When you pull your hand away from a burning-hot surface, the pain has often not yet been felt.
The decision was taken elsewhere, and faster.
The reflex
Some responses do not travel up to the brain before being carried out. They are processed in the spinal cord, which commands the movement directly.
The brain, for its part, is informed in parallel, and the pain is felt only afterwards.
That is why we sometimes have the impression of having pulled our hand away before it hurt. It is not merely an impression: it is often the real order of events.
The reflex arc follows a strict local loop:
- A nociceptive stimulus (a burn, a prick) stimulates a receptor in the skin.
- A sensory nerve carries the impulse to the dorsal horn of the spinal cord.
- An interneuron relays the information directly to a motor neuron.
- The motor neuron immediately triggers the contraction of the flexor muscle.1
The signal travels up to the cerebral cortex through the ascending pathways only in parallel with, or just after, this U-turn in the spinal cord.
One word covering two things
This site has set a trap here, without announcing it.
Our page on electricity explains that in a wire, electrons move more slowly than a snail — a few hundredths to a few tenths of a millimetre per second, depending on the wire and the current — while the electromagnetic signal travels at a large fraction of the speed of light.2
The nerve signal is said to be “electrical”. Should it then travel just as fast?
No. It propagates far more slowly: between 0.5 m/s (unmyelinated C fibres, which carry slow pain in particular) and 120 m/s (Aα proprioceptive fibres, wrapped in a myelin sheath).3
But the lesson is already there, and it is clear: the word “electrical” does not cover a single thing.
In a wire, it is an electromagnetic field that propagates around a metal conductor. In a nerve, the signal is bioelectrical in nature: it relies on an action potential generated by movements of ions (sodium Na⁺ and potassium K⁺) crossing the cell membrane through ion channels, step by step or from one node of Ranvier to the next.4
This site has already presented other words that, on their own, create a false belief. This one has the particularity of misleading by resemblance: the two phenomena are similar enough to bear the same name, and not similar enough to behave in the same way.
A consequence everyone has already felt
This difference in speed between fibres is not a textbook curiosity. Everyone has been able to experience it.
When you bump into something or prick yourself, you often feel two successive pains:
- first a sharp, clear, well-localised pain — it arrives through fast fibres wrapped in myelin;
- then, after a delay of a second or more, a dull, burning, diffuse pain — it arrives through the slow, bare fibres.5
The same blow, two messages, two arrivals. It is not the mind taking time to understand: it is two cables of different speeds carrying the same news.
The most surprising figure remains: in an ordinary sensory nerve, the slow, bare fibres make up about four-fifths of the total. In that nerve, most of the fibres are therefore slow fibres.6
The hand moves before we know it
On contact with something burning hot, the hand pulls away.
It often pulls away before the pain is felt.
This is not just a figure of speech. It is often the real order of events, and it is explained by the path the signal takes.
The short circuit
The signal leaves the skin, travels up a nerve to the spinal cord — inside the spinal column — and there it is immediately sent back to the muscles of the arm.
It does not go up to the brain before setting off again. The U-turn happens in the spinal cord.
The brain is alerted in parallel, but the pain is perceived only afterwards. It notes after the fact a movement it did not command.
The time saved is real, and that is the whole point: with a burn, a few tens of milliseconds count.
It is not a single muscle
The reflex is more organised than it seems.
When a person walking barefoot steps on a sharp object, the withdrawal of that leg triggers at the same time a stiffening of the other leg and of the muscles that keep the body standing. This is the crossed extensor reflex.7
Without it, the person would risk falling. The spinal cord does not merely withdraw a limb: it secures support on the other one while doing so.
And yet it can be stopped
This is the most interesting point.
The brain does not command this reflex. But it can inhibit it.
A person holding a very hot dish who knows it contains something precious can force themselves to put it down instead of dropping it. Descending motor pathways, which innervate the spinal motor neurons and interneurons, inhibit the automatic response.8
A right of veto without a right of command. This is not a nuance: it is a fairly accurate description of the relationship between the brain and this type of reflex.
