Molecular Mechanisms of Disease:1-15

Chemical and Voltage Dependent Message Transfer

  • Overview of Signal Transmission

    • The brain utilizes two primary methods for transmitting signals:

    1. Voltage Dependent Message Transfer

    2. Chemical Driven Message Transfer

Types of Neurotransmission

  • Chemical Driven Neurotransmission

    • Involves neurotransmitters (e.g., dopamine, serotonin) that are vital in regulating behaviors and emotions.

    • Functions notably in the synaptic cleft, where signals manifest through chemical interactions.

  • Voltage Dependent Neurotransmission

    • Relies on the movement of voltage across neuronal membranes.

    • Conducts electrical impulses from the cell body to the axon terminal.

    • Key difference: Channels for voltage transmission versus Receptors for chemical transmission.

Mechanism of Transmitting Signals

  • Neurotransmission Process

    • Chemical messages are released in the synapse (space between presynaptic and postsynaptic compartments).

    • Presynaptic neurons release neurotransmitters that bind to receptors on postsynaptic neurons.

    • This binding converts chemical signals back into electrical signals (voltage) in the postsynaptic compartment.

Brain Functions and Behavior Regulation

  • Cognitive and Emotional Processing

    • Different behaviors such as nervousness and sadness are regulated by various neurotransmitters.

    • Understanding these mechanisms aids in deciphering brain function and dysfunction.

Brain Structure Components

  • Structural Elements

    • Critical components include

    • Neuronal Cells: Primary functional units of the brain.

    • Microglia: Immune cells that activate in response to threats, providing protection against foreign substances.

    • Glial Cells: Provide support, nutrition, and insulation including oligoendrocytes and astrocytes.

Brain Division and Orientation

  • Understanding Brain Orientation

    • Brain and spinal cord can be divided along different axes for study purposes (e.g., medial-lateral, anterior-posterior).

    • Identification of sections is crucial for imaging techniques like CT or MRI.

Protection Mechanisms of the Brain

  • Brain Protection

    • The brain has multiple protective layers:

    • Dura Mater: Outer protective layer.

    • Arachnoid Mater: Middle layer.

    • Pia Mater: Inner layer adjacent to the brain.

  • Ventricles

    • The brain contains four ventricles filled with cerebrospinal fluid which cushions and protects it from damage.

    • Connects to the spinal cord via the central canal, assisting in nutrient transport.

Blood-Brain Barrier

  • Functional Role

    • A crucial feature that protects the brain by restricting harmful substances from entering.

    • Composed of astrocytes wrapping around blood vessel walls, distinguishing it from peripheral tissues.

    • Allows essential nutrients and oxygen while barring toxins and pathogens.

Major Brain Divisions

  • Forebrain: Responsible for complex behaviors, including thought and emotion.

  • Midbrain: Although small, it contains critical structures like dopamine neurons which are linked to various neurological conditions (e.g., Parkinson’s disease).

  • Hindbrain: Governs basic life functions such as breathing and heart rate.

Neuronal Communication and Action Potential

  • Action Potential Fundamentals

    • The primary mechanism for signals traveling along axons.

    • Resting State: Differentiation in charge across the membrane (positive outside, negative inside).

    • Triggers for Action Potential: Ion movement, particularly sodium (Na+) and potassium (K+) ions through ion channels.

Stages of Action Potential

  1. Resting Potential

    • Maintained by the sodium-potassium pump, exchanging Na+ for K+.

    • Pumps 3 sodium ions out for every 2 potassium ions in, establishing a resting potential around -70mV.

  2. Depolarization

    • Triggered when threshold potential is reached, causing the rapid influx of sodium ions, making the inside of the cell more positive.

  3. Repolarization

    • Sodium channels close and potassium channels open, allowing K+ to exit and restore negative charge inside the cell.

  4. Undershoot (Hyperpolarization)

    • Membrane potential temporarily becomes more negative than resting potential due to delayed closing of potassium channels.

Synaptic Transmission

  • Synapse Structure

    • The gap between neurons where neurotransmitters are released from the presynaptic terminal to bind at the postsynaptic receptors.

  • Transduction Mechanism

    • When the action potential reaches the axon terminal, it opens voltage-dependent calcium channels, leading to calcium influx.

    • Calcium triggers vesicles containing neurotransmitters to fuse with the presynaptic membrane, releasing neurotransmitters into the synapse.

  • Consequence on Postsynaptic Cell

    • Neurotransmitters bind to their specific receptors (often ion channels), resulting in the opening of sodium channels and the creation of a new action potential within the postsynaptic neuron.

Summary of Brain Functioning

  • The brain integrates vast inputs (sensory, emotional, and cognitive) and transmits outputs quickly through both chemical and electrical means.

  • Neurons not only communicate with rapid, electrical action potentials but also convey messages via chemical synapses, ensuring complex behavioral and functional responses.