Monday, June 10, 2013

Spinal Cord Anatomy

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. 
  • 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.


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 spinal ganglion, the ventral & dorsal nerve unites to form the spinal nerve

The part of spinal cord that gives off one pair of spinal nerve we call it  a segment of spinal cord. Each segment supply a certain area of the trunk and limbs

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) 





Internal structure
The spinal cord ,as seen In transverse section,  contains central grey matter and peripheral 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.
  • 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




  • In the large ventral horn lie the motor cells which give rise to the fibres of the anterior roots to control the movement of skeleton muscles of trunk and limbs
  • In the dorsal horn terminate many of the sensory fibres entering from the dorsal roots. 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 of 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


 The nerve fibers in the white matter are of two general types:

  1.   long , ascending fibers deliver sensory information to the brain
  2.   long, descending fibers deliver motor instructions from the brain

  
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

1-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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