organisation of the nervous...

Chapter

1Organisation of the nervous system

1. Subdivisions of the nervous systemThe nervous system is divided:

i) StructurallyThe central nervous system (CNS) – composed ofthe brain and spinal cord.The peripheral nervous system (PNS) – composedof nervous tissue relaying information betweenperipheral structures and the CNS.

ii) FunctionallyThe somatic nervous system (SNS) – providessensory and motor innervation to all parts of thebody except the viscera, smooth muscle andglands.The autonomic nervous system (ANS) – providessensory and motor innervation to the viscerasmooth muscle and glands.

2. Nervous tissueNervous tissue consists of:

i) NeuronsThese are nerve cells.They are composed of a nerve cell body,dendrites (projections from the cell body) and anaxon (carries impulses to and from the cell body).Axons may be surrounded inmyelin sheaths whichare enclosed by endoneurium, a connective tissuelayer.The perineurium encloses groups of axons to forma fascicle.The epineurium encloses groups of fascicles toform a peripheral nerve.

ii) NeurogliaNon-excitable tissue.Support the neurons structurally andmetabolically.

3. Organisation of nervous tissueWithin the CNS, collections of nerve cell bodies formthe grey matter; the interconnecting neural tissueforms the white matter.Within the PNS, collections of nerve cell bodies form aganglion; the interconnecting axons (held together bythe epineurium) form a peripheral nerve.Peripheral nerves may be cranial or spinal. There are:* 12 pairs of cranial nerves, which arise from the

brain and exit the cranium through the foramina ofthe skull.

* 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5lumbar, 5 sacral, 1 coccygeal), which arise from thespinal cord and exit through the intervertebralforamina.

4. Peripheral nervous system: anatomyof a spinal nerve

A typical spinal nerve originates from the spinal cordas dorsal (posterior) and ventral (anterior) rootletswhich unite to form a dorsal and ventral root respec-tively (Figure 1.1).* The dorsal fibres carry sensory (afferent) impulses

to the dorsal horn of the spinal cord. The cell bodiesof these nerves are found in the dorsal rootganglion.

* The ventral fibres carry motor (efferent) impulsesfrom the ventral horn of the spinal cord. The cellbodies of these fibres are found in the grey matterof the ventral horn.

The dorsal and ventral roots unite to form a mixedspinal nerve (motor and sensory) at the intervertebralforamen, at which point the dural covering of thenerve ceases. Any divisions of the nerve from thispoint distally will therefore carry both motor andsensory impulses.The spinal nerves of C1–7 emerge above their respectivevertebrae. C8 spinal nerve emerges below C7 vertebra. 1

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Moving inferiorly, all subsequent spinal nerves emergebelow their respective vertebrae (T1 spinal nerveemerges below T1 vertebra and so on).The spinal nerve then divides into dorsal and ventralprimary rami.* The dorsal primary ramus supplies the dorsal

structures – the synovial joints of the vertebralcolumn, musculature of the back and overlying skin.

* The ventral primary ramus supplies the ventralstructures – the anterolateral trunk and the limbs.

Grey rami communicantes join the dorsal and ven-tral primary rami from the sympathetic trunk (seesection 6, Sympathetic nervous system, below). Theprimary rami therefore carry somatic and auto-nomic, motor and sensory impulses.The ventral rami of the spinal nerves in the cervical,brachial, lumbar and sacrococcygeal regions unite toform somatic plexuses which supply the neck, arms,legs and legs/pelvis respectively.Autonomic plexuses are described in section 8, below.

Dorsal root

Ventral root

Dorsal root ganglion

Spinal nerve

Dorsal primary ramus

Ventral primary ramus

Grey ramus communicans

White ramus communicans

Sympathetic paravertebral ganglion

Sympathetic trunk

Figure 1.1 The anatomy of a spinal nerve. The spinal nerve itself is located at the intervertebral foramen (the foramen formed betweenthe pedicles of two adjacent vertebrae).

Chapter 1: Organisation of the nervous system

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Exceptions to the above anatomy* The first cervical dorsal primary ramus carries only

motor impulses.* The coccygeal dorsal primary ramus only supplies

the skin over the coccyx.

