CNS - spinal cord anatomy

• Spinal disc:

  • Nucleus pulposus - jelly like nucleus
  • Annulus fibrosis - tough and collagenous
  • Intervertebral disc herniation = nucleus pulposus ruptures through the annulus fibrosis
  • Normally displaces in the posterior-lateral direction and can irritate nearby spinal nerves
• There are 7 cervical vertebrae but 8 cervical nerves
• During the course of the spinal cord there are two points of enlargement:

  • Cervical enlargement at C4-T1 = origin of the brachial plexus
  • Lumbar enlargement at T11-L1 = origin of the lumbar and sacral plexuses
• A transverse section of the spinal cord reveals a district butterfly pattern of inner grey matter surrounded by white matter

  The grey matter is divided into four main columns: the dorsal horn, the intermediate column, the lateral horn and the ventral horn
• The dorsal (posterior) horn receives somatosensory information from the body that is then transmitted via the ascending pathways to the brain
• The ventral (anterior) horn largely contains motor neurones that exit the spinal canal to innervate skeletal muscles
• The white matter of the spinal cord = axons

  Grey matter = motor
• The ascending tracts of the spinal cord are pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex
• The ascending tracts consist of the dorsal column-medial lemniscal pathway and the spinothalamic pathway
• The DCML:

  • Carries the sensory modalities of fine touch, vibration and proprioception
  • Travels via the dorsal column of the spine
  • Signals decussate within the medulla oblongata
  • If there is a lesion in the spine the sensory loss with be ipsilateral
  • B12 deficiency
• The spinothalamic tract is also called the anterolateral system. Two separate pathways - anterior and lateral spinothalamic tracts:

  • Anterior tract - crude touch and pressure
  • Lateral tract - pain and temperature
  • Signals decussate within the spinal cord at the dorsal horn
  • If there is a lesion sensory loss will be contralateral
• Brown-sequard syndrome is the presence of a one sided lesion of the spinal cord - involves the DCML and anterolateral system

  • Ipsilateral loss of touch, vibration and proprioception
  • Contralateral loss of pain and temperature sensation
• The descending tracts are pathways which motor signals are sent from the brain to lower motor neurones. There are two major pathways:

  • Pyramidal tracts
  • Extrapyramidal tracts
• The pyramidal tracts are responsible for the voluntary control of the muscles, consists of:

  • Corticospinal tracts - muscles of the body
  • Corticobulbar tracts - muscles of the face
• The corticospinal tracts:

  • originate in the cortex and descend through the internal capsule - particularly susceptible to compression
  • Lateral corticospinal tract - fibres decussate within the medulla oblongata and travel to the ventral horn
  • Anterior corticospinal tract - remains ipsilateral until they decussate in the ventral horn
• Unilateral lesions of the corticospinal tracts:

  • Contralateral symptoms
  • Hypertonia
  • Hyperreflexia
  • Clonus
  • Babinski sign positive
  • Muscle weakness
• The corticobulbar tracts terminate at the cranial nerves. Most of the fibres innervate the muscles bilaterally, except:

  • Facial nerve - contralateral innervation of the lower quadrant of the face
  • Hypoglossal - contralateral innervation to the tongue
• The extrapyramidal tracts are responsible for the involuntary and automatic control of all musculature

  Muscle tone, balance and locomotion
• Extrapyramidal tract lesions are commonly seen in degenerative diseases, encephalitis and tumours. They result in various types of dyskinesia's or disorders of involuntary movement. – Parkinson's or Huntington's
• an upper motor neurone lesion will cause reduced power of muscles with a pyramidal pattern - extensors weaker than flexors in arms, flexors weaker in legs