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)
Bobby’s Bird Enjoys Being in Italy Everyday
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
Recap:
Parasympathetic 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 acetylcholine that is received by muscarinic receptors on the visceral effectors
Result: Visceral effector either stimulated or inhibited
Sympathetic neuron origin and outflow:
Originate from lateral horns of the spinal cord in spinal segments T1-L2, pre and post ganglionic neurons synapse in sympathetic chain in sympathetic ganglia where they can ascend or descend, postganglionic neurons innervate target structure. Get sent to innervate viscera
Parasympathetic neurons have cranial and sacral divisions and outflow and cranial via cranial nerves
Parasympathetic neurons innervation:
Note the regions innervated: Visceral structures in head thoracic and abdominal cavity. Para neurons have ganglia close to the structure innervated, sacral part of the spinal cord- viscera in the true pelvis (rectum, bladder, genital)
Sympathetic neuron innervation:
Sympathetic: Innervating pupil muscle to enact pupillary dilation, dilates blood vessels, increase heart rate, acts on bronchioles to dilate bronchioles acts on glands such as adrenal to secrete adrenaline, closes sphincters
Where is para/sympathetic ganglia found? Lengths?
- Sympathetic ganglia: found close to the spinal cord
- Parasympathetic: near/inside effector organ
- In Sympathetic: the preganglionic fibers are shorter than post
In parasympathetic: pre ganglionic nerve fibers are longer than post nerves fibers/axons
What happens in the action of moving a muscle?
receive blood from vessels, brain sends signal/action potential down spinal cord and preganglionic axon terminal and continues by sending signal to the ganglia, when signal reaches synapse inside ganglia, the signal reaches synapse inside ganglia, the nerve fiber releases a neurotransmitter called Ach - crosses synapse, stimulates neurons in postganglionic nerve fibers which then carry signal to effector organs e.g. smooth muscles of blood vessels in arms and legs. At the end of the post-ganglionic neurons, the fibers release a different neurotransmitter norepinephrine/noradrenaline (always released from post ganglionic nerve fiber as is crosses final synapse and creates a response)
The sympathetic trunk connect to the spinal nerve via the:
- Pre-ganglionic neuron: in via white ramus
Post-ganglionic neuron: out via grey ramus
Origin of sympathetic neurons:
- Preganglionic sympathetic neurons originate in the lateral horn if the spinal cord
- T1-L2 spinal segments
- Sometimes includes L3 due to the anatomical variation
- From here they exit the ventral root and enter the spinal nerve
The preganglionic axons leave spinal cord along with somatic efferent fibers at the same segmental level, the exit through intervertebral foramina and the myelinated axons pass to anterior root of spinal nerve in a short pathway called white ramus, before passing to the nearest sympathetic ganglionic the same side
Course of sympathetic neurons:
- The sympathetic trunk: Contain cell bodies of post ganglionic sympathetic neurons
- Allow the synapse of preganglionic and postganglionic sympathetic neurons
- Course of sympathetic neurons: Note synapses can occur at higher or lower levels in the trunks
When preganglionic neurons enter the sympathetic chain they can ascend or descend and synapse at the same level that it corresponding postganglionic neurons exit the sympathetic chain
Distribution of sympathetic neurons
- The sympathetic trunk: Receives preganglionic sympathetic neurons, some ascend, some descend
- Ganglia contains the synapse of pre and post ganglionic sympathetic neurons
Post ganglionic sympathetic neurons are then distributed to the body via peripheral nerves (spinal and cranial) and arteries
- Ganglia contains the synapse of pre and post ganglionic sympathetic neurons
Distribution of autonomic neurons:
- Autonomic neurons are required to reach target tissues around the body, they must travel with an anatomical structure
- Autonomic neurons must be transferred between peripheral nerves in order to reach effector organs
- Autonomic neurons may also travel with blood vessels to gain entry into a region
- Known as hitchhiking of nerve fibers
Can hitchhike with artery to get to the region of the head or where they would like to get to
Hitch-hiking of autonomic neurons:
- Post-ganglionic sympathetic neurons: Hitch-hike with the common carotid artery to enter the head and neck
- Then hitch-hike with peripheral nerves to reach a target structure
- Smooth muscle in blood vessels in head and neck, smooth muscle in glands (sweat)
- Parasympathetic neurons: Then hitch-hike with peripheral nerves to reach a target structure
- Smooth muscle in the pupil (pupillary constrictor), smooth muscle in glands (salivary)= example of cranial outflow
- Smooth muscle in glands (glands in genitalia) = example of sacral outflow
- Para neurons has cranial divisions so they must be contained in four of the 12 cranial nerves, when they get distributed some include pupils = cranial outflow
- Para neurons have sacral divisions, which go off to supply glands in pelvic region
Para neurons = cranial and sacral divisions
Course of sympathetic neurons in the head and neck:
- Sympathetic arise from thoracolumbar region of spinal cord, T1 spinal segment. Preganglionic neurons travel to sympathetic ganglia and descend up sympathetic chain to most superior one (superior cervical ganglion)
Post ganglionic are travelling out with the carotid artery up into the head
Which of the following is NOT an effect of sympathetic nervous system activation?
Constriction of bronchioles
Which of the following options is NOT a physiological response you would expect to occur when your ‘rest & digest’ response is activated?
Decreased glandular secretion of saliva
Where are the cell bodies of preganglionic sympathetic neurons located?
Thoracolumbar spinal cord
Which of the following best describes the pathway of sympathetic preganglionic neurons that synapses in the adrenal medulla?
They pass directly to the adrenal medulla without synapsing in a ganglion
Which of the following best describes the function of the sympathetic trunk?
Relaying sympathetic nerve signals between the spinal cord and target organs
Which neurotransmitter would be considered the ‘primary’ neurotransmitter involved in the parasympathetic nervous system
Acetylcholine
Which of the following options is correct with regard to (A) neuron type, and (B) neurotransmitter utilised?
(A) Post-ganglionic parasympathetic (B) Acetylcholine
What type of receptors do postganglionic sympathetic neurons primarily activate on visceral effectors?
Adrenergic receptors
