14 REFLEX ARCS
# 15 REFLEX ARCS
A reflex is a stereotyped, involuntary motor response to a sensory stimulus. Examiners love them because they package neuroanatomy, cranial nerves and cord segments into one testable concept. Name the afferent, efferent and centre — you can usually answer the question.
Components of a reflex arc
Every reflex uses the same five components:
➡ Receptor — detects the stimulus (muscle spindle, free nerve ending, photoreceptor).
➡ Afferent (sensory) limb — carries the signal to the CNS.
➡ Integrating centre — spinal cord segment or brainstem nucleus where the synapse occurs.
➡ Efferent (motor) limb — carries the command back out.
➡ Effector — the muscle or gland that produces the response.
👩⚕️ Damage anywhere along this chain abolishes the reflex. Localising a lesion is simply working out which link is broken.
Monosynaptic vs polysynaptic reflexes
| Feature | Monosynaptic | Polysynaptic |
|---|---|---|
| Synapses | One (afferent → motor neuron) | Two or more (interneurons) |
| Latency | Very short | Longer |
| Example | Stretch reflex (knee, biceps jerk) | Withdrawal, corneal, abdominal |
| Stimulus | Muscle stretch (Ia spindle afferent) | Noxious / cutaneous / visual / mucosal |
| Response | Same muscle contracts | Multi-muscle coordinated |
The stretch (myotatic) reflex is the simplest: a spindle senses lengthening, a Ia afferent synapses directly onto an alpha motor neuron in the ventral horn, and the muscle contracts. Antagonist reciprocal inhibition runs through an interneuron (polysynaptic).
The flexor withdrawal reflex is the classic polysynaptic example — step on a pin and interneurons coordinate ipsilateral flexion and crossed extension.
Spinal reflex levels
Deep tendon reflexes test specific cord segments — memorise in order.
| Reflex | Segment(s) | Peripheral nerve |
|---|---|---|
| Biceps jerk | C5–C6 | Musculocutaneous |
| Supinator (brachioradialis) | C5–C6 | Radial |
| Triceps jerk | C7 (–C8) | Radial |
| Abdominal reflexes (upper) | T8–T10 | Intercostal |
| Abdominal reflexes (lower) | T10–T12 | Intercostal / subcostal |
| Cremasteric reflex | L1–L2 | Ilioinguinal (afferent) / genitofemoral (efferent) |
| Knee jerk (patellar) | L3–L4 | Femoral |
| Ankle jerk (Achilles) | S1 (–S2) | Tibial |
| Plantar reflex | S1–S2 | Tibial |
| Bulbocavernosus | S2–S4 | Pudendal |
| Anal wink (anocutaneous) | S3–S4 | Inferior rectal |
👩⚕️ Counting mnemonic: S1,2 ankle; L3,4 knee; C5,6 biceps + supinator; C7 triceps; T8–12 abdominals. Ascending the cord, the numbers run in order.
Cremasteric and abdominal reflexes are superficial polysynaptic (cutaneous) reflexes — they are lost early in UMN disease, before deep tendon reflexes become brisk.
Cranial nerve reflexes
Highest-yield reflexes in the exam. Learn afferent, efferent and centre.
| Reflex | Afferent | Efferent | Centre |
|---|---|---|---|
| Pupillary light | CN II | CN III (parasympathetic) | Midbrain (pretectal + Edinger-Westphal nuclei) |
| Accommodation | CN II → visual cortex | CN III | Visual cortex → Edinger-Westphal |
| Corneal (blink) | CN V1 | CN VII | Pons |
| Jaw jerk | CN V3 | CN V3 (motor) | Pons (mesencephalic nucleus of V) |
| Gag | CN IX | CN X | Medulla |
| Baroreceptor | CN IX (carotid sinus) | CN X | Medulla (nucleus tractus solitarius) |
| Oculocardiac | CN V1 | CN X | Medulla |
Pupillary light reflex — the examiner's favourite
Light → retina → CN II → optic chiasm → bilateral pretectal nuclei → bilateral Edinger-Westphal nuclei → preganglionic parasympathetics in CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae.
