SOMATOSENSORY SYSTEM OVERVIEW

Definition

• The somatosensory system is a network of neurons that enables the sensation and perception of various stimuli including touch, pressure, pain, temperature, position, movement, and vibration.

Function

• This system relays sensory information from peripheral areas of the body to the brain via the spinal cord, brainstem, and thalamus for processing in the somatosensory cortex located in the parietal lobe.

RECEPTOR CELLS

Characteristics

Definition

• Specialized cells within sensory organs that sense stimuli as energy.

Function

• The structure of the receptor cells dictates the type of energy or chemicals they respond to.

• Receptor cells filter certain stimuli while focusing on key sensory inputs.

• They convert energy from stimuli into changes in electrical potential across their membranes, leading to action potentials.

Types of Receptors

• Stimuli can be touch, light, sight, sound, and temperature.

• Stimuli receptors may look different in different sense organs.

• Touch Stimuli: Includes light touch, pressure, vibration, and pain.

• Receptor Types:

  • Meissner's (tactile) corpuscles: detect light touch.

  • Pacinian (lamellated) corpuscles: detect pressure and vibration.

  • Ruffini corpuscles: respond to skin stretching.

  • Merkel's discs: detect light touch and form.

  • Free Nerve endings: pain and temperature

In-Depth Explanation of Receptor Types in the Somatosensory System

1. Meissner's Corpuscles (Tactile Corpuscles)

• Location: Primarily located in the upper dermis of hairless skin (e.g., palms, soles).

• Function: Specialized for detecting light touch and changes in texture. They are sensitive to vibrations and are critical for fine tactile discrimination.

• Characteristics: Rapidly adapting receptors, meaning they respond quickly to changes but stop firing with sustained pressure.

2. Pacinian Corpuscles (Lamellated Corpuscles)

• Location: Deep in the dermis and hypodermis, and also found in joints and some visceral organs.

• Function: Detects deep pressure and high-frequency vibrations. They play a role in feeling texture during the active exploration of objects.

• Characteristics: Rapidly adapting receptors, meaning they quickly react to the immediate changes in pressure or vibration, but do not respond to constant pressure for a long duration.

3. Ruffini Corpuscles (Bulbous Corpuscles)

• Location: Found in the deep layers of the skin and in joint capsules.

• Function: Respond to skin stretching and sustained pressure. They provide information regarding finger position and the shape of objects held in hand.

• Characteristics: Slowly adapting receptors, meaning they continue to fire as long as the stimulus is present, helping to detect static positions.

4. Merkel's Discs (Tactile Discs)

• Location: Located in the basal epidermis and hair follicles.

• Function: Responsible for detecting light touch and form perception, especially in textures and edges.

• Characteristics: Slowly adapting receptors; they provide a sustained response to pressure, leading to the perception of form and texture over time.

5. Free Nerve Endings

• Location: Found throughout the body, particularly in the skin and mucous membranes.

• Function: Respond to pain (nociception) and temperature. They can also detect some light touch stimuli.

• Characteristics: They are the most numerous type of sensory receptors and can respond to various stimuli, but are not specialized for any specific type of sensation. They adapt very slowly, which is why they are persistent in responding to pain signals.

PATHWAYS OF SENSORY INFORMATION

• Transmission Flow: From sensory endings in the skin to spinal cord, then to the thalamus, and ultimately to the primary somatosensory cortex (S1) in the postcentral gyrus of the parietal lobe.

• Neural Pathway Details:

  1. Sensory endings generate action potentials in sensory neurons.

  2. Signals enter the spinal cord, synapsing with interneurons.

  3. They project to the thalamus and then to the cerebral cortex for conscious perception.

  4. The upper motor neuron sends commands to lower motor neurons, triggering muscle responses.

DORSAL COLUMN PATHWAY

• Role: The pathway for touch information that travels via dorsal columns from the spinal cord to the brain.

• Process: Non-reflexive sensory information transfers from the dorsal part of the spinal cord to the brain, allowing for cognitive processing and perception.

• Extra: It is an ascending tract which follows a certain path (axons from sensory surface, spinal cord, medulla (crossing over), midbrain, thalamus, and then to the primary somatosensory cortex.).

SENSORY CORTEX STRUCTURE AND FUNCTION

• Somatosensory Cortex (S1): Buried in the postcentral gyrus; responsible for processing touch information from the opposite side of the body.

• Organizational Pattern: The S1 region contains a body map (homunculus or little man) proportional to the density of sensory receptors; areas with more receptors (e.g., hands and fingers) have larger representations than less sensitive areas (e.g., shoulders).

• Extra: S1 gets touch information from the opposite side of the body. It also has different areas for processing different sensory information.

PLASTICITY OF THE SOMATOSENSORY CORTEX

• Adaptations: The somatosensory cortex can change due to experience (e.g., larger cortical areas for musicians' fingers) or loss of limbs (adjacent areas take over the space once occupied by receptors for the missing limb).

SENSORY MODALITIES

• Modality: Refers to the different types of sensations detected, including:

  • Touch (touch receptors)

  • Hearing (sound waves)

  • Sight (light)

  • Smell (chemical signals)

  • Pain (nociception)

• Labelled Lines Theory: Proposes that different types of sensory modalities are processed through specific neural pathways, allowing the brain to distinguish among various stimuli efficiently.

SENSORY TRANSDUCTION

Definition: Sensory transduction is the process by which sensory stimuli are converted into electrical signals in the nervous system. This process allows the nervous system to interpret different forms of sensory input from the environment.

Mechanism:

• Receptor Cells: Specialized cells (receptors) respond to specific types of energy (like light, sound, or chemical signals) and convert these stimuli into electrical impulses.

• Signal Conversion: The energy from stimuli leads to changes in the receptor’s membrane potential, which generates action potentials. These action potentials travel along sensory neurons to the brain for processing.

• Receptor Types: Different receptors are adapted to different modalities, such as photoreceptors in the retina for light, mechanoreceptors for touch, and chemoreceptors for taste and smell.

Importance: Sensory transduction is essential for perception, enabling organisms to interact with and respond to their environment by detecting a vast array of sensations such