A link worth remembering
When a doctor taps below the knee with a small hammer and the leg lifts, they are not testing the brain first. They are testing a short circuit of the same kind (the stretch reflex, simpler still), which damaged descending pathways can nonetheless exaggerate.9
The point deserves to be noted: the word “reflex” covers a precise mechanism. It can be traced, measured, timed. That is not the case for every word used to talk about behaviour — the word “instinct”, for example, covers nothing so clear-cut.
Pain
A word on a difficult subject, and on how this site will treat it.
Pain is a complex sensory and emotional experience. It depends on nociception (the detection of the physical signal by nerve endings), but it is modulated at every level of the nervous system: in the spinal cord (as the gate control theory proposed as early as 1965), then in the brain by emotional, cognitive and memory-related contexts.10
Two people with the same tissue damage do not necessarily feel the same thing, and pain can persist or become intense without visible tissue damage (damage to the nervous system itself, or nociplastic pain).11
In keeping with this site’s charter:
this site will never write that a pain is imaginary, nor that it should be proportionate to a visible cause.
One last clarification
This page describes a nervous system.
Some people live with a spinal cord injury, a neuropathy or a neurological difference. Their bodies do not work exactly as described above — and that does not make them exceptions to a rule.
Variation is the fact. The general description is a simplification.
Note 1
A sharp object touching the foot causes the activation of Group III afferents of pain receptors. These afferents enter the spinal cord and then travel up the cord. A branch of the afferent innervates an excitatory interneuron in the lumbar region of the spinal cord, which then excites an alpha motor neuron that causes contraction of the thigh flexor muscle.
Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chapter 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston
In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned.
Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, 2nd ed., § 14.3 “Motor Responses”
Note 2
entrée 4.12
Entry 4.12
The facts concerning the speed of electrons in a wire are established in Entry 4.12, with their sources
Note 3
The fastest axons are the Aα or Group I type; they have the largest diameter and are heavily myelinated. […] Finally, the C fibers have the smallest diameter, are unmyelinated, and are the slowest at conducting action potentials. […] Conduction speed (m/sec) 80-120 (Fastest) 35-75 5-30 0.5-2 (Slowest) […] Aδ fibers carry pain and temperature sensation, and C fibers convey information about pain, temperature, itch, and chemoreception. […] Proprioceptive information from the skeletal muscles is sent to the spinal cord via Group I fibers.
Henley, Casey, Foundations of Neuroscience, Michigan State University Libraries, 2021
Note 4
When a nerve fibre carries an impulse it undergoes a rapid cycle of permeability changes which allow first sodium and then potassium ions to move down concentration gradients.
Hodgkin, Alan L., “The Ionic Basis of Nervous Conduction”, Nobel Lecture, 11 December 1963, Nobel Foundation, p. 33-48
Note 5
Pain caused by a needle, pin prick, skin cut, etc. - elicits a sharp, pricking quality, stinging pain sensation carried fast by the A delta fibers. The pain is precisely localized and of short duration. Pricking pain is also called fast pain, first pain or sensory pain. […] Some of these cell bodies give rise to myelinated axons (A delta fibers), […]
Dafny, Nachum, “Pain Principles”, chapter 6 of section 2, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston
It is followed several seconds later with additional pain sensation. […] Pain caused by inflammation, burned skin, etc., is carried by the C fibers (slowly conducted pain nerve fibers). This type of pain is a more diffuse, slower to onset, and longer in duration. […] C fibers (group IV fibers) are […] unmyelinated, have a slow conduction velocity (0.5-2.0 meters/sec)
Dafny, Nachum, “Pain Principles”, chapter 6 of section 2, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston
Using frequency analysis event related responses to burning pain can be detected in the EEG interval 1-2 seconds after laser stimulation.
