15. COMPARISON OF SOMATIC AND AUTONOMIC NERVOUS SYSTEMS

COMPARISON OF SOMATIC AND
AUTONOMIC NERVOUS SYSTEMS

• The somatic nervous system includes both sensory and motor neurons.

1. Sensory neurons convey input 

• from receptors for somatic senses (tactile, thermal, pain, and proprioceptive sensations; ) 
• and from receptors for the special senses (vision, hearing, taste, smell, and equilibrium; ). 
• All these sensations normally are consciously perceived. 

2. In turn, somatic motor neurons 

• innervate skeletal muscles––the effectors of the somatic nervous system––
• and produce voluntary movements.

• When a somatic motor neuron stimulates the muscle, it contracts; the effect always is excitation. 
• If somatic motor neurons cease to stimulate a muscle, the result is a paralyzed, limp muscle that has no muscle tone. 

• Although we are generally not conscious of breathing, the muscles that generate respiratory movements are also skeletal muscles controlled by somatic motor neurons. 

• If the respiratory motor neurons become inactive, breathing stops. 

• A few skeletal muscles, such as those in the middle ear, are controlled by reflexes and cannot be contracted voluntarily.

• The main input to the ANS comes from autonomic sensory neurons. 
• Mostly, these neurons are associated with interoceptors (IN-ter-o¯-sep -tors), 
• which are sensory receptors located in blood vessels, visceral organs, muscles, 
• and the nervous system that monitor conditions in the internal environment. 

Examples of interoceptors are 

• chemoreceptors that monitor blood CO2 level 
• and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels. 

• Unlike those triggered by a flower’s perfume, a beautiful painting, or a delicious meal, these sensory signals are not consciously perceived most of the time, although intense activation of interoceptors may produce conscious sensations. 

Two examples of perceived visceral sensations are 

1. pain sensations from damaged viscera and 
2. angina pectoris (chest pain) from inadequate blood flow to the heart.

• Input that influences the ANS also includes some sensations monitored by somatic sensory and special sensory neurons. 

For example, 

• somatic pain can produce dramatic changes in some autonomic activities.
• Autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues (cardiac muscle, smooth muscle, and glands). 

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

are examples of autonomic motor responses.

• Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their nerve supply is damaged.
• The heart continues to beat when it is removed for transplantation into another person,
•  smooth muscle in the lining of the gastrointestinal tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control.

• Most autonomic responses cannot be consciously altered to any great degree. 
• You probably cannot voluntarily slow your heartbeat to half its normal rate. 
• For this reason, some autonomic responses are the basis for polygraph (“lie detector”) tests. 
• However, practitioners of yoga or other techniques of meditation may learn how to regulate at least some of their autonomic activities through long practice. 

• Biofeedback, in which monitoring devices display information about a body function such as heart rate or blood pressure, enhances the ability to learn such conscious control. 
• Signals from the general somatic and special senses, acting via the limbic system, also influence responses of autonomic motor neurons. 
• Seeing a bike about to hit you, hearing squealing brakes of a nearby car, or being grabbed by an attacker would all increase the rate and force of your heartbeat.

•  the axon of a single, myelinated somatic motor neuron extends from the CNS all the way to the skeletal muscle fibers in its motor unit. 
• By contrast, most autonomic motor pathways consist of two motor neurons in series, that is, one following the other.

1. The first neuron has its cell body in the CNS; 

• its myelinated axon extends from the CNS to an autonomic ganglion. (Recall that a ganglion is a collection of neuronal cell bodies in the PNS.) 

2. The cell body of the second neuron is also in that same autonomic ganglion; 

• its unmyelinated axon extends directly from the ganglion to the effector (smooth muscle, cardiac muscle, or a gland). 

• Alternatively, in some autonomic pathways, the first motor neuron extends to specialized cells called chromaffin cells in the adrenal medullae (inner portion of the adrenal glands) rather than an autonomic ganglion.
•  In addition, all somatic motor neurons release only acetylcholine (ACh) as their neurotransmitter, 
• but autonomic motor neurons release either ACh or norepinephrine (NE).

• The output (motor) part of the ANS has two divisions: 

1. the sympathetic division 
2. and the parasympathetic division. 

• Most organs have dual innervation; 
• that is, they receive impulses from both sympathetic and parasympathetic neurons. 

In general,

• nerve impulses from one division of the ANS stimulate the organ to increase its activity (excitation), 
• and impulses from the other division decrease the organ’s activity (inhibition). 

For example,

• an increased rate of nerve impulses from the sympathetic division increases heart rate, 
• and an increased rate of nerve impulses from the parasympathetic division decreases heart rate.

• The sympathetic division is often called the fight-or-flight division.
• Sympathetic activities result in increased alertness and metabolic activities in order to prepare the body for an emergency situation. 

Responses to such situations, which may occur during physical activity or emotional stress, include 

• a rapid heart rate, 
• faster breathing rate, 
• dilation of the pupils, 
• dry mouth,
• sweaty but cool skin, 
• dilation of blood vessels to organs involved in combating stress (such as the heart and skeletal muscles),
• constriction of blood vessels to organs not involved in combating stress (for example, the gastrointestinal tract and kidneys),
• and the release of glucose from the liver.

The parasympathetic division is often referred to as  the restand- digest division 

• because its activities conserve and restore body energy during times of rest or digesting a meal. 
• The parasympathetic division conserves energy and replenishes nutrient stores. 

• Although both the sympathetic and parasympathetic divisions are concerned with maintaining health, they do so in dramatically different ways.

Comparison of the Somatic and Autonomic Nervous Systems
	SOMATIC NERVOUS SYSTEM	AUTONOMIC NERVOUS SYSTEM
Sensory input Control of motor output Motor neuron pathway Neurotransmitters and hormones  Effectors Responses	Somatic senses and special senses. Voluntary control from cerebral cortex, with contributions from basal ganglia, cerebellum, brain stem, and spinal cord. One-neuron pathway: Somatic motor neurons extending from CNS synapse directly with effector. All somatic motor neurons release ACh. Skeletal muscle. Contraction of skeletal muscle.	Mainly from interoceptors; some from somatic senses and special senses. Involuntary control from hypothalamus, limbic system, brain stem, and spinal cord; limited control from cerebral cortex. Usually two-neuron pathway: Preganglionic neurons extending from CNS synapse with postganglionic neurons in an autonomic ganglion, and postganglionic neurons extending from ganglion synapse with a visceral effector. Alternatively, preganglionic neurons may extend from CNS to synapse with chromaffin cells of adrenal medullae. All sympathetic and parasympathetic preganglionic neurons release acetylcholine(ACh). Most sympathetic postganglionic neurons release norepinephrine (NE); those to most sweat glands release ACh. All parasympathetic postganglionic neurons release ACh.  Chromaffin cells of adrenal medullae release epinephrine and norepinephrine. Smooth muscle, cardiac muscle, and glands. Contraction or relaxation of smooth muscle; increased or decreased rate and force of contraction of cardiac muscle; increased or decreased secretions of glands.