Overview
This lecture covers the peripheral nervous system (PNS), focusing on the structure and function of cranial nerves, sensory receptors, and nerve pathways to and from the central nervous system.
Peripheral Nervous System Overview
- The PNS connects the central nervous system (CNS), which includes the brain and spinal cord, to the rest of the body.
- Pathways to the CNS are "afferent" (sensory), and pathways from the CNS are "efferent" (motor).
- The PNS contains 12 pairs of cranial nerves and 31 pairs of spinal nerves.
Cranial Nerves
- There are 12 pairs of cranial nerves, numbered I–XII with Roman numerals.
- Most cranial nerves are mixed (sensory and motor), but some are only sensory or only motor.
- Students must know each nerve’s name, Roman numeral, and general function.
- I: Olfactory — sense of smell
- II: Optic — vision
- III: Oculomotor — eye movement
- IV: Trochlear — eye movement
- V: Trigeminal — facial sensation, chewing (three branches: ophthalmic, maxillary, mandibular)
- VI: Abducens — lateral eye movement
- VII: Facial — facial expression, taste (anterior 2/3 tongue)
- VIII: Vestibulocochlear — hearing and balance
- IX: Glossopharyngeal — taste (posterior tongue), swallowing
- X: Vagus — innervation of internal organs (viscera)
- XI: Accessory — movement of sternocleidomastoid and trapezius muscles
- XII: Hypoglossal — tongue movements for swallowing and speech
Sensory Receptors and Pathways
- The PNS links the body to the outside world by transmitting sensory and motor signals.
- Sensory receptors are classified by stimulus type (mechanical, thermal, photoreceptors, chemoreceptors, nociceptors), location (exteroceptors, interoceptors, proprioceptors), and complexity (simple or special senses).
- Simple receptors can be encapsulated or non-encapsulated; special senses (vision, hearing, smell, taste, balance) use complex receptors.
Neural Integration and Perception
- Sensation is awareness of a stimulus; perception is interpretation of that stimulus.
- Signal processing occurs at three levels: receptor (stimulus detected), circuit (ascending pathway), and perceptual (cortex).
- Sensory signals pass through first-order, second-order, and third-order neurons before reaching the somatosensory cortex.
- Features like spatial discrimination, magnitude estimation, and pattern recognition allow us to precisely identify stimuli.
- Pain perception signals potential tissue damage; threshold for pain is fixed, but tolerance varies.
Structure and Classification of Nerves
- Nerves have three connective tissue layers: endoneurium (inner), perineurium (around fascicles), and epineurium (outermost).
- Mixed nerves contain both sensory (afferent) and motor (efferent) fibers.
- Somatic and visceral fibers may be sensory (afferent) or motor (efferent); effectors are skeletal, smooth, cardiac muscle, and glands.
Nerve Regeneration
- Peripheral nerves can regenerate if damage is not severe; regeneration involves clearing debris, Schwann cell activity, and new myelin formation.
- CNS nerve fibers rarely regenerate due to growth-inhibiting proteins and scar tissue from astrocytes.
Key Terms & Definitions
- Afferent — sensory pathway carrying signals to the CNS.
- Efferent — motor pathway carrying signals from the CNS.
- Mixed nerves — contain both sensory and motor fibers.
- Exteroceptors — receptors that detect external stimuli.
- Interoceptors — receptors that detect internal stimuli.
- Proprioceptors — receptors in muscles, tendons, and joints for body position.
- Nociceptors — pain receptors.
- Somatosensory cortex — part of the cerebral cortex processing sensory input.
- Endoneurium — innermost nerve sheath.
- Perineurium — sheath around nerve fascicles.
- Epineurium — outermost sheath covering the entire nerve.
Action Items / Next Steps
- Memorize the names, Roman numerals, and general functions of all 12 cranial nerves.
- Review and understand sensory receptor classifications and neural integration concepts.
- Read the upcoming section on spinal nerves for Chapter 13, Part 2.