The Nervous System · Part Two · How It Senses and Moves
Lesson 21 / 61
Muscle Spindles and Proprioception: The Receptor and Its Reflex Loop
Where stretch becomes nerve impulses, and what the spinal cord does with them.
The muscle spindle is a stretch receptor threaded among the working fibers of a skeletal muscle, and it is the only receptor in the body issued its own motor supply. Pulling its equator opens Piezo2 channels in the sensory endings, and the traffic starting there is what the spinal cord reads as joint position and limb velocity. In the Unified Model of Tone, that motor supply is read as the gain a segment holds on its own report of length.
Intrafusal fibers per spindle
Up to a dozen, bag and chain types
Primary afferent
Group Ia, annulospiral ending, velocity sensitive
Secondary afferent
Group II, flower-spray ending, length sensitive
Tendon receptor
Golgi tendon organ, Ib afferent, in series
The receptors proprioception runs on
Proprioception runs on two muscle receptors that answer different questions. The muscle spindle, lying parallel to the working fibers, answers how long the muscle is and how fast that is changing. The Golgi tendon organ, in series at the tendon, answers what producing that length costs in force. Joint capsule and skin endings add angle and skin strain. How faithful the whole report is, and the five ways it fails, is the subject of the research page on input quality.
The gamma loop as a volume control
Gamma motor neurons supply the contractile poles inside the spindle, so the cord can raise a receptor's discharge without the joint moving at all. Alpha and gamma axons leave the cord together, so the setting is issued with the contraction command and never adjusted after the fact. The Unified Model of Tone reads a spindle whose gain no longer matches the work its muscle is doing as an input problem, not a muscle problem.
01Inside the spindle
The muscle spindle is a small muscle rebuilt as a sensor, threaded among the fibers that generate force
The poles of a spindle are contractile and its middle is not, and that split is the whole design. The larger intrafusal fibers run out of the ends of the capsule and anchor to the perimysium, the connective tissue sheath of the parent muscle. That anchoring puts the spindle in parallel with the extrafusal fibers doing the work. Lengthen the whole muscle and the equatorial region of the spindle is pulled along with it. The sensory endings sit in that equatorial zone, and nothing reaches the cord from this receptor that was not first a stretch across it.
Two populations of intrafusal fiber share the capsule. Nuclear bag fibers gather their nuclei into a central bag. Nuclear chain fibers line theirs up in a row. The division does real work, and stimulating single fusimotor axons in cat peroneus brevis showed how cleanly it runs. Of 153 responses recorded from primary endings, 67 percent were pure examples of dynamic or static action Emonet-Dénand 1977. Dynamic action arose from the bag1 fiber. Static action arose from the bag2 and chain fibers. Only 18 percent of responses gave a firm indication of admixture, so the velocity and length channels stay largely separate at the motor input as well as at the sensory output.
Human spindles are not built to one pattern
Liu found spindles in the deep muscles of the human neck missing the bag1 fiber, and others missing bag2, alongside conventional spindles carrying bag1, bag2 and chain fibers Liu 2003. Intrafusal fiber allotment and myosin heavy chain composition varied by muscle, which the authors read as functional specialization in the control of movement among different human muscles. A spindle is tuned to its parent muscle, so a muscle can be built to report velocity poorly and length well, or the reverse.
Spindle number scales with the square root of muscle mass
The exponent is one half, not the one third a volume relationship would give. It is a within-species result, demonstrable only in the one essentially complete sample, which is human, and the trend between species is isometric Banks 2006. Relative abundance cannot be read from spindles per gram, a measure the analysis calls grossly misleading. It is read from the residuals around that regression. Measured that way, the greatest relative abundance sits in axial muscles, including the muscles concerned with head position, and the least sits in the muscles of the shoulder girdle. The musculature is not reported evenly. The parts holding the spine and the head against gravity send the densest positional report to the cord.
