bsc2085L 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)

visceral reflexes

unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effects; controlled by ANS

visceral reflex arc

receptors, afferent neurons, interneurons, efferent neurons, effectors

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