Lab 9: Somatic Sensory Receptors

LAB 9: SOMATIC SENSORY RECEPTORS

CRANIAL NERVES AND SPINAL NERVES

  • The human body contains 12 cranial nerves:

    • Olfactory (I): Responsible for the sense of smell.

    • Optic (II): Responsible for vision.

    • Oculomotor (III): Controls eye movement and pupil size.

    • Trochlear (IV): Helps with eye movement.

    • Trigeminal (V): Responsible for facial sensation and motor functions such as biting and chewing.

    • Abducens (VI): Controls lateral eye movement.

    • Facial (VII): Responsible for facial expressions and taste sensations from the anterior two-thirds of the tongue.

    • Vestibulocochlear (VIII): Involved in hearing and balance.

    • Glossopharyngeal (IX): Taste and gag reflex; regulates blood pressure.

    • Vagus (X): Controls heart rate, gastrointestinal tract functions.

    • Accessory (XI): Provides motor command to sternocleidomastoid and trapezius muscles.

    • Hypoglossal (XII): Manages tongue movements for speech and swallowing.

SPINAL NERVES

  • 31 Spinal Nerves: Includes both sensory and motor fibers.

  • Division based on anatomy:

    • Cervical: C1 - C8

    • C1: Head and neck

    • C3: Diaphragm

    • C4: Deltoids, Biceps

    • C5: Wrist Extenders

    • C6:

    • C7: Triceps

    • C8: Hand

    • Thoracic: T1 - T12

    • T2 - T12: Thoracic Muscles

    • T6: Chest Muscles

    • T7: Abdominal Muscles

    • Lumbar: L1 - L5

    • L3: Leg Muscles

    • Sacral: S1 - S5

    • S2: Sacral nerves

    • S3: Bowel, Bladder

    • S4: Sexual Function

  • Coccygeal Nerve: Pertains to the coccyx (tailbone).

NERVOUS SYSTEM DIVISIONS

  • Central Nervous System (CNS): Comprising the brain and spinal cord.

  • Peripheral Nervous System (PNS): Comprising nervous tissue outside the CNS; includes:

    • Somatic Nervous System (SNS): Involved in voluntary control of skeletal muscles.

    • Autonomic Nervous System (ANS): Involved in involuntary functions; further divided into:

    • Parasympathetic Division: Promotes rest and digestion.

    • Sympathetic Division: Prepares the body for stress-related activities.

  • Afferent Division: Transmits sensory signals towards the CNS (Sensory Fibers).

  • Efferent Division: Transmits motor signals away from the CNS (Motor Fibers).

  • Mixed Spinal Nerves: Contain both afferent and efferent fibers.

SENSORY RECEPTORS

  • Receptors detect various stimuli:

    • Somatic Sensory Receptors: Monitor skeletal muscles, joints, and skin surface.

    • Visceral Sensory Receptors: Monitor internal organs.

    • Special Sensory Receptors: Monitor smell, taste, vision, balance, and hearing.

  • Types of receptors include:

    • Nociceptors: Pain detection.

    • Thermoreceptors: Temperature detection (heat and cold).

    • Mechanoreceptors: Detect touch, pressure, and vibration.

RECEPTORS IN SKIN

  • Various types of mechanoreceptors found in the skin:

    • Free Nerve Endings: Respond to pain, heat, and cold.

    • Merkel Disks: Detect light touch.

    • Meissner Corpuscles: Detect changes in texture and light touch.

    • Pacinian Corpuscles: Detect pressure and vibration.

    • Krause End Bulbs: Possibly involved in touch sensation.

    • Ruffini Endings: Respond to skin stretch and pressure.

RECEPTIVE FIELDS

  • Receptive Field: Area monitored by a single receptor; the ability to perceive stimuli depends on receptor density.

    • First Order Neurons: Sensory neurons that first detect a stimulus; axons carry the signal to the spinal cord.

DERMATOMES

  • Dermatome: An area of skin served by a single spinal nerve; all sensory information from a dermatome travels through the corresponding spinal nerve.

NEURONAL RELAY IN SENSORY PATHWAYS

  • Second Order Neurons: Interneurons that relay signals through the CNS.

  • Third Order Neurons: Located in the thalamus; carry signals to the somatosensory cortex.

  • Sensory information crosses the midline of the body as it travels to the brain.

RECEPTORS AND STIMULI ADAPTATION

  • Tonic Receptors: Always active, producing action potentials at a frequency related to stimulus intensity.

    • Change in stimulus causes a change in the rate of action potential generation.

  • Phasic Receptors: Normally inactive, generate action potentials only when conditions they sense change (i.e., during adaptation).

THERMORECEPTORS

  • Different thermoreceptors monitor warm and cold sensations:

    • More cold receptors compared to warm receptors:

    • Cold detection: up to 50°C (e.g., room temperature).

    • Warm detection: sensitive up to 100°C as well as adaptation to decreasing temperatures.

TWO POINT DISCRIMINATION TEST

  • A method used to test touch sensitivity:

    • For accurate discrimination of two closely spaced points, receptors need to be densely packed, resulting in small receptive fields.

  • Procedure includes:

    • Constant light pressure, no pain or blanching of skin.

    • Assessment is based on consistency over time.

  • Common Two Point Discrimination thresholds:

    • Fingertips: 2-8 mm

    • Lips: 2-4 mm

    • Palms: 8-12 mm

    • Forearm: 30-44 mm

SOMATOSENSORY CORTEX

  • The somatosensory cortex contains the cell bodies of CNS neurons that receive input from various body areas.

  • Represents a topographic map of the body, corresponding to the density of sensory receptors; areas with higher receptor density are represented more prominently.

FINE TOUCH SENSITIVITY

  • Evaluates how well subjects can detect shapes and translate them into visual contexts.