The spinal cord consists of a central core of grey matter (bodies of neurons) as H-shaped in appearance (2 anterior & 2 posterior horns with grey commissure) & an outer layer of white matter (nerve fibers) occupied by tracts.
The cord is divided into R. & L. halves by: -
Anterior median fissure or groove.
Posterior median sulcus or septum.
The 2 halves are connected together by a horizontal band of grey commissure traversed by the central canal.
Anterior horn: -contains lower motor neurons arranged in 3 columns as: -
Medial zone: - which supplies the muscles of the neck region & trunk (including intercostals & anterior abdominal wall musculature).
Central zone: - includes: -
Phrenic nucleus: -in the segments C3 – C5 of the spinal cord, it gives origin to the phrenic nerve.
Accessory nucleus: - in the upper 5 cervical segments, it gives origin to the spinal root of accessory nerve.
Lateral zone: - which is responsible for the formation of brachial & lumbo-sacral plexuses.
Posterior horn: - contains sensory nuclei as: -
Substantia gelatinosa: -at the apex of the posterior horn responsible for pain, temperature & touch sensation. It is seen along the entire length of the spinal cord. It gives origin to the lateral spino-thalamic tract.
Nucleus proprius:-in the middle of the horn, just anterior to substantia gelatinosa. It gives origin to the anterior spino-thalamic tract responsible for position & movement (proprioception), two points discrimination & vibration sensation.
Nucleusdorsalis (clark’s column): - at the of the posterior horn from the level of segments C8 – L3 (or L4) it gives origin to posterior spino-cerebellar tract. Some cells just medial to the clark’s nuclei gives origin to anterior spino-cerebellar tract.
Visceral afferent nuclei: -in the segments T1- L3 resposible for visceral afferent information.
Lateral grey horn: - in the following areas: -
T1 – L3 segments: - gives off to the thoraco-lumbar outflow which is the sympathetic component.
S2 – S4 segments: - gives off to sacral outflow as part of cranio-sacral outflow (parasympathetic).
Means a bundle of nerve fibers having a specific function, is also known as funiculi
The tracts in the posterior white column:
Gracile tract medially placed.
Guneate tract laterally placed.
These tracts convey proprioceptive sensation & discriminative fine touch.
They are of first order neurone.
Lateral white column tracts:-
1-lateral spino thalamic (2nd order venoues).
They cross to the opposite side in the white commissure.