The Resting Neuron

Resting Neuron

  • The plasma membrane of a resting neuron is polarized.
    • Inside: potassium ions (K⁺) → contributes to a negative charge.
    • Outside: sodium ions (Na⁺) → contributes to a positive charge.

Depolarization

  • Sodium (Na⁺) gates in the neuron membrane open.
  • Na⁺ ions rush into the axon, causing the inside of the neuron to become more positive.
  • This increase in positive charge activates the neuron to transmit a nerve impulse, known as an action potential.
  • After the influx of Na⁺, sodium gates close to halt further ion entry.

After the Nerve Impulse

Repolarization

  • Potassium ions (K⁺) exit the cell across the membrane.
  • This movement of K⁺ restores electrical conditions back to the resting state.
  • Despite this, the ions remain in the “wrong” places after repolarization.

Sodium-Potassium Pump

  • The sodium-potassium pump restores the concentrations of the ions to their resting states.
  • It utilizes energy in the form of ATP to function.
  • Once the pump has done its job, the neuron returns to the same state as the resting neuron.

Neurophysiology – Conductivity

  • An action potential reaches the end of the axon.
  • Axonal terminals release neurotransmitters.
  • These neurotransmitters diffuse across the synapse and bind to receptors on the next neuron.
  • If a sufficient amount of neurotransmitter binds, a nerve impulse occurs in the next neuron.
  • Neurotransmitters are subsequently broken down by enzymes or reabsorbed through a process called reuptake.

Myelinated vs Unmyelinated Fibers

  • Unmyelinated fibers: Impulse travels along the entire length of the axon.
  • Myelinated fibers: Use a process called saltatory conduction, whereby depolarization only occurs at the nodes of Ranvier.
  • Electrical current flows from node to node through extracellular fluid, enhancing speed of impulse.

Anesthetics

  • Local anesthetics work by preventing the opening of voltage-gated Na⁺ channels, which is essential for action potential generation.
  • Alcohol and various sedatives can exert similar effects by influencing neuronal activity.

Reflexes

  • Reflexes are considered rapid, predictable, involuntary responses to stimuli.

Types of Reflexes

  • Autonomic reflexes:
    • Regulate smooth muscle, cardiac muscle, glands, and involuntary functions like digestion, blood pressure, and sweating.
  • Somatic reflexes:
    • Stimulate skeletal muscles, such as the reflex of withdrawing a hand from a hot stove.

Reflex Arc

Elements of a Reflex Arc

  1. Sensory receptor: Reacts to a stimulus.
  2. Afferent neuron: Conveys information toward the central nervous system (CNS).
  3. CNS integration center: Contains synapses that process the incoming information.
  4. Efferent neuron: Leads away from the CNS, conveying information to effectors.
  5. Effector organ: The muscle or gland that is stimulated as a result of the