The suboccipital muscles, measured
No Golgi tendon organs were seen in any of the three small suboccipital muscles counted post mortem in human fetuses Kulkarni 2001. Those muscles carried 242 spindles per gram in obliquus capitis inferior, 190 in obliquus capitis superior and 98 in rectus capitis posterior, a figure the study reports without separating major from minor. Individual spindles measured 100 to 650 micrometers long and 50 to 250 micrometers across. These are fetal measurements, and per-gram density does not travel between muscles, so the counts work as contrasts within one study. Those muscles are positional instruments. What a report of that density is worth to the rest of the body is read on the research page on input quality.
02Findings
What the research shows
03Piezo2 and transduction
Piezo2 is the molecule that converts stretch of the spindle into proprioceptive traffic
Piezo2, a mechanically gated ion channel, sits in the sensory endings wrapping the spindle's equator and opens when the equator is pulled. It is expressed in the sensory endings of proprioceptors innervating both muscle spindles and Golgi tendon organs in mice. Two independent mouse lines lacking Piezo2 in proprioceptive neurons showed severely uncoordinated body movements and abnormal limb positions Woo 2015. Stretch-induced firing of proprioceptors was markedly reduced in those animals.
Losing that channel in a human costs the same circuitry. A 12-year-old girl with PIEZO2 deficiency had areflexia, impaired proprioception, decreased amplitude of sensory nerve action potentials, an absent H reflex and prolonged central conduction times Yamaguchi 2019. She carried congenital multiple contractures and progressive severe scoliosis. Sixteen patients from eight families had been described before her.
Remove one mechanically gated channel and the tendon reflex disappears, the sense of position goes with it, and the growing spine bends. Take out the stretch loop and the skeleton loses its shape.
Everything downstream runs on that conversion
Afferent conduction velocity, fusimotor gain, reflex latency and the whole interneuron network operate on impulses a mechanically gated channel produced. The Unified Model of Tone treats that channel as the first place the body's report on itself can be lost. It reads the scoliosis of PIEZO2 deficiency as what a skeleton does when the report never arrives: posture is assembled from afferent traffic, and connective tissue alone does not hold it.
04Velocity and length
Group Ia reports how fast a muscle is lengthening and group II reports how long it is
Group Ia fibers form annulospiral endings that wrap the central equatorial region, and primary spindle afferent and group Ia afferent name the same thing. Smaller group II fibers end toward either side of the nuclear region on the chain fibers, forming flower-spray endings, also called secondary spindle receptors. The primary runs larger and faster. Microneurography in man put the fastest of these afferents at 62 to 67 m/s and the slowest at 36 to 45 m/s Burke 1983. That volley was evoked by electrical stimulation of the sciatic nerve. How a volley of mixed conduction speeds spreads out along the nerve belongs to the peripheral nerve and reflex.
The two endings split the work along the time course of a stretch. Recording from ten primary endings in man, percussion produced two to five spike discharges per tap, with shortest interspike intervals of 4 to 7 ms Burke 1983. A single secondary ending merely altered its discharge pattern. The primary answers the tap. The secondary keeps reporting where the muscle currently sits, and that tonic message is what tells the brain a joint is being held still.
The same division appears during voluntary movement. Roll and Vedel recorded 15 primary and 5 secondary endings in man Roll 1982. Primary discharge frequency stayed almost constant through a movement and was mainly related to velocity. Secondary activity rose progressively, some of it tracking both movement velocity and joint angle.
What tendon vibration proves about the spindle code
Vibrating a tendon drives the muscle spindles of the attached muscle, and vibration trains at frequencies from 10 to 120 Hz produce illusory movement of a limb that is not moving. In human subjects the perceived velocity of that movement rose progressively from 10 Hz to a peak at 70 to 80 Hz, then fell again from 80 to 120 Hz Roll 1982. The direction felt is the one that would have stretched the vibrated muscle. Nothing about the joint changed. The afferent train changed, and the sense of position followed it.
