Spine J (N American Spine Society) 2002 (Sep); 2 (5): 357–371 ~ FULL TEXT
Joel G. Pickar, DC, PhD
Palmer Center for Chiropractic Research,
1000 Brady Street,
Davenport, IA 52803, USA.
BACKGROUND CONTEXT: Despite clinical evidence for the benefits of spinal manipulation and the apparent wide usage of it, the biological mechanisms underlying the effects of spinal manipulation are not known. Although this does not negate the clinical effects of spinal manipulation, it hinders acceptance by the wider scientific and health-care communities and hinders rational strategies for improving the delivery of spinal manipulation.
PURPOSE: The purpose of this review article is to examine the neurophysiological basis for the effects of spinal manipulation.
CONCLUSIONS: A review article discussing primarily basic science literature and clinically oriented basic science studies.
METHODS: This review article draws primarily from the peer-reviewed literature available on Medline. Several textbook publications and reports are referenced. A theoretical model is presented describing the relationships between spinal manipulation, segmental biomechanics, the nervous system and end-organ physiology. Experimental data for these relationships are presented.
RESULTS: Biomechanical changes caused by spinal manipulation are thought to have physiological consequences by means of their effects on the inflow of sensory information to the central nervous system. Muscle spindle afferents and Golgi tendon organ afferents are stimulated by spinal manipulation. Smaller-diameter sensory nerve fibers are likely activated, although this has not been demonstrated directly. Mechanical and chemical changes in the intervertebral foramen caused by a herniated intervertebral disc can affect the dorsal roots and dorsal root ganglia, but it is not known if spinal manipulation directly affects these changes. Individuals with herniated lumbar discs have shown clinical improvement in response to spinal manipulation. The phenomenon of central facilitation is known to increase the receptive field of central neurons, enabling either subthreshold or innocuous stimuli access to central pain pathways. Numerous studies show that spinal manipulation increases pain tolerance or its threshold. One mechanism underlying the effects of spinal manipulation may, therefore, be the manipulation's ability to alter central sensory processing by removing subthreshold mechanical or chemical stimuli from paraspinal tissues. Spinal manipulation is also thought to affect reflex neural outputs to both muscle and visceral organs. Substantial evidence demonstrates that spinal manipulation evokes paraspinal muscle reflexes and alters motoneuron excitability. The effects of spinal manipulation on these somatosomatic reflexes may be quite complex, producing excitatory and inhibitory effects. Whereas substantial information also shows that sensory input, especially noxious input, from paraspinal tissues can reflexively elicit sympathetic nerve activity, knowledge about spinal manipulation's effects on these reflexes and on end-organ function is more limited.
CONCLUSIONS: A theoretical framework exists from which hypotheses about the neurophysiological effects of spinal manipulation can be developed. An experimental body of evidence exists indicating that spinal manipulation impacts primary afferent neurons from paraspinal tissues, the motor control system and pain processing. Experimental work in this area is warranted and should be encouraged to help better understand mechanisms underlying the therapeutic scope of spinal manipulation.
From the FULL TEXT Article:
Recent reports estimate that 7.7% to 8.3% of the US population uses some form of complementary or alternative
medicine [1–3]. Approximately 30% to 40% of these individuals likely receive spinal manipulation . Strong evidence supports using spinal manipulation to help patients
with acute low back pain and neck pain [4, 5]. The benefits
of spinal manipulation for other disorders, such as chronic
low back pain and visceral disorders, are less clear, although
benefits have been noted [4, 6–8]. Despite the clinical evidence for the benefits of and the apparent wide usage of spinal manipulation, the biological mechanisms underlying the
effects of spinal manipulation are not known. Although this
does not negate the clinical effects of spinal manipulation, it
hinders acceptance by the wider scientific and health-care
communities and hinders rational strategies for improving
the delivery of spinal manipulation. The purpose of this review article is to examine the neurophysiological basis for
and the neurophysiological effects of spinal manipulation.
Biomechanical considerations of spinal manipulation
Spinal manipulation by its very nature is a mechanical
input to tissues of the vertebral column. Chiropractors deliver more than 90% of these manipulations in the United
States . Spinal manipulation is distinguished from spinal
mobilization in several ways . During spinal manipulation, the practitioner delivers a dynamic thrust (impulse) to
a specific vertebra. The clinician controls the velocity, magnitude and direction of the impulse . The art or skill of
spinal manipulation lies in the clinician’s ability to control
these three factors once the specific contact with a vertebra
is made. Mobilization techniques are sometimes used preparatory to the manipulation. Manipulation is also distinguished
from mobilization in that it is delivered at or near the end of
the physiological range of motion (the so-called paraphysiological range ) but not exceeding the anatomical limits of motion. A cracking or popping sound often, but not necessarily, accompanies the manipulation, because gapping the joint creates fluid cavitation [13, 14].
The most common form of spinal manipulation used by
chiropractors is the short-lever, high-velocity and lowamplitude thrust . The clinician usually delivers the
dynamic thrust through a short-lever arm by manually contacting paraspinal tissues overlying the spinous, transverse
or mammillary processes of the vertebra being manipulated.
Alternatively, the clinician contacts tissues overlying the
lamina or articular pillar of the vertebra. To manipulate the
pelvis, the iliac spine or the ischial spine is used . Spinal
manipulation may also be delivered through a long-lever
arm. While one hand may contact a specific area over the
vertebra being manipulated, the second hand contacts an
area of the body distant from the specific contact. Force is
developed through this long-lever arm. However, using a
short-lever arm applied directly over the vertebra minimizes
the force necessary to accomplish the manipulation  by
reducing the amount of compliant tissue through which the
force must be transmitted.
