General
- 4 Long Ascending (2) and Descending (2) Tracts
Tract | Topographical organization |
Dorsal column | Cervical fibers lateral, sacral fibers medial |
Lateral corticospinal | Cervical fibers central, sacral fibers most peripheral |
Lateral spinothalamic | Cervical fibers central, sacral fibers most peripheral |
2 ascending
General
- Large-diameter heavily myelinated afferents (1) enter medially through the posterior funiculus
- Tactile and proprioceptive information processing
- Small-diameter afferents (2) enter laterally near the substantia gelatinosa.
- Pain and temperature information processing
- These afferents then contact interneurons (3) and, in some cases, motor neurons (4) directly.
Spinothalamic tract
- Located:
- Lateral aspect of the tegmentum throughout the brainstem
- They occur in essentially the same position they occupy in the spinal cord
- Adjacent to the descending sympathetic tract
- Damage to the lateral tegmentum can injure both tracts simultaneously
- Ipsilateral Horner syndrome (descending sympathetic lesion) AND
- Contralateral hemisensory loss (spinothalamic lesion)
- Lateral and Anterior spinothalamic tracts
- Function:
- Pain
- Temperature
- Light touch sensation.
- Neural connections
- 1st order neurons from sensory organ
- 2nd order neurons from dorsal gray horn of the spinal cord → cross the midline in the anterior white commissure → ventral posterolateral (VPL) nucleus of the thalamus.
- 3rd order neurons VPL thalamus → postcentral gyrus .
Medial lemniscus
- Located
- In medulla
- Close to the midline
- Between the
- Posteriorly situated medial longitudinal fasciculus (MLF) and
- Anteriorly situated corticospinal and corticopontine tracts.
- In its rostral ascent,
- Medial lemniscus moves laterally but remains an important landmark of the medial aspect of the medulla and pons.
- Function
- Position sense
- Discriminative touch.
- Neural connections
- 1st order neurons
- Cuneate fasciculi (upper extremities)
- Gracile fasciculi (lower extremities)
- 2nd order neurons
- Synapse ipsilateraly with nucleus cuneatus and nucleus gracilis
- Ascend to the contralateral VPL thalamus.
- From here they ascend to the sensory cortex.
2 descending
Corticospinal tract
- Located
- Pons
- Midline
- Medulla
- Midline
- Just like the medial lemniscus
- Function
- Motor-related impulses
- Neural connections
- Cerebral cortex →
- Corona radiata →
- Posterior limb of internal capsule →
- The genu of the internal capsule may be affected in a patient with dysarthria and clumsy-hand syndrome
- Midbrain: Middle of the midbrain cerebral peduncle
- Adjacent to it are the corticopontine fibres
- Separate from direct corticopontine fibers, numerous collateral branches of the corticospinal fibers innervate the pontine nuclei, including those of the reticular formation.
- Pons-cranial: Decusscitating pontocerebellar fibres break corticospinal tract into small pieces
- Pontocerebellar fibres reach contralalteral cerebellar hemisphere via middle cerebellar peduncle
- Pons-caudal:
- Corticospinal fibers come together again and form the medullary pyramids
- Medulla
- 85% of corticospinal fibers cross the mid-line in the decussation of the pyramids to form the lateral corticospinal tract
- Where does the pyramidal decussation begin and where does it complete the decussation?
- The pyramidal decussation begins just below the obex and is not completed until some distance below the exit of the first cervical nerve root
- Spinal cord
- 15% of fibers continue in the uncrossed anterior corticospinal tract , which later decussates in the anterior commissure at cervical and upper thoracic levels
- Corticospinal tracts end in Laminae IV through IX (few fibers synapse directly with IX motor neurons in laminae IX) of the spinal gray matter.
- Synapsing with second-order, lower motor neurons (LMN) in the ventral horn at each level of the spinal cord.
Corticobulbar tract
- Located: medially.
- Associated with the medial lemniscus and the corticospinal tract
- Cerebral cortex to the lower brainstem.
- Destination to several motor cranial nerve nuclei,
- Trigeminal nuclei
- Bilateral symmetrical input from corticobulbar tracts
- Facial nuclei
- Caudal part of the facial motor nucleus has unilateral asymmetrical input
- Contralateral paralysis of lower facial muscles (sparing the forehead) is evidence of predominantly crossed corticobulbar innervation of part of the facial motor nucleus.
- Hypoglossal nuclei.
- Bilateral symmetrical input from corticobulbar tracts
- Bilateral symmetrical inputs
- For
- Laryngeal, pharyngeal, palatal, upper facial, extraocular, and muscles of mastication.
