Scratch reflex

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A shaggy dog demonstrates a scratch reflex. When she is scratched beneath her front leg, her back leg moves vigorously.

The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. [1] Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternatively, some sensory neurons can respond to a chemical stimulus that produces an itch sensation. During a scratch reflex, a nearby limb reaches toward and rubs against the site on the body surface that has been stimulated. The scratch reflex has been extensively studied to understand the functioning of neural networks in vertebrates. [2] Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated.

Contents

Scratch reflex in dogs

Most dogs will exhibit a scratch reflex when they are stimulated in the saddle region, which consists of the belly, sides, flanks, and back. These are the most common sites, but stimulation anywhere may be able to produce the reflex, such as the chest, ears, and even paws. Once stimulation of this area begins, the dog will begin to rhythmically "twitch" or "kick" their hind legs in an attempt to rid itself of the "irritant". Typically, only one of the hind legs will exhibit this reflex at a given time, however, it is possible for both legs to undergo the reflex at the same time. The intensity and rhythm of the reflex will vary depending on the intensity and speed of the stimulation.

The scratch reflex can commonly be triggered through various stimulations such as scratching, brushing, rubbing, or tapping a dog, although some techniques work better than others. For example, a majority of dogs will exhibit the reflex when scratched with fingernails, while only some with a stronger reflex might react to a lighter tapping.

Allergies and itchiness often play a role in the scratch reflex, with dogs who are already itchy before the additional stimulation often producing a stronger reflex than other dogs. It is common for dogs with flea infestations to have a strong reflex when stimulated at the base of the tail, due to the itchiness caused by the fleas.

Model systems and tools

Animal models and preparations

A brown bear scratching its body against a downed tree

A number of animal models have been used to study, understand and characterize the scratch reflex. These models include the turtle, cat, frog, dog, and a variety of other vertebrates. [1] [2] [3] [4] In these studies, researchers made use of spinal preparations, which involve a complete transection of the animal's spinal cord prior to experimentation. [1] Such preparations are used because the scratch reflex can be elicited and produced without the involvement of supraspinal structures. [1] Researchers focused predominantly on investigating spinal cord neural circuitry responsible for the generation of the scratch reflex, limiting the system of study.

In studies of spinal preparations, researchers have experimented with using preparations both with and without movement-related sensory inputs. [1] [2] In preparations with movement-related sensory inputs, the muscles and the motor neuron outputs to muscles are left intact, allowing sensory feedback from the moving limb. In preparations without movement-related sensory input, one of three strategies is used:

  1. the axons of sensory neurons are cut by dorsal root transection; or
  2. neuromuscular blockers are used to prevent contractions of muscles in response to motor neuron activity; or
  3. the spinal cord is isolated in a bath of physiological saline [1]

Recording techniques

Electromyographic (EMG) and electroneurographic (ENG) techniques are used to monitor and record from animals during experiments. [5] EMG recordings are used to record electrical activity directly from muscles. ENG recordings are used to record electrical activity from motor neurons and spinal cord neurons. [6] These techniques have enabled researchers to understand the neural circuitry of the scratch reflex on a single-cell level.

Characteristics

General

The scratch reflex is generally a rhythmic response. Results from animal studies have indicated that spinal neural networks are known as central pattern generators (CPGs) are responsible for the generation and maintenance of the scratch reflex. [1] [7] [8] [9] One feature of the scratch reflex is that supraspinal structures are not necessary for the generation of the reflex. The scratch response is programmed into the spinal cord and can be produced in spinal animals.[ citation needed ]

Another feature of the scratch reflex is that the spinal CPGs that generate and maintain the reflex is capable of producing the reflex in the absence of movement-related sensory feedback. [1] [2] This discovery was made while studying animals with silenced afferent neurons from the scratching limb, meaning no movement-related sensory feedback was available to the spinal circuits driving the scratch. These animals were capable of producing a functional scratch response, albeit diminished in accuracy. When afferent feedback is provided, the scratch response is more accurate in terms of accessing the stimulus site. Recordings indicate that feedback modulates the timing and intensity of scratching, in the form of phase and amplitude changes in nerve firing. [2]

In studying the scratch reflex, researchers have named a number of regions on the surface of the body as they relate to the reflex. [8] A pure form domain is a region on the surface of the body, that when stimulated, elicits only one form of the scratch reflex. A form is a movement-related strategy used by the animal to perform the scratch; for example, to scratch the upper back, humans are limited to one scratch form, involving the elbow raised above the shoulder to provide access to the upper back. In addition to pure-form domains, there also exist a number of transition zones, which can be successfully targeted by more than one form of the reflex, and which usually lie at the boundary of two pure-form domains.

