Week 7: The Autonomic Nervous System Flashcards
Sensory divisions of the ANS:
- Input to the ANS comes from visceral sensory neurons
- Sensory receptors located in blood vessels, visceral organs, muscles and the nervous system that monitor conditions in the internal environment
- Changes in these monitored conditions are not consciously perceived as they are with the somatic sensory neurons
e.g. chemoreceptors monitoring blood CO2 levels or mechanoreceptors that detect the degree in the stretch in the walls of organs
Motor divisions of the ANS:
- Output divisions of the ANS that controls visceral function e.g. heart, smooth muscle, glands
Autonomic = involuntary/outside of our control
Two motor neurons involved In the pathway that controls visceral effectors (target structure):
- Pre-ganglionic neurons
Post-ganglionic neurons
Two major subdivisions of the motor (afferent) division of the ANS:
- Sympathetic and parasympathetic
- Usually have opposing effects
Some visceral effectors are only innervated by one division
- Usually have opposing effects
Neurons of the ANS:
- There are two motor neurons involved in the pathway that controls visceral effectors:
Preganglionic neurons:- Cell bodies in the brainstem or spinal cord (CNS)
- Synapse with post-ganglionic neurons within an autonomic ganglion (PNS)
Post-ganglionic neurons: - Cell bodies in autonomic ganglia
Terminate (and therefore innervate) visceral effectors e.g. cardiac muscle, smooth muscle, glands
Sympathetic division
- Prepares the body for emergency situations
- It activates a series of physiological responses called the fight-or-flight responses
- e.g. you are hiking in the woods and you come across a huge bear and your fight-or-flight response kicks in
Heart and blood rate increases, blood flow diverted from digestive organs to skeletal muscle, glandular secretion of adrenaline increases + more
What is the neural travel in the sympathetic division?
- Pre-ganglionic neurons originate in the lateral horn of the grey matter in the spinal cord from spinal segments T1-L2, and generally either synapse with a post-ganglionic neuron in the sympathetic chain/trunk or on another prevertebral ganglion
- Post-ganglionic neurons originate either in the sympathetic chain or in a prevertebral ganglion (depending on the preganglionic neuron) and terminate in the visceral effector (target structure) e.g. cardiac muscle
- Sympathetic division has short pre-ganglionic neurons and long post ganglionic neurons due to pre-ganglionic synapse location
Parasympathetic division:
- Regulates parts of the body that conserve and restore energy e.g. digestion and absorption of food and the elimination of wastes
- These responses are called rest-and-digest responses
e.g. you have just eaten a meal and you lay down on the couch feeling full. Heart rate and blood pressure decrease, digestion activated e.g. peristaltic contractions of smooth muscle in the GIT enzymatic release from glands, glandular secretion of saliva increases + more
- These responses are called rest-and-digest responses
Neural travel in the parasympathetic division?
Preganglionic neurons originate in either:
- The nuclei (grey matter) of four cranial nerves in the brainstem (3, 7, 9, 10)
- The lateral horn of the grey matter in the spinal cord, from spinal segments S2-S4
Preganglionic neurons synapse with a post-ganglionic neuron in the terminal ganglia which are located close to or in the wall of the visceral organ.
Post-ganglionic neurons originate in terminal ganglia and terminate at the visceral effector e.g. cardiac muscle
Parasympathetic division has long pre-ganglionic neurons and short post ganglionic neurons due to pre-ganglionic synapse location
The sympathetic trunk (chain)
- A chain of ganglia located lateral to the vertebral bodies (outside the vertebral canal)
- Site of synapse for many pre-ganglionic neurons (onto post-ganglionic neurons)
- Has physical connections to the spinal nerves
White and grey ramus communicans connect the anterior/ventral ramus of the spinal nerve to the sympathetic trunk - Group pf ganglia located lateral to the vertebral bodies outside the vertebral canal
- Contain synapse of pre and post ganglionic
Processes connections to spinal nerves
Pathway of pre-ganglionic sympathetic neuron:
Preganglionic sympathetic neurons originate in the lateral horn of the spinal cord (T1-L2)
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A neuron exits the spinal cord via the anterior/ventral root and enters the spinal nerve
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It then travels into the anterior ramus and then into the white ramus communicans
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The neuron then enters a sympathetic ganglion and will synapse with a post-ganglionic neuron in one of four ways
A pre-ganglionic neuron will synapse with a post-ganglionic neuron in one of four ways:
- Synapse in the first ganglion it reaches
- May ascend or descent to a higher or lower ganglion before synapsing, won’t go outside the spinal segments its kept in
- May continue without synapsing, through the entire sympathetic trunk and instead synapse in a prevertebral ganglion
- May continue without synapsing, through the sympathetic trunk and prevertebral ganglia to synapse on the adrenal gland
Pathway of a post-ganglionic sympathetic neuron:
- After synapsing with a pre-ganglionic neuron, a post-ganglionic neuron will then exit their ganglion and travel to their visceral effectors
- There is no direct neural path for autonomic neurons, so post-ganglionic neurons need to travel or hitch hike with anatomical structure to reach their target
- They may travel with a peripheral spinal nerve: These neurons innervate visceral effectors in the skin of the neck, trunk and limbs, including sweat glands, smooth muscle in the blood vessels and arrector pili muscles of hair follicles
- They may travel with arteries: These neurons innervate visceral effectors in the skin of the face as well as other visceral effectors of the head
They may leave the sympathetic trunk and group together to form sympathetic nerves: These provide innervation to the heart and lungs
How do neurons talk to each other?
