subsynaptic membrane break down the neurotransmitter.
  3. Reuptake
    • The neurotransmitter is actively transported back into the presynaptic axon terminal through various mechanisms.

Grand Postsynaptic Potential

  • Temporal Summation
    • Involves the summation of several EPSPs that occur in close succession from a single presynaptic neuron without a refractory period.
  • Spatial Summation
    • Involves the summation of EPSPs originating simultaneously from multiple presynaptic neurons.

Action Potentials at the Axon Hillock

  • The threshold potential required to trigger an action potential is not uniform across the neuron.
  • The axon hillock possesses the lowest threshold potential, making it more susceptible to initiating an action potential due to:
    • A higher concentration of voltage-gated sodium channels.
    • Increased sensitivity to changes in membrane potential compared to dendrites or the rest of the cell body.

Neuropeptides as Neuromodulators

  • Defined as large molecules consisting of 2 to 40 amino acids.
  • Synthesized in the neuronal cell body within the rough endoplasmic reticulum.
  • Packaged in large, dense-core vesicles in the axon terminal.
  • Neuromodulators do not produce EPSPs or IPSPs directly but rather modulate synaptic activity.

Presynaptic Inhibition or Facilitation

  • This process influences synaptic effectiveness positively or negatively.
  • Facilitation
    • Results in an increased release of neurotransmitters.
  • Inhibition
    • Results in a decreased amount of neurotransmitter released.

Drug Interactions and their Mechanisms

  • Possible drug actions include:
    • Altering the synthesis, axonal transport, storage, or release of a neurotransmitter.
    • Modifying the interaction of neurotransmitters with postsynaptic receptors.
    • Influencing neurotransmitter reuptake or destruction.
    • Replacing deficient neurotransmitters with substitutes.
  • Examples of Drugs and Effects
    • Cocaine: Blocks the reuptake of dopamine at presynaptic terminals leading to prolonged effects of dopamine.
    • Strychnine: Competes with inhibitory neurotransmitter glycine at the postsynaptic receptor site, potentially leading to excessive excitability.
    • Tetanus Toxin: Prevents the release of the inhibitory neurotransmitter GABA, affecting skeletal muscle functions.

Neuronal Pathways

  • Convergence
    • A situation where a given neuron has multiple other neurons synapsing onto it, leading to combined input effects.
  • Divergence
    • When a single neuron synapses with and influences many other cells due to branching of axon terminals.

Chapter in Perspective: Focus on Homeostasis

  • All cells regulate the composition of ions in their internal environment.
  • Every cell has a resting membrane potential that is rigorously maintained for optimal cellular function.
  • Both nerve cells and muscle cells are electrically excitable with functions dependent on electrical activity changes.
  • Neurons are vital for communication within the body and relay sensory information about the surrounding environment.