Reflex arc

Also known as: reflex, stretch reflex, myotatic reflex, withdrawal reflex, crossed extensor reflex, deep tendon reflex

The neural pathway of a reflex: a rapid, involuntary, stereotyped response to a stimulus. It runs receptor → afferent neuron → integrating centre → efferent neuron → effector.

Overview

A reflex arc has five components: (1) a receptor; (2) a sensory (afferent) neuron, with its cell body in a DRG or cranial ganglion; (3) an integration centre in the CNS, either a single synapse (monosynaptic) or interneurons (polysynaptic); (4) a motor (efferent) neuron; and (5) an effector, which may be skeletal muscle (somatic reflex) or smooth muscle, cardiac muscle or gland (autonomic reflex, with a two-neuron efferent limb). Spinal reflexes survive transection above their segments, and descending pathways continuously modulate their gain.

The stretch (myotatic) reflex is the only monosynaptic reflex in humans and the basis of tendon jerks. A tap stretches the muscle and its muscle spindles. Ia afferents fire and synapse directly on α motor neurons of the same (homonymous) and synergist muscles, which contract. Simultaneously, Ia collaterals excite Ia inhibitory interneurons that inhibit the antagonist's motor neurons (reciprocal inhibition, disynaptic). γ motor neurons keep spindles taut during contraction (α–γ co-activation), so sensitivity is preserved.

The Golgi tendon (inverse myotatic) reflex works through Ib afferents from Golgi tendon organs, which sense tension. They excite inhibitory interneurons that relax the same muscle (autogenic inhibition), protecting against excessive force; this also explains the clasp-knife phenomenon. The withdrawal (flexor) reflex is polysynaptic. Nociceptor input recruits flexors of the stimulated limb across several segments. The crossed extensor reflex extends the opposite limb to support body weight.

Parts & subdivisions

Receptor

Detects the stimulus: the muscle spindle (stretch), Golgi tendon organ (tension) or nociceptor (withdrawal).

Afferent neuron

Pseudounipolar DRG neuron: Ia (spindle), Ib (tendon organ), or Aδ and C (nociceptive).

Integrating centre

Spinal or brainstem grey matter. Monosynaptic in the stretch reflex; polysynaptic, with excitatory and inhibitory interneurons, in all others.

Efferent neuron

The α motor neuron (somatic). For autonomic reflexes, a preganglionic plus a postganglionic neuron.

Effector

Skeletal muscle (somatic reflexes) or smooth muscle, cardiac muscle and glands (visceral reflexes such as the baroreceptor and micturition reflexes).

Reciprocal inhibition

Ia afferents excite agonist motor neurons and, via Ia inhibitory interneurons, inhibit antagonist motor neurons, so opposing muscles do not fight.

Crossed extensor reflex

Nociceptive input crosses the cord via commissural interneurons to excite contralateral extensors, so you can withdraw one foot from a tack while standing on the other.

Function

Fast, automatic protection, for example withdrawal from pain.

Maintenance of muscle tone and posture through stretch reflexes.

Automatic regulation of visceral function (baroreceptor, micturition and defecation reflexes).

Clinically, a test of the integrity of specific segments and of descending control.

Physiology

Reflex latency depends on conduction distance and the number of synapses, each adding ≈0.5 ms. The knee jerk's monosynaptic central delay is <1 ms. Descending pathways set reflex gain, mainly by tonic inhibition from the cortex and brainstem. UMN lesions remove this inhibition, giving hyperreflexia and clonus. LMN or afferent lesions break the arc, giving hypo- or areflexia.

Clinical significance

Interpreting tendon reflexes

Hyperreflexia, clonus and spreading indicate an UMN lesion (stroke, cord compression, MS). Absent reflexes indicate an LMN, root or afferent lesion (neuropathy, GBS, radiculopathy, cauda equina) or spinal shock. Brisk reflexes can be normal in an anxious patient; asymmetry and the plantar response help. Hypothyroidism causes slow-relaxing ("hung-up") ankle jerks.

Babinski sign

Stroking the lateral sole normally produces plantar flexion of the toes. In an UMN lesion the great toe extends and the other toes fan. It is normal in infants up to about 1–2 years, before corticospinal myelination is complete.

Test yourself

Which tendon reflex principally tests the S1 root?

Biceps
Triceps
Patellar (knee)
Achilles (ankle)

Answer: Achilles (ankle)

"S1, 2 buckle my shoe" refers to the ankle jerk. The knee jerk tests L3–L4, the biceps reflex C5–C6 and the triceps reflex C7–C8.

The afferent and efferent limbs of the corneal reflex are carried by:

II and III
V1 and VII
V2 and V3
IX and X

Answer: V1 and VII

Touching the cornea stimulates the nasociliary branch of V1. Both facial nerves (VII) then close the eyelids through orbicularis oculi, so the response is bilateral.