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Reflex arc

Reflex arc is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Reflex arc rather than just read about it. In short: A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain.

Reflex arc — main illustration
Reflex arc — illustration

Key takeaways

  • Reflex arc belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Reflex arc to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Reflex arc from memory before moving on to harder problems.

Reference excerpt

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action. There are two types: autonomic reflex arc (affecting inner organs) and somatic reflex arc (affecting muscles). Autonomic reflexes sometimes involve the spinal cord and some somatic reflexes are mediated more by the brain than the spinal cord. During a somatic reflex, nerve signals travel along the following pathway:

Somatic receptors in the skin, muscles and tendons Afferent nerve fibers carry signals from the somatic receptors to the posterior horn of the spinal cord or to the brainstem An integrating center, the point at which the neurons that compose the gray matter of the spinal cord or brainstem synapse Efferent nerve fibers carry motor nerve signals from the anterior horn to the muscles Effector muscle innervated by the efferent nerve fiber carries out the response. A reflex arc, then, is the pathway followed by nerves which (a.) carry sensory information from the receptor to the spinal cord, and then (b.) carry the response generated by the spinal cord to effector organs during a reflex action. The pathway taken by the nerve impulse to accomplish a reflex action is called the reflex arc.

Monosynaptic vs. polysynaptic

When a reflex arc in an animal consists of only one sensory neuron and one motor neuron, it is defined as monosynaptic, referring to the presence of a single chemical synapse. In the case of peripheral muscle reflexes (patellar reflex, achilles reflex), brief stimulation to the muscle spindle results in contraction of the agonist or effector muscle. By contrast, in polysynaptic reflex pathways, one or more interneurons connect afferent (sensory) and efferent (motor) signals. All but the most simple reflexes are polysynaptic, allowing processing or inhibition of polysynaptic reflexes within the brain.

The patellar reflex (aka "knee jerk")

When the patellar tendon is tapped just below the knee, the tap initiates an action potential in a specialized structure known as a muscle spindle located within the quadriceps. This action potential travels to the L3 and L4 nerve roots of the spinal cord, via a sensory axon which chemically communicates by releasing glutamate onto a motor nerve. The result of this motor nerve activity is contraction of the quadriceps muscle, leading to extension of the lower leg at the knee (i.e. the lower leg kicks forward). Ultimately, an improper patellar reflex may indicate an injury of the central nervous system. The sensory input from the quadriceps also activates local interneurons that release the inhibitory neurotransmitter glycine onto motor neurons of antagonist muscles, blocking their stimulation (in this case the hamstring muscles). The relaxation of the opposing muscle facilitates (by not opposing) the extension of the lower leg. In invertebrates reflex interneurons do not necessarily reside in the spinal cord, for example as in the lateral giant neuron of crayfish.

See also Lazarus sign

References

External links Ole Kæseler Andersen (1996). Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex (PDF) (PhD Thesis). Center for Sansory-Motor Interaction, Aalborg University. Archived from the original (PDF) on 2017-08-08. Retrieved 2015-10-22. Tutorial at wisc-online.com

Illustrations

Reflex arc: In a reflex arc, an action potential can bypass the brain for processing and uses dedicated neural pathways for faster processing. When a stimulus (A) is met at the signal from that stimulus will travel up the sensory neuron (B, in green) to the spinal column (C). There it will likely pass through a short interneuron (D, in purple) before continuing down a motor neuron (E, in blue) to the origin of the signal. Then, a contraction of the muscles (F, in red) is triggered, moving the bone (G).
In a reflex arc, an action potential can bypass the brain for processing and uses dedicated neural pathways for faster processing. When a stimulus (A) is met at the signal from that stimulus will travel up the sensory neuron (B, in green) to the spinal column (C). There it will likely pass through a short interneuron (D, in purple) before continuing down a motor neuron (E, in blue) to the origin of the signal. Then, a contraction of the muscles (F, in red) is triggered, moving the bone (G).
Reflex arc: Reflex arc demonstrated
Reflex arc demonstrated
Reflex arc: (A) Microscopic hairs etched along the tail of the decapod activate a somatic signal (2) in response to the presence of an environmental stimulus (1). (B) The action potential activated by the somatic interneuron (3) relays an impulse to the lateral giant (LG) interneuron (4). (C) The lateral giant interneuron executes a reflex by relaying impulses to various giant motor neurons (5) within the abdomen of the lobster. These muscular contractions result in the decapod being capable of successfully propelling itself through the water, away from the site of stimulus.
(A) Microscopic hairs etched along the tail of the decapod activate a somatic signal (2) in response to the presence of an environmental stimulus (1). (B) The action potential activated by the somatic interneuron (3) relays an impulse to the lateral giant (LG) interneuron (4). (C) The lateral giant interneuron executes a reflex by relaying impulses to various giant motor neurons (5) within the abdomen of the lobster. These muscular contractions result in the decapod being capable of successfully propelling itself through the water, away from the site of stimulus.

Worked examples

Example 1 — a first encounter with Reflex arc

Start with the simplest possible case. Write down what Reflex arc claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Reflex arc before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Reflex arc ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Reflex arc

In research
Reflex arc appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Reflex arc in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Reflex arc is common in secondary-school and first-year university syllabi. It links to neighbouring topics Reflexes, so understanding it makes those chapters shorter.
In everyday life
Look for Reflex arc outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Reflex arc in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Reflex arc means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Reflex arc out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Reflex arc in simple terms?

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons synapse in the spinal cord and the signal then travels through it into the brain.

Why does Reflex arc matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Reflex arc?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Reflex arc.

Tags

  • Reflexes

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