Introduction to the nervous system, Part 1: nervous system communication, sensory nervous systems, and motor neurophysiology.
The purpose of this two-part series is to introduce medical communicators to the anatomy and physiology of the human nervous system. The content is especially useful for individuals who do not have a background in biology or neuroscience. Since the nervous system controls many bodily functions and influences other functions, knowledge of the nervous system will aid medical communicators who work in fields such as endocrinology, cardiovascular medicine, and gastroenterology, to name a few. Clinical applications are discussed to highlight how certain disorders, traumas, and pharmaceuticals affect the nervous system. Part 1 provides basic information about nervous system communication, sensory nervous systems, and motor neurophysiology. Part 2, to be published in a future issue of the AMWA Journal, will discuss the autonomic nervous system and the central nervous system (CNS).
At the most fundamental level, the human nervous system communicates. It relays information between a source and a target, which can be separated by distances ranging from a few micrometers to nearly a meter. The nervous system simultaneously integrates information from many sources and responds by generating appropriate signals. Neurons are the nerve cells that conduct electrical signals. There are estimated to be more than 100 billion neurons of various form and function in the human body. Glial cells are the other main type of cell found in the nervous system. They primarily support the function of neurons by making a myelin sheath that covers the outer surface of neuron axons. Additionally, glial cells deliver nutrients to neurons and scavenge dead neuron sections.
In general, a neuron is composed of a cell body, which contains the nucleus and other essential organelles; an axon, which transmits the electrical signal away from the cell body toward the next neuron in the circuit; and dendrites, which accept a signal from a neighboring neuron and transmit the signal to the cell body. Glial cells contain a cell body and many extensions, referred to as processes, that extend from the cell body; however, these processes are not categorized as axons or dendrites.
Neurons do not act in isolation. It takes the coordinated communication of several neurons to perform even the most basic function. This coordinated action is achieved by a neural circuit, which is the assembly of neurons required to carry out a particular function. Afferent neurons carry neural signals toward the CNS. Efferent neurons carry neural signals away from the CNS. Interneurons form local connections between two other neurons in a neural circuit.
Neurons innervate most locations in the body, and they are involved in a wide variety of physiologic functions. The brain and spinal cord, which compose the CNS, are probably the most obvious anatomical components of the nervous system. The sensory nervous system comprises the eyes (visual), the ears (auditory and vestibular), the nose and tongue (chemical), and the skin receptors for pain, heat, and touch. Nerves that innervate skeletal muscles are part of the motor nervous system. Less obvious, but no less important, is the autonomic nervous system, which controls involuntary functions, such as breathing, heart rate, digestion, and sweating.
Basics of Communication
How do neurons communicate? They relay an electrical signal, called an action potential. The action potential originates at the cell body of a neuron and propagates along the axon. The action potential arises when channels in the neuron cell membrane open and allow electrically charged atoms, also known as ions, to flow into or out of the neuron. This ion flow causes a change in the electrical properties of the neuron. If the electrical change is large enough, an action potential can be triggered at the cell body and travel along the axon. Because most neurons in humans are myelinated (ie, covered with a myelin sheath from glial cells), action potentials propagate rapidly.
Clinical Applications Loss of myelin and myelin dysfunction cause clinically significant impairment within the nervous system. When the myelin is not functional, action potentials slow considerably or do not propagate at all. Multiple sclerosis is the most well-known disease caused by myelin dysfunction. The disease is characterized by damage to myelin in the CNS, which leads to scar formation on the neuron. Neuron demyelination and scarring cause a variety of symptoms, from numbness in the limbs to paralysis. (1)
Once an action potential has propagated the length of the axon, it reaches the presynaptic terminal of the neuron. It is at the synapse, the junction of the axon of one neuron with the dendrite of a neighboring neuron, where neural signals can pass from one neuron to another. When an action potential reaches the presynaptic terminal of a chemical synapse, a vesicle containing neurotransmitter fuses with the cell membrane facing the synapse. Once the vesicle has fused, neurotransmitter is released into the synaptic cleft (Figure 1).
