The Somatic Nervous System

This both transmits sensory information (afferent pathways) and innervates skeletal muscle (efferent pathways). The sensory cells are derived from the neural crest and are bipolar with their cell bodies lying in the dorsal root ganglia ( Note there is no synapse in dorsal root ganglia), while the motor cells grow out in the ventral root from the neural tube (i.e. single myelinated cells with no synapses before their end organs). Somatic nerves do not cross the midline ( Fig. 5.1 ).

Diagrammatic representation of a transected spinal cord showing the somatic and autonomic neurone pathways and the main tracts running within the cord.

Spinal nerves consist of:

• •
  

  Posterior primary rami sequentially supplying erector spinae and overlying skin

  
  
• •
  

  Anterior primary rami supply the rest of the body’s muscles and skin and are often involved in forming nerve plexuses before branching and joining together for more distal distribution (cervical, brachial, lumbar and sacral plexuses)

Autonomic nerves often ‘hitch a ride’ on these nerves (as they do on blood vessels).

The Autonomic Nervous System

Unlike the somatic nervous system, this is involuntary and regulates the body’s internal environment. It has the distinctive feature of comprising two neurones in its motor pathway which synapse outside the CNS: one neurone grows out from the CNS and is myelinated (preganglionic) while the postganglionic neurone (derived from neural crest cells) is unmyelinated. Two components (sympathetic and parasympathetic) form the autonomic system and tend to oppose each other to maintain internal homeostasis. The sympathetic nervous system originates from the thoracolumbar regions and the ganglia form a chain bilaterally down each side of the vertebral column, while the parasympathetic nerves have craniosacral outlets and their ganglia are situated distally near their target organs. These differences together with pharmacological features are illustrated in Fig. 5.2 .

Diagrammatic representation of the neurone arrangements and neurotransmitters of the somatic and autonomic nervous systems. CNS , Central nervous system.

In addition to these efferent autonomic motor neurones, there are afferent fibres which are conveyed via the sympathetic and parasympathetic nerves, but they are independent of them and do not relay in the ganglia. Like other sensory fibres, their cell bodies lie in the dorsal root ganglia from where they ascend centrally to the hypothalamus and thence to the orbital and frontal gyri of the cerebral cortex (see Fig. 5.1 ).

Clinical Application

In normal circumstances, we are unaware of autonomic afferent impulses but if sufficiently strong they can cause the sensation of visceral pain (intestinal colic, uterine pain, etc.) which can also produce referred pain in the dermatome of the relevant segmental supply (e.g. cervix S2 and S3, ovary T10 and T11, body of the uterus, lower thoracic and upper lumbar roots). Dermatomes are shown in Fig. 5.3 .

An approximate pattern of anterior and posterior dermatomes.

Sympathetic (Thoracolumbar) Nervous System

These cells are derived from the lateral horn of T1–L2 but the preganglionic fibres travel up and down to form a chain of ganglia extending from the cervical to the coccygeal region (i.e. the root value of the autonomic component may be different from the spinal component with which it emerges). Postganglionic neurones then form sympathetic nerve plexuses, the main ones being:

• •
  

  Cardiac plexus (below the aortic arch)

  
  
• •
  

  Pulmonary plexus (at the root of the lungs)

  
  
• •
  

  Coeliac plexus (on the coeliac axis and around the origin of the superior mesenteric artery)

  
  
• •
  

  Superior hypogastric plexus (anterior to the aortic bifurcation)

  
  
• •
  

  Inferior hypogastric plexus (lateral to the rectum, cervix and vaginal fornix)

The peripheral distribution of the sympathetic fibres includes branches for somatic distribution which travel with each spinal nerve to supply the corresponding segmental skin, and the visceral distribution which tends to reach its end organ by means of the arterial pathways.

