Spinal reflexes

The muscle spindle

• Lies in parallel with the main (extrafusal) fibres, inside a capsule.
• Intrafusal fibres: nuclear bag fibres (dynamic and static) and nuclear chain fibres (static).
• Sensory endings: group Ia (primary, annulospiral) respond to the rate and amount of stretch; group II (secondary, flower-spray) signal sustained length.
• Motor supply: gamma motor neurones contract the polar ends of intrafusal fibres, keeping the central sensory region taut. During voluntary contraction gamma neurones fire with alpha neurones (alpha–gamma coactivation), so the spindle stays sensitive while the muscle shortens. Gamma neurones produce no useful force; they set sensitivity.
• Spindles are densest in muscles needing fine control (hand, extraocular, neck).

The stretch (myotatic) reflex

Tapping a tendon stretches the muscle; spindle Ia afferents fire; they make monosynaptic excitatory synapses on alpha motor neurones of the same muscle, which contracts. At the same time Ia afferents excite Ia inhibitory interneurones that release glycine onto motor neurones of the antagonist, which relaxes: reciprocal inhibition (quadriceps contracts while hamstrings relax).

• The stretch reflex is the only monosynaptic reflex; it tests a specific segment.
• Strychnine (from kuchla, nux vomica seeds) blocks glycine receptors on motor neurones; tetanus toxin blocks glycine (and GABA) release from inhibitory interneurones. Both remove inhibition, so agonists and antagonists contract together: violent spasms triggered by noise or touch, opisthotonus and trismus, with full consciousness. Treatment is benzodiazepines, a quiet room and support.

Loss of lateral arm sensation, triceps weakness and an absent triceps jerk point to a C7 root (or radial nerve) lesion.

The Golgi tendon organ

• Lies in series at the musculotendinous junction and senses tension, mainly from active contraction.
• Group Ib afferents excite inhibitory interneurones that relax the same muscle (autogenic or inverse stretch reflex), limiting force. This is a disynaptic reflex.
• It contributes to the clasp-knife response in spasticity: resistance to passive stretch suddenly gives way.

Spindle and tendon organ compared

Other spinal reflexes

• Withdrawal (flexor) reflex: polysynaptic flexion away from a painful stimulus.
• Crossed extensor reflex: the opposite limb extends to bear weight.
• Superficial reflexes: abdominal (T8–T12), cremasteric (L1–L2), anal wink (S2–S4); lost in UMN lesions.
• Bulbocavernosus reflex (S2–S4): squeezing the glans or tugging a catheter contracts the anal sphincter. It is a sacral cord reflex and is the first to return, marking the end of spinal shock.

Reinforcement and reflex timing

• Jendrassik manoeuvre (pulling hooked fingers apart, clenching teeth) raises excitability of the reflex arc, partly through gamma drive and reduced presynaptic inhibition, bringing out a reflex that seemed absent. Healthy elderly people may have reduced ankle jerks, so reinforce first.
• Slow-relaxing reflexes in hypothyroidism: the contraction is normal but relaxation is slow because calcium reuptake into the sarcoplasmic reticulum (and myosin ATPase activity) is slowed. Hypothermia does the same.

Hyperreflexia and clonus after UMN lesions

• Descending pathways, especially reticulospinal, normally restrain gamma motor neurones and spinal interneurones. When they are lost, spindles become hypersensitive and inhibitory circuits weaken: tone and reflexes rise and the plantar becomes extensor.
• Clonus: a brisk dorsiflexion stretches the calf, the hyperactive stretch reflex contracts it, relaxation stretches it again, and the stretch reflex repeats itself rhythmically. Sustained clonus (for example with thoracic cord compression) is pathological; a few unsustained beats can be normal.
• Baclofen is a GABA-B agonist in the spinal cord that reduces excitatory transmitter release onto motor neurones, relieving spasticity (MS). Abrupt withdrawal, especially intrathecal, causes seizures, hallucinations and hyperthermia.

Spinal shock

Immediately after cord transection, everything below the lesion is flaccid and areflexic, because spinal neurones suddenly lose their tonic descending facilitation. Over days to weeks they become hyperexcitable: spasticity, hyperreflexia, extensor plantars and a small spastic reflex bladder appear. Return of the bulbocavernosus reflex marks the end of shock; do not call an injury complete before then.

Lost reflexes

• LMN or afferent lesions break the arc: peripheral neuropathy, tabes dorsalis, root lesions.
• Diabetic polyneuropathy is length-dependent: the longest axons degenerate first, so ankle jerks go before knee jerks and sensory loss starts in the feet.
• Guillain–Barré syndrome: early demyelination desynchronises and blocks the reflex volley, so reflexes are lost even while power is fair. Brisk reflexes in suspected GBS should prompt imaging for cord compression.

Primitive reflexes

Newborn reflexes (Moro, asymmetric tonic neck, grasp, rooting) are brainstem and spinal patterns that disappear by about 4–6 months as cortical inhibition matures. Their persistence at 10 months with stiff, scissoring legs after birth asphyxia indicates cerebral palsy. An asymmetric Moro in a newborn suggests brachial plexus injury or fractured clavicle.

Posture

Posture relies on stretch reflexes in antigravity muscles, with vestibular, visual and neck inputs integrated in the brainstem and cerebellum. The lateral vestibular (Deiters) nucleus, through the lateral vestibulospinal tract, and the pontine reticulospinal tract excite antigravity extensors; lesions above the vestibular nuclei release this drive (decerebrate rigidity).

Remember Spindle: parallel, length, Ia, contract. Tendon organ: series, tension, Ib, relax. Gamma sets the gain.

Exam trap The stretch reflex is monosynaptic; reciprocal inhibition of the antagonist uses a glycinergic Ia interneurone.

Exam trap Immediately after cord injury reflexes are absent (spinal shock), not brisk.

How FCPS asks about this

• Alpha–gamma coactivation: keeps spindles sensitive during shortening; gamma neurones make no useful force.
• Hamstrings relax during the knee jerk: Ia inhibitory interneurone (glycine); strychnine blocks glycine receptors.
• Golgi tendon organ: in series, senses tension, Ib afferents relax the same muscle.
• End of spinal shock: return of the bulbocavernosus reflex.
• Clonus: a self-repeating stretch reflex in UMN lesions.
• Ankle jerks lost before knee jerks in diabetes: longest axons degenerate first.
• Main nucleus for antigravity posture: lateral vestibular nucleus.