The neurotransmitter binds to specific receptors in the postsynaptic terminal, thereby acting as a "switch" for the postsynaptic cell. The switch signal can perform many tasks, including initiating an action potential in the postsynaptic neuron, causing the contraction of a skeletal muscle, or inhibiting neurotransmitter release. Not all neurotransmitter released by the presynaptic neuron is bound by the postsynaptic neuron. Most remaining neurotransmitter is degraded by local enzymes or reabsorbed by the presynaptic neuron to be used for subsequent cell-to-cell communication.
Clinical Applications Many psychotropic medications (eg, antidepressants, antianxiety drugs, stimulants) act to disrupt the regular pattern of neurotransmitter action in CNS neurons. Psychotropic drugs may increase neurotransmitter release from the presynaptic terminal, block neurotransmitter binding at the postsynaptic terminal, or enhance neurotransmitter binding to the receptors. For instance, a class of antianxiety drugs, benzodiazepines, increases the efficiency of g-aminobutyric acid (GABA) binding to receptors. GABA is an inhibitory neurotransmitter that decreases the excitability of neurons. Therefore, if more GABA binds at the postsynaptic neuron, less cell-to-cell communication will take place, slowing or calming many functions. (2)
SENSORY NERVOUS SYSTEMS
Through the senses, people transduce information from one modality (eg, sound) into another (action potential) to perceive the external and internal environments. The sensory systems contain highly specialized sensory neurons that send signals to the brain about both the type of stimulus and the strength of the stimulus.
Information about touch, pain, and temperature is transmitted by the somatosensory nervous system. Located in the skin are touch receptors, also known as mechanoreceptors, that allow people to determine the size, shape, and texture of objects. Specialized classes of mechanoreceptors respond preferentially to light touch, to textured surfaces moving over the skin, and to constant pressure. Different parts of the human body have different sensitivities for discriminating touch. Fingers have the highest sensitivity, as measured by two-point discrimination, a test documenting how well a person can distinguish between two items touching the skin. The trunk and proximal limbs have poor touch discrimination: Two stimuli must be separated by 3 to 4 cm on the trunk to be perceived as distinct.
There are also internal mechanoreceptors, termed proprioceptors, that offer information about where the body and limbs are located in space. Proprioceptors associated with skeletal muscle give information about muscle length, and those associated with tendons give information about muscle tension. Movements that require a high degree of precision (eg, fine manipulation with fingers) correlate with a higher density of proprioceptors in the corresponding muscles.
Pain information is transmitted by a different class of skin receptors, the nociceptors. Various subtypes of nociceptors transmit pain information that originates from different sources, such as noxious heat, chemicals, or touch. The brain interprets neural signals that originate in the nociceptors to form a more complete experience and response to the painful stimulus. Similar to the sensitivities for discriminating touch, different parts of the body have different sensitivities to painful stimuli. Interestingly, very few nociceptors are solely responsible for transmitting information about pain from internal organs (ie, the viscera). Usually visceral pain information is transmitted by neurons that also carry information from skin nociceptors. The colocation of both types of pain information results in referred pain--for example, pain signals originating from the heart are perceived as pain in the arm.
Humans rely most heavily on vision to transmit information about their surroundings. The eye and the visual system are very intricate and have a complex function, which can be distilled down to a few key steps. Light passes through the cornea and lens of the eye and is absorbed by photoreceptors (cones and rods) that are located in the retina at the back of the eye. The photoreceptors send electrical signals to retinal neurons, which, in turn, connect to the visual centers in the brain. Rods are very sensitive photoreceptors that are responsible for seeing in low-light conditions (eg, at night). Cones allow humans to see detailed images and color, with three types of cones that are each most sensitive to a particular color (red, green, and blue). Rods and cones generate a graded neural potential that is proportional in size to the amount of light that is absorbed. This response pattern is different from the traditional all-or-none action potential described above.
The density of photoreceptors is greatest in the central part of the retina, called the fovea. This is also the area on the retina where incoming light is focused by the lens. It is not surprising, then, that the fovea is crucial for seeing detail in images. Images are formed, in part, by the amount of light that is focused on a particular photoreceptor and the amount of light that is focused on neighboring photoreceptors. Brain structures for the visual system take information from retinal neurons about the quantity of light absorbed and process the signals to extract information about shape, movement, and color.
