Sensory Nuclei of spinal cord
Spinal cord has 9 laminae nuclei of spinal cord.
Nucleus Dorsomarginalis
Corresponds to lamina I. Extends the length of the spinal cord. Covers the dorsal and lateral surfaces of the dorsal horn of the spinal cord. Cells are spindle shaped.
Neurons receive afferent fibers from the dorsal root fibers and project their axons into dorsal and lateral funiculus from which they may ascend or descend to form inter segmental pathways for intraspinal integration of neural activity.
These fibers form the dorsolateral fasciculus of the propriospinal fiber system.
It receives the pain and thermal sensation .
It act as a low threshold mechanoceptors.
Nucleus Substantia Gelatinosa
Nucleus Substantia Gelatinosa Corresponds to laminae II and III of the spinal gray matter
Cells have small , spheroid or spindle shaped nucleus and little cytoplasm
They also have a rich dendritic tree
Axons of these cells turn either dorsally into dorsolateral fasciculus or ventrally. Some turn ventromedially and thereby reach the Substantia Gelatinosa of the other side
Substantia Gelatinosa is a closed neuronal system because all of its axons turn back and terminate within the nucleus of origin
Long range connections between different regions a re established over the lateral part and the dorsal part of the lateral propriospinal tract
These connections may bridge 5 to 7 segments of the spinal cord
Ipsilateral (same side) short range connections of 2 to 3mm distances are also seen
Axons originating in the Substantia Gelatinosa are always traced back to the Substantia Gelatinosa of the ipsilateral or contralateral side
The Substantia Gelatinosa has its outlet in the large cells that occupy the underlying lamina IV and project dendrites into the nucleus Substantia Gelatinosa.
It forms a chief association centre for pain, thermal, tactile sensory perception mediatory reflexes.
Nucleus Proprius
Nucleus Proprius forms the nuclear column that extends the length of the spinal cord and comprises lamina IV and lamina V
This nucleus serves as a origin of both crossed and uncrossed ascending tracts
The lateral portion of nucleus proprius is referred to as the reticular formation of the spinal cord.
It acts as a sensory pathway for light cutaneous mechanical stimulations (tactile, pain and thermal perception) and also from the Golgi tendon organ.
Nucleus dorsalis
Extension is limited to the thoracic and upper lumbar levels of the spinal cord
Occupies a medial portion of lamina VII
In cat the nucleus begins at the most cranial thoracic segments of the spinal cord
Cells of dorsal nucleus gives rise to ipsilateral dorsal spinocerebellar tract which terminates in the vermis of the cerebellum
This system has proprioceptive functions related to Golgi tendon organ and muscle spindle activity
The system also has two sub divisions activated exclusively by afferents from cutaneous receptors :
One informs the CNS about pressure on foot pads
The other has neurons activated by light touch , pressure and pinching of cutaneous areas
An additional dorsal spinocerebellar tract subdivision which is neither exteroceptive nor proprioceptive is seen. These are activated by the flexor reflex afferents
Proprioceptive cells of the nucleus dorsalis receive afferent information from the class Ia fibers from the muscle spindles and from class Ib of the Golgi tendon organs of skeletal muscles
A given neuron receives inhibitory influences mediated by group I afferents from both synergistic and antagonistic muscles
Synapses upon the cells of the dorsal nucleus by group Ia afferent fibers from muscle spindles are large compared to the synapses of group Ia fibers with alpha motor neurons
Though these synapses use the same neurotransmitter according to Dale’s principle , they have widely varied effects
Synaptic activity upon the dorsal nucleus requires less summation to discharge to discharge action potential
One bouton gives rise to a small series of action potential because of larger amount of transmitter agent released per action potential in the pre synaptic fiber or less diffusion of transmitter agent away from the reactive site
Both alpha motor neurons and neurons of dorsal nucleus exhibit long positive after-potentials
Discharge rate of alpha motor neurons as about 200 impulses per second and that of the dorsal nucleus neurons about 700 impulses per second
Action potential is brief – about 0.5 to 0.75 msec
Repetitive firing of neurons of the nucleus dorsalis is important in CNS function.
In primates it extends from the upper lumbar to lower cervical spinal cord, while in other animals it extends from upper lumbar to lower thoracic levels.
It receives the input signals from the muscle spindle through the dorsal root ganglion of the spinal cord, caudal to T1 spinal segment.
Nucleus Intermedialis and Nucleus intermediolateralis
Nucleus Intermedialis and Nucleus intermediolateralis receives the sensory signals from Golgi tendon organs up to cervical level in primates and up to thoracic level in other animals.
It also senses tactile, thermal and pain sensations along with nucleus proprius.