Reflex

• 1. Introduction

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  A reflex moves the body before the mind has time to weigh in. In biology, a reflex is an involuntary, unplanned response to a stimulus that unfolds in a fraction of a second, often with almost no help from the brain. Doctors use reflexes to read the health of the nervous system, tapping a tendon and watching how the muscle answers. Scientists have also found that reflexes are not as fixed as they look, and that the body can adjust their strength depending on what it is already doing. Where does this fast, involuntary wiring come from, and why does a modern reflex hammer trace back to a seventeenth-century philosopher's idea of the body as a machine?

• 2. Wiring The Reflex Arc

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  A reflex travels along a path called a reflex arc, a loop in which a sensory nerve carries the incoming signal and a motor nerve delivers the response. The simplest arcs, known as short-latency reflexes, cross only a single synapse between input and output. Long-latency reflexes route the same kind of signal across several synapses before a muscle ever moves. Because many of these signals never reach the brain, a body can flinch, brace, or steady itself with no conscious thought involved at all. Anatomists split reflex arcs into two families. Autonomic reflexes run through an older, more primitive branch of the nervous system, even though the word autonomic misleadingly suggests full automatic control. Skeletal, or somatic, reflexes run through a branch that appeared later in evolutionary history, the same branch that also carries voluntary movement. The feline righting reflex shows how precisely this wiring can be tuned. It reorients a falling cat's body in mid-air, giving the animal a chance to land safely on its feet. That same sensory-in, motor-out design scales up into far more elaborate reflexes, the kind a doctor can measure directly in an exam room.

• 3. Reading The Nervous System

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  Doctors watch the biceps reflex, tied to spinal levels C5 and C6, as one measure of whether the nervous system is working correctly. It belongs to a family called myotatic, or deep tendon, reflexes, muscle contractions triggered by a lengthwise stretch, and the strength of the response can be confirmed with electromyography. A weak or absent reflex usually points to a problem in the peripheral nerves, while an exaggerated one points toward the central nervous system instead. The same family includes the patellar reflex, at spinal levels L2 through L4 and better known as the knee-jerk, and the ankle jerk reflex at S1 and S2. A related but separate response, the tendon reflex, contracts a muscle when its tendon is struck directly rather than stretched. The Golgi tendon reflex runs in the opposite direction of the stretch reflex, relaxing the muscle instead of tightening it. Cranial nerves carry their own reflex circuits: the pupillary light reflex sends its sensory signal through nerve II and its motor response through nerve III, while the gag reflex uses nerves IX and X. To turn all of this into a single number, clinicians grade reflex strength on a scale from 0, meaning absent, to 4, meaning hyperactive with clonus, with 2 counted as normal. Some healthy people register every reflex at 1, and others at 3, without anything being wrong with them at all.

• 4. Reflexes Unique To Infants

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  A newborn's rooting reflex and sucking reflex disappear well before adulthood, and neither has an equivalent in a grown nervous system. Physiologists group these under primitive reflexes, automatic reactions that let an infant answer its environment before any learning has taken place. The list also includes the palmar grasp reflex, the asymmetrical and symmetrical tonic neck reflexes, the tonic labyrinthine reflex, the palmomental reflex, and the Moro reflex, which outside a clinical setting is simply called the startle reflex. None of these belong to the tendon reflexes a doctor tests with a hammer; they sit in a separate, earlier layer of the nervous system's design. Reflexes are not permanently locked in place once this early stage of life passes, and the body can reshape their strength depending on what it is already doing.

• 5. When Reflexes Bend

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  The stretch reflex shows how much a reflex can change depending on circumstance. At rest, stretching a muscle triggers the stretch reflex, contracting the muscle to resist the stretch and helping stabilize posture. During a voluntary movement, though, the strength of that same reflex drops, or its effect even reverses, so it stops fighting the movement the person is trying to make. Researchers do not fully understand where or how this adjustment happens, but they have found evidence that sensory neurons themselves are modulated during behavior, through a process called presynaptic inhibition. Interneurons in the spinal cord, along with signals descending from the brain, also shape how strongly a motor neuron answers sensory input. Breathing occupies a similarly strange middle ground. It normally runs as an involuntary reflex, yet a person can override it and hold their breath using the internal intercostal muscles. That flexibility stands in sharp contrast to how the reflex was first imagined, as a fixed, mechanical wiring diagram with no room to bend at all.

• 6. Descartes And The Reflex

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  The idea of the reflex reaches back to the philosopher Rene Descartes, who described it in his work Treatise on Man, published after his death in 1664. Descartes compared the body to a mechanical statue, arguing that sensory input could trigger a motor response automatically and deterministically, with no conscious thought required. The word reflex itself did not arrive until the nineteenth century, when the English physiologist Marshall Hall gave the concept its name and its science. Hall argued that involuntary movements triggered by outside stimuli were mediated by the spinal cord rather than the brain, setting them apart from voluntary, consciously controlled movement. He laid out the case in an 1833 paper, On the Reflex Function of the Medulla Oblongata and Medulla Spinalis, published in the Philosophical Transactions of the Royal Society, showing that these actions ran independently of the brain's conscious control. Hall's theory of a spinal, brain-independent circuit eventually gave doctors and scientists a whole vocabulary of named reflexes, from the cough reflex to the diving reflex, each one another loop of the very mechanism Descartes had pictured as a machine nearly two centuries before.