15. INTEGRATION AND CONTROL OF AUTONOMIC FUNCTIONS

15. INTEGRATION AND CONTROL
OF AUTONOMIC FUNCTIONS

Autonomic Reflexes

• Autonomic reflexes are responses that occur when nerve impulses pass through an autonomic reflex arc. 
• These reflexes play a key role in regulating controlled conditions in the body, such as 

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 gastrointestinal tract; 

defecation and urination, 

•  by regulating the opening and closing of sphincters.

The components of an autonomic reflex arc are as follows: 

1. Receptor. 

• Like the receptor in a somatic reflex arc , the receptor in an autonomic reflex arc is the distal end of a sensory neuron, 
• which responds to a stimulus 
• and produces a change that will ultimately trigger nerve impulses. 

• Autonomic sensory receptors are mostly associated with interoceptors.

2.  Sensory neuron. 

• Conducts nerve impulses from receptors to the CNS.

3. Integrating center. 

• Interneurons within the CNS relay signals from sensory neurons to motor neurons. 
• The main integrating centers for most autonomic reflexes are located in the hypothalamus and brain stem.
•  Some autonomic reflexes, such as those for urination and defecation, have integrating centers in the spinal cord.

4. Motor neurons. 

• Nerve impulses triggered by the integrating center propagate out of the CNS along motor neurons to an effector. 
• In an autonomic reflex arc, two motor neurons connect the CNS to an effector: 

a. The preganglionic neuron conducts motor impulses from the CNS to an autonomic ganglion, 

b. and the postganglionic neuron conducts motor impulses from an autonomic ganglion to an effector 

5. Effector. 


In an autonomic reflex arc, 

• the effectors are smooth muscle, cardiac muscle, and glands, 
• and the reflex is called an autonomic reflex.

Autonomic Control by Higher Centers

• 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. 

• Somatic or autonomic 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.

• The hypothalamus is the major control and integration center of the ANS. 
• The hypothalamus 

1. receives sensory input related to visceral functions, olfaction (smell), and gustation (taste), 
2. as well as changes in temperature, osmolarity, and levels of various substances in blood. 
3. It also receives input relating to emotions from the limbic system. 

• Output from the hypothalamus influences autonomic centers in both the brain stem (such as the cardiovascular, salivation, swallowing, and vomiting centers) and the spinal cord (such as the defecation and urination reflex centers in the sacral spinal cord).

• Anatomically, the hypothalamus is connected to both the sympathetic and parasympathetic divisions of the ANS by axons of neurons with dendrites and cell bodies in various hypothalamic nuclei. 

• The axons form tracts from the hypothalamus to parasympathetic and sympathetic nuclei in the brain stem and spinal cord through relays in the reticular formation. 

• The posterior and lateral parts of the hypothalamus control the sympathetic division. 
• Stimulation of these areas produces 

1. an increase in heart rate and force of contraction, 
2. a rise in blood pressure due to constriction of blood vessels, 
3. an increase in body temperature, 
4. dilation of the pupils, 
5. and inhibition of the gastrointestinal tract.

•  In contrast, the anterior and medial parts of the hypothalamus control the parasympathetic division.
• Stimulation of these areas results in 

1. a decrease in heart rate,
2. lowering of blood pressure, 
3. constriction of the pupils, 
4. and increased secretion and motility of the gastrointestinal tract.

Focus on Homeostasis: The Nervous System.

BODY SYSTEMS	CONTRIBUTION OF THE NERVOUS SYSTEM
For all body systems	Together with hormones from the endocrine system, nerve impulses provide communication and regulation of most body tissues.
Integumentary system	Sympathetic nerves of the autonomic nervous system (ANS) control contraction of smooth muscles attached to hair follicles and secretion of perspiration from sweat glands.
Skeletal system	Pain receptors in bone tissue warn of bone trauma or damage.
Muscular system	Somatic motor neurons receive instructions from motor areas of the brain and stimulate contraction of skeletal muscles to bring about body movements; basal ganglia and reticular formation set level of muscle tone; cerebellum coordinates skilled movements.
Endocrine system	Hypothalamus regulates secretion of hormones from anterior and posterior pituitary; ANS regulates secretion of hormones from adrenal medulla and pancreas.
Cardiovascular system	Cardiovascular center in the medulla oblongata provides nerve impulses to ANS that govern heart rate and the forcefulness of the heartbeat; nerve impulses from ANS also regulate blood pressure and blood flow through blood vessels.
Lymphatic system and immunity	Certain neurotransmitters help regulate immune responses; activity in nervous system may increase or decrease immune responses.
Respiratory. system	Respiratory areas in brain stem control breathing rate and depth; ANS helps regulate diameter of airways
Digestive system	ANS and enteric nervous system (ENS) help regulate digestion; parasympathetic division of ANS stimulates many digestive processes.
Urinary system	ANS helps regulate blood flow to kidneys, thereby influencing the rate of urine formation; brain and spinal cord centers govern emptying of the urinary bladder
Reproductive systems	Hypothalamus and limbic system govern a variety of sexual behaviors; ANS brings about erection of penis in males and clitoris in females and ejaculation of semen in males; hypothalamus regulates release of anterior pituitary hormones that control gonads (ovaries and testes); nerve impulses elicited by touch stimuli from suckling infant cause release of oxytocin and milk ejection in nursing mothers.