Organisation of Peripheral Nervous System & Spinal Cord

Note: Important for OSPE & read up foundation 1!

Peripheral nervous system

• Made up of
• nerve cell bodies
• nerve fibres
• cranial nerves
• spinal nerves
• Connects
• CNS with peripheral structure
• Function
• afferent (sensory) neuron
• carry impulse from sensory receptors to CNS
• efferent (motor) neuron
• carry information from CNS to muscles/glands

• Accessory nerve
• 2 parts
• cranial part
• spinal part
• arise from C5, C6
• Injury is possible

Spinal cord

• It is the major reflex centre & conduction pathway
• Length: 45 cm
• Extent:
• from foramen magnum
• to lower border of L1
• newborn: upper border of L3
• 2 enlargements
• cervical
• lumbar
• Midline demarcation
• anterior median fissure
• posterior median sulcus
• Cross section
• Gray mater
• anterior gray column
• posterior gray column
• gray commissure (central canal)
• lateral gray column
• T1 – L2
• S2,3,4
• White mater
• anterior funiculus
• lateral funiculus
• posterior funiculus

• Spinal meninges
• Dura mater
• dense, fibrous
• from foramen magnum –> S2
• above it epidural space
• below it subdural space (negligible)
• Arachnoid mater
• delicate, impermeable
• from foramen magnum –> S2
• Subarachnoid space
• cerebrospinal fluid here
• Pia mater
• fine, vascular
• from foramen magnum –> coccyx
• as filum terminale
• thickened on either side of nerve roots
• pairs of ligamentum denticulatum

• Denticulate ligament
• attached to pia mater
• actually pia mater extension
• helps in fixation of spinal cord
• Cauda equina
• bundle of lumbar & sacral nerve roots
• in the subarachnoid space
• caudal to termination of spinal cord
• looks like horse tails
• Conus medullaris
• ending of spinal cord
• lower tapering of spinal cord
• at lower border of L1
• from L2 – coccyx
• just spinal nerve roots
• extensions
• Filum terminate
• condensation of pia mater
• from lower end of spinal cord to coccyx
• attached to sacral
• fixation

Spinal column

• circular part of spinal cord
• Rootlets of 1 spinal nerve emerge
• 31 spinal segments – corresponding to spinal nerves
• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal

Blood supply

• Anterior spinal artery
• from vertebral artery
• supply anterior 2/3 of spinal cord
• Posterior spinal artery
• from posterior inferior cerebellar artery
• supply posterior 1/3 of spinal cord
• Radicular arteries
• reinforce the anterior & posterior spinal artery
• enter via intervertebral foramina
• from
• deep cervical artery
• intercostal artery
• lumbar artery
• running along spinal nerve roots
• mostly supply lower spinal cord
• Arteria Radicularis Magna (Artery of Adamkiewicz)
• major source of blood supply to
• lower 2/3 of spinal cord

• Clinical signs of spinal cord ischaemia
• flaccid
• paraplegia
• both lower limbs paralysed
• areflexia
• Absence of neurologic reflexes
• eg. knee jerk
• loss of pain
• loss of temperature sensation
• autonomic deficits
• atonic bladder

• Spinal cord is short , vertebral column is long
• spinal segment not the same level as vertebrae

VERTEBRAE	SPINAL SEGMENT	EXAMPLE
Cervical vertebrae	Add 1	C3 vertebrae – C4 spinal segment
Upper thoracic (T1 – T6)	Add 2	T4 vertebrae – T6 spinal segment
Lower thoracic (T7 – T9)	Add 3	T9 vertebrae – T12 spinal segment
T10 thoracic	L1 & L2
T11 thoracic	L3 & L4
T12 thoracic	L5
Lumbar 1	Sacral & coccygeal spinal segment

• How spinal cord can be affected?
• compression
• metastasis
• Intervertebral disc prolapse
• fracture
• Most common cervical spinal injuries involve C4/C5
• A person who has had a burst fracture of the C5 vertebral body
• injures C6 spinal segment
• wrist extensors weak
• sensation below C6 severely compromised
• injures C4 spinal roots
• exits the spinal column btwn C4 & C5 vertebrae
• loss of sensation in C4 dermatome
• weak deltoids (supplied by C4)

• Dermatome
• area of skin supplied by a single spinal nerve
• loss of sensation on a certain dermatome
• able to tell which spinal nerve affected

How are spinal nerves formed?