No single afferent is perceived
Human intraneural microstimulation of one muscle spindle afferent produces no conscious sensation at all, and the same is true of a single Ruffini ending in skin Macefield 2005. Stimulating a single joint afferent can evoke a sensation of joint rotation. Coactivation of many endings is what carries the meaningful information. The brain does not perceive the body directly. It infers position and movement from the traffic arriving out of spindle, tendon, joint and skin, and through the fusimotor system it can reset that traffic at will.
The spindle afferent supply thins after midlife
In human peripheral nerve, an average of 26 percent of myelinated tibial nerve fibers were positive for the alpha3 isoform of Na/K-ATPase, a marker of muscle spindle afferents Romanovsky 2015. The figure for spinal ganglion neuronal profiles was 17 percent. Relative frequency starts to fall during the fifth decade of life and approaches half of young adult values in 65-year-old subjects. The larger and more strongly labeled spindle afferents are proportionally the most vulnerable.
A quarter of a major peripheral nerve is given over to spindle report, and that share shrinks. Proprioceptive senses deteriorate with age, and the deterioration is associated with increased risk of falls in the elderly Proske 2012. A spindle supply working inside its healthy range leaves the nervous system free to lean on vision and the vestibular system when one source degrades. A supply that has drifted far outside that range stops supporting balance at all, and the falls follow.
05Gamma fusimotor gain
Gamma fusimotor neurons set how loudly the spindle reports before any stretch arrives
Gamma axons innervate the contractile poles of the intrafusal fibers. When they fire, the poles shorten and stretch the equatorial region from inside, raising the discharge of the annulospiral endings with the whole muscle at a fixed length. The equator can therefore be stretched two ways. Lengthening a relaxed muscle stretches it from outside. Fusimotor drive tightens the spindle from within.
That internal route solves a problem the receptor would otherwise face. When the parent muscle contracts, its extrafusal fibers shorten and slacken the spindle, unloading the sensory endings at the moment feedback matters most. Recording 31 units across more than 700 voluntary isometric contractions in man, Vallbo found afferent discharge increasing in spite of the mechanical unloading, which requires concomitant fusimotor activation Vallbo 1971. Spindle acceleration followed EMG onset by 10 to 50 ms. Alpha and gamma outflows leave the cord rigidly co-activated in voluntary contraction. Vallbo drew the limit himself: the fusimotor system does not participate in initiating voluntary contraction in man. Gamma drive accompanies the command, it does not start it.
Gain scales with the load being held
During isotonic position holding in man against loads up to 29 percent of maximum voluntary force, spindle firing rose linearly with the load. Across 18 afferents, 11 primary and 7 secondary, the pooled slope for primaries was 0.47 impulses per second per percent of maximum voluntary force Vallbo 1982. The corresponding figure for secondaries was 0.66 after correction for sampling bias. Scatter in the data points was considerable, and Vallbo's conclusion was a broad parallelism of skeletomotor and fusimotor activity without a rigid linkage to individual spindles. Fusimotor gain tracks the demand of the task, and it tracks it loosely.
Fusimotor traffic follows the state of the person; the afferent output follows the contraction
Ribot recorded twelve single efferent fibers in the lateral peroneal nerve of conscious humans, six skeletomotor and six fusimotor Ribot 1986. Fusimotor discharge changed with mental computation, with laughter and with the sound of hand clapping. The fusimotor fibers were generally spontaneously active in relaxed subjects. Clenching the fists, twisting the pinna, voluntary isometric contraction, passive stretch and environmental disturbance did the same. Those discharges were uncorrelated with any activation of extrafusal muscle fibers, tested with surface electromyography, deep tungsten electrodes and a 0.1 mN tension transducer. During spontaneous activity the subject could voluntarily stop the unit discharge. Six fusimotor fibers in a conscious human being is the whole of the record.
