Muscle tone regulation mechanism (Gamma loop)
Normally, muscle tone and posture maintenance are ensured by the coordinated work of neurons at various levels of the central nervous system. The main tool of this regulation is the so-called gamma loop.
The signal transmission sequence is as follows:
- Excitatory impulses from the association cortex of the cerebrum are transmitted to the structures of the extrapyramidal system.
- Next, the signal descends into the spinal cord, reaching the $\gamma$-motoneurons located in the anterior horns.
- From these neurons, impulses are directed to specific intrafusal fibers (elements of muscle spindles).
- The fibers contract, which leads to the excitation of the sensory endings of the spindle itself.
- The resulting afferent signal returns to the corresponding segment of the spinal cord to the $\alpha$-motoneurons.
- Activated $\alpha$-motoneurons cause the contraction of the working mass of the muscle—extrafusal fibers.
Tonic midbrain reactions
Midbrain structures are responsible for tone distribution through tonic reactions, which are broadly divided into static and statokinetic.
Static reactions operate when the body is not actively moving, but changes position:
- Postural reactions. They correct tone during isolated changes in head position. They are triggered by afferent signals from neck and trunk proprioceptors and labyrinthine receptors.
- Righting reactions. Their task is to return the body to a physiological posture when it is disturbed. Head holding is controlled by impulses from the retinal photoreceptors, the vestibular apparatus, the skin, and extraocular muscles. Signals from the skin and neck proprioceptors are critical for righting the trunk itself.
Statokinetic reactions are activated in response to body movement in space. During rotation, tone is redistributed thanks to the receptors of the semicircular canals, while during linear acceleration, the otolith organ plays a key role.
Postural restoration phenomena
The ability to restore posture is preserved even in decerebrate animals, which confirms the location of reflex centers at the brainstem level.
- Tactile righting reflex. It manifests in animals with artificially destroyed labyrinths. If such an animal is lifted into the air, it is unable to hold its head. However, upon contact with a hard surface, stimulation of skin tactile receptors sends a signal to the central nervous system. As a result, neck muscles contract, and the head assumes its habitual position.
- Lift phenomena. These are protective reactions to linear acceleration along the vertical axis. If the body moves downward with acceleration (the falling elevator effect), the tone of extensor muscles reflexively increases, preparing the musculoskeletal system for a hard landing. Conversely, during a sudden upward movement, the tone of flexor muscles dominates.
- Compensatory eye and head movements. They are necessary so that during any body movements, the visual image on the retina remains stable and focused.
Brainstem postural reflexes (Magnus experiments)
The significance of brain structures in posture formation was fundamentally proven by R. Magnus in experiments involving nerve pathway transection. By observing a decerebrate cat, one can evaluate how the position of the head rigidly determines the muscle tone of the limbs.
In the initial position, when the animal's head is positioned horizontally, tone is evenly distributed between the flexor and extensor muscles of all four limbs.
Dorsal flexion (head extension): When the head is raised upward, the tone of the extensors of the forelimbs increases sharply, causing them to stiffen in extension. Simultaneously, the tone of the flexors predominates in the hind limbs, causing them to buckle. Functionally, this creates an "observation posture" or prepares the animal for a jump: the center of gravity shifts backward, allowing the animal to survey the space above it.
Ventral flexion (head tilt downward): If the animal's head is lowered downward, the picture reverses. Flexors are activated in the forelimbs (elbows bend), and extensors in the hindlimbs (paws straighten). A posture characteristic of eating, laping water, or sniffing tracks is formed. In this case, the body's center of gravity is shifted forward.
Briefly: answers to topical questions
Which specific brainstem nuclei (midbrain, medulla oblongata) are involved in tonic postural reactions?
The sources explicitly name the following structures/nuclei associated with the regulation of posture, muscle tone, and balance:
- Red nuclei (nucleus ruber) of the midbrain—play a key role in the mechanisms of decerebrate rigidity; the rubrospinal tract participates in the regulation of muscle tone and movement coordination.
- Vestibular nuclei: Deiters', Schwalbe's, Roller's, Bechterew's. Via vestibulospinal connections, they participate in maintaining balance and muscle tone; the lateral vestibular nucleus of Deiters gives rise to the lateral vestibulospinal tract, which terminates on $\gamma$- and $\alpha$-motoneurons and facilitates extensor reflexes.
- Medial vestibular nucleus—gives rise to the medial vestibulospinal tract, which is involved in altering neck muscle tone during head position changes.
Through which descending tracts does the extrapyramidal system transmit impulses to alpha- and gamma-motoneurons?
Extrapyramidal descending pathways for which the sources explicitly indicate an effect on $\alpha$- and/or $\gamma$-motoneurons:
- Rubrospinal tract—excites flexor $\alpha$- and $\gamma$-motoneurons and inhibits extensor motoneurons.
- Vestibulospinal tract—activates extensor $\alpha$- and $\gamma$-motoneurons and inhibits flexor motoneurons; the lateral vestibulospinal tract terminates on spinal $\gamma$- and $\alpha$-motoneurons.
- Reticulospinal tract—exerts excitatory and inhibitory effects on spinal motoneurons; its influences converge on $\gamma$-motoneurons, and monosynaptic excitation of $\alpha$- and $\gamma$-motoneurons of proximal limb and trunk muscles is also indicated.
What is the difference between decerebrate and decorticate rigidity?
These types of pathological tone differ in the nature of muscle reactions, the level of the nervous system lesion, and the Glasgow Coma Scale score.
| Sign | Decorticate rigidity | Decerebrate rigidity |
|---|---|---|
| Motor response | Pathological flexion and adduction of the arms to the chest with extension of the legs | Pathological extension, adduction, and internal rotation of the arms, extension of the legs |
| Level of CNS lesion | Sign of a lesion above the brainstem | Sign of a deep brainstem lesion |
| GCS score | 3 points | 2 points |
To easily recall the postural reflexes from Magnus's experiments, imagine a cat. Looking up at a bird (dorsiflexion) — it sits on its hind legs (flexed) while pushing against the ground with its forelegs (straight). Looking down at a food bowl (ventral flexion) — it flexes its forelegs and extends its hind legs, elevating the pelvis.
Class and exam questions
Why do a person's legs reflexively straighten when an elevator moves downward?
This is a manifestation of the statokinetic reaction to linear acceleration (elevator reflex). During downward movement, the nervous system automatically increases extensor muscle tone to prepare the legs to cushion the impact in the event of a potential fall or landing.
What is the function of gamma motoneurons in posture regulation?
Gamma motor neurons innervate the intrafusal fibers of muscle spindles. By contracting these fibers, they increase the sensitivity of the spindle receptors, which ultimately leads to the reflex activation of primary alpha motor neurons and the maintenance of muscle tone.
Can an animal with inactivated labyrinths hold its head up?
Suspended in the air — no. However, if placed on a solid surface, the tactile righting reflex is triggered: stimulation of the trunk skin initiates a chain of reactions that restore neck muscle tone and normal head position.
What else to study on this topic
- Structural features of the extrapyramidal system and its influence on the gamma loop
- Differences in structure and function of intrafusal and extrafusal muscle fibers
- Anatomy of the vestibular apparatus: semicircular canals and otolith organs
- Mechanisms of compensatory eye movements during changes in body position
- Consequences of decerebration and the phenomenon of decerebrate rigidity
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