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.