Test 1- General NS Flashcards
The autonomic nervous system (ANS)
The autonomic nervous system (ANS) is the part of the nervous system that controls visceral functions, below the level of consciousness.
The ANS is divided into the sympathetic “Fight or Flight” and parasympathetic “Rest and Digest”.
somatic nervous system
In this it is different from the somatic nervous system which controls voluntary functions (like muscle movement)
In general, drugs that act on the autonomic nervous system can be either
In general, drugs that act on the autonomic nervous system can be either ‘mimetics’ which stimulate the system in question (parasympathomimetic, sympathomimetic) or ‘lytics’ which block the system (parasympatholytic, sympatholytic).
The parasympathetic nervous system (PSNS) originate
The parasympathetic nervous system (PSNS) originates in the central nervous system at the ‘ends’ of the spinal cord (craniosacral) with neurons originating with the cranial nerves III, VII, IX, and X cranially and originating from the sacral spinal segments (S2-S4) caudally.
The sympathetic nervous system (SNS) by contrast arises from
The sympathetic nervous system (SNS) by contrast arises from ‘middle’ section, the thoracolumbar spine
Both parts of the ANS consist of an origin in the CNS
Both parts of the ANS consist of an origin in the CNS that sends out an efferent nerve to synapse on an intermediate ganglion which sends a post-ganglionic neuron to the target tissue or organ.
In the parasympathetic system
In the parasympathetic system the pre-ganglionic neurons travel a long way before synapsing on their ganglion relatively closer to the target organ or tissue.
By contrast, the sympathetic system has
By contrast, the sympathetic system has short pre-ganglionic axons which synapse on the ganglia that form a chain alongside the spinal column and then send out relatively long post-synaptic axons to the end organ.
The predominate neurotransmitters at each of these intersections
The predominate neurotransmitters at each of these intersections varies. At the terminal synapses of the sympathetic neurons it is usually norepinephrine (adrenergic). At the terminal synapses of the parasympathetic neurons it is acetylcholine (cholinergic). Additionally all ganglionic synapses utilize acetylcholine, as do the terminal synapses of the somatic nerves (neuromuscular junction).
The two major adrenergic (sympathetic) receptors are
The two major adrenergic (sympathetic) receptors are α and β (which have further subdivisons – α-1, α-2, etc).
The two major cholinergic (parasympathetic) receptors are
The two major cholinergic (parasympathetic) receptors are Muscarinic and Nicotinic.
Nicotinic receptors are found at
Nicotinic receptors are found at all ganglionic and somatic synapses
Muscarinic receptors are found
Muscarinic receptors are found on the target tissues of the parasympathetic nervous system (and the less common post-ganglionic cholinergic of the SNS).
medullary outflow
The organs innervated by the cranial CNS (medullary outflow) are the eye, lacrimal gland, salivary glands, heart, lungs and upper GI tract.
sacral outflow
The caudal CNS (sacral outflow) supplies the lower GI tract, bladder and genitals.
exocytosis
When a nerve axon brings an action potential calcium channels open and the increased calcium ion concentration stimulates exocytosis of vesicles containing preformed neurotransmitters (for the PNS that is acetylcholine). The acetylcholine moves into the synapse and interacts with post-synaptic receptors (Nicotinic or Muscarinic).
cotransmission
Many neurons apparently respond to more than one neurotransmitter (cotransmission) which may explain why multiple drugs in combination can be particularly effective and why some drugs act on more than one neurotransmitter.
Nicotinic receptors
Nicotinic receptors are 5-unit ion channels with multiple subtypes. The significance of the subtypes is that they are different for different locations of nicotinic receptors (e.g. the neuromuscular junction, the ganglionic synapses, etc).
Muscarinic receptors come in five flavors:
- M1 – Excitatory – Neural (CNS stimulation, gastric acid secretion, increased GI motility)
- M3 – Excitatory – Glandular (Secretions, smooth muscle contraction, vasodilation)
- M5 – Excitatory – Salivary glands, iris, substantia nigra
- M2 – Inhibitory – Cardiac (cardiac and neural inhibition)
- M4 – Inhibitory – CNS/Smooth muscle
Acetylcholine
Acetylcholine has a large number of effects, depending on which receptor it is released at. Drugs will affect these various locations to different degrees, which is very useful clinically.
When ACH is released at the ganglia it stimulates
When released at the ganglia it stimulates the post-synaptic neuron to carry on the signal (sometimes this may be an adrenergic ganglion – remember, they use ACh too)
When Ach is released at the neuromuscular junction
When released at the neuromuscular junction it stimulates muscle contraction
When ACH is released in the CNS
When released in the CNS it can cause convulsions
When ACH is released at a parasympathetic terminal synapse
When released at a parasympathetic terminal synapse with the target organ it will facilitate
muscarinic signs:
o Diarrhea
o Urination (smooth muscle contraction in the bladder)
o Miosis o Bradycardia and Bronchoconstriction o Emesis (increased GI motility)
o Lacrimation o Salivation (increased secretions, also in airways)
parasympathomimetics.
Drugs that cause the same response ACh would are considered parasympathomimetics. These may be direct or indirect acting. The direct-acting drugs will interact directly with post-synaptic receptors (thus they are muscarinic agonists). Acetylcholine is, of course, the prototype of this.
Acetylcholinesterase
Acetylcholinesterase is an enzyme that breaks down acetylcholine and is important in stopping the stimulation of the post-synaptic nerve. They are membrane bound enzymes that bind acetylcholine and break it into choline and acetate which can then be recycled into more acetylcholine. Pseudocholinesterases are enzymes in plasma and other tissues that break down a number of esters.
