AN UNDERSTANDING OF THE NERVOUS SYSTEM
The Nervous System
AN UNDERSTANDING OF THE NERVOUS SYSTEM
A Description of the Nervous System
Student ID 80001023
Southern Cross International College
Path Education Group
(Malaysia)
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An Understanding of the Nervous System
The nervous system can be divided into several connected systems that function together. It monitors and controls almost every organ system through a series of positive and negative feedback loops. The nervous system is divided into the Central Nervous System (CNS) includes the brain and spinal cord and the Peripheral Nervous System (PNS) that connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons). At the centre of the nervous system is the brain. The brain sends and receives messages through a network of nerves.
The central nervous system is divided into two parts: the brain and the spinal cord. The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column.
The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system and motor nerve fibers that project to skeletal muscle. The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands.
The nerve system interacts with other body systems. All of the systems within the body interact with one another to keep an organism healthy. Although each system has specific functions, they are all interconnected and dependent on one another. The nervous system controls
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various organs of the body directly. The brain also receives information from many organs of the body and adjusts signals to these organs to maintain proper functioning.
Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them. The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body. Neurons are highly specialized cells that generate and transmit bioelectric impulses from one part of the body to another; the functional unit of the nervous system. A cell of the nerve tissue having a cell body input zone of dendrites and an output zone of an axon (of varying length). The electrochemical nerve impulse/message is transmitted by neurons. There are three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.
Sensory neurons carry signals from receptors and transmit information about the environment to processing centers in the brain and spinal cord. Neurons carrying messages from sensory receptors to the spinal cord and are sometimes referred to as an afferent neuron.
The central nervous system (CNS) is the division of the nervous system that includes the brain and spinal cord. The nerves of the body are organized into two major systems, that is the central nervous system (CNS), consisting of the brain and spinal cord, and another, the peripheral
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nervous system (PNS), the vast network of spinal and cranial nerves linking the body to the brain and spinal cord. The PNS is subdivided into the autonomic nervous system (involuntary control of internal organs, blood vessels, smooth and cardiac muscles), consisting of the sympathetic NS and parasympathetic NS the somatic nervous system (voluntary control of skin, bones, joints, and skeletal muscle). The two systems function together, with nerves from the periphery entering and becoming part of the central nervous system, and vice versa.
Motor neurons or efferent-sound neurons are the neurons the body uses to react to the environment. For example, if we touch a hot surface, then our body will make the hand move away from that surface by a motor neuron. Motor neurons also send impulses to the m=muscles. These neurons are called somatic neurons. Another motor neuron is the auitomomic neuron. The neuron controls the organs and heart. Motor neurons are neurons that receive signals from interneuron and transfer the signals to effector cells that produce a response. They are able to stimulate muscle cells throughout the body including the muscles of the heart, diaphragm, intestines, bladder, and glands. Sometimes these nerve cells that are connected to a muscle or gland is also known as effector neurons.
Interneuron which are neurons that process signals from one or more sensory neurons and relay signals to motor neurons, which are also known as connector neurons. It provides connections between sensory and motor neurons, as well as between themselves. The neurons of the central nervous system, including the brain, are all interneuron. Sensory neurons provide information from the environment to the body, for example, when we touch a hot surface; a sensory neuron informs our body that the temperature near our skin is raising.
The Central Nervous System (CNS) is effectively the centre of the nervous system, ...
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Interneuron which are neurons that process signals from one or more sensory neurons and relay signals to motor neurons, which are also known as connector neurons. It provides connections between sensory and motor neurons, as well as between themselves. The neurons of the central nervous system, including the brain, are all interneuron. Sensory neurons provide information from the environment to the body, for example, when we touch a hot surface; a sensory neuron informs our body that the temperature near our skin is raising.
The Central Nervous System (CNS) is effectively the centre of the nervous system, the part of it that processes the information received from the peripheral nervous system. The CNS
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consists of the brain and spinal cord. It is responsible for receiving and interpreting signals from the peripheral nervous system and also sends out signals to it, either consciously or unconsciously. This information highway called the nervous system consists of many nerve cells, also known as neurons.
The Peripheral Nervous System (PNS) consists of peripheral nerves and their receptors, nerve roots adjacent to the spinal cord, and the sensory, sympathetic and enteric neurons located in ganglia throughout the body. Of particular importance are the first-order sensory neurons within the dorsal root ganglia located in the epidural space, as these neurons are intimately involved in peripheral sensation and pain. The sympathetic division of the PNS is also critically important in maintaining normal homeostasis and physiologic function, and in our response to the environmental and psychological variables that affect our lives.
The nervous tissues are composed of nerve cells and their various processes, together with a supporting tissue called neuroglia, which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as nerve fibers. To the naked eye a difference is obvious between certain portions of the brain and medulla spinalis, viz., the gray substance and the white substance. The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers. It is in the former that nervous impressions are received, stored, and transformed into efferent impulses, and by the latter that they are conducted. Hence the gray substance forms the essential constituent of all the ganglionic centers, both those in the isolated ganglia and those aggregated in the brain and medulla spinalis; while the white substance forms the bulk of the commissural portions of the nerve centers and the peripheral nerves.