That is a direct recording of the efferent traffic, and the sensory side reads differently. Burke measured the variability of discharge in 39 spindle afferents from the pretibial muscles of normal human subjects Burke 1979. Of those, 24 held a sustained discharge at a mean of 11.1 Hz, with a mean coefficient of variation of 0.073. Burke concluded that no background fusimotor drive is functionally effective at the afferent output of a non-contracting muscle. The same paper states that the absence of fusimotor drive cannot be inferred from a regular afferent discharge, and that measurements of variability do not accurately reflect the level of fusimotor drive. That method reads the sensory side, and Ribot's electrodes read the efferent side. The two results describe one arrangement. Background fusimotor drive is present in the relaxed human, and it is too small to resolve in the output of a resting muscle.
Human spindles carry a background discharge related to the degree of stretch, at mean firing rates near 10 Hz Macefield 2018. Independent fusimotor control in humans is real, and modest next to the cat's. Mental arithmetic changes the traffic on the fusimotor fibers themselves, recorded directly, and the muscle need not be contracting for that to happen. The same movement made in two different states does not send the cord the same afferent stream.
Gain changes without corrupting the direction code
Jones recorded sixteen spindle afferents and one Golgi tendon organ afferent during voluntary wrist movements of about 5 degrees at 10 to 30 degrees per second in man. The preferred direction of a spindle afferent held the same during passive and active movement. Wrist joint position was remarkably well encoded in the ensemble spindle data, on the authors' own account of a limited data base from three extensor muscles Jones 2001. Fusimotor effects had little or no influence on directional tuning. Fusimotor drive changes the amount of traffic without changing what the traffic means.
06The stretch loop
The tendon jerk is the spindle's own circuit, and the cord multiplies the report before it answers
A tap on the patellar ligament stretches the quadriceps spindles, the primary endings fire a burst of group Ia impulses into the cord, and alpha motor neurons answer with a brief twitch. Both primary and secondary afferents discharge tonically at resting length and raise their rate when a new length is held. The speed and the reproducibility of that arc are why clinicians use it to probe the integrity of one segmental loop. What the reflex hammer actually reads, and why the arc is not the single synapse it is usually drawn as, is set out in the lesson on the peripheral nerve and reflex.
Reciprocal inhibition, and the cell that gates it
Branches of the same Ia fibers excite Ia inhibitory interneurons, which suppress the alpha motor neurons of the antagonist so a joint moves without the opposing muscle fighting the action. That disynaptic pathway is itself under control. Reciprocal inhibition induced in human tibialis anterior motor neurons by activation of soleus group I afferents is deeply depressed by activation of soleus-coupled Renshaw cells Baret 2003. Only Ia interneurons receive that recurrent inhibition, which is what identifies the pathway in the human experiment.
Renshaw cells as a gain regulator
Renshaw cells are excited by collaterals of alpha motor axons before those axons leave the cord, and they feed back onto the motor pool. Hultborn presents recurrent inhibition through motor axon collaterals and Renshaw cells as a variable gain regulator of motoneuron discharge Hultborn 2004. Dendritic persistent inward currents and recruitment gain sit alongside it as mechanisms that scale output across the pool for a fixed input. Hultborn offers the account as a hypothesis about how the cord sets input-output gain.
The cord does not pass the spindle report through unchanged. It multiplies that report by a factor it sets itself. Alpha and gamma commands leave together, so the cord issues an intended length at the same instant it issues the contraction. The group Ia traffic returning is the difference between the length requested and the length reached. The interneuron network exists to drive that difference toward zero. The segmental loop is the smallest complete instance of prediction in the body. When a segment holds a difference it never resolves, the Unified Model of Tone calls that a distortion of tone.
07The Golgi tendon organ
The Golgi tendon organ reports force, and it reports every motor unit contributing to it
The tendon organ sits in series with the muscle at the musculotendinous junction, where the spindle sits in parallel. It signals the tension a muscle develops when it contracts and the tension exerted on it during a stretch. Its Ib afferents drive autogenic inhibition, the reflex damping of a muscle by its own rising tension through inhibitory interneurons in the cord.
The tendon organ answers a single motor unit. Responses of cat soleus tendon organs to single motor unit contractions put the absolute force threshold as little as 4 mg Binder 1977. That is below the estimated minimum twitch force generated by individual soleus muscle fibers in the same animal. The authors concluded that this sensitivity ensures every twitch of every motor unit is reflected in the population of afferent signals projecting to the spinal cord. The tendon organ works as a continuous force meter, not as an emergency brake against tearing.
