What is the primary difference between the somatic nervous system and the autonomic nervous system?

PERIPHERAL NERVOUS SYSTEM (somatic/autonomic & sympathetic/parasympathetic)

Similarities:

The sympathetic nervous system (part of the autonomic nervous system) and the somatic nervous system respond to external stimuli. The sympathetic nervous system responds to external stimuli by preparing the body for fight or flight and the somatic nervous system responds to external stimuli (by carrying information from sensory receptors to the spinal cord and brain).

Differences:

The autonomic nervous system consists of two sub-components, whereas the somatic nervous system only has one.

The somatic nervous system has sensory and motor pathways, whereas the autonomic nervous system only has motor pathways.

The autonomic nervous system controls internal organs and glands, while the somatic nervous system controls muscles and movement.

The brain consists of multiple regions responsible for different functions, whereas the spinal cord has one main function.

Updated November 26, 2018

By Kevin Beck

The nervous system is what allows living things to gain and process information from the external environment and convert this information into instructions. Your five basic senses – touch, small, taste, vision and hearing – are rooted in your nervous system.

There are many ways to divide the nervous system up for purposes of study; for example, "afferent nerves of the right lower limb" would refer specifically to the afferent (sensory) nerves of your right thigh, calf and shin, and exclude the efferent (motor) nerves of those regions.

The nervous system can be divided into portions on the basis of anatomy, on the basis of function or using a combination of both. Most schemes begin by distinguishing between the central nervous system or CNS, which includes the brain and spinal cord, and the peripheral nervous system, which includes all other nervous-system tissue. The PNS in turn is divided into the somatic and autonomic nervous systems (SNS and ANS), with these terms translating to "voluntary" and "involuntary" respectively. Finally, the ANS can be divided into the parasympathetic and sympathetic nervous systems on the basis of the type of involuntary responses generated within each.

The somatic nervous system includes everything under your voluntary control as well as one involuntary function, the somatic reflex arc (this is what a doctor tests for when tapping the tendon under your knee with rubber hammer). The SNS includes both afferent (sensory) nerves that transmit various types of information (e.g., smells, pressure and pain) to the the brain for processing and efferent (motor) nerves that direct the muscles under your control, such as those in your legs and arms, to execute certain movements, such as throwing or running.

The nerves of the SNS are classified on the basis of location. For example, there are 12 pairs of cranial nerves, which originate in the head and supply the muscles of the eyes, throat and other areas within the head with both motor and sensory fibers; and 31 pairs of spinal nerves, all of which service the voluntary muscles of the trunk, pelvis, arms and legs. The neurotransmitter chemical acetylcholine is an excitatory neurotransmitter in the SNS, meaning that it tends to stimulate movements.

The autonomic nervous system vs somatic nervous system distinction is functional: While the somatic nervous system is under your conscious control, none of the autonomic nervous system is. Of course, the two systems interact, with involuntary nervous-system responses permitting more energetic purposeful movements and so on. The neurotransmitter chemical acetylcholine is an inhibitory neurotransmitter in the SNS, meaning that its presence tends to damp out movements. Digestion, the beating of your heart and various internal secretions results from activities of the ANS.

The sympathetic branch of the ANS has CNS components in the chest, abdomen and back. Its signals are processed in structures called peripheral ganglia (singular: ganglion) that lie close to the spinal cord.

The parasympathetic branch of the ANS has its CNS portion in the head and the lower end of the spinal cord. It also has peripheral ganglia, but these are close to the target organs of nervous signals rather than close to the spine.

Like the SNS, the ANS has its own kind of reflex arc. The sensory sides of the somatic and autonomic reflex arcs are essentially the same, but the motor sides are different. In a somatic reflex arc, the motor information passes unimpeded from the spinal cord to the target muscle. In an autonomic reflex arc, however, the efferent signal from the spinal cord passes through a peripheral ganglion and then to the target tissue, which is often the smooth muscle of internal organs.

Peripheral nervous system is the nervous system which is outside the brain and spinal cord. The main function of the PNS is to relay information between the central nervous system and the effector organs. Somatic and autonomic nervous system are the two parts of the peripheral nervous system (PNS).  The somatic nervous system is known as the voluntary nervous system and the autonomic nervous system is known as the involuntary nervous system. The main difference between somatic and autonomic nervous system is that somatic nervous system is involved in controlling voluntary muscular movements whereas the autonomic nervous system is involved in controlling involuntary muscular movements in the body

Key Areas Covered

1. What is Somatic Nervous System
      – Definition, Features, Types, Function
2. What is Autonomic Nervous System
      – Definition, Features, Types, Function
3. What are the Similarities Between Somatic and Peripheral Nervous System
      – Outline of Common Features
4. What is the Difference Between Somatic and Autonomic Nervous System
      – Comparison of Key Differences

Key terms: Afferent Neurons, Autonomic Nervous System (ANS), Cranial Nerves, Efferent Neurons, Involuntary Nervous System, Parasympathetic Nervous System, Reflex Arcs, Somatic Nervous System (SONS), Spinal nerves, Sympathetic Nervous System, Voluntary Nervous System

What is Somatic Nervous System

The somatic nervous system (SONS), also known as voluntary nervous system,  is a part of PNS and is involved in controlling the voluntary muscular movements of the body. It controls the skeletal muscles in the body according to the stimuli obtained from the sensory receptors of the body. On that account, the somatic nervous system is composed of both afferent and efferent nerves. The afferent neurons are responsible for carrying information from sensory receptors to the central nervous system. The efferent neurons are responsible for carrying information from the central nervous system to the effector organs. The afferent and efferent neurons are connected by interneurons at the central nervous system. The somatic nervous system is composed of two parts: cranial nerves and spinal nerves. The cranial nerves carry nerve impulses in and out of the brain. The spinal nerves carry nerve impulses in and out of the spinal cord. 

