Autonomic nervous system
The autonomic nervous system the vegetative nervous system, is a division of the peripheral nervous system that supplies smooth muscle and glands, thus influences the function of internal organs. The autonomic nervous system is a control system that acts unconsciously and regulates bodily functions such as the heart rate, respiratory rate, pupillary response and sexual arousal; this system is the primary mechanism in control of the fight-or-flight response. Within the brain, the autonomic nervous system is regulated by the hypothalamus. Autonomic functions include control of respiration, cardiac regulation, vasomotor activity, certain reflex actions such as coughing, sneezing and vomiting; those are subdivided into other areas and are linked to ANS subsystems and nervous systems external to the brain. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving ANS regulatory input from the limbic system to do so; the autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.
Some textbooks do not include the enteric nervous system as part of this system. The sympathetic nervous system is considered the "fight or flight" system, while the parasympathetic nervous system is considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more activated dampening system", but this has exceptions, such as in sexual arousal and orgasm, wherein both play a role. There are excitatory synapses between neurons. A third subsystem of neurons that have been named non-noradrenergic, non-cholinergic transmitters have been described and found to be integral in autonomic function, in particular in the gut and the lungs.
Although the ANS is known as the visceral nervous system, the ANS is only connected with the motor side. Most autonomous functions are involuntary but they can work in conjunction with the somatic nervous system which provides voluntary control; the autonomic nervous system is divided into the sympathetic nervous system and parasympathetic nervous system. The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3; the parasympathetic division has craniosacral “outflow”, meaning that the neurons begin at the cranial nerves and sacral spinal cord. The autonomic nervous system is unique in; the preganglionic, or first, neuron will begin at the “outflow” and will synapse at the postganglionic, or second, neuron's cell body. The postganglionic neuron will synapse at the target organ; the sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons: Paravertebral ganglia of the sympathetic chain cervical ganglia thoracic ganglia and rostral lumbar ganglia caudal lumbar ganglia and sacral gangliaPrevertebral ganglia Chromaffin cells of the adrenal medulla These ganglia provide the postganglionic neurons from which innervation of target organs follows.
Examples of splanchnic nerves are: Cervical cardiac nerves & thoracic visceral nerves, which synapse in the sympathetic chain Thoracic splanchnic nerves, which synapse in the prevertebral ganglia Lumbar splanchnic nerves, which synapse in the prevertebral ganglia Sacral splanchnic nerves, which synapse in the inferior hypogastric plexusThese all contain afferent nerves as well, known as GVA neurons. The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem or the sacral spinal cord; these are the preganglionic neurons, which synapse with postganglionic neurons in these locations: Parasympathetic ganglia of the head: Ciliary, Submandibular and Otic In or near the wall of an organ innervated by the Vagus or Sacral nerves These ganglia provide the postganglionic neurons from which innervations of target organs follows. Examples are: The postganglionic parasympathetic splanchnic nerves The vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs and stomach The sensory arm is composed of primary visceral sensory neurons found in the peripheral nervous system, in cranial sensory ganglia: the geniculate and nodose ganglia, appen
A reflex, or reflex action, is an involuntary and nearly instantaneous movement in response to a stimulus. A reflex is made possible by neural pathways called reflex arcs which can act on an impulse before that impulse reaches the brain; the reflex is an automatic response to a stimulus that does not receive or need conscious thought. Myotatic reflexes The myotatic reflexes, provide information on the integrity of the central nervous system and peripheral nervous system. Decreased reflexes indicate a peripheral problem, lively or exaggerated reflexes a central one. A stretch reflex is the contraction of a muscle in response to its lengthwise stretch. Biceps reflex Brachioradialis reflex Extensor digitorum reflex Triceps reflex Patellar reflex or knee-jerk reflex Ankle jerk reflex While the reflexes above are stimulated mechanically, the term H-reflex refers to the analogous reflex stimulated electrically, tonic vibration reflex for those stimulated to vibration. A tendon reflex is the contraction of a muscle in response to striking its tendon.
The Golgi tendon reflex is the inverse of a stretch reflex. Newborn babies have a number of other reflexes which are not seen in adults, referred to as primitive reflexes; these automatic reactions to stimuli enable infants to respond to the environment before any learning has taken place. They include: Asymmetrical tonic neck reflex Palmomental reflex Moro reflex known as the startle reflex Palmar grasp reflex Rooting reflex Sucking reflex Symmetrical tonic neck reflex Tonic labyrinthine reflex Other reflexes found in the central nervous system include: Abdominal reflexes Gastrocolic reflex Anocutaneous reflex Baroreflex Cough reflex Cremasteric reflex Diving reflex Muscular defense Photic sneeze reflex Scratch reflex Sneeze Startle reflex Withdrawal reflex Crossed extensor reflexMany of these reflexes are quite complex requiring a number of synapses in a number of different nuclei in the CNS. Others of these involve just a couple of synapses to function. Processes such as breathing and the maintenance of the heartbeat can be regarded as reflex actions, according to some definitions of the term.
