AUTONOMIC SYSTEM - LECTURE 1 Flashcards
Nerve and muscle cells are considered to be excitable tissues because
they are able to produce an electrical signal when exited
where does the autonomic nervous system come from
nervous system= peripheral system=efferent division =autonomic nervous system
the autonomic system is divided into
- sympathetic division
2. parasympathetic division
AS system innervates
- smooth muscles
- cardiac muscles
- glands(Exo/endocrine)
what is AS?
→ ANS regulates activities of systems not under voluntary control
e.g. respiration, circulation, digestion, body temperature, metabolism, sweating, some endocrine
AS helps maintain
maintain a constant internal body environment (homeostasis)
The cell bodies of the primary neurons (presynaptic/ preganglionic neurons) are located in the
the intermediolateral (IML) gray column of the spinal cord or in the brain stem nuclei
These cell bodies send axons which are usually
usually small-diameter, myelinated, relatively slow conducting B fibers, out to synapse with the secondary neuron (postsynaptic/ postganglionic, neuron) located in one of the autonomic ganglia.
From there, the postganglionic axon (mostly unmyelinated C fibers) passes to its target organ
Because the postganglionic fibers outnumber the preganglionic neurons by a ratio of about 32:1, a single preganglionic neuron may
control the autonomic functions of a rather extensive terminal area
comparison of the somatic nervous system from AS
- innervates the skeletal muscle.
no neuron on neuron synapse outside CNS.
may not be tonically active.
larger, thicker fibers – faster.
simple Arch-based NT communication
comparison of AS from somatic system
- usually more complex- requires 2nd messenger systems
→ receptor-activated → starts a chain of events
somatic AP vs AS AP
Somatic AP originate in CNS & sometimes (not often) from reflexes
Vs.
ANS actions originate from CNS, but most often through reflexes.
(in-built control system for homeostasis)
ANS pathway from CNS to target has a set (chain) of two fibers.
effects on effector organ from SS and AS
- SS can only stimulate
2. AS can stimulate and inhibit
length of neuron in SS
the neuron is very long, its cell body originates from the CNS, and its axon terminal releases ACH at the effector organ.
length of a neuron is AS- parasympathetic
long pre-ganglionic neuron releases ACH at the ganglion to the short post-ganglionic neuron in the ganglion which then releases the ACH at the effector organ
length of a neuron in AS- sympathetic
a pre-ganglionic neuron shorter than that of parasympathetic releases ACH at the ganglion to the post-ganglionic neuron that is longer than that of parasympathetic, which releases norepinephrine at the effector organ
length of neuron in the adrenal medulla
- the pre-ganglionic neuron is very very short.
- ! The adrenal medulla is essentially a sympathetic ganglion in which the postganglionic cells have lost their axons and secrete norepinephrine and epinephrine directly into the bloodstream. The cholinergic preganglionic neurons to these cells have consequently become the secretomotor nerve supply of this gland
Noradrenaline (NA) is the same as norepinephrine (NE
true
autonomic nerve pathway
the pre-ganglion neuron of which its cell body is in the CN releases pre-ganglionic neurotransmitter in the autonomic ganglion where synapse occurs
the neurotransmitter will then attach to the receptors of a post-ganglionic neuron using the lock and key method which then releases post-ganglionic neurotransmitter from varicosity to the receptors of an effector organ
ganglion is
a cluster of cell bodies that lie outside the CNS
varicosity
= areas of stored neurotransmitter- spread out for greater surface area compared to the axon terminal
endocrine gland
the hormone released into the bloodstream and reaches target receptors by blood (eg insulin)
exocrine gland
secretions not into the bloodstream but into the duct to get to target receptors (eg digestive enzymes of pancreas via a duct to the small intestine)
sympathetic system
SNS (thoracolumbar) division:
Preganglionic cell bodies are located in the intermediolateral (IML) gray column – 12 thoracic segments (T1 to T12) & upper 2 lumbar segments (L1 and L2) of the spinal cord
t1-t12 synapse at the sympathetic ganglion chain
L1-L2 synapse at the collateral ganglion
parasympathetic nervous system
sometimes called the craniosacral division (location of its preganglionic neurons).
preganglionic cell bodies in the gray matter of the brainstem (medial part of the oculomotor nucleus, Edinger–Westphal nucleus, superior & inferior salivatory nuclei) AND the middle three segments of the sacral cord (S2–4)
PNS nerves supply the visceral structures in the head via the oculomotor, facial, & glossopharyngeal nerves, & those in the thorax & upper abdomen via the vagus nerve.
