Download Chapter 15 the autonomic nervous system -

Chapter 15
the autonomic nervous system (ANS)
- controls many homeostatic mechanisms such as
heart rate, blood pressure, respiration, pupils, digestion,
metabolism, defecation, urination
- an involuntary system
- no ANS = death
- ANS = visceral motor system, has 2 parts, thoracic and
abdominopelvic
- target organs are viscera of the thoracic and abdominopelvic cavities
and glands, BV, and arrector pili muscles of the body wall
- if ANS modulation is absent many functions become hyperactive
- ANS is made up of 2 nervous systems
- the sympathetic and parasympathetic systems
- they have opposite effects on the body
- many ANS functions involve reflexes
- but unlike somatic reflexes which involve the cord
these reflexes involve a receptor, a neuron (in cord),
an interneuron, the brainstem, and an effector (response)
sympathetic = fight or flight, metabolism up, available sugar up,
= thoracolumbar division, short preganglionic fibers
and long postganglionic fibers which lead to the chain
of paravertebral ganglia
parasympathetic = craniosacral division, resting, digesting, calming,
= long preganglionic fibers and short post
ganglionic fibers
both systems always at work which produces an autonomic tone
where each system works to produce the best
outcome for the situation, both systems have
inhibitory and excitatory components
- for somatic reflexes the postganglionic fibers ended
at specific target cells
BUT
- in the ANS the postganglionic fibers end as a group or chain
of varicosities which release neurotransmitters at many
points and stimulate many cells at the same time
these two opposing nervous systems produce homeostasis
sympathetic system
- thoracolumbar division (T1 to L2)
- fibers from the spinal cord go to the sympathetic chain ganglia
- fibers from the chain move up and down to cover all of the body
neural pathways through sympathetic chain ganglia
- there are 3 possible paths for preganglionic fibers
- 1st a spinal path, in and out at the same level
- 2nd pre go or then post to form plexus prior to effectors
- 3rd presynaptic pass through T5 –T12 and form splanchnic
nerves and form a 2nd set of ganglia where they synapse
and send postsynaptic to effectors, also includes
presynaptic from L1 to Coc1
- these form the celiac, superior mesenteric, & inferior
mesenteric ganglia
- effectors in head & thoracic cavity use sympathetic nerves
- in the abdominopelvic cavity effectors use splanchnic nerves
- Info from one nerve to one effector never exists in the body
- every action involves integration from many nerves and centers
- within the sympathetic chain collateral fibers travel up and
down the ganglionic chain
- the result is that pre and post ganglionic fibers receive input
from other levels of the ganglia resulting in neural convergence,
meaning the output is the result of many inputs
- pre sympathetic fibers synapse with 10 to 20 postganglionic fibers
resulting in neural divergence which can effect more than 1 organ
the adrenal glands
- also known as the suprarenal glands
- adrenal cortex secretes steroids
- adrenal medulla is a sympathetic ganglion receiving
pre fibers and the postganglionic fibers do not have
dendrites or axons but
- they secrete hormones - 85% adrenaline (epinephrine)
and 15% noradrenaline (norepinephrine)
- these hormones have effects throughout the body
parasympathetic system (craniosacral division)
- pre fibers come from the midbrain, pons, and medulla oblongata
and S2 to S4 of spinal cord
- long pre which end in terminal ganglia in or near effector
- short post fibers in or to the effector
- less divergence than sympathetic only 1 pre to 5 post or less
- the parasympathetic is more selective than the sympathetic
- consists of cranial nerves III, VII, IX, X, and S2 to S4
- cranial nerve X through communicating fibers joins S2 to S4
III = oculomotor – lens and pupil, ciliary muscles, pupillary
constrictors through ciliary ganglion
VII = facial – tears, salivary, nasal glands (submandibular ganglion)
IX = glossopharyngeal – tympanic nerve, otic ganglion, parotid glands
X = vagus – 90% of all parasympathetic preganglion fibers,
form pulmonary, esophageal and cardiac plexii which send fibers
in or near target organs, at the lower esophagus the vagus sends
presynaptic fibers which form the abdominal aortic plexus which
innervates most abdominal organs, BV, glands,
- S2 to S4 send splanchnic through the inferior hypogastric plexus
where some synapse but most continue to innervate the lower
abdominal areas, bladder and genitals
the digestive system has a nervous system of its own
- the enteric nervous system
- does not originate from the brainstem
- has millions of fibers throughout the digestive tract
- has its own reflex arcs
- regulates motility and secretions
- proper functioning requires input from sym & para sympathetics
The enteric nervous system consists of some 500 million
neurons, one two-hundredth of the number of neurons in the
brain, and 5 times as many as the one hundred million neurons
in the spinal cord. The enteric nervous system is embedded in
the lining of the gasterointestinal system, beginning in the
esophagus and extending down to the anus.
