Autonomic Nervous System

Autonomic nervous system

• visceral efferent nerve fibers that innervate the heart, smooth muscle, and glands

• coordinates cardiovascular, respiratory, digestive, urinary, and reproductive functions

• works in conjunction with the endocrine system to maintain homeostasis in the body

• quick to respond to a stimulus. nut effects on ANS don’t last as long as the endocrine system

• autonomic nervous system influences other systems of the body because of its role in homeostasis

• DO NOT NEGLECT THIS CHAPTER FOR THE EXAM

Somatic efferent fibers

• somatic efferent fibers innervate skeletal muscles.

• one neuron pathway- cell body of a neuron that stimulates a skeletal muscle is found in the CNS

• two neuron pathway- In ANS, neuron with a cell body in CNS exits CNS and goes to ganglion where it forms a synapse with a second neuron

  • preganglionic neuron/first order neuron/central neuron- neuron that exits CNS

    • myelinated

  • postganglionic neuron/second order neuron- neuron that foes from the ganglion to the autonomic effector

    • unmyelinated

Divisions of ANS

• sympathetic and parasympathetic division

• dual innervation- when an organ has fibers from one division, usually has fibers from the other division

  • one division stimulates the organ and the other inhibits

  • allows ANS to maintain homeostasis

Autonomic Ganglions

• sympathetic chain (trunk) ganglion/ paravertebral/ lateral ganglia

  • series of ganglia that extend from the skull to the coccyx on each side of the vertebral column

  • part of the sympathetic division of ANS

• Collateral/ prevertebral ganglion-

  • located in front of the spinal column close to large arteries in the abdomen

  • associated with the sympathetic division of the ANS

• Terminal ganglion/intramural ganglion-

  • located near the effector (“terminal”- at the end of the path) or in the walls (“intramural”) of the effector

  • associated with the parasympathetic division

  • since ganglia are closer to target organ in PS division, preganglionic fibers of the PS division tend to be longer than those in the sympathetic division

Parasympathetic division

• dominant division during rest

• “rest and digest”

• usually slows activity of a target organ

• MAJOR EXCEPTION- digestive tract

• SLUDD- salivation, lacrimation, urination, digestion, defecation

  • activities that are stimulated

Parasympathetic Division Structures and Pathways

• cell bodies of preganglionic neurons of PS division (right side, pink) located in the nuclei of cranial nerves III, VII, IX, and X and the lateral gray horns of sacral nerves 2-4

• Parasympathetic division also called the craniosacral division

• preganglionic fiber may stimulate several postganglionic fibers, but they all usually go to the same target organ

• most of the cranial outflow (80%) of this division is through cranial nerve X, the vagus nerve

• note the long preganglionic fiber that synapses with the postganglionic fiber close to, or in the wall of, the target organ

Sympathetic division

• dominant division during periods of stress, exertion, or emergencies

• increases activity of a target organ

• causes activities associated with fear and anger'

• Heart rate increases, respiratory rate increases, pupils dilate

• “fight or flight”, “acute stress response”, “alarm reaction”

• survival mechanism that responds to potentially dangerous situations

• MAJOR EXCEPTION: digestion

Sympathetic Division Structures and Pathways

• cell bodies of the preganglionic neurons (left side, blue) located in the lateral gray horns of all of the thoracic nerves and the first two lumbar nerves

• sympathetic division also called the thoracolumbar division

• Preganglionic fiber synapses with several postganglionic fibers

  • may affect several target organs

  • some fibers synapse in the chain ganglia that lie beside the spinal cord, but other fibers pass through these ganglia to synapse in collateral ganglia

Rami

• rami- connections between spinal nerve and a ganglion of the sympathetic trunk

• White ramus-

  • contains myelinated axon of the preganglionic neuron

• Gray ramus

  • contains the unmyelinated postganglionic fiber

• note that some preganglionic fibers pass through the trunk and exit the ganglion as the splanchnic nerves

• these preganglionic fibers synapse in the collateral ganglia

Autonomic System Signaling Molecules

• preganglionic fibers of the ANS use Acetyl Choline as the transmitter

• postganglionic parasympathetic neurons also use acetyl choline as a transmitter

• Called CHOLINERGIC NEURONS

• Acetyl choline is broken down very quickly by the enzyme acetyl cholinesterase

• Two types of cholinergic receptors

  • nicotinic receptors- located on the postganglionic neurons

  • muscarinic receptors are on parasympathetic effectors

• adrenergic- almost all sympathetic postganglionic fibers (ignore sweat glands) are this

  • release noradrenalin/norepinephrine at the neuroeffector junction

  • alpha- and beta-adrenergic receptors on sympathetic effectors

  • noradrenalin is broken down slowly by COMT or MAO so the effects of sympathetic stimulation last longer than parasympathetic stimulation

Autonomic nervous stimulation exceptions

• supra renal gland/adrenal gland

  • gland that rests above kidney

  • divided into a cortex (outer) and medulla (inner)

  • adrenal medulla is the exception

    • does not have a two-neuron pathway

      • preganglionic neuron from CNS directly stimulates the adrenal medulla

      • Adrenal medulla does not have dual innervation

      • only has fibers from the sympathetic division

    • endocrine gland

      • secretes adrenalin (epinephrine) and noradrenalin into the blood

      • molecules bind to the alpha- and beta-adrenergic receptors of autonomic effectors

        • SYMPATHOMIMETIC- cause the same response as direct sympathetic stimulation

      • hormones important part of the alarm reaction

        • fact that these molecules are in the blood is another factor in causing the effects of the SNS to last longer than the PSNS

In the past, it was thought that the ANS was automatic, and not subject to control by the conscious mind.

• can be affected by higher centers of the brain

• biofeedback- people that are connected to a device can alter their heart rate or brain waves

• other individuals are able to inhibit SNS without being connected to a device

  • these techniques are applied through yoga or meditation and require long periods of training