Several laboratories have studied biomechanical features
of short-lever, high-velocity and low-amplitude manipulation. Herzog’s group  was the first to report the biomechanical features of a spinal manipulation in an indexed
journal. They identified two characteristics common to the
delivery of a spinal manipulation: 1) a preload force followed by 2) a larger impulse force. Using two chiropractors,
they quantified the preload and peak impulse forces applied
perpendicular to the contact point and the impulse duration
during manipulation of the sacroiliac joint. Preload load
forces ranged from 20 to 180 N, and peak forces ranged from
220 to 550 N. Often the preload was approximately 25% of
impulse load. The duration of the high-velocity impulse
ranged from 200 to 420 ms.
A number of studies have confirmed the force–time profile initially described by Hessel et al. . Herzog et al.
 showed the time to peak impulse was similar during
manipulation of the thoracic spine and sacroiliac joint (approximately 150 ms77 ms, mean SD). The perpendicularly applied preload and peak impulse forces were also similar
during spinal manipulations applied to the thoracic (13946 N
vs. 8878 N, respectively) and sacroiliac (32878 N vs.
399119 N, respectively) regions. Studies of the cervical
spine indicate that preload, peak impulse force and time to
peak impulse are less compared with the thoracic and lumbosacral spine [17–19]. Depending upon the type of cervical
manipulative technique used, preload forces range from 0 to
approximately 50 N, and peak impulse forces range from
approximately 40 N to approximately 120 N. The forces delivered during cervical manipulations develop faster than
during manipulation of the thoracic spine and sacroiliac joint.
Impulse duration lasts from approximately 30 ms to approximately 120 ms. The large variability in the applied forces and
durations should be recognized. The impact of this variability
on the biological mechanisms that could contribute to the
clinical effects of manipulation is unknown.
A complete understanding of the biomechanics of spinal manipulation requires knowing the manner in which
manipulative loads are transmitted to a specific vertebra.
Experimentally, this is substantially more difficult and more
complex compared with measuring applied loads. Transmitted loads may be different from applied loads because of the
effects of patient positioning and the contributions from inertial loads, loading moments and the active and passive
properties of the intervening connective and muscle tissues.
Triano and Schultz  calculated peak transmitted loads at
a lumbar segment by measuring loads transmitted to a force
plate placed under the subject. The force plate was capable
of transducing forces and moments about three orthogonal
axes. Peak forces transmitted to a lumbar segment during a
side posture spinal manipulation tended to be higher than
peak forces applied during a prone thoracic or sacroiliac
manipulation measured by Herzog et al. . Transmitted
impulse durations were similar to applied impulse durations
measured by Herzog et al. . Peak transmitted moments
were approximately three to four times less than peak transmitted forces. The transmitted loads were considered below
a threshold level capable of injuring the lumbar spine (see
 for further discussion).
In addition to applied and transmitted loads, the relative displacement or movement between contiguous vertebrae during
a spinal manipulation has been studied. Nathan and Keller 
measured intervertebral lumbar motion using pins inserted into
lumbar spinous processes. Manipulations were delivered using
a mechanical adjusting device (Activator Adjusting Instrument, Activator Methods International, Ltd., Phoenix, AZ
). Impulse duration using this device is approximately
5 ms, an impulse duration shorter than that from manual manipulation. Impulses delivered to the L2 spinous produced 1.62
mm1.06 mm peak axial displacement (in the longitudinal
plane), 0.480.1 mm shear displacement (in the transverse
plane) and 0.890.49° of rotation between L3 and L4 .
Smith et al.  measured similar vertebral displacements in
the lumbar spine of the dog. L2 translated 0.710.03 mm and
rotated 0.530.15° on L3 with impulse loads of 53 N. Gal et
al.  performed measurements in the thoracic spine, but their
results are difficult to compare with those reported above for
the lumbar spine. Nonetheless, the movements induced during
a spinal manipulative load suggest that mechanical processes
may play a role in the biological effects of spinal manipulation.
Neurophysiological and biomechanical mechanisms underlying the effects of spinal manipulation
Numerous theories have been proposed to explain the effects of spinal manipulation [25, 26]. A thread common to
many of these theories is that changes in the normal anatomical, physiological or biomechanical dynamics of contiguous
vertebrae can adversely affect function of the nervous system.
[27, 28]. Spinal manipulation is thought to correct these changes.
Accordingly, a number of biomechanical changes produced by vertebral movement during a spinal manipulation
have been hypothesized. The mechanical force introduced
into the vertebral column during a spinal manipulation may
directly alter segmental biomechanics by releasing trapped
meniscoids, releasing adhesions or by reducing distortion of
the annulus fibrosus [29–33]. In addition, individual motion
segments can buckle, thereby producing relatively large vertebral motions that achieve a new position of stable equilibrium . The mechanical changes elicited by manipulation
may provide sufficient energy to restore a buckled segment to
a lower energy level, thus reducing mechanical stress or
strain on soft and hard paraspinal tissues . A major consequence of these hypothesized mechanical changes elicited
by manipulation could be the restoration of zygapophyseal
joint mobility and joint play . In fact, authoritative discussion of spinal manipulation considers “the goal of manipulation to restore maximal, pain-free movement of the
musculoskeletal system” (from [35a] and see [31, 36, 37]).
Biomechanical changes caused by the manipulation are
thought to have physiological consequences by means of
their effects on the inflow of sensory information to the central nervous system [25, 36]. By releasing trapped meniscoids,
discal material or segmental adhesions, or by normalizing a
buckled segment, the mechanical input may ultimately reduce nociceptive input from receptive nerve endings in innervated paraspinal tissues. This would be consistent with the
observation that spinal manipulation is not painful when administered correctly. In addition, the mechanical thrust could
either stimulate or silence nonnociceptive, mechanosensitive
receptive nerve endings in paraspinal tissues, including skin,
muscle, tendons, ligaments, facet joints and intervertebral
disc [28, 38, 39]. These neural inputs may influence painproducing mechanisms as well as other physiological systems controlled or influenced by the nervous system.