Approximate segmental supplies:

• 
  

  T1–2 head and neck

  
  
• 
  

  T1–4 thoracic viscera

  
  
• 
  

  T2–5 upper limb

  
  
• 
  

  T4–L2 abdominal viscera

  
  
• 
  

  T10–L2 pelvic viscera

  
  
• 
  

  T11–L2 lower limb

Note : Thoracic, lumbar and sacral splanchnic nerves emerge from the sympathetic plexuses while the p elvic splanchnics, in contrast, are parasympathetic (S23 – nervi erigentes). These parasympathetic preganglionic fibres join the sympathetic fibres (from the inferior hypogastric plexus) for distribution within the pelvis and are described in more detail later.

Sympathetic Effects

These are essentially of fight and flight:

• •
  

  Vasoconstrictor (except to coronary arteries which it dilates)

  
  
• •
  

  Increases the heart rate

  
  
• •
  

  Dilates the bronchial tree

  
  
• •
  

  Relaxes the detrusor muscle

  
  
• •
  

  Contracts smooth muscle sphincters

  
  
• •
  

  Dilates the eye (by relaxing the ciliary muscle)

  
  
• •
  

  Relaxes the small intestine

The Adrenal Medulla

This is derived from neural crest cells and comprises approximately 10% of the adrenal gland (the remainder being the cortex). The myelinated preganglionic sympathetic fibres from the splanchnic nerves travel via the coeliac plexus and synapse directly with the medullary (chromaffin) cells which secrete catecholamines.

Parasympathetic (Craniosacral) Nervous System

Unlike the sympathetic, the parasympathetic system has no somatic distribution and is purely visceral. Four cranial nerves and two sacral roots are involved.

The Spinal Cord and Meninges

This extension of the CNS begins in the medulla oblongata at the foramen magnum and ends at L1/L2. The nerve roots that continue after the spinal cord has terminated at the conus medullaris comprise the cauda equina, while the filum terminale (the extension of pia mater) inserts into the coccyx. As the length of the spinal cord is shorter than the vertebral column, nerve roots arise at increasingly higher levels than their corresponding vertebrae and travel increasingly longer distances within the vertebral column before exiting from their respective vertebral foramina.

The membranes of the cord are termed the meninges and they comprise neuroepithelium of which there are three layers. From outside inwards these are:

• 1.
  

  Dura mater (under which lies the subdural space)

  
  
• 2.
  

  Arachnoid mater (under this is the subarachnoid space containing cerebrospinal fluid)

  
  
• 3.
  

  Pia mater

Both pia and arachnoid are continued out along the spinal nerve roots, while the dura forms a tough sheath for the cord ending at S2, and it extends out over each nerve root blending with its sheath.

The epidural (extradural) space lies between the dura mater and the spinal canal and is filled with fat and vessels (lymphatic and blood).

The spinal cord has neuronal cell bodies in its grey matter, and its external white matter comprises axonal tracts (see Fig. 5.1 ).

Afferent neurones include those for:

• •
  

  Touch and vibration – cell bodies in dorsal root ganglia, tracts in posterior columns

  
  
• •
  

  Pain and temperature – cell bodies in contralateral posterior horn, axons in lateral and anterior spinothalamic tracts

  
  
• •
  

  Proprioception – axons in lateral spinocerebellar tracts

Efferent neurones are motor and pass along the:

• •
  

  Lateral cerebrospinal (or corticospinal) tract. These neurones originate in the motor cortex but the fibres cross before descending in what is also referred to as the crossed pyramidal tract.

  
  
• •
  

  Anterior cerebrospinal (or direct pyramidal) tract. These neurones are uncrossed.

Spinal Nerve Roots and their Plexuses

Each pair of spinal nerves emerges from the vertebral column as illustrated in Fig. 5.1 , and branches proceed to supply the skin in a pattern which can be mapped out diagrammatically (see Fig. 5.3 ).

