HP6

Introduction to Reductionist Approach in Biology

  • The reductionist approach in biology involves understanding complex biological phenomena through simpler elements alongside principles from other disciplines like physics and chemistry.

    • Example: Impulse propagation in nerves is understood through electrical laws and the properties of membranes.

    • Limits of Reductionism:

    • Uncertainty about whether all aspects of life can be reduced to atomic and molecular terms.

    • Despite these limits, science must continue exploring this avenue.

    • Course Structure:

    • Basic principles of molecules and their interactions lead to physiology (nerves and muscles) and ultimately to complex organ systems.

Anatomy of the Nervous System

Major Components:

Central Nervous System (CNS):

  • Comprises the brain and spinal cord.

Peripheral Nervous System (PNS):

  • Composed of neurons connecting to muscles, sensory organs, and glands.

Enteric Nervous System:

  • Independent control of gastrointestinal functions.

Neuron Structure:

  • Not all neurons have the same appearance; focus here on typical motor neurons in the PNS.

Motor Neurons

  • Pathway: From CNS to skeletal muscles.

  • Structure:

    • Cell body (soma) with a nucleus.

    • Dendrites: Specialized projections for receiving incoming signals.

    • Action potentials are transmitted through the axon.

    • Diameter of Axons: Range from 1 to 20 µm in thickness; squid axons can exceed 1 mm.

    • Axon Characteristics:

    • Originate at the axon hillock.

    • Terminate at axon terminals, forming synapses with target cells (neurons, muscles, glands).

Classification of Neurons

  1. Afferent (Sensory) Neurons:

    • Transmit signals from sensory organs to the CNS.

  2. Efferent (Motor) Neurons:

    • Conduct signals from the CNS to muscles or glands.

  3. Interneurons:

    • Connect other neurons; primarily located in the CNS.

  • Neuron Morphology:

    • Multipolar Neurons: Characteristic with multiple dendrites and a single long axon.

    • Unipolar/Pseudounipolar Neurons: Cell bodies attached to the side of an axon.

    • Bipolar Neurons: Two extensions on opposite sides of the cell body.

  • Determinants of Neuron Shape:

    • Shape influenced by cytoskeletal composition and extracellular matrix attachments.

Action Potentials and Neural Transmission

Trigger Zones

  • High density of voltage-gated channels is essential for initiating action potentials.

  • Variability of Trigger Zones:

    • Can occur near the cell body (initial segment) or at the axon tip.

Mechanisms for Speeding Up Action Potentials

  1. Increased Axon Diameter:

    • Decreases electrical resistance.

  2. Myelination:

    • Axons wrapped in myelin (lipid-protein mixture from oligodendrocytes in CNS or Schwann cells in PNS).

    • Myelin acts as an electrical insulator, allowing action potentials to jump between nodes of Ranvier (saltatory conduction).

    • Speed of transmission is maximized in myelinated neurons because action potential generation occurs only at nodal points, thus increasing efficiency.

Synaptic Transmission

Function of Synapses

  • Convert action potential in presynaptic neuron into electric potential in postsynaptic cell (which can be a neuron or muscle).

    • Depolarization (closer to threshold) -> Excitatory Synapses

    • Hyperpolarization (further from threshold) -> Inhibitory Synapses

Types of Synapses

  1. Electrical Synapses:

    • Connected via gap junctions allowing ion movement directly between cells.

    • Fast but little control over signal processing.

  2. Chemical Synapses:

    • Typically involve neurotransmitter release from presynaptic neuron, diffusing across synaptic cleft to postsynaptic neuron.

    • Voltage-gated calcium channels facilitate neurotransmitter release in response to action potentials.

    • Example Neurotransmitters:

      • Acetylcholine: Stimulates muscle contractions.

      • Norepinephrine: In CNS and induces sympathetic nervous system effects.

      • Dopamine: Important for learning and pleasure.