In addition to controlling voluntary muscular movements, the somatic nervous system is also involved in controlling involuntary muscular movements called reflex arcs. In reflex arcs, the skeletal muscles work without coordinating with the central nervous system. The nerve pathways of the reflex arcs are connected to the spinal cord. There are two types of reflex arcs: autonomic reflex arcs and somatic reflex arcs. The autonomic reflex arcs control the involuntary actions of organs while somatic reflex arcs control that of skeletal muscles. The knee reflex is shown in figure 1.

What is Autonomic Nervous System

The autonomic nervous system (ANS) is another part of PNS, controlling the involuntary muscular movements of the body. The autonomic nervous system controls the muscular movements of the heart and other smooth muscle movements in the stomach, intestine, liver, kidney, lungs, and blood vessels as well as the glands such as sweat, salivary, and digestive glands. Thus, the autonomic nervous system controls the blood pressure, heart beat, digestion, metabolism, body temperature, urination, and homeostasis. The innervation of the autonomic nervous system is shown in figure 2.

Figure 2: Innervation of the autonomic nervous system

The two components of the autonomic nervous system are sympathetic nervous system and parasympathetic nervous system. Sympathetic and parasympathetic nervous systems have opposite effects on the above-mentioned processes. The sympathetic nervous system prepares the body for fight-or-flight response under stressful conditions. Thus, the heart rate is increased, airways are dilated, and the energy-demanding processes such as digestion and urination are stopped. The parasympathetic nervous system maintains the body at rest. It decreases the blood pressure and heart rate while inducing the secretion, digestion, and urination.  

Similarities Between Somatic and Autonomic Nervous System

• Both somatic and autonomic nervous systems are two components of the peripheral nervous system.
• Both somatic and autonomic nervous systems are involved in controlling muscular movements of the body.

Difference Between Somatic and Autonomic Nervous System

Definition

Somatic Nervous System: The somatic nervous system is the part of the nervous system which controls the voluntary movements in the body

Autonomic Nervous System: The autonomic nervous system is the part of the nervous system which regulates the involuntary movements in the body.

Known as

Somatic Nervous System: This is also known as the voluntary nervous system.

Autonomic Nervous System: This is also known as the involuntary nervous system.

Innervation

Somatic Nervous System: This innervates the voluntary skeletal muscles.

Autonomic Nervous System: This innervates involuntary smooth muscles, cardiac muscles, and glands.

Stimuli

Somatic Nervous System: Smell, noise, taste, light, touch, pain, and temperature are the sensory stimuli detected by the somatic nervous system.

Autonomic Nervous System: The blood pressure, salinity, and pH are the sensory stimuli detected by the autonomic nervous system.

Excitatory/Inhibitory Response

Somatic Nervous System: The somatic nervous system always causes the excitatory response at the effector.

Autonomic Nervous System: The autonomic nervous system may cause either excitatory or inhibitory response at the effector.

Neurotransmitter

Somatic Nervous System: The somatic nervous system releases acetylcholine at the effector.

Autonomic Nervous System: The autonomic nervous system releases acetylcholine or norepinephrine at the effector.

Number of Neurons in the Efferent Path

Somatic Nervous System: The somatic nervous system is composed of a single neuron between the CNS and the effector organ.

Autonomic Nervous System: The autonomic nervous system is composed of two neurons with a single synapse between the CNS and the effector organ.

Structure of Nerve Fibers

Somatic Nervous System: This is composed of thick myelinated nerve fibers.

Autonomic Nervous System: This is composed of both thin and thick myelinated nerve fibers.

Branches

Somatic Nervous System: Spinal and cranial nerves are the two branches of the somatic nervous system.

Autonomic Nervous System: Sympathetic and parasympathetic nerves are the two branches of the autonomic nervous system.

Functions

Somatic Nervous System: Posture and movement are the functions of the somatic nervous system.

Autonomic Nervous System: Visceral functions such as secretion and control of metabolism are the functions of the autonomic nervous system.

Conclusion

Somatic and autonomic nervous systems are two components of the peripheral nervous system of the body. Both nervous systems are involved in controlling the functions of the body based on the internal and external stimuli. The somatic nervous system is composed of spinal and cranial nerves. The autonomic nervous system is composed of sympathetic and parasympathetic nervous systems. The somatic nervous system controls the voluntary muscular movements and the reflex arcs. The autonomic nervous system controls the involuntary movements of the body. Thus, the main difference between somatic and autonomic nervous systems is the type of movements controlled by each of them.

Reference:

1.” Somatic Nervous System: Definition, Function & Example.” Study.com. N.p., n.d. Web. Available here. 03 July 2017. 
2. Kendra Cherry. “What Is the Somatic Nervous System?” Verywell. N.p., n.d. Web. Available here. 03 July 2017. 
3. “Overview of the Autonomic Nervous System – Brain, Spinal Cord, and Nerve Disorders.” MSD Manual Consumer Version. N.p., n.d. Web. Available here. 04 July 2017.

Image Courtesy:

1. “Patellar-knee-reflex” By ChristinaT3 at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “1503 Connections of the Parasympathetic Nervous System” By OpenStax College – Anatomy & Physiology, Connexions Web site. June 19, 2013. (CC BY 3.0) via Commons Wikimedia