Reflexes

Reflexes are involuntary, nearly instantaneous movements in response to specific stimuli, produced by reflex arcs that connect a sensory input directly or via interneurons to an appropriate motor output. A reflex arc typically includes a receptor, a sensory neuron, one or more interneurons in the central nervous system, a motor neuron, and an effector such as a muscle. Many reflexes are spinal in origin and can operate without conscious brain input, while others are cranial and involve brainstem circuits.

Reflexes can be classified as monosynaptic or polysynaptic. Monosynaptic stretch reflexes involve a single synapse between the sensory and motor neurons, as in the patellar tendon reflex. Polysynaptic reflexes involve interneurons and multiple synapses, as in withdrawal from a painful stimulus or the crossed extensor reflex. Visceral or autonomic reflexes govern internal organs and include pupil constriction, digestive responses, and cardiovascular adjustments.

In development, humans exhibit primitive reflexes in infancy (such as sucking, rooting, and grasp), which typically recede as cortical control matures. Reflexes can be modulated by higher brain centers and may be influenced by attention, emotion, fatigue, and pharmacology.

Clinically, reflex testing assesses nervous system integrity. Absent or diminished reflexes can indicate peripheral nerve or muscle disease, while hyperreflexia or spasticity can suggest upper motor neuron involvement. An abnormal plantar reflex in adults (Babinski sign) may indicate pyramidal tract damage.

Reflexes play a fundamental role in rapid protection and motor coordination, while also illustrating the interaction between automatic responses and higher-level neural control.