Lecture 15: The Autonomic Nervous Syetem

Question 1

Define autonomic nervous system and explain its relationship to the peripheral nervous system

Answer 1

The ANS consists of motor neurons that:
-innervate smooth and cardiac muscle and glands
-operate via subconscious control
Other names:
Involuntary nervous system
-general visceral motor system

Question 2

Compare the somatic and autonomic nervous systems relative to effectors, efferent pathways

Answer 2

The two systems differ in effectors of the somatic division go to skeletal muscles whilst effectors of autonomic go to smooth and cardiac muscles and glands
The efferent pathways of the somatic division have myelinated fibres whilst the efferent pathways of autonomic div are unmyelinated fibres

Question 3

Division of the autonomic nervous system:

1. Sympathetic
2. Parasympathetic

Answer 3

They have dual innervation
-almost all visceral organs are serves by both divs, but they cause opposite effects
Exceptions include: blood vessels, erector pilli muscles, and sweat glands (these are purely sympathetic)

Question 4

What is the role of the parasympathetic division?

Answer 4

Promotes maintenance activities and conserves body energy.
It’s so shown by:
-lower blood pressure, heart rate and respiratory rates
-high gastrointestinal tract activity
-pupils are constricted and lenses are accommodated for close vision

Question 5

What is the role of the sympathetic division?

Answer 5

Mobilizes the body during activity; it’s the fight or flight system
Promotes adjustments during exercise,nor when threatened
-blood flow is pushed to skeletal muscles and heart
-bronchioles dilate
-liver releases glucose
-pupils dilate

Question 6

Autonomic nervous system anatomy:
Compare their origin of fibres, length of fibres and location of ganglia in both the sympathetic and parasympathetic nervous systems

Answer 6

Sympathetic:
Origins of fibres: thoracolumbar region of the spinal cord
Length of fibres: short preganglionic and long postganglionic
Location of ganglia: close to the spinal cord
Parasympathetic:
Origin of fibres: Brain and sacral spinal cord (craniosacral)
Length of fibres: long preganglionic and short postganglionic
Location of ganglia: in visceral effector organs

Question 7

Identify the cranial outflow of nerves, their ganglia and effector organs of this section of the parasympathetic division
First the cranial outflow
Then the sacral outflow

Answer 7

Oculomotor (3)- ganglia (ciliary)- effector (eye)
Facial (7)- ganglia (pterygopalatine and submandibular)- effector organs (salivary, nasal, and lacrimal glands.
Glossopharyngeal (9)- ganglia (otic) effector (parotid salivary glands)
Vagus (10)- ganglia (within the walls of target organs) organ- (heart, lungs, and most visceral organs)

Now sacral outflow
Cranial nerve is S2-S4 -ganglia (within the walls of the target organs)- effector (large intestine, urinary bladder, ureters and reproductive organs

Question 8

Sympathetic (thoracolumbar) division

Answer 8

• neurons are in spinal cord segments T1-L2
• sympathetic neurons produce the lateral horns of the spinal cord
• fibres enter the sympathetic trunk ganglia

Question 9

Pathways with synapses in chain ganglia

Answer 9

-postganglionic axons enter the ventral rami via the gray rami communicantes
These fibres innervate: 
-sweat glands
-erector pili muscles 
-vascular smooth muscle

Question 10

Sympathetic pathways to the head

Answer 10

Fibres emerge from T1-T4 and synapse in the superior cervical ganglion

These fibres:

• innervate skin and blood vessels in the head
• stimulate dilator muscle of the iris
• inhibit and salivary glands

Question 11

Sympathetic pathways to the thorax

Answer 11

Preganglionic fibres emerge from T1-T6 and synapse in the cervical trunk ganglia 
These fibres innervate: 
-Heart via the cardiac plexus
-thyroid gland and the skin 
-lungs and esophagus

Question 12

Sympathetic pathways to the abdomen

Answer 12

• Fibres from T5-L2 travel through the thoracic splanchnic nerves
• synapses occur in the celiac and superior mesenteric ganglia
• these fibres serve the stomach, intestines, liver, spleen and kidneys

Question 13

Sympathetic pathways to the pelvis

Answer 13

• preganglionic fibres from T10-L2 travel via the lumbar and sacral splanchnic nerves
• synapses occur in the inferior mesenteric and hypogastric ganglia
• postganglionic fibres serve the distal half of the large intestine, the urinary bladder, and the reproductive organs

Question 14

Pathways with synapses in the adrenal medulla

Answer 14

• Some preganglionic fibres pass directly to the adrenal medulla without synapsing
• upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood

Question 15

Visceral reflexes

Answer 15

• Visceral afferent pain travel along the same pathways as somatic pain fibres, contributing to the phenomenon of referred pain
• pain stimuli arising in the viscera are perceived as somatic in origin

Question 16

Interactions of the autonomic divisions

Answer 16

-Most viscera organs have dual innervation
-dynamic antagonism allows for precise control of visceral activity
Sympathetic division increases heart and respiratory rates and inhibits digestion and elimination
Parasympathetic division decreases heart and respiratory rates, and allows for digestion and the discarding of wastes.

Question 17

What is sympathetic tone?

Answer 17

Sympathetic division controls blood pressure, even at rest
Sympathetic tone (vasomotor tone)
-keeps the blood vessels in a continual state of partial constriction

Question 18

What is parasympathetic tone?

Answer 18

Parasympathetic division normally dominates the heart and smooth muscle if digestive and urinary tract organs
-slows the heart
-dictates normal activity levels of the digestive and urinary tracts
The sympathetic division can override these effects during times of stress

Question 19

Cooperative effects

Answer 19

Best seen in control of the external genitalia

• parasympathetic fibres cause vasodilation; are responsible for erection of penis or clitoris
• Sympathetic fibres cause ejaculation of semen in males and reflex contraction of a females vagina

Question 20

What are the unique roles of the sympathetic division?

Answer 20

The adrenal medulla, sweat glands, arrector pilli muscles, kidneys, and most blood vessels receive only sympathetic fibres.
The sympathetic division controls 
-thermoregulatory responses to heat
-metabolic blood glucose levels 
-mobilizes fats for use as fuels

Question 21

Localised versus diffuse effects

Answer 21

Parasympathetic division: short lived, highly localised control over effectors
Sympathetic division: long-lasting, body’s idle effects