Notes on Spinal Cord

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Spinal Cord Location:

• Housed within vertebral canal and continuous with medulla; ends as conus medullaris.

Spinal Cord Horns:

Dorsal/posterior Horn:

• Sensory.

Ventral/anterior Horn:

• Alpha and gamma motorneurons.

Spinal Cord Motorneurons:

Alpha motorneurons:

• Innervate skeletal muscle.

Gamma motorneurons:

• Innervate muscle spindle.

Skeletal Muscle Contraction:

• Voluntary contraction of skeletal muscle initiated by upper and lower motor neurons working in unison.

Corticospinal tract:

• Motor function.

• Crossing at brain stem: caudal medulla-spinal cord junction.

• Two neurons used: UMN and LMN.

• Voluntary movements of distal extremities.

• Enters brain stem at pyramidal decussation in lower medulla.

• Lesions above pyramidal decussation: muscle weakness on contralateral side.

• Lesions below pyramidal decussation: muscle weakness on ipsilateral side.

Reflex innervation of skeletal muscle:

• Sensory neuron stimulated, produces motor response.

• Skeletal muscle reflex: muscle has sensory function; muscle has motor function.

• Alpha motor neurons innervates motor unit.

• Gamma motor neurons innervate muscle spindle (sensory function).

Muscle stretch:

• Eg., knee jerk.

• Tapping patellar ligament stretches muscle spindles.

• Ia afferents activated; afferent impulses transmitted to chord.

• Some impulses monosynaptically stimulate and activate alpha motorneurons; knee jerked.

Inverse muscle stretch reflex:

• Golgi tendon organs used.

• Ib afferent neurons activated.

• Polysynaptically facilitate antagonists.

• Activated with sudden stretch of muscle; overstretch of muscle inhibited.

LMN lesion:

• Hypotonicity; paresis; absent muscle reflexes; fasciculations; fibrillations; atrophy; flaccid paralysis.

UMN lesion:

• Spastic paresis; hypertoniciy; hyperreflexia; Babinski reflex present.

Dorsal column-medical lemniscal system:

• Proprioception-stereognosis (conscious).

• Enters brain stem at N. gracilis of N. cuneatus (lower part of medulla).

• Discriminative touch, conscious proprioception, vibratory, and pressure.

• Three neurons used.

Primary neuron:

• Lowermost; cell body located in dorsal root ganglion; supplied by fasciculus gracilis (from lower) and fasciculus cuneatus (from upper).

Secondary neuron:

• Middle; cell body located in brain stem or spinal cord; in nucleus gracilis and nucleus cuneatus.

Third neuron:

• Uppermost; cell body located in thalamus.

Lesions above N. gracilis of N. cuneatus:

• Affects contralateral side.

Lesions below N. gracilis of N. cuneatus:

• Affects ipsilateral side.

Dorsal column-medical lemniscal lesion:

• + Romberg test: standing, placing feet together, and swaying with eyes closed.

Cerebellar lesion:

• Standing, placing feet together, and swaying with eyes open.

Astereognosis:

• Can't recognize object only by touch.

Anterolateral (spinothalamic tract) system:

• Light touch.

• Pain, temperature, and crude touch.

• Pain/temperature fibers enter spinal cord via A-delta and C or class III and class IV dorsal root fibers; travel through tract of Lissaure before entering the dorsal horn in the spinal cord.

• Analgesia begins on contralateral side 1-2 segments below lesion.

• Three neurons used.

Primary neuron:

• Cell body located in dorsal root ganglion.

Secondary neuron:

• Cell body located in dorsal horn gray matter.

Third neuron:

• Cell body located in thalamus.

Spinocerebellar pathways:

• Unconscious proprioception.

• Cuneocerebellar: upper extremities and upper trunk; Cell bodies are located in medulla in external cuneate nucleus.

• Dorsal spinocerebellar: lower extremities and lower trunk; Cell bodies are located in Clarke's nucleus in spinal cord.

• Seen in Friedreich ataxia, an autosomal dominant disease.

Uncommon lesions:

• Lesion affects Ipsilateral side.

Spinal cord lesions:

• Brown-Sequard syndrome: lesion in corticospinal, dorsal columns, and spinothalamic tracts.

• Findings: two ipsilateral and one contralateral sign.

Polio:

• LMN destruction.

ALS:

• UMN and LMN dysfunction.

Anterior Spinal Artery occlusion:

• No blood supply to corticospinal and spinothalamic tracts.

• Bilateral paresis and loss of pain and temperature below lesion.

Syringomyelia:

• Progressive cavitation of central canal, usually in cervical spinal cord.

• Early: Bilateral pain and temperature sensation loss in hands and forearms.

• Late: Horner syndrome.

Tabes dorsalis:

• Caused by neurosyphilis: dorsal roots bilaterally degenerated; secondary degeneration of dorsal columns.

• Dysfunctional proprioceptive pathways.

• High step stride.

• Argyll-Robertson pupils.

Multiple Sclerosis:

• Demyelinating disease of CNS.

Additional Notes:

• Brown-Sequard's syndrome: motor deficit on right body; decreased sensations and pain on left body; diminished vibratory and position sense on right body.

• Dorsal root afferents of spinal cord division: medical division consists of type C fibers and lateral division consists of type A fibers.

• C-fibers in spinal gray matter: substance P (pain).

Additional Readings:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem

Neurology Videos

1. Video of Neurology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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