Researchers have also developed terms to describe the scratch reflex movements themselves. [8] A pure movement is one in which only one form of the scratch response is utilized to respond to the stimulus. A switch movement occurs in a transition zone and is characterized by the smooth switching between two different scratch forms in response to the stimulus. A hybrid movement is observed and occurs at transition zones as well, and is characterized by two rubs during each scratch cycle, where each rub is derived from one pure-form movement. Research on hybrid and switch movements at transition zones indicates that the CPGs responsible for scratch generation are modular and share interneurons. For this reason, in both the switch and hybrid movements, the path of the moving limb is smooth and uninterrupted.

Studies from EMG recordings have indicated that reciprocal inhibition between hip-related interneurons in the CPG for the scratch reflex is not necessary for the production and maintenance of the hip-flexor rhythm that is a key part of the scratch reflex. [8] This research further supports the findings on the switch and hybrid movements, which suggest a modular organization of unit generator CPGs used in combination to achieve a task.

Another general aspect of the scratch response is that the response continues even after afferent input from the stimulated zone ceases. [9] For a few seconds after the cessation of the scratch, the neural networks involved in the generation of the scratch reflex remain in a state of heightened sensitivity. During this period of increased excitability, stimuli normally too weak to trigger a scratch response are capable of eliciting a scratch response in a site-specific manner. That is, stimuli, too weak to elicit the scratch response when applied in a rested preparation, are capable of eliciting the scratch response during the period of increased excitability just following a scratch response. This excitability is due, in part, to the long-time constant of NMDA receptors. Research has also shown that voltage-gated calcium channels have a role in the increased excitability of spinal neurons. [2]

Spinal

As described in the general characteristics above, the scratch reflex is programmed into the neural circuitry of the spinal cord. Initial experiments on the scratch reflex in dogs revealed that the spinal cord has circuits capable of summing inputs. This ability of the spinal cord was discovered when stimuli, on their own too weak to generate a response, was capable of eliciting a scratch response when applied in quick succession. [4]

Additionally, studies involving successive spinal transections in a turtle model have identified that spinal CPGs are distributed throughout the spinal segments asymmetrically. [7] Furthermore, the site specificity of the scratch response indicates that the spinal circuitry also has a built-in map of the body. This allows the spinal CPGs to generate a scratch response targeted to the site of the stimulus independent of supraspinal structures.

Research into form selection has revealed that form selection is also intrinsic to the spinal cord. [7] More recent research suggests that form selection is accomplished using the summed activities of populations of broadly tuned interneurons shared by various unit CPGs. [10] Additionally, intracellular recordings have illustrated that motor neurons receive at least two types of inputs from spinal CPGs. These inputs include inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs), meaning that scratch CPGs are responsible for both the activation and deactivation of muscles during the scratch response.

Very recent research suggests that the scratch reflex shares interneurons and CPGs with other locomotor tasks such as walking and swimming. [10] The findings from these studies also suggests that mutual inhibition between networks may play a role in behavioral choice in the spinal cord. This finding is supported by earlier observations on the scratch reflex, which indicate that the scratch reflex was particularly difficult to induce in animals already involved in a different locomotive task, such as walking or swimming. [2]

Supraspinal

While the scratch reflex can be produced without supraspinal structures, research indicates that neurons in the motor cortex play a role in the modulation of the scratch reflex as well. [3] Stimulation of pyramidal tract neurons has been found to modulate the timing and intensity of scratch reflex. Furthermore, extensive research has identified the involvement of supraspinal structures in the modulation of the rhythmic elements of the scratch reflex. The current theory is that efference copies from CPGs travel to the cerebellum via spinocerebellar pathways. These signals then modulate the activity of the cerebellar cortex and nuclei, which in turn regulate descending tract neurons in the vestibulospinal, reticulospinal, and rubrospinal tracts. [11] [12] [13] [14] Presently, there is not much else known about the specifics of supraspinal control of the scratch reflex.

See also

Related Research Articles

<span class="mw-page-title-main">Motor neuron</span> Nerve cell sending impulse to muscle

A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.

<span class="mw-page-title-main">Somatic nervous system</span> Part of the peripheral nervous system

The somatic nervous system (SNS), or voluntary nervous system is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.

<span class="mw-page-title-main">Muscle spindle</span> Innervated muscle structure involved in reflex actions and proprioception

Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.

<span class="mw-page-title-main">Grey column</span>

The grey column refers to a somewhat ridge-shaped mass of grey matter in the spinal cord. This presents as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which are visible in cross-section of the spinal cord.

