Somatic Sensory System Notes

The Somatic Sensory System

Somatic Sensations

  • Somatic sensations include:
    • Touch
    • Pain
    • Temperature
    • Proprioception
  • Each sensation is a modality of sensation.
  • Sensory receptors mediate stimuli into changes in membrane potential, transmitted as action potentials to the spinal cord and higher brain centers.

Sensory Modalities

  • Different modalities of sensation (touch, pain, temperature, proprioception) are experienced because:
    • Each nerve fiber carrying information for a specific sensory modality terminates in a specific part of the CNS.
    • Different brain regions receive and interpret different sensations.

Sensory Fiber Transmission

  • Sensory fibers transmit nerve impulses as action potentials.
  • Different sensory receptors exhibit varying sensitivities to different types of sensory stimuli.
  • Activation of a sensory receptor results in a change in membrane potential, known as a receptor potential.

Types of Sensory Receptors

  • Mechanoreceptors: Detect physical distortion (touch, pressure).
  • Nociceptors: Detect painful stimuli.
  • Thermoreceptors: Detect changes in temperature.
  • Chemoreceptors: Detect chemicals (irritating or non-irritating).
  • Electromagnetic receptors: Detect light (e.g., in the retina).

Receptor Activation

  • Mechanical deformation
  • Application of a chemical
  • Change in temperature
  • Electromagnetic radiation

Stimulus and Action Potentials

  • A stimulus causes channels to open, leading to ion movement (current flow) and a depolarizing receptor potential.
  • If the depolarizing current is large enough, an action potential is generated.

Skin

  • Two layers:
    • Epidermis
    • Dermis
  • Two types:
    • Hairy
    • Hairless (glabrous)
  • The skin is the largest sensory organ.

Sensory Receptors in the Somatic Sensory System

  • Largest collection of sensory receptors.
  • Innervated by:
    • Large-diameter myelinated A-type fibers
    • Small-diameter unmyelinated C-type fibers
  • Adaptation:
    • Rapidly adapting: Pacinian corpuscles, Meissner’s corpuscles
    • Slowly adapting: Ruffini’s endings, Merkel’s disks
  • Receptive Fields:
    • Large: Pacinian corpuscles, Ruffini’s endings
    • Small: Meissner’s corpuscles, Merkel’s disks

Stimulus and Receptor Activation

  • A single stimulus can activate several receptors.
  • Different mechanical sensitivities of mechanoreceptors mediate different sensations.

Two-Point Discrimination

  • Ability to discriminate two distinct points of stimuli (spatial resolution).
  • Varies across the body; fingertips have the highest resolution.
  • Reasons for high resolution in fingertips:
    • Higher density of mechanoreceptors
    • Small receptive fields
    • More cortex devoted to sensory information from fingertips

Peripheral Nerves

  • Axons carry information from the periphery (PNS) to the spinal cord and brain (CNS).
  • Peripheral nerve composition:
    • Afferent fibers (sensory)
    • Efferent fibers (motor)
  • A peripheral nerve consists of many nerve fibers (axons) and blood vessels.

Axon Classification

  • Axons from skin:
    • Aα:
      • Diameter: 20-13 μm
      • Speed: 80-120 m/sec
      • Receptor type: Proprioceptors
    • Aβ:
      • Diameter: 12-6 μm
      • Speed: 35-75 m/sec
      • Receptor type: Mechanoreceptors
    • Aδ:
      • Diameter: 5-1 μm
      • Speed: 5-30 m/sec
      • Receptor type: Pain/Temp
    • C:
      • Diameter: 0.2-1.5 μm
      • Speed: 0.5-2 m/sec
      • Receptor type: Pain/Temp
  • Axons from muscles:
    • Group I: Similar to Aα
    • Group II: Similar to Aβ
    • Group III: Similar to Aδ
    • Group IV: Similar to C
  • Peripheral nerves associated with sensory receptors in the skin are named using the letters “A” and “C” and Greek letters; while those associated with muscles are classified into groups, using Roman numerals.

Spinal Cord Connection

  • Peripheral nerves connect with the spinal cord through 31 paired spinal nerves.
  • The spinal cord connects to the brain via the brain stem.
  • Spinal cord divisions: 4 groups
  • 1st order neurons (PAFs) synapse with 2nd order neurons that ascend to higher centers.

Spinal Nerve Segmentation

  • Each spinal nerve entering the spinal cord is segmented into a “spinal segment” corresponding to each pair of spinal nerves.
  • Each spinal nerve bifurcates into a dorsal root and a ventral root.
  • Dorsal roots carry afferent (sensory) fibers; ventral roots carry efferent (motor) fibers.
  • The DRG contains the cell bodies (somas) of PAFs.
  • Most 2nd order neurons receiving sensory input lie within the dorsal horn.
  • The cell bodies of efferent motor fibers lie within the ventral horns.

