10 differences between somatic and autonomic nervous system

The peripheral nervous system consists of the nerves that branch out from the brain and spinal cord. These nerves form the communication network between the CNS and the body parts. The peripheral nervous system is further subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that go to the skin and muscles and is involved in conscious activities. The autonomic nervous system consists of nerves that connect the CNS to the visceral organs such as the heart, stomach, and intestines. It mediates unconscious activities.

Structure of a Nerve

A connective tissue sheath called the epineurium surrounds each nerve. Each bundle of nerve fibers is called a fasciculus and is surrounded by a layer of connective tissue called the perineurium. Within the fasciculus, each individual nerve fiber, with its myelin and neurilemma, is surrounded by connective tissue called the endoneurium. A nerve may also have blood vessels enclosed in its connective tissue wrappings.

Cranial Nerves

Twelve pairs of cranial nerves emerge from the inferior surface of the brain. All of these nerves, except the vagus nerve, pass through foramina of the skull to innervate structures in the head, neck, and facial region.

The cranial nerves are designated both by name and by Roman numerals, according to the order in which they appear on the inferior surface of the brain. Most of the nerves have both sensory and motor components. Three of the nerves are associated with the special senses of smell, vision, hearing, and equilibrium and have only sensory fibers. Five other nerves are primarily motor in function but do have some sensory fibers for proprioception. The remaining four nerves consist of significant amounts of both sensory and motor fibers.

Acoustic neuromas are benign fibrous growths that arise from the balance nerve, also called the eighth cranial nerve or vestibulocochlear nerve. These tumors are non-malignant, meaning that they do not spread or metastasize to other parts of the body. The location of these tumors is deep inside the skull, adjacent to vital brain centers in the brain stem. As the tumors enlarge, they involve surrounding structures which have to do with vital functions. In the majority of cases, these tumors grow slowly over a period of years. In other cases, the growth rate is more rapid and patients develop symptoms at a faster pace. Usually, the symptoms are mild and many patients are not diagnosed until some time after their tumor has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans.

Spinal Nerves

Thirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair of nerves corresponds to a segment of the cord and they are named accordingly. This means there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.

Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. The cell bodies of the sensory neurons are in the dorsal root ganglion, but the motor neuron cell bodies are in the gray matter. The two roots join to form the spinal nerve just before the nerve leaves the vertebral column. Because all spinal nerves have both sensory and motor components, they are all mixed nerves.

Autonomic Nervous System

The autonomic nervous system is a visceral efferent system, which means it sends motor impulses to the visceral organs. It functions automatically and continuously, without conscious effort, to innervate smooth muscle, cardiac muscle, and glands. It is concerned with heart rate, breathing rate, blood pressure, body temperature, and other visceral activities that work together to maintain homeostasis.

The autonomic nervous system has two parts, the sympathetic division and the parasympathetic division. Many visceral organs are supplied with fibers from both divisions. In this case, one stimulates and the other inhibits. This antagonistic functional relationship serves as a balance to help maintain homeostasis.

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.

Divisions of the Human Nervous System

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

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

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.

The Autonomic Reflex Arc

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.

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