The Thalami

The two thalami lie laterally in the diencephalon forming the lateral walls of the third ventricle. The internal capsule lies laterally, separating them from the basal ganglia. The thalamus is sensory in function and relays impulses on to the cerebral cortex via the internal capsule. It also connects to the hypothalamus.

The Hypothalamus

The hypothalamus is also in the diencephalon forming the floor of the third ventricle and is concerned with the autonomic nervous system. It contains many cell types, in particular the supraoptic and paraventricular nuclei whose axons connect it to the posterior lobe of the pituitary via the pituitary stalk. It also connects with the basal nuclei caudally and via long axons to the sympathetic and parasympathetic cells in the lateral horns of the spinal cord.

The Pineal Gland

The pineal gland lies posterior to the thalamus at the posterior end of the third ventricle and is innervated by the sympathetic nervous system. It is most active at night, produces melatonin and tends to have an inhibitory effect on other endocrine glands and gonads. It calcifies with age and may be visible on a skull x-ray after the age of 40 years.

The Pituitary Gland

The pituitary gland is composed of two parts; both are derived from ectodermal tissue but of different origins:

• •
  

  The small posterior pituitary is derived from a downgrowth of ectodermal neural plate and these neurones have their cell stations in the hypothalamus. These neurosecretory cells produce oxytocin and antidiuretic hormone (adh)

  
  
• •
  

  The larger anterior pituitary (pars tuberalis) forms from Rathke’s pouch growing up from the roof of the mouth and consists of glandular cells:

  
  
  
  
  • •
    

    chromophobes – account for 50% of the anterior pituitary

    
    
  • •
    

    eosinophilic/acidophilic cells produce growth hormone (GH) and prolactin

    
    
  • •
    

    basophilic cells produce adrenocorticotrophic hormones (ACTH), follicle stimulating hormones (FSH), luteinizing hormones (LH) and thyroid stimulating hormones (TSH)

  
  

This gland occupies the pituitary fossa with:

• •
  

  The diaphragma sellae and optic chiasma above

  
  
• •
  

  The cavernous sinuses laterally

  
  
• •
  

  The body of the sphenoid below

Lymphatic Vessels

The extracellular tissues of the body are constantly gaining fluid and debris (from capillary leakage, cell death, etc.) and the function of the lymphatics is to remove this and return it to the venous circulation. The lymphatic capillaries have the same basic structure as vascular capillaries but their distribution is not uniform throughout the body. The lymphatics in the limbs tend to be superficial, while those of the viscera tend to drain via channels on the posterior abdominal and thoracic walls.

The lymphatic vessels return the lymph to the venous system via two main channels:

• •
  

  The right lymphatic duct drains the right thorax, upper limb, head and neck

  
  
• •
  

  The thoracic duct drains all lymph from the lower half of the body

The pre- and para-aortic lymphatics drain into the cisterna chyli which is an elongated sac-like vessel that lies over the body of L1 and L2 behind the inferior vena cava (IVC) and between the aorta and the azygous vein. It becomes the thoracic duct as it ascends through the diaphragm at the level of T12. It starts on the right side of the oesophagus, but as it ascends through the thorax the thoracic duct passes behind the oesophagus (at T5) to reach its left side, then superiorly it passes over the left subclavian artery and the dome of the left pleura to drain into the confluence of the left subclavian with the left internal jugular veins.

Lymphatics, like blood vessels (and unlike somatic nerves), can cross the midline, but in contrast they pass to and from lymph nodes (afferent and efferent lymphatics) and they comprise an anastomosing low-pressure system.

Lymphatic Tissue

These comprise concentrations of lymphocytes and occur in mucosal and submucosal collections in the gut (e.g. Peyer’s patches in the ileum) as well as in the thymus, the spleen and lymph nodes themselves.

The anatomical clinical importance of this system relates to the drainage patterns of each group of nodes, which is summarised in Table 5.1 , but also described for the individual organs in their relevant regional anatomy sections.

