Somatosensation Overview Quiz

Somatosensation Overview

• Somatosensation encompasses all sensations except sight, hearing, taste, smell, and vestibular equilibrium.
• It's controlled by cranial nerves and special senses.
• Somatosensation can be conscious or unconscious.

Types of Somatosensation

• Touch: Crude (general) or discriminative (precise), pressure, and vibration.
• Temperature: Perceptions of hot and cold.
• Proprioception: Internal sense of body position and movement in space, not based on skin input.
• Nociception: Sensory signaling regarding pain; doesn't necessarily result in perceived pain.

Somatosensory System Components

• Receptors: Specialized structures within the skin and other tissues.
• Primary Afferent Neurons (First-Order Fibers): Transmit signals from receptors to the spinal cord, with cell bodies located in the dorsal root ganglion (DRG).
• Prethalamic Relay Neurons (Second-Order Fibers): Connect the first-order neurons and the thalamus.
• Decussation: Point where sensory pathways cross from one side of the body to the other side of the central nervous system (CNS).
• Thalamic Relay Neurons (Third-Order Fibers): Transmit signals from the thalamus to the cerebral cortex.
• Cerebral Cortex: Site where sensory information is interpreted and perceived as sensation.

Primary Afferent Neurons (First-Order Fibers)

• Peripheral process connects to receptors, central process enters the CNS and synapses with prethalamic relay neurons.

Prethalamic Relay and Decussation (Second-Order Neurons)

• Located in the spinal cord or brainstem.
• Relay signals and decussate before projecting to the thalamus.

Thalamic Relay and Cerebral Cortex (Third-Order Neurons)

• Located in specific thalamic nuclei.
• Transmit signals to the cerebral cortex via thalamocortical projection fibers in the internal capsule.

Receptors: Sensory End Organs

• Convert environmental energy into electrical signals (receptor potential).
• Can be unimodal or polymodal depending on the type of stimulus converted.

Classification of Receptors

• Structure:

  • Free nerve endings (e.g., nociceptors).
  • Encapsulated receptors (e.g., Pacinian corpuscles, Golgi tendon organs).

• Stimulus Source:

  • Exteroreceptors (external environment, skin).
  • Interoreceptors (internal environment, organs).
  • Proprioceptors (internal body position and movement).

• Rate of Adaptation:

  • Slowly adapting (tonic): Continuously fire as long as stimulus is present.
  • Rapidly adapting (phasic): Stop firing soon after stimulus.

• Type of Stimulus:

  • Mechanoreceptors (respond to mechanical deformation).
  • Thermoreceptors (respond to temperature changes).
  • Nociceptors (respond to tissue damage).
  • Chemoreceptors (respond to chemicals).

Specific Types of Receptors

• Nociceptors and Thermal Receptors: Free nerve endings, most numerous in skin.
• Superficial Mechanoreceptors (skin): Provide external environment information.
• Deep Mechanoreceptors (proprioceptors): Provide information about body position in space.
• Muscle Spindles: Specialized receptors in skeletal muscle that are sensitive to muscle stretch and length.
• Golgi Tendon Organs: Sensitive to muscle tension, located at the junction between muscles and tendons.
• Joint Receptors: Detect mechanical deformation in joints.
  • Ruffini's endings (capsule stretch).
  • Pacinian corpuscles (capsule movement).

Cutaneous Receptor Fields

• Area of skin that activates a primary afferent neuron with stimulus.
• Smallest in areas with high innervation density (e.g., hands and feet).
• Larger proximally, smaller distally.

Primary Afferents and Their Receptors

• Myelinated or unmyelinated fibers; conduction speeds vary depending on myelination and diameter.
• Types include Ia (fastest, proprioceptors), Ib, II (A beta), A delta, and C (slowest, nociceptors).

Conscious Sensory Pathways

• Dorsal Column-Medial Lemniscus (DCML) System: For fine touch, proprioception, and vibration.
• Spinothalamic Tract: For pain, temperature, and crude touch.
• Pathways differ based on the type of sensation and where they decussate.

DCML Pathway: Dorsal Columns (Posterior Tracts)

• Fasciculus Gracilis: Carries lower body information.
• Fasciculus Cuneatus: Carries upper body information.
• Decussation occurs in the medulla.

Spinothalamic Tract: Anterolateral System

• Carries pain, temperature, and crude touch information.
• Decussation occurs in the spinal cord.

Thalamus

• Sensory "relay station" that processes sensory and motor information.
• Specific nuclei receive and project to the cortex. (VPL for body, VPM for face).

Somatosensory System Injuries

• Lesions can occur anywhere, affecting sensation.
• Symptoms vary based on the location and extent of the injury.

Nociception and Pain

• Nociceptors signal tissue damage, but pain is the perceived experience of that signal.
• Different pathways relay information based on pain nature (fast/slow).

Pain Modulation

• Brain possesses systems to modulate pain responses.

Referred Pain

• Pain perceived from a different location than the origin.

Clinical Connections (Fibromyalgia, CRPS)

• Described conditions involving altered sensory processing and pain.

Pain Assessment

• Patient's self-report is crucial. Various scales help measure pain objectively.
• Pain history is essential.

Sensory Exam

• Tools and techniques used in assessing somatosensory function.