Unit 2.3 somatosensory Btech

Overview of Sensory Systems

  • Somatosensory systems include:

    • Visual system

    • Auditory and vestibular systems

    • Chemical senses

  • Sensory receptors detect stimuli, which are processed in the CNS, leading to motor responses.

Types of Sensations

  • General Sensations

    • Somatic Sensations: Include tactile, thermal and pain sensations.

    • Visceral Sensations: Associated with internal organs, e.g. baroreception and chemoreception.

  • Special Sensations:

    • Vision

    • Audition (Hearing)

    • Olfaction (Smell)

    • Gustation (Taste)

Classification of Sensations

General Sensations

  • Superficial Sensations:

    • Touch

    • Temperature

    • Pain

    • Synthetic senses

  • Deep Sensations:

    • Pain

    • Proprioception

Somatic Sensations

  • Include:

    • Tactile sensations: touch, pressure, vibration, itch, tickle

    • Thermal sensations: heat, cold

    • Pain sensations: general pain

    • Proprioceptive sensations: joint/muscle position and limb movement

  • Receptors are located in:

    • Skin, mucous membranes

    • Muscles, tendons, joints

Classification of Somatic Senses

Physiologic Classification

  1. Mechanoreceptive Sensation: Touch, pressure, vibration, tickle

  2. Thermoreceptive Sensation: Cold and warmth

  3. Pain Sensation: Injury to tissues

Other Classifications

  1. Exteroreceptive Senses: From body's surface

  2. Proprioceptive Senses: Body and joint position

  3. Visceral Senses: From internal organs

  4. Deep Sensations: From deep tissues

Tactile Sensations

  • Touch, pressure, and vibration are often classified separately but share receptor types.

Differences:

  1. Touch typically stimulated by receptors in the skin.

  2. Pressure results from deeper tissue deformation.

  3. Vibration from rapid sensory signal repetition.

Tactile Receptors

Types of Tactile Receptors

  • Free nerve endings: Detect touch/pressure.

  • Meissner’s corpuscle: Sensitive touch receptor in skin, responds rapidly to movement.

Expanded Receptor Types:

  • Krause corpuscles

  • Pacinian corpuscles: Detect rapid tissue compression; adapt quickly.

  • Ruffini endings: Slow-adapting; respond to sustained deformation.

  • Merkel’s discs: Provide continuous touch sensation and texture recognition.

Mechanoreceptive Free Nerve Endings

  • Found mostly in skin; responsible for tickle and itch sensations—transmitted via type C fibers.

Sensory Pathways to Central Nervous System

  • All somatic signals enter the spinal cord via dorsal roots.

Two Main Pathways:

  1. Dorsal Column–Medial Lemniscal System:

    • Carries touch and proprioceptive signals.

    • Signals ascend to the brain without interruption until the medulla, where they cross sides.

  2. Anterolateral System:

    • Carries pain and thermal sensations,

    • Signals cross sides immediately after entering the spinal cord.

Dorsal Column-Medial Lemniscal Pathway

  • This pathway supports high localization, fine intensity gradation, and transmits vibrations and postural sensations.

Anterolateral Pathway

  • Processes pain, temperature, crude touch, tickle, and itch sensations with lower localization precision.

Somatosensory Cortex

  • Composed of two areas:

    • Somatosensory Area I: High degree of localization—important for identifying touch sensations across the body.

    • Somatosensory Area II: Integrates sensations but less localization specificity.

Cortical Representation

  • Body areas represented on the cortex vary in size; lips and fingertips have large areas due to high sensitivity.

Somatosensory Association Areas

  • Integrates signals from various sources, including other sensory cortices.

  • Involved in recognition and memory of sensations

Functions of Primary Somatosensory Cortex

  • Responsible for awareness and processing of somatic sensations.

  • Involved in recognizing tactile inputs, pain, and temperature.