Nervous System, Somatic
The somatic nervous system (SNS) is a division of the peripheral nervous system (PNS). The SNS controls voluntary activities, such as movement of skeletal muscles. It includes both sensory and motor nerves. Sensory nerves convey nerve impulses from the sense organs to the central nervous system (CNS), while motor nerves convey nerve impulses from the CNS to skeletal muscle effectors.
All nervous tissue—including that of the SNS—consists of two main cell types: neurons and glial cells. Neurons transmit nerve signals and are surrounded by glial cells, that provide mechanical and physical support as well as electrical insulation between neurons.
A neuron consists of a cell body, the soma, which contains the nucleus and surrounding cytoplasm, several short thread-like projections, called dendrites, and of one long filament, called the axon. The dendrites receive information from other nearby cells and transmit the signals to the soma and the axon carries signals away from the neuron. Both axons and dendrites are surrounded by a white protective coating called the myelin sheath. The average adult brain contains about 100 billion neurons. Neurons are also the longest cells of the body, a single axon can be several feet long. There are two types of neurons found in the SNS: sensory neurons, which typically have long dendrites and short axons, and carry messages from sensory receptors to the CNS, and motor neurons, which have a long axon and short dendrites and transmit signals from the CNS to muscles or glands.
The nervous system
The nervous system of the human body is divided into the central nervous system (CNS), consisting of the spinal cord and brain, and the peripheral nervous system (PNS), consisting of all the nerves that connect the CNS with organs, muscles, blood vessels and glands. The PNS is subdivided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). The ANS is further divided by function into sympathetic and parasympathetic systems.
The somatic nervous system (SNS)
The somatic nervous system (SNS) consists of sensory and motor nerve divisions. The sensory division, also called the afferent division, contains neurons that receive signals from the tendons, joints, skin, skeletal muscles, eyes, nose, ears and tongue, and many other tissues and organs. These signals are conveyed to the cranial and spinal nerves. The motor division, also called the efferent division, contains pathways that go from the brain stem and spinal cord to the lower motor neurons of the cranial and spinal nerves. When these nerves are stimulated, they cause the skeletal muscles to contract. This is called voluntary contraction of the skeletal muscles.
The nerves of the sensory-somatic system are:
THE CRANIAL NERVES (12 PAIRS).
- olfactory nerve, a sensory nerve for the sense of smell
- optic nerve, a sensory nerve for vision
- oculomotor nerve, a motor nerve for eyelid and eyeball muscle control
- trochlear nerve, a motor nerve for eyeball muscle control
- trigeminal nerve, a mixed nerve, the sensory part for facial and mouth sensation and the motor part for chewing
- abducens nerve, a motor nerve for eyeball movement control
- facial nerve, a mixed nerve, the sensory part for taste and the motor part for the control of facial muscles and salivary glands
- auditory nerve, a sensory nerve for hearing and balance control
Axon—Long filament of a neuron that carries outgoing electrical signals from the cell body towards target cells. Each neuron has one axon, which can be longer than a foot. Neurons communicate with each other by transmitting signals from branches located at the end of their axons. At the end of the axons, nerve impulses are transmitted to other nerve cells or to effector organs.
Brachial plexus—A group of lower neck and upper back spinal nerves supplying the arm, forearm and hand.
Brain stem—Lowest part of the brain that connects with the spinal cord. It is a complicated neural center with several neuronal pathways between the cerebrum, spinal cord, cerebellum, and motor and sensory functions of the head and neck. It consists of the medulla oblongata, the part responsible for cardiac and respiratory control, the midbrain, which is involved in basic, involuntary body functions, and the pons, where some cranial nerves originate.
Central nervous system (CNS)—One of two major divisions of the nervous system. The CNS consists of the brain, the cranial nerves and the spinal cord.
Cranial nerve—In humans, there are 12 cranial nerves. They are connected to the brain stem and basically 'run' the head as well as help regulate the organs of the thoracic and abdominal cavities.
Dendrites—Threadlike extensions of the cytoplasm of a neuron.
Effector—Any molecule, chemical, organ, structure or agent that regulates a pathway by changing the pathway's reaction rate.
Ganglia—A mass of nerve tissue or a group of neurons.
Mechanoreceptors—Receptors specialized to detect mechanical signals and relay that information centrally in the nervous system. Mechanoreceptors include hair cells involved in hearing and balance.
Myelin—The substance making up the protective sheath of nerve axons.
Nervous system—The entire system of nerve tissue in the body. It includes the brain, the brain stem, the spinal cord, the nerves and the ganglia, and is divided into the peripheral nervous system (PNS) and the central nervous system (CNS).
Neurons—Cells of the nervous system. Usually consist of a cell body, the soma, that contains the nucleus and the surrounding cytoplasm; several short thread-like projections (dendrites); and one long filament (the axon)
Neuropathy—A general term describing functional disorders and/or abnormal changes in the peripheral nervous system. If the involvement is in one nerve it is called mononeuropathy, and if in several nerves, mononeuropathy multiplex.
Oculomotor nerve—Cranial nerve responsible for motor enervation of the upper eyelid muscle, the extraocular muscle and the eye pupil muscle.
Parasympathetic nervous system—One of the two divisions of the autonomic nervous system. Parasympathetic nerves emerge from the skull as fibres from the oculomotor, facial, glossopharyngeal and vagus nerves and from the sacral region of the spinal cord.
Peripheral nerves—The nerves outside of the brain and spinal cord, including the autonomic, cranial, and spinal nerves. These nerves contain cells other than neurons and connective tissue as well as axons.
