Ch. 15 - Autonomic Nervous System

Autonomic Nervous System (ANS)

a motor nervous system that controls glands, cardiac muscle, and smooth muscle

• Also called the visceral motor system

• Carries out actions involuntarily

  • Visceral effectors do not depend on the ANS to function, only to adjust their activity to the body’s needs

• ANS is considered the efferent pathway

5 Primary Organs of the ANS:

• Viscera of thoracic and abdominal cavities

• Some structures of the body wall

• Cutaneous blood vessels

• Sweat glands

• Piloerector muscles

Autonomic Nervous System (ANS)

a motor nervous system that controls glands, cardiac muscle, and smooth muscle

• Also called the visceral motor system

• Carries out actions involuntarily

  • Visceral effectors do not depend on the ANS to function, only to adjust their activity to the body’s needs

• ANS is considered the efferent pathway

5 Primary Organs of the ANS:

• Viscera of thoracic and abdominal cavities

• Some structures of the body wall

• Cutaneous blood vessels

• Sweat glands

• Piloerector muscles

Denervation Hypersensitivity

exaggerated responses of cardiac and smooth muscle if autonomic nerves are severed

Visceral Reflexes

unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors

Reflex Arc

1. Receptors are nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli

2. Afferent neurons take signal from receptors to the CNS

3. Interneurons in the CNS make up the integrating center and integrate the info from the afferent neurons

4. Efferent neurons carry motor signals away from the CNS to effector organs

5. Effectors carry out the end response

Sympathetic Division

• Prepares body for physical activity: exercise, trauma, arousal, competition, anger, or fear

• Increases heart rate, BP, airflow, blood glucose levels, etc.

• Reduces blood flow to the skin and digestive tract

• how we deal with stress

• “Fight-or-flight”

Parasympathetic Division

• Calms many body functions, reducing energy expenditure and assists in bodily maintenance

• Digestion and waste elimination

• “Resting and digesting” state

• is relatively selective in stimulation of target organ

  • less neural divergence than the sympathetic division

• Cranial nerves it involves are:

  • Oculomotor nerve (III)

  • Facial nerve (VII)

  • Glossopharyngeal nerve (IX)

  • Vagus nerve (X)

    • vagus nerve is especially recognized

Autonomic Tone

The normal background rate of activity that represents the balance of the parasympathetic and sympathetic systems according to the body’s needs

• Parasympathetic tone

  • Maintains smooth muscle tone in intestines

  • Holds resting heart rate down to about 70 to 80 beats per minute

• Sympathetic tone

  • Keeps most blood vessels partially constricted and maintains blood pressure

The parasympathetic and sympathetic divisions have….

opposite effects

Somatic Pathway

A motor neuron from the brainstem or spinal cord issues a myelinated axon that reaches all the way to skeletal muscle

Autonomic Pathway

Signal must travel across 2 neurons to get to the target organ

• Must cross a synapse where these two neurons meet in an autonomic ganglion

• has synapses with a postganglionic neuron whose axon extends the rest of the way to the target cell

Nerve Fibers of the Sympathetic Division

have relatively short preganglionic and long postganglionic fibers

• Preganglionic neurosomas are in the lateral horns and nearby regions of the spinal cord’s gray matter

• Fibers exit spinal cord through spinal nerves T1 to L2

• Lead to nearby sympathetic chain of ganglia (paravertebral ganglia)

Sympathetic Chain of Ganglia (paravertebral ganglia)

a series of ganglia next to both sides of the vertebral column from cervical to coccygeal levels

• Sympathetic nerve fibers are distributed to every level of the body

• the series of ganglia extends up longitudinally

• Each paravertebral ganglion is connected to a spinal nerve by two branches called communicating rami

Preganglionic Fibers in Sympathetic Division

small myelinated fibers that travel from the spinal nerve to the ganglion using the white communicating ramus

• myelinated

Postganglionic fibers

leave the ganglion using the gray communicating ramus

• unmyelinated

• Forms a bridge back to the spinal nerve

• extend the rest of the way to the target organ

What are the 3 courses the post ganglionic fibers could follow after entering the sympathetic chain?

1. Some end in ganglia which enter and synapse immediately with a postganglionic neuron

2. Some travel up or down the chain and synapse in ganglia at other levels

3. Some pass through the chain without synapsing and continue as splanchnic nerves

Adrenal Glands

Paired adrenal (suprarenal) glands located on the superior poles of the kidneys

• Each is two glands with different functions

Adrenal Cortex

Outer layer of adrenal gland

• Secretes steroid hormones

Adrenal Medulla

Inner core of adrenal gland

• Essentially a sympathetic ganglion made of modified postganglionic neurons (without fibers)

• Secretes a mixture of hormones into bloodstream

  • Catecholamines—85% epinephrine (adrenaline) and 15% norepinephrine (noradrenaline)

Preganglionic Neurons of the Parasympathetic Division

• Origins of long preganglionic neurons:

  • Midbrain, pons, and medulla

  • Sacral spinal cord segments S2 to S4

• Preganglionic fibers end in terminal ganglia in or near the target organs

• Has long preganglionic fibers and short postganglionic fibers

Enteric Nervous System

the nervous system of the digestive tract

• Does not arise from the brainstem or spinal cord (no CNS components)

• Innervates smooth muscle glands

• Composed of millions of neurons found in the walls of the digestive tract

• Has its own reflex arcs

• Regulates movement of esophagus, stomach, and intestines and secretion of digestive enzymes and acid

• Normal digestive function also requires regulation by sympathetic and parasympathetic systems