The segmental arrangement of the primary rami leadsto a segmental innervation of the overlying skin. Thearea of skin supplied by a single pair of spinal nerves isknown as a dermatome (Figure 1.2). Note the absenceof C1 dermatome.

C2

C3

C2

C3

C4

C4

C5C5

C6 C6

C7 C7C8

C8

T1T1

T2T2

T3

T3T4

T4

T5

T5T6

T6T7

T7T8

T8

T9

T9

T10

T10T11

T11

T12

T12

L1

L1L2

L2

L3

L3L4

L4

L5

L5

L5S1

S1

S2

S1S2

S3

S2

S1

S3

S5S4

Figure 1.2 The dermatomes. Note:there is no C1 dermatome.


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5. Organisation of the autonomic nervous systemSubdivided anatomically and functionally into thesympathetic and parasympathetic nervous systems.Both conduct impulses from the CNS via two neuronsin series:* The first (presynaptic or preganglionic)

neuron has its cell body in the greymatter of the CNSand synapses only with the second neuron.

* The second (postsynaptic or postganglionic)neuron has its cell body in an autonomic ganglionlocated outside of the CNS. Its fibres terminate inthe effector organs.

Neurotransmitters:* Between the pre- and postganglionic neurons –

acetylcholine.* Between the postganglionic neuron and the

effector – acetylcholine in the parasympatheticnervous system, noradrenaline in the sympatheticnervous system.

6. Sympathetic nervous systemThe cell bodies of the sympathetic preganglionic neuronsare found in the intermediolateral cell columns of thespinal cord. These columns are found in the lateral hornsof the greymatter (Figure 2.7), between segments T1 andL2/3 of the spinal cord (‘thoracolumbar’ outflow).The fibres leave the spinal cord through the ventralroots and enter the ventral primary rami.Almost immediately, the fibres leave the ventralprimary rami via white rami communicantes andenter the sympathetic trunk, which is a series oflinked sympathetic paravertebral ganglia. There are3 cervical (superior, middle and inferior cervical gan-glia), 12 thoracic, 4 lumbar and 4 sacral ganglia withinthe sympathetic trunk (Figure 1.3). In 80% of people,the inferior cervical ganglion is fused with the firstthoracic ganglion at the level of C7–T1 to form thestellate ganglion (described in more detail inChapter 4, The nerves of the neck, section 3).The preganglionic fibres then follow one of fourpathways:Pathway 1. Synapse with a postganglionic neuron

at a paravertebral ganglion in the sympathetictrunk at the same spinal level.

Pathway 2. Ascend or descend within the sympathetictrunk before synapsing with a postganglionicneuron at a paravertebral ganglion at a higher orlower spinal level. This explains why, although the

input to the sympathetic trunk comes fromsegments T1–L2/3, the trunk itself runs from C1 tothe coccyx.

Pathway 3. Pass through the sympathetic trunkwithout synapsing. These neurons, known asthe greater, lesser, least and lumbar splanchnicnerves, synapse with a postsynaptic neuron atone of the prevertebral ganglia. Theprevertebral ganglia, located about the origins ofthe branches of the abdominal aorta or in thepelvis, are:* Coeliac ganglion – supplied by the greater

splanchnic nerves (T5–9) and the lessersplanchnic nerves (T10–11).

* Superior mesenteric ganglion – supplied by thelesser splanchnic nerves (T10–11).

* Renal ganglion – supplied by the leastsplanchnic nerves (T12).

* Inferior mesenteric ganglion – supplied by thelumbar splanchnic nerves (L1–2).

Pathway 4. Fibres innervating the suprarenal gland alsopass through the sympathetic trunk withoutsynapsing. These fibres then pass through the coeliacprevertebral ganglion, again without synapsing,before innervating the medulla of the suprarenalgland. The suprarenal gland therefore acts as aspecial type of postganglionic neuron, releasing itsneurotransmitter into the bloodstream instead ofonto an effector organ.

Fibres supplying:* The head, neck and limbs follow pathway 2

(ascending within the sympathetic trunk). Moredetail is supplied in Chapter 4, The nerves of theneck, section 3.