Dermatomes

  • Each spinal nerve innervates a particular segment of skin called a dermatome.
  • There is a one-to-one correspondence between dermatomes and spinal segments.
  • There is overlap between adjacent spinal nerves; thus, cutting a single dorsal root does not eliminate all sensation in the corresponding dermatome.

Varicella Zoster Virus

  • Varicella (chicken pox) is caused by the herpes zoster virus.
  • Following initial infection, the virus remains dormant in the dorsal root ganglia (DRG).
  • Reactivation results in shingles, affecting the skin (dermatome) supplied by the infected DRG.
  • The rash can involve more than one dermatome.

Tactile Information Pathway (DCML)

  • Tactile information is transmitted to the brain via the Dorsal Column-Medial Lemniscus (DCML) pathway.
  • Aβ fibers enter the ipsilateral dorsal column (white matter) medial to the dorsal horn and ascend (along with 2nd order axons from the spinal gray matter) to terminate in the dorsal column nuclei (DCN) in the medulla.
  • In the DCN, axons decussate before ascending via the medial lemniscus to synapse onto 3rd order neurons in the ventral posterior (VP) nucleus of the thalamus.
  • These 3rd order neurons project to the primary somatosensory cortex (S1), mostly terminating in the postcentral gyrus.

Facial Sensations

  • Facial sensations are primarily supplied by the trigeminal nerve (CN V).
  • Some sensory information from the head is supplied by other cranial nerves.
  • Axons synapse onto 2nd order neurons in the ipsilateral trigeminal nucleus in the pons.
  • Axons from the trigeminal nucleus decussate and ascend to the VP nucleus of the thalamus, synapsing onto 3rd order neurons that project to the somatosensory (S1) cortex.

Somatosensory Cortex (S1)

  • Much of the complex processing associated with somatosensations occurs in the cerebral cortex, specifically the primary somatosensory cortex (S1) (aka, Brodmann’s area 3b) in the parietal lobes.
  • The body’s surface sensations can be mapped onto the brain, creating a somatotopic map.
  • The caricature is not scaled to the human body but represents the density of sensory receptors from that area, reflecting the importance of sensory input.

Nociceptors and Pain

  • Somatic sensory receptors called nociceptors transmit information about pain to the CNS.
  • These are mostly free-ending small unmyelinated C-type fibers, but also include Aδ-type fibers. Aβ-type fibers may play a role in neuropathic pain.
  • Pain is defined as the perception of unpleasant sensory or emotional experience associated with actual or potential damage; nociception is the process that provides the information that results in pain. (The International Association for the Study of Pain (IASP) defines “nociception” as the neural process of encoding noxious stimuli).
  • Transduction involves ion channels activated by various stimuli.
  • Types of receptors: polymodal, mechanical, thermal, and chemical.

Pain Perception

  • Nociceptor activation produces two different perceptions of pain:
    • Fast, sharp first pain (caused by Aδ fiber activation).
    • Duller, longer-lasting second pain (caused by C fiber activation).

Hyperalgesia

  • Hyperalgesia is a reduced threshold to pain - increased pain - resulting from a normally innocuous stimulus.
  • Primary hyperalgesia occurs within the area of tissue damage; secondary hyperalgesia occurs in the supersensitive region surrounding the damaged area.
  • Tissue damage results in the release of substances that modulate nociceptor excitability:
    • Substance P, bradykinin, prostaglandins, histamine, and ATP
  • Substance P, synthesized by nociceptors, can sensitize nearby nociceptors, resulting in secondary hyperalgesia.
  • The mechanisms associated with hyperalgesia and pain are complex and much remains unknown.

Pain Pathways

  • Spinothalamic tract (STT):
    • Includes the anterior STT and the lateral STT.
    • Fibers terminate in several nuclei of the thalamus, including the posterior portion of the ventral medial nucleus (VMpo).
  • Spinoreticular tract (SRT)
  • Spinomesencephalic tract (SMT)
  • Trigeminothalamic tract

Descending Pain Control

  • These pathways exert much control over pain signals and their perception.
  • Many targets of descending input are the dorsal horns of the spinal cord and the trigeminal nucleus.
  • Signals from the periaqueductal gray (PAG) play a large role in modulating pain signals in the dorsal horn.
  • Two systems are the rostral ventromedial medulla (RVM) and the dorsolateral pontine tegmentum (DLPT).

Gate Control Theory

  • Proposes that pain signals can be modulated in the spinal cord before ascending upwards toward higher brain centers.

Tactile vs. Pain Sensations

  • Tactile Sensations (DCML):
    • Decussation first occurs in the medulla.
    • Uses mainly large-myelinated Aβ fibers.
    • High degree of spatial orientation.
    • Axons travel to the medulla, synapse, and decussate, then continue upward through the brain stem to the thalamus.
  • Pain Sensations (ALS):
    • Decussation occurs immediately after entering the spinal cord.
    • Uses mainly small-unmyelinated C fibers and small myelinated Aδ fibers.
    • Much less spatial orientation.
    • Axons terminate at all levels of the lower brain stem and in the thalamus.