Spinal tracts Flashcards
Ascending pathway vs Descending pathway
Ascending = sensory pathway to brain
Descending = motor pathway to periphery
The specific ascending pathways that transmit information from somatic receptors (Skin, skeletal muscle, tendons and joints) go to where in the brain?
somatosensory cortex
Ascending pathways
DCML
Spinothalamic
Spinocerebellar
Descending tracts
Corticospinal (pyramidal)
Corticobulbar (pyramidal)
Extra pyramidal tracts
What is the DCML for?
Fine touch, vibration, proprioception
DCML: Information travels …
via dorsal columns in the spinal cord then is transmitted through medial lemniscus in brainstem
DCML
First order neurons carry sensory information from peripheral nerves to the medulla. (FG + FC)
Second order neurons carry information from gracilis nuclei to 3rd order neurons and decussate in the medulla and travel to CONTRALATERAL THALAMUS.
Third order neurons transmit information to thalamus and ipsilateral sensory cortex.
DCML: Signals from upper limbs (T6 and above)
Fasciculus cuneatus
DCML: Signals from lower limbs
Fasciculus gracilis
Spinothalamic/Anterolateral Tracts consists of …
Anterior spinothalamic tract (MEDIAL)
= crude touch, pressure
Lateral spinothalamic tract
= pain, temperature
Pathways are the same for both tracts and they run alongisde each other
Spinothalamic
First order neurons arise from sensory receptors in the periphery. They enter the spinal cord and synapse at the tip of dorsal horn.
Second order neurons carry info from dorsal horn to thalamus.
The fibres DECUSSATE in spinal cord.
From thalamus -> ipsilateral primary sensory cortex.
Spinocerebellar tracts
Posterior spinocerebellar
Cuneocerebellar
Anterior spinocerebellar
Rostral spinocerebellar
Posterior spinocerebellar tract:
From lower limbs to ipsilateral cerebellum
Cuneocerebellar tract:
From upper limbs to ipsilateral cerebellum
Anterior spinocerebellar tract:
From lower limbs to ipsilateral cerebellum
Fibres in this tract decussate twice
Rostral spinocerebellar tract
From upper limbs to ipsilateral cerebellum
Clinical relevance
Injury to DCML
- A lesion of the dorsal column medial lemniscus pathway causes a loss of proprioception and fine touch
- However, a small number of tactile fibres travel within the anterolateral system, and so the patient is still able to perform tasks requiring tactile information processing
- If the lesion occurs in the spinal cord, the sensory loss will be ipsilateral because decussation occurs in the medulla oblongata
Clinical relevance
Injury to spinothalamic tract
- Injury to the anterolateral system will produce an impairment of pain and temperature sensation.
- In contrast to DCML lesions, this sensory loss will be contralateral because the spinothalamic tracts decussate within the spinal cord
Brown-Sequard Syndrome
- A hemisection (one sided lesion) of the spinal cord
- This is most often due to traumatic injury, and involves both the anterolateral system and DCML pathway
- DCML - Ipsilateral loss of touch, vibration and proprioception
- Spinothalamic - contralateral loss of pain and temperature sensation
Clinical relevance
Injury to spinocerebellar tracts
- Lesions of the spinocerebellar tracts present with an ipsilateral loss of muscle co-ordination
- However, the spinocerebellar pathways are unlikely to be damaged in ‘isolation’ - there is likely to be additional injury to the descending motor tracts
- This will cause muscle weakness or paralysis, and usually masks the loss of muscle co-ordination
Descending tracts
Pyramidal
- corticospinal
- corticobulbar
Extrapyramidal
- Vestibulospinal
- Reticulospinal
- Rubrospinal
- Tectospinal
Pyramidal tracts
Originate in the cerebral cortex and carru motor fibres to spinal cord and brainstem.
Responsible for voluntary control of musculature.
Extrapyramidal tracts
Originate in brainstem and carry motor fibres to spinal cord.
Responsible for involuntary and autonomic control of musculature.
Where does corticospinal tract begin?
Cerebral cortex
Corticospinal tracts inputs
Primary motor cortex
Premotor cortex
Supplementary cortex
Pathway of corticospinal tract
Cortex -> Descends through internal capsule -> Crus cerebri -> Pons -> Medulla
inputs -> internal capsule -> synapse in medulla -> decussation of 85% of fibres -> down spinal cord -> synapses at ventral horn
In the caudal part of the medulla, the tract divides into 2:
- Lateral corticospinal tract
- > Decussates at medulla and then descends, terminating in the ventral horn. - Anterior corticospinal tract
- > Remains ipsilateral to the spinal cord, then decussates at ventral horn and terminates in the ventral horn of the upper thoracic levels and synapses there.
Clinical relevance of corticospinal tract
The internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as ‘capsular stroke’. This could cause a lesion in the descending tracts.
Where does corticobulbar tract begin?