Figure 1 diagrams the theoretical relationships between spinal manipulation, segmental biomechanics, the nervous system and end-organ physiology. A biomechanical alteration
between vertebral segments hypothetically produces a biomechanical overload the effects of which may alter the signaling properties of mechanically or chemically sensitive
neurons in paraspinal tissues. These changes in sensory input are thought to modify neural integration either by directly affecting reflex activity and/or by affecting central
neural integration within motor, nociceptive and possibly
autonomic neuronal pools. Either of these changes in sensory input may elicit changes in efferent somatomotor and
visceromotor activity. Pain, discomfort, altered muscle function or altered visceromotor activities comprise the signs or
symptoms that might cause patients to seek spinal manipulation. Spinal manipulation, then, theoretically alters the inflow
of sensory signals from paraspinal tissues in a manner that
improves physiological function. This explanation comprises
one of the most rational neurophysiological bases for the
mechanisms underlying the effects of spinal manipulation.
Experimental efforts to understand sensory processing from
paraspinal tissues and the effects of spinal manipulation on
this sensory processing is receiving increasing attention, as
described below. Each of the following sections addresses a
component of the theoretical relationship depicted in Fig. 1
with each section’s number corresponding to a numbered
component in the figure.
1. The effects of spinal manipulation on sensory receptors
in paraspinal tissues
Group I and II afferents (proprioceptive afferents)
Korr  proposed that spinal manipulation increases
joint mobility by producing a barrage of impulses in muscle
spindle afferents and smaller-diameter afferents ultimately
silencing facilitated γ motoneurons. Figure 2 shows the neural circuitry of the γ loop. He hypothesized that γ-motoneuron discharge is elevated in muscles of vertebral segments responding to spinal manipulation. The high gain of the γ loop would impair joint mobility by sensitizing the stretch reflex to abnormally small changes in muscle length. Korr
further hypothesized that spinal manipulation stimulates
muscle spindle afferents, that is, Group Ia and possibly
Group II afferents (Table 1). The barrage of impulses from
these afferents produced by the spinal manipulation would
reduce the gain of the γ loop through an undetermined neural pathway. Although portions of this mechanism remain
speculative, the contribution of proprioceptive afferents to
spine function and the neurophysiological effects of spinal
manipulation on these afferents are receiving increasing attention.
The importance of paraspinal proprioceptive input to the
function of the vertebral column, and of the lumbar spine in
particular, has been demonstrated recently in humans. Several
studies indicate that muscle spindle input from the lumbar
multifidus helps to accurately position the pelvis and lumbosacral spine. Healthy individuals can accurately reposition
their lumbosacral spine, but their repositioning ability is impaired when the multifidus muscle is vibrated . Vibration
stimulates muscle spindles and creates a sensory illusion that
the multifidus is stretched and therefore that the spine is
flexed more than it actually is. The repositioning error occurs
because of the misperception of vertebral position. Interestingly, lumbosacral-repositioning ability is impaired in individuals with a history of low back pain, even in the absence
of vibration . This finding was associated with altered
proprioceptive input from muscle spindles . In addition,
paraspinal muscles in individuals with a history of low back
pain also have longer response times to sudden loads, which
also suggests the presence of abnormal paraspinal proprioceptive input in these individuals [42–44].
Two experimental models have been developed recently
that should enhance our neurophysiological understanding
of the lumbar and cervical spine in general and of spinal manipulation specifically [45, 46]. The experimental preparations
enable the recording of neural activity from paraspinal tissues
under conditions where controlled mechanical loads can be
applied to an individual vertebra. The discharge properties of
primary afferents with receptive fields in paraspinal tissues
and the effects of these sensory inputs on neurons in the spinal cord can be determined. The preparations isolate the
spinous process of a cervical  or lumbar  vertebra and
use a servo-driven motor to control the displacement of or
force applied to the spinous process. These preparations will
enable neurophysiological studies not possible in humans.
Recent findings using one of the experimental models
described above  demonstrate that spinal manipulation
modifies the discharge of Group I and II afferents. Pickar
and Wheeler  recorded single-unit activity from muscle
spindle and Golgi tendon organ afferents having receptive
fields in the lumbar multifidus and longissimus muscles
while applying a spinal manipulative-like load to a lumbar
vertebra. Golgi tendon organ afferents were generally silent
at rest and were activated more by the impulsive thrust of a
spinal manipulation than by the static load preparatory to
the thrust. Their silence resumed at the end of the manipulation. Muscle spindles generally had a resting discharge that
also increased more to the impulse than to the preload
(200% compared with 30%). The spindles were silenced for
1.3 seconds on average after the manipulative impulse. In
addition, a presumed Pacinian corpuscle responded to the impulse of a manipulative-like load but not to loads with a slower
force–time profile. The discharge of these three types of afferents may represent a portion of the neural discharge recorded by Colloca et al.  during spinal manipulation in an anesthetized human patient undergoing an L4–L5 laminectomy. They
recorded multiunit activity from the intact S1 nerve root
during spinal manipulations of the lumbosacral region using
a low-force, short-duration (impulse) loading apparatus (ie,
Activator Adjusting Instrument ).
Group III and IV afferents
Electrophysiological recordings from Group III and IV
afferents innervating the spine of rats, rabbits and cats have
helped us understand the mechanical and chemical stimuli
that can excite the receptive endings of sensory paraspinal
neurons. Cavanaugh et al.  recorded afferent activity from
the medial branch of the dorsal primary rami after removing
the superficial and deep lower back muscles in the rat. Gentle
probing of the facet capsule elicited a slowly adapting discharge, whereas forceful pulling on the supraspinous ligament
elicited a slowly adapting discharge from afferents in the lumbar spine. The forces applied to these tissues were not quantified. The afferents from which Cavanaugh et al.  recorded
were likely slowly conducting, that is, Group III and/or Group
IV afferents, but classification based on single-unit conduction
velocities was not obtained. Pickar and McLain  recorded
single-unit activity from Group III (conduction velocity,
9.06.6 m/s) and Group IV (conduction velocity, 1.50.5 m/s)
afferents with receptive fields in feline lumbar paraspinal tissues. They measured the response of these small-diameter
neurons to movement of the L5–L6 facet joint. The majority
of afferents, including seven with receptive fields in or near
the facet joint capsule, responded in a graded fashion to the
direction of a nonnoxious load applied to the joint. Yamashita
et al.  found that only 20% of Group III afferents in and
around the lumbar facet joint had high mechanical thresholds
(greater than 8.5 gm), as determined with von Frey–like hairs.
