The Nervous System and Neuronal Communication

The Nervous System

  • Psychologists study the nervous system to understand biological basis of human mind.
  • The nervous system consists of:
    • Glial cells (glia)
    • Neurons

Glial Cells

  • Outnumber neurons ten to one.
  • Traditionally thought to have a supportive role to neurons.
  • Functions:
    • Provide scaffolding for the nervous system.
    • Help neurons align for communication.
    • Insulate neurons.
    • Transport nutrients and waste.
    • Mediate immune responses.

Neurons

  • Serve as interconnected information processors.
  • Essential for nervous system tasks.

Neuron Structure

  • Central building blocks of the nervous system.
  • 100 billion at birth.
  • Composed of different parts, each with specialized function.
    • Semipermeable Membrane
      • Outer surface of a neuron.
      • Allows small and uncharged molecules to pass through.
      • Stops larger or highly charged molecules.
    • Soma (cell body)
      • Contains the nucleus.
      • Signals are transmitted electrically across the soma.
    • Dendrites
      • Branching extensions from the soma.
      • Serve as input sites, receiving signals from other neurons.
    • Axon
      • Major extension from the soma.
      • Ranges in length from a fraction of an inch to several feet.
      • Ends at multiple terminal buttons.
    • Myelin Sheath
      • Glial cells form a fatty substance that coats the axon.
      • Acts as an insulator.
      • Increases the speed at which the signal travels.
      • Loss of insulation can be detrimental.
    • Terminal Buttons
      • Located at the end of the axon.
      • Contain synaptic vesicles that house neurotransmitters.
    • Synaptic Vesicles
      • House neurotransmitters, the chemical messengers of the nervous system.
      • Release neurotransmitters into the synapse.
    • Synapse
      • Small space between two neurons.
      • Important site for communication between neurons.
    • Neurotransmitters
      • Chemical messengers that travel across the synapse.
      • Bind with corresponding receptors on the dendrite of an adjacent neuron.
    • Receptors
      • Proteins on the cell surface where neurotransmitters attach.
      • Vary in shape, with different shapes matching different neurotransmitters.
      • Lock-and-key relationship: specific neurotransmitters fit specific receptors.

Multiple Sclerosis (MS)

  • Autoimmune disorder.
  • Involves loss of myelin sheath on axons.
  • Interferes with electrical signals.
  • Leads to symptoms like dizziness, fatigue, loss of motor control, and sexual dysfunction.
  • No known cure but treatments can modify the course and manage symptoms.

Neuronal Communication

  • The neuron exists in a fluid environment.
    • Extracellular fluid surrounds the neuron.
    • Intracellular fluid (cytoplasm) is contained within the neuron.
  • The neuronal membrane keeps these fluids separate.
  • The membrane potential is the difference in charge across the membrane, providing energy for the signal.
  • Semipermeable nature of the membrane restricts movement of charged molecules (ions).
    • Results in some charged particles concentrating inside or outside the cell.

Resting Potential

  • Neuron membrane's potential held in readiness between signals.
  • Ions line up on either side of the cell membrane.
  • Ready to rush across when the neuron becomes active and the membrane opens its gates.
  • Sodium-potassium pump allows movement of ions across the membrane.
  • Ions in high concentration areas move to low concentration areas.
  • Positive ions move to areas with a negative charge.
  • Sodium ({Na}^{+}) is at higher concentrations outside the cell and tends to move in.
  • Potassium ({K}^{+}) is more concentrated inside the cell and tends to move out.
  • Inside of the cell is slightly negatively charged compared to the outside, adding and additional force on sodium.

Action Potential

  • Neuron receives signals at the dendrites.
  • Neurotransmitters from an adjacent neuron bind to receptors.
  • Small pores/gates open on the neuronal membrane.
  • Sodium ({Na}^{+}) ions move into the cell.
  • Internal charge of the cell becomes more positive.
  • If the charge reaches the threshold of excitation, the neuron becomes active and the action potential begins.
  • Many additional pores open, causing a massive influx of ({Na}^{+}) ions.
  • Huge positive spike in the membrane potential (peak action potential).
  • At the peak, the sodium gates close and the potassium gates open.
  • Positively charged potassium ions leave, cell begins repolarization.
  • Cell hyperpolarizes, becoming more negative than the resting potential, then returns to resting potential.

Electrical Signal

  • Moves down the axon like a wave.
  • Sodium ions diffuse to the next section of the axon.
  • Raises the charge past the threshold of excitation.
  • Triggers a new influx of sodium ions.
  • Action potential moves all the way down the axon to the terminal buttons.
  • All-or-none phenomenon: incoming signal is either sufficient or insufficient to reach the threshold of excitation.
  • The action potential is recreated, or propagated, at its full strength at every point along the axon.
  • When the action potential arrives at the terminal button, the synaptic vesicles release their neurotransmitters into the synapse.
  • The neurotransmitters travel across the synapse and bind to receptors on the dendrites of the adjacent neuron, and the process repeats itself in the new neuron.

Clearing the Synapse

  • Excess neurotransmitters drift away, are broken down, or are reabsorbed in reuptake.
  • Reuptake is the process of neurotransmitters being pumped back into the neuron that released it to clear the synapse.
  • Clearing the synapse provides a clear “on” and “off” state and regulates neurotransmitter production.

Electrochemical Event

  • Neuronal communication is an electrochemical event.
    • The movement of the action potential down the axon is an electrical event.
    • The movement of the neurotransmitter across the synaptic space is the chemical portion.

Neurotransmitters and Drugs

  • Different types of neurotransmitters are released by different neurons.
  • Psychologists with a biological perspective believe that psychological disorders are associated with neurotransmitter imbalances.
  • Psychotropic medications restore neurotransmitter balance to treat psychiatric symptoms.

Psychoactive Drugs

  • Act as agonists or antagonists for a given neurotransmitter system.
    • Agonists mimic a neurotransmitter and strengthen its effects.
    • Antagonists block or impede neurotransmitter activity.
    • Agonist and antagonist drugs correct neurotransmitter imbalances.

Parkinson's Disease

  • Associated with low levels of dopamine.
  • Dopamine agonists are used as a treatment strategy.

Schizophrenia

  • Certain symptoms are associated with overactive dopamine neurotransmission.
  • Antipsychotics, which are antagonists for dopamine, are used to treat these symptoms.

Reuptake Inhibitors

  • Prevent unused neurotransmitters from being transported back to the neuron.
  • Leaves more neurotransmitters in the synapse for longer, increasing its effects.

Depression

  • Linked with reduced serotonin levels.
  • Selective serotonin reuptake inhibitors (SSRIs) are used to strengthen the effect of serotonin.
    • Examples: Prozac, Paxil, and Zoloft.

Important considerations

  • Psychotropic drugs are not instant solutions.
  • It can take weeks to see improvement.
  • Many drugs have negative side effects.
  • Individuals vary in their response to drugs.
  • Combining drug therapy with psychological and/or behavioral therapies can be more effective.