* The thorax follow pathway 1.* The abdominal structures follow pathway 3.* The pelvic structures follow pathway 2 (descending

within the sympathetic trunk).

Postsynaptic fibres originating from the paraver-tebral ganglia (pathways 1 or 2) pass out of thesympathetic trunk through grey rami communicantesand re-join the ventral primary rami. From here,the fibres join all branches of the spinal nerve.Postganglionic fibres originating from the preverte-bral ganglia (pathway 3) form plexuses which trackalong the branches of the abdominal aorta to reachtheir effector sites (see section 8, Autonomic plexuses,below).


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Superior cervical ganglion

Middle cervical ganglion

Inferior cervical ganglion

12 thoracic ganglia

4 lumbar ganglia

4 sacral ganglia

Sympathetic chain(input T1- L2/3)

Spinal cord

b

a

c

d

e

f

g

i

h

j

k

Figure 1.3 Schematic anatomy of the sympathetic nervous system.a – Ascending sympathetic supply to head (follows arterial supply); follows pathway 2 (see text)b – Cardiac innervation, superficial cardiac plexus (left superior cervical ganglion) and deep cardiac plexus (all cervical ganglia (with the

exception of the left superior cervical ganglion) and the upper four thoracic ganglia)c – Pulmonary innervation (via pulmonary plexus) inferior cervical ganglion and the upper four or five thoracic ganglia.d – Greater splanchnic nerve (T5–9)e – Lesser splanchnic nerve (T10–11)f – Least splanchnic nerve (T12)g – Lumbar splanchnic nerve (L1–2)h – Coeliac ganglioni – Superior mesenteric ganglionj – Renal ganglionk – Inferior mesenteric ganglion

The plexi (cardiac and pulmonary) also have parasympathetic innervation (not shown); the ganglia do not. The inferior cervical ganglion may befused with the first thoracic ganglion to form the stellate ganglion (not shown for clarity).


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7. Parasympathetic nervous systemThe cell bodies of the presynaptic neurons are ‘cranio-sacral’ in origin.

i) Cranial outflowThese preganglionic fibres have their cell bodies inthe grey matter of the brainstem. They leave theCNS in cranial nerves III, VII, IX and X andsynapse with ganglia located in or on the walls ofthe effector organs (Figure 1.4). These ganglia are:* Ciliary ganglion – receives CN

III (pupillary constriction andaccommodation).

* Pterygopalatine ganglion –receives CN VII (secretomotor to thelacrimal gland).

* Submandibular ganglion – receives CN VII(secretomotor to the submandibular andsublingual glands).

* Otic ganglion – receives CN IX (secretomotorto the parotid gland).

The vagus nerve (CN X) originates fromthree nuclei found in the medulla oblongata. It isresponsible for the parasympathetic innervation tothe thorax and abdominal viscera as far distally asthe splenic flexure. Preganglionic fibres exit the

skull through the jugular foramen. It formsseveral plexuses with the sympathetic nervoussystem (see section 8, Autonomic plexuses, below),before synapsing in ganglia in or on the walls ofeffector organs. The vagus nerve is described inmore detail in Chapter 3, The cranial nerves,section 10.

ii) Sacral outflowThese preganglionic fibres have their cell bodiesin the lateral horns of the grey matter of the spinalcord between S2 and S4. The fibres leave the CNSin the ventral roots of spinal nerves S2–4. Thepreganglionic fibres then leave the ventral rami aspelvic splanchnic nerves to synapse with ganglia inor on the walls of the descending andsigmoid colons and the inferior hypogastric plexus(see section 8, Autonomic plexuses, below). Thesefibres supply the pelvic viscera.

Given that the parasympathetic ganglia are on orclose to the walls of the effector organs, it can beseen that the preganglionic fibres must be long andthe postganglionic fibres short. This is in contrastto the sympathetic nervous system, where, given thepara- or prevertebral location of the ganglia, thereverse is true.