This latter finding contrasts with afferents studied in the cervical spine, where almost all Group III afferents studied had
high mechanical thresholds . However, Yamashita et al.
 further showed that substance P increases the resting discharge and decreases the von Frey threshold by +80% and
–30%, respectively, of the afferents in and around the lumbar
facet joint. This suggests that inflammation may decrease
the mechanical thresholds of receptive endings around a lumbar facet joint. Again, this contrasts with the discharge properties of Group III afferents in the cervical region, which were
not sensitive to the inflammatory mediator bradykinin . To
date, there have been no studies investigating the effects of spinal manipulation on the discharge properties of small-diameter, thinly myelinated and unmyelinated sensory neurons innervating paraspinal tissues.
The studies cited above make it reasonable to think that
spinal manipulation may add a novel sensory input or remove
a source of aberrant input. Gillette  presented a speculative
yet comprehensive analysis of the receptive nerve endings potentially affected by spinal manipulation. He suggested that
40 types of mechanoreceptive endings in the skin and deep
tissues of the paraspinal region could be activated, because
they have mechanical thresholds below the level of mechanical
force applied during a manipulation. The mechanoreceptors
include proprioceptors (muscle spindles, both primary and secondary endings and Golgi tendon organs), low-threshold mechanoreceptors, high-threshold mechanoreceptors, high-threshold
mechanonociceptors and high-threshold polymodal nociceptors
. Thus, all classifications of sensory neurons, that is, Group Ia, Ib, II, III and IV fibers (Table 1), could be affected, theoretically, by spinal manipulation.
2. The effects of spinal manipulation on neural tissue
within the intervertebral foramen
The spinal roots within the intervertebral foramen (IVF)
possess unusual anatomical properties, having less connective tissue support and protection compared with peripheral
nerve [53, 54]. As the peripheral nerve trunk enters the IVF,
its epineurium separates from the trunk and becomes continuous with the dura mater. Perineurium surrounding individual fascicles is lost as the fascicles separate into ventral
and dorsal roots. Endoneurium surrounding the individual
Schwann cells that ensheath both the myelinated and unmyelinated axons continue into the nerve roots, but the endoneurium’s collagen content becomes less dense and is no longer
organized as a protective sheath . In addition, the density
of Na+ channels in the soma and initial segment of dorsal
root ganglia cells is relatively high, suggesting these regions
may be unusually excitable . These properties may render
neural tissue within the IVF vulnerable to effects of mechanical
compression and the chemical environment produced by
changes in the intervertebral disc or facet joints .
Substantial evidence demonstrates that the dorsal roots
(DRs) and dorsal root ganglia (DRG) are more susceptible to
the effects of mechanical compression than are the axons of
peripheral nerves, because impaired or altered function is
produced at substantially lower pressures [57, 58]. Compressive loads as low as 10 mg applied rapidly to the DRs slightly
increases the discharge of Group I, II, III and IV afferents
. Slowly repeated loads or gradually increasing loads
produce conduction block [58, 59]. Maintained compressive
pressures as low as 20 mm Hg applied to the DRs cause conduction block . Although the DRs are not as sensitive as
the DRGs to mechanical pressure, prior mechanical injury
greatly increases resting DR discharge. In contrast, only
slight mechanical compression applied to the DRG is sufficient to produce large, prolonged increases in the discharge of
Group I, II, III and IV afferents even in the absence of prior
mechanical injury [58, 59, 61].
Mechanical compression of the DRs or DRG, in addition to
altering impulse-based neural transmission (ie, action potentials), may alter non–impulse-based mechanisms (eg, axoplasmic transport). This biological concept was introduced into the literature of spinal manipulation nearly a quarter century ago . Applying as little as 10 mm Hg pressure to the DRs reduces by 20% to 30% nutritional transport to the peripheral
axons as measured by tracer-labeled glucose . DR compression reduces the transport rate of the neuropeptide subtance P but not vasointestinal peptide . In addition, DRG
compression increases endoneurial fluid pressure and is accompanied by edema and hemorrhage within the DRG .
Compression studies, like those described above, laid experimental groundwork for investigating how herniated intervertebral discs affect nerve root function. Clearly, the
idea that a herniated disc could directly compress the DRs
or DRG is straightforward. Recently, pressure between a
herniated disc and the nerve root was measured in 34 humans undergoing surgery for lumbar disc herniation .
Mean pressures of 53 mm Hg (range, 7 to 256 mm Hg) were
measured. A second idea describing how herniated intervertebral discs could affect nerve root function suggests that its effects are mediated indirectly by the release of neuroactive
chemicals . This mechanism would help explain the
common observation that, even in the absence of compression, herniated discs are accompanied by neurological findings. Recent studies demonstrate that the application of nucleus pulposus to a lumbar nerve root causes mechanical hyperalgesia in the distal limb and causes swelling in and decreased blood flow to the DRG [67, 68]. In addition, phospholipase A2 (PLA2 ), an inflammatory mediator associated with disc herniation [66, 69], is neurotoxic in high doses to Group I, II, III and IV . In moderate doses it increases mechanical sensitivity of the DRs, producing long-lasting discharge, and it increases the discharge of previously silent DRG cells [61, 70].
