Spinal Cord

What is the basic structure of the spinal cord?

• Central core of nerve cells – grey matter – surrounded by fibre tracts – white matter

What does the grey matter contain?

• Posterior dorsal horn and anterior ventral horn

What is the white matter divided into?

• Dorsal and lateral and anterior or medial (ventral) columns

What are the functions of dorsal and ventral respectively?

• Sensory and motor

What are the 3 layers of meninges the spinal cord is covered by?

• Thick dura

• Fine arachnoid

• Pia mater

What do sensory afferent fibres do?

• Enter the spinal cord from dorsal root ganglion cells trough dorsal spinal nerve roots

What do motor efferent fibres do?

• Leave the spina cord from motor neurons in the ventral horn through ventral spinal nerve roots

3 steps of the development of the spinal cord

• Neural plate (ectodermal)

• Neural groove

• Neural tube and neural crest

What is the notochord?

• Patterning centre which tells tissue around it what to become

• When it is not there

What are morphogens?

• Signalling molecules which form cross repressive interactions

What do bone morphogenetic proteins do (dorsal)?

• Specify the roof plate and subsequent dorsal horn formation

• Comes from overlaying ectoderm

What do sonic hedgehog protein do (ventral)?

• Specify floorplate and ventral motor neurons

• From notochord

What is the segmental organisation of the spinal cord like?

• Paired dorsal and ventral roots repeated 30 times with each spinal nerve passing through a notch between the vertebrae

What are the 4 groups that the 30 spinal segments are divided into?

• Cervical thoracic lumbar and sacral

Where is the grey matter largest?

• At the cervical and lumber enlargements reflecting the large number of neurons responsible for the innervation of the limbs

Where is the amount of white matter greatest?

• At the rostral (top) end of the cord

Why do ascending tracts increase in size as one moves up the cord?

• Sensory fibres are added to the cord from caudal to rostral

What are the 4 main functional zones in the dorsal horn?

• An outer tract of fine unmyelinated fibres: Lissauer’s tract.

• A marginal zone of grey matter where many nociceptive fibres end (lamina

• The substantia gelatinosa which are interneurons (lamina II/III)

• The ‘main sensory nucleus’ (nucleus proprius) – projection neurons which send axons which ascend the cord in the anterolateral column of grey matter (lamina IV/V).

What is the main ascending sensory pathway?

• Dorsal column

• Goes to cognitive awareness and unconscious cerebellum

What does the dorsal column do?

• Touch mostly (and proprioception)

What are the two types of ascending tracts in the dorsal column system?

• Gracile fascicle (arms)

• Cuneate fascicle (legs)

What does the anterolateral system do?

• Pain and temperature

Where does the anterolateral system ascend?

• On the contralateral side and slower than the dorsal column

What do the spino-cerebellar tracts do?

• Proprioception

What is the main descending system?

• The corticospinal tract which arises from both somatosensory and motor cortex and descends to the caudal medulla

• It suppresses reflexes to activate flexors in voluntary movement
  o Suppresses flexors involved in involuntary movements

What is a dermatome?

• Map of body in motor and sensory space that reflects the somite-induced segmentation of outgrowing spinal nerves

What can a dermatome be used to?

• Determine level of spinal cord injury from trauma to radiculopathy

What are the 2 types of motor neurons?

• Gamma and somatic

What are the 4 types of sensory afferent fibres?

• A alpha

• A beta

• A delta

• C

What are A alpha and A beta fibres used for?

• Touch and proprioception

• Fast and precise signalling for touch and awareness of surroundings

What are A delta and C fibres used for?

• Information about temperature and noxious stimuli

• Less precise but not needed just need awareness

• Pathways are about amplification and can be modulated (have lots of local synapses)

What is proprioception?

• A sense of body position essential for locomotion and balance

What are the muscle receptors involved in proprioception?

• Muscle spindles

• Golgi tendon organs

What happens in proprioception?

• Large myelinated fibres giver afferent innervation to muscle spindles which means rapid feedback and adjustment of motor neuron activity and muscle length

What do proprioceptors synapse onto?

• Motor neurons (for stretch reflex) in dorsal horn
  o Send collateral branches which ascend in the dorsal columns to inform higher centres (conscious proprioception)

What is glabrous skin?

• Ridges of fingers which vibrate or fluctuate

What are the 4 different types of mechanoreceptors

• Meissner’s corpuscle

• Merkel cells

• Pacinian corpuscle

• Ruffini endings

What does acupuncture target?

• Pacinian corpuscle

• Stimulation and vibration will feed into dorsal cord and can relieve pain

What does amplification of nociceptive afferent input happen via?

• Lissauer’s tract

What is the spinothalamic tract part of?

• The anterolateral column