Somatosensory System Overview

Somatosensory System

• The primary somatosensory cortex is located dorsally to the central sulcus
• It contains separate regions representing different body parts, known as somatotropic maps
• Some somatotropic areas are larger than others, relative to the size of the body part; for example, the thumb and lower lip have larger areas of representation.
• Reconstructing the body from the somatotropic maps in the somatosensory and motor cortex creates a "homunculus"
• Two types of dorsal root afferents exist (those carrying signals from the skin to the spinal cord)
  • Touch and pressure:
    • Detected by mechanosensory fibers using mechanoreceptors
    • Signals stay on the same side of the spinal cord
  • Pain and temperature:
    • Detected by nociceptive afferents using free nerve endings
    • Signals cross over to the opposite side of the spinal cord

Mechanosensory Fiber and Nociceptive Afferent

• Mechanosensory fibers detect touch and pressure using receptors in the skin.
• Nociceptors detect pain and temperature. They are activated by potentially harmful stimuli and free nerve endings are involved in their mechanoreception
• Touch is detected by a change in pressure on the skin
• The change in phospholipid pressure in the membrane leaflet causes opening/closing of ion channels

Receptive Fields

• Receptive fields are important in sensory processing
• Large receptive fields have low sensory precision in the stimulation area, meaning any stimulus in a large area will result in a response.
• Large receptive fields lead to difficulty discriminating stimuli within the field.

Types of Mechanoreceptors

• Pacinian corpuscles detect general pressure on the skin
• Ruffini corpuscles detect the stretching of skin, important for motion detection
• Meissner corpuscles are common mechanoreceptors in the hand, and are highly sensitive but less precise than Merkel cells
• Merkel cells are located at the fingertips, with small receptive fields and high precision
• Different types of mechanoreceptors have different sensitivities and rapidly/slowly adapting responses.

Proprioception

• Proprioception is the body's sense of its position in space.
• Receptor types for proprioception are present in muscles and tendons
• Muscle spindles detect changes in muscle length
• Golgi tendon organs detect changes in muscle tension
• This system continually maps limb position.
• Afferents of proprioceptors project to the ventral horn for reflex responses and through Clarke's nucleus to the cerebellum for movement timing, and to the thalamus
• Signals travel to the thalamus and stay on the same side of the spinal cord

Dermatomes and Facial Sensory Input

• A dermatome is the area of skin innervated by a single spinal nerve root.
• Facial sensory input is mediated by the trigeminal nerve (cranial nerve V)
• Somatotropic maps are within individuals; digits are processed separately by slow and fast adapting inputs

Cortical Representation Plasticity

• Cortical reorganization can occur in the brain's representation of bodily sensations, leading to adjustments in the somatosensory maps.
• This can happen, for example, after training complex tasks or following limb amputations

Nociceptors and Pain

• Nociceptors responding only to strong stimuli
• Pain perception is mediated by slow fibers
  • TRPV1 receptor is a heat receptor and pain receptor
  • Aδ fibers are lightly myelinated (fast pain)
  • C fibers are unmyelinated and thin (slow pain)
• Pain stimuli can be thermal, mechanical, or chemical

Referred Pain

• Visceral pain from internal organs is projected onto the skin
• Visceral pain neurons project onto dorsal horn neurons that convey cutaneous pain
• Pain from the esophagus or heart may be perceived as pain in the chest or arm.

Pain Modulation

• Descending tracts from brain regions like the Raphe nuclei and Locus Coeruleus suppress pain signals.
• Local mechanosensory input (like rubbing) can also help suppress pain.
• Endocannabinoids and endorphins suppress pain signals in the spinal cord.

Explore the intricate details of the somatosensory system, including the primary somatosensory cortex and the representation of body parts through somatotropic maps. Learn about mechanosensory fibers and nociceptive afferents that contribute to our sense of touch, pressure, pain, and temperature.