The neurons of the ENS are collected into two types of ganglia:
myenteric and submucosal plexuses. Myenteric plexuses are
located between the inner and outer layers of the GI muscles,
while submucosal plexuses are located in the submucosa.
Function
The ENS is capable of autonomous functions such as the coordination of reflexes; although it receives considerable
innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord. Its
study is the focus of neurogastroenterology.
ENS function can be damaged by ischemia. Transplantation, previously described as a theoretical possibility,has been a
clinical reality in the United States since 2011 and is regularly performed at some hospitals.
ComplexityThe enteric nervous system has been described as a "second brain" for several reasons. The enteric nervous
system can operate autonomously. It normally communicates with the central nervous system (CNS) through the
parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However,
vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function.
In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make
the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input. The
sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control
peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes. The enteric nervous system
also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such as
acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the
body's dopamine, which is currently being studied to further our understanding of its utility in the brain.
The enteric nervous system has the capacity to alter its response depending on such factors as bulk and nutrient
composition. In addition, ENS contains support cells which are similar to astroglia of the brain and a diffusion barrier around
the capillaries surrounding ganglia which is similar to the blood–brain barrier of cerebral blood vessels.
neurotransmitters
-the chemicals which open up the membranes and
make the NS function
-the chemical by which nerves communicate
-some are always inhibitory or excitatory
-some are either inhibitory or excitatory depending
on site of action or
-all ANS fibers secrete ACh or NE
acetylcholine (ACh)
- ACh is released by sym and parasym and post parasym fibers
- muscarinic receptors on target cells
- nicotinic receptors - ACh from pre fibers and always excitatory
norepinephrine (NE)
- from almost all sym postganglionic nerve fibers
- alpha1 adrenergic fibers are usually excitatory
- alpha2 adrenergic fibers are inhibitory
- beta1 adrenergics fibers are usually inhibitory
- beta2 are excitatory and inhibitory
sympathetic affects last longer
- parasym release ACh which is quickly destroyed by AChE in seconds
- sym release of NE is reabsorbed, diffuses away, or destroyed by MAO
- NE as it diffuses is destroyed by cathecol-O-methyltransferase (COMT)
- a significant amount of NE passes into the bloodstream and can effect
the body for minutes to hours before liver destruction
the ANS is also affected by
neuropeptides – modulate affects of ACh and NE
enkalphins – bind to opiod receptors, regulate nociception (pain)
substance P – immediate defense to stress, repair, and survival
neuropeptide Y – increase food intake, fat storage, reduce anxiety,
stress and pain, affects circadian rhythms, lowers blood
pressure, epileptic seizures, and alcohol intake
somatostatin – GH inhibitor, from enteric NS, reduces acid secretion
neurotensin – regulates LH and prolactin release, enteric system
release, many endocrine functions modulated
gonadotropin releasing hormone – regulates LH & FSH & gonadotropins
release in the brain and other endocrine modulating affects
nitric oxide – vasodilator, neuroscience, physiology, immunology
antagonistic behavior is good in the nervous system
- much of the viscera has excitatory and inhibitory fibers
- these opposite affects are cooperative in nature when the
sym and parasym act on different effectors for a
unified response or the desired effect
- in some cases the antagonistic affects are equal but
- in some areas one system has much more influence than
the other (in heart sym is stronger than parasym)
- not all areas have dual innervation (adrenal medulla, sweat
glands, piloerector muscles, many BV have sym fibers
only)
central control of ANS
- cerebrum = limbic, sensory, mental
- hypothalamus = integrations of emotions and primary
functions, which are then sent to brainstem, cranial
nerves or sym spinal cord fibers
- midbrain = pons and medulla oblongata receive parasym
fibers from reticular system
- spinal cord = integrates ANS reflexes but brain can override