Chapter 15: The Autonomic Nervous System

Efferent Pathways and Ganglia

- Axons of most preganglionic neurons run from the CNS to synapse in a peripheral autonomic ganglion with a postganglionic neuron - Axons of postganglionic neurons run from the ganglion to the effectors (cardiac and smooth muscle fibers and glands) Preganglionic axons are lightly myelinated thin fibers - Postganglionic axons are even thinner and are unmyelinated - Conduction through the autonomic chain is slower than through the somatic motor - Many pre and postganglionic fibers are incorporated into spinal or cranial nerves for most of their course - Remember that autonomic ganglion are motor ganglia, containing the cell bodies of motor neurons - They are sites of synapse and information transmission from pre to postganglionic neurons - Also note that the somatic motor division lacks ganglia entirely

Somatic Nervous System

- Cell bodies are within the central nervous system - Axons extend to the muscles they serve - Somatic motor fibers are thick, heavily myelinated - Type A fibers that conduct impulses very rapidly

Role of Sympathetic Division

- During exercise the sympathetic division also promotes physiological adjustments - Visceral blood supply is diminished - Blood is shunted to working musculature - Bronchioles of the lungs dilate to increase ventilation - Liver releases more sugar into blood stream to support metabolism - Its activity is evident when we are excited or find ourselves in emergency or threatening situations (frightened) - Pounding heart; rapid, deep breathing; cold, sweaty skin; and dilated eyes are signs - Also changes in brain wave patterns - Its function is to provide the optimal conditions for an appropriate response to some threat (run / see / think

Divisions of ANS

- Generally the two divisions have chains of two motor neurons that innervate same visceral organs but cause essentially opposite effects - If one division stimulates certain smooth muscle to contract or a gland to secrete, the other division inhibits that action - Through this process of dual innervation the two systems counterbalance each other - The sympathetic part mobilizes the body during extreme situations (such as fear, exercise or rage) - The parasympathetic division allows us to unwind as it performs maintenance activities and conserves body energy - Both the sympathetic and parasympathetic divisions issue from the brain and spinal cord - Two neuron pathways are shown for both divisions - Solid lines indicate preganglionic axons while broken lines indicate postganglionic axons - In addition to the functional differences between the parasympathetic and sympathetic divisions , there are also anatomical and biochemical differences - The two divisions issue from different regions of the CNS - The sympathetic can also be called the thoracolumbar division because its fibers emerge from the thoracic and lumbar parts of the spinal cord - The parasympathetic division can also be termed the craniospinal division because its fibers emerge from the brain and spinal cord (sacral) - Therefore, all sympathetic ganglia lie near the spinal cord and vertebral column, and all parasympathetic ganglia lie far from the CNS, in or near the organs innervated - The third anatomical difference between the two divisions is that sympathetic axons branch profusely, while parasympathetic fibers do not - Extensive branching allows each sympathetic neuron to influence a number of different visceral organs, enabling many organs to mobilize simultaneously during the "fight, flight or fright" response - Parasympathetic effects, by contrast are more localized and discrete - The main biochemical difference between the two divisions involves the neurotransmitter release by the postganglionic axons - In the sympathetic division, most postganglionic axons release norepinephrine (also called noradrenaline) these fibers are termed adrenergic - The postganglionic neurotransmitter in the parasympathetic division is acetycholine (Ach) these fibers are termed cholinergic

Overlap of Somatic & Autonomic

- Higher brain centers regulate and coordinate both somatic and visceral motor activities - Nearby spinal nerves and many cranial nerves contain both somatic and autonomic fibers - Most of the body's adaptations to changing internal and external conditions involve both skeletal activity and enhanced response of visceral organs