It stays relatively insensitive to tension applied passively by stretching, because the mechanical load reaching the receptor differs sharply between passive and active conditions. Tendon organs, and possibly spindles as well, carry the senses of force and heaviness Proske 2012. The spindle watches length and the rate at which length changes. The tendon organ watches the force being spent to get there.
Not every muscle carries both receptors
No Golgi tendon organs were seen in the human fetal suboccipital muscles that carried 98 to 242 spindles per gram Kulkarni 2001. A muscle can be built to report position without being built to report force. Where that split exists, the cord receives length and velocity from a region and receives almost nothing about the tension being produced there.
What a contract-relax hold actually does
Contract-relax and hold-relax stretching exploit the depression that follows a voluntary contraction, and the window has been measured. In 16 healthy subjects performing isometric plantar flexion at 65 to 75 percent of maximum, H reflexes fell by a mean of 67 percent within 0.05 seconds Moore 1991. Maximal depression of 83.3 percent ran from 0.1 to 1 second after the contraction. Reflexes recovered to 70 percent of control amplitudes by 5 seconds and reached 90 percent by 10.05 seconds. The authors described a strong but brief neuromuscular inhibition. The usable window is about one second wide.
Autogenic Ib inhibition is not what produces it. After isometric preconditioning at 30 and 60 percent of maximum in ten human subjects, responses to both mechanical and electrical stimulation were depressed during force relaxation Gollhofer 1998. The mechanically evoked stretch response returned to control values in under 400 ms. Recovery after electrical stimulation was delayed. The authors attributed the modulation to presynaptic inhibition, with a possible peripheral contribution from alpha-gamma coactivation altering receptor stiffness. The reflex window is brief. Any change in a muscle that outlasts it is held somewhere other than the reflex.
08Thrust and paraspinal spindle
A thrust reaches paraspinal spindles directly, and how fast it is delivered matters more than how far it travels
A thrust arrives at a receptor bed that is already firing, at a gain the cord set before the contact was made. What the receptor does with the contact turns on how fast it is delivered. Discharge in lumbar paraspinal spindle afferents rose curvilinearly as thrust duration shortened, with a concave-up inflection near 100 ms, in single-unit recordings from deeply anesthetized cats Pickar 2007. That inflection sits inside the clinical high-velocity low-amplitude range the same study defines as duration under 150 ms, segmental translation under 2 mm, rotation under 4 degrees and applied force of 220 to 889 N.
The amplitude result runs against intuition. A 1 mm thrust drove spindle afferents at 6.2 spikes per second per mm per second against 3.3 for a 2 mm thrust Pickar 2007. The smaller displacement was nearly twice as effective per unit of velocity. The authors attribute the paradox to small-signal and large-signal range properties of muscle spindles.
Force-controlled work supplies the mechanism. Half-sine force-time profiles of 25 to 800 ms were delivered at 33, 66 or 100 percent of body weight to anesthetized cats. Paraspinal spindle discharge climbed once thrust duration fell below roughly 200 ms Pickar 2006. The authors concluded that a manipulation loads paraspinal spindles at a rate where velocity sensitivity predominates over length sensitivity. Afterward, muscle spindle afferents often became silent, and thrust duration had little effect on how long that silence lasted. The muscles recorded were multifidus and longissimus, in cats.
Why a smaller, faster thrust reaches the spindle harder
A fast thrust addresses the group Ia dynamic channel of the paraspinal muscle spindle and leaves a quiet period in the segmental afferent stream behind it. In deeply anesthetized cats, 1 mm delivered fast outreports 2 mm delivered slow, 6.2 against 3.3 spikes per second per mm per second, and the response curve inflects near 100 ms Pickar 2007.
