APPP 11: Peripheral Nervous System and Autonomic Nervous System Flashcards
What is the nervous system divided into anatomically?
- peripheral nervous system (PNS)
- central nervous system (CNS)
What is the PNS divided into?
- autonomic nervous system (involuntary)
- somatic nervous system (voluntary)
What is the autonomic nervous system divided into?
- parasympathetic nervous system
- sympathetic nervous system
What is the somatic nervous system divided into?
- sensory
- motor
How many nerves does the PNS have?
- 12 cranial nerves (ventral surface of the brain)
- 31 spinal nerves
Are nerves of the PNS myelinated (formed by surrounding Schwann cells) or unmyelinated?
can be either
What type of information do spinal nerves of the PNS contain?
both afferent and efferent information
What type of information do cranial nerves of the PNS contain?
only afferent information
What does the somatic nervous system of the PNS innervate?
skeletal muscle
What does the autonomic nervous system of the PNS innervate?
- glands
- neurons of the gastrointestinal tract
- cardiac and smooth muscles of glandular tissue
What is cranial nerve I?
olfactory nerve
- smell
What is cranial nerve II?
optic nerve
- vision
What is cranial nerve III?
oculomotor nerve
- control of eye movement – precise movement
- pupil constriction – focusing
What is cranial nerve IV?
trochlear nerve
- control of eye movement – move down and out
What is cranial nerve V?
trigeminal nerve – 3 branches (V1, V2, V3)
- touch, pain
- muscles for chewing
- V1: general sensory innervation to cornea, bridge of nose, scalp, and forehead
- V2: sensory innervation to the nasal cavity, sinuses, and maxillary teeth
- V3: sensory innervation of the lower part of the face (tongue, mandibular teeth, motor innervation of muscles of mastication)
What is cranial nerve VI?
abducens nerve
- control of eye movement – directs pupil laterally
What is cranial nerve VII?
facial nerve
- taste – anterior 2/3 of tongue
- sound
- facial expression, submandibular and sublingual salivary glands
What is cranial nerve VIII?
vestibulocochlear nerve
- hearing
- balance – equilibrium
What is cranial nerve IX?
glossopharyngeal nerve
- taste – posterior 1/3 of tongue
- gag reflex
- swallowing
- also motor innervation to parotid gland (saliva)
- also somatosensory information from tongue, tonsils, pharynx
What are the 5 tastes?
- sweet
- sour
- bitter
- salty
- umami (amino acid)
What is cranial nerve X?
vagus nerve
- has the most extensive distribution in the body
- supplies parasympathetic innervation (digestion, breathing, heart rate)
- slows heart reate
- increases secretions
What is cranial nerve XI?
spinal accessory nerve
- head movement – head turning
What is cranial nerve XII?
hypoglossal nerve
- tongue movement
What is the autonomic nervous system involve in?
- arousal (fight or flight), sympathetic
- calm (relax and digest), parasympathetic
What are the cranial nerves involved in the parasympathetic nervous system?
- oculomotor (III)
- facial (VII)
- glossopharyngeal (IX)
- vagus (X)
Which spinal nerves are involved in the sympathetic nervous system?
- thoracic
- lumbar
Describe the 2-neuron system of the autonomic nervous system.
- cells of origin lie within the CNS
- synapse occurs in the autonomic ganglia located outside the CNS
- preganglionic fibre utilizes a neurotransmitter to signal the postganglionic fibre
- postganglionic fibre uses a neurotransmitter to signal the effector organ
- these two neurotransmitters are not always the same
Where is the ganglia and what is the major neurotransmitter of the parasympathetic division?
- parasympathetic ganglia are usually located in or near target organs
- major neurotransmitter is acetylcholine
What is a result of parasympathetic activation
- slows heart rate
- decreases blood pressure
- constricts pupils
- stimulates digestive system activity
- promotes emptying of bladder and rectum
Where is the ganglia and what is the major neurotransmitter of the sympathetic division?
- sympathetic ganglia are usually located in proximal ganglion
- major neurotransmitter is acetylcholine and norepinephrine
What is a result of sympathetic activation?
- increases heart rate
- increases blood pressure
- dilates pupils
- decrease digestive system activity
What are the neurotransmitters of the ANS and what effect do they have?
acetylcholine and norepinephrine
- both can have excitatory and inhibitory actions
- mode of action depends on the receptor present on the target neuron or tissue
What are nicotinic receptors of the ANS and how do they work?
- ligand-gated ion channels
- responsive to ACh
- activation always causes a rapid increase in cellular permeability to Na+ and Ca2+
- depolarization and excitation
What are muscarinic receptors of the ANS and how do they work?