- Via action potentials (electrical signals)
- Action potentials move form one neuron to the next across the synapse (space between axon terminal of one neuron and dendrites of another neuron)
While action potentials are electrical, most signals cross the synapse in chemical form (neurotransmitters)
- Action potentials move form one neuron to the next across the synapse (space between axon terminal of one neuron and dendrites of another neuron)
What is a neurotransmitter?
- Chemical messenger molecules that are synthesized, stored and released by the neuron
- Can be functionally classified by their mode of action and their resulting effect
- Can exert excitatory post-synaptic potential (EPSP) or inhibitory post-synaptic potential (IPSP)
- Depend on which receptor the neurotransmitter binds to on a post-synaptic neuron
A post-synaptic neuron will either be excited and continue the message by generating its own action potential or it will be inhibited and the message will stop there
What are common Neurotransmitters? Are they excitatory, inhibitory or both
ANAD SGG
Acetylcholine (both)
Noradrenaline/Norepinephrine (Both
Adrenaline/epinephrine (excitatory)
Dopamine (both)
Serotonin (inhibitory)
GABA (inhibitory)
Glutamate (excitatory)
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There are two major neurotransmitters of the ANS:
Acetylcholine (ACh)
- Used by pre-ganglionic neurons in both sympathetic and parasympathetic divisions
- Used by post-ganglionic neurons of the parasympathetic neurons of the parasympathetic division
Noradrenaline/norepinephrine (NE)
- Used by post-ganglionic neurons of the sympathetic division
Released from the adrenal medulla as a hormone that enters the blood stream
Receptors of the NS:
Cholinergic receptors (receptors of acetylcholine)
- Nicotinic
- Muscarinic
Adrenergic receptors (receptors of noradrenaline/norepinephrine)
- Alpha
Beta
Cholinergic receptors of the ANS:
- Receptors of Acetylcholine (nicotinic or muscarinic)
Recall that acetylcholine is:- Used by pre-ganglionic neurons in both sympathetic and parasympathetic divisions
Used by post-ganglionic neurons of the parasympathetic division
- Used by pre-ganglionic neurons in both sympathetic and parasympathetic divisions
Nicotinic receptors:
- Found on dendrites of all post-ganglionic neurons of both sympathetic and parasympathetic divisions
- Found within the adrenal medulla
Are ALWAYS excitatory - Located on all post ganglionic neurons
- Para and symp neurons
- Adrenal gland
- Skeletal muscle cells (only in somatic nervous system)
Always stimulatory
- Found within the adrenal medulla
Muscarinic receptors:
- Found on visceral effector cells of the parasympathetic division
Effect can be excitatory or inhibitory- Located on all effector cells stimulated by postganglionic cholinergic fibers (all parasympathetic)
Can be inhibitory or stimulatory
- Located on all effector cells stimulated by postganglionic cholinergic fibers (all parasympathetic)
Adrenergic receptors of the ANS:
- Receptors of noradrenaline/norepinephrine Alpha or Beta and subdivisions of each
- Used by post-ganglionic neurons of the sympathetic division
Released from the adrenal medulla as a hormone that enters the bloodstream
- Used by post-ganglionic neurons of the sympathetic division
Alpha/beta receptors:
- Found on visceral effector cells of the sympathetic division
Effect can be excitatory or inhibitory
Recap:
Sympathetic division:
- Pre-ganglionic neuron leaves the spinal cord and synapses with post-ganglionic neuron, releasing acetylcholine across the synaptic space
- Nicotinic receptors on the post-ganglionic neuron are excited and continue to send the signal down toward the visceral effector
- Post-ganglionic neurons release noradrenaline/norepinephrine that is received by alpha or beta receptors on the visceral effector
Result: Visceral effector either stimulated or inhibited