Autonomic Nervous System Flashcards
How do efferent autonomic fibres and skeletal fibres differ?
skeletal pass directly to effector, while autonomic pass to autonomic ganglia outside the CNS - giving rise to preganglionic and postganglionic fibers.
Sympathetic vs Parasympathetic
Sympathetic prepares the body for activity while para actions are more discrete, restorative in general e.g. digestion, slowing of the heart rate. PARAchute
Describe the structure of the sympathetic nervous system
Originates in the thoracic and lumbar spinal cord as sympathetic preganglionic neurons, the axons of which travel out the ventral horns alongside somatic motor efferents, to sympathetic ganglia via the white rami communicantes. Here they synapse with sympathetic neurons of which the post-ganglionic axons project to effectors via the gray rami communicantes and segmental spinal nerves.
Describe the organisation of the sympathetic ganglia
they’re distributed segmentally either side of the spine, connected by longitudinal fibres to form sympathetic trunks. Preganglionic fibres to abdominal organs join into splanchnic nerves which synapse at the celiac, superior and inferior mesenteric ganglia.
How is sympathetic innervation of the adrenal medulla different?
The thoracic sympathetic preganglionic fibres pass via the splanchnic nerves to the adrenal medulla and synapse directly with chromaffin cells, which are homologous with postganglionic neurons, sharing properties like AP generation and catecholamine secretion.
Name some organs innervated by sympathetic postganglionic fibres
Eye, lungs, heart, gut, salivary glands. Muscle of: blood vessels, piloerectors. Fibres are long as ganglia are close to the spinal cord
Explain the difference between grey and white rami
the sympathetic preganglionic axons are myelinated, while the postganglionic aren’t, giving the preganglionic white rami its white appearance and vice versa for grey.
Describe the origin of the preganglionic parasympathetic neurons.
The parasympathetic preganglionic neurons have cell bodies in the brain stem and the sacral spinal cord. Thus preganglionic fibres emerge as part of the oculomotor, glossopharyngeal, facial and vagus nerve cranial outflow, and the sacral outflow. Axons pass to parasympathetic ganglia.
Describe the structure of the parasympathetic nervous system
Long preganglionic axons travel from the CNS to parasympathetic ganglia, usually located close to or within target organs, meaning postganglionic fibres are short. Innervates: eye, salivary glands, genitalia, gut, heart, lungs and more.
What blood vessels have parasympathetic innervation?
parasympathetic vasodilator fibres innervate the salivary glands, exocrine pancreas, gastrointestinal mucosa, genital erectile tissue and cerebral and coronary arteries. All other blood vessels are only innervated by sympathetic fibres.
What is the importance of dual autonomic innervation?
Many organs are innervated by both sympathetic and parasympathetic nerves. As these two are usually antagonistic, this allows delicate control of an organs function by modulating the action of both.
Which organs only have sympathetic innervation?
Spleen, sweat glands, pilomotor muscles of skin hairs and and the adrenal medulla and most blood vessels (among others)
Describe the autonomic innervation of muscles in the eye
parasympathetic nervous system alone control the focusing of the eye with ciliary muscles, and constriction of the pupils by constrictor pupillae. Sympathetic stimulation of the dilator pupillae dilates the iris - not dual innervation but functional antagonism.
What is autonomic tone?
describes a basal level of activity in the tissues innervated by the autonomic nervous system - e.g. slight constriction of blood vessels due to sympathetic tone. Heart under parasympathetic vagal tone.
What is the enteric nervous system?
This refers to neurons present in the walls of the GI tracts that act independently of their autonomic supply. Organised into the submucosal and myenteric plexus.
What neurotransmitter do preganglionic autonomic neurons secrete and what receptors does this bind to?
Acetylcholine binds to nicotinic receptors
What neurotransmitter do postganglionic autonomic neurons secrete and what receptors does this bind to?
parasympathetic fibres secrete Ach which binds to muscarinic receptors, while sympathetic fibres secrete norepinephrine, apart from those innervating sweat glands and pilomotor muscles which secrete Ach onto muscarinic receptors
Describe M1 Muscarinic receptors
mainly on CNS and peripheral ganglia neurons + gastric parietal cells. Generally excitatory, e.g. gastric acid secretion following vagus nerve stimulation
Describe M2 Muscarinic receptors
occur in the heart and on both CNS and PNS nerve terminals. E.g. slowing of heart following vagus nerve stimulation
Describe M3 Muscarinic receptors
Mostly located on secretory glands and on smooth muscle. Generally excitatory e.g. contraction of visceral smooth muscle.
Describe the types of adrenergic receptor
Alpha adrenoreceptors are generally excitatory and cause contraction in smooth muscle (apart from gut wall). Beta adrenoreceptors can be excitatory or inhibitory e.g. B1 stimulates cardiac muscle while B2 relaxes bronchial smooth muscle.
Describe the ascending autonomic pathways
Some visceral afferents enter the dorsal horn with somatic fibres and synapse at the segmental level, with second-order fibres ascending via the spinothalamic tract. Other visceral afferents like from arterial baroreceptors reach the brainstem via vagus nerves.
Describe the autonomic reflexes
A specific reflex elicited by stimulation of a visceral afferent - these reflexes can be segmental or involve the brain. E.g. baroreceptor reflex, lung inflation reflex, micturition reflex. In response to perceived danger, the defense reaction is triggered by the hypothalamus, which overrides normal autonomic reflexes.
What is the role of the hypothalamus in terms of autonomic control?
Mainly concerned with homeostasis. Receives inputs from the retina, the chemical sense organs, the somatic senses and visceral afferents, as well as brain regions like the limbic system and cerebral cortex. Plays an important role in regulation of temperature, osmolality, feeding and drinking and reproductive activity.