- there are 5 distinct subtypes, each produced by a different gene – these variants have distinct anatomic locations in the periphery and CNS and differing chemical specificities
- are metabotropic receptors (GPCRs) – can inhibit or excite postsynaptic neurons depending on their coupling to G-protein α-subunits
What happens when a signaling molecule binds to a G-Protein Coupled Receptor (GPCR)?
results in G protein activation, which in turn triggers the production of any number of second messengers
Is the Gs protein family excitatory or inhibitory?
excitatory
Is the Gi protein family excitatory or inhibitory?
inhibitory
Is the Gq protein family excitatory or inhibitory?
excitatory
Is the G12 protein family excitatory or inhibitory?
excitatory
Muscarinic Receptors
M1
- location
- postreceptor mechanism
- location: nerves
- postreceptor mechanism: IP3, DAG cascade
Muscarinic Receptors
M2
- location
- postreceptor mechanism
- location: heart, nerves, smooth muscle
- postreceptor mechanism: inhibition of cAMP production, activation of K+ channels
Muscarinic Receptors
M3
- location
- postreceptor mechanism
- location: glands, smooth muscle, endothelium
- postreceptor mechanism: IP3, DAG cascade
Muscarinic Receptors
M4
- location
- postreceptor mechanism
- location: CNS
- postreceptor mechanism: inhibition of cAMP production
Muscarinic Receptors
M5
- location
- postreceptor mechanism
- location: CNS
- postreceptor mechanism: IP3, DAG cascade
Which muscarinic receptors are associated with acetylcholine?
- M1
- M4
What is acetylcholine?
neurotransmitter released at both the target organs of primarily the parasympathetic system, but also the sympathetic system, that act on muscarinic receptors
Which muscarinic receptors cause excitation?
M1, M3, and M4
- phosphotidyl inositol
- IP3, DAG
Which muscarinic receptors cause inhibition?
M2 and M4
- adenylyl cyclase
- cAMP
What is norepinephrine?
neurotransmitter released at most sympathetic postganglionic neuroeffector sites
What receptors does norepinephrine act via?
- alpha receptors – α1, α2
- beta receptors – β1, β2, β3
How do α1 receptors work?
- increase 1,4,5-triphosphate (IP3) and diacylglycerol (DAG)
- IP3 promotes release of Ca2+ from intracellular stores
- DAG activates protein kinase C
- EXCITATION
How do α2 receptors work?
- inhibit adenylyl cyclase
- INHIBITION
How do β receptors work?
- β1 increases cAMP – ie. heart
- β2 increases cAMP – ie. bronchial smooth muscle
- β3 increases cAMP – ie. adipose tissue, bladder
- tissue specific actions: heart, β receptor activation increases the influx of calcium across the cell membrane and its sequestration inside the cell – increases force of contraction
Sympathetic vs. Parasympathetic Division
Cell of Origin (Pre-Ganglionic Neuron)
- sympathetic: thoracic and upper lumbar spinal cord
- parasympathetic: brainstem (cranial nerves) and sacral spinal cord
Sympathetic vs. Parasympathetic Division
Number of Nerve Fibres
- sympathetic: many
- parasympathetic: few – vagus (X) nerve is the major nerve of this division
Sympathetic vs. Parasympathetic Division
Cell of Origin
- sympathetic: short axon
- parasympathetic: long axon
Sympathetic vs. Parasympathetic Division
Ganglion Location
- sympathetic: near spinal cord
- parasympathetic: near organ
Sympathetic vs. Parasympathetic Division
Ganglion Neuron
- sympathetic: long axon
- parasympathetic: short axon
Sympathetic vs. Parasympathetic Division
Major Neurotransmitter
- sympathetic: acetylcholine and norepinephrine
- parasympathetic: acetylcholine
Sympathetic vs. Parasympathetic Division
Function
- sympathetic: fight or flight, arousing
- parasympathetic: rest and digest, calming
Lung Function
What modulates airway tone?
vagus nerve
Lung Function
What happens in patients with asthma, COPD, chronic cough?
- in the airways, acetylcholine is released from efferent endings of the vagus nerve fibre
- ACh acts on M3 receptors – stimulates muscle cells to contract
- results in high constriction
- muscarinic antagonists block this effect – less constriction
What is syncope?
fainting or passing out, temporary and sudden loss of consciousness
- caused by a temporary drop in blood pressure or drop in heart rate – temporary drop in the amount of blood that flows to the brain
What are the causes of syncope?
- age-related cardiovascular changes – prevalence of syncope increases with age (> 20% in ages above 75)
- neural-mediated etiologies
Who is at risk for syncope?
- older adults that have reduced vascular tone
- people who overreact to certain triggers such as the sight of blood or extreme emotional distress
- sympathetic withdrawal and vagus nerve overstimulation, excess acetylcholine release
Describe who syncope occurs.