<span class="mw-page-title-main">Reflex arc</span> Neural pathway which controls a reflex

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. 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.

The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. The reflex rapidly coordinates the contractions of all the flexor muscles and the relaxations of the extensors in that limb causing sudden withdrawal from the potentially damaging stimulus. Spinal reflexes are often monosynaptic and are mediated by a simple reflex arc. A withdrawal reflex is mediated by a polysynaptic reflex resulting in the stimulation of many motor neurons in order to give a quick response.

Central pattern generators (CPGs) are self-organizing biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. They are the source of the tightly-coupled patterns of neural activity that drive rhythmic and stereotyped motor behaviors like walking, swimming, breathing, or chewing. The ability to function without input from higher brain areas still requires modulatory inputs, and their outputs are not fixed. Flexibility in response to sensory input is a fundamental quality of CPG-driven behavior. To be classified as a rhythmic generator, a CPG requires:

  1. "two or more processes that interact such that each process sequentially increases and decreases, and
  2. that, as a result of this interaction, the system repeatedly returns to its starting condition."
<span class="mw-page-title-main">Escape reflex</span>

Escape reflex, or escape behavior, is any kind of escape response found in an animal when it is presented with an unwanted stimulus. It is a simple reflectory reaction in response to stimuli indicative of danger, that initiates an escape motion of an animal. The escape response has been found to be processed in the telencephalon.

<span class="mw-page-title-main">Vestibulospinal tract</span>

The vestibulospinal tract is a neural tract in the central nervous system. Specifically, it is a component of the extrapyramidal system and is classified as a component of the medial pathway. Like other descending motor pathways, the vestibulospinal fibers of the tract relay information from nuclei to motor neurons. The vestibular nuclei receive information through the vestibulocochlear nerve about changes in the orientation of the head. The nuclei relay motor commands through the vestibulospinal tract. The function of these motor commands is to alter muscle tone, extend, and change the position of the limbs and head with the goal of supporting posture and maintaining balance of the body and head.

<span class="mw-page-title-main">Alpha motor neuron</span>

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

The Mauthner cells are a pair of big and easily identifiable neurons located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex. The cells are also notable for their unusual use of both chemical and electrical synapses.

The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-induced. The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon. When the tension is extreme, the inhibition can be so great it overcomes the excitatory effects on the muscle's alpha motoneurons causing the muscle to suddenly relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.

<span class="mw-page-title-main">Spinal locomotion</span>

Spinal locomotion results from intricate dynamic interactions between a central program in lower thoracolumbar spine and proprioceptive feedback from body in the absence of central control by brain as in complete spinal cord injury (SCI). Following SCI, the spinal circuitry below the lesion site does not become silent rather it continues to maintain active and functional neuronal properties although in a modified manner.

<span class="mw-page-title-main">Neural substrate of locomotor central pattern generators in mammals</span>

Central pattern generators are biological neural networks organized to produce any rhythmic output without requiring a rhythmic input. In mammals, locomotor CPGs are organized in the lumbar and cervical segments of the spinal cord, and are used to control rhythmic muscle output in the arms and legs. Certain areas of the brain initiate the descending neural pathways that ultimately control and modulate the CPG signals. In addition to this direct control, there exist different feedback loops that coordinate the limbs for efficient locomotion and allow for the switching of gaits under appropriate circumstances.

<span class="mw-page-title-main">Spinal interneuron</span>

A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.

<span class="mw-page-title-main">Cutaneous reflex in human locomotion</span>

Cutaneous, superficial, or skin reflexes, are activated by skin receptors and play a valuable role in locomotion, providing quick responses to unexpected environmental challenges. They have been shown to be important in responses to obstacles or stumbling, in preparing for visually challenging terrain, and for assistance in making adjustments when instability is introduced. In addition to the role in normal locomotion, cutaneous reflexes are being studied for their potential in enhancing rehabilitation therapy (physiotherapy) for people with gait abnormalities.

<span class="mw-page-title-main">Golgi tendon organ</span> Proprioceptive sensory receptor organ that senses changes in muscle tension

The Golgi tendon organ (GTO) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a muscle and its tendon known as the musculotendinous junction also known as the myotendinous junction. It provides the sensory component of the Golgi tendon reflex.

Postural control refers to the maintenance of body posture in space. The central nervous system interprets sensory input to produce motor output that maintains upright posture. Sensory information used for postural control largely comes from visual, proprioceptive, and vestibular systems. While the ability to regulate posture in vertebrates was previously thought to be a mostly automatic task, controlled by circuits in the spinal cord and brainstem, it is now clear that cortical areas are also involved, updating motor commands based on the state of the body and environment.