In medicine, reflexes are used to assess the health of the nervous system. Doctors will grade the activity of a reflex on a scale from 0 to 4. While 2+ is considered normal, some healthy individuals are hypo-reflexive and register all reflexes at 1+, while others are hyper-reflexive and register all reflexes at 3+. List of reflexes All-or-none law Automatic behavior Conditioned reflex Instinct Jumping Frenchmen of Maine Voluntary action Preflexes
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
The soma, neurocyton, or cell body is the bulbous, non-process portion of a neuron or other brain cell type, containing the cell nucleus. The word'soma' comes from the Greek'σῶμα', meaning'body'. Although it is used to refer to neurons, it can refer to other cell types as well, including astrocytes and microglia. There are many different specialized types of neurons, their sizes vary from as small as about 5 micrometres to over 10 millimetre for some of the smallest and largest neurons of invertebrates, respectively; the soma of a neuron contains many organelles, including granules called Nissl granules, which are composed of rough endoplasmic reticulum and free polyribosomes. The cell nucleus is a key feature of the soma; the nucleus is the source of most of the RNA, produced in neurons. In general, most proteins are produced from mRNAs; this creates a challenge for supplying new proteins to axon endings that can be a meter or more away from the soma. Axons contain microtubule-associated motor proteins that transport protein-containing vesicles between the soma and the synapses at the axon terminals.
Such transport of molecules away from the soma maintains critical cell functions. The axon hillock is a specialized domain of the neuronal cell body. A high amount of protein synthesis occurs in this region, as it contains a large number of Nissl granules and polyribosomes. Within the axon hillock, materials are sorted as either items that will enter the axon or will remain in the soma. In addition, the axon hillock has a specialized plasma membrane that contains large numbers of voltage-gated ion channels, since this is most the site of action potential initiation; the survival of some sensory neurons depends on axon terminals making contact with sources of survival factors that prevent apoptosis. The survival factors are neurotrophic factors, including molecules such as nerve growth factor. NGF interacts with receptors at axon terminals, this produces a signal that must be transported up the length of the axon to the nucleus. A current theory of how such survival signals are sent from axon endings to the soma includes the idea that NGF receptors are endocytosed from the surface of axon tips and that such endocytotic vesicles are transported up the axon.
Histology image: 3_09 at the University of Oklahoma Health Sciences Center - "Slide 3 Spinal cord"
Denervation is any loss of nerve supply regardless of the cause. If the nerves lost to denervation are part of the neuronal communication to a specific function in the body altered or a loss of physiological functioning can occur. Denervation can be a symptom of a disorder like ALS and post-polio syndrome. Additionally, it can be a useful surgical technique to alleviate major negative symptoms, such as in renal denervation. Denervation can have many harmful side effects such as increased risk of infection and tissue dysfunction; the loss of nerve supply can result from a surgical procedure. Denervation may be the result of nerve injury; the three main types of nerve injury are neurapraxia and neurotmesis. These three types distinguish between the severity of the nerve damage and the potential for recovery after the damage. After an injury in which some nerves are damaged, the brain has shown capabilities in rewiring or rearranging neuronal circuitry; this plasticity allows for the brain to compensate for the loss in neuronal communication resulting from injury.
Denervation processes have a strong association with the symptoms seen in post-polio syndrome. Those with post polio syndrome are undergoing a constant process of reinnervation; this process leads to increased motor unit areas over time. The motor unit areas soon increase to a point where reinnervation is no longer possible causing an uncompensated denervation of motor units which leads to muscle atrophy and loss of muscular strength. Following an acute polio infection diagnosis symptoms such as fatiguability, general weakness and pain are believed to be correlated to muscle denervation. Much like post-polio syndrome, amyotrophic lateral sclerosis has similar symptoms of motor neuron degeneration leading to general weakness and in some cases paralysis; the type of symptoms experienced can depend on which particular areas of the body experience the loss in nerve supply. This process of denervation is however different from post-polio syndrome in that it only involves upper and lower motor neuron degeneration and does not experience a process of constant reinnervaiton and denervation.
