Timothy C. Hain, MD • Page last modified: August 23, 2020
|Rajin (god of thunder) illustrating tonic neck reflex. This is a postural reflex. Source: Wikipedia commons. Artist - -Tawaraya Sotatsu.|
Automatic body movements reflect a overlay of several "synergies" -- The main ones discussed in the classic leterature are ones related to body positioning alone (basic postural reflexes), and ones related to vestibular input (labyrinthine or vestibular spinal reflexes). When we stand or move, our body tends to automatically assume particular postures based on the combination of these responses.
The Tonic neck reflex describes the automatic positioning of the limbs in response to a movement of the head on trunk (neck). The tonic neck reflex can be observed in animals that have both been decerebrated as well as had their vestibular system eliminated. In such an animal there is no voluntary or vestibular input.
When the head is rotated in the yaw (around the up-down axis of the body), there is an increase in the extensor tone on the two limbs on the side of the nose (jaw), and a decrease in extension and flexion on the opposite side. Magnus called the jaw limbs "Kieferbein" and the skull limbs, "Schadelbein". The illustration above shows the TNR in art (see below).
There are also similar reflexes elicited when the head is rolled towards one sholder. In most animals (including humans), when the head is moved in pitch on the trunk, dorsiflexion induces extension of the legs and arms, while flexion of the head induces flexion of the arms and head.
The TNR, by itself, is driven simply by head-on trunk position. As it is not driven by any input related to body stability in space, it does not stabilize the body. However, it is relevant because it combines with the other vestibular reflexes.
The purpose of the vestibulospinal reflex (VSR) is to stabilize the body. The VSR is an assemblage of several reflexes named according to the timing (dynamic vs. static or tonic) and sensory input (canal, otolith or both). While terminology varies among authors, the term VSR usually also implies motor output to skeletal muscle below the neck, or in other words, it excludes the neck reflex which is called the vestibulocollic reflex or VCR (see following). As an example of a vestibulospinal reflex, let us examine the sequence of events involved in generating a labyrinthine reflex.
1. When the head is tilted (rolled) to one side, both the canals and otoliths are stimulated.
2. The vestibular nerve and vestibular nucleus are activated.
3. Impulses are transmitted via the lateral and medial vestibulospinal tracts to the spinal cord.
4. Extensor activity is induced on the side to which the head is inclined, and flexor activity is induced on the opposite side.
When the body is pitched, extensor tone changes according to the position of the head with respect to horizontal. Extensor tone is maximal when the angle of the head is 45 degrees with respect to horizontal (i.e. head is nose up as well as an additional 45 degrees towards upright). Extensor tone is minimal when the head is nose-down and pointing an additional 45 degrees down (Magnus, 1924).
There is also a "righting reflex". When the position of the head or body changes, reflex movements occur that tend to return the head or body to the normal posture. The input to this reflex is vestibular, vision and somatosensation -- it is not purely a vestibular reflex.
The Vestibulocollic Reflex (VCR)
The vestibulocollic reflex acts on the neck musculature in order to stabilize the head. Reflex head movement counters the movement sensed by the otoliths or semicircular canals. The neural pathways mediating this reflex are as yet uncertain.
The Cervicoocular Reflex (COR)
The COR consists of eye movements driven by neck proprioceptors. As the COR can supplement the VOR under certain circumstances it becomes relevant when considering recovery from vestibular lesions. Normally, the gain of the COR is very low (Peterson, 1988) but the COR is facilitated when the vestibular apparatus is injured (Kasai and Zee, 1979),.
The Cervicospinal Reflex (CSR)
The cervicospinal reflex, also known as the tonic neck reflex (TNR), is defined as changes in limb position driven by neck afferent activity. Analogous to the COR which interacts with the VOR, the CSR can supplement or interfere with the VSR. Two pathways are thought to mediate these reflex signals, an excitatory pathway from the lateral vestibular nucleus and an inhibitory pathway from the medial part of the medullary reticular formation. Their activity leads to extension of the limb on the side to which the chin is pointed and flexion of the limb on the contralateral side. Vestibular receptors influence both of these systems by modulating the firing of medullary neurons in a pattern opposite to that elicited by neck receptors. The interaction between the effects on the body of vestibular and neck inputs tend to cancel one another when the head moves freely on the body so that posture remains stable. (Pompeiano, 1988)
The Cervicocollic Reflex (CCR)
The cervicocollic reflex is a cervical reflex that stabilizes the head on the body. Afferent sensory changes caused by changes in neck position, create opposition to that stretch by reflexive contractions of neck muscles. The reflex was once thought to be primarily a monosynaptic one, however, long-loop influences are now being investigated. Like the COR, the CCR may be facilitated after labyrinthine loss. Cervicocollic reflexes have longer latencies (67.4 ms) than vestibulocollic reflexes (24.5) msec in humans, which gives normal individuals an advantage in head righting over labyrinthine defective subjects (Ito et al, 1997).
Other somatosensory mechanisms appear to be involved in postural responses driven by vestibular circuitry as well. Bles and associates documented somatosensory induced nystagmus (so-called "stepping around nystagmus").(Bles et al, 1984) Interestingly, the subjects with bilateral vestibular loss developed a more pronounced nystagmus than did normal subjects. This implies that subjects with bilateral vestibular loss use somatosensory information to a greater level than controls.
Dr. Fukuda (1984) provided a beautiful and instructive discussion of how vestibulospinal reflexes are found in art. The illustration above of Rajin is found on the cover of his textbook on vestibulospinal reflexes. According to Dr. Fukuda's discussion, there is a "perfect" expression of the tonic neck reflex with extension of the two jaw limbs, and flexion of the two skull limbs. Dr. Fukuda poitns out that in this picture, not only is the head turned to the left but the eyes also deviate to the left.
This book also provides a discussion of the TNR in other japanese sculptures and works of art.