Scratch reflex
Encyclopedia
The scratch reflex
is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternately, 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 network
s in vertebrates. Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated. This article will focus on the neurological aspects of the reflex.
In studies of spinal preparations, researchers have experimented using preparations both with and without movement-related sensory inputs . In reparations 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, either of three strategies are 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 .
(ENG) techniques are used to monitor and record from animals during experiments . 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 . These techniques have enabled researchers to understand the neural circuitry of the scratch reflex on a single-cell level.
s (CPGs) are responsible for the
generation and maintenance of the scratch reflex . One fascinating discovery about 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.
Another equally fascinating feature of the scratch reflex is that the spinal CPGs which generate and maintain the reflex are capable of producing the reflex in the absence of movement-related sensory
feedback . 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. Amazingly, 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 .
In studying the scratch reflex, researchers have named a number of regions on the surface of the body as they relate to the reflex . 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 . 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 . This research further supports the findings on 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 . 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 .
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, were capable of eliciting a scratch response when
applied in a quick succession .
Additionally, studies involving successive spinal transections in a
turtle model have identified that spinal CPGs are distributed throughout
the spinal segments asymmetrically . 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 . More recent research suggests that form
selection is accomplished using the summed activities of populations
of broadly tuned interneurons shared by various unit
CPGs . Additionally, intracellular recordings have illustrated that
motor neurons receive at least two types of inputs from spinal
CPGs. These inputs include inhibitory postsynaptic potential
s
(IPSPs) and excitatory postsynaptic potential
s (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 . 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 .
structures, research indicates that neurons in the motor cortex play a
role in the modulation of the scratch reflex as well . 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 . Presently, there is not much
else known about the specifics of supraspinal control of the scratch
reflex .
the behavior of vertebrate neural networks. This reflex serves as an
excellent model for such research because the circuitry required to
generate this response has been found to exist within the spinal
cord. This allows researchers to access, record from, and
manipulate the circuitry involved in the scratch reflex to
characterize neural networks. There still exist large voids in our
understanding of biological neural networks, even with a system as
well defined as the scratch reflex.
Reflex action
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternately, some sensory neurons can respond to a chemical stimulus that produces an itch
Itch
Itch is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to classify it as any one type of sensory experience. Modern science has shown that itch has many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response...
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 network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...
s in vertebrates. Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated. This article will focus on the neurological aspects of the reflex.
Animal Models and Preparations
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. In these studies, researchers made use of spinal preparations, which involve a complete transection of the animal's spinal cord prior to experimentation . Such preparations are used because the scratch reflex can be elicited and produced without the involvement of supraspinal structures . 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 using preparations both with and without movement-related sensory inputs . In reparations 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, either of three strategies are 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 .
Recording Techniques
Electromyographic (EMG) and electroneurographicElectroneurogram
An electroneurogram is a method used to visualize directly recorded electrical activity of neurons in the central nervous system or the peripheral nervous system . The acronym ENG is often used. An electroneurogram is similar to an electromyogram , but the later is used to visualize muscular...
(ENG) techniques are used to monitor and record from animals during experiments . 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 . These techniques have enabled researchers to understand the neural circuitry of the scratch reflex on a single-cell level.
General Characteristics
The scratch reflex is generally a rhythmic response. Results from animal studies have indicated that spinal neural networks known as central pattern generatorCentral pattern generator
Central pattern generators are neural networks that produce rhythmic patterned outputs without sensory feedback. CPGs have been shown to produce rhythmic outputs resembling normal "rhythmic motor pattern production" even in isolation from motor and sensory feedback from limbs and other muscle...
s (CPGs) are responsible for the
generation and maintenance of the scratch reflex . One fascinating discovery about 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.
Another equally fascinating feature of the scratch reflex is that the spinal CPGs which generate and maintain the reflex are capable of producing the reflex in the absence of movement-related sensory
feedback . 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. Amazingly, 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 .
In studying the scratch reflex, researchers have named a number of regions on the surface of the body as they relate to the reflex . 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 . 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
Reciprocal inhibition
“ When the central nervous system sends a message to the agonist to contract, the tension in the antagonist is inhibited by impulses from motor neurons, and thus must simultaneously relax. This neural phenomenon is called reciprocal inhibition. This information can be used to ease the pain of an...
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 . This research further supports the findings on 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 . 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 .
Spinal Characteristics
As described in the general characteristics above, the scratch reflexis 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, were capable of eliciting a scratch response when
applied in a quick succession .
Additionally, studies involving successive spinal transections in a
turtle model have identified that spinal CPGs are distributed throughout
the spinal segments asymmetrically . 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 . More recent research suggests that form
selection is accomplished using the summed activities of populations
of broadly tuned interneurons shared by various unit
CPGs . Additionally, intracellular recordings have illustrated that
motor neurons receive at least two types of inputs from spinal
CPGs. These inputs include inhibitory postsynaptic potential
Inhibitory postsynaptic potential
An inhibitory postsynaptic potential is a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron or α-motoneuron...
s
(IPSPs) and excitatory postsynaptic potential
Excitatory postsynaptic potential
In neuroscience, an excitatory postsynaptic potential is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels...
s (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 . 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 .
Supraspinal Characteristics
While the scratch reflex can be produced without supraspinalstructures, research indicates that neurons in the motor cortex play a
role in the modulation of the scratch reflex as well . 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 . Presently, there is not much
else known about the specifics of supraspinal control of the scratch
reflex .
Conclusion
The scratch reflex has been extensively studied to understandthe behavior of vertebrate neural networks. This reflex serves as an
excellent model for such research because the circuitry required to
generate this response has been found to exist within the spinal
cord. This allows researchers to access, record from, and
manipulate the circuitry involved in the scratch reflex to
characterize neural networks. There still exist large voids in our
understanding of biological neural networks, even with a system as
well defined as the scratch reflex.