bsc2085 lesson 19

somatic nervous system (efferent)

- operates under conscious control

- controls skeletal muscles

- seldom affect long-term survival

autonomic nervous system (visceral motor system)

- operates without conscious instructions

- controls visceral effectors such as cardiac and smooth muscles, glands, adipocytes

- coordinates organ system functions

primary target organs of ANS

- viscera of thoracic and abdominal cavities

- some structures of the body wall- cutaneous blood vessels, sweat glands, and arrector muscles

autonomic system carries out actions involuntarily

- visceral effectors don't depend on ANS to function, but use it to adjust their activity to body's needs

- severing ANS nerves to smooth/cardiac muscles results in exaggerated responses (denervation hypersensitivity)

parasympathetic division

calms many body functions reducing energy expenditure and assists in bodily maintenance; rest and digest

autonomic tone

- normal background rate of activity that represents the balance of the 2 systems according to body's needs

- body doesn't just alternate between two states (action v rest)

parasympathetic tone

- maintains smooth muscle tone in intestines

- holds resting heart rate down to about 70-80 bpm

examples of sympathetic tone

keeps most blood vessels partially constricted and maintains blood pressure

divisions of ans

neither division has universally excitatory nor inhibitory effects; usually they have opposing effects

ANS output pathways

- ANS has components in central and peripheral nervous systems

- control nuclei in hypothalamus and other brainstem regions

- motor neurons can be located in the spinal cord and peripheral ganglia

- nerve fibers that travel through the cranial and spinal nerves

somatic motor pathway

a motor neuron from brainstem or spinal cord issues a myelinated axon that reaches all the way to skeletal muscle

autonomic motor pathway

a signal must travel across 2 neurons to get to the target organ, and cross a synapse where these two neurons meet in an autonomic ganglion

First neuron in autonomic pathway

- cell body in the brainstem or spinal cord

- its axon (preganglionic fiber) extends to autonomic ganglion

ganglion of autonomic pathway

- at this point, first neuron synapses with second (post/ganglionic neuron)

- ACh is the neurotransmitter released by the first neuron

second neuron in autonomic pathway

- cell body located in autonomic ganglion

- its axon (postganglionic fiber) extends to target cells

- neurotransmitter released is either ACh or NE

Difference in somatic and autonomic efferent pathways

- in somatic pathway, the motor neuron is in the brain stem/spinal cord and extends an axon all the way to targeted skeletal muscle

- in the autonomic pathway, the motor neuron is in the CNS and synapses with another motor neuron which then extends an exon to target cell

sympathetic motor vs parasympathetic motor innervation

in sympathetic, ganglion is close after spinal cord, in parasympathetic it's all the way against postganglionic fiber or close to it

Anatomy of sympathetic division

- its pathways arise from thoracic and lumbar regions of the spinal cord

- (aka thoracolumbar division)

- relatively short preganglionic and long post ganglionic fibers

General pathway of sympathetic division

- cell bodies of preganglionic neurons in lateral horns and nearby regions of spinal cord gray matter; fibers exit at T1 to L2

- lead to nearby paravertebral ganglia from C to Co levels

- usually 3C, 11T, 4L, 4S, 1Co

- nerve fibers distributed to every level of body

- axons leave spinal cord and enter ventral roots of segments before terminating on ganglionic neurons

7 responses to increased sympathetic activity

- heightened mental alertness

- increased metabolic rate

- reduced digestive and urinary functions

- energy reserves activated

- increased respiratory rate and respiratory passageways dilate

- increased heart rate and blood pressure

- sweat glands activated

3 possible areas of ganglionic neurons (bus stops)

- sympathetic chain ganglia

- collateral ganglia

- adrenal medullae

Sympathetic chain ganglia

- on both sides on vertebral column

- in body wall

- inside thoracic cavity

- in head

- in limbs

Collateral ganglia

- anterior to vertebral bodies

- contain ganglionic neurons that innervate tissues and organs in abdominopelvic cavity

- preganglionic axons that synapse here are from motor neurons in CNS- axons don't make any synapses in sympathetic chain on their way to collateral ganglia

Adrenal medullae (suprarenal medullae)

- very short axons (ganglionic neurons are endocrine cells); innervated by axons from CNS that travel thru sympathetic chain and collateral ganglia

- when stimulated, release neurotransmitters into blood stream (not at synapse)