• At each spinal segment
• 1 pair of nerves exit to the left
• 1 pair of nerves exit to the right
• Dorsal root & ventral root meet
• forming spinal nerve
• Comes out through intervertebral foramina
• Spinal nerve divides into

1. Dorsal ramus
2. Ventral ramus
3. Meningeal ramus
4. Rami communicating to sympathetic chain

 

What are plexuses?

• Ventral rami of spinal nerves interconnect to form several networks of nerves
• Plexuses (5 in total)
• Cervical
• C1 – C4
• Brachial
• C5 – C8, T1
• Lumbar
• L1 – L4
• Sacral
• L4 – L5, S1 – S3
• Coccyx
• S4 – S5, Coccygeal 1
• T2 – T12 do not form plexus
• innervate muscles & skin of thoracic, abdominal wall
• as intercostal nerves
• In a plexus, nerve fibres of different spinal nerves are sorted & recombined
• Peripheral nerves coming out of plexuses are composed of
• fibres from several spinal nerve rami
• eg median nerve made up of
• C5, C6, C7, C8, T1

• Question:
• What are the roots forming the brachial plexus?
• Ventral rami of
• must mention this
• C4 5 6 7 and T1

• Components of a typical spinal nerve
• general somatic afferent
• general somatic efferent
• general visceral afferent
• general visceral efferent

Branches of spinal nerve communicating to sympathetic trunk

• Gray rami communicantes
• White rami communicantes
• Posterior primary ramus
• dorsal cutaneous branch
• muscular branch to erector spinae
• articular to joints of vertebral column
• Anterior primary ramus
• lateral cutaneous branch
• anterior cutaneous branch
• proximal muscular branch
• distal muscular branch

Coverings

• Epineurium
• outer
• Perineurium
• middle
• Endoneurium
• inner

• 3 layers
• neuroepithelial layer
• mantle layer
• marginal layer
• Ventral thickening
• basal plate –> anterior horn cells
• Dorsal thickening
• alar plate –> posterior horn cells
• Boundary (centre)
• sulcus limitans

• Neural crests
• detached from neural tube ectoderm
• forms dorsal root ganglion

Lumbar puncture

• Between L3 & L4 vertebrae
• Structures pierced
• skin
• thoracolumbar fascia
• interspinous, supraspinous ligament
• ligamentum flavum
• dura mater
• arachnoid mater
• subarachnoid space

http://rch.org.au/clinicalguide/cpg.cfm?doc_id=5178
*lumbar puncture – read up for OSPE

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Questions

• The following statements concerning meninges of the spinal cord are correc EXCEPT:
• Dura mater is a delicate impermeable membrane F
• Pia mater is a vascular membrane that closely invests the cord T
• Arachnoid mater continues along spinal nerve roots for a short extent T (until S2)
• Dura mater is continuous above through foramen magnum with cranial dura T
• Extradural space contains internal vertebral venous plexus T
• The following statements concerning the arterial supply of the spinal cord are true EXCEPT:
• Posterior spinal arteries supply posterior 3rd of spinal cord T
• Veins communicate with veins of brain and venous sinuses
• Arteria radicularis magna (Artery of Adamkiewicz) arises in the upper thoracic region of from arch of aorta F
• Anterior spinal artery is single but usually arises from vertebral artery T
• Spinal arteries are reinforced by radicular arteries, which are branches of local arteries T
• The following statements concerning the spinal cord are correct EXCEPT:
• Spinal cord has a cervical enlargement for the brachial plexus T
• It possess 34 pairs of spinal nerve F (31 pairs)
• In the adult it ends at lower border of 1st lumbar vertebra T
• Ligamentum denticulatum anchors spinal cord to the dura mater along each side T
• Central canal communicates with the 4th ventricle of brain T
• The following statements concerning CSF are correct EXCEPT:
• Normal pressure is between 60 & 150 mm of water T
• Normal glucose content is 50 to 85 mg per 100 ml T
• It is a clear colorless fluid T
• The cells are polymorphonuclear leucocytes F (mononuclear)
• Compression of internal jugular vein (Queckenstedt sign) can diagnose blockage of subarachnoid space by tumour T