Clinical Applications Age-related macular degeneration (AMD) is the most common cause of vision impairment in adults older than 50 years. Vision loss occurs when the retinal pigment epithelium (the nonneuronal layer of the retina) begins to degenerate, which leads to degeneration of the neighboring layer of rods and cones. The epithelium and photoreceptor degeneration occurs only in the macula region, which is near the center of the retina and includes the fovea. Therefore, AMD results in the loss of detailed central vision and the preservation of peripheral vision (Figure 2). Although there is no cure for AMD, therapies exist for "wet" AMD, in which abnormal blood vessels growing behind the epithelium leak blood or fluid that damages the photoreceptors. (3)
Auditory and Vestibular Systems
Hearing and balance are very important senses that can be underappreciated until they are not functioning normally. The auditory system facilitates communication with the world by allowing humans to perceive sounds and interpret their significance. Sound waves enter the ear canal and vibrate a thin membrane, called the tympanic membrane. The vibrations of the tympanic membrane cause movement in a series of bones, which, in turn, move another thin membrane, the oval window. Movement at the oval window causes movement of the fluid contained in the snail shell-shaped cochlea, which is where sound energy is converted into neural signals. The cochlear sensory receptors, the hair cells, transmit electrical signals to cochlear neurons in response to the fluid movement.
The cochlea has been described as a frequency analyzer because it is organized such that neurons at different locations along the length of the cochlea (imagine the snail shell unwound) respond maximally to different sound frequencies (ie, pitches). This frequency organization is maintained throughout many of the auditory brain structures. Cochlear neurons transmit information about the intensity (loudness) of the sound to central auditory regions in part by the number of action potentials that are generated.
Clinical Applications Individuals with sensorineural hearing loss (SNHL) have lost the function of most cochlear hair cells, although some of the cochlear neurons remain and are functional. Patients with SNHL have severe hearing impairments that are not helped by hearing aids, which amplify sound and rely on a complete sensory pathway in the cochlea. However, cochlear implants (devices surgically implanted into the cochlea) can restore sound perception to most patients with SNHL. Cochlear implants deliver electrical impulses directly to the remaining cochlear neurons. The goal is to mimic the input that would have been received by the dysfunctional hair cells. (4)
The vestibular system has a primary role in the senses of balance and body position. Similar to the auditory system, the vestibular system relies on fluid movement to initiate action potentials. In fact, the vestibular system and the auditory system share a fluid connection, which is evident when vertigo (a vestibular disorder) accompanies an ear infection (an auditory disorder). There are two main components of the vestibular system, the semicircular canals and the otolith organs. The three semicircular canals are responsible for detecting rotations of the head (in three dimensions). The vestibular neurons that innervate the semicircular canals generate a constant rate of action potentials when the body is stationary. Thus, the neurons transmit information about the speed and direction of rotation by either increasing or decreasing the rate of action potentials. The otolith organs detect head tilt and linear movement (imagine the sensation of riding an elevator).
The senses of taste and smell are closely related and are responsible for most human interaction with chemicals in the environment. The olfactory system detects odors when an odorant (ie, an odor molecule) comes in contact with a sheet of cells lining the inside of the nose, the olfactory epithelium. Sensory cells housed within the olfactory epithelium have receptors that, when bound by certain odorants, will cause a neural response. The particular pattern of neural responses arising from different olfactory cells provides a neural signature that is unique for each odorant. Exposure to certain odorants can cause physiologic responses, such as salivation and hormonal fluctuations. The olfactory system is one of the least studied sensory systems in humans; therefore, much less is known about its organization and function.
Taste sensations begin at the taste buds, located on papillae on the tongue. It is estimated that each person has 5,000 to 10,000 taste buds. When a tastant (ie, a chemical in food) interacts with taste buds, neural signals are transmitted along the gustatory neuron to the brainstem. Human taste sensation primarily detects flavors such as salt, sweet, bitter, and sour, although humans can perceive other tastes such as fatty, umami, and pungent. A much larger range of chemicals can be detected through smelling than through tasting. Different locations on the tongue have higher sensitivities to different tastant classes, but it is a common misconception that certain areas of the tongue are exclusively used to perceive certain tastes.
