Ascending & Descending spinal tracts

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General characteristics of NS pathways

CNS uses pathways to communicate with peripheral structures

Conduct either sensory or motor information

WM of brainstem + SC

Use cranial & spinal nerves to transmit info peripherally

i) Ascending & Descending tracts

Ascending = sensory (Spino)

from body to brain - pain, tactile, thermal, proprioception, pressure (See Spinal cord lec. 2. Internal anatomy, ii) White Matter - ascending & descending tracts)

Descending = motor (Cortico)

from brain (Cerebral cortex & brainstem) to muscles or glands - regulate skeletal activity (as above)

ii) Decussation

Movement of neurons from one side to the other

>90% of pathways decussate

Reason behind why R brain processes L body info & vice versa

Key words: Contralateral & Ipsilateral

iii) Pathway composition

Sensory pathways composed of 3 neurons:

Primary neurons - info from sensory ending to SC

Secondary neurons - info from SC to thalamus (decussation will occur @ some level in SC)

Tertiary neurons - Thalamus to somatosensory cortex

Motor pathways composed of 2 neurons:

Upper motor (1st order neuron) - info from motor cortex to SC (decussation occurs somewhere in SC) - cell body located either in cerebral cortex or a nucleus in brainstem

Lower motor (2nd & 3rd order neuron) - 2nd = interneuron, 3rd = info from SC to muscle/gland - cell body located either in Ant. gray matter or brainstem cranial nerve nucleus

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Somatosensory pathways

i) Types

There are 3 types of somatosensory pathways in SC:

Posterior column - medial lemniscus pathway

Spinothalamic pathway

​​​​​​​Spinocerebellar pathway

a) Posterior column - medial lemniscus pathway

What does lemniscus mean?

Lemniscus = bundle of 2ndary sensory nerve fibres in brainstem - Greek for ribbon

Where is the PCML pathway located?

Spinal cord, brainstem, diencephalon & cerebral cortex

What information PCML pathway bring?

​​​​​​​Concious sensory info - proprioception, touch, precise pressure & vibration

Posterior funiculus (Primary neurons)

Fasiculus gracilis:

Located throughout entire length of SC

Detects sense info from sacral, lumbar, & lower six thoracic SNs

Fasiculus cuneatus

​​​​​​​Located from level of T-6 & above

Detects sense info from upper 6 thoracic & all cervical SNs

SC tracts will ascend ipsilaterally through either one of these structures!

Medulla Oblongta (Secondary neurons)

Fibres coming from Fac G & Fac C will synapse with neurons in Nucleus Gracilis & Nucleus Cuneatus respectively

Decussation of 2ndary neurons (internal arcuate fibres)

Entry into Medial Lemniscus pathway

Ascension through Pons & Midbrain to Thalamus

Ventral Posterolateral Nucleus Thalamus (Tertiary neurons)

Tertiary neuron travels through posterior limb of internal capsule

Through corona radiata

Terminates @ post-central gyrus (primary somatosensory cortex)

Cuneocerebellar tract

Transmits unconcious proprioceptive information from the upper limbs (FC path!) to the ipsilateral cerebellum (through inf. cerebellar peduncle)

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b) Spinothalamic pathway

Where is the spinothalamic pathway located?

SC, brainstem, diencephalon & post-central gyrus

What is the ST path composed of?

2 tracts - Anterior spinothalamic & posterior spinothalamic

What info ST pathway bring?

Depends on tract

Anterior ST tract brings conscious info in regards crude touch & pressure

Posterior ST tract "" pain & temperature

Substantia Gelatinosa (Primary neurons)

Peripheral sensory receptors enter SC

Ascend 1 or 2 vertebral levels

Synapse in dorsolateral horn of gray SC matter (Sub G)

Spinothalamic tract (Secondary neurons)

Decussation across SC

Form the Anterior & Lateral spinothalamic tracts (together, in conjunction with the spinotectal tract (controls head & eye movements), form the Spinal Lemniscus)

Ascension to thalamus

VPNT (Tertiary neurons)

Tertiary neuron passes through Post. limb internal capsule

Through corona radiata

Terminates @ post central gyrus (primary somatosensory cortex)

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c) Spinocerebellar pathway

Where is the spinocerebellar pathway located?

