Spinal cord anatomy

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Introduction

The Nervous System is divided into two main divisions. 

These are:

1. Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)

Anatomy of the Spinal Cord

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. The lower part of the spinal canal contains the lower lumbar and sacral nerves known as the Cauda Equina.

• Sensory Nerve Fibres enter the Spinal Cord via the Dorsal (Posterior) Root. The cell bodies for these neurons are situated in the Dorsal Root Ganglia.

• Motor and Preganglionic Autonomic Fibres exit via the Ventral (Anterior) Root.

This short video clip gives an overview of spinal cord anatomy.

Associated Pathways

Ascending Sensory Pathway

Spinothalamic Tract

• From Dorsal horn laminae I,III,IV,V. crosses midline in spinal cord, projects to brain stem and contr-lateral thalamus. Conveys pain and temperature.

Dorsal Column Medial Lemniscal Pathway

• Afferents from mechanoreceptors, muscle and joint receptors. terminates in dorsal column nuclei of medulla. Forms medial lemniscus at this level and synapses in ventroposterior nucleus of thalamus. Conveys proprioception, light touch and vibration.

Spinocerebellar Tract

• From spinal cord interneurons. It has two tracts a) Dorsal SCT relays via inferior cerebellar peduncle and b) VCT relays via superior cerebellar peduncle to the cerebellum. It conveys proprioceptive information and on-going activity in the spinal cord interneurons.

Descending Motor Pathways

Corticospinal (Pyramidal) Tract

From the motor cortex, premotor cortex, and somatosensory cortex. Has a role in sensory processing and fractionated finger movements.

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.

Vestibulospinal Tract

Originates from Deiters nucleus in the medulla and innervates the extensor and axial muscles. It is involved in balance control and posture.

Reticulospinal Tract

This tract begins in the caudal reticular formation in the pons and medulla. 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.

Spinal Motorneurons

Alpha and Gamma motorneurons (MNs) are both found in the ventral (anterior) horn. 

Alpha motorneurons are the largest motor 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.

References

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