Spinal cord

Question 1

Outline the spinal cord.

Answer 1

• The spinal cord, or spinal medulla, is a cylinder, somewhat flattened from front to back, whose lower end tapers into a cone.
• Ventrally it possesses a deep midline groove, the anterior median fissure, and dorsally it shows a shallow posterior median sulcus, from which a posterior median septum of neuroglia extends into its substance.
• In the fetus the spinal cord extends to the lower limit of the spinal dura mater at the level of S2 vertebra.
• The spinal dura continues to reach this level throughout life, but the spinal cord becomes relatively shorter, which is to say that the bony spinal column and the dura mater grow more rapidly than the spinal cord.
• Thus at birth the conus medullaris lies opposite L3 vertebra and does not reach its permanent level opposite L1 or L2 until about the age of 20 years.
• The spinal nerve roots, especially those of the lumbar and sacral segments, thus come to slope more and more steeply downwards.
• The spinal cord possesses two symmetrical enlargements which occupy the segments of the limb plexuses: as the cervical enlargement (C5 to T1) for the brachial plexus and the lumbosacral enlargement (L2 to S3) for the lumbar and sacral plexuses.
• Their levels measured by vertebrae are, of course, quite different; the cervical enlargement is approximately opposite vertebrae C3 to T1, but the lumbosacral extends only from T9 to L1.
• Both enlargements are due to the greatly increased mass of motor cells in the anterior horns of grey matter in these situations.

Question 2

Outline the spinal nerve roots.

Answer 2

• The anterior and posterior roots of the spinal nerves unite within the intervertebral foramina.
• Within the subarachnoid space the nerve roots are attached to the spinal cord each by a series of rootlets. * Each anterior root is formed by three or four rootlets which emerge irregularly along the anterolateral surface of the spinal cord.
• Each posterior root is formed by several rootlets, attached vertically to the posterolateral surface of the cord.
• A short distance from the cord the rootlets are combined into a single root.
• The anterior and posterior roots pass from the cord to their appropriate intervertebral foramina, where each evaginates the dura mater separately before uniting to form the mixed spinal nerve.
• The ganglion on the posterior nerve root lies in the intervertebral foramen, within the tubular evagination of dura and arachnoid immediately proximal to the point of union of anterior and posterior nerve roots.
• However, the posterior root ganglia of cervical nerves lie partly lateral to the intervertebral foramina, behind and in contact with the vertebral artery; the ganglion of the first cervical nerve, however, lies on the posterior arch of the atlas. * For all levels from C1 to L1 vertebrae the anterior and posterior nerve roots pass in front of and behind the denticulate ligament respectively, and evaginate the dura mater between the denticulations.
• In conformity with the shortness of the spinal cord, the lower a nerve root the more steeply it slopes down to the intervertebral foramen.
• The upper cervical roots are horizontal, the upper thoracic roots first slope down to their point of evagination of the meninges only to become kinked upwards at an angle to reach their foramen. Below L1 vertebra the roots pass almost vertically downwards through the subarachnoid space, forming the cauda equina. * The filum terminale (pia mater) extends down from the tip of the conus medullaris among the nerve roots of the cauda.
• The roots of the spinal part of the accessory nerve emerge from the lateral surface of the upper five or six segments of the cord, behind the denticulate ligament. They unite into a single trunk which passes upwards through the foramen magnum into the cranium to join the cranial root.

Question 3

Describe the internal structure of the spinal cord.

Answer 3

• The spinal cord consists of a central mass of grey matter (cell bodies), in the form of a vertically grooved column surrounding the central canal, enclosed in a cylindrical mass of white matter (fibres).
• It is almost divided into two halves by the anterior median fissure and the posterior median septum.
• The septum extends forwards as far as the grey commissure (the central limb of the H in cross-section) which connects the grey matter of the right and left halves of the cord, and contains the central canal.
• This is the tiny downward continuation of the cavity of the fourth ventricle and like it lined by ependyma. It extends into the upper few millimetres of the filum terminale.
• The anterior fissure does not completely separate the white matter - a narrow white commissure lies anterior to the grey.
• On account of the shape of the grey matter and the attachment of nerve roots, the grey and white matter of the right and left halves is divisible into anterior, lateral and posterior parts, referred to as horns for the grey matter and columns for the white matter.
• The shapes of the grey horns and white columns in sections enables the three regions to be distinguished.
• The anterior grey horns are largest in the cervical and lumbar regions and the posterior white columns largest in the cervical regions.

Question 4

Outline the grey matter of the spinal cord.