Cranial Outflow

- Most of the terminal ganglia are not individually named; instead they are collectively called intramural ganglia, literally ganglia "within the walls" - As the vagus nerves passes into the thorax, they send branches to autonomic plexuses: Cardiac plexuses Pulmonary plexuses Esophageal plexuses - When the vagus nerves reach the esophagus, their fibers intermingle to form the anterior and posterior vagal trunks - Each trunk carries fibers from both vagus nerves - The vagal trunks ride the esophagus down to enter the abdominal cavity - They send fibers to form the aortic plexuses (formed by the celiac, superior mesenteric and hypogastric) - Abdominal organs which receive vagal innervation include the liver, gallbladder, stomach, small intestine, kidneys, pancreas, and the proximal half of the large intestine - The rest of the cavity are innervated by the sacral outflow

Comparison of ANS & PNS

- Recall that the somatic motor system innervates skeletal muscles - Each somatic motor neuron runs from the central nervous system all the way to the muscle being innervated, and that each motor unit consists of a single neuron plus the skeletal muscle cells it innervates - Typical somatic motor axons are thick, heavily myelinated fibers that conduct nerve impulses rapidly - A second difference between the two divisions is that sympathetic pathways have short preganglionic fibers and long postganglionic fibers - Parasympathetic pathways in contrast have long preganglionic fibers and short postganglionic fibers

Cranial Outflow: Vagus Nerves (X)

- The major portion of the parasympathetic cranial outflow is via the vagus nerves - The two vagus nerves account for an estimated 90% of all preganglionic parasympathetic fibers in the body - They provide fibers to the neck and contribute to nerve plexuses that serve virtually every organ in the thoracic and abdominal cavity - The vagus nerve fibers arise from the dorsal motor of the medulla and terminate by synapsing in terminal ganglia that are usually located in the walls of the target organ

Autonomic Nervous System

- The motor unit is a two neuron chain - The cell body of the first neuron, the preganglionic neuron, resides in the brain or spinal cord - Its axon, the preganglionic axon, synapses with the second motor neuron, the post- ganglionic neuron, in an autonomic ganglion outside the central nervous system - The postganglionic axon then extends to the effector organ

Parasympathetic Division

- The parasympathetic division is most effective in non-stressful situations - This division is chiefly concerned with keeping body energy use as low as possible, even as it directs body processes such as digestion and elimination - Resting and digesting division - The preganglionic neurons synapse with the ganglionic neurons located in terminal ganglia - Very short postganglionic axons issue from the terminal ganglia and synapse with effector cells in their immediate area - Several cranial nerves contain outflow of the parasympathetic - Preganglionic fibers run in the oculomotor, facial, glossopharyngeal, and vagus nerve

Cranial Outflow: Facial Nerves VII

- The parasympathetic fibers of the facial nerves stimulate the secretory activity of many large glands of the head - The pathway activates the nasal glands and the lacrimal glands of the eyes - The preganglionic fibers then run to synapse with ganglionic neurons in the pterygopalatine ganglion stimulating the submandibular and sublingual salivary glands

Cranial Outflow: Oculomotor Nerve III

- The parasympathetic fibers of the oculomotor nerves innervate smooth muscles of the eye - Constrictor muscles of iris cause pupil to constrict - Ciliary muscle within the orbits of the eye controls lens shape for visual focusing - Allow the eye to focus on close objects in the visual field

Cranial Outflow: Glossopharyngeal (IX)

- The parasympathetic nerves originate in the medulla and activate the parotid salivary gland

Sacral Outflow

- The sacral outflow arises from neurons located in the lateral horn of the spinal cord at S2 - S4 - The axons of these neurons run in the ventral roots of the spinal nerves to the ventral rami - From the ventral rami the neurons branch to form the pelvic splanchnic nerves - Most neurons synapse in the intramural ganglia located in the walls of the distal large intestine, urinary bladder and reproductive organs

Anatomy of ANS

- The sympathetic and parasympathetic divisions are distinguished by Unique sites of origin: - Parasympathetic fibers emerge from the brain and from the spinal cord at the sacral level - Sympathetic fibers originate from the thoracic and lumbar regions of the spinal cord Different lengths of their fibers: - Parasympathetic division has long preganglionic and short postganglionic fibers - Sympathetic is the opposite with short preganglionic and long postganglionic fibers Location of their ganglia: - Most parasympathetic ganglia are located in the visceral effector organs - Sympathetic ganglia lie close to the spinal cord

