Spinal cord anatomy

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Introduction

The nervous system is divided into the central nervous system (brain and spinal cord considered upper motor neurons) and the peripheral nervous system (nerves that enter and exit the spinal cord considered lower motor neurons).[1] Information to and from the muscles, glands, organs and sensory receptors are carried through the peripheral nervous system, which is divided into the autonomic nervous system carrying information to and from the organs, and the somatic nervous system carrying information to and from the muscles and the external environment. [2][3] The autonomic nervous system consists of the parasympathetic nervous system that governs resting function and the sympathetic nervous system that governs excitatory functions. The spinal cord and peripheral nerves provide all impulses to control muscle contraction, cardiac rhythm, pain and other bodily functions so therefore any lesion to the spinal cord prevents or reduces transmission of this information to and from the brain to the peripheries, affecting movement, sensation and visceral function. [3]

Nervous System

Spinal Column

The spine or vertebral column bears the weight of the head, neck, trunk and upper extremities. The adult vertebral column typically consists of 33 vertebrae arranged in five regions, which provide support and protection for the spinal cord.  It consists of seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, five sacral and four coccygeal vertebrae, The five sacral vertebrae are fused in adults to form the sacrum, and the four coccygeal vertebrae are fused to form the coccyx. Its uppermost vertebrae incorporating the atlas and axis articulate with the head, and the lower most portion, the sacrum articulates with the pelvis. Each pair of adjacent vertebrae is separated by a semi-rigid intervertebral disk, which provides a level of flexibility to the vertebral column. Its flexibility is greatest in the cervical region, and lowest in the thoracic region. [4]

Spinal Canal

Formed by the vertebral foramina of the vertebral bodies the spinal canal is bound anteriorly by the vertebral bodies and posteriorly by the laminae (vertebral arches) with reinforcement at the walls through the intervertebral disks and the anterior and posterior longitudinal ligaments. Containing the spinal cord, meninges, blood vessels, spinal nerve roots and surrounding fatty and connective tissues the diameter varies from 12 to 22 mm in the cervical region and from 22 to 25 mm in the lumbar region. [2][4]

Spinal Cord

The spinal cord is the major conduit and reflex centre between the peripheral nerves and the brain, and transmits motor information from the brain to the muscles, tissues and organs, and sensory information from these areas back to the brain. [2][3] The spinal cord in an adult is approximately a 45 cm long, cylindrical structure that is slightly flattened anteriorly and posteriorly. [2] The spinal cord lies within the vertebral canal, extending from the foramen magnum to the lowest border of the first lumbar vertebra. It is enlarged at two sites, the cervical and lumbar region. Its upper end is continuous with the medulla; the transition is defined to occur just above the level of exit of the first pair of cervical nerves. Its tapering lower end, the conus medullaris, terminates at the level of the L3 vertebra in neonates, and at the level of the L1-2 intervertebral disk in adults. The conus medullaris is continuous at its lower end with the threadlike filum terminale, composed mainly of glial and connective tissue, which, in turn, runs through the lumbar sac amidst the posterior and anterior roots of the spinal nerves, collectively called the cauda equina (“horse’s tail”), and then attaches to the dorsal surface of the coccyx. [4]

  • Sensory Nerve Fibres enter the Spinal Cord via the Posterior (Dorsal) Root. The cell bodies for these neurons are situated in the Dorsal Root Ganglia.
  • Motor and Preganglionic Autonomic Fibres exit via the Anterior (Ventral) Root.
Spinal Column [5]
Spinal Cord Sectional Anatomy [6]

Spinal Nerves

Emerging from the spinal cord are 31 pairs of anterior and posterior nerve roots. The cervical, thoracic, lumbar, and sacral portions of the spinal cord are defined according to the segmental division of the vertebral column and spinal nerves. There are eight cervical*, twelve thoracic, five lumbar, five sacral and one coccygeal. At each level an anterior pair of nerve roots carries motor nerves, while a posterior pair of nerve roots carries only sensory nerves. The anterior and posterior roots join to form two spinal nerves, one on either side of the spine, which then exit the vertebral canal through the intervertebral foramina. Once outside the intervertebral foramina they form peripheral nerves. [4]