- sympathetic withdrawal and vagus nerve overstimulation, excess acetylcholine released
- heart rate slows and blood vessels dilate, making it harder for blood to defeat gravity and be pumped to the brain
- decrease in blood flow to the brain
- brain is deprived of oxygen
- loss of consciousness/fainting episode
(body is trying to restore blood pressure)
Syncope has also been reported in people after vaccination.
- fainting is more common in adolescents than in children or adults – thus is more common after vaccination with HPV, MCV4, and Tdap
- immediate fainting episodes following vaccination procedures is triggered by pain or anxiety, not the contents of the vaccines
- while not serious, falling is the primary concern
- prevent falls by having patients lie down or sit during the procedure
- clinicians can give patients drinks and snacks to prevent some fainting
- patients who faint after vaccination will recover after a few minutes, and clinicians should observe patients for at last 15 minutes after vaccination (recommendation of the CDC)
What drugs are commonly implicated in syncope?
- agents that reduce blood pressure – ie. antihypertensives, diuretics, nitrates
- serotonin agonists
What are the treatments for syncope?
- elderly: modifying or discontinuing medications known to cause syncope is often the only necessary intervention
- serotonin antagonists (ie. fluoxetine) have been reported to be effective
What is dopamine β hydroxylase deficiency?
a rare genetic syndrome characterized by the complete absence of norepinephrine in the peripheral and central nervous system
- deficiency in the enzyme that converts dopamine to norepinephrine
- results in progressive sympathetic denervation, but normal cholinergic innervation
What is the only effective treatment for dopamine β hydroxylase deficiency?
involves administration of the drug L-threo-3,4-dihydroxyphenylserine (DOPS, droxidopa)
- DOPS is converted directly into NE via a decarboxylase, thereby bypassing DβH
What nerves does the somatic nervous system (voluntary nervous system) contain?
- afferent nerves made of sensory neurons that inform the central nervous system about our 5 senses
- efferent nerves which contain motor neurons responsible for voluntary movements, such as walking or lifting an object
Where are cell bodies of the SNS located?
either in the brainstem or the spinal cord
- extremely long course as they do not synapse after they leave the CNS until they are at their termination in skeletal muscle
Autonomic Nervous System
Adenoreceptors
What type of receptors does the SNS have?
nicotinic receptors
SNS – Acetylcholine
- influx of Na+ changes membrane potential
- Ca2+ channels at the sarcoplasmic reticulum (SR) open and leads to an increase in cytoplasmic Ca2+
- Ca2+ binds troponin, actin active site is exposed, myosin binds and pulls actin filaments
What are the nerves in the sensory system?
afferent nerves made of sensory neurons inform the nervous system about our 5 senses
Sensory System
What is the specificity theory?
sensory neurons are sensitive to only one form of stimulation and encode only one perceptual quality
Sensory System
What do mechanoreceptors sense?
tactile or discriminative touch
Sensory System
What do thermoreceptors sense?
thermal or temperature sense
Sensory System
What do chemoreceptors sense?
itch (and other senses)
Sensory System
What do nociceptors sense?
pain
Sensory System
Describe sensory fibres
nerve fibres contain afferent fibres of varying conduction velocities that have specific sensory functions
Sensory System
What are the 2 types of cutaneous mechanoreceptors?
- hair follicle afferents
- Merkel cells
Sensory System
Describe hair follicle afferents.
- detect light touch
- rapidly adapting (send information related to changing stimuli)
Sensory System
Describe Merkel cells.
- slowly adapting (send information about ongoing stimulation)
- important for spatial resolution, pressure sensation, and roughness
Sensory System
Describe thermal sensitivity.
- highest on face, more sensitive to cold than heat – cold fibres activated with very small change in temperature
- cold-specific – mostly C-fibres in humans
- cold allodynia – lower cold pain threshold – can be a symptom of peripheral neuropathies due to chemotherapy, a consequence of stroke
Sensory System
Describe cold sensation
- rare cold fibres are also activated by noxious heat (ie. severe burn) – paradoxical cold sensation produced by heat
- menthol activates cold fibres
Sensory System
Describe itch (pruritus).
- frequently occurring, but poorly understood somatosensory experience
- an actual or perceived disruption to the skin that produces a desire to scratch
- scratching can often lessen or remove the itch sensation
- itch is now considered an independent sensation from pain, although until recently it was felt to be part of the pain continuum
- associated with numerous allergic diseases – ie. atopic eczema
- itch is proposed to be transmitted from the skin to the spinal cord by C fibres (unmyelinated, < 2 m/s)
- broadly categorized as having either a histamine-dependent (acute) or histamine-independent (chronic) pathogenesis
- acute itch is responsive to typical antihistmaines, but chronic itch is not
