Somatosensory System Overview

Somatosensory System

• The somatosensory system is responsible for touch, pain, temperature, pressure, and proprioception.
• It is not considered one of the special senses, but rather a system made up of receptors broadly distributed throughout the body that responds to many kinds of stimuli.

Touch Receptors

• Mechanoreceptors are the most common type of somatosensory receptor, responsible for touch sensations.
• Mechanoreceptors include:
  • Pacinian corpuscles: respond to vibrations, both high and low frequency, and pressure.
  • Ruffini's endings: respond to stretching and sliding.
  • Meissner's corpuscles: respond to pressure, force, and fine touch.
  • Merkel's disks: respond to light touch.
  • Krause end bulbs: respond to temperature, but their role in temperature perception is not fully understood.
• Mechanosensitive ion channels convert mechanical force into changes in ionic current.
• Different ion channels can be sensitive to physical stretching of the lipid membrane or deformation of the cell's cytoskeleton.

Primary Afferent Axons

• Primary afferent axons of the somatosensory system are classified as Aa, Ab, Ad, and C axons.
• Ab axons mediate touch sensations, while C fibers mediate pain, temperature, and itch.

Spinal Cord Organization

• The spinal cord is divided into four sections: cervical (head/neck), thoracic (chest/abdomen), lumbar (lower abdomen/upper leg), and sacral (lower limbs).
• Spinal segments are organized into dermatomes, which are areas of skin where sensory information is relayed to a specific spinal segment nerve.

Ascending Somatosensory Pathways

• Sensory information travels up the spinal cord via a chain of three neurons.
  • First-order neuron: conducts impulses from receptors to the spinal cord or medulla. It synapses with the second-order neuron.
  • Second-order neuron: an interneuron located in the dorsal horn of the spinal cord or medullary nuclei. Its axon extends to the thalamus or cerebellum.
  • Third-order neuron: also an interneuron, located in the thalamus. Its axon extends to the somatosensory cortex.

Dorsal Column-Medial Lemniscus Pathway (DCML)

• The DCML pathway transmits information about discriminative touch, vibrations, pressure, and proprioception to the somatosensory cortex.
• It decussates (crosses over) at the medulla oblongata.

Trigeminal Touch Pathway

• The trigeminal touch pathway conveys somatosensory information from the face.
• This pathway also carries information from the dura mater.
• It decussates at the level of entry of the trigeminal nerve (cranial nerve V) at the trigeminal nuclei in the brainstem.

Somatic Sensory Areas of Cortex

• The primary somatosensory cortex (S1) is located in Brodmann areas 1, 2, and 3.
  • Area 3a: proprioception
  • Area 3b: basic touch sensations
  • Area 1: texture
  • Area 2: shape and size; also contributes to proprioception

Cortical Somatotopy

• The somatosensory cortex has a map called the homunculus that represents the body disproportionately, reflecting the different sensitivities of various body parts.
• The homunculus demonstrates that areas with more receptors, like the hands and face, have larger cortical representations.

Somatotopy in Other Animals

• Other animals, like cats and mice with whiskers, have somatotopic maps in their cortex as well.
• Some animals, such as owl monkeys, have somatotopic maps in more than one cortical region.

Somatotopic Map Plasticity

• Somatotopic maps can adjust to changes, like amputation, to prevent "wasted" brain space.
• If digits are removed or overstimulated, the somatotopic map will change, demonstrating neuroplasticity.

Posterior Parietal Cortex

• The posterior parietal cortex is involved in integrating somatic sensation with visual stimuli, movement planning, and attention.
• Damage to the posterior parietal cortex can cause neurological disorders such as:
  • Agnosia: inability to recognize stimuli from senses
  • Astereognosia: inability to identify objects by feel
  • Neglect syndrome: one side of the body or sensory space is neglected.

Pain

• Pain is a feeling of soreness, aching, or throbbing. Nociception is the sensory process that triggers pain.
• Nociceptors are the receptors for pain.
• Nociceptors are typically polymodal, meaning they can be stimulated by multiple types of stimuli, including:
  • Strong mechanical stimulation
  • Temperature extremes
  • Oxygen deprivation
  • Chemicals

Nociception

• Damaged cells release substances that can activate nociceptors such as:
  • Proteases (which activate bradykinin), ATP, potassium ions
  • Histamine

Types of Nociceptors

• There are three types of nociceptors based on their stimulus:
  • Mechanical: respond to physical forces like stabbing, punching, or stretching.
  • Thermal: respond to hot and cold temperatures.
  • Chemical: respond to pH changes or specific ligands.

Explore the intricate workings of the somatosensory system, including its role in touch, pain, and temperature perception. Understand the different types of mechanoreceptors and their specific responses to stimuli. This quiz will deepen your knowledge of how our body perceives various sensations.