Whereas increasing evidence demonstrates that the mechanical and chemical consequences of a herniated disc can affect
neural tissue within the IVF, no studies were found investigating the effects of spinal manipulation on the mechanical or
chemical environment of the IVF. Whether spinal manipulation
can alter neural function by mechanically changing compressional pressures or reducing the concentration of metabolites in
the IVF is unknown. However, several case studies [35, 71, 72]
and randomized clinical studies [73, 74] show that spinal manipulation of patients with herniated intervertebral discs can
be followed by clinical improvements. These findings warrant further investigation. Without adequate basic science
studies, it will be difficult to determine the mechanism of
action underlying observed clinical improvements.
3. The effects of spinal manipulation on central facilitation
Central facilitation (also called central sensitization) refers to the increased excitability or enhanced responsiveness of dorsal horn neurons to an afferent input. Central facilitation can be manifested by increased spontaneous central neural activity, by enhanced discharge of central neurons to an afferent input or by a change in the receptive field properties of central neurons .
Denslow et al.  were one of the first groups of investigators to systematically study the neural organization of
tender areas in paraspinal tissues. Their findings lead to one
of the predominant rationales for the clinical use of spinal
manipulation, namely, the premise that persistent alterations
in normal sensory input from a functional spinal unit increases
the excitability of neuronal cells or circuits in the spinal cord
[25, 36, 76]. They observed that muscles with firm texture,
which accompany postural abnormalities, show electromyographic (EMG) characteristics different from muscles with
normal texture. Either spontaneous EMG activity was present
or EMG activity could be induced unlike the normal area
[77, 78]. In subsequent studies, Denslow et al. [76, 79] showed
that reflex erector spinae activity evoked by pressure placed
against paraspinal tissues varied between subjects and between vertebral segments. The patterns they observed suggested that motoneurons could be held in a facilitated state
because of sensory bombardment from segmentally related
paraspinal structures. The motor reflex thresholds also correlated with pain thresholds, further suggesting that some sensory pathways were also sensitized or facilitated in the abnormal segment .
We currently know that the phenomenon of central facilitation increases the receptive field of central neurons and
allows innocuous mechanical stimuli access to central pain
pathways . In other words, subthreshold mechanical
stimuli may initiate pain, because central neurons have become sensitized. Removal of these subthreshold stimuli
should be clinically beneficial. One mechanism underlying
the clinical effects of spinal manipulation may be the removal of subthreshold stimuli induced by changes in joint
movement or joint play (see previous section: Neurophysiological and biomechanical mechanisms underlying the effects of spinal manipulation). In addition, nonnoxious mechanical inputs themselves can also have therapeutic effect.
The gate control theory of Melzack and Wall  drew attention to the active role of the dorsal horn of the spinal
cord. The dorsal horn is not simply a passive relay station
for sensory messages but can modulate the messages as
well. Numerous studies inspired by Melzack and Wall’s
theory clearly demonstrate that nonnoxious mechanical inputs travelling by means of the large, myelinated A fiber
neurons can inhibit the response of dorsal horn neurons to
nociceptive stimuli from C fibers (reviewed in ). Natural activation of A-α and A-β fibers (Table 1) has been
shown to reduce chronic pain and increase pain threshold
levels (reviewed in ). If such a gate mechanism contributes to the effects of spinal manipulation, the means by
which such a short-lasting nonnoxious mechanical input
produces a long-lasting effect needs to be understood.
Effects on pain and pain processing
Numerous studies suggest that spinal manipulation alters
central processing of innocuous, mechanical stimuli, because pain tolerance or threshold levels increase. In patients
with low back pain, Glover et al.  examined areas of
lumbar skin that were painful to a pinprick. Fifteen minutes
after spinal manipulation of the lumbar region, the size of
the area from which the pinpricks evoked pain was reduced
compared with the control group receiving detuned short-wave
therapy. Terrett and Vernon  quantified the reduction in
pain sensitivity after spinal manipulation. They established a
model of pain sensation using graded, electrical stimulation of
cutaneous paraspinal tissues. A blinded observer assessed the
minimal current necessary to evoke pain (pain threshold)
and the maximal tolerable current that evoked pain (pain
tolerance) in subjects with tender regions of the thoracic
spine. Spinal manipulation significantly increased (1.5-fold)
pain tolerance levels within 30 seconds. Over the next 9.5
minutes, tolerance levels progressively increased (up to 2.4-
fold; Figure 3).
Continued efforts to determine and quantify the effects of
spinal manipulation on nociceptive processing have made use
of the pressure algometer. The reliability and validity of this
pressure gauge have been demonstrated [85, 86]. Vernon 
measured changes in the pressure/pain threshold after spinal
manipulation using this device sensation. The pressure/pain
threshold represents the magnitude of the pressure at which
the subject reports that the sensation of pain changes to a
sensation of tenderness. In this case study, spinal manipulation increased the average pressure/pain threshold of six
tender spots in the neck region by approximately 50% (from
2 kg/cm2 to 2.9 kg/cm2). In a study of the lumbar spine, neither spinal manipulation nor spinal mobilization changed the pressure/pain thresholds at three standardized locations in patients with chronic mechanical low back pain . The standardized locations were myofascial trigger points associated with low back pain but were not necessarily clinically
relevant (ie, tender) to the patient. These latter results, when
compared with those from Vernon’s study , could suggest that physiological responses to spinal manipulation are
specific to regions of the vertebral column. Alternatively, the
results suggest that the neurophysiological effects of spinal
manipulation on pain processing will be understood only
when symptomatic sites are chosen based on their degree of
tenderness or painfulness to the patient. Overall, the findings
are provocative and warrant continued investigation. If spinal manipulation initiates changes in the central facilitatory
state of the spinal cord, then understanding the relationship
between biomechanical inputs to and the neurophysiological responses from paraspinal tissues will enable us to optimize the delivery of these manipulations.
