Sensory Pathways in the Human Body

Ataxia

• Ataxia is a degenerative disease of the nervous system, characterized by symptoms similar to being drunk, such as slurred speech, stumbling, falling, and incoordination.

Types of Fibers

• Type Aβ: associated with mechanoreceptors, responsible for touch and pressure sensation
• Type Aδ: associated with thermoceptors, responsible for cold and pain sensation
• Type C: associated with free nerve endings, responsible for pain and temperature sensation

Somatosensory System

• The ability to identify objects by handling them without looking at them depends on intact touch and pressure sensation
• Thermoceptive sensation:
  • Freezing cold: <15°C
  • Cold: 15-20°C
  • Cool: 20-30°C
  • Indifferent (warm or cold): 30-38°C
  • Warm: 38-43°C
  • Hot: 43-47°C
  • Burning hot: 47-59°C
• Thermoreceptors:
  • Cold receptors: stimulated by cold temperatures
  • Warm receptors: stimulated by warm temperatures
• Nociceptors:
  • Pain receptors: located immediately under the skin
  • Free nerve endings of Type Aδ and some Type C: stimulated by extreme degrees of temperature
• Vibratory sensation:
  • Depends on the combination of cutaneous and separate neural pathways
  • Pattern of stimulation and cortical components are synthesized
• Two-Point Discrimination:
  • Ability to perceive two touch stimuli as two separate sensations at a minimum distance
  • Minimal distance by which two touch stimuli must be separated to be perceived as separate is called the "Two-Point Threshold"
• Stereognosis:
  • Ability to recognize objects by touch

Insular Cortex and Fasciculus

• The Insular Cortex is responsible for temperature sensation
• Fasciculus Cuneatus: receives signals from the upper extremities
• Fasciculus Gracilis: receives signals from the lower extremities

Somatosensory Areas

• Primary Somatosensory Area (Brodmann Area 3, 1, and 2): responsible for processing sensory information
• Accessory Somatosensory Area (Brodmann Area 5 and 7): responsible for processing sensory information
• The "map" of the body is called the Sensory Homunculus, with the largest areas representing the face, hands, and fingers

Dorsal Column or Lemniscal Pathway

• Fine touch and pressure sensation
• Vibratory sensation
• Position sensation
• Spinothalamic or Anterolateral Tract: responsible for pain and temperature sensation

Pain Sensation

• Epicritic pain: sharp, pricking pain, easily localized
• Protopathic pain: poorly localized, unpleasant, and associated with nausea and autonomic symptoms
• Hyperalgesia: increased sensitivity of pain receptors
• Allodynia: when normally innocuous stimuli cause pain
• Excruciating pain: difficult to treat, often radiates or referred to other areas

Sensory Pathways

• The signals travel from the receptors to the Somatosensory Areas 1 and 2 along the pathways
• 1st Order Neurons: primary afferent neurons that receive the transduced signal and send the information to the CNS
• 2nd Order Neurons: located in the Spinal Cord or Brainstem, receive information from one or more primary afferent neurons and transmit it to the Thalamus
• 3rd Order Neurons: located in the relay nuclei of the Thalamus, encoded sensory information ascends to the Cerebral Cortex
• Higher Order Neurons: located in the appropriate sensory area of the Cerebral Cortex, result in a conscious perception of the stimulus

Injuries and Abnormalities

• Ataxia: manifestation of Cerebellum dysfunction
• Brown-Sequard Syndrome: half of the Spinal Cord is transected, manifestations include ipsilateral paralysis, loss of proprioceptive/vibration sensation, and contralateral loss of pain/thermal sensations
• Syringomyelia: loss of ability for extreme hot and cold sensations, loss of pain and temperature sensations in the back and arms
• Tabes Dorsalis: caused by demyelination of sensory neurons secondary to an untreated syphilis infection, degenerating nerves are in the Dorsal Columns of the Spinal Cord
• Paresthesia: shooting/burning pains, pricking sensations
• Hypoesthesia: diminished tactile sensations
• Glossodynia: burning or tingling sensation of the lips, mouth, and tongue