Neurological Reflexes and Somatosensory Pathways Explained

Understanding Reflexes

A reflex is an involuntary motor response. The reflex arc follows this path: receptor → afferent nerve → CNS → efferent nerve → effector.

Deep Tendon Reflexes (Monosynaptic)

• Biceps (C5–C6, musculocutaneous): Elbow flexion.
• Triceps (C6–C7, radial): Elbow extension.
• Brachioradial (C5–C6, radial): Elbow flexion.
• Patellar (L3–L4, femoral): Knee extension.
• Achilles (S1–S2, tibial): Plantar flexion.

Superficial Reflexes (Polysynaptic)

• Corneal: CN V–VII.
• Abdominal: T7–T12.
• Cremasteric: L1–L2.
• Anal: S4–S5.

Plantar Reflex and Lesions

The normal plantar reflex results in toe flexion. A Babinski sign (big toe extension + fanning) indicates an Upper Motor Neuron (UMN) lesion.

• UMN Lesions: Hyperreflexia, clonus, Babinski sign.
• LMN Lesions: Hyporeflexia or areflexia, hypotonia.

Pathological reflexes: Oral automatisms (snout, suck, palmomental) suggest frontal lobe involvement. Reflex asymmetry is always considered pathological.

Somatosensory Pathways

These pathways convey touch, pain, temperature, vibration, and proprioception.

Receptors

• Mechanoreceptors: Touch, pressure, vibration (Meissner, Merkel, Pacinian, Ruffini).
• Proprioceptors: Position, movement (muscle spindle, Golgi tendon organ).
• Nociceptors: Pain.
• Thermoreceptors: Temperature.

DCML (Posterior Columns)

Modalities include fine touch, vibration, proprioception, and 2-point discrimination. The 1° neuron enters via the posterior root and ascends ipsilaterally via the fasciculus gracilis (lower, medial) or fasciculus cuneatus (upper, lateral). It synapses in the medulla, decussates, and ascends as the medial lemniscus to the VPL and cortex.

Spinothalamic (Anterolateral)

Modalities include pain, temperature, and crude touch. The 1° neuron enters via the posterior root, synapses in the dorsal horn, decussates in the spinal cord, and ascends contralaterally to the VPL and cortex.

Spinal Cord and Brainstem Lesions

Spinal ganglion lesion: Severe pain and paresthesias in the affected dermatome.

Spinal Cord Sensory Syndromes

• Posterior horn: Ipsilateral pain and temperature loss at the level.
• Posterior columns: Loss of vibration and proprioception; sensory ataxia; positive Romberg test.
• Brown-Séquard (hemisection): Ipsilateral DCML loss, contralateral pain/temperature loss, and ipsilateral UMN signs below the lesion.
• Complete transverse lesion: All modalities lost below the lesion.

Brainstem Lesions

• Lateral medullary (Wallenberg): Ipsilateral face pain/temp loss; contralateral body pain/temp loss.
• Medial medullary: Contralateral loss of position/vibration.
• Upper brainstem: Contralateral loss of all sensory modalities (face and body).

Nociception and Pain Mechanisms

Nociceptors are free nerve endings categorized by stimulus: Mechanical (Aδ), Thermal (Aδ), Polymodal (C), and Silent (inflammation).

Nerve Fibers

• Aδ: Lightly myelinated, fast, sharp, well-localized (“1st pain”).
• C: Unmyelinated, slow, dull/burning (“2nd pain”), emotional.

Types of Pain

• Nociceptive: Somatic (sharp, localized) or Visceral (dull, poorly localized, referred).
• Neuropathic: Burning, electric, shock-like; associated with allodynia and hyperalgesia.

Sensitization: Peripheral sensitization lowers the threshold via inflammatory mediators, while central sensitization (wind-up) involves NMDA receptors.

Pain Modulation: Descending inhibition involves the PAG and RVM using 5-HT, NA, and endorphins. Opioids increase this inhibition.

Referred Pain: Occurs due to visceral and somatic convergence (e.g., diaphragm to shoulder, heart to left arm/jaw).