Autonomic Nervous System

Chapter 15

Autonomic Nervous System

Autonomic Nervous System

- Regulate activity of smooth muscle, cardiac muscle & certain glands- Structures involved

General visceral afferent neuronsGeneral visceral efferent neuronsIntegration center within the brain

- Receives input from limbic system and other regions of the cerebrum

Basic Anatomy of ANS

• Preganglionic neuron– cell body in brain or spinal cord

– axon is myelinated type B fiber that extends to autonomic ganglion

• Postganglionic neuron– cell body lies outside the CNS in an autonomic ganglion

– axon is unmyelinated type C fiber that terminates in a visceral effector

Autonomic vs Somatic NS

Somatic nervous systemconsciously perceived sensations

excitation of skeletal muscle

one neuron connects CNS to organ

Autonomic nervous systemunconsciously perceived visceral sensations involuntary inhibition or excitation of smooth muscle, cardiac muscle or glandular secretiontwo neurons needed to connect CNS to organ

preganglionic and postganglionic neurons

Somatic/Autonomic Nervous System

Sympathetic Ganglia

Trunk(chain) ganglia- near vertebral column on either side

Prevertebral(collateral)

ganglia-

anterior to vertebral column close to large abdominal arteries.

Sympathetic Division of

the ANS

Sympathetic Division Structure• Sympathetic preganglionic neurons pass to the

sympathetic trunk. They may connect to postganglionic neurons in the following ways:

1. May synapse with postganglionic neurons in the ganglion it first reaches.

2. May ascend or descend to a higher of lower ganglion before synapsing with postganglionic neurons.

3. May continue, without synapsing, through the sympathetic trunk ganglion to a prevertebral ganglion and synapse with postganglionic neurons.

4. May pass through sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cell of adrenal medulla.

Pathways of Sympathetic

Fibers

Parasympathetic Division of the

ANS

Parasympathetic Ganglia

terminal ganglia close to or in the wall of organs

Parasympathetic Division Structure•Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia.

• The cranial outflow consists of preganglionic axons that extend from the brain stem in four cranial nerves. – The cranial outflow consists of four pairs of ganglia and the

plexuses associated with the vagus nerve (X).• Ciliary ganglia• Pterygopalatine ganglia• Submandidibular ganglia• Otic ganglia

• The sacral parasympathetic outflow consists of preganglionic axons in the anterior roots of the 2nd through 4th sacral nerves and they form the pelvic splanchnic nerve.

Autonomic PlexusesCardiac plexus

Pulmonary plexus

Celiac (solar) plexus

Superior mesenteric

Inferior mesenteric

Hypogastric

Renal plexus

Pathway from Spinal Cord to Sympathetic Trunk Ganglia:

• Preganglionic axons → anterior root of a spinal nerve → white ramus → sympathetic trunk ganglion.

• White rami communicantes: structures containing sympathetic preganglionic axons that connect the anterior ramus of the spinal nerve with the ganglia of the sympathetic trunk.

Organization of Sympathetic Trunk Ganglia

• Sympathetic trunk ganglia: 3 cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral and 1 coccygeal.

• Postganglionic neurons from the

- superior cervical region: head and heart.

- middle cervical ganglion and the inferior cervical ganglion: heart.

• Thoracic sympathetic trunk- heart, lungs, and bronchi.

• Axons leave the sympathetic trunk in 4 possible ways:

- spinal nerves

- cephalic periarterial nerves

- sympathetic nerves

- splanchnic nerves

Pathways from Sympathetic Trunk Ganglia to Visceral Effectors

Spinal nerves

• Gray ramus: Axons of some postganglionic neurons leave the sympathetic trunk by entering a short pathway called a gray ramus and merge with the anterior ramus of a spinal nerve.

• Gray rami communicantes: structures containing sympathetic postganglionic axons that connect the ganglia of the sympathetic trunk to spinal nerves.

Cephalic Periarterial Nerves

• Some sympathetic preganglionic neurons that enter the sympathetic trunk ascend to the superior cervical ganglion where they synapse with postganglionic neurons. Some of these leave the sympathetic trunk by forming cephalic periarterial nerves.

