The Autonomic Nervous System Flashcards
What is the ANS?
ANS is a subdivision of the peripheral nervous system and is not under conscious control. Controls non-skeletal peripheral functions: Cardiac muscle, smooth muscle, internal organs, and skin.
Describe PNS structure
The PNS (spinal and cranial nerves) is comprised of 2 divisions: Afferent and Efferent divisions. The afferent division contains somatic sensory and visceral sensory neurons which take information to the brain about internal and external stimuli. The efferent division contains somatic motor and visceral motor neurons. The visceral can be further subdivided to sympathetic and parasympathetic divisions.
Describe the sympathetic and parasympathetic divisions
Parasympathetic: Involved in rest and digest response or routine maintenance. Sympathetic: Involved in fight or flight response and mobilisation + increased metabolism. These often innervate the same tissues and have opposing/antagonistic effects. However, don’t always innervate the same tissues – SNS controls blood vessel tone mainly, controlling both constriction and dilation.
Describe autonomic neuron associations
Visceral motor nuclei originate in the hypothalamus and project to the brainstem or spinal cord where they synapse with autonomic neurons (parasympathetic or sympathetic). Autonomic neurons consist of a pre-ganglionic and post-ganglionic neuron.
Describe the structure of sympathetic and parasympathetic neurons
Parasympathetic nervous system: Consist of long pre-ganglionic fibres where ganglions are close to or embedded within the effector tissue. Short post-ganglionic fibres.
Sympathetic nervous system: Short pre-ganglionic fibres with ganglion close to spinal cord. Long post-ganglionic fibres. Usually more coordinated control as lots of sympathetic nerves controlled by single signal. Lots of ganglions in SNS exist in sympathetic trunks which are long chains running parallel to the spinal cord with lots of sympathetic nerves coming out of the spinal cord, synapsing with these sympathetic trunks.
What is different about the adrenal gland?
Adrenal gland is an exception to the two neuron system as it doesn’t have a ganglion – innervated by only sympathetic nerve.
Where do synapses exist and how do these differ?
Synapses exist between:
• Pre and post-ganglionic neurones
• Post-ganglionic neurone and the effector tissue
• Sympathetic nerve innervating adrenal gland
All pre-ganglionic nerve releases acetylcholine so cholinergic synapses. Post-ganglion also releases acetylcholine in the PARASYMPATHETIC NERVOUS SYSTEM. Post-ganglion in SYMPATHETIC NERVOUS SYSTEM releases noradrenaline into effector tissue.
How does the adrenal gland respond?
Adrenal (adrenal medulla) gland secretes a hormone and not a neurotransmitter – secretes adrenaline and small amount of dopamine into bloodstream which is then transported in tissue.
How is function in the gut and lungs regulated?
Complex nervous system exists within the gut called the enteric nervous system which can respond to gut function and stimuli received within the gut without engaging the brain.
There are no sympathetic neurones innervating the lung tissue – however, lungs can bronchodilate so shows there must be some level of control the SNS has on the lung. This is controlled by adrenaline produced by adrenal gland – adrenaline passes up to lung via diffusion from blood and causes bronchodilation.
What is the ANS’s main role?
ANS is a regulatory system that controls many of the body’s organ systems and homeostatic mechanisms – effects are involuntary and reflex responses to visceral stimuli
Important responses:
• Baroreceptor reflex – control of blood pressure and heart rate
• Pupillary reflex – pupil size changed in response to light
• Enteric nervous system adds layer of complexity to GI responses
• Micturition reflex
Describe the micturition reflex
Bladder pressure detected: parasympathetic contracts the detrusor muscle which controls the bladder while sympathetic controls internal sphincter
Pressure slowly builds so when there is moderate pressure in the bladder, sympathetic in control and keeps internal sphincter closed but when pressure reaches a certain threshold, parasympathetic disinhibited and sympathetic inhibited. PNS causes contraction of detrusor and SNS is inhibited so internal sphincter relaxes and urine can leave the bladder.
Somatic nervous system provides voluntary level of control on emptying bladder as there is a second sphincter under voluntary control.
What is the role of nicotinic acetylcholine receptors?
Ion channel receptors mediate effects in the autonomic nervous system as they have fast response – nicotinic acetylcholine receptor mediate all fast excitatory and inhibitory transmission. Nicotinic acetylcholine receptors are also found in the adrenal gland. Nicotinic acetylcholine receptors mediate the responses to acetylcholine released from preganglionic fibres at all autonomic ganglia. Also, mediate the response to acetylcholine released by sympathetic nerves innervating the adrenal medulla.
What are the receptors found in post-ganglionic neurons?
Muscarinic (G-Protein coupled acetylcholine receptors) and Adrenergic (G-Protein coupled noradrenaline receptors) have slow responses as more complex intracellular signalling pathway. Not as fast as ion-linked channels.
Muscarinic receptors are found in post-ganglionic fibres of parasympathetic system.
Adrenergic receptors are found in post-ganglionic receptors of sympathetic system.
Describe neurotransmitter biosynthesis and metabolism
- Precursor enters presynaptic nerve which is then converted into neurotransmitters and packaged into vesicles.
- Action potential stimulates calcium influx causing exocytosis, so neurotransmitter released into synaptic cleft.
- Neurotransmitters bind to receptors producing postsynaptic action. However, must be transient and NTs must be inactivated.
- Neurotransmitter removed either by uptake into pre-synaptic terminal or glial cell; this can be metabolised in the synapse prior to uptake.
Describe acetylcholine biosynthesis and metabolism
- Choline and acetyl CoA enzymatically converted by choline acetyltransferase in presynaptic neurone to form acetylcholine
- Packaged into vesicles and action potential causes calcium ion influx
- Vesicles dock and fuse with membrane releasing ACh into synapse where it activates muscarinic/nicotinic receptors
- Metabolised by acetylcholinesterase in the synapse where it is rapidly degraded into components – choline taken up into presynaptic terminal by choline uptake protein or into glial cell.