Fetal Circulation and Changes after Birth

The Aorta

The aorta ( Fig. 5.4 ) enters the abdomen behind the diaphragm between its crura at T12 and descends to divide into the common iliac arteries at L4. It has three ventral branches which give rise to the portal circulation, while the other branches are systemic.

The abdominal aorta and its branches.

The Inferior Vena CAVA

The common iliac veins join to form the IVC ( Fig. 5.5 ) behind the right external iliac artery at L5. The IVC ascends through the abdomen on the right of the aorta piercing the central tendon of the diaphragm at T8. It receives:

• •
  

  Segmental lumbar veins

  
  
• •
  

  The right gonadal vein (the left gonadal vein drains into left renal vein)

  
  
• •
  

  The renal and suprarenal veins

  
  
• •
  

  The hepatic veins

  
  
• •
  

  The inferior phrenic veins

The inferior vena cava and its tributaries.

Collateral Venous Drainage Pathways

There is an extensive network of potential collateral circulations which open when thrombosis of the IVC occurs.

Superficial venous channels which can eventually drain to the SVC are:

• •
  

  Epigastric

  
  
• •
  

  Circumflex iliac

  
  
• •
  

  Superficial epigastric and lateral thoracic (via thoracoepigastric vein)

  
  
• •
  

  Internal thoracic

  
  
• •
  

  Posterior intercostals

  
  
• •
  

  External pudendal

  
  
• •
  

  Lumbovertebral

Deep channels which provide deep anastomoses are:

• •
  

  Azygous

  
  
• •
  

  Hemiazygos

  
  
• •
  

  Lumbar

The vertebral venous plexus also provides effective collateral circulation between IVC and SVC.

The Portal Venous Drainage and Portosystemic Venous Anastomoses

The portal venous system drains blood to the liver from the abdominal part of the alimentary canal (except the anus), the spleen, pancreas and gall bladder. The superior and inferior mesenteric veins join the splenic vein behind the pancreas to form the portal vein which carries blood to the liver, which in turn is drained by the hepatic veins which pass into the IVC. This pathway may be obstructed causing portal hypertension and then collaterals open up between the portal and the systemic venous systems:

• •
  

  Lower oesophagus – tributaries of: left gastric with hemiazygos/azygous

  
  
• •
  

  Anal wall – superior rectal with middle and inferior rectal

  
  
• •
  

  Caput medusa – tributary from left branch of portal vein (paraumbilical) with epigastrics

  
  
• •
  

  Retroperitoneal veins of abdominal wall with veins of the ascending colon and the bare area of the liver

  
  
• •
  

  Very rarely a patent ductus venosus

Vertebral Column

Venous drainage from both the internal and the external vertebral plexus drain to regional segmental veins providing potential communication with systems which also drain segmentally. This is a largely valveless system and therefore the spread of malignancy is possible (especially likely from breast, uterus, prostate and thyroid):

• •
  

  Pelvic viscera via the lateral sacral vessels

  
  
• •
  

  Abdomen via the lumbar veins

  
  
• •
  

  Breast via the posterior intercostals

  
  
• •
  

  Neck via the vertebral vein

Types of Joint

• •
  

  Fibrous (bone/fibrous tissue/bone) (e.g. skull sutures although these ossify in later life)

  
  
• •
  

  Cartilaginous:

  
  
  
  
  • •
    

    primary (bone/hyaline cartilage/bone) (e.g. epiphyses or costochondral junctions)

    
    
  • •
    

    secondary (bone/hyaline cartilage/fibrocartilage/hyaline cartilage/bone) – these only occur in the midline (e.g. pubic symphysis, intervertebral joints)

  
  
  
  
• •
  

  Synovial joints that allow movement (e.g. hip joint). The sacroiliac joint is also a synovial joint but atypical in that the movement allowed is extremely limited

The Vertebral Column

The vertebral column has 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal). The five sacral vertebrae are fused to form the sacrum, and the coccygeal components can be variably fused.

