autonomic nervous system

what are the components of the ANS?

• sympathetic

• parasympathetic

• enteric nervous system

what is the ANS?

• autonomic = self-governing

  • a collection of motor neurones (efferents) in the central & peripheral nervous system that innervate smooth muscle, cardiac muscle and glands to regulate their activity

  • it plays a key role in homeostasis as it helps regulate cardiac function, blood pressure, body temperature, energy balance…

what is the enteric nervous system?

• extensive neural network in walls of the gut

• consists mainly of the myenteric plexus and the submucosal plexus

• works largely autonomously to control the activity of the gastrointestinal tract

  • bidirectional communication with CNS

  • influence from sympathetic & parasympathetic inputs

what is the difference between the sympathetic & parasympathetic nervous systems?

• sympathetic = fight/flight

• parasympathetic = rest & digest

  • dominates at rest, but sympathetic still performs important regulatory functions (e.g. blood pressure control)

what is the ANS system an example of?

• visceromotor system - acts to regulate the env.

  • smooth muscle/ involuntary

how many spinal nerves are there?

• 31

how are autonomic nerves organised?

CNS - preganglionic fibre - synapse in ganglion - postganglionic fibre - target (effector)

describe the PNS.

• originates in the brain stem (in cranium) and sacral spinal cord

• sacral

  • preganglionic neurones do not join with the spinal nerve

  • innervate bladder. urinary tract, reproductive tract

• cranial nerves (brainstem)

what are the cranial nerves?

• preganglionic parasympathetic axons travel with the somatic fibres in cranial nerves & supply.

  • vagus (X) - smooth muscle & glands of the throat

  • occulomotor (III) - pupil size, lens shape

  • facial (VII) - salivary glands

  • glossopharyngeal (XI) - salivary glands

describe the anatomy of parasympathetic neurones.

• preganglionic fibres = long and unbranched

• innervates ganglia located on or close to their target organs

• postganglionic neurones have short projections to innervate their target organs/tissue

describe the sympathetic nervous system.

• originates thoracic & lumbar spinal cord

  • preganglionic neurones are typically short & unbranched

  • ganglia located near vertebral column & spinal cord

  • postganglionic neurones have long, unbranched axons that innervate target organs

  • many travel with spinal nerves

  • outflow is from thoracic & lumbar spinal cord (T1 - L2/3)

  • sympathetic afferents project throughout the body

describe the sympathetic chain & collateral ganglia

• sympathetic preganglionic neurones arise from the lateral horn of the spinal grey matter

• their axons leave the spinal cord through the ventral root and enter the sympathetic chain

• then they do this:

  • synapse with postganglionic neurones in the sympathetic chain at the same level

  • travel up/down the sympathetic chain before synapsing in a ganglion

  • leave the sympathetic chain to innervate a collateral ganglion

  • leave the sympathetic chain to innervate the adrenal gland

what is the sympathetic chain?

extends ganglia into the neck and sacrum, providing sympathetic innervation to broader more rostral and caudal regions

what does the adrenal medulla contain?

chromaffin cells

describe the release of adrenaline & noradrenaline from chromaffin cells.

• preganglionic fibres pass through the sympathetic chain

• ganglionic synaptic transmission:

  • ACh release from preganglionic fibres

  • activation of nAChRs on chromaffin cells

• chromaffin cells secrete catecholamines into capillaries in adrenal gland

  • 80% adrenaline

  • 20% noradrenaline

• circulating Adr/NA act as hormones

describe neurotransmission in preganglionic neurones.

ACh released preganglionic neurones of BOTH the sympathetic & parasympathetic systems

• transmission occurs at the synapses in the ganglia

describe neurotransmission in postganglionic neurones

ACh released by the parasympathetic postganglionic neurones

• NA released by sympathetic postganglionic neurones (2 exceptions: adrenal medulla, sweat glands)

• volume transmission occurs at target/effector tissues (no true synapse is present)

how is volume transmitted from axon varicosities at target tissue?

• target cells of ANS do not specialised receptor sites - neurotransmitter release occurs at multiple sites along axon

• neurotransmitter may diffuse a large distance before reaching receptors - more diffuse control than the somatic motor system (less spatial precision, greater range of effect)

what are cholinergic receptors?

• Nicotine Acetylcholine Receptors = nAChR - ionotropic receptor, cation channel, excitatory, depolarise target cell

  • activated by nicotine, inhibited by curare, hexamethonium

• Muscarinic Acetylcholine Receptors = nAChR - metabotropic receptor, g-protein coupled receptor, effect on target cell depends on coupling of receptor

  • activated by muscarine, inhibited by atropine

what are muscarinic receptor subtypes?

• M1 - generally excitatory - stimulation of vagus nerve causes gastric acid secretion

• M2 - generally inhibitory - in heart and on nerve terminals of neurones

  • e.g. bradycardia

• M3 - generally excitatory - in secretory glands and smooth muscle

  • causes sweat production

what are adrenergic receptors?

• 2 major classes of adrenergic receptors - both metabotropic

  • alpha - usually excitatory/stimulators to target cell - a1 agonists contract vascular smooth muscle

  • beta - either excitatory or inhibitory

    • b1 - excitatory - stimulates cardiac muscle, b blockers used to treat hypertension

    • b2 - inhibitory - relaxes bronchial smooth muscle, adrenergic agonists used to treat asthma

    • b3 - excitatory effect on adipose tissue

how does the ANS regulate the heart?

• heart rate (chonotropic)

  • sympathetic- increase HR due to effect on SA node

  • parasympathetic- decrease

• conduction rate (dromotropic)

  • sympathetic - increase conduction rate due to effect on AV node

  • parasympathetic- decrease

• contractility (inotropic)

  • sympathetic- large increase in force of ventricle contraction

  • parasympathetic- small decrease

how are pupils regulated?

• constriction - parasympathetic stimulation of circular muscle

• dilation - sympathetic stimulation of radial muscle

  • oculomotor nerve (III)

how is the ANS regulated by the brain?

• hypothalamus is a core element of homeostatic physiological and behavioural processes

• recognition of threat in forebrain areas - cortex, limbic system - behavioural responses preparation begins

• descending signals to ANS initiate appropriate autonomic responses

• coordinated sympathetic & parasympathetic activity involved

• behaviour and physiological responses are aligned to promote survival