<span class="mw-page-title-main">Eberhard Fetz</span> American neuroscientist, academic and researcher

Eberhard Erich Fetz is an American neuroscientist, academic and researcher. He is a Professor of Physiology and Biophysics and DXARTS at the University of Washington.

References

  1. 1 2 3 4 5 6 7 8 Stein, P. S. G. (1983). The vertebrate scratch reflex. Symposia of the Society for Experimental Biology(37), 383–403
  2. 1 2 3 4 5 6 7 Stein P (2005). "Neuronal control of turtle hindlimb motor rhythms". Journal of Comparative Physiology A. 191 (3): 213–229. doi:10.1007/s00359-004-0568-6. PMID   15452660. S2CID   23455981.
  3. 1 2 Sirota M. G., Pavlova G. A., Beloozerova I. N. (2006). "Activity of the motor cortex during scratching". Journal of Neurophysiology. 95 (2): 753–765. doi:10.1152/jn.00050.2005. PMID   16236789.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. 1 2 Sherrington CS (1906a). "Observations on the scratch-reflex in the spinal dog". J Physiol. 34 (1–2): 1–50. doi:10.1113/jphysiol.1906.sp001139. PMC   1465804 . PMID   16992835.
  5. ↑ Robertson, G. A.; Stein, P. S. G. (October 1988). "Synaptic control of hindlimb motoneurones during three forms of the fictive scratch reflex in the turtle". Journal of Physiology. 404: 101–128. doi:10.1113/jphysiol.1988.sp017281. PMC   1190817 . PMID   3253428.
  6. ↑ Earhart G. M., Stein P. S. G. (2000). "Step, swim, and scratch motor patterns in the turtle". Journal of Neurophysiology. 84 (5): 2181–2190. doi: 10.1152/jn.2000.84.5.2181 . PMID   11067964.
  7. 1 2 3 Mortin, L. I.; Stein, P. S. G. (1989). "Spinal cord segments containing key elements of the central pattern generators for three forms of scratch reflex in the turtle". Journal of Neuroscience. 9 (7): 2285–2296. doi: 10.1523/jneurosci.09-07-02285.1989 . PMC   6569757 . PMID   2746329.
  8. 1 2 3 4 Stein, P.S.G. (2003) Scratch reflex. The Handbook of Brain Theory and Neural Networks 2nd Edition, ed. by M.A. Arbib, MIT Press, Cambridge, pp. 999–1001
  9. 1 2 Stein, P.S.G. (2004) Scratch reflex. In Encyclopedia of Neuroscience, Third Edition CD-ROM, ed. by G. Adelman and B.H. Smith, Elsevier, Oxford.
  10. 1 2 Berkowitz A (2008). "Physiology and morphology of shared and specialized spinal interneurons for locomotion and scratching". Journal of Neurophysiology. 99 (6): 2887–2901. doi:10.1152/jn.90235.2008. PMID   18385486.
  11. ↑ Arshavsky Y. I., Gelfand I. M., Orlovsky G. N., Pavlova G. A. (1978a). "Messages conveyed by descending tracts during scratching in the cat. I. Activity of vestibulospinal neurons". Brain Research. 159 (1): 99–110. doi:10.1016/0006-8993(78)90112-9. PMID   728803. S2CID   24647334.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ↑ Arshavsky Y. I., Gelfand I. M., Orlovsky G. N., Pavlova G. A. (1978b). "Messages Conveyed by Spinocerebellar Pathways During Scratching in Cat .1. Activity of Neurons of Lateral Reticular Nucleus". Brain Research. 151 (3): 479–491. doi:10.1016/0006-8993(78)91081-8. PMID   667626. S2CID   43601332.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ↑ Arshavsky Y. I., Gelfand I. M., Orlovsky G. N., Pavlova G. A. (1978c). "Messages Conveyed by Spinocerebellar Pathways During Scratching in Cat .2. Activity of Neurons of Ventral Spinocerebellar Tract". Brain Research. 151 (3): 493–506. doi:10.1016/0006-8993(78)91082-x. PMID   667627. S2CID   38643070.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. ↑ Arshavsky Y. I., Orlovsky G. N., Pavlova G. A., Perret C. (1978). "Messages Conveyed by Descending Tracts During Scratching in the Cat .2. Activity of Rubrospinal Neurons". Brain Research. 159 (1): 111–123. doi:10.1016/0006-8993(78)90113-0. PMID   728791. S2CID   23733278.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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