Ascending and Descending Tracts

What are tracts?

The tracts are the  collection of axons (nerve fibers) in the central nervous system (CNS) that connect masses of grey matter. These nerve fibers have same origin, course, termination and function. Tracts are also referred as Fasciculi and Lemnisci.  There are two types of tracts:

• Ascending tracts / sensory tracts – carry exteroceptive and proprioceptive sensations  to higher centers of brain.
• Descending tracts / motor tracts – carry  motor impulses from  higher brain centers to  motor nuclei of cranial nerves and anterior horn neurons of spinal cord.

What are the characteristic features of ascending tracts?

The characteristic features of ascending tracts are:

•  The ascending tracts to cerebral cortex  (posterior column tracts, spinothalamic tracts)
  • involve a chain of 3 neurons ( 1st order, 2nd order and 3rd order neuron) to reach the cerebral cortex.
  • originate from the  exteroceptive and proprioceptive receptors.
  • terminate  mainly in the primary sensory  cerebral cortex (Brodmann area number ( 3,1,2).
  • has their 1st order neurons located outside the CNS,  either in the dorsal root ganglion of spinal cord or sensory ganglion of  cranial nerves ( exception is mesencephalic nucleus of trigeminal nerve, it contains 1st order  sensory neurons for proprioception and is located in the midbrain).
  • have  their 2nd order neurons located in dorsal horn of spinal cord or medulla oblongata. It is the axon of 2nd order neuron that crosses to the opposite side (as the body is  represented contralaterally in the cerebral cortex.
  • have  their 3rd order neuron located in the ventral posterior nucleus of thalamus.
  • ascending tracts carry the sensory information from the body and external environment to the cerebral cortex  that is used for conscious awareness of the various sensations  as well as in figuring out the  appropriate response to the stimuli.
• The ascending tracts  to the  cerebellum (spinocerebellar tracts)
  • involve a chain of 2 neurons to reach the cerebellum
  • originate from the  proprioceptive receptors.
  • terminate in the cerebellum.
  • have their 1st order neurons located outside the CNS,  in the dorsal root ganglion of spinal cord.
  •  have their 2nd order neurons located in dorsal horn of spinal cord or medulla oblongata.
  •  the representation of the body in the cerebellum is ipsilateral.
  • carry the unconscious proprioception  to the cerebellum  which is used for  postural control and coordination of movements.

Name the  main ascending tracts and write the sensory modalities carried by them.

Draw labelled diagram to show the origin course and termination of ascending tracts.

 Posterior Column Tract

Lateral SpinothalamicTract

  Spinocerebellar Tracts

What are the characteristic features of descending tracts?

The characteristic features of descending tracts are:

• descending tracts involve 2 neurons ( upper and lower motor neurons).
• the motor neurons located in the anterior horn of spinal cord and motor nuclei of cranial nerves are referred to as lower motor neurons.
• the  neurons present in the various parts of the brain which influence the activity of lower motor neurons are referred to as upper motor neurons.The control is usually contralateral.
• descending  tracts arising from motor and premotor  area of cerebral cortex constitute pyramidal  tracts (lateral and anterior corticospinal tracts).
• all  other descending  tracts pathways having their origin in subcortical areas  are referred to as extrapyramidal tracts (tectospinal, rubrospinal, reticulospinal, olivospinal and vestibulospinal).

Name the important descending tracts and write their functions.

Draw labelled diagram to show the origin, course and termination of corticospinal tracts.

What are the differences between upper motor neuron  and lower motor neuron?

Babinski’s sign

Dorsiflexion of great toe with fanning of lateral toes on scratching lateral aspect of plantar surface of foot.  In healthy/normal  adults  the toes are planter flexed.