A null result the model predicts
Chemical stimulation of small-diameter afferents did not change spindle behavior in the lumbar spine. Bradykinin or capsaicin was applied to chemosensitive afferents in L5 and L7 multifidus. L6 spindle responses to ramp-and-hold vertebral loads of 25 to 100 percent of body weight were unchanged, in both alpha-chloralose-anesthetized and unanesthetized decerebrate cats Kang 2001. The authors concluded the data do not support fusimotor reflexes contributing to muscle spasm in the lumbar spine, and noted that studies in the cervical spine do support the hypothesis.
The Unified Model of Tone reads that regional split as input meeting tone. A lumbar segment does not convert nociceptive traffic into spindle gain the way the neck does. The neck is also where relative spindle abundance is highest Banks 2006 and where human spindles missing a bag fiber have been found Liu 2003. The model predicts that the correspondence holds as a rule. Regions with the densest and most specialized spindle supply are the regions where chemical and nociceptive input most readily changes fusimotor gain. The lumbar null result is the low end of that gradient.
What the model reads in a single fusimotor fiber
Fusimotor gain and the autonomic outflow of a segment are not two dials in the Unified Model of Tone. Turn one and the other has already turned. A fusimotor fiber in a conscious human changing its rate at the sound of hand clapping catches that at the receptor, with nothing moved and nothing loaded Ribot 1986. The clinical consequence follows from the numbers already on this page. An input aimed at a segment moves the rate at which that segment reports itself, and 1 mm delivered fast moves it harder than 2 mm delivered slow.
The gain on proprioception is set by the state of the person, not by the movement alone.
09Tone
How this system expresses tone
Every part of the nervous system expresses all of tone. In the muscle spindle three of its foundations are directly measurable, in the afferent record and in the efferent record alike.
Gain
Fusimotor traffic in six human fibers changed with mental computation and laughter, with no extrafusal activity detectable on a 0.1 mN tension transducer.
Prediction
Spindle firing accelerates 10 to 50 ms after EMG onset in man, late enough that the cord has committed to a target length before the report returns.
Input quality
A 1 mm paraspinal thrust drove spindle afferents at 6.2 spikes per second per mm per second, nearly twice the 2 mm thrust in anesthetized cats.
Set point: alpha and gamma outflow together hold a target length the cord defends against load, with primary firing rising 0.47 impulses per second for each percent of maximum voluntary force. Oscillation: primary endings answer a tap with two to five spikes at intervals of 4 to 7 ms, and tendon vibration near 70 to 80 Hz drives them hard enough to invent a movement. Time course: reflex depression measured in tenths of a second sits on top of a spindle afferent population that thins across decades. Constraint: a spindle reports only what its intrafusal allotment allows, and human deep neck spindles lacking bag1 or bag2 fibers cannot carry the full velocity and length code. Load: holding a position taxes the channel continuously, since firing scales with the force held out to 29 percent of maximum voluntary force. Coupling: one Ia fiber excites its own muscle and inhibits the antagonist through Ia interneurons, and Renshaw cells gate that reciprocal pathway from the motor side.
10Across the library
How this page relates to the rest of the library
Go there for the fidelity of the report and the five ways it fails, degraded, noisy, mismatched, absent and never established. Stay here for the capsule, the afferent classes and the gamma loop.
A limb held in a cast disconnects from the brain's movement network within 48 hours, and the cortex serving it thins measurably within about sixteen days.
Where the group Ia afferent terminates and the interneuron network that multiplies its report lives, including the descending traffic that sets reflex gain from above.
The extrafusal side of the same muscle: how a motor command becomes force, and the motor unit whose every twitch the tendon organ reports.
What the reflex hammer actually reads: percussion of the human soleus tendon sends a volley lasting 25 to 30 ms, carried by fibers of many conduction speeds.
Why a developing nervous system needs to move to build its map, and what the spindle supply contributes to a child's posture and coordination.
The same receptors under athletic load, where joint position sense decides landing mechanics and where fatigue degrades the report first.
11Frequently asked
Questions about this topic
What is a muscle spindle?