Skeletal muscles are the large muscles most commonly associated with movement. These muscles, composed of individual muscle fibers, attach to bones and facilitate joint movement. To accomplish joint movement, a muscle fiber receives a neural signal from an innervating alpha motor neuron. This neural signal causes contraction of the skeletal muscle, which typically causes movement of a joint. Each muscle fiber is innervated by one alpha motor neuron, but each alpha motor neuron innervates many muscle fibers. Therefore, with a signal originating from one alpha motor neuron, the coordinated contraction of many muscle fibers occurs. Each alpha motor neuron and its associated muscle fibers is considered one motor unit. The force with which a particular muscle contracts is variable and regulated by the number of motor units that are recruited.
For most movements, a pair of muscles evokes opposite actions on a joint. When the flexor muscle contracts, the joint closes; when the extensor muscle contracts, the joint opens. To achieve any particular joint movement, one muscle must contract and the other must lengthen in a coordinated fashion.
Clinical Applications Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig disease, is a disease marked by progressive degeneration of the alpha motor neurons and motor neurons that originate in the brain. The disease affects 1 to 3 people per 100,000, and the cause of the disease is unknown in almost all cases. When the motor neurons degenerate, they no longer transmit signals to the innervating muscles; the result is muscle weakness, muscle atrophy, and eventually the inability to initiate voluntary movement. Most patients with ALS are affected in their limbs first, showing muscle weakness or stiffness. Currently, there is no cure, but one US Food and Drug Administration-approved drug, riluzole, slows the progression of ALS. (5)
Reflex movements are automatic movements that occur in response to certain stimuli. Take, for instance, the leg extension reflex that occurs when the knee is tapped with a small rubber reflex hammer. To execute this leg extension, a muscle stretch receptor in the extensor muscle, which is stretched in response to the knee tap, sends a signal through a sensory neuron to the spinal cord. In the spinal cord, the sensory neuron activates a motor neuron innervating the stretched extensor muscle, causing the extensor muscle to contract. At the same time, the sensory neuron signal inactivates the flexor muscle in the back of the leg, causing the flexor muscle to relax.
Many usual body movements involve groups of sensory and motor neurons that form circuits through the spinal cord. Research in paralyzed animals has shown that walking movements can be completely restored by controlled stimulation in the spinal cord alone. Several "higher" areas of motor control in the CNS are involved in coordinating, adjusting, and deciding about movements. They will be discussed in Part 2 of this series.
With a basic understanding of how a neuron generates and propagates a signal, medical communicators can delve into topics that deal with signal alterations, such as pharmaceuticals that influence neurotransmitters. Although each sensory system is specialized to respond to a particular stimulus, the systems have much in common, such as how peripheral information is transmitted and how stimuli are discriminated.
action potential--The electrical signal that travels from the cell body of a neuron to the axon and allows a neuron to communicate with neighboring neurons or effector cells.
axon--The long process of a neuron that conducts an action potential to the presynaptic terminal.
dendrite--A fingerlike process of the neuron that receives information from neighboring neurons.
glial cell--A type of neural cell that primarily produces myelin and acts as a support cell for the nervous system.
ion--Electrically charged atoms or molecules.
neuron--A type of neural cell that is specialized to conduct action potentials and transmit information to other cells.
neurotransmitter--A chemical that is released into the synapse by a neuron and can then bind to receptors on other neurons.
synapse--The connection between two neurons that allows cell-to-cell communication and into which neurotransmitter is typically released.
Author disclosure: The author notes that she has no commercial associations that may pose a conflict of interest in relation to this article.
Author contact: email@example.com References
(1.) Compston A, Coles A. Multiple sclerosis. Lancet. 2008;372(9648):1502-1517.
(2.) Davis KL, Charney D, Coyle JT, Nemeroff C, eds. Neuropsychopharmacology--The Fifth Generation of Progress. Philadelphia, PA: Lippincott, Williams, & Wilkins; 2002.
(3.) Jager RD, Mieler WF, Miller JW. Age-related macular degeneration. N Eng J Med. 2008;358(24):2606-2617.
(4.) Dorman MF, Wilson BS. The design and function of cochlear implants. Am Sci. 2004;92(5):436-445.
(5.) Mitchell JD, Borasio GD. Amyotrophic lateral sclerosis. Lancet. 2007;369(9578):2031-2041.
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By Agnella Izzo Matic, PhD/Principal, AIM Biomedical, Evanston, IL
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|Title Annotation:||SCIENCE SERIES|
|Author:||Matic, Agnella Izzo|
|Publication:||American Medical Writers Association Journal|
|Date:||Mar 1, 2014|
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