Neuroanat 4_Ascending and descending tracts_2023_Student - Tagged (1).pdf

Ascending and descending tracts of the spinal cord Professor Andrew Dilley Previous lecture • Ventricles of brain • Cerebrospinal fluid production • Circulation and absorption of cerebrospinal fluid • Function of cerebrospinal fluid • Disorders of the ventricular system and cerebrospinal fluid ...

Ascending and descending tracts of the spinal cord Professor Andrew Dilley Previous lecture • Ventricles of brain • Cerebrospinal fluid production • Circulation and absorption of cerebrospinal fluid • Function of cerebrospinal fluid • Disorders of the ventricular system and cerebrospinal fluid Spinal cord anatomy • Consists of grey matter (horns) and white matter (columns/tract/funiculi) Posterior median sulcus Thoracic spinal cord: Dorsal horn Dorsal column Lateral horn Lateral column Ventral horn • Lateral horn in thoracic region only Ventral column Anterior median fissure Grey matter organisation • Excellent organization of the dorsal, ventral and lateral horns Dorsal horn - Neurons receiving sensory input Lateral horn - Preganglionic sympathetic neurons (thoracic only) Ventral horn - Motor neuronal cell bodies Ventral Also interneurons between/within levels Expanded grey matter • Grey matter is expanded at levels that supply the limbs C5 Cervical enlargement T2 L1 L2 Lumbosacral enlargement S2 White matter organisation • Contains tracts AFFERENT Long ascending tracts carry afferent (sensory) impulses to centres within the brain Long descending tracts carry efferent (motor) impulses from centres within brain EFFERENT Tracts to/from cerebral hemispheres – crossed (i.e. left cerebral hemisphere controls right side of body) Organisation of the tracts • Excellent organization or the ascending and descending tracts Dorsal column - Ascending tracts Lateral column - Descending and ascending tracts Ventral column - Descending tracts Ventral Blue = Ascending Red = Descending Ascending tracts Ascending (sensory) tracts • Two types of sensory information carried in ascending tracts: Proprioceptive - Information originating from inside the body (from muscles, joints, tendons) Exteroceptive - Information originating from outside the body (pain, temperature, touch) Anatomy of the ascending tracts • Often three neurons in circuit: 1. First order (primary sensory) neuron - Enters spinal cord via dorsal root 3rd order 2. Second order neuron - Ascends spinal cord or brainstem 3. Third order neuron 2nd order - Projects to the cortex 1st order Dorsal column-medial lemniscus pathway Sensory modality Light (discriminative) touch / vibration (From cutaneous mechanoreceptors) Proprioception (From muscle spindles (length), Golgi tendon organs (tension), joints) Provides brain with positional information Dorsal column-medial lemniscus pathway First order neurons First-order neurons • Enter spinal cord and ascend dorsal column on same side within the: Fasciculus gracilis (from lower limb) - Medial Dorsal Fasciculus cuneatus (from upper limb)* - Lateral [*No fasciculus cuneatus below mid thoracic spinal cord] • Fibres ascend dorsal column uncrossed - Longest neurons in body Dorsal column-medial lemniscus pathway Cervical segment of spinal cord Topographical organisation • First-order neurons synapse on second-order neurons in the medulla: Fasciculus gracilis terminates in nucleus gracilis (gracile) - Lower limb Gracile tubercle Fasciculus cuneatus terminates in nucleus cuneatus (cuneate) Cuneate tubercle - Upper limb Medulla DORSAL SURFACE (CEREBELLUM REMOVED) Dorsal column-medial lemniscus pathway Second and third order neurons Second-order neurons • Cross in medulla and ascend to thalamus - Form medial lemniscus (ribbon-like) Dorsal Nucleus gracilis (Lower limb) Nucleus cuneatus (Upper limb) Medial lemniscus Third-order neurons • Project from thalamus to somatosensory cortex Dorsal column-medial lemniscus pathway Dorsal column-medial lemniscus pathway Internal capsule Thalamus Nucleus gracilis Medial lemniscus Nucleus cuneatus Fasciculus gracilis Fasciculus cuneatus Upper limb Dorsal column-medial lemniscus pathway Lower limb Damage to dorsal column R Lesion on one side of spinal cord Lesion E.g., in multiple sclerosis •Loss • L of tactile discrimination + proprioception on same side Symptoms include sensory ataxia Loss of coordination and balance due to lack of proprioceptive input (i.e., no positional information) [Balance is dependent on visual cues] Clinical test: Romberg’s sign - Severe swaying on standing with eyes closed/feet together Dorsal column-medial lemniscus pathway Spinothalamic tract Sensory modality Pain (From nociceptors) Thermal Course (non-discriminative) touch Spinothalamic tract First order neurons First-order neurons • Enter spinal cord and form tract of Lissauer - Collateral branches given off at tip of dorsal horn - Run up or down 1-2 spinal segments • Synapse in dorsal horn with second-order neurons Spinothalamic tract Tract of Lissauer Dorsal