The effect of spinal manipulation on pain could also be mediated by the neuroendocrine system. The endogenous opiate system is known to modify pain processes , and a number of therapeutic modalities, including acupuncture , transcutaneous nerve stimulation  and exercise , are thought to exert pain-relieving effects through activation of this system. Several studies have investigated the effect on spinal manipulation on circulating levels of β-endorphin. The findings have been inconsistent for possible reasons discussed by Rosner . Vernon et al.  reported an 8% increase in plasma β-endorphin levels 5 minutes after spinal manipulation but
not after control interventions. Christian et al.  did not
find any change in plasma β-endorphin levels, but their assay would have been unable to detect an 8% increase because their between-assay variation was greater than the
8%. On the other hand, Sanders et al.  did not find any
change in plasma β-endorphin levels despite a reduction in
the visual analog pain scale in the group receiving spinal manipulation. Anti–pain-producing effects of β-endorphin can
be mediated by their ability to bind to membrane-bound receptors on sensory nerve endings in the periphery as well as
to receptors in the spinal cord and brain. However, the relationship between circulating levels of β-endorphin and the release of β-endorphin in the spinal cord is not known . Thus, while the experiments cited may indicate a response
mediated by peripheral receptors, the effects of spinal manipulation on β-endorphin release within the central nervous system are unknown.
4. The effects of spinal manipulation on somatosomatic (muscle) reflexes
Substantial evidence demonstrates that spinal manipulation evokes paraspinal muscle reflexes and alters motoneuron excitability. In asymptomatic patients. Herzog’s group
[98, 99] showed that posterior to anterior spinal manipulative treatments applied to the cervical, thoracic lumbar and
sacroiliac regions increased paraspinal EMG activity in a
pattern related to the region of the spine that was manipulated. The EMG response latencies occur within 50 to 200
ms after initiation of the manipulative thrust. Similarly, spinal manipulation using an Activator Adjusting Instrument
applied to a transverse process elicits paraspinal EMG activity at the same segmental level but within 2 to 3 ms .
Colloca and Keller  confirmed these latter findings in
symptomatic patients with low back pain. In addition, they
reported that the increased EMG activity, while beginning
within 2 to 3 ms of the manipulation, reached its peak within
50 to 100 ms. EMG activity representing a strong reflex response in terms of peak amplitude was relatively long lived
(greater than 273 ms), whereas EMG activity representing
weak reflex responses was more short lived (less than 273 ms).
Paraspinal EMG responses were greatest in magnitude
when the manipulation was delivered close to the electrode
site and, interestingly, the more chronic the low back pain,
the less the EMG response. It is important to note that the
EMG electrodes were not placed relative to any physical
finding associated with the low back, for example, a presumed site of muscle spasm or a site of muscle pain or tenderness.
The effect of spinal manipulation on paraspinal muscle
activity is not only excitatory. In one symptomatic patient
with spontaneous muscle activity in the thoracic spine,
Suter et al.  observed reduced paraspinal EMG activity
within 1 second after a thoracic spinal manipulation. DeVocht obtained similar findings in a symptomatic patient
with low back pain (Figure 4, unpublished observations). He placed EMG electrodes over palpably taut lumbar paraspinal muscles and often observed a decrease in spontaneous
EMG activity after spinal manipulation using an Activator
Adjusting Instrument and treatment protocol. The decreased
muscle activity did not occur instantaneously.
The effects of spinal manipulation on somatomotor activity
may be quite complex, producing excitatory and inhibitory effects. It is worth noting that many of the individual human
studies cited above were performed on either symptomatic or
asymptomatic individuals but not both. In addition, EMG recordings were sometimes obtained from standardized sites
and in other studies were obtained relative to clinical findings
of taut muscle fibers. Paradoxical findings may be reconciled if
future studies compare the effects of spinal manipulation on
symptomatic versus asymptomatic subjects and on anatomical
sites with clinically identified or quantified signs. Clearly, the potential for spinal manipulation to inhibit motor activity can be determined only under experimental conditions where muscle activity is spontaneously present or has been evoked.
The effects of spinal manipulation on paraspinal EMG
activity may be associated with increases in muscle strength
measured after spinal manipulation. Suter et al.  studied symptomatic patients with sacroiliac joint dysfunction,
anterior knee pain and evidence of motor inhibition to knee
extensor muscles. A side posture spinal manipulation applied to the sacroiliac joint significantly decreased the inhibition of the knee extensors on the side of the body to which
the manipulation was applied. Similarly, Keller and Colloca
found that erector spinae isometric strength (assessed using
EMG activity) was increased after spinal manipulation compared with sham manipulation . In neurophysiological
terms, these two studies indicate that spinal manipulation improves muscle function either through facilitation or disinhibition of neural pathways.
A series of studies has sought to understand how spinal
manipulation affects central processing of motor control information. The studies indicate that spinal manipulation can
both increase the excitability of motor pathways in the spinal cord and depress the inflow of sensory information from
muscle spindles. In asymptomatic patients, Dishman et al.
 showed that spinal manipulation increases central motor excitability (Figure 5). EMG activity from gastrocnemius
muscle evoked by direct activation of descending corticospinal
tracts using transcranial magnetic stimulation was larger
after lumbar spinal manipulation compared with simply
positioning the patient but not applying the manipulation.
Spinal manipulation also depresses the H reflex. Manipulation
applied to the sacroiliac joint in a posterior to anterior direction decreased the magnitude of the tibial nerve H reflex for up to 15 minutes in asymptomatic humans . Similarly,
side-posture lumbar manipulation of L5–S1 joint inhibited
the H reflex from the tibial nerve . The effects of mobilization alone applied to the same joint were similar, but the effects of manipulation tended to be greater. After manipulation alone, the inhibition lasted for approximately 20 seconds but lasted up to 1 minute when manipulation was preceded by spinal mobilization. These contrasting effects on EMG activity, between methodologies using motor evoked potentials versus the H reflex, may reflect the differential effects of sensory input evoked by spinal manipulation on postsynaptic processing versus presynaptic inhibition, respectively (see, for extensive discussion, Dishman et al. .
