autonomic nervous system

  • Autonomic nervous system= self-governing 

  • Visceral nervous system= control over internal organs 

  • Subconscious control (e.g. heart rate, pupil diameter, blood vessel contractility, hormonal secretions, gastrointestinal motility) 

  • Homeostasis 

Autonomic nervous system subdivision  

  • Somatic nervous system= conscious control of skeletal muscle  

  • Autonomic nervous system= unconscious control of cardiac, smooth muscle and glands 

  • Parasympathetic/sympathetic division is anatomical (based on CNS location of cell body of preganglionic nerve) 

Organisation of somatic nervous system  

  • Axons of somatic nervous system have a single myelinated axon extending from CNS to the effector (skeletal muscle) 

Organisation of the autonomic nervous system  

  • Axons of the autonomic nervous system have: a two-neuron chain 

  1. The preganglionic neuron extends to ganglion  

  2. The postganglionic neuron extends from ganglion to an effector organ  

  • Ganglion= cell bodies of many peripheral autonomic neurones occur in clusters and form swellings on nerve trunks  

Motor (efferent) pathways of ANS  

Neurotransmitter of the autonomic nervous system  

  • Preganglionic nerves= Acetylcholine (ACh) cholinergic transmission 

  • Postganglionic nerves= Acetylcholine (ACh) cholinergic transmission, Noradrenaline (NA) noradgerenic transmission  

  • Pre and postganglionic nerves= non-adrenergic non-cholinergic neurotransmission (NANC), co-transmitters= neurotransmitters and neuromodulators  

Autonomic nervous system  

  • Conveys all outputs from the central nervous system (CNS) to rest of the body except motor innervation of skeletal muscle  

  • Regulates mostly non-voluntary processes in homeostasis : heartbeat, contraction of smooth muscle of various organs and blood vessels, exocrine (and some endocrine) secretions, energy metabolism (e.g. liver and skeletal muscle) 

  • Sympathetic and parasympathetic systems function separately and have opposing effects in some states but not other  

  • Sympathetic= increases in stress (flight-or-fight response e.g. pupils dialted, hair erected, increased blood sugars) 

  • Parasympathetic= predominates during satiation and repose ( rest and digest) 

  • Both systems exert continuous physiological control under normal conditions, at neither extreme- cooperate 

  • Influenced by sensory information via control centres in brain  

Parasympathetic nervous system  

  • The parasympathetic ganglia lie near to their effector organs or within their walls and are often very small 

  • Preganglionic axons emerge from cranial and sacral regions of CNS and from synapses in ganglia near to/adjacent to/within effector tissues 

  • Sacral nerves= form pelvic plexuses containing scattered ganglia and some ganglia within tissue- pelvic and abdominal viscera  

  • Preganglionic fibres are long and postganglionic fibres are short

  •   Also known as craniosacral division  

  • Role in keeping body energy use low: 

  • Predominates in individuals in relaxed states 

  • Blood pressure, heart rate and respiratory rates low 

  • Gastrointestinal activity is high  

  • Rest-and-digest 

  • Inhibitory effect on many tissues and organs  

Organisation 

Parasympathetic neuroeffector pathway  

  • Acetylcholine (ACh) 

Neurotransmitter synthesis, action and degradation at cholinergic synapse  

  • ACh acts post-synaptically on nicotinic receptors at ganglionic synapse (also present at neuromuscular junction in striated muscle) 

  • ACh acts post-junctionally on muscarinic receptors at the effector cell (e.g. heart) 

  Parasympathetic control of the heart 

  • Postganglionic nerves release ACh which acts at M2 muscarinic receptors to Decrease heart function: 

  • Sinoatrial (SA) node (pacemaker)=> decreases heart rate (bradycardia) 

  • Atrial muscle=> decrease contractility (force and duration) 

  • Atrioventricular (AV) node (conducts electrical impulses)=> decreased rate of conduction 

  • Cardiac arrest= atropine (muscarinic antagonist) used for resuscitation 

  • Parasympathetic control of pupil diameter  

    • Sympathetic nervous system activation= radial muscles contract= pupil dilation/mydriasis 

