When an object hits our knees, they jerk up, and when hot metal touches our hands, they instinctively jerk away. These automatic reactions, known as reflexes, can occur before our brain even processes them.
Different Complexities of Reflexes
Monosynaptic reflexes only contain a sensory (converts external stimuli into internal stimuli) and a motor neuron (uses the internal stimuli to control muscles).
Polysynaptic reflexes contain relay neurons (allow sensory and motor neurons to communicate) in addition to sensory and motor neurons.
Most reflex arcs consist of only three neurons. The stimulus triggers the receptors on our skin, which induces the impulse in a sensory neuron. The impulse then travels to the spinal cord by a relay neuron, which in turn makes a synapse with a motor neuron to carry out the reflex.
Note that most reflexes don’t involve any input from the brain. The impulse generally travels to only the spinal cord, or even just the local area where the stimulus was received. However, the brain can prevent reflexes from occurring. For example, the desire to protect an expensive vase from breaking after tripping is stronger than the reflex to break the fall with our hands. The brain processes the dangers of both options and chooses the best one within 10 to 30 milliseconds.
Examples of Common Reflexes
Accommodation reflex: the visual reflex in order to transition from looking at nearby objects to faraway objects, and vice versa. It includes the convergence (the eyes move and point inwards) of both eyes, the change of lens shape by ciliary muscles, and pupillary constriction (the pupil shrinks).
Pupillary light reflex: Constricts the pupil after the stimulus (light) flashes near it. The constriction is achieved through innervation of the iris sphincter muscle, and aims to protect the eye from excessively bright light.
Knee jerk reflex: a sharp tap on the patellar tendon (right below the kneecap) will result in a sudden kicking up motion. The tap marginally stretches the quadriceps (front of the thighs), which triggers their subsequent contraction.
Cough reflex: Coughing protects airways from inhaling foreign bodies and by clearing excessive bronchial secretions. After the cough receptors are stimulated, they send impulses through sensory neurons (ie. the vagus nerve) in the medulla oblongata (where they are processed), sent on by motor neurons and initiate the reflex.
Acoustic reflex: Protects our ears from loud sounds and from hearing ourselves. The stapedius muscle, which is attached to the stapes (a bone in the middle of their ear that vibrates to transmit signals), decreases the amount that the stapes vibrates when a loud sound occurs. Additionally, the tensor tympani muscle, which is attached to the eardrum (a hole in the tissue that separates the ear canal and the middle ear), draws the eardrum backwards, also reducing its movement. Both of these muscles have the end goal of reducing the sounds that transmit to the inner ear, ultimately shielding it from potential damage.
References
"12.10A: Components of a Reflex Arc." LibreTexts, UC Davis, med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Anatomy_and_Physiology_(Boundless)/12%3A_Peripheral_Nervous_System/12.10%3A_Reflexes/12.10A%3A__Components_of_a_Reflex_Arc#:~:text=Reflexes%20do%20not%20require%20involvement,is%20called%20a%20reflex%20arc. Accessed 13 Dec. 2023.
Andrani F, Aiello M, Bertorelli G, Crisafulli E, Chetta A. Cough, a vital reflex. mechanisms, determinants and measurements. Acta Biomed. 2019 Jan 15;89(4):477-480. doi: 10.23750/abm.v89i4.6182. PMID: 30657115; PMCID: PMC6502102.
Belliveau, Alison P., et al. "Pupillary Light Reflex." National Library of Medicine, StatPearls Publishing, 25 July 2023, www.ncbi.nlm.nih.gov/books/NBK537180/. Accessed 13 Dec. 2023.
The Editors of Encyclopaedia Britannica. "Knee-jerk reflex." Encyclopaedia Britannica, www.britannica.com/science/knee-jerk-reflex. Accessed 13 Dec. 2023.
Motlagh, Mahsaw, and Ragi Geetha. "Physiology, Accommodation." National Library of Medicine, StatPearls Publishing, 15 Nov. 2022, www.ncbi.nlm.nih.gov/books/NBK542189/. Accessed 13 Dec. 2023.
"A Quick Look at Reflexes." University of Rochester Medical Center, www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=1&ContentID=562. Accessed 13 Dec. 2023.
Ross, Mark. "Understanding the Acoustic Reflex: What It Is and How It Works." Hearing Research, Health Check, www.hearingresearch.org/acoustic-reflex/. Accessed 14 Dec. 2023.
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