*While there are eight pairs of cervical spinal nerves there are only seven cervical vertebrae. This disparity occurs because the first pair of cervical spinal nerves exits above the first cervical vertebra just below the skull. However, the eighth pair of cervical spinal nerves exits below the last cervical vertebra.* [2]

Spinalnerves.jpg

Spinal Motorneurons

Alpha and Gamma motorneurons (MNs) are both found in the ventral (anterior) horn. Alpha motorneurons are the largest moor neurons in the nervous system. They innervate skeletal muscle. Gamma Motorneurons innervate intrafusal muscle fibres of the muscle spindle. Motor neurons are arranged somatotopically across the ventral horn. The more medially placed MNs innervate proximal muscles while laterally placed MNs innervate distal muscles.

Blood Supply

The majority of the spinal cord blood supply is provided by the segmental spinal arteries, with further supply coming from the vertebral arteries via a single anterior spinal artery and paired posterior spinal arteries. The segmental and spinal arteries are linked by numerous anastomoses. [4][2]

Segmental Arteries

The segmental arteries give off radicular branches, cervical and thoracolumbar, which enter the intervertebral foramen and supply the anterior and posterior roots and spinal ganglion of the corresponding level. The spinal cord itself is supplied by unpaired medullary arteries that originate from segmental arteries.[4][2]

Spinal Arteries

The spinal arteries, arising from the vertebral artery, run longitudinally down the spinal cord. The unpaired anterior spinal artery lies in the anterior median fissure of the spinal cord and extends from the level of the lower brainstem to the tip of the conus medullaris, with the diameter steadily increasing below the T2 level. It supplies the ventral medial surface of the medulla and anterior 2/3 of the spinal cord. The paired posterior spinal arteries supply the dorsal columns and the all but the base of the dorsal horns bilaterally. There are reinforcing branches from other arteries along the length of the cord and numerous anastomoses of the spinal arteries produce a vasocorona around the spinal cord. If occlusion occurs, it is normally of the anterior spinal artery, producing loss of power and spinothalamic sensory deficit, but dorsal column sensory capabilities are maintained.[4][2]

Spinal Veins

Blood from within the spinal cord travels through the intramedullary veins, to the anterior and posterior spinal veins, which form a reticulated network in the pia mater around the circumference of the cord throughout its length. The anterior two-thirds of the gray matter drain via the anterior spinal vein, while the posterior and lateral spinal veins drain the rest of the spinal cord. These vessels empty by way of the radicular veins into the external and internal vertebral venous plexuses, groups of valveless veins that extend from the coccyx to the base of the skull. The vertebral and deep cervical veins drain venous blood from the cervical spine into the superior vena cava; the posterior intercostal and lumbar veins drain venous supply from the thoracic and lumbar spine into the azygos and hemiazygos veins; and the median and lateral sacral veins drain venous supply from the sacrum into the common iliac vein.[4][2]

Associated Pathways

Ascending Sensory Pathway

Information detected by sensory receptors in the periphery is transmitted along ascending neurol tracts in the spinal cord. Located in the white matter of the spinal cord, the ascending sensory tracts arise from either the the cells of the spinal ganglia or the intrinsic neurons within the grey matter that receive primary sensory input. There are many sensory tracts and pathways carrying different types of sensory information from the periphery to the cerebral cortex. In humans the major sensory pathways include;

Spinothalamic Tract

The spinothalamic tracts sit within the dorsal horn laminae I,III,IV,Vof the spinal cord. Most of the fibres cross the midline at or near the level they enter the spinal cord. The lateral spinothalamic tract carries information about pain and temperature, and the anterior spinothalamic tract carries information about crude touch.

Dorsal Column or Medial Lemniscal Pathway

The largest ascending tract, the fasciculi gracilis and cuneatus, arise from the spinal ganglion cells and ascend in the dorsal funiculus to the medulla oblongata. The gracile and cuneate tracts carry information about proprioception and light touch. The gracile tract is positioned medially and predominantly carries sensory fibres from the lower body below T6, while the cuneate tractis positioned laterally and predominantly carries fibres from the upper body above T6. The fibres within the gracile and cuneate tracts cross completey in the brainstem and form the medial lemniscus, which in turn projects to the anteriobasal nuclear complex of the thalamus. Conveys proprioception, light touch and vibration.