- functions as hormones to affect target cells throughout body

- releases NE and E into bloodstream

Adrenal (suprarenal) glands

- paired glands located on superior poles of kidneys; each is two glands with different functions

- adrenal cortex

- adrenal medulla

adrenal cortex

outer layer, secretes steroid hormones

adrenal medulla

- inner core; essentially a sympathetic ganglion

- consists of modified postganglionic neurons without dendrites; axons

- stimulated by preganglionic sympathetic neurons

- secretes a mixture of hormones into bloodstream: catecholamines- 85% epi, 15% nor

stimulation of sympathetic preganglionic neurons

- releases ACh at synapses with ganglionic neurons

- ACh is always released at cholinergic synapses

- always excitatory on ganglionic neurons

stimulation of sympathetic ganglionic neurons

release neurotransmitters from the post ganglionic axons at specific target organs- mostly release NE, but some ganglionic neurons release ACh

ganglionic neurons: axon terminals

- form branching networks of telodendria instead of sympathetic terminals

- telodendria form sympathetic varicosities

- resemble string of pearls

- swollen segment packed with neurotransmitter vesicles

- pass along or near surface of effector cells

- no specialize postsynaptic membranes

- membrane receptors on surfaces of target cells

- if release NE- adrenergic neuron

- if release ACh- cholinergic (located in body wall, skin, brain, and skeletal muscles)

Sympathetic activation

- activates entire sympathetic division during crisis

- releasing NE at peripheral synapses

- target specific effectors, smooth muscle fibers in blood vessels of skin

- distribute E and NE thru body in bloodstream

- controlled by sympathetic centers in hypothalamus

- effects aren't limited to PNS, alters CNS activity

Changes caused by sympathetic activation

- increased alertness- stimulates RAS

- feelings of energy and euphora

- change in breathing- higher activity in respiratory and CV centers of pons and medulla

- elevation in muscle tone

- mobilization of energy reserves

Craniosacral division

Another name for parasympathetic division because of pathway origins

Parasympathetic division

has long preganglionic and short postganglionic fibers

Pathway of Parasympathetic Division

- cell bodies of preganglionic neurons are in midbrain, stem, or segments S2-S4

- preganglionic fibers travel through cranial or sacral nerves, ending in terminal ganglia (in/near) target organs

- synapse with ganglionic neurons in ganglia

- postganglionic fibers cover the rest of distance to target organ

Intramural ganglion

Embedded within organ wall

Cranial nerves involved with parasympathetic division

III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus)

Control visceral structures in the head

CN III, VII, and IX

Vagus nerve

- 75% of all parasympathetic outflow

- Innervates structures in neck, thoracic, and abdominopelvic cavities

Parasympathetic preganglionic fibers exit

Leave spinal cord at sacral level as pelvic nerves

Neuromuscular and Neuroglandular Junctions of Parasympathetic Division

- small with narrow synaptic clefts

- all release ACH as neurotransmitter

- effects of stimulation are short lived (seconds)

Inactivation of ACh in Parasympathetic division

- by acetylcholinesterase at synapse

- by cholinesterase in surrounding tissues

Responses to Increased Parasympathetic Activity

- Decreased metabolic rate

- Decreased heart rate and blood pressure

- Increased secretion by salivary and digestive glands

- Increased motility and blood flow in digestive tract

- Stimulation of urination and defecation

Enteric plexus

- nervous network of digestive tract

- 500 million neurons found in walls of the digestive tract

- doesn't arise from brainstem or spinal cord

- innervates smooth muscle, glands

- has own ganglia and reflex arcs

- regulates motility of esophagus, stomach, intestines, and secretions of enzymes and acid

- regulated by sympathetic and parasympathetic systems

reasons why ANS has opposite effects on an organ

- sympathetic and parasympathetic fibers secrete diff neurotransmitters

- diff receptors on target cells respond differently to same neurotransmitter

ACh

- secreted by preganglionic sympathetic neurons

- secreted by preganglionic parasympathetic neurons

- secreted by postganglionic parasympathetic neurons

Cholinergic receptor

binds ACh

Cholinergic sympathetic terminals

- innervate sweat glands of skin and blood vessels of skeletal muscles and brain

- stimulate sweat gland secretion and dilate blood vessels to skeletal muscles and brain

Categories of ACh receptors

- muscarinic receptors

- nicotinic receptors

Muscarinic receptors

- found on cardiac muscle, smooth muscle, gland cells with cholinergic innervation