Arendt-Nielsen, Lars, “Second Pain Event Related Potentials to Argon Laser Stimuli: Recording and Quantification”, Journal of Neurology, Neurosurgery & Psychiatry, vol. 53, no. 5, 1990, p. 405-410
Note 6
Unmyelinated nerve fibers are one of the cardinal constituents of sural nerve. They are about three to five times more numerous than myelinated nerve fibers and are considered to represent postganglionic sympathetic efferents as well as sensory afferents subserving such modalities as pain and temperature.
Kanda, Takashi, “Pathological Changes of Human Unmyelinated Nerve Fibers: A Review”, Histology and Histopathology, vol. 15, no. 1, 2000, p. 313-324
Note 7
Flexion of the stimulated limb is also accompanied by an opposite reaction in the contralateral limb […] the contralateral extensor muscles are excited while flexor muscles are inhibited […] This crossed extension reflex serves to enhance postural support […]
Purves, Dale, Augustine, George J., Fitzpatrick, David et al. (eds), Neuroscience, 2nd ed., Sinauer Associates, Sunderland (Massachusetts), 2001, “Flexion Reflex Pathways”
Note 8
Descending motor pathways arise from multiple regions of the brain and send axons down the spinal cord that innervate alpha motor neurons, gamma motor neurons, and interneurons. […] In this case, the descending pathways inhibited the flexor response.
Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chap. 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston
Note 9
synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction
Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University
Signs that suggest a UMN lesion include muscle weakness, strong deep tendon reflexes […]
Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University
Note 10
The Gate Theory of Pain, published by Ronald Melzack and Patrick Wall in Science in 1965, was formulated to provide a mechanism for coding the nociceptive component of cutaneous sensory input. […] It incorporated recently discovered mechanisms of presynaptic control of synaptic transmission from large and small sensory afferents […]. Other important features included the convergence of small and large sensory inputs on spinal neurons that transmitted the sensory information to the forebrain
Mendell, Lorne M., “Constructing and Deconstructing the Gate Theory of Pain”, Pain, vol. 155, no. 2, 2014, p. 210-216
Note 11
Pain caused by a lesion or disease of the somatosensory nervous system. […] Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.
International Association for the Study of Pain, “Terminology”
Pain caused by a lesion or disease of the somatosensory nervous system.
International Association for the Study of Pain, “Terminology”
Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage […] or evidence for disease or lesion of the somatosensory system causing the pain.
International Association for the Study of Pain, “Terminology”
Notes and sources
- ↑
Passage An interneuron relays the information directly to a motor neuron.
A sharp object touching the foot causes the activation of Group III afferents of pain receptors. These afferents enter the spinal cord and then travel up the cord. A branch of the afferent innervates an excitatory interneuron in the lumbar region of the spinal cord, which then excites an alpha motor neuron that causes contraction of the thigh flexor muscle.
Source Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chapter 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, § 2.1 (“Spinal Reflexes”), subheading “Flexor reflex”.
Open access open the source · checked on 27 October 2026
Passage A sensory nerve carries the impulse to the dorsal horn of the spinal cord
In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned.
Source Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, 2nd ed., § 14.3 “Motor Responses”, § 14.3 (“Motor Responses”), “Reflexes” subsection.
Open access open the source · checked on 27 October 2026
- ↑
Passage electrons move more slowly than a snail […] while the electromagnetic signal travels at a large fraction of the speed of light
entrée 4.12
Editors’ translation Entry 4.12
Source The facts concerning the speed of electrons in a wire are established in Entry 4.12, with their sources, fact established elsewhere with its sources.
Established in other articles, with their sources · checked on 9 September 2026
- ↑
Passage between 0.5 m/s (unmyelinated C fibres, which carry slow pain in particular) and 120 m/s (Aα proprioceptive fibres, wrapped in a myelin sheath)
The fastest axons are the Aα or Group I type; they have the largest diameter and are heavily myelinated. […] Finally, the C fibers have the smallest diameter, are unmyelinated, and are the slowest at conducting action potentials. […] Conduction speed (m/sec) 80-120 (Fastest) 35-75 5-30 0.5-2 (Slowest) […] Aδ fibers carry pain and temperature sensation, and C fibers convey information about pain, temperature, itch, and chemoreception. […] Proprioceptive information from the skeletal muscles is sent to the spinal cord via Group I fibers.
