Peripheral Nervous System and Neurons

The Peripheral Nervous System

  • Definition: The Peripheral Nervous System (PNS) co-ordinates the transmission and reception of impulses between the body and the Central Nervous System (CNS).
Components of the Peripheral Nervous System
  • Spinal Nerves: 31 pairs
    • 8 Cervical
    • 12 Thoracic
    • 5 Lumbar
    • 5 Sacral
    • 1 Coccygeal
  • Cranial Nerves: 12 pairs
  • Autonomic Nervous System: Coordinates involuntary bodily functions.
Neurones
  • Definition: Highly modified cells capable of initiating, relaying, and redirecting electrical impulses by converting chemical energy into electrical energy.
Functions of Neurones
  • Control of muscle activity
  • Regulation of gland activity
  • Support and protection of the nervous system.
Structure of a Neurone
  • Dendrites: Extensions carrying impulses toward the cell body.
  • Cell Body/Soma: Contains organelles and nucleus.
  • Axon Hillock: The area where the impulse is initiated.
  • Myelin Sheath: Insulation that allows for high-speed impulse transmission.
  • Node of Ranvier: Gaps in the myelin sheath that help in the conduction of nerve impulses.
  • Schwann Cells: Cells that form the myelin sheath.
  • Axon Terminal: End of the axon where neurotransmitter release occurs.
Transmission of Impulses
  • The electrical signal travels along the axon where sodium (Na+Na^+) and potassium (K+K^+) ions exchange places due to permeability changes in the Myelin sheath.
  • At the synapse, synaptic vesicles discharge neurotransmitters to transmit impulses to the next neurone.
Neurotransmitters
  • Acetylcholine (ACh): Mainly involved in muscle contraction; binds to sodium and potassium to generate a nerve impulse.
  • Adrenaline and Noradrenaline: Involved in the fight-or-flight response; rapidly reabsorbed at the synapse to control excitation.
  • Other Neurotransmitters:
    • Serotonin: Stabilizes mood; influences sleep, eating, and digestion.
    • Dopamine: Associated with pleasure and cognitive functions.
    • Histamine: Affects various bodily functions, including smooth muscle contraction and blood vessel dilation.
Nerve Types in the PNS
  • Afferent Neurones (Sensory): Unipolar neurones that convey information from the periphery to the CNS.
  • Efferent Neurones:
    • Somatic Nerves: Voluntary movements to skeletal muscles.
    • Autonomic Nerves: Involuntary functions to glands and muscles.
Spinal Reflexes
  • Reflex actions are automatic responses requiring:
    • A receptor
    • A sensory neurone
    • An integrating center (CNS)
    • A motor neurone
    • An effector (muscle or gland).
  • Types of reflexes:
    • Spinal Reflexes: Quick, automatic responses.
    • Cortical Reflexes: Under conscious control; can override spinal reflexes.
    • Autonomic Reflexes: Immediate reactions that bypass conscious processing.
Autonomic Nervous System (ANS)
  • Function: Controls automatic body functions below the cerebral level.
  • Components:
    • Sympathetic (Fight or Flight): Increases heart rate, blood pressure.
    • Parasympathetic (Rest and Digest): Decreases heart rate, promotes digestion.
Nociception (Pain Perception)
  • Transduction: Begins when nociceptors respond to harmful stimuli.
  • Transmission: Involves three stages:
    1. From the site of injury to the dorsal horn of the spinal cord.
    2. From the spinal cord to the brain stem.
    3. From spinal cord connections to the thalamus and higher brain areas.
  • Perception: The conscious experience of pain involves emotional and sensory interpretation.
  • Responses to Pain:
    • Somatosensory Cortex: Processes and identifies pain intensity and location.
    • Reticular System: An autonomic and motor response to pain.
    • Limbic System: Governs emotional responses to pain.
Pain Modulation
  • Analgesics: Drugs that relieve pain without loss of consciousness by blocking pain pathways.
  • Anaesthetics: Drugs that depress CNS to provide loss of sensory modalities, including consciousness.
Summary
  • The PNS is crucial for relaying information between the body and CNS, involving complex networks of neurones and neurotransmitters. The functioning of the nervous system can be influenced by both internal and external factors, necessitating a solid understanding of its components and operations for effective medical practice in fields such as Paramedic Science.