The Autonomic Nervous System Flashcards
What are the 2 major divisions of the autonomic nervous system ?
The sympathetic nervous system and the parasympathetic nervous system
What are pre-ganglionic fibres ?
The fibres going from the CNS to the autonomic ganglia
What are post-ganglionic fibres ?
Fibres going from the ganglia to the target organs
Where do sympathetic fibres operate ?
They leave the CNS from the thoracic and lumbar regions (T1 - L2) or the thoracolumbar division
Where do parasympathetic fibres operate ?
They leave the CNS from the brain stem and sacral regions or the craniosacral division
What is the adrenal medulla ?
It is a specialised part of the sympathetic nervous system
It forms an endocrine gland
What is autonomic ‘tone’ ?
When autonomic innovation provides a basal level of activity in the tissues it innovates
Can autonomic tone be changed ?
It can either be increased or decreased to modulate activity of specific tissues
Summarise somatic nerves
Somatic nerves (motor neurones) go to voluntary effectors (skeletal muscle)
They only have 1 synapse
They use myelinated axons
The neurotransmitter is Ach.
Summarise autonomic nerves
Autonomic nerves go to non-voluntary effectors – such as smooth muscle, heart and glands . Neurotransmitters include ACh at the ganglia for both sympathetic and parasympathetic nervous system and noradrenaline at the post-ganglionic sympathetic nervous system
What is the American name for adrenaline ?
Epinephrine
What is the American name for noradrenaline ?
Norepinephrine
Summarise autonomic ganglia
In the sympathetic nervous system > there are short pre-ganglionic myelinated fibres or long post-ganglionic unmyelinated fibres and the ganglia are close to the spinal cord
In the parasympathetic nervous system > there are long pre-ganglionic fibres and short post-ganglionic fibres and the ganglia are close to the innervated structure
Summarise nicotinic acetylcholine receptors
- activated by acetylcholine
- ion channel linked
- blocked by hexamethonium
Summarise transmission at parasympathetic postganglionic nerve endings
Acetylcholine can increase or decrease the activity of the effector -
• heart - ACh decreases the rate of beating
• salivary glands - ACh increases rate of secretion
Summarise muscarinic acetylcholine receptors
- stimulated by muscarine
- blocked by atropine
- G protein linked
What does atropine do ?
It prevents salivary and bronchial secretions during surgery and also increases pupil size
Summarise transmission at sympathetic post-ganglionic nerve endings
- transmitter is noradrenaline
- it can increase or decrease the activity of the effector
What is tyrosine used in the synthesis of ?
- dopamine
- adrenaline
- noradrenaline
- NOT acetylcholine
How does noradrenaline turn into adrenaline ?
It is converted by the enzyme PNMT
Summarise adrenoreceptors
- receptors for noradrenaline
- divided into α, β1 and β2 receptors
- potencies of adrenaline, noradrenaline and isoprenaline are used to distinguish between adrenoreceptors
How is acetylcholine removed ?
Acetylcholine is hydrolysed by acetylcholinesterase into acetate and choline
How is noradrenaline removed ?
It is removed via uptake mechanisms involving monoamine oxidase and COMT
Summarise sympathetic cholinergic fibres
- release acetylcholine
- innervate sweat glands
- innervate blood vessels in the skeletal muscle
Summarise cotransmission in the autonomic nervous system
Cotransmission is the release of more than one neurotransmitter at once
- one neurotransmitter usually acts as a neuromodulator - it modulates the action of the other transmitter instead of exerting a direct effect on the effector organ
Give an example of contransmission in the ANS
Parasympathetic innervation by salivary glands
- VIP and acetylcholine are secreted
- VIP is secreted due to high frequency stimulation
- acetylcholine is secreted due to low frequency stimulation
- they are both stored in different vesicles but released due to different frequencies of stimulation
Summarise fight or flight responses
- mediated by activation of the sympathetic nervous system
- triggered by sensory input
- triggered by memory of stressful event
What is sympathetic or parasympathetic tone ?
When both the sympathetic and parasympathetic nervous systems are normally partially active
What are the effects of adrenaline in the cardiovascular system ?
- increases heart rate and force of contraction
- dilates blood vessels in skeletal muscles
- constricts blood vessels elsewhere
- increases blood pressure and muscle blood flow
What are the metabolic effects of adrenaline ?
- glycogenolysis - increase in blood glucose
- lipolysis - increase in blood fatty acids
- calorigenic effect - increased metabolic rate in many tissues
Describe innervation of the smooth muscle
There is diffuse contact between nerve endings and the target muscle
The sympathetic postganglionic fibres and parasympathetic preganglionic fibres have varicosities
The varicosities secrete neurotransmitter into the space surrounding the target tissue
Why are autonomic reflexes important ?
They play an important role in maintaining homeostasis of the body
Summarise the autonomic reflex arc
Sensory neurone > CNS > autonomic pre and postganglionic neurones > effector
- long reflexes involve the CNS
- short reflexes bypass the CNS
Describe the light reflex
- it is a parasympathetic reflex
- when light is shone in the eye, the pupil shrinks and this is known as the direct pupillary response
- optic fibres > brain > oculomotor nerve
Describe the role of sympathetic reflexes in thermoregulation
When there is an increase in body temperature :
- increased activity in the central thermoreceptors via the hypothalamus
- reflex increases sympathetic discharge to sweat glands = sweating
- reflex decreases sympathetic discharge to cutaneous blood vessels = vasodilation in skin
- leads to cooling (response)
What does the adrenal medulla secrete ?
80% adrenaline and 20% noradrenaline
