Functional morphology of CNS

Question 1

What are the two main ascending pathways that reach the somatosensory region of the cerbral cortex?

Answer 1

1. Leminscal system
2. Extraleminscal system

Question 2

Leminscal system:

Answer 2

• Transmits info that provides high degree of spatial discrimination of touch , pressure , vibrations and muscle and join proprioception
• No interneurons

Question 3

Leminscal system components:

Answer 3

• Glial septum appears in dorsal fun at midthoracic that divides it into two parts - medial - gracile fasciculus and lateral- cuneate fasciculus

Question 4

Gracile fasciculus contains:

Answer 4

axons from PL and caudal trunk

Question 5

Cuneate fasciculus contains:

Answer 5

axons from forelimb and cranial trunk and neck

Question 6

Neurons are in nuclei of cuneate and gracile- these are?

Answer 6

• Axons of second stage neurons leave ventral aspects of nuclei and at once decussate to opposite side and turn rostrally as the medial lemniscus

Question 7

What is the route of the medial lemniscus?

Answer 7

• Runs cranially to reach the caudoventral nuclear complex of the thalamus (MCV)
• Synapses with thalamus, axons of the third stage neurons travel within white matter projection called thalamic radiation to the somatosensory area of the cerebral cortex
• At the level of BS, ML is joined by axons transmitting somatic sensory info from lateral cervical nuc, nuc of the descending tract of the trigeminal nerve and the rostral sensory nuc of the trigeminal nerve after decussation within the meten

Question 8

What is the extralemniscal system most important for?

Answer 8

• Pain response
• Basic pathway: Branches of the primary sensory axons synapse on interneurons location in dorsal gray within a seg or two of axonal entry into SC, processed by interneurons and leaves dorsal horn, Axons of these second stage neurons pass into white matter and into higher brain centers
• These ascending systems can be divided into medial and lateral groups

Question 9

What is the tract of the medial group of the extralem system?

Answer 9

• Tracts of medial group project and synpase upon regions throughout the brainstem up to level of diencephalon

Question 10

What tracts are the medial group of extralem system made up of?

Answer 10

1. Spinothalamic tract: projects to the medial and intralaminar thalamic nuclei
2. Spinoreticular tract: composed pf axon bundles located bilaterally within the ventral and ventrolateral region of the spinal white matter that synapse within ret. formation of the brainstem to rostral thalamus
3. Loosely organized group of ascending propriospinal axons that originate and end repeatedly in the spinal gray matter forming a mulitisynaptic relay system

Question 11

What tracts make up the lateral group of the extralem pathway?

Answer 11

• Tracts porjecting onto the medial caudoventral nuclear complex of the thalamus (like lem pathway)
• Neospinothalamic tract
• Spinocervicothalamic system
• Second order dorsal column pathway

Question 12

What is the neospinothalamic tract?

Answer 12

• Extralem pathway
• Constitutes the classic pain tract of primates
• Entirely crossed and ascends within SC white matter on the ventrolateral aspect of the SC and then through the brainstem toward MCV

Question 13

What is spinocervicothalamic tract?

Answer 13

• Second order neurons from spinal interneurons ascend ipsi as the spinocervical tract and synapse in the lateral cervical nuc, located at junction of SC and BS
• Third order neurons arise from this nuc cross midline and follow medial lem to end in the MCV (overlap with neospino tract)

Question 15

What is the third system of the extralem pathways found in cats?

Answer 15

• Composed of second order axons from spinal interneurons that ascend as a component of the spinal dorsal columns alongside the axons of the primary afferent neurons of the lem system
• Post-synaptic, pain conveying axons of this third system synapse in ipsi dorsal column nuclei
• Neurons in the dorsal column nuclei, third order axons cross the midline and run to MCV

Question 16

What is path for sensory info from head and neck in extralem system?

Answer 16

• Second order axons arise from caudal part of the descending trigeminal nuc
• Axons either join the lateral system and ascend to synpase in the MCV or join the medial system and ascend to reticular formation of thalamus
• Third order axons arising in the thalamus project to an area of the somatosensory cortex rostral to area allocated for lem system

Question 17

What are subconscious proprioceptive pathways?

Answer 17

• End in cerebellum and do not make its way to cortex
• Axons of the second stage neurons will travel cranially as the dorsal and ventral spinocerebellar tracts in the most lateral white matter
• Dorsal- stim of muscle spindles
• Ventral- tendon receptors
• Only from trunk and hindlimbs

Question 18

What is the reticular formation?

Answer 18

• Extends throughout the brainstem as diffuse arrangement of neurons interspersed with axon tracts
• Connected to all projection systems within the CNS and has reciprocal connections with the major integration centers within the brain
• *Plays imp role in modulating activities of these integration centers
• Plays a role in motor control as well

Question 19

Where is the reticular formation?

Answer 19

• Occupies large part of brainstem
• Forms main substance of medulla, pons, midbrain
• Reaches thalamus and contributes a nuclear group
• Extends to cervical part of spinal cord

Question 20

What are the parts of the reticular formation?

Answer 20

• Periaqueductal gray - medial part, located adjacent to the ventricular system
• Ascending reticular activating system (ARAS)