Peripheral Nervous System (PNS) Overview

Introduction to PNS

• Discusses features of the PNS, including receptors and sensors in peripheral organs.
• Covers sensory and motor components, and their pathways.

Organization of Nervous System

• Central Nervous System (CNS): Brain and spinal cord.
• Peripheral Nervous System (PNS): All neural structures outside CNS; includes cranial and spinal nerves, ganglia.
• Functions to relay sensory information to the CNS and motor information from the CNS.

PNS Divisions

• Sensory (Afferent) Division: Carries sensory info to the CNS.
  • Somatic Sensory: From skin, muscles.
  • Visceral Sensory: From organs like cardiac, smooth muscle.
• Motor (Efferent) Division: Carries motor info from the CNS.
  • Somatic Motor Division: Voluntary, controls skeletal muscles.
  • Autonomic Motor Division (ANS): Involuntary, controls cardiac/smooth muscles, glands.
    • Sympathetic & Parasympathetic Divisions: Covered in detail later.

Sensory Receptors

• Detect changes in external environment (stimuli).
• Classification by Stimulus Type:
  • Mechanoreceptors: Touch, pressure, vibration, stretch.
  • Thermoreceptors: Temperature changes.
  • Photoreceptors: Light (vision).
  • Chemoreceptors: Chemicals (smell, taste).
  • Nociceptors: Pain-causing stimuli.
• Classification by Location:
  • Exteroceptors: External stimuli (skin, senses).
  • Interoceptors: Internal stimuli (organs, blood vessels).
  • Proprioceptors: Stretch in muscles, tendons, joints.
• Classification by Structure:
  • General Sense Receptors: Simplistic (free nerve endings) or encapsulated.
  • Special Sense Receptors: Complex structures (e.g., retina, cochlea).

Pathways and Processing

• Sensory Integration Levels:
  • Receptor Level: Detection of stimulus, conversion to graded potential.
  • Circuit Level: Pathway to CNS, involves ascending/descending tracts.
  • Perceptual Level: Processing in somatosensory cortex.
• Somatotopy: Mapping of body’s sensory/motor regions in brain.

Nerve Types

• Cranial Nerves: Originate from the brain.
• Spinal Nerves: Originate from the spinal cord.
• Sensory Nerves: Carry sensory inputs.
• Motor Nerves: Carry motor outputs.
• Mixed Nerves: Carry both sensory and motor signals.

Motor Control

• Motor Endings: Neurons activating target cells (effectors).
• Levels of Motor Activity Control:
  • Segmental Level: Spinal cord, reflex activities.
  • Projection Level: Motor cortex, brainstem.
  • Pre-command Level: Cerebellum, basal nuclei, blueprint of motor output.

Reflexes

• Types of Reflexes:
  • Intrinsic Reflexes: Inborn, e.g., posture, pain avoidance.
  • Acquired Reflexes: Learned through practice, e.g., driving.
• Reflex Arc Components: Receptor, sensory neuron, integration center, motor neuron, effector.
• Somatic Reflexes: Affect skeletal muscles, e.g., stretch reflex, tendon reflex.
  • Stretch Reflex: Detects muscle length changes via muscle spindle, causes contraction.
  • Tendon Reflex: Detects tension via tendon organ, prevents excessive contraction.

Conclusion

• Integration of CNS and PNS functions is crucial for understanding sensory inputs and motor outputs.
• Importance of reflex pathways in diagnosing nerve or spinal cord issues.