I

ch 13

Poliomyelitis and ALS

  • Poliomyelitis (polio) is a viral disease that affects the nervous system, leading to muscle weakness and paralysis.
  • Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons, resulting in muscle atrophy and eventual loss of voluntary movement.

Spinal Nerves

  • Composed of numerous nerve fibers.
  • Nerve vs. Nerve Fiber:
    • A nerve is a bundle of nerve fibers (axons) that transmit signals between the brain and the body.
    • A nerve fiber typically refers to a single axon.
  • Connective Tissues in Nerves:
    • Endoneurium: Surrounds each individual nerve fiber.
    • Perineurium: Encloses groups of nerve fibers, forming fascicles.
    • Epineurium: The outer layer that encases the entire nerve.
  • Types of Nerves:
    • Mixed Nerve: Contains both sensory and motor fibers.
    • Sensory Nerve: Contains only sensory fibers carrying signals to the CNS.
    • Motor Nerve: Contains only motor fibers carrying signals away from the CNS.
  • There are 31 pairs of spinal nerves:
    • 8 cervical
    • 12 thoracic
    • 5 lumbar
    • 5 sacral
    • 1 coccygeal
  • Ganglion: A cluster of neuron cell bodies (soma) located outside (usually) the CNS.
  • Proximal Branches:
    • Dorsal Root: Carries sensory information to the spinal cord.
    • Ventral Root: Carries motor information away from the spinal cord.
  • Distal Branches:
    • Dorsal Ramus: Innervates the muscles and skin of the back.
    • Ventral Ramus: Innervates the anterior and lateral trunk, as well as the limbs.
    • Meningeal Fibers: Supplies the vertebral column with fibers.
  • Clinical Insight (Shingles): Refer to Clinical Insight 13.3 (pg 494) to understand the impact of shingles on the nervous system.

Nerve Plexuses

  • Definition: A network of intersecting nerves.
  • Names of 5 Spinal Nerve Plexuses and Examples:
    • Cervical Plexus: Phrenic nerve - innervates the diaphragm.
    • Brachial Plexus: Radial nerve - innervates the triceps brachii.
    • Lumbar Plexus: Femoral nerve - innervates the quadriceps (knee extenders).
    • Sacral Plexus: Sciatic nerve - innervates hamstrings (knee flexors) via a branch (tibial nerve).
    • Coccygeal Plexus: (no specific example needed as per instructions).

Somatic Reflexes

  • Definition of a Reflex: An involuntary response to a stimuli that occurs without conscious thought.
  • Differences from Other Motor Movements: Reflexes are automatic and typically occur faster than voluntary actions.
  • 4 Important Properties of a Reflex:
    1. Rapid: Reflexes involve few neurons and synapses, resulting in quick responses.
    2. Automatic: Reflexes operate independently of conscious thought.
    3. Involuntary: Reflex actions cannot be consciously controlled.
    4. Stereotyped: The same reflex occurs in response to the same stimulus each time.
  • 5 Components of a Reflex Arc:
    1. Receptor: Senses a stimulus.
    2. Sensory Neuron: Sends signal to the CNS.
    3. Integration Center: Processes the information (could be monosynaptic or polysynaptic).
    4. Motor Neuron: Carries the response away from the CNS.
    5. Effector: Executes the response (muscle or gland).
  • Absent Component in Monosynaptic Reflex: The integration center is a single synapse and may not have a distinct interneuron.