Autonomic Nervous System

The autonomic nervous system operates to…

regulate the activities of the smooth muscle, cardiac muscle and glands

Somatic nervous system consists of…

somatic motor neurons that innervate the skeletal muscles of the body

When a somatic motor neuron stimulates a skeletal muscle…

it contracts; the effect is always excitation

Functions of Somatic Vs. Autonomic:

Somatic:

-stimulates the skeletal muscles to contract (excitation)

-usually operates under voluntary control

-somatic motor neurons involved in posture, balance, breathing, and somatic reflexes are controlled by integrating centers in the brainstem and spinal cord (involuntary)

-can also receive sensory input from sensory neurons that convey information for somatic senses (tactile, thermal, pain, and proprioceptive sensations) or the special senses (sight, hearing, taste, smell, and equilibrium)

Autonomic:

-regulates visceral activities within the effector tissues

-operates without conscious control

-receive sensory input from sensory neurons associated with interoceptors (sensory receptors located in blood vessels, visceral organs, muscles, and the nervous system that monitor conditions in the internal environment)

The autonomic nervous system is the part of the nervous system that…

regulates cardiac muscle, smooth muscle, and glands (often referred to as visceral effectors)

The autonomic nervous system consists of autonomic motor neurons that…

regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, and glands).

Examples of autonomic motor responses:

Changes in the diameter of the pupils, dilation and constriction of blood vessels, and adjustment of the rate and force of the heartbeat

Unlike skeletal muscle, tissues innervated by the ANS often…

function to some extent even if their nerve supply is damaged.

The ANS consists of two main divisions:

The Sympathetic and parasympathetic nervous system

Dual innervation

concept by which most organs of the body receive impulses from sympathetic and parasympathetic neurons.

One division excites while the other inhibits an organs activity, For example:

Neurons of the sympathetic nervous system increase heart rate, and neurons of the parasympathetic nervous system slow it down.

The sympathetic nervous system promotes the…

fight-or-flight response, which prepares the body for emergency situations.

The parasympathetic nervous system promotes the…

rest-and-digest activities, which conserve and restore body energy during times of rest or digesting a meal.

The third division of the ANS is known as the…

Enteric plexuses which are embedded in the submucosa and muscular layer of the digestive canal; they govern motility and secretions of the digestive canal. (operation is involuntary)

Most autonomic motor pathways consist of two motor neurons in series…

One following the other; preganglionic and postganglionic neurons

The first neuron (preganglionic)…

has its cell body in the CNS (brain or spinal cord); its myelinated axon extends from the CNS to an autonomic ganglion

The second ganglionic (postganglionic)…

is also in the same autonomic ganglion as the preganglionic; its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland).

The first motor neuron can also extend to…

specialized cells called chromaffin cells in the adrenal medullae (inner portion of the adrenal glands) rather than an autonomic ganglion.

Chromaffin cells secrete…

the neurotransmitters epinephrine and norepinephrine (NE)

All somatic motor neurons release only…

acetylcholine (ACh) as their neurotransmitter, but autonomic motor neurons release either ACh or norepinephrine (NE).

The axon of a preganglionic neuron is a…

small-diameter, myelinated type B fiber that usually extends to an autonomic ganglion, where it synapses with a postganglionic neuron

The axon of a postganglionic neuron is a…

small-diameter, unmyelinated type C fiber that terminates in a visceral effector.

The postganglionic neuron is located…

outside of the CNS and PNS, its cell body and dendrites are located in an autonomic ganglion, where it forms synapses with one or more preganglionic axons.

Preganglionic neurons convey nerve impulses from the CNS to…

autonomic ganglia, and postganglionic neurons relay the impulses from autonomic ganglia to visceral effectors

In the sympathetic division, the preganglionic neurons have their cell bodies in the…

lateral horns of the gray matter in the 12 thoracic segments and the first two (sometimes three) lumbar segments of the spinal cord

The sympathetic division is also called the…

Thoracolumbar division, and the axons of the sympathetic preganglionic neurons are known as the thoracolumbar outflow.

Cell bodies of preganglionic neurons of the parasympathetic division are located in…

the nuclei of four cranial nerves in the brainstem (III, VII, IX, and X) and in the lateral gray matter of the second through fourth sacral segments of the spinal cord

The parasympathetic division is also known as the…

Craniosacral division, and the axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow.

The sympathetic ganglia are the sites of…

synapses between sympathetic preganglionic and postganglionic neurons.

The two major types of sympathetic ganglia are the…

sympathetic trunk ganglia and the prevertebral ganglia

Sympathetic trunk ganglia

A cluster of cell bodies of sympathetic postganglionic neurons lateral to the vertebral column, close to the body of a vertebra. These ganglia extend inferiorly through the neck, thorax, and abdomen to the coccyx. Also called vertebral chain ganglia or paravertebral ganglia.

Because the sympathetic trunk ganglia are near the spinal cord…

most sympathetic preganglionic axons are short and most sympathetic postganglionic axons are long

Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above the…

diaphragm, such as the head, neck, shoulders, and heart. They are the superior, middle, and inferior cervical ganglia.

Prevertebral (collateral) ganglia

A cluster of cell bodies of postganglionic sympathetic neurons anterior to the spinal column and close to large abdominal arteries. Also called a collateral ganglion.

The five major prevertebral ganglia are…

1. celiac ganglion

2. superior mesenteric ganglion

3. inferior mesenteric ganglion

4. aorticorenal ganglion

5. renal ganglion

Preganglionic axons of the parasympathetic division synapse with…

postganglionic neurons in parasympathetic (intramural) ganglia, most are located close to or within the wall of a visceral organ

Parasympathetic ganglia in the head consist of:

ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, and otic ganglion

Once axons of sympathetic preganglionic neurons pass to sympathetic trunk ganglia, they may connect with postganglionic neurons in one of the following ways…

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

2. axon may ascend or descend to a higher or lower ganglion before synapsing with postganglionic neurons. The axons of incoming sympathetic preganglionic neurons pass up or down the sympathetic trunk from ganglion to ganglion.