Because the pretectal nucleus projects to both EW nuclei, light in one eye constricts both pupils — direct in the lit eye, consensual in the other.
👩⚕️ "2 in, 3 out." A blind eye still gives a consensual response when its fellow is lit; a CN III palsy fixes that pupil dilated whichever side you shine.
Accommodation reflex
Far-to-near focusing gives a triad: pupil constriction, lens thickening (ciliary muscle), convergence (medial recti). All CN III-driven, but routed via the visual cortex first. Hence the Argyll Robertson pupil in neurosyphilis — accommodates but does not react to light (dorsal midbrain afferent selectively damaged).
Corneal reflex
V1 carries corneal sensation to the spinal trigeminal nucleus in the pons; interneurons project bilaterally to both facial nuclei, driving bilateral orbicularis oculi contraction via CN VII.
A unilateral CN V lesion abolishes blink on either side when the affected cornea is touched (no afferent input). A CN VII lesion abolishes blink on its own side whichever cornea you touch. 👩⚕️ "5 in, 7 out."
Jaw jerk
Unique — CN V is both afferent and efferent. Masseter spindles signal via V3 to the mesencephalic nucleus of V (the only primary sensory neurons inside the CNS), projecting to the motor nucleus of V. An exaggerated jaw jerk = bilateral UMN lesion above the pons (pseudobulbar palsy).
Gag and baroreflex
Gag: 👩⚕️ "9 in, 10 out" — CN IX afferent from posterior pharynx, CN X efferent to pharyngeal constrictors. The baroreflex uses the same pair from the carotid sinus: BP rise stretches baroreceptors, CN IX fires to nucleus tractus solitarius, CN X slows the heart. Hence carotid sinus massage for SVT, and syncope from tight collars.
UMN vs LMN signs
A central question in every reflex SBA is: is the lesion upper or lower motor neuron?
| Feature | Upper motor neuron | Lower motor neuron |
|---|---|---|
| Tone | Increased (spasticity, clasp-knife) | Decreased (flaccid) |
| Reflexes | Hyperreflexia, clonus | Diminished or absent |
| Plantar response | Extensor (Babinski positive) | Flexor or absent |
| Power | Pyramidal weakness (extensors weak in arm, flexors weak in leg) | Segmental weakness |
| Wasting | Mild (disuse) | Marked |
| Fasciculations | Absent | Present |
| Superficial abdominal/cremasteric reflex | Absent (lost early) | Lost only if local segment damaged |
👩⚕️ An inverted reflex localises the lesion — e.g. absent biceps jerk with brisk triceps jerk = LMN damage at C5–C6 plus UMN signs below. Babinski is normal up to 12 months (incomplete myelination); in adults it signals corticospinal disease.
Clinical applications
- Spinal cord injury: return of the bulbocavernosus reflex ends spinal shock. Absent anal wink + saddle anaesthesia = cauda equina — urgent MRI.
- Brainstem death: absent pupillary, corneal, gag and oculovestibular reflexes are core criteria.
- Oculocardiac reflex (V1 in, X out): extraocular traction → bradycardia in squint surgery — release traction, give atropine.
- Carotid surgery: sinus manipulation triggers baroreflex bradycardia — infiltrate with local anaesthetic.
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Test yourself
A patient presents with asymmetrical pupils and abnormal light reflex. Where is the lesion most likely?
- ((Edinger-Westphal nucleus::☑️ Midbrain parasympathetic nucleus — efferent limb via CN III to sphincter pupillae.))
- ((Hippocampus::Memory consolidation; no role in pupillary pathway.))
- ((Lateral geniculate body::Visual relay for conscious vision; reflex fibres leave the tract before LGN.))
- ((Locus coeruleus::Pontine noradrenergic nucleus for arousal; not pupillary.))
- ((Posterior limb of internal capsule::Carries corticospinal fibres — lesion gives hemiplegia, not pupil signs.))
👩⚕️ "2 in, 3 out" — CN II afferent, CN III efferent.
What is the afferent limb of the gag reflex?