One possible mechanism contributing to spinal manipulation’s inhibitory effects on the H reflex and on spontaneous paraspinal EMG activity is suggested by recent experiments. Sensory input from facet joint tissues stimulated
during spinal manipulation might reflexively decrease
paraspinal muscle activity. Indahl et al.  elicited reflex
longissimus and multifidus muscle (EMG) activity by
electrically stimulating the intervertebral disc in a porcine
preparation. Stretching the facet joint by injecting 1 ml
physiological saline abolished the EMG activity.
There is reason to believe that stretching the facet joint
capsule and surrounding tissues likely occurs during spinal
manipulation, although this has received little study .
Using magnetic resonance imaging scans in human subjects, Cramer et al.  demonstrated that a side-posture
spinal manipulation, accompanied by cavitation, gaps the
facet joints. The synovial space of the lumbar facet joints increased in width by up to 0.7 mm in individuals receiving
manipulation compared with nonmanipulated controls. The
length of time between manipulation and the magnetic resonance imaging scan was not reported. In a study of the metacarpophalangeal joint, 5 minutes after cavitation joint separation was still increased by 0.4 mm and did not return to
precavitation dimensions until 10 minutes after “cracking”
. It remains to be shown if joint separations of these
magnitudes are sufficient to load the facet joint tissues. If
so, this raises the possibility that tissues surrounding the
facet joint could be stretched for periods of time longer than
the duration of the manipulation itself. Graded sensory input
from tissues surrounding the facet joint  could elicit reflex muscle responses similar to that measured by Indahl et
Changes in muscle spindle input produced by spinal manipulation could also contribute to the inhibition of somatosomatic reflexes. Using magnetic stimulation, Zhu et al. [110, 111] stimulated lumbar paraspinal muscles and recorded the evoked cerebral potentials. Stimulation of paraspinal muscle spindles using vibration reduced the magnitude of the cerebral potentials. Similarly, muscle spasm in human patients reduced the magnitude of the paraspinal muscle–evoked cerebral potentials. Spinal manipulation reversed these effects, improving muscle spasm and restoring the magnitude of the evoked cerebral potentials , suggesting that increased sensory input from paraspinal muscle spindles during muscle spasm may contribute to the reduced magnitude of the evoked cerebral potentials. It is worthwhile recalling Korr’s ideas  that spinal manipulation increases joint mobility by producing a barrage of impulses in muscle spindle afferents and smaller-diameter afferents, ultimately silencing facilitated motoneurons (see previous section: The effects of spinal manipulation on sensory neurons innervating paraspinal tissues; Group I and II afferents [proprioceptive afferents]).
At first it seems counterintuitive that muscle spindle discharge is increased during muscle spasm, because one could
anticipate muscle shortening and spindle unloading during
spasm. However, extensive studies from Proske’s laboratory
(reviewed in ) show that a maintained joint position or
maintained muscle shortening, even for short durations, alters
muscle spindle sensitivity to subsequent joint movement or
muscle stretch. For example, from a given muscle length, muscle spindles respond more to a slow stretch when a leg muscle
has previously been held at a shortened length compared with
having been previously held at a long length for as little as 10
seconds . Recently, Pickar and Kang  observed the
same phenomenon in the lumbar longissimus and multifidus
muscles (Figure 6). Muscle spindle activity in response to a slow vertebral translation that stretched the muscle spindle
depended on whether the muscle had previously been shortened for as little as 5 seconds (by linearly displacing the L6 vertebra dorsalward) or had previously been stretched (by linearly displacing the L6 vertebra ventralward). If paraspinal
muscle spasm results in muscle shortening, or if segmental
buckling results in muscle shortening ipsilaterally and muscle lengthening contralaterally, then for the same change in
muscle length subsequent stretch or vibration of the affected
muscles would increase spindle discharge more than expected. Because spinal manipulation has been shown to stimulate muscle spindles (Figure 7), spinal manipulation may normalize spindle biomechanics and return muscle spindle discharge to normal.
5. The effects of spinal manipulation on somatovisceral reflexes
A number of animal experiments provide evidence supporting the link between altered paraspinal sensory input
and a somatovisceral change shown in Fig. 1. Sensory input
from paraspinal tissues can evoke visceral reflexes affecting
the sympathetic nervous system and may alter end-organ
function. In general, nonnoxious paraspinal sensory input
appears to have an inhibitory effect on sympathetic outflow,
whereas noxious input appears to have an excitatory effect.
However, insufficient experiments have been conducted to
determine the regional variation of this effect, that is, the
change in sympathetic outflow to different organs. Nonetheless, the data are provocative, indicating that neural input
from axial tissues can evoke somatovisceral reflexes.
Sato and Swenson  applied a nonnoxious mechanical stimulus to several vertebrae in the thoracic and lumbar spine of rats by applying a force to the lateral aspects of their spinous processes. Renal and adrenal sympathetic nerve activities were recorded. Because the paraspinal musculature was removed, the sensory input was derived presumably from the facet joints, intervertebral discs and/or intervertebral ligaments. The mechanical stimulus reflexively decreased the level of renal and adrenal sympathetic nerve activity by 25% to 40%. The stimuli were short in duration (approximately 30 seconds), and the responses attenuated rapidly. The sensory input from the paraspinal tissues had access to centers at least as high as the upper cervical spinal cord, because C1–C2 spinal cord transection abolished the inhibition. Sato and Swenson concluded that nonnoxious mechanical stimuli applied to the spine reflexively inhibit the level of sympathetic nerve activity by means of a supraspinal reflex.
