Peripheral Nervous System: General Sensations and Sensory Pathways

General Sensations

• Sensory impulses reach the CNS and integrate with other arriving and stored information.
• Integration occurs in the spinal cord, brain stem, cerebellum, basal nuclei, and cerebral cortex.
• Sensations result in conscious perception or subconscious awareness of environmental changes.
• Complex integrative functions include wakefulness, sleep, learning, and memory.

Sensory Modalities

• Each unique sensation type (touch, pain, vision) is a sensory modality.
• Somatic senses: tactile, thermal, pain, and proprioception.
• Visceral senses: conditions within internal organs.
• Sensation process:
  • Stimulation of sensory receptor within its receptive field.
  • Transduction: stimulus energy converted into a graded potential.
  • Generation of nerve impulses: graded potential reaches threshold triggering nerve impulses.
  • Integration: CNS integrates sensory impulses, resulting in conscious sensation.

Sensory Receptors

• Classes based on:
  • Microscopic structure: free nerve endings vs. encapsulated endings.
  • Location: exteroceptors vs. interoceptors (visceroceptors).
  • Stimulus type: nociceptors, mechanoreceptors, etc.
• Free Nerve Endings: bare dendrites for pain, temp, tickle, itch and some touch. Note the generator potential.
• Encapsulated Nerve Endings: dendrites enclosed in connective tissue capsule for pressure, vibration and some touch. Generator potential
• Separate Cells: sensory receptors for some special senses. Produce Receptor Potentials which synapse with sensory neurons.
• Generator potentials in free and encapsulated nerve endings trigger nerve impulses in first-order neurons.
• Receptor potentials in separate cells trigger neurotransmitter release, producing postsynaptic potentials (PSPs) in first-order neurons.
• Receptors by location:
  • Exteroceptors: external surface; respond to external stimuli.
  • Interoceptors: blood vessels, organs; usually not consciously perceived.
  • Proprioceptors: muscles, tendons, joints, inner ear; body position, muscle length, joint movement.
• Receptors by activation:
  • Mechanoreceptors: deformation, stretching, bending.
  • Thermoreceptors: temperature changes.
  • Nociceptors: painful stimuli, tissue damage.
  • Photoreceptors: light.
  • Chemoreceptors: chemicals.
  • Osmoreceptors: osmotic pressure.
• Adaptation: generator or receptor potential decreases during sustained stimulus.
• Rapidly adapting receptors: pressure, touch, smell.
• Slowly adapting receptors: pain, body position, blood chemical composition.

Somatic Sensations

• Arise from receptors in skin, mucous membranes, muscles, tendons, joints, and inner ear.
• Four modalities: tactile, thermal, pain, and proprioceptive.
• Fiber Types
  • A fibers: large, fast, myelinated; touch, pressure, joint position. (27-290mph)
  • B fibers: medium size and speed, myelinated; visceral sensory & autonomic preganglionic neurons. (34mph)
  • C fibers: small, slow, unmyelinated; sensory and autonomic motor neurons; pain, touch, pressure, hot/cold. (1-4mph)

Tactile Sensations

• Touch, pressure, vibration, itch, and tickle.
• Mechanoreceptors (A fibers) mediate touch, pressure, and vibration.
• Free nerve endings (C fibers) mediate itch and tickle.
• Touch:
  • Rapidly adapting: Meissner corpuscles, hair root plexuses.
  • Slowly adapting: Type I (tactile discs), Type II (Ruffini corpuscles).
• Pressure: sustained sensation over a larger area; Type 1 mechanoreceptors, Pacinian corpuscles.
• Vibration: rapidly repetitive signals; Meissner corpuscles (lower frequency), Pacinian corpuscles (higher frequency).
• Itch: chemicals stimulating free nerve endings.
• Tickle: free nerve endings, typically requires external touch.

Thermal Sensations

• Thermoreceptors are free nerve endings.
• Cold receptors: stratum basale; (50-95^"); rapidly adapting.
• Warm receptors: dermis; (86-113^"); rapidly adapting.

Pain Sensations

• Nociceptors: free nerve endings activated by tissue damage.
• Fast pain (acute, sharp): myelinated A fibers, NOT felt in deeper tissues. (0.1s)
• Slow pain (burning, aching): unmyelinated C fibers, in skin, deeper tissues and internal organs. (>1.0 s)
• Superficial somatic pain: skin receptors.
• Deep somatic pain: receptors in skeletal muscles, joints, tendons, and fascia.
• Visceral pain: nociceptors in visceral organs; can result in referred pain.

Proprioception

• Awareness of body position and movement (kinesthesia).
• Proprioceptors in muscles, tendons, joints, and inner ear.

Somatic Sensory Pathways

• Postcentral gyri are primary somatosensory area.
• Pathways consist of three neurons: first, second, and third order.
  • First-order neurons: somatic receptors to brain stem or spinal cord; cell body in dorsal root ganglia (DRG); unipolar.
  • Second-order neurons: brain stem and spinal cord to thalamus; axons decussate.
  • Third-order neurons: thalamus to primary somatosensory area of cortex.
• Thalamus functions as a major relay station.

Somatic Motor Pathways

• The cerebellum is key for maintaining proper posture and balance and performs 4 activities:
  • Monitoring intentions for movement/ Planned movements.
  • Monitoring actual movement: receives input from proprioceptors.
  • Comparing command signals with sensory information.
  • Sending out corrective feedback.

Integrative Functions of the Cerebrum

• Wakefulness and sleep: relies on the reticular activating system (RAS).
• Learning is the ability to acquire new information or skills through instruction or experience.
  *Memory is the process by which information already acquired through learning is stored and retrieved.
  *Immediate- for only a few seconds.
  *Short-term- a few seconds to minutes
  *Long-term memory- lasts from days to years.
• Plasticity- For experience to become part of memory, it must produce changes in the brain. The capability for change associated with learning is called.
• Amnesia: refers to the lack or loss of memory.
  • Anterograde amnesia- inability to form new memories
  • Retrograde amnesia- inability to recall the past.