Back to the debate: sternalis muscle / Volver al debate: músculo esternal

The sternalis muscle (SM) is an anatomical variant found in the anterior thoracic wall. While the attachment sites of SM are generally agreed upon, the innervation and function of this muscle are not well established. Cadaveric and surgical explorations to date report that SM is innervated by either the pectoral nerves or the anterior branches of the intercostal nerves, or a combination of both. Knowledge of SM is relevant to health care providers specialising in imaging and/or surgery of the anterior thoracic wall. This paper aims to raise awareness in the medical community of the clinical relevance of SM through two case reports and a brief literature review.

El músculo esternal (ME) es una variante anatómica en la pared torácica anterior. Mientras que los sitios de fijación del ME estan acordados, la inervación y la función de este músculo no están bien establecida. Exploraciones cadavéricas y quirúrgicas han informado que el ME está inervado por los nervios pectorales o ramos anteriores de los nervios intercostales, o una combinación de ambos. El conocimiento del SE es relevante para los proveedores de atención de salud especializada de imágenes y/o cirugía de la pared torácica anterior. Este documento tiene como objetivo crear conciencia en la comunidad médica de la relevancia clínica de ME a través de dos reportes de caso y una breve revisión bibliográfica.

Neurotization of motor nerves innervating the lower extremity by utilizing the lower intercostal nerves.

An animal model in the rat was developed to study the reinnervation of ventral roots contributing to lower-extremity nerves by use of intercostal nerves. Intercostal nerves and distal cauda equina roots were anastomosed, using a collagen tube and microsurgical technique. Most experimental animals could lift their previously paralyzed legs and could walk with a severe limp by 9 months postoperatively. Recordings of nerve action potentials (NAPs) and muscle action potentials (MAPs) indicated that the intercostal and sciatic nerves had some functional connections. Histologic analysis 12 months after repair demonstrated axonal regeneration extending from the intercostal nerves to and down lumbar ventral roots. Most of the regenerated fibers were moderately well-myelinated. Connections between the neurons of the anterior horn cells in the lower thoracic spinal cord and the reinnervated sciatic nerve were confirmed by retrograde tracer, using fast blue.

Rat intercostal nerves as experimental nerve grafts.

In this study, we dissected and measured both sets of intercostal nerves including lengths, diameters, and axon counts in 12 adult rats to provide data applicable to experimental nerve graft research. Dissections showed that total lengths of intercostal nerves from the spinal bifurcation to their last arborizations near the midline ranged from 10 to 89 mm, and diameters ranged from less than 0.1 to 0.5 mm from the thinnest to the thickest part. The segment of easiest dissection was the part between the spinal bifurcation and the lateral cutaneous branch. This part was 4-27 mm (mean, 13.3 mm) long and had an almost constant diameter of 0.18-0.5 mm (mean, 0.32 mm). Counts ranged from 201 to 566 axons/ nerve. The segment proximal to the lateral cutaneous branch was the most convenient part to be harvested as a nerve graft, especially in the 8th to 12th intercostal nerves. These nerves could serve as sources for experimental grafts.

Morphometric evidence from C-synapses for phased Nissl body response in alpha-motoneurones retrogradely intoxicated with diphtheria toxin.

Diphtheria toxin (DTX) kills cells by inactivating ribosomal translocation and when used to retrogradely intoxicate cat intercostal motoneurones produces marked morphological alterations in Nissl bodies, including those specifically sited postsynaptic to C-type axon terminals. Here, qualitative examinations of 'intoxicated' postsynaptic Nissl bodies reveal a progressive structural alteration marked by rER dilatation, rER lamellae fragmentation but retention of both the highly ordered multilamellate organization and ribosomal attachment until final stages of Nissl body dissolution. Morphometric results identified 3 broad phases to the postintoxication response which differed in the degree of rER cisternal dilation, and the numerical and spatial relationships between rER-lamellae, rER-bound ribosomes and rER-associated polyribosomes. These phases reflect the known molecular basis of diphtheritic toxicity and contrast with the fast developing Nissl body reaction associated with the neurotoxin ricin which also invokes ribosomal dysfunction and has been used to mimic certain features of motor neurone disease. The cytopathology of DTX and ricin are compared in the Discussion.

The onset and rate of myelination in six peripheral and autonomic nerves of the rat.

A light and electron microscopic study was carried out of the numbers of myelinated fibres in 6 nerves of the rat for 7 age groups from birth to 73 weeks. The hypoglossal nerve and the mandibular branch of the facial nerve had short and early myelination periods, essentially complete by the second week. The glossopharyngeal nerve and the sympathetic rami communicantes myelinated late and over a protracted period. Myelination of the rami communicantes continued up to 20 weeks, followed by a marked loss of fibres in the 73 week animals. Intercostal and saphenous nerves had intermediary patterns. There was evidence of subpopulations myelinating at different times. Measurements of myelin sheath thickness showed variations of relative sheath thickness with age, between nerves and for subpopulations of nerves. Late myelination corresponded to relatively thin myelin sheaths. Statistical two-stage-density cluster analysis by computer was used for analysing complex fibre populations. The developmental changes of three subpopulations of the intercostal nerve are documented. Nerves also differed in their rates of axon growth. The increment in axon calibre was small and late for sympathetic fibres. Intercostal and facial nerve fibres had rapid axon growth with different growth rates for subpopulations.

Variance in relative internode length (l/d) in the rat and its presumed significance for the safety factor and neuropathy.

(1) Previous work has shown that the quotient l/d (internode length/fiber diameter) is the product of independent variations in internode length (increasing with fiber elongation) and in axon caliber (whose growth is not as yet well understood). Using these guiding principles, the distribution of l/d in 19 nerves and 7 roots of the rat was determined. (2) Each nerve or root showed a linear decrease of the l/d with fiber caliber, the thin fibers always having relatively longer internodes than the thick ones. Comparing nerves, the highest l/d was found for extremity nerves, particularly those in the hindlimbs. Nerves of the trunk (phrenic, intercostal) had lower l/d with further decreases for the extracranical branches of cranical nerves, the intercranial roots and a minimum for the acoustic nerve. In the facial nerve, the l/d of the intraosseous portions was distinctly lower than that of its branches in the face. Ventral roots showed a cranio-caudal increase in their l/d. For some fiber systems, e.g. cranial nerves, the l/d reflected variance in axon caliber, their internode length being relatively uniform. For others, e.g. roots, the l/d was dominated by their large variation in internode length. (3) Variation of the l/d in fiber populations may correlate with such parameters as internodal conduction time, different sensitivities of thick and thin fibers and, particularly, the safety factor. It is also proposed that the regional variation in the l/d is a critical parameter affecting the regional vulnerability of fiber populations to polyneuropathies or to radiculopathies.

The non-myelinated fibers of the phrenic and the intercostal nerves in the cat.

The aim of this work was obtaining quantitative data relative to the non-myelinated fibers of some thoracic nerves of the cat. The study has been done both through the utilization of the light and the electron microscope. A comparative analysis of the results obtained from the muscle nerves (external intercostal nerve and phrenic nerve), cutaneous nerve (lateral collateral branch of the internal intercostal nerve) and mixed nerves (internal intercostal) has been made. The C muscle fibers have a smaller diameter than the C cutaneous fibers. Some of the non-myelinated fibers of the phrenic nerve are probably involved in the innervation of the pleura or of the peritoneum. The relative number of non-myelinated fibers varies according to the nerves. The factor linking the diameter of the non-myelinated fibers to their conduction velocity is approximately equal to 0.7.