Lecture 12 Study Notes

BIO 203: Lecture 12: Neurophysiology, part 2

Recap of Previous Material

  • Topics covered in Chapter 12, part 1 (12.1-12.3):

    • Division of the nervous system by function:

    • Stimulus: Detection of changes in the environment.

    • Integration: Processing of information to make decisions.

    • Response: The actions taken as a result of integration.

    • Control divisions:

    • Somatic: Voluntary control over skeletal muscles.

    • Autonomic: Involuntary control over internal organs.

    • Enteric: Autonomous control over the gastrointestinal system.

    • Cell types in the nervous system and their functions:

    • Neurons: Basic signaling units of the nervous system that transmit impulses.

    • Glia (glial cells): Supportive cells which maintain homeostasis, form myelin, and provide support and protection for neurons.

    • Examples of neuronal communication loops.

Importance of Myelination

  • Why increase myelination?

    • Enhanced conduction speed and efficiency of electrical impulses along neurons.

Signs of Demyelination

  • Symptoms associated with loss of myelin:

    • Vision problems

    • Muscle weakness

    • Fatigue

    • Memory problems

    • Mood changes

    • Numbness and tingling.

Demyelinating Disorders (CNS)

  1. Inflammatory/immune disorders:

    • Multiple Sclerosis (MS)

    • Optic Neuritis

    • Acute Disseminated Encephalomyelitis (ADEM)

    • Paraneoplastic encephalomyelitis

    • Various autoimmune diseases (e.g., rheumatoid arthritis, systemic lupus erythematosus, Behçet's disease, Sjögren's syndrome).

    • Infectious diseases:

      • HIV, Progressive Multifocal Leukoencephalopathy (PML), Lyme disease, Neurosyphilis, HTLV-1.

  2. Granulomatous diseases:

    • Sarcoidosis, Wegener granulomatosis, Lymphoid granulomatosis.

  3. Myelin disorders:

    • Metachromatic leukodystrophy

    • Adrenoleukodystrophy/adrenomyeloneuropathy

    • Globoid cell (Krabbe's) leukodystrophy

    • Alexander disease

    • Canavan disease.

  4. Toxic/metabolic disorders:

    • Vitamin B12 deficiency

    • Central pontine myelinolysis

    • Carbon monoxide poisoning

    • Radiation exposure.

Increasing Myelination

  • Research findings:

    • A study by Yoon et al. (2016) indicated that a high-fat diet, when coupled with exercise training, increases myelin protein expression.

    • Exercise training protects against the loss of oligodendrocyte progenitor cells and mature oligodendrocytes induced by a high-fat diet.

    • Neuroprotective factors safeguard against detrimental effects on neural health.

Learning Objectives for Today

  • Cover topics in Chapter 12, parts 2 (12.4-12.5):

    • Fundamentals of neurophysiology

    • The action potential

    • Mechanisms of cell communication

    • Role of electrical impulses in releasing chemical messages

    • Understanding graded potentials and neurotransmitter release.

The Action Potential

  • Historical Context:

    • Discovery of "animal electricity" by Luigi Galvani (~1780).

    • Demonstrated that muscle contractions could be induced by electrical energy.

    • His publication "De Viribus Electricitatis in Motu Musculari Commentarius" (1791) posited that electrical forces are fundamental to living organisms.

  • Galvani vs. Volta:

    • Galvani’s work suggested intrinsic electrical activity in nerves; Volta contested this with the origin of muscle contractions being due to metals he used.

  • Key Discoveries:

    • Galvani determined that nerves conduct electricity and that the phenomenon involved a fluid known as "animal spirits."

    • Volta developed the first constant current battery, the Voltaic pile.

Mechanisms of Action Potential

  • Current generated through electrical stimulation of nerves is vital for nervous function.

  • Neuron Structure:

    • Neurons consist of dendrites, cell body (soma), axon, oligodendrocyte, nodes of Ranvier, myelin sheath, and synapse.

    • Note: Not every neuron is myelinated.

Action Potential Initiation

  • Process of reaching action potential involves several steps:

    1. A stimulus from another neuron leads to depolarization towards threshold potential.

    2. If the threshold is reached, all Na+ channels open, leading to membrane depolarization ("all or nothing" phenomenon).

    3. Peak action potential is characterized by K+ channels opening, Na+ channels closing, resulting in repolarization.

    4. Membrane hyperpolarizes as K+ continues to exit; refractory period begins where the membrane cannot fire again.

    5. Na+/K+ pump restores the resting potential after K+ channels close.

Membrane Potential Schooling

  • Membrane potentials ($V_m$) reflect the ion charge difference across the membrane, typically resting around
    $-70 mV$.

  • Internal and external concentrations for key ions are as follows:

    • extbf{Intracellular (inside the cell)}:

    • [K+] = 155 mM

    • [Na+] = 12 mM

    • extbf{Extracellular (outside the cell)}:

    • [K+] = 4 mM

    • [Na+] = 145 mM

Nernst Equation and Ion Movement

  • The equilibrium potential (E) for ions is dictated by their concentration on each side of the membrane and their permeability.

  • Nernst Equation: Eion=racRTFZimesextlnrac[extionoutside][extioninside]E_{ion} = rac{RT}{FZ} imes ext{ln} rac{[ ext{ion outside}]}{[ ext{ion inside}]}

    • (Details on memorization are not necessary, per lecture).

  • Consideration of multiple ions and their respective reversal potentials (e.g., K+ = -88 mV, Na+ = +60 mV, etc.) helps visualize their flow at various phases of the action potential.

Types of Synapses

  • Chemical Synapses:

    • Involve neurotransmitter release from the presynaptic element, traversing the synaptic cleft, and then binding to postsynaptic receptors.

  • Electrical Synapses:

    • Direct physical connections (gap junctions) between cells, allowing for rapid communication, crucial in synchrony, such as in cardiac tissue signaling.

Neurotransmitter Systems Overview

  • Groups of neurotransmitters:

    1. Cholinergics

    • Nicotinic and muscarinic receptors (acetylcholine as the endogenous neurotransmitter).

    • Nicotinic receptors function as ligand-gated ion channels; muscarinic receptors are G protein-coupled receptors (GPCRs).

    1. Amino Acids

    • Glutamate, GABA (gamma-aminobutyric acid), and glycine (excitatory and inhibitory roles, respectively).

    1. Biogenic Amines

    • Examples include serotonin, dopamine, norepinephrine, and epinephrine; GPCR interactions.

    1. Neuropeptides

    • Small proteins that consist of chains of amino acids; these include met-enkephalin and vasoactive intestinal peptide.

Summary Points

  • The action potential: It is fundamental for understanding neuron signaling and communication.

  • Myelination: Enhances speed and efficiency of action potentials through saltatory conduction, which preserves the signal.

  • Membrane potentials: Are critical for neurotransmitter release and neuronal communication, influenced by ion distributions.

  • Neurotransmitter types and receptor interactions: Provide a framework for understanding excitatory vs. inhibitory neurotransmission and the effects on target cells.

Next Lecture: Paper 2 due on Canvas.