Autonomic Nervous System Flashcards
ANS
The central nervous system (CNS) sends commands to the rest of the body via “autonomic pathways”to control functions that are essential to survival but are undertaken without conscious thought or effort.
- It’s not about being automatic it’s about being autonomous.
- It’s self-governed
ANS in history
Langley (early 1900s) coined the term “Autonomic Nervous System” and defined it as purely “motor”, excluding any afferent component. Sensory information relevant to autonomic control (eg degree of bladder filling, blood pressure etc) merely travels through autonomic outflows and nerve cell bodies are present in dorsal root ganglia (part of the somatic system) and in specialized sensory ganglia on particular cranial nerves.
Autonomics system motor neurons
Consists of collections of motor neurons (with cell bodies present in ganglia) in the head, neck, thorax, abdomen, and pelvis, and the axonal connections of these neurons
ANS MOTOR neurons Vs somatic motor neurons
Incorporates CNS components, including brainstem and spinal autonomic preganglionic neurons that project to the autonomic motor neurons in the peripheral ganglia
In the ANS, preganglionic motor neurons are clearly distinguished from somatic motor neurons which project from the CNS directly to the innervated tissue (skeletal muscle) without any intervening ganglia
Sensory component of ANS
Communication from organs or tissues to the brain is mediated via afferent neurons with cell bodies in the dorsal root ganglia (near the spinal cord) or in the nodose or petrosal ganglia of the lower cranial nerves (located in the neck). These visceral afferent neurons have a central process that projects into the dorsal horn of the spinal cord (from the DRG) or into sensory nuclei (eg the nucleus tractus solitarius in the dorsal medulla oblongata) in the brainstem from the cranial ganglia
Preganglionic neurons and post ganglionic neuron in sympathetic NS
preganglionic neurons lie in the thoraco-lumbar spinal
cord. Postganglionic axons arise from cell bodies
in ganglia that are either:
- arranged in a chain running parallel to the spinal cord (paravertebral) OR
- in isolated ganglia situated in the abdomen (prevertebral
Preganglionic neurons and post ganglionic neuron in Parasympathetic NS
Preganglionic neurons lie in the cranial brainstem
region or in the sacral part of the spinal cord.
Post ganglionic fibres arise from ganglionic neurons
situated close to or in target end organs
Preganglionic neurons and post ganglionic neuron in Enteric NS
A complex network of ganglia in the walls of the stomach and small intestine. These networks
lack preganglionic input and CAN operate independently of the CNS. DO contain sensory
neurons that interact with interneurons that connect with enteric motor neurons (+/-)
Somatic vs Autonomic Motor Outflows
Somatic motoneurons have no intervening synapse
Neurotransmitter releases in Sympathetic NS
“Sympathetic” outflows have an intervening synapse where preganglionic neurons use ACh as a
transmitter and postganglionic fibres use Nad (NE)
Sympathetic preganglionic neurons are myelinated and
postganglionics are unmyelinated
Enteric NS structure
The enteric nervous system is structurally and functionally organized into afferent neurons, interneurons and motoneurons, with characteristic projections and neurochemical profiles. There are some projections from afferent cell bodies within the enteric nervous system to neurons in autonomic ganglia that project back to the gut, but projections to other parts of the autonomic nervous system are sparse or absent.
Realities about division of ANS
Neither division is ever activated in its entirety
Each division has a series of distinct functional
pathways that can be activated by the CNS
either independently or in patterns depending
on homeostatic need
Sympathetic nervous system transmitters
preganglionic - acetylcholine, Ach, (fast excitatory transmitter)
- ACh acts on a nicotinic receptor (different from muscle)
- range of peptide slow excitatory transmitters
Postganglionic - Noradrenaline as the primary transmitter
- Co-transmitters ATP, Neuropeptide Y, galanin, somatostatin, opioids
- ACh is the exception in the case of the innervation of the sweat glands
Parasympathetic nervous system transmitters
preganglionic - acetylcholine, ACh, (fast excitatory transmitter)
Postganglionic - ACh and nitric oxide (NO) as main non peptide transmitters
- peptide co-transmitters including vasoactive intestinal peptide (VIP),
calcitonin gene related peptide (CGRP), somatostatin, opioids ++
Cranial Parasympathetic Pathways(excluding the vagus)
directed mainly to structures in the
head and neck
with synaptic relays in ganglia
(mainly) at a distance from
target organs (compared with
those in the vagus)
Parasympathetic preganglionic axonal outflow to facial structures and in the neck is very precise
Parasympathetic preganglionic axonal outflow to thoracic, abdominal and pelvic structures is diverse
