Demystifying the Nervous System

Neurons and Neurotransmitters

  • Neuron functions: reception, integration, transmission, and transfer of information.
  • Neurons are electrochemically active cells; Action Potentials are the flow of positively charged ions across the membrane.
  • Neurons synthesize and transport neurotransmitters in cellular organelles for cell-to-cell interaction.
  • Neurotransmitters can be excitatory or inhibitory at the postsynaptic cell and cause receptor shape changes to restart the signaling process.
  • Neurons transmit signals via synapses (presynaptic terminal releases neurotransmitter into the synaptic cleft; postsynaptic receptor responds).

Structure of a Neuron

  • Dendrite: input site; receives and transfers information to the cell body.
  • Cell body (Soma): processing site; synthesizes neurotransmitters.
  • Axon: output site; sends impulses away from the cell body to the next target (neuron, muscle, or gland).
  • Axon terminals: transmitting site; where neurotransmitters are released.
  • Node of Ranvier: gaps in the myelin sheath that enable saltatory conduction.
  • Myelin sheath: lipid/protein layer that insulates the axon and increases conduction speed; speed depends on myelin presence and axon diameter.
  • Structure names (from diagram): DENDRITE → NUCLEUS → AXON → CELL BODY → NODE OF RANVIER → MYELIN SHEATH → AXON TERMINALS.

Glial Cells

  • Glial cells are support cells in the nervous system; roles include:
    • Forming myelin
    • Protection and support
    • Regulation
  • Glial cell types by location:
    • CNS glia: Astrocyte, Microglia, Ependymal cell, Oligodendrocyte
    • PNS glia: Satellite cell, Schwann cell
    • Basic functions include insulation/myelination, immune surveillance/phagocytosis, and creation of CSF.

Myelin Sheath and Saltatory Conduction

  • Myelin sheaths insulate axons and speed conduction.
  • Conduction speed is increased by myelination and by larger axon diameter.
  • Saltatory conduction: the nerve impulse hops from node to node (Nodes of Ranvier).
  • Nodes of Ranvier provide interruptions that facilitate rapid conduction.

Action Potentials and Ion Movement

  • Action Potential flow involves Na+ and K+ movement across the membrane.
  • A resting membrane potential exists when the inside is more negative than the outside.
  • Local potentials are the initial changes from a stimulus.
  • During an action potential:
    • Na+ channels open, depolarizing the membrane.
    • At peak, Na+ channels close and K+ channels open, causing repolarization (K+ efflux).
    • The membrane hyperpolarizes briefly before returning to resting potential.
  • Key visual cue from slide:
    • Repolarization step: Na+ channels close, K+ channels open; K+ diffusion reverts the potential toward resting.
  • Membrane potential values referenced in the material:
    • Resting membrane potential is around -70 mV.
    • During repolarization, potential can rise toward +30 mV before returning toward resting.
    • Action Potential peak is approximately +30 mV as shown in the schematic.

Movement of Na+ and K+; Resting Potential and Local Potentials

  • At rest, the inside of the neuron is more negative than the outside; this is the resting membrane potential.
  • Local potential is the initial change caused by a stimulus.
  • An action potential is an all-or-none event: it occurs only if the stimulus reaches the threshold; amplitude does not depend on stimulus strength.
  • Refractory period: during repolarization the membrane is unresponsive to stimuli.
  • All-or-none principle summary:
    • Threshold stimulus is required to trigger an AP.
    • Once triggered, the AP has a constant amplitude; stronger stimuli do not produce a larger AP.
  • Possible representation: V<em>rest ≈ −70 mV,V</em>peak ≈ +30 mV.V<em>{rest} \,\approx\, -70\text{ mV}, \quad V</em>{peak} \,\approx\, +30\text{ mV}.

Neurotransmitter Release and Synapses

  • An electrical signal (action potential) reaches the presynaptic terminal.
  • The AP is all-or-none and triggers the release of neurotransmitters into the synaptic cleft.
  • Neurotransmitters can be excitatory or inhibitory, causing receptor changes on the postsynaptic cell and starting the process anew.
  • Structure of a typical chemical synapse includes:
    • Synaptic vesicle
    • Voltage-gated Ca2+ channels
    • Axon terminal
    • Receptor on postsynaptic membrane
    • Postsynaptic density and the synaptic cleft
    • Reuptake pump for neurotransmitter clearance
  • The postsynaptic cell can be a neuron, muscle, gland, or organ.

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

  • CNS components: brain and spinal cord; neurons and neuroglia.
  • Neuroglia: non-neuronal cells that maintain home