      • Endorphins: Pain inhibition and feelings of euphoria.

Mechanism of Neurotransmitter Release

  • Neurotransmitters stored in vesicles (40-50 nm in diameter) at axon terminals.

  • Release upon calcium influx; neurotransmitters quickly diffuse across the synaptic cleft (~20-50 nm wide).

  • Reuptake and Degradation:

    • Neurotransmitters must be cleared after action to prevent prolonged activation.

      • Reabsorption or enzymatic degradation (e.g., acetylcholine by acetylcholinesterase).

    • Acetylcholine is split into choline and acetate for recycling.

Postsynaptic Response

Neurotransmitter Receptors

  1. Ionotropic Receptors:

    • Function as ligand-gated ion channels. Open in response to neurotransmitter binding.

  2. Metabotropic Receptors:

    • Cause cellular signaling that usually activates separate ion channels via signaling pathways (slower but longer-lasting effects).

  • Outcomes:

    • Excitatory Postsynaptic Potential (EPSP):

    • Depolarization brought about by opening Na+ channels.

    • Inhibitory Postsynaptic Potential (IPSP):

    • Hyperpolarization caused by opening K+ or Cl− channels.

Termination of Neurotransmission

  • Neurotransmitter clearance is vital to cease postsynaptic activation.

  • Methods include:

    • Diffusion away from the synapse.

    • Active transport mechanisms (e.g., Na+ -coupled reabsorption for norepinephrine and glutamate).

    • Enzymatic breakdown of neurotransmitters (e.g., Ach by acetylcholinesterase).

Receptor Desensitization

  • Prolonged stimulation may lead to receptor desensitization due to mechanisms like phosphorylation.

  • Summary of effects from neurotransmitter activation:

    • Excitatory Synapses: Na+ channels open.

    • Inhibitory Synapses: Na+ channels close while K+ and Cl− channels open.

Integration of Signals in Neurons

  • Neurons receive inputs from multiple sources (both stimulatory and inhibitory).

  • Graded potentials weaken as they travel across the cell body and converge at the trigger zone.

  • Action potentials are generated only if the summation reaches a threshold at the trigger zone, resulting in a binary output (action potential or none).

Chloride Channel Functions

  • Activation of chloride channels opposes depolarization, contributing to inhibition within the neuron.

  • High chloride permeability stabilizes neuronal resting potential around its equilibrium.

Acetylcholine and Its Receptors

  • Acetylcholine is pivotal in efferent neurons, with receptors classified as cholinergic.

    • Nicotinic Receptors: Ionotropic; facilitate action potential transmission.

    • Muscarinic Receptors: Metabotropic; longer-lasting effects impacting parasympathetic nervous functions.

Toxicological Effects on Neurotransmission

  • Neurotransmitter transmission can be inhibited via various methods:

    • Sodium channel inhibition, calcium channel inhibition, or preventing neurotransmitter release.

    • Consequences include muscle paralysis or abnormal stimulation affecting respiratory muscles.

Natural Toxins and Their Mechanisms

  1. Tetrodotoxin:

    • Blocks voltage-gated sodium channels.

  2. Conotoxin:

    • Inhibits calcium channels, impeding neurotransmitter release.

  3. α-Bungarotoxin:

    • Blocks nicotinic receptors at the neuromuscular junction.

  4. D-Tubocurarine:

    • Inhibits acetylcholine binding at the postsynaptic receptor.

  5. Botulinum and Tetanus Toxins:

    • Both prevent neurotransmitter release but have opposing effects (muscle weakness vs. muscle spasms).

Ischemia and Excitotoxicity

  • Conditions like brain ischemia lead to excess glutamate release due to ATP depletion, resulting in neuronal damage.

  • NMDA receptors increase calcium permeability, exacerbating cell injury during excitotoxic events.

  • Potential interventions: Calcium channel blockers or NMDA antagonists may mitigate effects, although effectiveness varies.