Source Henley, Casey, Foundations of Neuroscience, Michigan State University Libraries, 2021, chapter “Somatosensory Systems”, section “Primary Afferent Axons” and Table 21.1 (“Diameter and conduction speed of primary afferent axons”, row “Conduction speed (m/sec)”).
Open access open the source · checked on 27 October 2026
- ↑
Passage it relies on an action potential generated by movements of ions (sodium Na⁺ and potassium K⁺) crossing the cell membrane
When a nerve fibre carries an impulse it undergoes a rapid cycle of permeability changes which allow first sodium and then potassium ions to move down concentration gradients.
Source Hodgkin, Alan L., “The Ionic Basis of Nervous Conduction”, Nobel Lecture, 11 December 1963, Nobel Foundation, p. 33-48, p. 43, 1st paragraph.
Open access open the source · checked on 27 October 2026
- ↑
Passage first a sharp, clear, well-localised pain — it arrives through fast fibres wrapped in myelin
Pain caused by a needle, pin prick, skin cut, etc. - elicits a sharp, pricking quality, stinging pain sensation carried fast by the A delta fibers. The pain is precisely localized and of short duration. Pricking pain is also called fast pain, first pain or sensory pain. […] Some of these cell bodies give rise to myelinated axons (A delta fibers), […]
Source Dafny, Nachum, “Pain Principles”, chapter 6 of section 2, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, sections 6.7 (“Classification of Pain”) and 6.4 (“Pain Fibers”).
Open access open the source · checked on 27 October 2026
Passage then, after a delay of a second or more, a dull, burning, diffuse pain — it arrives through the slow, bare fibres
It is followed several seconds later with additional pain sensation. […] Pain caused by inflammation, burned skin, etc., is carried by the C fibers (slowly conducted pain nerve fibers). This type of pain is a more diffuse, slower to onset, and longer in duration. […] C fibers (group IV fibers) are […] unmyelinated, have a slow conduction velocity (0.5-2.0 meters/sec)
Source Dafny, Nachum, “Pain Principles”, chapter 6 of section 2, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, sections 6.5 (“Double Pain Sensations”), 6.7 (“Burning pain or soreness pain”) and 6.4 (“Pain Fibers”).
Open access open the source · checked on 27 October 2026
Passage after a delay of a second or more
Using frequency analysis event related responses to burning pain can be detected in the EEG interval 1-2 seconds after laser stimulation.
Source Arendt-Nielsen, Lars, “Second Pain Event Related Potentials to Argon Laser Stimuli: Recording and Quantification”, Journal of Neurology, Neurosurgery & Psychiatry, vol. 53, no. 5, 1990, p. 405-410, abstract.
Abstract open access open the source · checked on 27 October 2026
- ↑
Passage in an ordinary sensory nerve, the slow, bare fibres make up about four-fifths of the total
Unmyelinated nerve fibers are one of the cardinal constituents of sural nerve. They are about three to five times more numerous than myelinated nerve fibers and are considered to represent postganglionic sympathetic efferents as well as sensory afferents subserving such modalities as pain and temperature.
Source Kanda, Takashi, “Pathological Changes of Human Unmyelinated Nerve Fibers: A Review”, Histology and Histopathology, vol. 15, no. 1, 2000, p. 313-324, “Introduction”, 1st paragraph.
Open access open the source · checked on 27 October 2026
- ↑
Passage the withdrawal of that leg triggers at the same time a stiffening of the other leg […] This is the crossed extensor reflex.
Flexion of the stimulated limb is also accompanied by an opposite reaction in the contralateral limb […] the contralateral extensor muscles are excited while flexor muscles are inhibited […] This crossed extension reflex serves to enhance postural support […]
Source Purves, Dale, Augustine, George J., Fitzpatrick, David et al. (eds), Neuroscience, 2nd ed., Sinauer Associates, Sunderland (Massachusetts), 2001, “Flexion Reflex Pathways”, section “Flexion Reflex Pathways”, 1st paragraph.