There are 31 pairs of spinal nerves whose nerve roots travel variable distances within the vertebral column to exit the spine by passing across the disc of the vertebra above (therefore problems with, e.g. L4 disc will affect L5 nerve root).

The Pelvis

The bony pelvis comprises the sacrum and the os innominatum.

• •
  

  The sacrum is composed of five fused vertebrae (with four sacral foramina). It articulates with the fifth lumbar vertebra above, the coccyx below and the ilium laterally.

  
  
• •
  

  The os innominatum is made up of three bones: ilium, pubis and ischium, which are joined by cartilage in the young, but by bone in adulthood. They meet in a Y-shaped junction in the acetabulum to which they all contribute.

Obstetric Pelvic Definitions and Dimensions

The pelvic inlet is oval being widest transversely, the pelvic mid-cavity is circular, while the outlet is oval being widest anteroposteriorly. Normally, the fetal head enters the pelvis transversely due to the shape of the inlet and subsequent rotation of the fetal head during the descent through the pelvis in labour takes advantage of the bony dimensions, but the rotation itself is caused by the muscular pelvic gutter ( Table 5.2 ).

The Pelvic Outlet

The pelvic outlet is widest from front to back and lies between:

• •
  

  The lower border of the symphysis pubis anteriorly

  
  
• •
  

  The ischial tuberosities laterally

  
  
• •
  

  The tip of the last sacral vertebra posteriorly

The true obstetric conjugate extends from the sacral promontory to the upper border of the pubic symphysis. The diagonal conjugate extends from the sacral promontory to the lower border of the pubic symphysis. The important landmarks of the pelvis are indicated in Figs. 5.6 and 5.7 .

Important landmarks of the pelvis.

Lateral view of the pelvis showing the obstetric conjugates.

Variations in Pelvic Shape

( Fig. 5.8 )

Diagrammatic representation of different pelvic shapes.

• •
  

  Gynaecoid – normal female

  
  
• •
  

  Android – normal male

  
  
• •
  

  Anthropoid – the pelvic brim is longer anteroposteriorly than transversely

  
  
• •
  

  Platypelloid – the pelvic brim is much wider transversely and foreshortened anteroposteriorly

  
  
• •
  

  Rachitic pelvis – typical of rickets and the result of vitamin D deficiency. The sacral promontory projects forwards reducing the anteroposterior diameter

  
  
• •
  

  The contracted pelvis – can be symmetrical associated with a small stature, or asymmetrical due to a variety of disease processes

  
  
• •
  

  A narrow (gothic) subpubic arch foreshortens the effective pelvic outlet because the narrow anterior triangle (the waste space of Morrison) cannot accommodate the fetal head. In such circumstances, more space is required posteriorly to enable vaginal delivery ( Fig. 5.9 ).

  
  

  
  

  
  

  

  
  

  
  

  Illustration of the effect of a narrow subpubic arch and the waste space of Morrison.

  
  

Surface Anatomy

Knowledge of the surface anatomy of the chest can be extremely valuable clinically:

• •
  

  The angle of Louis , which is the ridge produced by the manubriosternal joint, lies at the level of thoracic vertebra T4, but more useful is the site of the second costochondral junction marking the second rib from which subsequent intercostal spaces can be defined. These features also mark the upper limit of the surface markings of the heart.

  
  
• •
  

  The 4th intercostal space marks the dome of the diaphragm and the uppermost edge of the liver.

RIBS

Ribs generate a negative pressure for respiration (−5 to −15 mmHg)

• •
  

  True ribs (ribs 1 to 7) articulate with the sternum

  
  
• •
  

  False ribs (8 to 10) – their costal cartilages articulate with the rib above

  
  
• •
  

  Floating ribs (11 and 12) have muscle attachments only.

The intercostal muscles which comprise three layers – external, internal and innermost – run between neighbouring ribs. The neurovascular bundles run along the lower inside border of each rib (i.e. superiorly in each intercostal space) between the internal and innermost intercostal muscles.