Second and third order neurons Dorsal Second-order neurons Spinothalamic tract • Cross in dorsal horn at each level • Ascend in anterolateral column to thalamus Fibres from lower limb – Lateral in tract Fibres from upper limb – Medial in tract Third-order neurons • Project from thalamus to somatosensory cortex Spinothalamic tract Spinothalamic tract Internal capsule Thalamus Tract of Lissauer Anterolateral column Spinothalamic tract Upper limb Lower limb Damage to anterolateral column R Lesion on one side of spinal cord • Lesion Loss of pain, temperature and crude touch on opposite side Outer tract injury (e.g., cord compression due to herniated disk) - Loss of lower limb pain first (fibres sit laterally) Inner tract injury (e.g., grey matter tumour) - Loss of upper limb pain first (fibres sit medially) From lower limb Spinothalamic tract L From upper limb Spinocerebellar tracts Sensory modality Unconscious muscle proprioception (From muscle spindles (length), golgi tendon organs (tension)) For smooth motor coordination Spinocerebellar tracts Spinocerebellar tract anatomy • Only two neurons in pathway • Comprises of multiple tracts that carry proprioceptive information from: Trunk and lower limb [Dorsal and ventral spinocerebellar tracts] Upper limb [Cuneocerebellar tract – via fasciculus cuneatus) • Tracts terminate in the cerebellum on the same side (Left cerebellum controls left side of body) Spinocerebellar tracts Dorsal and ventral spinocerebellar tracts Dorsal First-order neurons Cross back Dorsal • Synapse in dorsal horn Second-order neurons Ventral Dorsal spinocerebellar tract Ventral spinocerebellar tract • Ascend in lateral column to cerebellum - Very fast axons Dorsal SCT Lesion on one side of spinal cord - Uncoordinated lower limb muscular activity on same side Spinocerebellar tracts Although rarely damaged in isolation L Lesion Descending tracts Descending tracts • Control of muscular activity • Many descending tracts • Mainly from cerebral cortex or brainstem • Grouped into pyramidal or extrapyramidal Corticospinal tract Voluntary motor pathway • Pyramidal tract • 2 neurons in circuit: horn Corticospinal tract 1. Upper motor (premotor) neurons - From cerebral cortex to ventral 2. Lower motor neurons - From ventral horn to skeletal Pyramidal tract Trunk Arm Ha nd ce Fa Primary motor cortex Foot Le g • Upper motor axons pass through the pyramids of the medulla Internal capsule Cerebral peduncle Pons Pyramids of medulla Lateral / Anterior Corticospinal tract Corticospinal tract Motor homunculus Pyramids of decussation • Within the pyramids of the medulla, nerve fibres decussate Lateral corticospinal tract Ventral Pyramid Pyramids of decussation 80% cross midline Lateral corticospinal tract 20% on same side Anterior corticospinal tract Corticospinal tract Ventral (cross in spinal cord – supply axial/trunk muscles) Lower motor neuron organisation • Excellent topographical organisation of lower motor neurons in ventral horn Lateral columns Upper limb Ventral horn Distal limb Proximal limb Trunk Corticospinal tract Lower limb Ventral Anterior columns Medial Anterolateral Posterolateral To trunk To proximal limb (arm/thigh) To distal limb (hand/foot) Motor neuron disease • Disruption of the corticospinal tract Upper motor neuron disease × • Degeneration of upper motor neurons - Spastic paralysis (increased muscle tone) - Overactive tendon reflexes - No significant muscle atrophy E.g., Following a stroke Above pyramids: Opposite side Corticospinal tract Below pyramids: Same side × Motor neuron disease Lower motor neuron disease • Degeneration of/damage to lower motor neurons (in ventral horn or periphery) - Flaccid paralysis (no muscle tone) - No tendon reflexes - Muscle atrophy × E.g., Spinal muscular atrophy Corticospinal tract Amyotrophic lateral sclerosis • Selectively affects lower and upper motor neurons Progressive muscle weakness and atrophy but mind intact • Symptoms initially in limbs or bulbar signs (speech and swallowing difficulties) • Spasticity present when upper motor neurons affected • Short life span (~5 years – due to respiratory failure) Affects 4-5 in 100,000 Corticospinal tract Lou Gehrig’s disease Extrapyramidal tracts • Do not pass through the pyramids • Many descending extrapyramidal tracts Dorsal ** * Midbrain Rubrospinal *** Pons Medulla * (from red nucleus [midbrain]) ** Reticulospinal (from reticular formation [pons]) *** Vestibulospinal (from vestibular nuclei [medulla]) Ventral Important for maintaining posture and regulating reflexes Clinical: Lesions to extrapyramidal tracts cause movement disorders (e.g. dyskinesia [involuntary muscle movements], dystonia [involuntary muscle contractions]) Summary • Ascending tracts Dorsal column-medial lemniscus (touch, proprioception) Spinothalamic (pain) Spinocerebellar (unconscious proprioception) • Descending tracts Corticospinal (voluntary motor) Extrapyramidal tracts (posture, regulating involuntary movements) Next Lecture: Autonomic pathways