Neuro Physiology

Action Potential and Neuron Structure

• Neuron Types

  • Neuron: Single nerve cell

  • Nerve: Bundle of neurons

• Neuron Composition

  • Dendritic Region:

    • Increases surface area for receiving signals

    • Sends signals toward cell body

  • Cell Body:

    • Houses the nucleus and organelles

  • Axon Hillock:

    • Junction where the axon meets the cell body; neuron trigger zone

  • Axon:

    • Nerve “fiber” conducting impulses away from the cell body

  • Axon Terminals:

    • Forms synapses with other neurons or effector organs

    • Releases chemical messengers

• Types of Neurons

  • Bipolar, Pseudo-unipolar, Multipolar

• Dyneins and Kinesins

  • Dyneins: Carry recycled vesicles and chemical messengers back toward the cell body

  • Kinesins: Transport nutrients, enzymes, and organelles away from the cell body

  • Microtubules: Serve as the “railway” for vesicle transport

Membrane Potential

• Membrane Potential

  • Plasma membranes of all living cells exhibit a membrane potential (electrically polarized)

  • Separation of opposite charges across plasma membrane

  • Arises from differences in concentration and permeability of key ions

• Excitable Cells:

  • Nerve and muscle cells can produce rapid, transient changes in their membrane potential

• Resting Membrane Potential:

  • Constant potential in non-excitable and excitable tissues at rest

  • Measured by placing one electrode inside the cell, and one outside to show the potential difference

Ion Movement and Resting Membrane Potential

• Movement of Ions

  • Dependent on:

    • Permeability: Controlled by ion channels

    • Electrical Gradient: Positive charge attracted to negative

    • Concentration Gradient: Movement from high to low concentration

• Nernst Equation:

  • Describes equilibrium potential for an ion

  • Example for Na+: Equilibrium at +60 mV; K+: Equilibrium at -89 mV

• Mechanisms Maintaining Resting Membrane Potential:

  • Na+/K+ ATPase Pump: Establishes and maintains gradients

  • Permeability Increases: Greater permeability to K+ promotes outward leakage

  • Impermeable Membrane: Keeps anions (negatively charged ions) inside the membrane

Membrane States and Neural Communication

• Membrane States

  • Polarization: Any state other than 0 mV

  • Depolarization: Membrane potential becomes less negative

  • Repolarization: Return to resting potential after depolarization

  • Hyperpolarization: Membrane potential becomes more negative

• Neural Communication:

  • Two types of potential changes:

    • Graded Potentials:

      • Short-distance signals initiated by stimuli (mechanical, chemical, electrical) in dendrites

      • Examples: Postsynaptic potentials, receptor potentials, and end-plate potentials

    • Action Potentials:

      • Serve as long-distance signals characterized by brief, rapid potential changes

      • Na+ and K+ gates play essential roles

Characteristics of Graded and Action Potentials

• Graded Potentials:

  • Local: Die away quickly

  • Summation: Can add together to increase amplitude

  • Properties:

    • Can vary in size, may be excitatory or inhibitory

    • No refractory period

• Action Potentials:

  • Description: Brief, rapid, and large potential changes

  • Phases include resting, depolarization, repolarization, hyperpolarization

  • Triggered once graded potentials reach a threshold of -55 mV

  • Na+ gates open: Na+ rushes in (reaches +30 mV); K+ gates open: K+ rushes out to repolarize

Refractory Period and Propagation of Action Potentials

• Refractory Period

  • Absolute Refractory Period: No second action potential is possible

  • Relative Refractory Period: A second action potential is possible with a stronger stimulus

• Propagation

  • Self-propagating: An impulse in one region triggers neighboring regions

  • Uni-directional Movement: Ensured by the refractory period affecting propagation

• Conduction Types:

  • Contiguous Conduction: In unmyelinated fibers

  • Saltatory Conduction: In myelinated fibers (~50 times faster)

Myelination and Conduction Velocity

• Myelin:

  • Fatty insulator primarily made of lipids, formed by oligodendrocytes (CNS) and Schwann cells (PNS)

• Multiple Sclerosis:

  • Loss of myelin; results in decreased impulse speed and muscle coordination

• Factors Affecting Conduction:

  • Neuron diameter, myelination, temperature

• Comparison: Frog nerves vs. human nerves; A-delta fibers vs. C fibers

Regeneration and Synaptic Transmission

• Regeneration of Nerve Fibers

  • Location-dependent: Schwann cells in PNS promote regeneration; oligodendrocytes in CNS inhibit it

• Synapses

  • Junctions between neurons for interaction

  • Process:

    • Action potential arrival, voltage-gated Ca2+ channels open, leading to neurotransmitter release into the synapse

Neuronal Communication: Convergence, Divergence, and Synaptic Anatomy

• Convergence and Divergence:

  • Convergence: Multiple inputs onto one neuron

  • Divergence: One neuron synapsing with many

• Anatomy of Synapses

  • Presynaptic Neuron: Conducts action potentials toward synapse

  • Postsynaptic Neuron: Receives the signal and propagates action potentials away

  • Synaptic Cleft: Space between neurons

Neurotransmitter Types and Functions

• Postsynaptic Potential Factors:

  • Calcium levels, neurotransmitter levels, sensitivity changes

• Neurotransmitters:

  • Common examples: Acetylcholine, dopamine, serotonin, epinephrine

• Neuropeptides:

  • Larger molecules (2-40 amino acids); examples include Substance P, endorphins

Specific Chemical Effects on Synaptic Transmission

• Synaptic Drug Interactions:

  • Modify synthesis, transport, storage, or release of neurotransmitters

  • Agonists (like morphine) mimic neurotransmitters; antagonists (like atropine) block receptor activation

• Examples of Drugs:

  • Cocaine blocks dopamine reuptake, strychnine competes with glycine

Neurotoxins and Their Effects

• Bacterial Toxins:

  • Tetanus Toxin: Prevents GABA release, affecting muscle control

  • Botulism: Interferes with excitatory neurotransmitter release, causing paralysis

• Neurotoxins:

  • Batrachotoxin: Lowers neuronal firing threshold; muscles contract and eventually lead to paralysis

  • Black Mamba Snake Toxin: Prolongs action potentials, leading to muscle tremors and respiratory failure

Neurotransmitter Disruption and Injury Responses

• Extracellular K+ Increases (e.g., KCl injection):

  • Raises intracellular concentration, leading to higher likelihood of depolarization and potential seizures

• Curare: Competes with ACh at receptors, resulting in paralysis

• Tetrodotoxin: Inhibits Na+ gates, leading to paralysis and loss of sensation

• Box Jellyfish Toxin: Causes hyperkalemia, leading to cardiovascular collapse

Sensory Systems Overview

• Sensory Receptors:

  • Specialized neuron endings or separate cells signaling to afferent neurons

  • Receptor fields and characteristics impact sensitivity and localization

  • Pain and protective mechanisms influenced by nociceptors and neuropeptides

Eye Structure and Function

• Eye Components:

  • Outer Layers: Sclera, choroid, retina

  • Iris: Controls the amount of light entering the eye

• Light Refraction:

  • Cornea and lens major contributors; conditions like glaucoma involve pressure build-up

• Accommodation:

  • Ciliary muscles adjust lens shape for near and far vision

Phototransduction and Retinal Structure

• Photoreceptors:

  • Rods (low light, less acuity) vs. Cones (high light, high acuity)

• Color Blindness:

  • Results from deficiencies in particular color cones

Auditory and Vestibular Systems

• Hearing Function:

  • Involves the outer ear, middle ear ossicles, and inner ear cochlea

• Balance:

  • Vestibular apparatus detects body position and movement through acceleration and otoliths

Conclusion

• Understanding these physiological concepts of neuronal structure, function, potential types, synaptic transmission, and sensory processing is crucial for comprehending the nervous system's role in overall human physiology.