The sacral outflow supplies the pelvic viscera via branches of S2-S4 spinal nerves.
The parasympathetic distribution is confined entirely to visceral structures
cranial nerves
- oculomotor (III)
- facial (VII)
- glossopharyngeal (IX)
- vagus (X)
Note the differences in the origin of fibers in the two divisions of ANS
(PNS: cell bodies in the brainstem and sacrum, whereas SNS: thoracic (T1-T12) and lumbar (L1-L2) segments of the spinal cord)
PNS and SNS have similar pathways autonomic nervous system pathways: ie, preganglionic, postganglionic, but the differences are in the length of fibers and origin of fibers.
PNS nerves supply the visceral structures in the head and thorax and upper abdomen via
the oculomotor, facial, & glossopharyngeal nerves
& those in the thorax & upper abdomen via the vagus nerve.
The sacral outflow supplies the pelvic viscera via
via branches of S2-S4 spinal nerves.
The parasympathetic distribution is confined entirely to visceral structures
when both the SNS and PNS are partially active
this is known the as sympathetic and parasympathetic tone
tonically active
PNS and ANS can have antagonist (opposite) effects or complementary effects
SNS activation
SNS activation is also known as the “Fight or flight” response or the “fight, flight or freeze” response as it prepares the body for physical activity/coping with stress/emergency situations.
“freeze” (or acting as if you are dead), is also a reaction that one may choose for survival under circumstances of excessive stress- e.g. if one sees a wild animal close by, they may freeze (so as not to attract attention or be seen as a threat to the animal.
Stimuli include: stress, low blood pressure (BP), low blood glucose (BG), anxiety
stimulation of SNS leads to
mobilization of energy, ↑ CVS function, ↑ respiratory function,↓ digestion
i.e. SNS = catabolic nervous system.
dilates the pupils (letting more light into the eyes), accelerates the heartbeat & raises the BP (providing better perfusion of the vital organs & muscles), & constricts the BVs of the skin (which limits bleeding from wounds).
Noradrenaline (NA) discharge due to SNS activation also leads to elevated plasma glucose & free fatty acid levels (supplying more energy)
PNS activation
PNS- concerned with the vegetative aspects of day-to-day living.
PNS dominates in quiet, relaxed situations- PNS is the body’s response to a “rest and digest” state.
PNS is important for maintenance/normal functioning.
Eg, PNS action favors digestion and absorption of food by increasing the activity of the intestinal musculature, increasing gastric secretion, and relaxing the pyloric sphincter.
For this reason, the cholinergic PNS division is sometimes called the anabolic nervous system
Stimuli: high blood glucose and other metabolites, restful situations
→ conserve and store energy, ↑ digestion, ↓ CVS function, and ↓ respiratory function
PNS and SNS
Bearing in mind that TONE will change according to one’s current state: fight/flight (SNS) or rest/digest (PNS).
Say, for example, you were taking a hike and you have just come across a snake in your path.
This will obviously put you into fight/flight mode- this means SNS will take over (so there will no longer be balance between the two divisions) and you will get all the physiological SNS responses (which we will go through shortly) and your PNS responses will “back down”
(which makes sense, since, mostly the two ANS divisions have opposite effects (are antagonistic)- which you will learn as we move on).
In this circumstance (with the snake), when the SNS takes over, the SNS “dominates” – known as “sympathetic dominance”.
On the other hand, when one is relaxed (e.g. reading a book), they are in a state of “Parasympathetic dominance”.
