Basic mechanisms of posture formation
The posture of a human or animal is formed through a combination of reflexes that continuously redistribute muscle tone. The leading role in this process is played by the excitation of skeletal muscle proprioceptors (especially neck and trunk muscles) and signals from the vestibular apparatus.
At the brainstem level, the interaction of two structures is of key importance:
- Vestibular nuclei: send centrifugal commands that activate $\alpha$- and $\gamma$-motoneurons of extensor muscles. Simultaneously, reciprocal inhibition of flexor motoneurons occurs. This allows the body to counteract gravity.
- Reticular formation of the medulla oblongata: acts in the opposite direction. Its neurons activate $\alpha$- and $\gamma$-motoneurons of flexors while simultaneously inhibiting the activity of extensor motoneurons.
Static postural reflexes
These reflexes are necessary to maintain a comfortable posture (for example, when a human or animal lies, sits, or stands). The main task here is the adequate redistribution of muscle tone.
The sources of afferent (sensory) information for these reflexes are:
- The labyrinths of the vestibular apparatus.
- Neck muscle proprioceptors, which report the position of the head relative to the trunk.
- Receptors of the skin, joints, and muscles throughout the body.
- The visual system, which helps assess the structure of the environment.
It is interesting to observe the dependence of muscle tone on head position using animals as an example. If the head is raised, the nervous system increases the tone of the extensors of the forelimbs and the flexors of the hind limbs. If the head is lowered, the opposite reaction occurs: the tone of the extensors of the hind legs and the flexors of the forelegs increases.
Static righting reflexes
This group of reflexes is activated when the body needs to return from an unstable position to a stable one (for example, when transitioning from lying to standing). The mechanism is triggered by the stimulation of vestibular labyrinths, as well as by afferent input from skin receptors and neck muscles.
The key factor for initiating the righting reaction is a change in head position. The process proceeds as a chain reaction:
- First, the lying animal raises its head and fixes it in the horizontal plane (this is the so-called labyrinthine righting reflex).
- As soon as the head has assumed the correct position, neck-tonic reflexes are engaged, and information from trunk receptors is integrated.
- A large-scale redistribution of muscle tone occurs.
- As a result, the animal stands up on its limbs.
If we consider a mesencephalic animal (with transection at the midbrain level), we can notice a disruption in posture-maintenance mechanisms under external load. If such an animal is lightly pressed from above with a board, its head drops because the compensatory mechanisms do not function fully.
Statokinetic reflexes
These reflexes are associated with changes in muscle tone during body movement in space (during rectilinear or rotational motion). Their main effect is the redistribution of somatic muscle tone to maintain dynamic balance.
A characteristic manifestation of statokinetic reflexes during circular motion is nystagmus (of the head and eyes). It consists of two components:
- Slow phase: the eyes and head smoothly move in the direction opposite to the rotation. This is necessary for the temporary fixation of gaze on objects.
- Fast phase: a sharp jerk of the eyes and head back to return to the initial state.
Orienting and startle reactions
Orienting reflexes are realized with the participation of neurons in the corpora quadrigemina nuclei located in the midbrain. The superior colliculus contains primary visual centers, and the inferior colliculus contains primary auditory centers. The reaction includes listening, eye globe movement, regulation of pupil diameter, and turning of the auricles (clearly noticeable in animals).
In addition to orienting reflexes, there are startle reactions (the so-called startle response). They occur in response to sudden strong auditory or visual stimuli. Their main function is the immediate mobilization of the muscular apparatus, preparing the organism for flight or fight. It is important to note that the cerebral cortex exercises control over these reactions, exerting a modulating influence: it is capable of both enhancing and weakening the severity of the startle response.
Briefly: answers to topical questions
Which descending spinal cord tracts directly transmit impulses for posture maintenance?
To maintain posture and regulate muscle tone, sources indicate the following descending pathways:
- Vestibulospinal tract — responsible for maintaining balance and posture, activates extensor motoneurons, and is associated with the tone of antigravity musculature.
- Rubrospinal tract — participates in the regulation of muscle tone and movement coordination; the integrity of the rubrospinal tract is necessary for righting reflexes and maintaining balance.
- Reticulospinal tract — regulates the activity level of the spinal cord and muscle tone.
What is the role of the cerebellum in the regulation of muscle tone and balance maintenance?
The cerebellum regulates motor function on a feedback principle, receiving sensory input from muscle and joint receptors. The following participate in the regulation of tone and posture:
- Medial zone (vermis) (Vermis) — plays a leading role in establishing initial body posture and maintaining tone. Sends impulses to Deiters' vestibular nucleus and the reticular formation. Isolated damage to the vermis causes hypertonia.
- Intermediate zone — acts as a comparator, receiving afferentation about the current posture and comparing it with the expected one, rapidly correcting deviations and adjusting movement.
- Cerebellar cortex — its damage leads to hypotonia and increased fatigue.
What is decerebrate rigidity, and between which structures does the transection occur?
Decerebrate rigidity is a pathological condition manifested by a sharp increase in extensor muscle tone and the loss of the ability to maintain balance. The animal can be placed on its feet, but it cannot hold a posture.
The condition occurs when brainstem centers are separated from the inhibitory influences of overlying structures. According to sources, the transection can be located:
- In the brainstem between the superior and inferior colliculi.
- At the level of the upper border of the pons (separation of the medulla and pons from higher divisions).
A key role in controlling rigidity is played by the red nuclei (nucleus ruber); brainstem transection below them eliminates rigidity.
For memorizing brainstem control: Vestibular nuclei "Right" (activate extensors to fight gravity), while the Reticular formation "Flexes" (activates flexors).
Class and exam questions
How does muscle tone change in animals when the head is raised?
When the head is raised, the nervous system redistributes tone: tension increases in the extensor muscles of the forelimbs and the flexor muscles of the hindlimbs.
What is the chain-stretching reflex (righting reflex)?
This is the sequence of actions to return to a stable position: first, the head is positioned horizontally (labyrinthine reflex), then neck-tonic reflexes are engaged, tone is redistributed, and the animal stands up.
What phases of nystagmus are distinguished?
Nystagmus consists of a slow phase (movement in the direction opposite to rotation to fixate gaze) and a fast phase (a sharp jerk returning the eyes to their initial position).
Why does the body need a startle response?
The startle response (arising from a strong sudden stimulus) is necessary for the immediate mobilization of the muscular apparatus for active actions — preparing for flight or fight.
What else to study on this topic
- Interaction of $\alpha$- and $\gamma$-motoneurons in tone regulation
- Functions of the corpora quadrigemina nuclei (superior and inferior colliculi)
- Modulating influence of the cerebral cortex on startle reactions
- Features of reflexes in a mesencephalic animal
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