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Oculomotor nucleus

CN III

Orbit

a

b

c

d

e

f

g

h

Lacrimal, submandibular & sublingual glands

Parotid gland

CN VII

CN IX

CN X

Superior salivary nucleus of VII

Inferior salivary nucleus of IXDorsal nucleus of X

Ganglia: Cervical,

T1-4

Ganglia: Inferior Cervical, T1-4/5

Greater & lesser splanchnic nerves

Coeliac plexus, lumbar paravertebral ganglia

Superior hypogastric plexus

Pelvic splanchnic nerves S2-4

Figure 1.4 Schematic anatomy of the parasympathetic nervous system. Parasympathetic nerves are shown in light blue, sympatheticnerves in orange, ganglia in yellow and plexuses in green. The origin of the sympathetic fibres contributing to the autonomic plexuses is also shown.

a – Ciliary ganglionb – Pterygopalatine ganglion (to the lacrimal glands) and submandibular ganglion (to the submandibular and sublingual glands)c – Otic gangliond – Cardiac plexuse – Pulmonary plexusf – Coeliac plexusg – Aortic plexush – Inferior hypogastric plexus






8. Autonomic plexusesAutonomic plexuses receive sympathetic and para-sympathetic nerves from the pathways describedabove and distribute these to the viscera. The majorplexuses are:

i) Superficial cardiac plexusFound anterior to the pulmonary artery. It givesbranches to the deep cardiac plexus and thepulmonary plexus.Receives sympathetic innervation from theleft superior cervical ganglion and parasympatheticinnervation from the left vagus nerve.

ii) Deep cardiac plexusFound anterior to the tracheal bifurcation. Itinnervates the heart.Receives sympathetic innervation from the cervicalganglia (with the exception of the left superiorcervical ganglion) and the upper four thoracicganglia.Receives parasympathetic innervation from theright vagus nerve.

iii) Pulmonary plexusFound mostly posterior but also anterior to theroots of the lungs. Innervates the lungs and visceralpleura.Receives sympathetic innervation from inferiorcervical ganglion and the upper four or fivethoracic ganglia.

Receives parasympathetic innervation from thevagus nerve, which synapses with postganglionicfibres in the plexus itself and those found along thewalls of the bronchial tree.

iv) Coeliac plexusConsists of the coeliac ganglia (see section 6,Sympathetic nervous system, above) and a seriesof interconnecting fibres of sympathetic andparasympathetic origin, which innervate theforegut.There are two coeliac plexuses found to the left andright of the coeliac arterial trunk at its origin fromthe aorta at the level of L1. The relations of theplexuses are:* Superior – crura of the diaphragm.* Inferior – coeliac arterial trunk and

its branches (hepatic, splenic and left gastricarteries).

* Anterior – inferior vena cava (on theright) pancreas and splenic artery (on the left).

* Posterior – abdominal aorta.Receives preganglionic sympathetic fibresfrom the greater and lesser splanchnicnerves, which originate from thoracicsegments T5–9 and T10–11 respectively.These nerves synapse with postganglionic fibresin the coeliac ganglion. It also receivesthe coeliac branch from the posterior vagaltrunk, derived mainly from the right vagusnerve.


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Coeliac plexus block

Introduction

Provides relief of pain carried by sympathetic nerves from the pancreas, stomach, liver, gall bladder, spleen, kidneys,small bowel and proximal two-thirds of the large bowel. It may be performed acutely during surgery for post-operative pain relief and electively for treatment of chronic pain and cancer pain, when an initial diagnostic localanaesthetic block may be followed by a neurolytic block.

Indications

Any condition causing severe upper abdominal visceral pain from the above organs that has failed to respondto standard treatments.

Specific contraindications

* Large aortic aneurysm.* Where there is a possibility of seeding metastasis around the plexus.

Pre-procedure checks

Intravenous fluids are administered to reduce the degree of hypotension following the block due to dilation of theupper abdominal vessels.

Technique

X-ray screening.

Landmark

The patient is in the prone position. Local anaesthetic is infiltrated just below the tip of the 12th rib and the needleadvanced, using x-ray screening in two planes, until it contacts the body of L1. The needle is then withdrawn slightlyand redirected past the side of the vertebra forwards into the area of the coeliac plexus, avoiding the aorta andinferior vena cava. Radio-opaque dye is injected to confirm correct placement. A second injection from the other sideis often required to achieve good spread.

Figure 1.6 X-ray screening for a coeliac plexus block.


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