Reflex Action and Reflex Arc

A reflex action is a rapid, automatic, involuntary and highly stereotyped response of the body to a particular stimulus. It is 'involuntary' in the specific sense that it does not require, and typically happens faster than, any conscious decision by the cerebral cortex — indeed the very speed and reliability of a reflex response comes precisely from the fact that the higher, decision-making centres of the brain are bypassed for the immediate response, even though the CNS (usually the spinal cord) is still very much involved in processing the signal and issuing the motor command. This is quite different from a voluntary action, such as deciding to pick up a pen, which does require the cerebral cortex to receive the sensory information, weigh it, decide on a course of action and then send a deliberate motor command — a much slower process because of the extra distance the signal must travel (up to the brain and back down) and the extra processing involved.

The fixed neural pathway along which a reflex response travels is called a reflex arc, and every reflex arc, whatever the particular reflex, is built from the same five components acting in the same basic sequence. (1) A receptor detects the stimulus — this might be a sensory ending in the skin detecting a painful or hot stimulus, or a muscle spindle within a muscle detecting that the muscle has been stretched — and converts the stimulus into a nerve impulse. (2) A sensory (afferent) neuron carries this impulse from the receptor into the central nervous system, usually entering the spinal cord through its dorsal (posterior) root. (3) Within the CNS, an integrating centre processes the incoming signal; in many spinal reflexes this integrating centre is simply the grey matter of the spinal cord itself (sometimes involving one or more interneurons), which is precisely what allows the response to be organised without the impulse needing to travel up to the brain. (4) A motor (efferent) neuron carries the outgoing command from the integrating centre to the target organ, usually leaving the spinal cord through its ventral (anterior) root. (5) Finally, the effector — typically a muscle, which contracts, or a gland, which secretes — actually carries out the response.

Reflex arcs are further classified by how many synapses lie between the sensory neuron and the motor neuron. In a monosynaptic reflex arc, the sensory neuron synapses directly onto the motor neuron with just a single synapse in between, making it the fastest possible type of reflex; the knee-jerk (patellar) reflex is the classic example. In a polysynaptic reflex arc, one or more interneurons are interposed between the sensory and motor neurons, allowing more complex processing — for instance, coordinating several different muscles at once, or simultaneously exciting one muscle group while inhibiting an opposing one — at the cost of being marginally slower than a monosynaptic arc because of the extra synapse(s) the signal must cross; the withdrawal reflex, in which a hand is pulled sharply away from a painfully hot object while the opposing muscles are inhibited to allow smooth withdrawal, is a typical polysynaptic reflex.

The knee-jerk reflex is worth tracing through step by step as the standard worked example of a monosynaptic reflex arc. A sharp tap just below the kneecap stretches the tendon of the quadriceps (thigh) muscle and, with it, the muscle itself; this stretch is detected by muscle-spindle receptors embedded within the quadriceps. The resulting impulse travels along a sensory neuron whose fibre enters the spinal cord through the dorsal root at the appropriate spinal segment. Inside the spinal cord's grey matter, this sensory neuron makes a single, direct synapse onto a motor neuron supplying the very same quadriceps muscle — there is no interneuron in between, which is exactly what makes this arc monosynaptic. The motor neuron's fibre leaves the cord through the ventral root and travels back to the quadriceps, where it causes the muscle to contract, producing the familiar sudden kick of the lower leg. At the same time, a parallel inhibitory pathway (via an interneuron) relaxes the antagonistic hamstring muscle at the back of the thigh, so that the leg can extend smoothly rather than fighting against its own opposing muscle. …