- act through second-messenger systems

- receptor subtypes exist, providing contrasting effects on organs

- indirect effect

Example of contracting receptor subtype

ACh excites intestinal smooth muscle, but inhibits cardiac muscle

Nicotinic receptors

- Found on ganglionic neuron cell bodies in autonomic ganglia, on cells of adrenal medulla, and neuromuscular junction of skeletal muscle fibers

- Binding of ACh to these receptors is always excitatory

- Open ligand-gated Na+ ion channels, excitatory postsynaptic potential

- Direct effect

Nicotine

- Binds to nicotinic receptors in sym and parasym ganglion cells

- targets autonomic ganglia and skeletal neuromuscular junctions

Nicotine posioning

- ingesting/absorbing 50mg or more

- vomiting

- diarrhea

- high blood pressure

- tachycardia

- sweating

- profuse salivation

- convulsions

- coma or death

Muscarine

- toxin produced from some poisonous mushrooms

- binds to muscarinic receptors

- targets parasympathetic neuromuscular/glandular junctions

Muscarine poisoning

- salivation

- nausea

- vomiting

- diarrhea

- constriction of airways

- low blood pressure

- bradycardia

Norepinephrine

- Secreted by nearly all sympathetic ganglionic neurons

- nerve fibers that secrete it are adrenergic fibers

- receptors are called adrenergic receptors

Alpha-adrenergic receptors

- usually excitatory

- two subclasses use different second messengers (a1 and a2)

Beta-adrenergic receptors

- usually inhibitory

- two subclasses with different effects, but both act through cAMP as a second messenger (b1 and b2)

Autonomic effects on glandular secretion

- often an indirect result of their effect on blood vessels

- increased blood flow, increased secretion

- decreased blood flow, decreased secretion

_____ effects tend to last longer than ____ effects

Sympathetic; Parasympathetic

Why do sympathetic effects last longer?

- NE released by sympathetic fibers an be reabsorbed by axon terminal, diffuse, or pass into blood

- ACh from parasympathetic fibers is quickly broken down at synapse

Other neurotransmitters

Some ANS fibers release other neurotransmitters in addition to ACh or NE

Dual innervation

- most viscera receive nerve fibers from both parasympathetic and sympathetic divisions

- not always equal influences

Antagonistic dual innervation causes opposing effects

- can be exerted through dual innervated of same effector cells

- can be exerted because each division innervates diff cells

Cooperative dual innervation

- Act on different effectors to produce a unified overall effect

- saliva production: para stimulates serous cell secretion and sym stimulates mucous cell secretion

control without dual innervation

Some effectors receive only sympathetic fibers

Some effectors receive only sympathetic fibers

- Adrenal medulla

- arrector muscles

- sweat glands

- many blood vessels.

Regulation of blood pressure and routes of blood flow

- Baseline sympathetic tone- vasomotor tone

- Increase in firing frequency=vasoconstriction

- Decrease in firing frequency=vasodilation

- can also shift blood from one organ to another

Blood vessels in stress

- dilate to skeletal muscles and heart

- to skin constrict

ANS regulated by

- cerebral cortex and limbic system

- hypothalamus

- midbrain, pons, medulla

- spinal cord

Cerebral cortex and limbic system

emotions influence the ANS because of the connections between our limbic system and hypothalamus

Hypothalamus

contains nuclei for primitive functions- hunger, thirst, sex

Midbrain, pons, medulla oblongata

contain nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes

Spinal cord

- Reflexes for defecation and micturition

- If damaged, controlled by ANS reflexes

Neuropharmacology

study of effects of drugs on nervous system

Sympathomimetics

- enhance sympathetic activity

- stimulate receptors or increase norepinephrine release

- cold medicines that dilate bronchioles or constrict nasal blood vessels

Sympatholytics

- suppress sympathetic activity

- block receptors or inhibit norepinephrine release

- beta-blockers reduce high BP interfering with NE and epi effects

Parasympathomimetics

enhance parasympathetic effects

Parasympatholytics

suppress parasympathetic effects

Prozac

blocks reuptake of serotonin to prolong mood-elevating effect

MAO inhibitors

Block monoamine oxidase to slow breakdown of monoamine neurotransmitters

Caffeine

competes with adenosine by binding to its receptors

Effectors that only receive sympathetic fibers

adrenal medulla, arrector muscles, sweat glands, many blood vessels