Peripheral nervous system (PNS)—One of the two major divisions of the nervous system. The PNS consists of the somatic nervous system (SNS), which controls voluntary activities, and of the autonomic nervous system (ANS), which controls regulatory activities. The ANS is further divided into sympathetic and parasympathetic systems.
Plexus—A network or group of nerves.
Sensory cells—Cells that contain receptors on their surface.
Sensory nerve—A nerve that receives input from sensory cells, such as the skin mechanoreceptors or the muscle receptors.
Spinal cord—Elongated part of the central nervous system that lies in the vertebral column and from which the spinal nerves emerge.
Sympathetic nervous system—One of the two divisions of the autonomic nervous system. The sympathetic neurons have their cell bodies in the thoracic and lumbar regions of the spinal cord and connect to the paravertebral chain of sympathetic ganglia. They innervate heart and blood vessels, sweat glands, organs and the adrenal medulla.
- glossopharyngeal, a mixed nerve, the sensory part for taste and the motor part for the control of swallowing
- vagus, a mixed nerve, main PNS nerve that controls the gut, heart and larynx
- accessory, a motor nerve for swallowing and moving the head and shoulders
- hypoglossal, a motor nerve for the control of tongue muscles
THE SPINAL NERVES (31 PAIRS). All of the spinal nerves are mixed nerves containing both sensory and motor neurons. They consist of eight cervical, 12 thoracic, five lumbar, five sacral, and one coccygeal. In spinal nerves, some nerves fibers are ascending, meaning that they carry messages to the brain, while others are descending, meaning that they carry messages from the brain.
Sensory input to the nervous system occurs through the senses, which are: vision, taste, smell, touch and hearing, also called the special senses. Additional input is provided by the somatic senses, which are pain, temperature, and pressure. This sensory input uses sensors, also called sensory receptors. The major sensory receptors are:
- mechanoreceptors that respond to hearing and stretching
- photoreceptors that are sensitive to light
- chemoreceptors that respond mostly to smell and taste
- thermoreceptors that are sensitive to changes in temperature
- electroreceptors that detect electrical currents in the environment
The major function of the SNS is the voluntary control of the muscle system of the body and the processing of sensory information to the CNS. All conscious knowledge of the external world and all the motor activity performed by the body to respond to it operates through the SNS.
Role in human health
The overall role of the nervous system is to act as an internal communications system that allows the body to react to environmental changes and to perform all activities required to maintain life. The PNS is the message carrier between the CNS and the rest of the body and it can not function with an impaired SNS. Thus, the role of the SNS in human health is crucial.
Common diseases and disorders
Somatic nervous system diseases are diseases of the peripheral nerves that are external to the brain and spinal cord. Thus, they include diseases of the nerve roots, ganglia, sensory and motor nerves. A functional disorder and/or abnormal change that occurs in any region of the peripheral nervous system is called a neuropathy. If the involvement is in one nerve only, it is called a mononeuropathy, and if in several nerves, mononeuropathy multiplex or polyneuropathy. The most common disorders are the following:
- Brachial plexus neuropathies: Diseases of the peripheral nerve components of the brachial plexus, a group of lower neck and upper back spinal nerves supplying the arm, forearm and hand. Symptoms include local pain, muscle weakness, and decreased sensation (hypesthesia) in the upper extremity.
- Cranial nerve diseases: Disorders and diseases of the cranial nerves.
- Cranial nerve neoplasms: Benign or cancerous growth in cranial nerve tissues. Examples are: acoustic neuroma, optic nerve glioma, optic nerve meningioma.
- Diabetic neuropathies: Peripheral and cranial nerve disorders that are associated with diabetes. A common condition associated with diabetic neuropathy includes third nerve palsy, which affects the oculomotor nerve.
- Guillain-Barre syndrome: An acute inflammatory autoimmune neuritis caused by the body attacking the myelin coating of its own peripheral nerves. The syndrome often occurs as a result of viral or bacterial infection, surgery, immunization, lymphoma, or exposure to toxins.
- Mononeuropathies: Disease or trauma involving a single peripheral nerve. Mononeuropathies result from a wide variety of causes such as traumatic injury; nerve compression, and connective tissue diseases.
- Myasthenia gravis (MG): MG (and also the less common Lambert-Eaton syndrome) are neuromuscular junction diseases, that is, diseases affecting how nerve impulses are transmitted to muscle at the neuromuscular junction. They are autoimmune diseases, meaning that the body generates an immune system attack against its own skeletal muscles.
- Nerve compression syndromes: These syndromes are due to the compression of nerves or nerve roots from internal or external causes and result in the blocking of nerve impulses due to myelin sheath or axon damage.
- Neuralgia: Neuralgias are disorders of the cranial nerves that result in intense or aching pain occuring along a peripheral or cranial nerve. Neuralgias are associated ciated with all of the cranial nerves: trigeminal neuralgia in the facial area, glossopharyngeal neuralgia in the throat, occipital neuralgia in the rear and side of the head, geniculate neuralgia in the ear, and vegal neuralgia in the jaw.
- Neuritis: Inflammation of a peripheral or cranial nerve.
- Peripheral nervous system neoplasms: Benign or cancerous growths that arise from peripheral nerve tissue. They include neurofibromas, granular cell tumors and malignant peripheral nerve sheath tumors.
- Trigeminal neuralgia (TN): Most common neuralgia. It affects the fifth cranial (trigeminal) nerve and causes episodes of intense, stabbing, electric shock-like pain in the areas of the face where the branches of the nerve are distributed, that is lips, eyes, nose, scalp, forehead, upper jaw, and lower jaw.
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Monique Laberge, Ph.D.