A muscle spindle is a fluid-filled capsule holding up to a dozen small intrafusal muscle fibers, lying parallel to the working fibers of a skeletal muscle. Its poles are contractile and its middle is not. Stretching the whole muscle pulls the spindle's equatorial region, where the sensory endings sit, and those endings fire. Group Ia afferents wrap that equator and report the rate of lengthening. Group II afferents end nearby on the chain fibers and report the length being held. Both send their traffic to the spinal cord.
What is the difference between group Ia and group II afferents?
Group Ia and group II afferents both leave the muscle spindle, and they divide a stretch between speed and position. Group Ia fibers form annulospiral endings on the spindle's central region and are the larger, faster line, with the fastest human muscle afferents conducting at 62 to 67 m/s. They answer the dynamic phase of a stretch, producing two to five spikes per tap during tendon percussion. Group II fibers form flower-spray endings on the chain fibers and hold a steady discharge encoding the length the muscle currently sits at.
What do gamma motor neurons do?
Gamma fusimotor neurons supply the contractile poles of the intrafusal fibers inside the muscle spindle. Firing them shortens the poles and stretches the spindle's sensory region from within, raising afferent discharge with the whole muscle at a fixed length. That keeps the spindle reporting during a contraction, when the shortening muscle would otherwise slacken it. In human recordings, spindle discharge rose despite mechanical unloading, and firing accelerated 10 to 50 ms after EMG onset. Gamma drive accompanies a voluntary contraction, it does not initiate one.
Why does a clinician tap the tendon with a reflex hammer?
The tap stretches the muscle spindles of the tested muscle, the primary endings fire a burst of group Ia impulses into the spinal cord, and alpha motor neurons answer with a brief twitch. The response probes one segmental loop. In man, percussion of a tendon draws two to five spikes per tap from a primary spindle ending, at shortest interspike intervals of 4 to 7 ms. What that volley becomes once it reaches the cord is set out in the lesson on the peripheral nerve and reflex.
What is the difference between a muscle spindle and a Golgi tendon organ?
The muscle spindle reports position and the Golgi tendon organ reports force. The spindle lies parallel to the working muscle fibers and signals length and the rate at which length is changing. The tendon organ sits in series at the tendon and signals the tension the muscle is producing. It stays relatively insensitive to passive stretch and extremely sensitive to active pull, with an absolute force threshold calculated as low as 4 mg in cat soleus. Its Ib afferents drive autogenic inhibition, damping a muscle as its own tension rises.
Why does proprioception get worse with age?
The afferent supply thins. In human peripheral nerve, an average of 26 percent of myelinated tibial nerve fibers carry the alpha3 isoform of Na/K-ATPase, a marker of muscle spindle afferents. Their relative frequency begins falling during the fifth decade of life and approaches half of young adult values by 65, with the largest and most strongly labeled spindle afferents proportionally the most vulnerable. Proprioceptive senses deteriorate with age, and that deterioration is associated with increased risk of falls in the elderly.
Does a spinal adjustment reach the muscle spindles?
Single-unit recordings from lumbar paraspinal spindle afferents in deeply anesthetized cats answer that directly. A high-velocity thrust raised discharge, and the response climbed steeply as thrust duration fell toward 100 ms and below. A 1 mm thrust drove afferents at 6.2 spikes per second per mm per second against 3.3 for a 2 mm thrust. Afterward the afferents often fell silent for a period. A manipulation loads the spindle at a rate where its velocity sensitivity predominates over its length sensitivity.
Why contract a muscle before stretching it?
A voluntary contraction leaves the reflex loop briefly depressed. In 16 healthy subjects holding an isometric plantar flexion at 65 to 75 percent of maximum, H reflexes fell 67 percent within 0.05 seconds. Maximal depression of 83.3 percent ran from 0.1 to 1 second after the contraction. Reflexes returned to 90 percent of control by 10.05 seconds. The usable window is about one second wide. The depression is attributed to presynaptic inhibition, not to autogenic inhibition from the tendon organ.
12The sources
References
Sources: primary literature, linked inline.