Anterior spinal cord consists of Lower motor neurons.
Lateral spinal cord consists of Corticospinal tract descending from cortex to spinal (lower)motor neurons.
Lateral spinal cord also consists of ascending spinothalamic tracts which is responsible for pain & temperature from OPPOSITE SIDE of the body. Crossing spinothalamic fibers pass in front of central canal. Spinothalamic tracts from lumbar nerves are lateral while those from higher segment of nerves ( thoracic & cervical) are medically located. So lesions within the centre of cord ( like syringomyrlia or Ependymoma ) will affect upper body spinothalamic fibers first & lesions compressing from outside affect spinothalamic from lower body first ( meningioma etc).
Posterior spinal cord consists of Dorsal Colmn fibers ( position & vibration)of the SAME SIDE. These fibers of lumbar nerves are medial & those from thoracic & cervical nerves enter laterally in the posterior cord. So lesions from inside will affect lumbar fibers first & upper body will be affected later, where as lesions compressing from outside will affect the upper body first & lower body later ( this is opposite to that of spinothalamic tracts,READ AGAIN PLEASE). So spinothalamic fibers from lower body are lateral while those from upper body are medial whereas dorsal Colmn from lower body are lateral & from upper body are medial.
So MASS LESIONS FROM WITHIN THE CORD WILL AFFECT UPPER BODY PAIN & TEMPERATURE FIRST & POSITION VIBRATION OF LOWER BODY FIRST. COMPRESSIVE LESIONS FROM OUTSIDE WILL AFFECT PAIN TEMPERATURE FROM LOWER BODY FIRST & POSITION VIBRATION OF UPPER BODY FIRST.
Anterior 2/3rd of cord ( anterior & lateral ) are supplied by anterior spinal artery, so anterior spinal artery blockage will cause damage to lower motor neurons, corticospinal tract of same side & spinothalamic tract.
Posterior 1/3rd is supplied by posterior spinal artery, so blockage of this artery will affect dorsal Colmn tracts ( position & vibration ).
Motor roots exit from anterior horn ( lower) motor neurons. these motor roots then make motor nerves to supply Muscles via Neuromuscular junctions.
SENSORY NERVES from body relay in DORSAL ROOT GANGLION & then enter in posterior side of the cord. From here there are THREE OPTIONS for sensory fibers. pain & temperature fibers cross in front of central canal & goes to the opposite side to run upward in lateral side of the (OPPOSITE) cord as Spinothalamic tract.While position & vibration fibers remain on the same side and ascend upward in posterior cord ( dorsal Colmn) of the SAME side as Posterior/Dorsal Column Tract which will cross to opposite side at the beginning of Brain stem.
Some of these sensory fibres don’t ascend upward to brain but connect posterior cord neurons( SENSORY) to the anterior horn (LOWER MOTOR) neurons & hence complete the REFLEX ARC which is responsible for marinating reflexes & tone of the muscles. So TONE & REDLEXES are maintained by sensory input & lower motor neurons.
Lateral spinal Cord also has sympathetic neurons ( from T1 to L2 segment of the spinal cord). Where as sacral segments of spinal cord has Parasympathetic neurons for bladder, rectum , anal canal & sex organs.higher input to the sacral cord coming from frontal lobe to the Sacral cord runs close to midline.So any midline cord lesions or any lesion affecting the whole wideth of the cord at any level thus interrupting the higher input to the sacral cord or damage to the sacral spinal cord itself or damage to the sacral roots are affected, we can have sexual dysfunction & urinary/fecal problem. Lesions affecting the cord from outside or incomplete wide the cord may not affect sphincters control as higher input runs close to midline, where as central lesions affect sphincters very early. So SPHINCTER disturbance means SPINAL CORD unless proven. While spinal cord still can be affected without sphincter involvement.
Spinal cord has Sensory & Motor Segments.
Sensory segments are C2-4 supplies neck. C5-T1 supplied upper limb. T2-T12 supplies chest & abdomen uptill Inguinal Ligament. L1-S2 supplies Lower limbs. S2-4 supplied sex organs & sphincter areas.
Motor segments are less specified but a few are as follows. C5 shoulder, C5-6 Biceps & Brachioradialis. C6-7 triceps. T1 hand muscles. L2-4 Quadriceps & knee jerk. L5 & S1 ankle jerk.
So finding a definite sensory level above the Inguinal Ligament indicates spinal cord disease.
Sensory loss upto neck mean Cervical cord or lower brain stem disease. Sensory loss upto upper limbs mean cervical cord disease.sensory loss upto chest or abdomen means thoracic cord disease.
Sensory loss upto inguinal ligament means Lumbar cord. Where as sensory loss sparing L1 area ( area of upper 1/3rd of anterior thigh) means Cauda Equina where as sensory loss below knee means peripheral nerves ( will write a separate post on it).
Spinal roots come out of spinal cord & exit through the corresponding vertebral formaena. Spinal cord ends at L2 Vertebrae. So roots from L2 onward ( L2 to S4) has to hang out & then run downward within the spinal canal to reach their corresponding vertebrae for exit. These roots from L2-S4 which are hanging & running downward in the spinal canal surrounded by Lumbar & sacral vertebrae are called as Cauda Equina. So cauda Equina consists of roots from L2-S4, IT’S NOT SPINAL CORD.
CONUS MEDULLARIS IS SACRAL CORD WHICH CONTAINS EXTRA NEURONS DUE TO PRESENCE OF PATASYMAPTHETIC NEURONS meant for sphincters & sexual functions ( S2-4).
Will write a next post on Clinical feature of the Cord ( But this basic is important to understand the next post).