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The spinal cord is a thin, tubular bundle of nerves that is an extension of the central nervous system from the brain and is enclosed in and protected by the bony vertebral column. The main function of the spinal cord is transmission of neural inputs between the periphery and the brain.

Sensory Organization

Somatosensory organization is divided into a touch/proprioception/vibration sensory pathway and a pain/temperature sensory pathway, which are more formally known as the dorsal column-medial lemniscus tract and the spinothalamic tract, respectively. Each of these sensory pathways utilizes three different neurons to get from the sensory receptors to the cerebral cortex. These neurons are designated primary, secondary and tertiary sensory neurons. The primary neuron will have its cell body in the dorsal root ganglia and then its axon projects into the spinal cord. In the case of the touch/proprioception/vibration sensory pathway, the primary neuron enters the spinal cord and travels in the dorsal column. If the neuron enters below level T6, the neuron will travel in the fasciculus gracilis - the most medial part of the column. Above level T6, the neuron will enter the fasciculus cuneatus - lateral to the fasiculus gracilis. The primary axons reach the caudal medulla, they will leave their respective fasiculi and will enter and synapse on secondary neurons with the nucleus gracilis and the nucleus cuneatus, respectively. At this point, the seconday neuronal axons will decussate and will continue to ascend as the medial leminiscus. They will run all the way up to the VPL nucleus of the thalamus. They will synapse there on the tertiary neurons. From there, the tertiary neurons will ascend via the posterior limb of the internal capsule to the post central gyrus, or Brodmann's Area 3,1,2.

The pain/temperature sensory pathway differs from that of the touch/proprioception/vibration pathway. The pain neurons will enter as primary neurons and will ascend 1-2 levels before synapsing in the substantia gelatinosa. The tract that ascends those 1-2 levels before synapsing is known as Lissauer's tract. After synapsing, the secondary neurons will cross decussate and ascend as the spinothalamic tract in the anterior lateral portion of the spinal cord. Hence, the spinothalamic tract is also known as the anterior lateral system (ALS). The tract will ascend all the way to the VPL of the thalamus where it will synapse on the tertiary neurons. The tertiary neuronal axon will then project via the posterior limb of the internal capsule to the post-central gyrus or Broadmann's Area 3,1,2.

It should be noted that the pain fibers in the ALS can also deviate in their pathway towards the VPL. In one pathway, the axons can project towards the reticular formation in the midbrain. The reticular formation will then project to a number of places including the hippocampus (to create memories about the pain), to the centromedian nucleus (to cause diffuse, non-specific pain) and the various places on the cortex. The third place that the neurons can project to is the periaqueductal gray in the pons. The neurons form the periaqueductal gray will then project to the nucleus raphe magnus which then projects back down to where the pain signal is coming in from and inhibits it. This will reduce the pain sensation to some degree.

The anterior corticospinal tract descends down ipsilaterally in the anterior column where the axons will emerge and either synapse on lower motor neurons, known as ventromedial (VM) lower motor neurons, in the ventral horn in an ipsilateral fashion, or will descussate at the anterior white commissure where they will synapse on VM lower motor neurons in a contralateral fashion. 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 axial motor function. This means they are in charge of large, postural muscles. These lower motor neurons, unlike those of the DL, are located in the ventral horn all the way throughout the spinal cord.

Additional images

• Diagrams of the spinal cord.
• Cross-section through the spinal cord at the mid-thoracic level.
• Cross-sections of the spinal cord at varying levels.

See also

• cauda equina
• conus medullaris
• meninges
• spinal nerves
• lumbar puncture

External links

• Spinal Cord Histology - A multitude of great Images from the University of Cincinnati
• Spinal Cord Injuries Australia (formerly Australian Quadriplegic Association AQA, established 1967) provides information about the disability and services for affected people, advocacy, accommodation, employment, peer support.
• Spinal Cord Medical Notes - Online medical notes on the Spinal Cord
• eMedicine: Spinal Cord, Topographical and Functional Anatomy
• WebMD. May 17, 2005. Spina Bifida - Topic Overview Information about Spina Bifida in fetuses and throughout adulthood. WebMD children's health. Retrieved March 19, 2007.

Template:Vertebral column and spinal cord

• Spinal cord