Monday, April 11, 2011

Anatomy of Spinal Cord

EXTERNAL FEATURES

It is the lower elongated cylindrical part of the CNS and connect with the brain above
It occupies upper 2/3rd of the vertebral canal
It extends from the foraman magnum to the lower border of the L1 or upper border of the L 2
In neonates, the spinal cord extends approximately to vertebra LIII, but can reach as low as vertebra LIV.
It is about 45 cm in length
The lower end is called the conus medullaris
From here a prolongation of pia mater, the filum terminale descends to be attached to the back of the coccyx.


It has six external longitudinal fissures and sulci. They are a deep anterior median fissure, a shallow posterior median sulcus, a pair of anterolateral sulcus and a pair of posterolateral sulcus lying lateral to them.

The anterolateral sulcus and posterolateral sulcus marks the sites of attachment for the anterior and posterior roots of each pair of spinal nerves.

It also has two enlargement called cervical and lumbosacral enlargement which represent the aggregations of cell bodies that supply the muscles and skin of upper limbs and lower limbs.



The spinal cord gives off 31 pairs of spinal nerves. 8 cervical,12 thoracic, 5 lumbar, 5 sacral, & 1 coccygeal, each nerve is attached to the spinal cord by 2 roots, ventral ( motor) , & dorsal ( sensory ), each dorsal root bears a ganglion, the ventral & dorsal nerve unites to form the nerve trunk, which soon divides into ventral & dorsal rami

The part of spinal cord that gives off one pair of spinal nerve we call it a segment of spinal cord

Below the lower end of the spinal cord the roots of spinal nerves form a bundle known as the cauda equina (because of resemblance to the tail of a horse)



In transverse section of the cord is seen the central canal around which is the H-shaped grey matter, surrounded in turn by the white matter which contains the long ascending and descending tracts.
Gray matter

When seen in transverse section the grey matter of the spinal cord forms an “ H “ shaped mass and is surrounded by white matter.
It is divisible into 1) Ventral Horn(column ) 2) Dorsal Horn (column ) 3) intermediate zone .In some parts of spinal cord it presents as a lateral horn (column ) 4)The right & left halves of spinal cord are connected across the midline by the grey commisure, which is traversed by the central canal


The Ventral horn contains several nuclei with large motor neurons to control the movement of skeleton muscles of trunk and limbs

The dorsal horn is dominated by neurons that respond to sensory stimulation.
All incoming sensory fibers in spinal nerves enter the dorsolateral part of the cord adjacent to the dorsal horn in a dorsal root. Neurons in the dorsal horn project to higher levels of the CNS to carry sensations to them
Sympathetic and parasympathetic neurons locate in the intermediate zone espacially in the thoracic and sacral part of spinal cord.

White matter

The white matter of the sp cord is divisible into right & left halves in front by the anterior median fissure & behind by the posterior median sulcus

Each half of the white matter is divided into 1) post funiculus 2)lateral funiculus 3)anterior funiculus

The white matter of the rt & lft sides is continuous across the midline through the anterior white commisure which lies ant to the grey commisure

The nerve fibers in the white matter are of two general types:
1. long , ascending fibers projecting to the upper parts of the brain
2. long, descending fibers projecting from the upper parts of the brain to the spinal gray matter

ASCENDING TRACTS

1.Fasiculus gracilis and Fasiculus cuneatus
They convey sensory fibres subserving fine
touch and proprioception (position sense), mostly
uncrossed, to the gracile and cuneate nuclei in the
medulla oblongata.

2 . The lateral and anterior spinothalamic tracts
They transmit information to the thalamus about pain, temperature and crude touch




DESCENDING TRACTS

Corticospinal tracts – descends from the cerebral cortex to the sp cord. It consists of 2 parts 1) the lat corticospinal tract, which lies in the lateral funiculus. 2) the ant corticospinal tract which lies in the ant funiculus
The corticospinal tracts are responsible for voluntary movements control.

2 . Rubrospinal tracts
3 . Olivospinal tracts

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