Sympathetic Division

- The sympathetic division is responsible for the "fight, flight, or fright" response - Its activity is evident during vigorous exercise, excitement, or emergencies - Physiological changes like a pounding heart, fast and deep breathing, dilated eye pupils, and cold, sweaty skin are signs of the mobilization of the sympathetic division, which help us survive danger - Sympathetic responses prepare our bodies to cope with physiological stressors - While sympathetic response may increases the capacities of some systems they may in fact inhibit "non-essential" functions such as digestion and urinary tract motility - The sympathetic system also innervates blood vessels, sending signals to the smooth muscles in their walls - Even though sympathetic input causes the smooth muscle in some vessels (in skeletal muscle) to relax so that the vessel dilates, the bulk of sympathetic input signals smooth muscle in blood vessels to contract, producing vasoconstriction - Vasoconstriction results in the narrowing of vessel diameter which forces the heart to work harder to pump blood around the vascular circuit - As a result sympathetic activity results in blood pressure to rise during excitement and stress - The sympathetic division innervates more organs - It supplies not only the visceral organs in the internal body cavities, but also the visceral structures in the superficial part of the body: Sweat glands Arrector pili Arteries and veins - All preganglionic fibers in the sympathetic division arise from cell bodies of preganglionic neurons located in spinal cord segments from T1 through L2 - It is also called the thoracolumbar - After leaving the cord via the ventral root, the preganglionic sympathetic fibers pass through a white ramus communicans to enter the adjoining chain (paravertebral) ganglion forming part of the sympathetic trunk or chain - The sympathetic trunks flank each side of the vertebral column and appear as strands of white beads - Although the sympathetic trunks extend from the neck to the pelvis, sympathetic fibers arise only from the thoracic and lumbar spinal cord segments - The ganglia vary in size, position, and number, but there are typically 23 ganglia in each sympathetic chain... 3 cervical 11 thoracic 4 lumbar 4 sacral 1 coccygeal

Visceral Reflexes

- The visceral sensory neurons are the first link in the autonomic reflexes - These neurons send information concerning chemical changes, stretch, and irritation of the viscera Visceral reflex arcs have essentially the same components as somatic reflex arcs: - Receptor - Sensory neuron - Integration center - Motor neuron - Effector - Visceral reflex arcs differ in that they have a two-neuron chain - Nearly all sympathetic and parasympathetic fibers are accompanied by afferent fibers conducting sensory impulses from glands or muscular structures - Thus, peripheral processes of visceral sensory neurons are found in cranial nerves, VII, IX, and X, the splanchnic nerves, and the sympathetic trunk, as well as the spinal nerves - Like sensory neurons serving somatic structures (skeletal muscles and skin) - The cell bodies of visceral sensory neurons are located in the sensory ganglia of associated cranial nerves or in the dorsal root ganglia of the spinal cord - Visceral sensory reflexes are also found within sympathetic ganglia where synapses with preganglionic neurons occur - Complete three-neuron reflex arcs (sensory, motor, and intrinsic neurons) exist within the walls of the gastrointestinal tract - Enteric nervous system - Controls gastrointestinal activity - The fact that visceral pain travels along the same pathways as somatic pain fibers helps to explain the phenomenon of referred pain in which pain stimuli arising in the viscera is perceived as somatic in origin - A heart attack may produce a sensation of pain that radiates to the superior thoracic wall and along the medial aspect of the left arm - Since the same spinal segments (T1-T5) innervate both the heart and the regions to which pain signals from heart tissue are referred, the brain interprets most such inputs as coming from the somatic pathway - Additional cutaneous areas to which visceral pain is referred

Sympathetic Division 2

Once a preganglionic axon reaches a paravertebral ganglion one of three things can happen to it: 1. It can synapse with a ganglionic neuron within the same chain ganglion 2. It can ascend or descend the sympathetic chain to synapse in another chain ganglion 3. It can pass through the chain ganglion and emerge from the sympathetic chain without synapsing