Medical Sciences - Neural Signalling Notes

Introduction to Neural Signalling

  • Neural signalling involves the transmission of information through the nervous system via neurons.

Structure of Neurons

  • Dendrites: Receive signals.
  • Cell Body: Integration center.
  • Axon: Relays signals as action potentials.
  • Synapses: Facilitate communication between neurons; can be chemical or electrical.
  • Functional Classification:
    • Sensory neurons
    • Motor neurons
    • Interneurons (relay neurons)

Phases of Nerve Action Potential

  1. Resting Phase: Membrane potential is approximately -70 mV (resting membrane potential).
  2. Depolarization:
    • Triggered by excitatory stimuli leading to a membrane potential increase and crossing a threshold of approximately -55 mV.
    • Voltage-gated Na+ channels open, allowing Na+ ions to flow in, shifting the potential to about +30 mV.
  3. Repolarization:
    • Inactivation of Na+ channels occurs as K+ channels open, allowing K+ to flow out, decreasing membrane potential.
  4. Hyperpolarization:
    • A brief overshoot occurs due to excessive efflux of K+.
  5. Restoration:
    • Na+/K+ ATPase restores gradients; membrane potential returns to resting.

Propagation of Action Potentials

  • Myelinated Axons: Myelin sheath speeds up propagation by insulating the axon.
  • Unmyelinated Axons: Action potentials propagate slower via continuous conduction.

Refractory Periods

  • Absolute Refractory Period: A neuron cannot fire another action potential.
    • Ensures unidirectional propagation.
  • Relative Refractory Period: A stronger-than-normal stimulus can generate an action potential.

``All-or-Nothing'' Principle

  • Action potentials are generated at full amplitude or not at all; increased frequency corresponds to a larger stimulus, not amplitude.

Factors Affecting Action Potentials

  • Motor Neuron Disease (MND):
    • Example: Amyotrophic Lateral Sclerosis (ALS)
    • Characterized by muscle weakness and eventual paralysis. Changes occur due to degeneration of motoneurons, which disrupt signal transmission.
  • Multiple Sclerosis (MS):
    • Autoimmune disorder affecting CNS myelin, leading to impaired signal conduction. Symptoms vary widely.
  • Guillain-Barre Syndrome:
    • Affects PNS, leading to muscle weakness caused by autoimmune response, often triggered by infections.

Sensory Receptors

  • Modified nerve endings tuned to detect specific energies (mechanical, thermal, chemical).
  • Mechanoreceptors: Sense touch/pressure.
  • Thermoreceptors: Sense temperature changes.
  • Photoreceptors: Sense light.
  • Nociceptors: Sense pain.

Sensory Transduction Example: Pacinian Corpuscle

  • Located in tissues around sensory neurons; detects pressure in a series of connective tissue layers.
  • Conversion of mechanical pressure into electrical signals via stretch-mediated Na+ channels.

Muscle Proprioceptors

  1. Muscle Spindles: Detect muscle stretch and initiate reflex contraction to protect from overstretching.
  2. Golgi Tendon Organs: Respond to excessive tension and signal for relaxation to prevent damage.

Reflex Arcs

  • Automatic, rapid responses to stimuli crucial for protection and coordination.
  • Components:
    1. Receptor
    2. Sensory neuron
    3. Interneuron
    4. Motor neuron
    5. Effector

Types of Reflex Actions

  • Stretch Reflex (e.g., knee-jerk): Muscle spindle detects stretch, triggers contraction without interneurons (monosynaptic).
  • Golgi Tendon Reflex: Prevents excessive contraction; inhibits muscle activity when tension is too high.