    • Parasympathetic nervous system activation= circular muscle contracts= pupil constriction/miosis (due to acetylcholine at M3 receptors) 

     

    Parasympathetic control of focusing 

    • Ciliary muscle relaxes= suspensory ligaments contract and lens flattened allowing for distance focus  

    • Ciliary muscle contracts= due to ACh action at M3 receptors causing suspensory ligaments to relax causing lens to get fatter allowing for near vision/accomodation 

    • Cycloplegia= paralysis of ciliary muscle so loss of accomodation  

    Clinical modulation of pupil diameter  

     

    • Mydriatic drugs= eye inspection and surgery 

    • Muscarinic antagonist= passive dilation/mydriasis (loss of drive to constrictor pupillae) and cycloplegia (loss of accommodation to ciliary muscle) 

    • Antiglaucoma and miotic drugs= glaucoma and rapid pupil closure (surgery) 

    • Muscarinic agonist 

    • Pressure in eye builds up due to production of fluid (aqueous humour) and lack of drainage through canal of Schlemm 

    • Dilated pupil= iris occludes canal  

    • Muscarinic agonist constricts pupil/miosis=> moves away from canal of Schlemm to increase outflow and decrease intraocular pressure  

     

    • Anticholinesterases= enhancement of parasympathetic: miosis and accommodation (due to contraction of ciliary muscle)  

    Parasympathetic co-transmission  

     

     

Sympathetic nervous system  

  • Ganglia form a chain which is located immediately outside of the vertebral column  

  • Preganglionic sympathetic axons entering sympathetic chains terminate in: 

  1. Paravertebral sympathetic chains= both pre and postganglionic axons may run for some distance up or down the sympathetic chain before forming a synapse 

  2. Prevertebral ganglia/plexuses= in the abdominal cavity, 3 main: coeliac ganglion, superior mesenteric ganglion, hypogastric ganglion 

  3. Adrenal medulla=some preganglionic fibres emerging from 10th and 11th thoracic segments run in greater splanchnic nerve and terminate on chromaffin cells in medullae of adrenal glands (chromaffin cells analogous to sympathetic ganglia cells 

  • Sympathetic preganglionic fibres are short and postganglionic fibres are relatively long  

  • Preganglionic sympathetic axons entering sympathetic chains terminate in: 

  1. Paravertebral sympathetic chain= pre- and postganglionic axons may run for some distance up or down the sympathetic chain before forming synapse/emerging 

  2. Prevertebral ganglia/plexuses= in the abdominal cavity, 3 main: coeliac ganglion, superior mesenteric ganglion, hypogastric/inferior mesenteric ganglion  

  3. Adrenal medulla= some preganglionic fibres emerging from 10th and 11th thoracic segments run in greater splanchnic nerve and terminate on chromaffin cells in the medullae of adrenal glands (chromaffin cells analogous to sympathetic ganglia cells) 

  • Sympathetic preganglionic fibres are short and postganglionic fibres are long  

Most sympathetic neuroeffector pathway  

  • Acetylcholine (ACh) and noradrenaline (NA) 

Other sympathetic neuroeffector pathways 

Sympathetic neurotransmitter release  

Sympathetic control of pupil diameter 

  • Radial muscle/dilator pupillae= smooth muscle arranged radially  

  • Circular muscle/constrictor pupillae= smooth muscle arranged concentrically, regulated by parasympathetic nervous system  

 

  • Sympathetic nerve action=> radial muscle contraction=> pupil dilation/ MYDRIASIS 

  1. Noradrenaline (NA)=> alpha receptors on radial muscle=> radial muscle contraction=> mydriasis (dilation)  

Clinical use of sympathetic drugs in the eye 

  • Mydriatic drugs= eye inspection and surgery  

  • Sympathomimetic= mydriasis due to contraction of dilator pupillae  

  • Antiglaucoma drugs 

  • Sympathomimetic 

  • in glaucoma pressure builds up due to production of aqueous humour and lack of drainage through canal of Schlemm  

  • Vasoconstriction caused by sympathomimetic impairs secretion of aqueous humour from ciliary body and facilitates reabsorption from canal of Schlemm easing the pressure 

Sympathetic cotransmission 

  • NA + NPY + ATP