Clinical Assessment and Physiology of Human Reflexes

I. Reflex Activity: Definition and Physiology

• Reflex Definition: An involuntary, automatic response to a peripheral stimulus, involving either motor or secretory action.
• Function: Crucial for adaptation to internal and external changes.
• Dependence: Requires the integrity of the reflex arc; disruption at any level abolishes the reflex.

II. The Reflex Arc: Components

The reflex arc consists of five essential components:

1. Receptor Organ: Transforms a stimulus into nerve impulses.
2. Afferent Neuron: Conducts the impulse to the Central Nervous System (CNS).
3. Integration Center: Typically located in the spinal cord or brainstem; processes signals via synapses.
4. Efferent (Motor) Neuron: Sends the command from the CNS to the effector.
5. Effector: The muscle or gland that executes the response.

Additional Concepts

• Synapse: A neurochemical junction (e.g., involving Acetylcholine (Ach) or GABA).
• Reflexogenic Area: The specific zone where receptor stimulation elicits a reflex response.

III. Reflex Classification

A. Based on Receptor Localization

• Exteroceptive: Stimulus originates from the skin or mucosa.
• Interoceptive: Stimulus originates from internal organs.
• Proprioceptive: Stimulus originates from muscles, tendons, or ligaments.

B. Based on Synaptic Connections

• Monosynaptic: Direct connection between afferent and efferent neurons.
• Polysynaptic: Involves one or more interneurons.

C. Based on CNS Location

• Spinal reflexes
• Brainstem reflexes
• Cortical reflexes

D. Based on Efferent Organs

• Somatic: Involves skeletal muscle.
• Autonomic: Involves glands and smooth muscles.

E. Clinical Classification

• Superficial reflexes
• Deep Tendon (Myotatic) reflexes
• Visceral reflexes
• Pathologic (Abnormal) reflexes

IV. Reflex Testing: Methodology

• Testing requires a systematic and symmetrical assessment.
• Always compare the left side versus the right side.
• Reflex testing provides objective, low-effort diagnostic information.

V. Superficial Reflexes (Polysynaptic)

1. Mucosal Reflexes

• Corneal Reflex: Blinking response (mediated by Trigeminal and Facial nerves).
• Pharyngeal (Gag) Reflex: Soft palate contraction (mediated by Glossopharyngeal and Vagus nerves).
• Nasal Reflex: Sneezing response.

2. Abdominal Reflexes

Segmental mapping:

• Upper: T7–T8
• Middle: T9–T10
• Lower: T11–T12

Absence of abdominal reflexes is considered an early sign of Upper Motor Neuron (UMN) lesion.

3. Cremasteric Reflex

Stimulation of the inner thigh leads to testis elevation (mediated by L1-L2 via the Genitofemoral nerve).

4. Plantar Reflex

• Normal Response: Toe flexion.
• Pathologic Response: The Babinski sign (great toe extension and fanning of the other toes).

VI. Deep Tendon Reflexes (Monosynaptic Stretch Reflexes)

Sensory Receptors Involved

• Muscle Spindles: Detect muscle stretch and rate of change in length.
• Golgi Tendon Organs: Monitor muscle tension.

Underlying Mechanisms

• Reciprocal Inhibition: Antagonist muscles are simultaneously inhibited during agonist contraction.
• Autogenic Inhibition: Mediated via Golgi afferents to prevent excessive muscle contraction or tension.

Clinical Examples and Nerve Roots

• Biceps Reflex: C5–C6 (Musculocutaneous nerve)
• Triceps Reflex: C7–C8 (Radial nerve)
• Patellar Reflex (Knee Jerk): L2–L4 (Femoral nerve)
• Achilles Reflex (Ankle Jerk): S1–S2 (Tibial nerve)
• Jaw Jerk Reflex: Trigeminal nerve
• Hoffman/Trömner Signs: Pathological finger flexion tests (often grouped with pathological reflexes, but listed here as examples of testing methods).

VII. Reflex Abnormalities

A. Quantitative Changes in Reflex Grading

• Hyporeflexia Causes: Lower Motor Neuron (LMN) lesion, myopathy, or vitamin deficiency.
• Hyperreflexia Causes: Upper Motor Neuron (UMN) lesion, anxiety, or thyrotoxicosis.

B. Qualitative Changes (Pathological Reflexes)

1. Primitive Reflexes

These reflexes are normal in infants but are abnormal if present in adults, often indicating diffuse cerebral dysfunction.

• Sucking, Snouting, Rooting
• Palmar-mental, Grasp, Moro, Tonic Neck

2. Extensor Plantar Response Group

• The Babinski Sign: Indicates a corticospinal tract lesion.
• Similar pathological responses are elicited by the Chaddock, Oppenheim, and Gordon maneuvers.

3. Pathologic Finger/Toe Flexion (Rossolimo Group)

Tapping elicits abnormal flexion (e.g., Hoffman Sign, Mendel-Bechterew Reflex).

4. Spinal Automatism

Reflexes present following complete spinal cord transection (e.g., Marie-Foix and Remak's reflexes).

VIII. Clinical Significance of Reflex Findings

• Asymmetry: Always indicates an abnormal finding requiring further investigation.
• Absent Deep Tendon Reflexes: Suggests a Lower Motor Neuron (LMN) issue or interruption of the reflex arc.
• Increased Tendon Reflexes (Hyperreflexia): Suggests an Upper Motor Neuron (UMN) lesion.
• Absent Superficial Reflexes combined with Exaggerated Deep Reflexes: Highly indicative of a Pyramidal Tract Lesion.
• Pathologic Reflexes (e.g., Babinski sign): Always abnormal in adults and signify neurological dysfunction.