Spinocerebellar Tract

From spinal cord interneurons. It has two tracts a) Posterior Spinocerebellatr Tract, which relays via inferior cerebellar peduncle and b) Anterior Spinocerebellar Tract relays via superior cerebellar peduncle to the cerebellum. It conveys proprioceptive information and on-going activity in the spinal cord interneurons.

Descending Motor Pathways

The descending pathways, convey the motor commands and are involved in the control of movement. In humans, the major motor pathways include: 

Corticospinal (Pyramidal) Tract

Carries information from the cerebral cortex to the spinal cord. Also called the pyramidal tract as it is the only point where all the fibres are collected together without contamination by other fibre tracts in the medullary pyramids of the brain stem. Primary motor cortex is the main source of input to this tract, but the premotor and supplementary motor cortex also contribute fibres. Its projections are primarily contralateral and have a strong influence on the activity of groups of spinal motoneurones, which innervate distal muscles of the hands and feet. Most of the fibers, approximately 85%, cross the midline in the decussation of the pyramids in the brain stem and then descend through the spinal cord in the lateral corticospinal tract, while the other 15% cross within the spinal cord at the level they terminate and are carried within the medial corticospinal tract. According to Harvey (2008) the cervical upper motor neurons are centrally located within the corticospinal tract while the lumbar and sacral neurons are peripherally located, which explains neurological patterns of loss seen with certain types of incomplete spinal cord injuries where the peripheral rim of the spinal cord is undamaged 

Reticulospinal Tract

This tract begins in the caudal reticular formation in the pons and medulla. The fibres from the medullary portion descend in the dorsolateral funiculus of the cord near the corticospinal fibres, whereas the fibres from the pontine region travel in the ventromedial portion of the spinal cord. These pathways are predominantly bilateral and have the largest density of projections to axial and proximal muscles. Provides both excitable and inhibitory effects on the interneurons in the spinal cord, and to a lesser extent, it also acts on the motor neurons. Its main action is to dampen down activity in the spinal cord. without this pathway, there is increased extensor tone observed.

Vestibulospinal Tract

Originates from Deiters nucleus in the medulla and innervates the extensor and axial muscles. The vestibulospinal tracts receive limited input from the cortical motor areas as they arise in the vestibular nuclei, which receive main input from the balance organs of the ear. Their projections to the spinal cord are mostly bilateral and to proximal and axial muscles. It is involved in balance control and posture.

Rubrospinal Tract

Originates from the magnocellular part of the red nucleus in the brain. It projects towards common structures with the CoST, particularly those involved with distal motor control. There is debate as to how significant this tract is in humans, is quite prominent in the cat. 


Autonomic Pathways

The spinal cord not only carries motor and sensory nerves but also autonomic nerves (see Figure 1.4). Sympathetic nerves exit the vertebral canal via thoraco-lumbar spinal nerves, and parasympathetic nerves exit via sacral spinal nerves. Consequently, patients with cervical lesions lose supraspinal control of the entire sympathetic nervous systemand of the sacral part of the parasympathetic nervous system. Patients with thoracic, lumbar or sacral lesions lose varying amounts of supraspinal control of the sympathetic and parasympathetic nervous system as determined by the level of the lesion. Some parasympathetic fibres are carried within cranial nerves and are unaffected by spinal cord injury. 

[7]

References

  1. Barker; Barasi; Neal. Neuroscience at a glance; Blackwell science Ltd; 1999
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Moore KL, Agur AM, Dalley AF. Essential Clinical Anatomy. Philadelphia: Lippincott Williams & Wilkins; 2002 Mar.
  3. 3.0 3.1 3.2 Francisco de Assis Aquino Gondim et al., Topographic and Functional Anatomy of the Spinal Cord, Medshape, 2015
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Rohkamm Reinhard. Color Atlas of Neurology. Thieme; 2014 Sep 2.
  5. Designed by Freepik at http://www.freepik.com
  6. Spinal CordBlausen Medical. Retrieved on 26 January 2016.
  7. Handwritten tutorials. Spinal Pathways 1 - Spinal Cord Anatomy and Organisation. Available from: http://www.youtube.com/watch?v=5B87zsAKmWc [last accessed 29/08/16]