Spinal Cord

Cross references:    Sensory System     Sensory Input     
Posterior Horn of the Spinal Cord    
Dorsal Root Ganglion    
Pseudounipolar Neuron   
  Spinobulbar Tract    Spinocerebellar Tract        
Spinothalamic Tract       Spinal Lemniscus         Spinal Locomotor Generator    

The Spinal Cord, Spinal Nerves, and the Autonomic Nervous System 
"Internuncial Neurons travel between segments, sending projections up to the brain stem and cerebellum. They project in an ascending, not descending manner. "

Note:  Since I'm particularly interested in the influence that sensory input has on locomotion, I'm going to focus my attention on that part of the spinal cord which serves as the interface between sensation and movement: the spinal interneurons. 

Spinal interneuron - Wikipedia 
    "A spinal interneuron is an interneuron found in the spinal cord that relays signals between afferent neurons and efferent neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration.[1] Most interneurons are found in the grey column a region of grey matter in the spinal cord."  

Spinal interneuron @
Anatomy and physiology of animals Relation btw sensory, relay & motor neurons.jpgSpinal Interneuron, labeled here as just a "relay neuron", integrates sensory-motor input.   



The grey column of the spinal cord appears to have groups of small neurons, often referred to as spinal interneurons, that are neither primary sensory cells nor motor neurons.[2] The versatile properties of these spinal interneurons cover a wide range of activities. Their functions include processing sensory input, modulating motor neuron activity, coordinating activity at different spinal levels, and relaying sensory or proprioceptive data to the brain. There has been extensive research on the identification and characterization of the spinal cord interneurons based on factors such as location, size, structure, connectivity, and function.[2] Generally, it is difficult to characterize every aspect of the neuronal anatomy a vertebrate's spinal cord. This difficulty arises due to the extent of complexity observed in the structure,connectivity, and morphology of neurons. For instance, in the spinal cord of a 19 day embryo rat, at least 17 different subclasses of interneurons with ipsilateral axon projections were found. In addition, 18 types of commissural interneurons have been identified on the basis of morphology and location.[3][4]


In particular, the cell bodies of the spinal interneurons are found in the grey matter of the spinal cord, which also contains the motor neurons. In 1952, the grey matter of the cat's spinal cord was investigated, and it was shown to have ten distinct zones referred to as Rexed laminae. Eventually, the lamination pattern was also observed in several species including humans. 

    Rexed laminae VII and VIII are locations where most of the interneurons are found.[5]  

Anterior grey column (Ventral horn) - Wiki 
    "The anterior grey column (also called the anterior cornu, anterior horn of spinal cord or ventral horn) is the front column of grey matter in the spinal cord. It is one of the three grey columns. The anterior grey column contains motor neurons that affect the skeletal muscles while the posterior grey column receives information regarding touch and sensation. The anterior grey column is the column where the cell bodies of alpha motor neurons are located.


Posterior grey column (Dorsal horn) - Wiki 
    "The posterior grey column (posterior cornu, dorsal horn, spinal dorsal horn posterior horn) of the spinal cord is one of the three grey columns of the spinal cord. It receives several types of sensory information from the body, including fine touch, proprioception, and vibration. This information is sent from receptors of the skin, bones, and joints through sensory neurons whose cell bodies lie in the dorsal root ganglion.



The posterior grey column is subdivided into laminae I-VI

The other laminae are located in other regions of grey matter in the spinal cord.    "  

Spinal cord (Wiki) 
    "The brain and spinal cord together make up the central nervous system (CNS)."  


Somatosensory organization:

    Somatosensory organization is divided into the dorsal column-medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the pain/temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex. These neurons are designated primary, secondary and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.   @


    In the dorsal column-medial leminiscus tract, a primary neuron's axon enters the spinal cord and then enters the dorsal column. If the primary axon enters below spinal level T6, the axon travels in the fasciculus gracilis, the medial part of the column. If the axon enters above level T6, then it travels in the fasciculus cuneatus, which is lateral to the fasciculus gracilis. Either way, the primary axon ascends to the lower medulla, where it leaves its fasciculus and synapses with a secondary neuron in one of the dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it took. At this point, the secondary axon leaves its nucleus and passes anteriorly and medially. The collection of secondary axons that do this are known as internal arcuate fibers. The internal arcuate fibers decussate and continue ascending as the contralateral medial lemniscus. Secondary axons from the medial lemniscus finally terminate in the ventral posterolateral nucleus (VPL) of the thalamus, where they synapse with tertiary neurons. From there, tertiary neurons ascend via the posterior limb of the internal capsule and end in the primary sensory cortex.

    The anterolateral system works somewhat differently. Its primary neurons axons enter the spinal cord and then ascend one to two levels before synapsing in the substantia gelatinosa. The tract that ascends before synapsing is known as Lissauer's tract. After synapsing, secondary axons decussate and ascend in the anterior lateral portion of the spinal cord as the spinothalamic tract. This tract ascends all the way to the VPL, where it synapses on tertiary neurons. Tertiary neuronal axons then travel to the primary sensory cortex via the posterior limb of the internal capsule.

Motor organization:

    The corticospinal tract serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei.

    Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6 and then descend in the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract. These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract. These axons also synapse with lower motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord (via the anterior white commissure) right before synapsing.

    The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal tract and the reticulospinal tract. The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract.

    The function of lower motor neurons can be divided into two different groups: the lateral corticospinal tract and the anterior cortical spinal tract. The lateral tract contains upper motor neuronal axons which synapse on dorsal lateral (DL) lower motor neurons. The DL neurons are involved in distal limb control. Therefore, these DL neurons are found specifically only in the cervical and lumbosacral enlargements within the spinal cord. There is no decussation in the lateral corticospinal tract after the decussation at the medullary pyramids.

    The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure where they synapse on VM lower motor neurons contralaterally . The tectospinal, vestibulospinal and reticulospinal descend ipsilaterally in the anterior column but do not synapse across the anterior white commissure. Rather, they only synapse on VM lower motor neurons ipsilaterally. The VM lower motor neurons control the large, postural muscles of the axial skeleton. These lower motor neurons, unlike those of the DL, are located in the ventral horn all the way throughout the spinal cord.   

Medulla spinalis - Section - English.svg

SubC: Spinal Cord
160109 - 1216