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Innervation of the complexus ("hatching") muscle of the chick
Insights
The avian hatching muscle receives dual innervation from both brain and spinal cord nuclei, a unique finding in neck muscles. This complex innervation is linked to head movements crucial for hatching and sensory feedback.
Area of Science:
- Neuroscience
- Developmental Biology
- Avian Anatomy
Background:
- The avian complexus muscle, or "hatching" muscle, exhibits significant temporary growth (pseudohypertrophy) near hatching.
- This muscle is essential for the head movements required during the hatching process.
Purpose of the Study:
- To investigate the neural innervation of the avian complexus muscle in hatchling chicks.
- To identify the specific brain and spinal cord nuclei that innervate this specialized muscle.
Main Methods:
- Horseradish peroxidase (HRP) injections into the complexus muscle.
- Tracing motor and sensory pathways using HRP transport techniques.
- Histological analysis of neural connections.
Main Results:
- Motor innervation originates from the cervical motor column (C1-C6) as expected.
- Unexpectedly, dual innervation was identified from the spinal accessory nucleus, nucleus supraspinalis, and facial motor nuclei.
- Primary afferent fibers project from the hatching muscle to the vestibular complex, particularly the tangential nucleus.
Conclusions:
- This study reveals the first documented instance of dual brain and spinal cord innervation for a neck muscle.
- The extensive sensory feedback pathway to the vestibular complex suggests a role in coordinating hatching movements.
- Synaptic changes in the tangential nucleus coinciding with hatching warrant further investigation into its functional relationship with the hatching muscle.
Abstract:
The complexus muscle of avians, also known as the "hatching" muscle, is notable for the dramatic, transient pseudohypertrophy which it undergoes around the time of hatching. The muscle is believed to be involved in specific dorsal and lateral head movements used for hatching. Innervation of the complexus muscle was studied in the hatchling chick by using horseradish peroxidase (HRP). HRP injections of the muscle showed that motor innervation arose, as expected, from the cervical motor column (C1-C6). However, additional innervation was also discovered; the spinal accessory nucleus (column of von Lenhossek), the nucleus supraspinalis, and the dorsal and ventral facial motor nuclei all contributed efferents to the hatching muscle. This observation constitutes the first description of dual innervation of a neck muscle by nuclei in both the brain and spinal cord. In addition, transganglionic transport of HRP revealed labelled primary afferent fibers from the hatching muscle ascending in the dorsal columns and terminating extensively within the vestibular complex, especially on the principal cells of the tangential nucleus. The tangential nucleus itself undergoes synaptic changes at the time of hatching. Possible functional relations between the tangential nucleus and the hatching muscle are discussed.