Answer 4

• The posterior horn stretches to the surface of the cord, but the anterior horn falls short of it.
• The cervical and lumbosacral enlargements are due to a great increase in the number of anterior horn cells, to provide fibres for the great nerve plexuses.
• The more medial anterior horn cells are concerned with the innervation of trunk musculature, with the more lateral cells supplying the limbs.
• The more ventral cells of the lateral group supply proximal limb muscles, and the more dorsal cells innervate the more distal limb muscles.
• Between the limb enlargements, from segments T1 to L2, there is a small lateral horn, containing preganglionic sympathetic cell bodies. Their axons pass out in the anterior nerve roots and enter the spinal nerves from T1 to L2 which they leave in the white rami communicantes passing to the sympathetic trunk.
• A similar group of cells forms the small lateral horn in sacral segments 2–4; these are preganglionic parasympathetic cell bodies whose axons leave in those sacral anterior nerve roots and emerge distally from the sacral nerves as pelvic splanchnic nerves.
• The cells of the grey matter in each half of the cord lie in specific functional groups or laminae, designated by the Roman numerals I to X.
• Among the more important cell groups in the various laminae are:
• Those of lamina II which constitute the gelatinous substance
• The cells of lamina V are a main source of anterolateral tract (spinothalamic and spinoreticular) fibres.
• Lamina VII contains in its medial part the thoracic nucleus and laterally the thoracolumbar (sympathetic) and sacral (parasympathetic) lateral horn cells.
• Other cells of lamina VII, together with those of lamina VIII, are interneurons involved in coordinating motor activity and projecting to lamina IX; lamina IX contains the alpha and gamma motor neurons which innervate skeletal muscle.

Question 5

Outline the white matter of the spinal cord.

Answer 5

• In each half of the cord the posterior white column lies between the posterior median septum and the posterior grey horn.
• It is wholly occupied by the ascending fibres of the gracile and cuneate tracts, the pathways for touch and some associated sensations.
• The rest of the white matter forms the lateral and anterior white columns, the emerging anterior nerve roots providing a convenient dividing line between the two.
• Both these columns contain long ascending and descending tracts. In addition, short intersegmental fibres run up and down adjacent to the central grey matter, forming communications between segments.
• Except in the posterior columns, there is much intermingling of fibres and there are no sharp boundaries to tracts.

Question 6

Describe afferent pathways.

Answer 6

• There are three possible destinations for all incoming fibres:
• The cortex of the opposite cerebral hemisphere, via thalamic relay, for conscious sensation.
• The cerebellum, for muscular coordination.
• The brainstem or spinal cord, for reflex actions.
• As a guiding principle it is usually stated that afferent impulses are conveyed to the cerebral cortex by three groups of neurons.
• While this is a convenient concept and accurate enough for the main long-fibre components, it takes no account of the vast numbers of short interneurons in the neuronal pathways.
• The cell bodies of the first group of neurons (first neurons or first order neurons) lie outside the central nervous system: in the posterior root ganglia of spinal nerves or the equivalent ganglia of cranial nerves.
• The cell bodies of the second neurons are in the spinal cord or brainstem, and those of the third neurons are in the thalamus.

Question 7

Describe efferent pathways.

Answer 7

• For the control of the skeletal muscles supplied by spinal nerves, two main systems of neurons are involved.
• One can be called the direct corticospinal pathway, consisting essentially of two groups of neurons:
• The cell bodies of the first are in the cerebral cortex, and their fibres extend through the internal capsule and brainstem to the anterior horn cells of the cord; these are the corticospinal fibres. The corresponding corticonuclear fibres go to the motor nuclei of cranial nerves. These fibres (often called pyramidal) with their cortical cell bodies are known as the upper motor neurons.
• The second neurons are the anterior horn cells and their axons which end as the motor endplates on skeletal muscle fibres. These neurons (and those of the motor nuclei of cranial nerves) are called the lower motor neurons.
• This sequence does not take account of interneurons that participate in this pathway.
• The other system involves a whole series of neurons on the way to anterior horn cells.
• For example, cortical cells may send their axons to pontine nuclei, from which fibres run to the cerebellum which in turn may project to the red, reticular, vestibular and olivary nuclei, all of which can communicate with the same anterior horn cells as pyramidal fibres.
• Other cortical fibres run to the basal nuclei (corpus striatum) and thalamus, with projections from them to the red, reticular and subthalamic nuclei and the substantia nigra.
• These brainstem nuclei can also be activated by reflex sensory pathways (e.g. light and sound).
• From some of these brainstem groups fibres descend to synapse with the same anterior horn cells that have received corticospinal fibres.
• None of these fibres has passed through the pyramid of the medulla (hence the name extrapyramidal) and because none has passed uninterruptedly from the cortex to anterior horn cells they are also called indirect corticospinal pathways.