We covered the key anatomical structures of the brain in the previous article. Now we will look to extend downwards towards the spinal cord and one of the key tracts connecting it’s various anatomical structures to the brain.
Gross Anatomy of the Spine
Typically, the term ‘spinal cord’ tends to initiate thoughts of the vertebral column seen in many pictures or memories of vertebral segments such as ‘cervical’, ‘thoracic’, ‘lumbar’, ‘sacrum’ and ‘coccyx’. Well, it certainly isn’t far off, as the column and segments are all related EXCEPT: the spinal cord sits within the vertebrae as an extension of the central nervous system (CNS) from the brain stem.
The spinal cord occupies 2/3rd of the spinal canal within the vertebral column, approximately 42-45cm in length and ~1.2cm wide in adults (Figure 1). The width of the cord specifically widens at the cervical enlargement and lumbosacral enlargement, representing the larger amount of lower motor neurons to provide origins of the nerves travelling out towards the upper and lower extremeties.
Quick tip: Just like with vertebral segments, spinal cord and nerves are split into cervical, thoracic, lumbar, sacral and cocccygeal regions. The primary differences are:
- The spinal cord is split into 30 segments unlike the vertebrae (33).
- The cervical spine consists of 8 segments (nerves) as opposed to 7 vertebrae.
- Similar to the motor and sensory homonculus, the use of spinal nerve classifications allows for neuroscientists to map the spinal cord in terms of sensory and motor innervation points. These are known as dermatomes and myotomes respectively (Figure 2).
Leading on from the lumbosacral enlargement, the inferior (distal) end of the cord is known as the conus medullaris which ends at the L1/L2 vertebrae and continues as extended branches of nerve fibers known as the cauda equina followed by further extensions named the filum terminale (Figure 3).
Cross sectional anatomy of the spinal cord
Before we begin here, let me try help you define a few anatomical terms:
Sulcus: As in the previous article, think of a crease or a fold (like within the brain).
Fissure: Long, narrow opening or line of breakage/splitting.
Commissure: Where two areas connect. Think of the end of a fissure or the point of at which the two sides of the spinal cord meet.
Septum: Think of a central partition between two sides such as in between the nostril or the heart.
Now that we have a rough idea of what these words generally mean, let us break down the cross section of a spinal cord!
A H or ‘butterfly’ like formation of gray matter surrounded by white matter separated by the shallow dorsal (posterior) median sulcus. As a result, this creates a right and left divide of the spinal cord (dorsal median septum) until the dorsal gray commissure. In an almost symetrical fashion, the ventral (anterior) midsection of the spinal cord is separated, however by the much deeper ventral median fissure whilst ending at the ventral white commissure. In addition, a tunnel like structure within the centre of the gray matter runs all the way down the cord (central canal), lined with ependymal cells allowing the flow of cerebrospinal fluid to and from the 4th ventricle. Now try visualise this paragraph using Figures 4 & 5.
The most dorsal areas of gray matter are named the dorsal horns while the most ventral areas of gray matter are the ventral horns. The area in between is either known as the lateral horns, or has also been named the intermediate gray matter. Just like in the brain, the differentiation between gray and white matter is based on which regions contain higher levels of lipid-rich myelin or neuronal cell bodies. The white matter is once again split into dorsal (posterior) or ventral (anterior) regions, however these are known as the dorsal, ventral and lateral columns. As can be seen in any cross sectional diagram, there are also wire-like structures protruding from the white matter columns, seemingly in line with each of the ventral and dorsal horns. These are the ventral and dorsal roots (initially rootlets) and eventually form the root ganglions.
The dorsal horns largely contain interneurons (relay neurons) as well as neuronal cell bodies in relation to sensory input, therefore project towards the ascending tracts (afferent neurons) further up the CNS via the dorsal roots. Inversely, the ventral horns largely contain the cell bodies of lower motor neurons and project towards the ventral roots towards skeletal muscle. The lateral horns are also known as intermediate gray matter due to having shared characteristics of ventral and dorsal horns, however, are also crucial in autonomic function due to containing cell bodies of autonomic neurons.
Quick tip: The H-shape or horns of gray matter vary in size and shape based on spinal segments. Proportion of gray to white matter increases within the cervical and lumbar enlargements representing larger amounts of neuronal cell bodies. Dorsal to ventral horn proportions vary within each segment also (Figure 6).
Surrounding the gray matter, the white matter contains columns (funiculi) which contain multiple bundles of myelinated and unmyelinated nerve fibers. Bundles of fast conducting fibers (myelinated) travelling for a particular distance (ascending/descending) with the addition of glial cells, astrocytes and oligodendrocytes in between, are termed fasciculi. When a particular set of fasciculi has a common function, this is known as a tract.
Various Tracts within Spinal White Matter
The various tracts and pathways found within the white matter of the spinal cord can be generally split into two categories: 1) Descending fiber systems and 2) Ascending fiber systems. Here, I will aim to briefly summarize all of these tracts and pathways, their primary functions, origin and location within the spinal cord. I will then go on to elaborate on the corticospinal tract specifically and it’s relation to skeletal muscle contraction.
Table 1 summarizes the various descending fiber systems
Table 2 summarizes the various ascending fiber systems
The Corticospinal Tracts
Between the cerebral cortex and spinal cord lie large bundles (>1million) of mostly myelinated axons that descend through the brain stem to eventually innervate skeletal muscles. The corticospinal tract is the largest descending motor pathway (though not the only pathway) for skeletal muscle contraction, hence is crucial in the field of neuromuscular physiology.
The journey begins with upper motor neurons found within the primary motor cortex (over half), premotor cortex and somatosensory cortex that travel down towards the various structures within the brain stem:
- They initally descend through the midbrain as part of large fiber bundles called the cerebral peduncles.
- The tract continues through to the medulla forming two large collections of axons known as the medullary pyramids or pyramidal tracts.
- Upon entry through the pyramids, approximately 90% of the axons will decussate (cross over) into the lateral white columns of the spinal cord of the contralateral side. This means it will either innervate the contralateral skeletal muscle directly (through lower motor neurons at ventral horns) or indirectly (through interneurons and subsequently lower motor neurons thereafter). This crossover is known as pyramidal decusssation.
- This is known as the LATERAL CORTICOSPINAL TRACT and is responsible for innervation and voluntary contraction of distal skeletal muscle as well as highly skilled/fine motor movement. It is is the largest descending motor pathway in the nervous system.
- Approximately 10% do not decussate intially and continue through the ventral white colu-
mns within the spinal cord. These fibres continue to travel through the ventral columns until they reach the level of the muscle required to be innervated. This is done by decussating at the ventral white commissure then projecting to an interneuron or lower motor neuron firing out at the contralateral ventral rootlets.
- This is known as the ANTERIOR CORTICOSPINAL TRACT and is responsible for innervation and voluntary contraction of skeletal muscle at the level of the spinal cord itself e.g. trunk, neck and shoulder muscles.
- A small perecentage of corticospinal tract fibers play a role in posture control, whilst some terminate at the dorsal columns and play a role in afferent input to the brain to allow filter certain sensory stimuli.
That concludes my summary of spinal cord anatomy and tracts for today! Stay tuned for the following articles/topics within the NM System series:
- Neuronal and Glial Cells
- Neurotransmission & Membrane Potentials: Excitatory or Inhibitory?
- Motor Units – Recruitment & Synchronization.
- NM Contraction (Skeletal Muscle anatomy & Sliding Filament theory).
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