Autonomic Nervous System PDF

Autonomic nervous system: [Organisation of the nervous system: ] A diagram of nervous system Description automatically generated [The autonomic nervous system (ANS) contributes to homeostasis - function:] Definition of homeostasis -- the physiological process which the internal systems of the bo...

Autonomic nervous system: [Organisation of the nervous system: ] A diagram of nervous system Description automatically generated [The autonomic nervous system (ANS) contributes to homeostasis - function:] Definition of homeostasis -- the physiological process which the internal systems of the body are maintained at equilibrium despite variations in external conditions. Examples of parameters regulated by homeostasis: - Body temperature - Blood pressure - Hydration levels - Acid-base balance The ANS operates without us being aware of it. The ANS is involuntary, and we have no direct conscious control over its actions. [The ANS is divided into three divisions:] 1. Sympathetic (active) - "fight or flight for active/stressful situations i.e: - increased heart rate - dilation of pupils (let more light into the eye) and bronchioles (more air into the lungs -- more oxygen in the blood) - blood flow diverted away from skin and viscera to striated and cardiac muscle 2. Parasympathetic (passive) -- preparing the body for maintenance activities/rest i.e.: - Decreased heart rate - Bronchiolar and pupillary constriction - Blood directed to viscera - Increased gut mobility -- peristalsis + secretion of mucus + digestive enzymes. 3. Enteric - Embedded in the lining of the gastrointestinal tract - Regulates gut motility and secretion of mucous and digestive enzymes [The sympathetic nervous system is a 2-neuron relay:] - 2-neurones = preganglionic + postganglionic neurone - Preganglionic neurones are found in the spinal cord -- between levels T1 and L2 - Preganglionic cell bodies are found in the intermediolateral horn - Preganglionic neuron axons are **short**, lightly myelinated out of the spinal cord and synapse with the post ganglionic neurone. - In two types of ganglia -- most commonly in the paravertebral ganglia (sympathetic chain of ganglia that run parallel to the spinal cord) or in the prevertebral ganglia -- found in the abdomen. - Preganglionic neurone has a cell body in the spinal cord and releases acetylcholine **(Ach)** as a neurotransmitter. - Postganglionic neuron axons are **long** and non-myelinated -- project their targets i.e. smooth muscle glands and cardiac muscle. - 90% of postganglionic neurons release noradrenaline **(NA)** as a neurotransmitter - 10% use acetylcholine as a neurotransmitter (these mainly project to sweat glands and piloerector smooth muscle in the skin) - Piloerector muscles -- attached to the hairs on the skin and when they contract they raise the hairs.  [The parasympathetic nervous system is also a 2-neuron relay:] Much of the parasympathetic nervous system is associated with cranial nerves - Made up of preganglionic neurones in the hindbrain + spinal cord synapse with a postganglionic neurone which is outside the brain and spinal cord. - Most parasympathetic preganglionic neurons are located in the visceral efferent motor nuclei of cranial nerves: III (midbrain), VII (pons), IX (pons/medulla) and X (medulla) - III = oculomotor nerve - VII = facial nerve - IX = pharyngeal nerve - X = vagus nerve - Parasympathetic preganglionic neurons are also located in the lateral grey matter of the sacral spinal cord (S2-S4) - Preganglionic parasympathetic neurons have long lightly myelinated axons that project into the peripheral tissues make synaptic contact with a post ganglionic neurone therefore synthesising and releasing acetylcholine (Ach) as a neurotransmitter - Some postganglionic parasympathetic neurons are in discrete ganglia close to their target organs (cranial nerves III, VII and IX) - Differ from the sympathetic nervous system where most of the ganglia are close to the spinal cord. - Many postganglionic parasympathetic neurons are in loose plexi (not in the ganglia) imbedded in their target organs (CN X and sacral outflow) - Postganglionic parasympathetic neurons have short non-myelinated axons that release Ach as a neurotransmitter - Usually travel a short distance A diagram of a bone Description automatically generated [Preganglionic sympathetic axons take multiple routes from the spinal cord to reach postganglionic neurons.]  - Preganglionic sympathetic axons emerge from the spinal cord and can take several routes to reach the ganglia that contains neurones that they are going to synapse with. - Simplest route = through intermedio-lateral horn (IML): 1. Axon projection through the ventral rootlets. 2. Ventral root 3. White ramus -- short, myelinated fibre tract 4. Synaptic contact with a post ganglion in a sympathetic chain (paravertebral sympathetic ganglia) - The post ganglion is at the same axial level as the preganglion. 5. Axon projects through the grey ramus 6. Spinal nerve -- travels to the target of the post ganglion after this. - Alternative route: 1. Ventral rootlets 2. Ventral root 3. White ramus 4. Instead of synapsing with the first parasympathetic ganglia, the axon travels **up or down to make synaptic contact with a post ganglionic neurone in the caudal or rostral level.** - Alternative route (2): Prevertebral ganglia 1. Ventral rootlets 2. Ventral root 3. White ramus 4. Splanchnic nerve 5. Sympathetic chain ganglia 6. Project into the prevertebral sympathetic ganglia -- target the abdomen/pelvis - Three types: celiac, superior and inferior mesenteric ganglia - Dorsal root ganglion visceral sensory neurones innervate most of the organs/viscera 1. Travel through the sympathetic nerve in the abdomen 2. Splanchnic nerve 3. White ramus 4. Dorsal root ganglion [Sympathetic actions] A diagram of the human body Description automatically generated - The sympathetic chain/trunk extends below the upper lumbar levels -- even though the preganglionic neurones are only found in T1 -- L2. - Sympathetic chain on both sides of the spinal cord coalesces -- form a loop through the coccygeal sympathetic ganglia. - Some cervical level sympathetic ganglia is part of the sympathetic chain -- superior, medial and inferior