The Diaphragm

This is a musculotendinous structure which separates the thorax from the abdomen. It arises from:

• •
  

  The xiphisternum

  
  
• •
  

  The lower six ribs and their costal cartilages

  
  
• •
  

  The medial and lateral arcuate ligaments

  
  
• •
  

  The first three lumbar vertebrae on the right/first two on the left (right and left crus) and fuses into a trifoliate central tendon below the pericardium.

The motor nerve supply is from the phrenic nerve (C3-5), and sensory supply is from the lower six intercostal nerves. The blood supply comes from the lower intercostal arteries superiorly, and the phrenic arteries (branches of aorta) inferiorly.

The three main openings in the diaphragm and their vertebral levels are as follows:

• 1.
  

  The aortic opening at the level of T12 transmits the aorta with the thoracic duct and the azygous vein (from left to right).

  
  
• 2.
  

  The oesophageal opening which passes through the right crus of the diaphragm at the level of T10, and also transmits the left gastric artery and both vagi.

  
  
• 3.
  

  The IVC runs through the central tendon at the level of T8 together with the right phrenic nerve.

Other structures which penetrate the diaphragm include the greater and lesser splanchnic nerves and the sympathetic chain.

Surface Anatomy

• 
  

  The transpyloric plane is an important landmark because of its anatomical relationships. It lies a patient hand-breadth below their xiphoid and is at the level of the first lumbar vertebra and the ninth costal cartilage and marks the termination of the spinal cord. Structures in this plane include the:

  
  
  
  
  • •
    

    Pylorus of stomach

    
    
  • •
    

    Duodenojejunal flexure

    
    
  • •
    

    Fundus of the gall bladder

    
    
  • •
    

    Renal hila

    
    
  • •
    

    Neck of pancreas.

  
  
  
  
• 
  

  The subcostal plane joins the lowest costal margins on both sides and marks the tenth rib and the level of the third lumbar vertebra.

  
  
• 
  

  The plane of the iliac crests marks the bifurcation of the abdominal aorta at the level of the fourth lumbar vertebra.

  
  
• 
  

  The umbilicus is an inconsistent landmark, but in the slim adult lies at the lower part of the third lumbar vertebra, the third part of the duodenum and the origin of the inferior mesenteric artery.

  
  
• 
  

  McBurney’s point lies two-thirds laterally along a line drawn from the umbilicus to the anterior superior iliac spine. It guides the positioning for an appendicectomy incision (non-pregnant) and needle entry for a paracentesis must pass lateral to this point to avoid the inferior epigastric vessels.

  
  
• 
  

  Langer’s (cleavage or tension) lines of the skin result from the collagen fibre arrangements, and incisions placed along these heal with minimum scarring. On the anterior abdominal wall, they lie transversely.

  
  
• 
  

  The dermatomes of the anterior abdominal wall are relevant in situations of referred pain, and in the assessment of regional anaesthesia. They are illustrated in Fig. 5.3 .

The Abdominal Wall

This is essentially muscular, maintaining tone and imposing a positive intra-abdominal pressure (+5 mmHg), despite respiration.

• •
  

  A muscular cylinder joins two bony rings (costal margin and pelvis) which are joined/splinted apart by the vertebral column

  
  
• •
  

  The superior bony ring is closed off by the muscular diaphragm

  
  
• •
  

  The inferior ring is closed off by the muscular pelvic ‘diaphragm’/pelvic floor.

Clinical Application

At laparoscopy the intra-abdominal pressure should always be noted together with its fluctuation with respirations. The Veress needle and trocar should be angled inferiorly at 45 degrees from the umbilicus in the midline, thus avoiding the aorta (which has already terminated) and the iliac vessels (which have diverged).

The muscles of the abdominal wall can be thought of as straight (anterior and posterior) and flat (lateral) muscles ( Fig. 5.11 ).

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