3. axon may continue, without synapsing, through the sympathetic trunk ganglion to end at a prevertebral ganglion and synapse with postganglionic neurons there.

4. axon may pass, without synapsing, through the sympathetic trunk ganglion and a prevertebral ganglion and then extend to chromaffin cells of the suprarenal medullae that are functionally similar to sympathetic postganglionic neurons

A single sympathetic preganglionic fiber has many…

axon collaterals (branches) and may synapse with 20 or more postganglionic neurons

After exiting their ganglia, the postganglionic axons…

typically terminate in several visceral effectors

Autonomic Plexus

A network of sympathetic and parasympathetic axons; examples are the cardiac, celiac, and pelvic plexuses, which are located in the thorax, abdomen, and pelvis, respectively.

Cell bodies of sympathetic preganglionic neurons are part of the…

lateral gray horns of all thoracic segments and of the first two lumbar segments of the spinal cord

The preganglionic axons leave the…

spinal cord along with the somatic motor neurons at the same segmental level. After exiting through the intervertebral foramina, the myelinated preganglionic sympathetic axons pass into the anterior root of a spinal nerve and enter a short pathway called a white ramus before passing to the nearest sympathetic trunk ganglion on the same side

The paired sympathetic trunk ganglia are arranged…

anterior and lateral to the vertebral column, one on either side. Typically, there are 3 cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral sympathetic trunk ganglia, and 1 coccygeal ganglion.

Axons leave the sympathetic trunk in four possible ways…

(1) They can enter spinal nerves

(2) they can form cephalic periarterial nerves

(3) they can form sympathetic nerves

(4) they can form splanchnic (relating to viscera, especially on the abdomen) nerves

Structure of the parasympathetic division:

A single sympathetic preganglionic fiber has many axon collaterals (branches) and may synapse with 20 or more postganglionic neurons.

Cell bodies of parasympathetic preganglionic neurons are found in…

nuclei in the brainstem and in the lateral gray matter of the second through fourth sacral segments of the spinal cord

The cranial outflow has four pairs of ganglia and the ganglia associated with the vagus (X) nerve:

1. ciliary ganglia

2. pterygopalatine ganglia

3. submandibular

4. otic ganglia

The ciliary ganglia lie lateral to…

each optic nerve near the posterior aspect of the orbit. Preganglionic axons pass with the oculomotor (III) nerves to the ciliary ganglia. Postganglionic axons from the ganglia innervate smooth muscle fibers in the eyeball.

The pterygopalatine ganglia are located lateral to the…

sphenopalatine foramen, between the sphenoid and palatine bones. They receive preganglionic axons from the facial (VII) nerve and send postganglionic axons to the nasal mucosa, palate, pharynx, and lacrimal glands.

The submandibular ganglia are found near the…

ducts of the submandibular salivary glands. They receive preganglionic axons from the facial nerves and send postganglionic axons to the submandibular and sublingual salivary glands.

The otic ganglia are situated just inferior to…

each foramen ovale. They receive preganglionic axons from the glossopharyngeal (IX) nerves and send postganglionic axons to the parotid salivary glands.

Preganglionic axons that leave the brain as part of the vagus (X) nerves carry…

nearly 80% of the total craniosacral outflow. Vagal axons extend to many terminal ganglia in the thorax and abdomen.

Cholinergic neurons release the neurotransmitter…

acetylcholine (ACh)

In the ANS, the cholinergic neurons include:

(1) all sympathetic and parasympathetic preganglionic neurons

(2) sympathetic postganglionic neurons that innervate most sweat glands

(3) all parasympathetic postganglionic neurons

The two types of cholinergic receptors are…

nicotinic receptors and muscarinic receptors

Nicotinic receptors are present in…

the plasma membrane of dendrites and cell bodies of both sympathetic and parasympathetic postganglionic neurons, the plasma membranes of chromaffin cells of the suprarenal medullae, and in the motor end plate at the neuromuscular junction.

Muscarinic receptors are present in…

the plasma membranes of all effectors (smooth muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic axons. Most sweat glands receive their innervation from cholinergic sympathetic postganglionic neurons and possess muscarinic receptors

Adrenergic neurons release the neurotransmitter…

norepinephrine (noradrenalin)

Most sympathetic postganglionic neurons are…

adrenergic

The two main types of adrenergic receptors are…

alpha (a) receptors and beta (b) receptors which are found on visceral effectors innervated by most sympathetic postganglionic axons

Autonomic tone

The balance between sympathetic and parasympathetic activity, which is regulated by the hypothalamus. Typically, the hypothalamus turns up one divisions tone as it turns down the others tone

Autonomic visceral reflexes

responses that occur when nerve impulses pass through an autonomic reflex arc.

Types of autonomic visceral reflexes

• Blood pressure by adjusting heart rate, force of ventricular contraction, and blood vessel diameter

• Digestion by adjusting the motility (movement) and muscle tone of the digestive canal

• Defecation and urination, by regulating the opening and closing of sphincters.

The components of an autonomic reflex arc are:

• sensory receptor

• sensory neuron

• integrating center

• motor neurons

• effector

Normally, we are not aware of muscular contractions of our digestive organs, our heartbeat, changes in the diameter of our blood vessels, and pupil dilation and constriction because…

the integrating centers for these autonomic responses are in the spinal cord or the lower regions of the brain. Sensory neurons deliver input to these centers, and autonomic motor neurons provide output that adjusts activity in the visceral effector, usually without our conscious perception.