Study Notes on Touch and Proprioception

Touch and Proprioception

Sensation

  • Definition: Conversion of light, sound, touch, taste, or smell into a neural signal.

  • Process:

    • Information is conveyed to the brain via AFFERENT signaling.

    • In the brain, the signal represents:

    • QUALITATIVE nature of the stimulus (what kind of stimulus is it?)

    • QUANTITATIVE nature of the stimulus (how much of it is there?)

  • Focus: The somatosensory system which includes:

    • Touch

    • Pressure

    • Vibration

    • Limb position

    • Temperature

    • Itch

Potential Study Resources

  • Peripheral somatosensation: Khan Academy resource for MCAT preparation available here.

Mechanosensory Afferents

  • Mechanosensory afferents are detected by receptors that transmit signals to the brain regarding various stimuli like pressure and vibration.

    • Example: Dorsal Root Ganglion (DRG)

    • Structure includes:

      • Terminal branch (peripheral branch)

      • Central branch

      • Cell body which is pseudounipolar

      • The axon bifurcates with one branch innervating peripheral tissue and the other connecting to the central nervous system.

Types of Sensory Receptors

  1. Ruffini's endings: Detects stretch.

  2. Pacinian corpuscles: Detects deep pressure and vibration.

  3. Krause's end bulbs: Involved in sensing cold temperatures.

  4. Meissner's corpuscles: Sensitive to light touch and vibration.

  5. Merkel disks: Responsible for sensing steady pressure and texture.

  6. Root hair plexus: Detects hair movement.

  7. Free nerve endings: Responsible for detecting temperature, pain, and crude touch.

Sensory Function

  • Understanding each receptor type's response:

    • Temperature: Detected by free nerve endings and Krause's end bulbs.

    • Pressure sensitive: Detected by Ruffini's endings and Pacinian corpuscles.

    • Fine touch: Detected by Meissner's corpuscles and Merkel disks.

    • Pain: Detected via free nerve endings.

Receptor Adaptation and Fields

  • Adaptation types:

    • Rapidly Adapting (e.g., Meissner and Pacinian corpuscles): Responds quickly to changes in stimuli but not to constant stimuli.

    • Slowly Adapting (e.g., Merkel cells, Ruffini endings): Continuous response to sustained stimuli.

  • Receptive field: The area in which a stimulus will affect a receptor, differing by receptor type (small for fine touch and large for deep pressure).

Sensory Pathways

  • Sensory Transduction: The process by which sensory stimuli are converted into electrical signals.

    • Involves changes in the permeability of cation channels leading to depolarization and creating a receptor potential.

    • Important ion channels involved are Piezo1 and Piezo2.

  • Somatosensation Processing Pathways:

    • Dorsal Column (DC)/ Medial Lemniscal (ML) pathway for mechanosensory information.

    • Anterolateral system for pain and temperature.

  • Trigeminal System: Processes mechanosensory, pain, and temperature information for the face.

Proprioception

  • Definition: The sense of body position and movement.

  • Involves receptors having a low-threshold mechanoreceptor that facilitates the understanding of limb positions and movements.

  • Joint Receptors Types:

    • Resemble LTMRs (Low-Threshold Mechanoreceptors) that innervate the skin. There are five types of joint receptors.

    • Important for motor control, primarily in fine motor areas (e.g., fingers).

  • Activated with great mechanical force influencing the sensation of pain rather than proprioception in less critical areas.

Proprioception Mechanism

  • Information from mechanoreceptors about the position of limbs is picked up and sent to the CNS. Encompasses:

    • Golgi tendon organs (tension)

    • Muscle spindle fibers (muscle stretch)

Importance in Neurological Conditions

  • Implications in conditions like:

    • Friedreich's ataxia: interfacing with loss of large myelinated neurons in DRG.

    • Parkinson's disease: Related to dopamine cell loss.

  • Propriosensory dysfunctions notable in Autism Spectrum Disorder leading to altered sensory perceptions.

Class Activities

  • A group exercise is planned for hands-on experiences with mechanoreceptors.

  • Discussion planned in the next class regarding sensations of pain and temperature processing procedures.

Future Topics

  • Review and prepare for next discussions on pain and temperature processing including the somatosensory implications in behavioral scenarios.