Lateral aspect of primary motor cortex
Inputs corticobulbar
lateral aspect of primary motor cortex
premotor cortex
supplementary cortex
Pathway of corticobulbar
Cortex → Descend through internal capsule → Crus cerebri → Brainstem → Terminate and synapse on motor nuclei of cranial nerves (acting on facial and neck muscles)
Most fibres innervate motor neurones bilaterally. What are the exceptions?
facial nerve
hypoglossal nerve
Clinical relevance
corticobulbar
It is important to understand the organisation of the corticobulbar fibres. Many of these fibres innervate the motor neurones bilaterally. For example, fibres from the left primary cortex act as upper motor neurones for the right and left trochlear nerves.
Exceptions
Facial nerve
⇒ Upper motor neurones for CN VII have a contralateral innervation
⇒ Only affects muscles in lower quadrant of the face (below eyes)
Hypoglossal nerve
⇒ Only provides contralateral innervation
Where do extrapyramidal tracts originate?
brainstem
Vestibulospinal tract
- Arise from vestibular nuclei
- Medial and lateral tracts
- Supply ipsilateral information
- Controls balance and posture
Reticulospinal tract
Medial tract
- Arises from the pons
- Facilitates voluntary movement
- Increases muscle tone
Lateral tract
- Arises from the medulla
- Inhibits voluntary movement
- Decreases muscle tone
Rubrospinal tract
- Arises from red nucleus
- Fibres decussate and then descend
- Plays a role in the fine control of hand movement
Tectospinal tract
- Arises from superior colliculus
- Decussate and then enters spinal cord
- Co-ordinates movements of the head in relation to vision stimuli
Damage to Corticospinal tracts
The pyramidal tracts are susceptible to damage because they extend almost the whole length of the central nervous system. They are particularly vulnerable as they pass through the internal capsule, a common site of cerebrovascular accidents.
- If there is only UNILATERAL lesion of the left or right corticospinal tract, symptoms will appear on the contralateral side of the body.
Signs of a UMN lesion
- Hypertonia - increased muscle tone
- Hyperreflexia - increased muscle reflexes
- Clonus - involuntary, rhythmic muscle contractions
- Babinski sign - extension of the hallux in response to blunt stimulation of the sole of the foot
- Muscle weakness
Damage to corticobulbar tracts
Due to bilateral nature of majority of corticobulbar tracts, a unilateral lesion usually results in mild muscle weakness.
However, not all cranial nerves receive bilateral input, and so there are a few exceptions:
Hypoglossal
- A lesion to the upper motor neurons for hypoglossal will result in SPASTIC PARALYSIS of the contralateral genioglossus.
= CONTRALATERAL TONGUE DEVIATION
contrast to LMN lesion; IPSILATERAL tongue deviation (to damaged side)
Facial :
A lesion to upper motor neurons for facial nerve will result in spastic paralysis of the muscles in the CONTRALATERAL LOWER QUADRANT OF FACE.
Damage to extrapyramidal tract
Extrapyramidal tract lesions are commonly seen in:
- Degenerative diseases
- Encephalitis
- Tumours
They result in various types of dyskinesia or disorders of involuntary movement
White matter tracts
Commissural tracts
Association tracts
Projection tracts
Commissural tracts
Travel from one cerebral hemisphere to another via commissures.
Most of these tracts travel through the CORPUS CALLOSUM.
Association tracts
Connect different areas of the same brain hemisphere.
Projection tracts
Extend from high brain regions to areas deep within the brain and spinal cord, relaying information from the cerebrum to the rest of the body.
White matter injuries
When you suffer an injury to ONLY white matter, you will retain movement and sensation, but your brain may have difficulty coordinating and sending signals.
- Challenges with ‘executive’ function (ability to co-ordinate, plan and monitor your thought process)
- Difficulty co-ordinating muscle movements; shaky or uncoordinated.
- Problems with memory, spatial reasoning and planning.
- Difficulty controlling your emotions, or changes in psychological health.
Multiple sclerosis
MS occurs when the brain’s white matter steadily breaks down, and interferes with movement and muscle co-ordination.
Dorsal horn of spinal cord
POSTERIOR
Somatosensory
Ascending pathways TO brain
Ventral horn of spinal cord
ANTERIOR
Motor
Exit spinal cord to innervate skeletal muscle
Intermediate column and lateral horn
innervate visceral and pelvic organs
Substantia gelatinosa
Located at top of dorsal horn
Pain, temperature and light touch sensation TO brain
Nucleus proprius
Neck of dorsal horn
Mechanical and temperature sensation TO brain
Dorsal nucleus of Clarke
Most dorso-medial nuclei
Unconscious proprioceptive information to the brain
C8-L3 spinal segments
Front of arm spinal
C5
C6
Back of arm spinal
C7
C8
Diaphragm spinal
C3
C4
C5
Erection of penis spinal
S2
S3
S4
Ankle jerk reflex
S1
S2
1, 2 buckle my shoe
Knee jerk reflex
S3
S4
3, 4 kick down the door
Wrists and biceps reflex
C5
C6
5, 6 pick up sticks
Triceps reflex
C7
C8
7,8 push open the gate
