Q5 LONG ANSWER 10M ★ High-yield

Define reflex action. Classify reflexes and describe their properties. Describe the stretch reflex and the inverse stretch reflex.

Physiology · 1st Professional — Pre-Clinical

Definitions

Reflex action = an involuntary, stereotyped, purposive response to a stimulus, mediated through the central nervous system.Reflex arc = the anatomical pathway of a reflex.

Components of the Reflex Arc

Receptor → Afferent (sensory) neurone → Centre (with or without interneurones) → Efferent (motor) neurone → Effector

Classification

Basis	Types
Number of synapses	Monosynaptic (stretch reflex — the only one) and polysynaptic
Development	Unconditioned (inborn) and conditioned (acquired — Pavlov)
Site of receptor	Superficial (skin — plantar, abdominal, corneal) and deep (tendon — knee, ankle, biceps)
Response	Somatic and visceral (autonomic)
Level of centre	Spinal, medullary, midbrain, cortical

Properties of Reflexes

Adequate stimulus — each reflex has its own
Reflex time — the delay is chiefly synaptic delay; shortest in monosynaptic reflexes
Summation, temporal and spatial
Occlusion and subliminal fringe
Reciprocal innervation — agonist contracts while antagonist relaxes
Final common path (Sherrington) — the anterior horn cell is the final route for all influences
Fatigue — at the synapse
Rebound phenomenon and after-discharge
Irradiation — a strong stimulus spreads to more motor neurones

Stretch Reflex (myotatic Reflex)

Stretch reflex = contraction of a muscle in response to its own stretch.It is the only monosynaptic reflex in the body.

Muscle stretched (tendon tapped) → Muscle spindle stimulated → Ia afferent fibre → directly excites α motor neurone (one synapse) → Extrafusal fibres contract → Muscle shortens

Two components — phasic (dynamic), tested clinically as the tendon jerk; and tonic (static), which maintains muscle tone and posture
Reflex time is only 19–24 ms for the knee jerk
Reciprocal inhibition of the antagonist occurs through an interneurone

Inverse Stretch Reflex

Receptor — Golgi tendon organ, in the tendon, arranged in series with the muscle
Stimulated by tension, whereas the spindle responds to length
Afferent — Ib fibre; disynaptic, through an inhibitory interneurone
Response — relaxation of the same muscle — autogenic inhibition
Function — protects against excessive tension; produces the clasp-knife effect in spasticity

Withdrawal (flexor) Reflex

Polysynaptic, protective; a painful stimulus causes flexion and withdrawal
Shows irradiation, after-discharge and fatigue
Crossed extensor reflex — the opposite limb extends to support the body weight

Superficial and Visceral Reflexes

Reflex	Stimulus	Segment
Corneal	Touching the cornea	Pons (V afferent, VII efferent)
Abdominal	Stroking the abdominal wall	T7–T12
Cremasteric	Stroking the inner thigh	L1–L2
Plantar	Stroking the lateral sole	L5–S2
Anal	Pricking the perianal skin	S4–S5
Light reflex	Light in the eye	Midbrain

Superficial reflexes are polysynaptic and are lost in both upper and lower motor neurone lesions — unlike deep reflexes, which are exaggerated in UMN and lost in LMN lesions

Conditioned Reflexes

Acquired by learning, not inborn (Pavlov)
A conditioned stimulus (bell) is repeatedly paired with an unconditioned stimulus (food) until the bell alone produces the response
Requires an intact cerebral cortex
Extinction occurs if the conditioned stimulus is repeatedly given without reinforcement
Basis of habit formation and learning

Applied Aspects

Tendon jerks test the integrity of the reflex arc and localise the level — biceps C5–C6, triceps C7–C8, knee L2–L4, ankle S1–S2
Exaggerated jerks → UMN lesion (loss of descending inhibition); lost jerks → LMN lesion or interruption of the arc anywhere
Jendrassik manoeuvre — clenching the teeth or pulling interlocked fingers reinforces a sluggish jerk by raising γ motor neurone activity
Clonus — rhythmic contractions on sustained stretch; indicates a pyramidal lesion

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