Gray matter and white matter

Gray matter and white matter

Gray matter

Two types of cells– Motor neurons and Internuncial neurons.

1. Motor neurons

Motor neurons project their axons into the PNS 

• They are of 3 types
  1. Somatic or Alpha motor neurons – supplying skeletal muscles
  2. Autonomic neurons – supplying visceral organs
  3. Intrafusal or Gamma motor neurons – supplying special receptors called muscle spindles
     • Somatic or Alpha motor neurons show Nissel substance in their cytoplasm
     • Internuncial neuron processes remain within the confines of the CNS
     • They vary considerably in their shape, size , organization
     • Two classes according to the course of their projection of their fibers within the nervous system
     • Commissural Internuncial neurons whose fibers cross the midline
     • Non Commissural Internuncial neurons or association neurons whose fibers do not cross the mid line
     • Association neurons are called so because by their connections they correlate activities within various portions of the nervous system
     • Gray matter of the spinal cord is divided into many nuclei on anatomical basis
     • Many portions of the CNS are said to composed of cell laminae which are groups of similar cells arranged together in a particular order

2. Internuncial Neuron function

• Classified into 2 types based on their synaptic capacity :
  1. Excitatory Internuncial Neurons – exert a depolarizing influence on the postsynaptic with which they have contact
  2. Inhibitory Internuncial Neurons – exert hyper polarizing influence on the neurons with which they have contact

• With the production of presynaptic inhibition, Internuncial Neuron produces inhibition by depolarizing the presynaptic terminals of other neurons.
• The presence of two types of Internuncial neurons ensure that activity and signaling inside the spinal cord remain flexible
• Reciprocal innervation
  • A phenomenon exhibited by afferent nerves to the spinal cord on somatic efferent nerve activities
  • When motor neurons supplying a skeletal muscle are depolarized by excitatory interneurons or by dorsal root afferents, the motor neurons supplying muscles that are antagonistic to this muscle are hyperpolarized by inhibitory interneurons.
  • All voluntary motor activity is made meaningful because of this integration by inhibitory and excitatory interneurons
• Interneurons are classified based on their pattern of convergence into 5 basic groups
• They can have one or two or three muscle groups projecting muscle spindle fibers to them
• One synergistic muscle group :
• Some of these are excited by exteroceptive afferent sources especially from the foot pads or hair in between the toes
• Two different muscle groups : Usually antagonists acting on the same joint
• Or they may be synergists  acting at neighboring joints
• Or antagonists acting at neighboring joints
• Few are influenced by sensory information from the skin (cutaneous afferents )
• Three muscle groups are also seen
• Interneurons form the system that is within the spinal cord
• Interneurons are capable of prolonged rapid discharge of action potentials following afferent stimulation
• Interneurons play an important role as convergence systems for sensory (afferent ) fibers of sensory modalities
• The convergence systems are not necessarily anatomical but rather are ones that are active at that time
• Spinal reflexes usually involve interneurons except some autonomic reflex arcs through the spinal cord and the myotatic reflex
• The corticospinal, rubrospinal, vestibulospinal, tectospinal, interstitiospinal and reticulospinal motor pathways for somatic and autonomous controls exert their influence through these interneurons
• Many interneurons are organized to perform specific functions
• Alpha motor neurons give rise to large fibers which innervate skeletal muscle
• These fibers also give rise to collateral branches which reenter spinal gray matter to synapse upon inhibitory interneurons
• These Internuncial neurons are called Renshaw cells
• They control the duration, intensity and distribution for motor neuron discharge
• Thus they function in localization of reflex motor activity
• Disinhibition : The above neurons in certain cases increase the firing of motor neurons inhibitory to the alpha motor neuron
• Discharge of Renshaw cells is inhibited by stimulation of the cutaneous and muscle afferents
• A group of neurons within the nucleus intermedialis and nucleus proprius give rise to the ascending fiber system
• These neurons give rise to ventral and rostral spino cerebellar tracts , the spino cervical tracts, the spino olivary tract and spino reticular tracts

white matter

white matter forms the tract systems of the spinal cord containing myelinated nerves, and they transmit sensory impulses through ascending tracts and the motor impulses through the descending tracts between the spinal cord and the higher centres of the brain.

• The white matter is divided into paired dorsal funiculi, ventral funiculi and two lateral funiculi
  • Ascending (Sensory) tracts of spinal cord
  • Dorsal funiculus
    • Is composed of two ascending fiber tracts
    • These terminate in the medulla oblongata
    • The fasciculus gracilis terminates on the nucleus gracilis of the medulla
    • The fasciculus cuneatus terminates on the nucleus cuneatus and nucleus cuneatus lateralis of the medulla
    • But many of the fibers of the Dorsal funiculus terminate in the gray matter of the spinal cord before the rest enter the medulla
    • These fibers support intersegmental reflex activity and a portion them terminate on the cells of the nucleus proprius , nucleus dorsalis and nucleus intermedialis, and these further give rise to another system that ascends like the spinocerebellar, spinobulbar and spinothalamic tracts
    • The fasciculus gracilis and fasciculus cuneatus are also arranged somatotropically
    • Ascending fibers from the coccygeal, sacral, lumbar and thoracic levels comprise the fasciculus gracilis
    • Fibers from the cervical and upper cervical (T1) comprise the fasciculus cuneatus
    • As these fibers represent sensory information , the ipsilateral surface of the body is represented topographically within the dorsal funiculus
    • The fibers of the Dorsal funiculus convey impulses from joint receptors, muscle spindle and golgi tendon organ, cutaneous receptors for tactile and pressure sensations and so relay sensations related to joint movement and position.
    • Fibers from joint afferents and skin afferents terminate in the spinal cord for reflex mechanisms or within the nucleus gracilis and nucleus cuneatus of medulla for relay to thalamus and then cortex.