- ((Glossopharyngeal nerve (CN IX)::☑️ CN IX senses posterior pharynx; CN X is the efferent limb.))
- ((Vagus nerve (CN X)::This is the EFFERENT limb — pharyngeal muscle contraction.))
- ((Trigeminal nerve (CN V)::Afferent for corneal and jaw jerk reflexes, not the gag.))
- ((Hypoglossal nerve (CN XII)::Pure motor to tongue — no sensory component.))
- ((Facial nerve (CN VII)::Taste and facial expression; not part of the gag arc.))
A patient has an absent corneal reflex on the left. Touching the left cornea produces no blink on either side, but touching the right cornea produces bilateral blinking. Where is the lesion?
- ((Left trigeminal nerve (CN V)::☑️ Afferent limb broken on the left; right side still drives bilateral blink.))
- ((Left facial nerve (CN VII)::Would abolish only LEFT blink regardless of which cornea is stimulated.))
- ((Right trigeminal nerve (CN V)::Would abolish responses when touching the RIGHT cornea, not the left.))
- ((Pons::Possible, but a pure peripheral pattern localises to the nerve, not the brainstem.))
- ((Left optic nerve (CN II)::CN II serves the pupillary light reflex, not the corneal.))
👩⚕️ "5 in, 7 out." Trace where the response fails to localise the lesion.
A 45-year-old man has an absent jaw jerk reflex. Which nerve supplies BOTH the afferent and efferent limbs?
- ((Trigeminal nerve (CN V)::☑️ V3 carries spindle afferents AND motor to muscles of mastication.))
- ((Facial nerve (CN VII)::Supplies muscles of facial expression, not mastication.))
- ((Glossopharyngeal nerve (CN IX)::Afferent for gag reflex; no masticatory role.))
- ((Hypoglossal nerve (CN XII)::Pure motor to tongue.))
- ((Accessory nerve (CN XI)::Supplies SCM and trapezius only.))
👩⚕️ Exaggerated jaw jerk = bilateral UMN lesion above the pons (pseudobulbar palsy).
Which spinal level mediates the cremasteric reflex?
- ((L1–L2::☑️ Ilioinguinal afferent, genital branch of genitofemoral efferent.))
- ((S1–S2::Ankle jerk level.))
- ((L3–L4::Knee jerk level.))
- ((T8–T12::Abdominal reflexes.))
- ((S3–S4::Anal wink and bulbocavernosus.))
A patient with a suspected spinal cord injury has an absent anal wink reflex. Which nerve roots are being tested?
- ((S3–S4::☑️ Inferior rectal nerve — afferent and efferent; conus/cauda lesions abolish it.))
- ((L1–L2::Cremasteric reflex.))
- ((L3–L4::Patellar tendon reflex.))
- ((S1–S2::Ankle jerk — close, but not the anal wink.))
- ((T10–T12::Lower abdominal reflexes — far too high for sacral.))
👩⚕️ Return of the bulbocavernosus reflex marks the end of spinal shock; persisting deficits after that are usually permanent.
Revision summary
- Five components: receptor → afferent → centre → efferent → effector. Lesion anywhere abolishes the reflex.
- Monosynaptic = stretch (knee, ankle, biceps). Polysynaptic = everything else.
- Spinal levels (ascending): S1,2 ankle; L3,4 knee; L1,2 cremaster; T8–12 abdominals; C5,6 biceps + supinator; C7 triceps; S3,4 anal wink.
- Cranial reflexes: 2 in/3 out light; 5 in/7 out corneal; V in/V out jaw jerk; 9 in/10 out gag and baroreflex.
- Light reflex is bilateral (pretectal projects to both EW nuclei). Argyll Robertson = accommodation preserved, light reflex lost.
- UMN = hyperreflexia, clonus, Babinski up, no fasciculations. LMN = absent reflexes, flaccid, wasting, fasciculations.
- Inverted reflex localises the lesion to the segment of the absent reflex with UMN signs below.
- Cauda equina red flag: absent anal wink + saddle anaesthesia → urgent MRI.