M1i / 20B05 / 15A20: Anatomy of Sympathetic Nervous System

20B05: Exam Report

Describe the anatomy (70% marks) and effects (30% marks) of the sympathetic nervous system

51% of candidates passed this question.

Most candidates had a suitable structure to their answers, those without a clear organisation of thought tended to gain fewer marks. In many cases incorrect information or limited detail, particularly around the anatomical organisation prevented higher marks.

15A20: Exam Report

Describe the anatomy of the sympathetic nervous system.

25% of candidates passed this question.

A definition of the sympathetic system, followed by a systematic description of the central sympathetic centres; what happens at the spinal cord; the anatomy of the pre and post ganglionic fibres would have been awarded with a pass mark. Additional  information about the sympathetic ganglia and the neurotransmitters involved would have rounded off a good answer.

Many answers lacked anatomical detail and described the actions (function) of the sympathetic system which was not asked for.

Most answers lacked any structure. The most common reason for not passing this question was that significant sections of the anatomy from central to peripheral were not mentioned. Most had a simple sketch understanding of the question asked but could not add enough of the next layer to be awarded a pass mark.

M1i / 20B05 / 15A20: Describe the anatomy of the sympathetic nervous system

Definition:

  • SNS is a division of the ANS which is activated in fight-or-flight  
  • ANS is the efferent pathway controlling the action of involuntary organs & tissues in order to maintain homeostasis.
Nervous System

Sympathetic Anatomy

Preganglionic Neurons

  • Myelinated slow conducting B fibres
  • Cell bodies in intermediolateral horn
  • Arise from T1 – L2
  • Preganglionic fibre leaves spinal cord via ventral root
  • Passes via white ramus into sympathetic chain
  • Sympathetic chain = 22 pairs of ganglia
  • From sympathetic chain:
    1. Synapse with post-ganglionic cell bodies at level they entered
    2. Pass up/down to another level of symp. chain and synapse with that post-ganglionic cell body
    3. Pass through the symp. chain & synapse with a collateral ganglion

Postganglionic Fibres

  • Unmyelinated C fibres
  • Either:
    1. Runs along blood vessels to supply head, neck, thorax & viscera
    2. Re-enters spinal n’s via grey rami to supply vessels, sweat glands, piloerector m.
    3. From collateral ganglia synapse close to viscera or in adrenal medulla

NOTE: adrenal medulla is essentially a symp. ganglion in which postganglionic cells have lost their axons & secetes adrenaline, DA, NA directly into the bloodstream

Sympathetic Neurons System Function

The ANS controls the visceral function of the body & is under involuntary control.

The SNS maintains homeostasis during stress.

SNS
    1. Sympathetic neurons originate from preganglionic neurons in grey matter of spinal cord
    2. From T1 – L2
    3. Exists spinal cord by ventral roots
    4. Leave spinal nerve as white rami
    5. Synapse with postganglionic neuron
    6. Postganglionic fibres leave as grey rami & join spinal/visceral nerves to innervate target organ
    7. Sympathetic chain divides into 4 segments:

Cervical (3 ganglia) →  head, neck, thorax

Thoracic (T1 – T5) →  aortic, cardiac, pulmonary plexi

                 (T5 – T9) →  greater & lesser splanchnic n., renal plexi

Lumbar →  coeliac plexus

Pelvic →  pelvic plexus

Characteristic

Sympathetic

Origin

Thoracolumbar

Fibre length

Short preganglionic, long postganglionic

Preganglionic fibre

Myelinated B fibres

Ganglia location

Close to CNS

Preganglionic NT

Ach

Ganglia receptors

Nicotinic

Postganglionic fibre

Unmyelinated C fibre

Postganglionic NT

NA (most), ACh (some)

Postganglionic receptors

Adrenergic or muscarinic

Function

Prepares for stress & intense muscle activity

Organ

Action

Receptor

Heart

+ inotropy

+ chronotropy

+ chonotropy

β1

β1 + β2

Arteries

VC

VD

α1

β2

Veins

VC

VD

α1

β2

Lung

Bronchial smooth m. relaxation

β2

GI

↓motility

Sphincter contraction

β2

α

Liver

Glycogenolysis

Gluconeogenesis

Lipolysis

β2 + α

Kidney

Renin release

β1

Bladder

Detrusor relaxation

Sphincter contraction

Β2

α

Uterus

Relaxation

Contraction

Β2

α1

Eye

Pupil dilation

Ciliary relaxation

α1

Β2