cervical ganglia. - Major target of the sympathetic system is the skin -- covers the entire body i.e. piloerector muscles, sweat glands, blood vessels and capillary beds (vasoconstriction) - Activity of post-ganglionic sympathetic neurones in the cervical ganglion leads to **pupil dilation** in the **eye** and **eyelid retraction** -- allows for more light in the eye which is important in a flight or fight response - Activity of post-ganglionic sympathetic neurones **increases** **saliva viscosity** -- doesn't change the volume of saliva produced - Heart = **increased heart rate, force of contraction and dilation of the coronary arteries** -- better blood supply to the heart which is working very hard. - Adrenal medulla -- stimulates the release of adrenaline + noradrenaline into the blood stream, large scale which occurs in the flight or fight response - Innervated by preganglionic sympathetic neurones. [Parasympathetic actions:]  - Cranial nerve III = ciliary ganglion - Cranial nerve VII = pterygopalatine ganglion (lacrimal gland) + submandibular ganglion (submandibular salivary glands). - Cranial nerve IX = otic ganglion (parotid salivary gland) - Cranial X = long nerve that travels throughout the body and the preganglionic nerve travels through various plexi to innervate postganglionic nerves that are imbedded in the site of target tissue. - Pelvic nerves = sacral outflow that innervates the kidney, bladder, rectum and the sexual organs. [ANS neurotransmitters] -- neurotransmitters that the neurones use. A diagram of parasympathetics Description automatically generated The effects of adrenaline/NA on sympathetic targets are dependent upon which adrenergic receptor types targets express: - Adrenergic receptors are G-protein-coupled receptors (GPCRs) -- induce slow acting and prolonged postsynaptic responses after adrenaline/NA binding - Different receptor associated G protein (G~q~, G~i~, G~s~) induce the activation of different intracellular signalling pathways after ligand binding. [Five main adrenergic receptor types:]  - Phospholipase C = convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate - Gs -- adrenergic receptors bind to their ligands which activates adenylate cyclase + increase cAMP production. - Lipolysis = breakdown of lipids in adipose tissue. - Observed that depending on which receptor binding occurs to, can result in different effects -- same neurotransmitter. [Multiple receptors also mediate the actions of acetylcholine in the ANS:] A diagram of a protein source Description automatically generated [Organization of the enteric nervous system (ENS):] Cross section of the GI tract:  - Coordinated activity or contraction of the longitudinal and circular smooth muscle layers that enable peristalsis of the gut. - Two nerve plexi: - Myenteric plexus -- found between the circular + longitudinal muscle layers - Submucosal plexus -- found in the submucosa [Physiology of the ENS:] - The myenteric and submucosal plexi contain sensory neurons, interneurons and autonomic motor neurons - Sensory neurons in the myenteric plexus measure the degree of stretch in smooth muscles -- longitudinal + smooth muscle - Sensory neurons in the submucosal plexus measure the chemical composition of the lumen - Excitatory and inhibitory visceromotor neurons (autonomic motor neurones) work together to coordinate peristaltic movement of the gut by regulating contraction and relaxation of the gut. - The submucosal plexus has secretomotor neurons (autonomic motor neurones) promote mucous release into the lumen - Interneurons connect sensory and motor neurons - The enteric nervous system functions largely independently of the brain but has regulatory input from parasympathetic *(vagus nerve)* and sympathetic *(prevertebral ganglia)* neurons - More than 30 neurotransmitters are used in the ENS (eg. Ach, serotonin, dopamine, neuropeptides and NO) A diagram of a nervous system Description automatically generated [Regulation of autonomic nervous system activity:] - Sensory feedback from the target tissues/organs innervated by autonomic neurons plays a major role in regulating the activity of the autonomic nervous system - Visceral sensory neurons innervate the target tissues of the ANS - visceral sensory DRG neurons -- found in dorsal root ganglion - visceral sensory neurons of the petrosal ganglion (associated with cranial nerve IX) - visceral sensory neurons of the nodose ganglion (associated with cranial nerve X) [Most visceral sensory information is sent to the nucleus of the solitary tract in the medulla]  - The centrally projecting axons of petrosal (CN IX) and nodose (CN X) sensory neurons (afferents) directly innervate the visceral sensory division of the nucleus of the solitary tract (NST) in the medulla - NST is divided into two regions: - Upper region is associated with taste sensation - Lower region is associated with processing visceral sensory information. - Visceral sensory neurons associated with spinal nerves (DRG neurons) indirectly innervate the NST - DRG neurones project axons into deeper layers of the dorsal horn where they make synaptic connection with projection interneurons cross the midline travel up the spinal cord through the medulla + innervates the NST. [Visceral sensory input contributes to both local autonomic reflexes and widespread changes in ANS activity.] A diagram of a structure Description automatically generated - Sensory feedback is provided by visceral sensory DRG neurones and visceral sensory nerves from the cranial nerves IX and X. - Information is sent to the nucleus of the solitary tract which relays information after processing to the preganglionic ANS neurones in the brainstem and spinal cord -- NST modulates the activity - Leads to a change in output which means a change in the excitability of the post ganglionic neurones in the autonomic ganglion plexi -- modules the impact the post ganglionic neurones have on the target tissues. - NST can also relay information to higher centres of the autonomic nervous system which are the brainstem and the nuclei of the reticular formation which can later change the output to preganglionic + postganglionic neurones. - NST can also send sensory information to higher brain areas -- separate system to above.