• Serve visceral effectors in the skin of the face and head.

Sympathetic Nerves

• Some axons of the postganglionic neurons leave the trunk by forming sympathetic nerves.

• Innervate the heart and lungs.

Splanchnic Nerves

• Some sympathetic preganglionic axons pass through the sympathetic trunk without terminating in it. Beyond the trunk they form nerves called splanchnic nerves which extend to prevertebral ganglia.

• T5-T9 or T10- Greater splanchnic nerve.

• T10-T11- Lesser splanchnic nerve.

• L1-L4- Lumbar splanchnic nerve.

Cranial Parasympathetic Outflow

• The cranial outflow has four pairs of ganglia and are associated with the vagus nerve.1. Ciliary ganglia-

2. Pterygopalatine ganglia-

3. Submandibular ganglia-

4. Otic ganglia-

• Vagus nerve carries nearly 80% of the total craniosacral flow.

Sacral Parasympathetic Outflow

• Consists of S2-S4.

• Pelvic splanchnic nerves

Neurotransmitters and Receptors

Adrenergic and Cholinergic Receptors

Sympathetic vs. Parasympathetic

Physiological Effects of the ANS

• Most body organs receive dual innervation– innervation by both sympathetic & parasympathetic

• Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity levels

• Some organs have only sympathetic innervation– sweat glands, adrenal medulla, arrector pili mm &

many blood vessels – controlled by regulation of the “tone” of the

sympathetic system

Sympathetic Responses• Dominance by the sympathetic system is caused by

physical or emotional stress -- “E situations”– emergency, embarrassment, excitement, exercise

• Alarm reaction = flight or fight response– dilation of pupils– increase of heart rate, force of contraction & BP– decrease in blood flow to nonessential organs– increase in blood flow to skeletal & cardiac muscle– airways dilate & respiratory rate increases– blood glucose level increase

• Longer lasting than parasympathetic due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland

Parasympathetic Responses• Enhance “rest-and-digest” activities• Mechanisms that help conserve and restore body energy

during times of rest• Normally dominate over sympathetic impulses• SLUDD type responses = salivation, lacrimation,

urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil

• Paradoxical fear when there is no escape route or no way to win– causes massive activation of parasympathetic division

– loss of control over urination and defecation

Sympathetic vs. Parasympathetic

Autonomic Reflexes

• Autonomic reflexes occur over autonomic reflex arcs. Components of reflex arc:– sensory receptor– sensory neuron– integrating center– motor neurons (pre & postganglionic) – visceral effectors

• Unconscious sensations and responses– changes in blood pressure, digestive functions etc– filling & emptying of bladder or defecation

Hypothalamus is major control center– input: emotions and visceral sensory information

• smell, taste, temperature, osmolarity of blood, etc

– output: to nuclei in brainstem and spinal cord– posterior & lateral portions control sympathetic NS

• increase heart rate, inhibition GI tract, increase temperature

– anterior & medial portions control parasympathetic NS

• decrease in heart rate, lower blood pressure, increased GI tract secretion and mobility

Control of Autonomic Functioning

Autonomic vs Somatic NS - Review

• Somatic nervous system– consciously perceived

sensations

– excitation of skeletal muscle

– one neuron connects CNS to organ

•Autonomic nervous system–unconsciously perceived visceral sensations

–involuntary inhibition or excitation of smooth muscle, cardiac muscle or glandular secretion

–two neurons needed to connect CNS to organ

•preganglionic and postganglionic neurons

Autonomic vs Somatic NS - Review

Autonomic Dysreflexia• Exaggerated response of sympathetic NS in cases of

spinal cord injury above T6• Certain sensory impulses trigger mass stimulation of

sympathetic nerves below the injury• Result

– vasoconstriction which elevates blood pressure– parasympathetic NS tries to compensate by slowing heart rate

& dilating blood vessels above the injury– pounding headaches, sweating warm skin above the injury and

cool dry skin below– can cause seizures, strokes & heart attacks