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Neuroglia, the peculiar ground substance in which are imbedded the true nervous constituents of the brain and medulla spinalis, consists of cells and fibers. Some of the cells are stellate in shape, with ill-defined cell body, and their fine processes become neuroglia fibers, which extend radially and unbranched among the nerve cells and fibers which they aid in supporting. Other cells give off fibers which branch repeatedly. Some of the fibers start from the epithelial cells lining the ventricles of the brain and central canal of the medulla spinalis, and pass through the nervous tissue, branching repeatedly to end in slight enlargements on the pia mater. Thus, neuroglia is evidently a connective tissue in function but is not so in development; it is ectodermal in origin, whereas all connective tissues are mesodermal.
Nerve cells are largely aggregated in the gray substance of the brain and medulla spinalis, but smaller collections of these cells also form the swellings, called ganglia, seen on many nerves. The nerve cells vary in shape and size, and have one or more processes. They may be divided for purposes of description into three groups, according to the number of processes which they possess.
(1) Unipolar cells, which are found in the spinal ganglia; the single process, after a short course, divides in a T-shaped manner.
(2) Bipolar cells, also found in the spinal ganglia, when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes come off from opposite poles of the cell, and the cell then assumes a spindle shape; in other cells both processes emerge at the same point. In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled spirally around the nerve process which is issuing from the cell.
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(3) Multipolar cells, which are pyramidal or stellate in shape, and characterized by their large size and by the numerous processes which issue from them. The processes are of two kinds: one of them is termed the axis-cylinder process or axon because it becomes the axis-cylinder of a nerve fiber. The others are termed the protoplasmic processes or dendrons; they begin to divide and subdivide soon after they emerge from the cell, and finally end in minute twigs and become lost among the other elements of the nervous tissue.
To understanding the nerve message, there is the plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a positive charge, inside has a negative charge. The charge difference is a resting potential and is measured in millivolts. Passage of ions across the cell membrane passes the electrical charge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (resting potential). Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump. The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients.
Action potential is a reversal of the electrical potential in the plasma membrane of a neuron that occurs when a nerve cell is stimulated; caused by rapid changes in membrane permeability to sodium and potassium. Changed polarity of the membrane is the action potential, results in propagation of the nerve impulse along the membrane. An action potential is a temporary reversal of the electrical potential along the membrane for a few milliseconds. Sodium gates and
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potassium gates open in the membrane to allow their respective ions to cross. Sodium and potassium ions reverse positions by passing through membrane protein channel gates that can be opened or closed to control ion passage. Sodium crosses first. At the height of the membrane potential reversal, potassium channels open to allow potassium ions to pass to the outside of the membrane. Potassium crosses second, resulting in changed ionic distributions, which must be reset by the continuously running sodium-potassium pump. Eventually enough potassium ions pass to the outside to restore the membrane charges to those of the original resting potential. The cell begins then to pump the ions back to their original sides of the membrane.
The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period, during which the membrane cannot be stimulated. It prevents the message from being transmitted backward along the membrane.
Synapse is the junction between an axon and an adjacent neuron, which is between a nerve cell and another cell. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vessicles clustered at the tip of the axon. Synapses usually occur between the axon of a pre-synaptic neuron & a dendrite or cell body of a post-synaptic neuron. At a synapse, the end of the axon is 'swollen' and referred to as an end bulb or synaptic knob. Within the end bulb are found lots of synaptic vesicles (which contain neurotransmitter chemicals) and mitochondria (which provide ATP to make more neurotransmitter). Between the end bulb and the dendrite (or cell body) of the post-synaptic neuron, there is a gap commonly referred to as the synaptic cleft. So, pre- and post-
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synaptic membranes do not actually come in contact. That means that the impulse cannot be transmitted directly. Rather, the impulse is transmitted by the release of chemicals called chemical transmitters (or neurotransmitters).
When an impulse arrives at the end bulb, the end bulb membrane becomes more permeable to calcium. Calcium diffuses into the end bulb & activates enzymes that cause the synaptic vesicles to move toward the synaptic cleft. Some vesicles fuse with the membrane and release their neurotransmitter (a good example of exocytosis). The neurotransmitter molecules diffuse across the cleft and fit into receptor sites in the postsynaptic membrane. When these sites are filled, sodium channels (also called, as in the figure above, chemically gated ion channels) open & permit an inward diffusion of sodium ions. It, of course, causes the membrane potential to become less negative (or, in other words, to approach the threshold potential). If enough neurotransmitter is released, and enough sodium channels are opened, then the membrane potential will reach threshold. If so, an action potential occurs and spreads along the membrane of the post-synaptic neuron (in other words, the impulse will be transmitted). Of course, if insufficient neurotransmitter is released, the impulse will not be transmitted.
Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft. Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action potential, others are inhibitory. Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding
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to receptor molecules on the next cell, prompting transmission of the message along that cell's membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses. Once in the cleft, neurotransmitters are active for only a short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.
Diseases that affect the function of signal transmission can have serious consequences. Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting as a substitute neurotransmitter) but cannot reverse the progression of the disease.
The bacterium Clostridium tetani produces a toxin that prevents the release of GABA. GABA is important in control of skeletal muscles. Without this control chemical, regulation of muscle contraction is lost; it can be fatal when it effects the muscles used in breathing. Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes the progressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in the synapses: cocaine, LSD, caffeine, and insecticides.
The autonomic nervous system consists of sensory neurons and motor neurons that run between the central nervous system (especially the hypothalamus and medulla oblongata) and various internal organs such as the heart, lungs, viscera, and glands. It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. The contraction of both smooth muscle and cardiac muscle is controlled by motor neurons of the autonomic system.
The actions of the autonomic nervous system are largely involuntary (in contrast to those
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of the sensory-somatic system). It also differs from the sensory-somatic system is using two groups of motor neurons to stimulate the effectors instead of one. The first, the preganglionic neurons, arise in the CNS and run to a ganglion in the body. Here they synapse with postganglionic neurons, which run to the effector organ (cardiac muscle, smooth muscle, or a gland). The autonomic nervous system has two subdivisions, the sympathetic nervous system and the parasympathetic nervous system.
Most body organs/systems are enervated by both sympathetic and parasympathetic nerves, and these have opposite effects on the various organs. For example, the sympathetic NS prepares for action by increasing heart and respiration rates by telling the liver to release stored glycogen as sugar, and by decreasing digestive processes. Conversely, the parasympathetic NS stores energy by slowing heart and respiration rates, by telling the liver to store up sugar as glycogen, and by increasing digestive processes.
The brain and nervous system combine to form a communication and computation system that allows an organism to regulate internal function and to react to external stimuli. The study of the brain and nervous system can encompass a broad array of disciplines from the study of single signaling molecules to the overall response of an organism to a given set of behavioral or regulatory challenges. The brain consists of the cerebrum which is the large, anterior portion; the cerebellum which is the wrinkled-looking, posterior part; the pons which is the closest, larger bulge at the top of the spinal cord; the medulla which is the farther, smaller bulge between the pons and the top of the spinal cord; then the spinal cord starts after the medulla. Also note under the cerebrum, the optic chiasma, the place where the optic nerves cross to the other side of the brain. The cerebellum, medulla, and pons are collectively referred to as the hindbrain. Many of their functions are involved in homeostasis, coordination of movement, and maintenance/control
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of breathing and heart rate. While a stroke in the cerebrum might result in partial paralysis, a stroke in the hind brain is actually, potentially more dangerous because it could knock out coordination of the cerebrum's activities, or worse yet, automatic control of breathing and/or heart beat. The midbrain is responsible for receiving and integrating of information and sending/routing that information to other appropriate parts of the brain. The forebrain is composed of the cerebrum and related parts, and functions in pattern and image formation, memory, learning, emotion, and motor control. In addition, the right side functions more in artistic and spatial concepts, while the left side controls speech, language, and calculations. Keep in mind that motor skills are controlled by the opposite half of the brain, thus a left-brain stroke would cause paralysis on the right side of the body. Also, a left-brain stroke might cause problems with speech while a right-brain stroke is more likely to cause abnormal/inappropriate emotional responses.
Nerves carry messages back and forth between the brain and other parts of the body. All of our nerves together make up the nervous system. Some nerves tell the brain what is happening in the body. For example, when we step on a tack, the nerve in our foot tells the brain about the pain. Other nerves tell the body what to do. For example, nerves from the brain tell our stomach when it is time to move food into your intestines.
Every minute of every day, our nervous system is sending and receiving countless messages about what happens both inside and around our body. Right now, our nervous system is receiving sensory input from our eyes about the words on the screen, from our ears about the sound of the computer, from our skin about the feel of our clothes, etc. At the same time, our brain is receiving information from sensors that monitor our heart rate, blood pressure, levels of oxygen and the contents of our stomach and intestines. Our brain then interprets all of these
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signals, which allows for an understanding of the words on the screen, the recognition of the noise as computer noise, and the development of motor responses such as moving our eyeballs, changing positions in our chair, and decreasing or increasing our heart rate and digestion. In short, our nervous system coordinates all the activities of our body. In general, the nervous system as a whole is a system capable of so many sophisticated and complicated functions that can be extremely complex. This essay cannot possibly present all the information about the nervous system and it will probably take a few trips through the nervous system before the pieces fall into place. To sum it up, the nervous system is in charge of directing and overlooking all bodily functions - keeping us alive and healthy, fighting off diseases and infections, and healing us after we have sustained injury.
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