Open access open the source · checked on 27 October 2026
- ↑
Passage Descending motor pathways, which innervate the spinal motor neurons and interneurons, inhibit the automatic response.
Descending motor pathways arise from multiple regions of the brain and send axons down the spinal cord that innervate alpha motor neurons, gamma motor neurons, and interneurons. […] In this case, the descending pathways inhibited the flexor response.
Source Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chap. 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, § 2.2 (“Descending Motor Pathways”, subheading “Parallel and Serial Processing”) and § 2.3 (“Influences of Descending Pathways on Spinal Circuits”, subheading “Reflex modulation”, hot-dish example).
Open access open the source · checked on 27 October 2026
- ↑
Passage They are testing a short circuit of the same kind (the stretch reflex, simpler still)
synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction
Source Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, chapter 16.4, “The Sensory and Motor Exams” (2nd ed.), the stretch reflex.
Open access open the source · checked on 24 October 2026
Passage which damaged descending pathways can nonetheless exaggerate
Signs that suggest a UMN lesion include muscle weakness, strong deep tendon reflexes […]
Source Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, chapter 16.4, “The Sensory and Motor Exams” (2nd ed.), comparison of upper and lower motor neuron lesions.
Open access open the source · checked on 26 October 2026
- ↑
Passage it is modulated at every level of the nervous system: in the spinal cord (as the gate control theory proposed as early as 1965
The Gate Theory of Pain, published by Ronald Melzack and Patrick Wall in Science in 1965, was formulated to provide a mechanism for coding the nociceptive component of cutaneous sensory input. […] It incorporated recently discovered mechanisms of presynaptic control of synaptic transmission from large and small sensory afferents […]. Other important features included the convergence of small and large sensory inputs on spinal neurons that transmitted the sensory information to the forebrain
Source Mendell, Lorne M., “Constructing and Deconstructing the Gate Theory of Pain”, Pain, vol. 155, no. 2, 2014, p. 210-216, abstract.
Open access open the source · checked on 27 October 2026
- ↑
Passage pain can persist or become intense without visible tissue damage (damage to the nervous system itself
Pain caused by a lesion or disease of the somatosensory nervous system. […] Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.
Source International Association for the Study of Pain, “Terminology”, definitions of “Neuropathic pain” and “Nociplastic pain”.
Open access open the source · checked on 27 October 2026
Passage damage to the nervous system itself, or nociplastic pain
Pain caused by a lesion or disease of the somatosensory nervous system.
Source International Association for the Study of Pain, “Terminology”, definition of “Neuropathic pain”.
Open access open the source · checked on 24 October 2026
Passage or nociplastic pain
Pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage […] or evidence for disease or lesion of the somatosensory system causing the pain.
Source International Association for the Study of Pain, “Terminology”, definition of “Nociplastic pain”.
Open access open the source · checked on 24 October 2026
Bibliography
18
- Arendt-Nielsen, Lars, “Second Pain Event Related Potentials to Argon Laser Stimuli: Recording and Quantification”, Journal of Neurology, Neurosurgery & Psychiatry, vol. 53, no. 5, 1990, p. 405-410, abstract open access (burning pain, carried by C fibres, detected one to two seconds after the stimulus). https://doi.org/10.1136/jnnp.53.5.405
- Baron, Ralf, Binder, Andreas and Wasner, Gunnar, “Neuropathic Pain: Diagnosis, Pathophysiological Mechanisms, and Treatment”, The Lancet Neurology, vol. 9, 2010, p. 807-819, paywalled (neuropathic pain, caused by a lesion or disease of the nervous system).
- Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, 2ᵉ éd., open access (the stretch reflex, in which the sensory fibre acts directly on the motor neuron; brisk reflexes as a sign of damage to the descending motor pathways). https://openstax.org/books/anatomy-and-physiology-2e/pages/16-4-the-sensory-and-motor-exams
- Betts, J. Gordon et al., Anatomy and Physiology, OpenStax, Rice University, 2nd ed., § 14.3 “Motor Responses”, open access (the withdrawal reflex: the sensory axon synapsing on an interneuron whose cell body lies in the dorsal horn of the spinal cord). https://openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses
- Dafny, Nachum, “Pain Principles”, chapter 6 of section 2, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, open access (double pain; burning, diffuse pain carried by slow, unmyelinated C fibres). https://nba.uth.tmc.edu/neuroscience/m/s2/chapter06.html
- Henley, Casey, Foundations of Neuroscience, Michigan State University Libraries, 2021, open access (university textbook under a CC BY-NC-SA 4.0 licence; chapter “Somatosensory Systems”: diameter and conduction speed of afferent fibres, from 0.5-2 m/s for C fibres to 80-120 m/s for Aα fibres). https://openbooks.lib.msu.edu/neuroscience/chapter/somatosensory-systems/
- Hodgkin, Alan L. and Huxley, Andrew F., “A Quantitative Description of Membrane Current and Its Application to Conduction and Excitation in Nerve”, The Journal of Physiology, vol. 117, 1952, p. 500-544, open access (the model of the action potential and the role of sodium and potassium ions).
- Hodgkin, Alan L., “The Ionic Basis of Nervous Conduction”, Nobel Lecture, 11 December 1963, Nobel Foundation, p. 33-48, open access (the nerve impulse as a cycle of permeability changes letting sodium then potassium through). https://www.nobelprize.org/uploads/2018/06/hodgkin-lecture.pdf
- International Association for the Study of Pain, “Terminology”, open access (the definitions of neuropathic pain and nociplastic pain). https://www.iasp-pain.org/resources/terminology/
- Kanda, Takashi, “Pathological Changes of Human Unmyelinated Nerve Fibers: A Review”, Histology and Histopathology, vol. 15, no. 1, 2000, p. 313-324, open access (in the sural nerve, a sensory cutaneous nerve, unmyelinated fibres are three to five times more numerous than myelinated ones). https://revistas.um.es/hh/article/view/127951
- Kandel, Eric R. et al. (ed.), Principles of Neural Science, McGraw-Hill, 2021, in libraries (spinal reflexes; conduction speeds by fibre type; double pain, fast and slow; the proportion of unmyelinated fibres in a sensory nerve).
- Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chap. 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, open access. https://nba.uth.tmc.edu/neuroscience/m/s3/chapter02.html
- Knierim, James, “Spinal Reflexes and Descending Motor Pathways”, chapter 2 of section 3, in Neuroscience Online: An Electronic Textbook for the Neurosciences, Department of Neurobiology and Anatomy, McGovern Medical School, UTHealth Houston, open access (the flexor reflex: the pain afferent, the excitatory interneuron and the alpha motor neuron of the flexor). https://nba.uth.tmc.edu/neuroscience/m/s3/chapter02.html
- Melzack, Ronald and Wall, Patrick D., “Pain Mechanisms: A New Theory”, Science, vol. 150, 1965, p. 971-979, paywalled (the gate control theory).
- Mendell, Lorne M., “Constructing and Deconstructing the Gate Theory of Pain”, Pain, vol. 155, no. 2, 2014, p. 210-216, open access. https://pmc.ncbi.nlm.nih.gov/articles/PMC4009371/
- Purves, Dale, Augustine, George J., Fitzpatrick, David et al. (eds), Neuroscience, 2nd ed., Sinauer Associates, Sunderland (Massachusetts), 2001, “Flexion Reflex Pathways”, open access on NCBI Bookshelf (the crossed extension reflex in humans and its role in maintaining posture). https://www.ncbi.nlm.nih.gov/books/NBK11091/
- Sherrington, Charles S., The Integrative Action of the Nervous System, Yale University Press, 1906, public domain (the crossed extensor reflex and reciprocal inhibition).
- The facts concerning the speed of electrons in a wire are established in Entry 4.12, with their sources.
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