Wednesday, June 30, 2010

The Autonomic Nervous System (mainly)

We have two main parts of our peripheral nervous system.  There is the somatic nervous system and the autonomic nervous system.  The autonomic nervous system is then broken down into two systems: the parasympathetic and the sympathetic systems.

Here is a quick overview of the autonomic and somatic...
The autonomic nervous system operates via reflex arcs (it's involuntary and unconscious).  The sensory input comes from the organs and the somatic nervous system.  Motor output is via both the parasympathetic and the sympathetic nervous systems.  It is regulated by the hypothalamus and brain stem.  The function is for involuntary control of vital organs.  These include viscera and cardiac/smooth muscles and glands. The autonomic nervous system regulates visceral activities by either increasing/decreasing (exciting/inhibiting) ongoing activities.
The somatic nervous system controls the skin, muscles and joints.  Sensations are consciously perceived.  There are special senses (taste, smell, hearing, equilibrium, and vision) and somatic senses (touch, temperature, pressure, pain from body).  The motor neurons innervate skeletal muscle to produce conscious, voluntary movements.  Motor neurons are always excitatory. 
A comparison:
The SNS is a single efferent.  The efferents release acetylcholine. 
The ANS has two efferents in a series - to and from the ganglion.  The first neuron cell body is in the Central nervous system (preganglionic body).  It is a mylinated axon that extends to the cell body of the second efferent.  Efferents release acetylcholine.  The first motor neuron may extend to the adrenal medullae instead of an autonomic ganglion.  The second neuron cell body is in the ganglion.  It is a nonmyelinated axon that extends to an effector.  The efferents here will release either acetylcholine or norepinepherine. 

Autonomic Divisions: most nerves are dually innervated as one division acts as the brake and the other is the gas
Sympathetic Division is our flight or fight response:
speeds up metabolism
speeds up heart rate
speeds up breathing
originates in the thoracic and lumbar segments.

Parasympathetic is our rest and digest response
slows down metabolism
slows down heart rate
slows down breathing
originates in the brain and sacral segments

The sympathetic division is called the thoracolumbar division.  The preganglionic cell bodies are in the lateral gray horns of the 12 thoracic and 2 lumbar segments.   Then, the preganglionic fibers leave the spinal nerves through white rami and enter paravertebral (autonomic) ganglia.  Paravertebral ganglia and fibers that connect them make up the sympathetic trunk.  Postganglionic fibers extend from ganglia to viscera.  The fibers pass through gray rami and return to a spinal nerve before proceeding to an effector.  The exception, as previously mentioned is to the adrenal medulla.  A single supathetic preganglion fibner has many axon collaterals and may sunapse with 20 or more postganglionic neurons.  The postganglionic axons typically terminate in several visceral effectors and therefore the effects of a sympathetic stimulation are more widespread than the effects of a parasympathetic stimulation.

The parasympathetic division is also known as the craniosacral division. The preganglionic cell bodies in the cranial nerves (only III, IV, IX, X) and lateral gray horns of the 2nd-4th sacral segments of the cord.  The presynaptic neuron usually only synapses with 4-5 postsynaptic neurons at the viscera.  The ganglia are terminal ganglia that lie very, very close to or actually within a visceral organ.  They have short postganglionic fibers that continue to their specific muscle or glands.

Cholinergic Neurons and Receptors
Cholinergic neurons release acetylcholine.  All of the preganglionic neurons release acetylcholine.  All parasympathetic postganglionic neurons release acetylcholine.  Sympathetic postganglionic neurons that innervate most sweat glands release acetylcholine. 
Cholinergic receptors are membrane proteins the postsynaptic cell's plasma membrane that binds to acetylcholine.  Nicotinin receptors are fast and are on the postganglionic neuron.  This causes excitation.  Muscarinic receptors are slow and can excite or inhibit depending on the cell that has the receptor. 

Adrenergic Neurons and Receptors
Adrenergic neurons release norepinepherine.  They include most sympathetic postganglionic neurons.  The adrenal medulla also releases epinephrine. The main types of adrenergic receptors are alpha (a1, in arteries and causes vasoconstriction) and beta (b1 in heart and b2 in lungs; causes bronchodilation and increased heart rate).  The effects triggered by adrenergic neurons are typically longer lasting than those triggered by cholinergic neurons. 

The sympathetic response is fight or flight.  This is also known as increased stress causes increased flight-or-fight response.  More ATP is produced, the pupils are dilated, the heart rate and blood pressures increase, airways dilate and constriction of blood vessels that supply the kidneys and the gastrointestinal tract.  There is an increase in supply of blood to skeletal muscles, cardiac muscle, liver and adipose tissue.  Glycogenolysis and lypolysis levels increase, causing a rise in blood glucose levels.  This is the shoot reflex, such as in orgasm. 

The parasympathetic response is our rest and digest response.  It is the point reflex - genital engorgement.  We conserve and restore body energy, increase our a digestive and urinary function and decrease body functions that support physical activity. 

To control our autonomic nervous system, we use the hypothalamus, medulla oblongata and the limbic system.  The hypothalamus is our autonomic tone.  It is a balance between the sympathetic and parasympathetic activity.  The hypothalamus controls visceral functions including body temperature, hunger, thirst, and water and electrolyte balance.  The medulla oblongata modulates the hypothalamus signals regarding cardiac, vasomotor and respiratory activities.  The limbic system and cerebral cortex send input to the cerebellum regarding emotional response. 

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