University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

304

Pages:

3
Author:

Mariam A. A.

SNS SYSTEMS • • • Preganglionic neurons located in lateral horn of gray matter → 1st thoracic (T1) to 3rd lumbar (L3) spinal cord Preganglionic neurons arranged in rostral-to-caudal viscerotopic organisation ACh: excitatory neurotransmitter of preganglionic neurons Mechanism: • • • • Preganglionic axons exit spinal cord via ventral root of a spinal verve Separates from ventral root and forms a white ramus Enters a paravertebral ganglia (sympathetic chain) Synapses onto postganglionic neurons at the ganglion on the same level, at different level, or pass through without synapsing to form a splanchnic nerve which synapses with postganglionic neurons within the prevertebral ganglia Prevertebral/collateral ganglia • • • • Situated between sympathetic trunk (paravertebral ganglia) and target organ Not paired or segmentally arranged Occur only in abdomen and pelvis All lie anterior to vertebral column and mostly on the abdominal aorta Autonomic plexuses • • Complex, interconnected neural networks formed by ganglia and axons (sympathetic and parasympathetic) found close to major arteries Named according to organs they supply/blood vessels along which they are found e.g. cardiac plexus Adrenal medulla • • • • Unique variant of sympathetic nervous system Modified sympathetic ganglion whose cells are similar to sympathetic postganglionic neurons Preganglionic autonomic nerve fibres synapse directly in the gland Secrets adrenaline and noradrenaline directly into bloodstream Postganglionic neurons • • • Axons of postganglionic neurons are unmyelinated – exit ganglion via grey ramus Long and travel to tissues via peripheral nerves or major blood vessels Noradrenaline (excitatory transmitter) supplies target tissue except sweat glands adrenal medulla • ACh is excitatory neurotransmitter of postganglionic sudomotor neurons – innervate sweat glands and adrenal medulla (secrets adrenaline) Fight or flight response • • • • • • • ↑ rate and force of cardiac contraction ↑ vasoconstriction of inactive muscle → ↑ BP shunted to skeletal and cardiac muscle ↑ sweat release ↑ pupil dilation ↑ bronchodilation (relaxes airways) ↑ glucose (fuel for body) ↑ adrenaline SNS working in the body • • • • Can engage different components together or separately Exercise: increase in cardiac sympathetic drive, increase in muscle vasoconstrictor drive, decrease cutaneous vasoconstrictor drive, increase in sweat release Arousal: co-activation of cutaneous vasoconstrictor, sudomotor neurons and cardiac sympathetic neurons, long term stress can increase muscle vasoconstrictor drive, leading to hypertension When things go wrong – cardiovascular and cardiorespiratory disease e.g. heart failure, chronic obstructive pulmonary disease, obstructive sleep apnoea syndrome, renovascular hypertension, pregnancy-induced hypertension, essential hypertension, metabolic syndrome SNS and spinal cord injury • Muscle, cardiac, renal, splanchnic and cutaneous vasoconstrictor outflow is abolished below lesion in spinal cord injury, resulting in postural (orthostatic) hypotension and compromised thermoregulation • Sensory inputs below lesion can reflexly excite vasoconstrictor neurons through segmental reflex circuits, leading to uncontrolled increases in blood pressure Autonomic dysreflexia: SCI lesions T6 and above – life threatening, medical emergency