Budgell et al. [116, 117] also stimulated paraspinal structures
using noxious and nonnoxious chemical stimuli. Injections
were placed into the lumbar facet joints or lumbar interspinous
tissues. Blood pressure and sciatic nerve blood flow were measured . A small volume (20 ul) of a nonnoxious chemical
(physiological saline 0.9%) injected into the interspinous
ligament produced a depressor response and a concomitant
decrease in sciatic nerve blood flow. A similar volume of
low-dose capsaicin (2 ug), which activates nociceptive neurons
, caused an initial increase in blood pressure and sciatic
nerve blood flow. However, when injected into the facet joint,
capsaicin produced a depressor response. The results from the
interspinous ligament are consistent with the suggestion offered by Sato and Swenson  that stimulation of receptive endings sensitive to innocuous mechanical stimuli in the paraspinal tissues produce inhibitory somaticsympathetic reflexes. The findings from the facet joints suggested to the authors that capsaicin might more effectively produce innocuous mechanical changes in the facet joint compared with the interspinous ligament by increasing the permeability of the synovial membrane’s microvasculature. Similar to the cardiovascular effects produced by capsaicin injection into the lumbar interspinous ligament, capsaicin injection into the lumbar interspinous tissues also increased adrenal sympathetic nerve activity and catecholamine secretion , whereas physiological saline injection had no effect. Thus, noxious stimulation of paraspinal tissues can produce excitatory somatic-sympathetic reflexes.
More recently, Pickar et al. , in a preliminary report, showed that mustard oil, a nociceptive substance that
also produces inflammation, injected into the lumbar multifidus muscle increases the discharge of sympathetic nerves
to the kidney and spleen. The response is a reflex mediated
by segmental branches of the dorsal ramus and is integrated
by centers at least as high as the upper cervical spinal cord.
This reflex organization is similar to that found by Sato and
Swenson  for the sympathetic nerves to the kidney and
adrenal gland. Interestingly, animal studies have also shown
that increased splenic sympathetic nerve discharge is immunosuppressive, decreasing the number of natural killer cells
released. Somatovisceral reflex stimulation of the sympathetic outflow to the spleen may contribute to the depressed
levels of natural killer cells measured in individuals with
low back pain .
Mechanical stimulation of paraspinal tissues can be sufficient to inhibit gastric motility. Myoelectric activity from
the wall of the gastrointestinal tract in conscious rabbits was
decreased by sustained (2.5 minutes) mechanical inputs
. In these experiments, it was unclear if the mechanical
stimulation was noxious or innocuous, but the inhibition of
gastric motility was greatest when the mechanical stimulation
was applied to the sixth thoracic vertebra, and it decreased as
the mechanical stimulation was applied further cranial or caudal. These results were confirmed by Budgell and Suzuki
. Noxious chemical stimulation inhibited gastric motility, and the effect tended to be greater when the stimulus was
applied to the mid-thoracic region compared with the lumbar
region. In addition, the inhibitory response was shown to be
a reflex predominated by changes in sympathetic outflow
and to a lesser extent vagal outflow.
It is important to note that these studies do not provide evidence for the unique potential of paraspinal tissues to elicit somatosympathetic reflexes. Substantial evidence shows that
noxious stimulation of tissues in the appendicular skeleton also
evokes somatosympathetic reflexes , but nothing is
known about the relative magnitudes of somatosympathetic reflexes elicited by axial versus appendicular tissues. Although
the data on gastric motility suggest segmental specificity, it is
not certain the degree to which segmental input from paraspinal tissues produce regionally specific changes in sympathetic
Very few laboratory or clinically oriented basic science
studies have been conducted to determine the effects of spinal manipulation on the sympathetic nervous system. Recently, Budgell and Hirano  measured changes in heart
rate variability after upper cervical versus sham spinal manipulation. Power spectral analysis of heart rate variability
showed that manipulation increased the ratio of low frequency to high frequency components indicating a possible shift in the balance of autonomic control of the heart toward the parasympathetic nervous system.
Spinal manipulation may alter the response of immunologic
cells as well as the production of immunomodulatory and neuromodulatory cytokines. In a series of studies on human subjects in the 1990s, Brennan et al. [120, 125, 126] showed that
spinal manipulation — but not sham manipulation nor soft tissue
massage — primed polymorphonuclear leukocytes (PMNs) and monocytes. Spinal manipulation enhanced the respiratory burst (a marker for phagocytic activity) of these white blood cells to a particulate challenge. The mechanism is unclear, although speculation on the role of substance P was discussed. Spinal manipulation also primed the polymorphonuclear leukocytes for enhanced production of cytokines as determined by the release of tumor necrosis factor in response to endotoxin challenge. The priming effect was short lived, being greater 15 minutes after manipulation compared with 30 and 45 minutes. The biological consequence of these changes have yet to be investigated, but their changes suggested their potential use, at least, as markers of successful spinal manipulation.
A theoretical framework has been presented for understanding the neurophysiological effects of spinal manipulation. The reasons underlying the biomechanical changes in the vertebral column are hypothesized to affect neural input, subsequently altering central processing and affecting reflex somatomotor or somatovisceral output. Table 2 summarizes the evidence for the theoretical relationships presented in this review. Spinal manipulation evokes changes in the neuromusculoskeletal system. The experimental evidence indicates that the impulse load of a spinal manipulation impacts proprioceptive primary afferent neurons from paraspinal tissues. In addition, spinal manipulation can affect pain processing, possibly by altering the central facilitated state of the spinal cord, and can affect the motor control system. Animal experiments show that sensory input from paraspinal tissues has the capacity to reflexively alter the neural outflow to the autonomic nervous system. However, the effects of spinal manipulation on the autonomic nervous system are less well investigated The neurophysiological evidence demonstrating physiological effects produced by spinal manipulation is growing. More than one mechanism likely explains the effects of spinal manipulation. During the past 10 to 20 years, novel experimental approaches have been developed to investigate both the effects of and the mechanisms underlying spinal manipulation. Neurophysiological studies of the spine using animal models are difficult, if for no other reason than the paraspinal tissues of interest directly overlie the central nervous system and the distances between paraspinal tissues
and the spinal cord are short. Several experimental models
have offered solutions to this difficulty. Continued work in
this area will help us better understand the therapeutic
mechanisms impacted through spinal manipulation.
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