- Unilateral corticobulbar lesions: supply of these muscles cause only mild signs of paresis,
- Bilateral corticobulbar lesions are usually significant (pseudobulbar palsy).
- Other ways the cortex communicates with the brainstem motor nuclei
- Corticoreticular fibers
- Innervate neurons of the reticular formation,
- Relay impulses indirectly from the cortex to the motor cranial nerve nuclei.
Other less important descending tracts
Tectospinal tracts
- Aka colliculospinal tract
- Tract that coordinates head and eye movements.
- Responsible for motor impulses that arise from one side of the midbrain to muscles on the opposite side of the body (contralateral).
- Mediate reflex postural movements of the head in response to visual > auditory stimuli.
- Part of the extrapyramidal system
- Connects the midbrain tectum (superior colliculi) → cervical regions of the spinal cord
- Pathway: afferents from the visual nuclei (primarily the oculomotor nuclei complex) → superior colliculus → projects to the
- Contralateral (decussating dorsal to the mesencephalic duct)
- Ipsilateral portion of the first cervical neuromeres of the spinal cord,
- Terminate in Rexed laminae VI, VII, and VIII to coordinate head, neck, and eye movements,
- Oculomotor and trochlear nuclei in the midbrain
- Abducens nucleus in the caudal portion of the pons.
Connection between Brainstem and Cerebellum
3 cerebellar peduncles, connect the cerebellum to the brainstem.
- Superior cerebellar peduncles: cerebellum = midbrain
- Middle cerebellar peduncles: cerebellum = pons
- Inferior cerebellar peduncles: cerebellum = medulla
Brainstem nuclei sending afferents to the cerebellum
Pontine nuclei
- Corticopontine fibers: cerebral cortex → internal capsule → cerebral peduncles → pontine nuclei .
- Enter cerebellum: middle cerebellar peduncle
- Terminate
- Pontine nuclei send fibres that crosses midline → contralateral cerebellar hemisphere.
- Function
- Initiation and planning of movements.
- Vision:
- The pontine nuclei relay information from the lateral geniculate body, the superior colliculus, and the striate cortex concerning vision.
Inferior olivary nucleus
- Location
- Rostral medulla.
- Receives input from widespread areas of the central nervous system,
- Spinal cord,
- Brainstem,
- Cerebral cortex.
- Enter cerebellum:
- Inferior cerebellar peduncle
- Terminate
- Multiple sensory and motor cross the midline → contralateral cerebellar hemisphere.
Vestibular nuclei
- Location
- Rostral medulla.
- Enter cerebellum:
- Inferior cerebellar peduncle
- Terminate
- Ipsilateral cerebellar hemisphere
- Function
- Providing information about eye movements, head movements, and changes in the position of the head.
Reticular formation (Reticulocerebellar fibers)
- Originating in the
- Medulla
- Pons
- Enter cerebellum:
- Inferior cerebellar peduncles
- Termination
- Bilateral cerebellar hemisphere
- Function
- Integrate and relay information received from widespread parts of the central nervous system, including the spinal cord and higher regions of the brain.
Brainstem nuclei receiving efferents to from cerebellum.
Red nucleus
- Location
- In midbrain tegmentum
- Dorsal to the substantia nigra.
- Origin
- Contralateral cerebellar hemisphere
- Crosses midline at cerebellar peduncle
- Exit cerebellum
- Superior cerebellar peduncle
- Crosses midline twice
- Once at the cerebellar peduncle
- Another at the rubrospinal tract (efferent of red nucleus)
- Function
- Influence flexor muscle tone on the ipsilateral side of the body.
- Rubrospinal tract
- Function
- Primitive grasp reflexes
- Responsible for crawling
- When pyramidal tracts are more developed pyramidal tracts takes over more finer movements
- If there is injury to the regions above the red nucleus it can lead to decorticate posturing
- Forebrain (motor cortex, premotor cortex, corona radiata, internal capsule), diencephalon (thalamus), or rostral midbrain
- Red nucleus to the spinal motor neurons
Vestibular nuclei
- Receive input from the ipsilateral cerebellar hemisphere via the inferior cerebellar peduncle.
- Function
- Equilibrium and control of axial musculature (extensive muscle tone) via the uncrossed vestibulospinal tract,
- Coordination of eye movements via the uncrossed medial longitudinal fasciculus.
- Causes decerebrate posturing
Reticular formation (via Cerebelloreticular fibers)
- Location
- Pons
- Medulla
- Exit cerebellum
- Inferior cerebellar peduncle.
- Terminate
- Ipsilateral reticular formation
- The reticular formation, in turn, influences extensor muscle tone via the uncrossed pontine and medullary reticulospinal tract.
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