SC, MO, Pons & Cerebellum

What is the SC path composed of?

2 tracts - Anterior & Posterior spinocerebellar tracts

What information SC path bring?

​​​​​​​Tract dependent

Anterior SCt = unconscious proprioceptive info from trunk, upper limbs & lower limbs

Posterior SCt = unconscious proprioceptive info from trunk & lower limbs

This info helps cerebellum regulate posture

** SPINOCEREBELLAR TRACT IS ONLY TRACT COMPOSED OF 2 SENSORY NEURONS!!**

Nucleus Dorsalis (Primary neurons)

Proprioceptive input transmitted into SC through Dorsal root

Synapses with Nucleus Dorsalis

Anterior Spinocerebellar fibres decussate to oppposite side of cord - forms Anterior spinocerebellar tract

Posterior spinocerebellar fibres do not decussate - forms Posterior spinocerebellar tract

Both tracts ascend same side SC

Cerebellar cortex (Secondary neurons)

Ant. & Post. cerebellar pathways ascend on same side SC

Post. SCt goes as far as MO - connects to cerebellar nuclei through inferior cerebellar peduncle

Ant. SCt goes as far as midbrain - fibres descend & enter through superior cerebellar peduncle - fibres decussate again to contralateral side

Anterior spinocerebellar tract decussates twice & terminates ipsilateral to its origin!!

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Motor pathways

i) Types

2 types of somatic motor pathways

Conscious control skeletal muscles - Pyramidal paths

Unconscious control skeletal muscles - Extrapyramidal paths

a) Corticospinal tract (Pyramidal path)

Conscious muscle control

Consists of 2 pathways:

Anterior corticospinal tract

Lateral corticospinal tract

Both tracts form thick bulges called pyramids on anterior aspect of MO

Upper motor neurons begin cerebral cortex

Receive input from primary motor cortex, pre-motor cortex & somatosensory area (to small degree)

Both descend through internal capsule

Internal capsule susceptible to compression from haemorrhagic bleeds - could cause lesion of descending tracts!

Pass through crus cerebri, pons & into MO

@ most caudal part MO, tract divides into Ant. & Lat. tracts

Anterior corticospinal tract

Does not decussate at pyramids!

Descends into SC on ipsilateral side

Decussate with either interneurons or lower motor neurons in Ant. gray matter SC

Terminates ventral horn cervical & upper thoracic levels

Lower motor neurons will go on to controls axial skeletal muscles (includes thigh)

Lateral corticospinal tract

Decussates at pyramids! - ~90% fibres

Descend SC on contralateral side - lateral funiculi

Also terminate in ventral horn GM, but at all levels SC!

Lower motor neurons will then go on to control skilled limb movements (fingers, hands, leg & foot)

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b) Extra-pyramidal tracts

Upper motor neurons originate brainstem nuclei

Control involuntary & automatic muscle responses (tone, balance, posture, locomotion)

Composed of a Lateral & Medial pathway ​​​​​​​

Lateral pathway

Precise discrete movements & tone in flexor limb muscles

Consists of rubrospinal tracts

Rubrospinal tract

Originates red nucleus

Fibres decussate & descend SC

Thought to have role in fine control hand movements

Medial pathway

Muscle tone & gross movements of head, neck, proximal limb & trunk muscles

Consists of reticulospinal, tectospinal & vestibulospinal tracts

Reticulospinal tract

Medial tract arises from pons

Facilitates voluntary movements

Increases muscle tone

Lateral tract arises from medulla

Inhibits voluntary movements

Reduces muscle tone

Tectospinal tract

Begins at superior colliculus (optic nerve input region)

Neurons decussate & enter SC

Terminate @ cervical levels SC

Co-ordinate head movements in relation visual stimuli

Vestibulospinal tract

Medial & lateral paths arise from vestibular nuclei (balance)

Convey this info to SC - do not decussate

Control 'anti-gravity' balance & posture muscles (arm flexors & leg extensors) via lower MNs

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