In addition to peripheral nerve injury, denervation is used as a medical procedure for various benefits resulting from eliminating nerve supply to a specific area of the body. In renal denervation, the procedure involves using radio frequency or ultrasound to remove sympathetic nerve supply to the wall of the kidney with the intention of reducing blood pressure and treating chronic hypertension. However, renal denervation is used less in recent years due to new evidence suggesting that blood pressure is not reduced after the procedure and there are recommendations against using the procedure since there has been little proof to show that renal denervation leads to reduced blood pressure. Other prevalent surgical procedures involve intentionally reducing nerve supply to treat a variety of disorders. In a sympathectomy, a sympathetic ganglion is surgically removed to treat hyperhidrosis, or excessive sweating. In a vagotomy, the vagus nerve is surgically removed to treat peptic ulcer disease through reducing stomach acid.
In a rhizotomy, nerve fibers in the spinal cord are removed in the hopes of eliminating chronic muscle pain. In regard to skeletal muscle denervation there are two distinct diagnoses: entrapment and compressive neuropathies or non-entrapment neuropathies. Entrapment and compressive neuropathy syndromes occur due to compression and/or constriction on a specific location for a segment of a single nerve or multiple nerve sites; this entrapment or compression can be diagnosed based on multiple factors including physical examination, electrodiagnostic test and clinical history. Following denervation, muscular atrophy and degeneration occurs within affected skeletal muscle tissue. Within the skeletal tissue is observable progressive loss of weight of denervated muscles as well as reduction in muscle fiber size and quantity; these muscles exhibit a slowing of contraction speed, a reduction of developed tension, twitch force. Magnetic resonance imaging and high-resolution ultrasonography are two clinical imaging examinations performed to classify the different diagnoses.
Ultrasonography is advantageous with the evaluation of peripheral nerve resolutions while Magnetic Resonance Imaging is more sensitive in regard to signal intensity changes of the muscle. Denervation affects the muscle activation process, brought on by the development and propagation of an action potential and the ensuing release of calcium, it is found that there is an increase with calcium reuptake because of changes within sarcoplasmic reticulum morphology and structure. As a result there is a decrease in amplitude and velocity of impulse conduction with an increase in muscle spike duration. In clinical and experimental studies there is an observed increase in muscle excitability in electrical currents involving chemical actions, while there is a decrease in excitability to current associated with electrical induction in denervated muscles. Changes in the resting membrane potential involving denervated muscles presents mild depolarization when a muscle contraction stimulus is present. While there is no immediate change involving resting and action potential, there is an increase with membrane resistance.
After prolonged denervation, it is revealed that resting membrane potential over time is reduced while action potentials progressively decreased and become slower. Acetylcholine is a neurotransmitter that becomes supersensitive in the presence of denervated muscle. Upon injection of acetylcholine, a slower contractile response, drastically under action potential threshold, is elicited. De
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization, it inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold. Hyperpolarization is caused by efflux of K+ through K+ channels, or influx of Cl– through Cl– channels. On the other hand, influx of cations, e.g. Na+ through Na+ channels or Ca2+ through Ca2+ channels, inhibits hyperpolarization. If a cell has Na+ or Ca2+ currents at rest inhibition of those currents will result in a hyperpolarization; this voltage-gated ion channel response is. In neurons, the cell enters a state of hyperpolarization following the generation of an action potential. While hyperpolarized, the neuron is in a refractory period that lasts 2 milliseconds, during which the neuron is unable to generate subsequent action potentials. Sodium-potassium ATPases redistribute K+ and Na+ ions until the membrane potential is back to its resting potential of around –70 millivolts, at which point the neuron is once again ready to transmit another action potential.
Voltage gated ion channels respond to changes in the membrane potential. Voltage gated potassium and sodium channels are key component for generating the action potential as well as hyper-polarization; these channels work by selecting an ion based on electrostatic attraction or repulsion allowing the ion to bind to the channel. This releases the water molecule attached to the channel and the ion is passed through the pore. Voltage gated sodium channels close again; this means the channel either is open or not, there is no part way open. Sometimes the channel closes but is able to be reopened right away, known as channel gating, or it can be closed without being able to be reopened right away, known as channel inactivation. At resting potential, both the voltage gated sodium and potassium channels are closed but as the cell membrane becomes depolarized the voltage gated sodium channels begin to open up and the neuron begins to depolarize, creating a current feedback loop known as the Hodgkin cycle.
However, potassium ions move out of the cell and if the original depolarization event was not significant enough the neuron does not generate an action potential. If all the sodium channels are open, however the neuron becomes ten times more permeable to sodium than potassium depolarizing the cell to a peak of +40 mV. At this level the sodium channels begin to voltage gated potassium channels begin to open; this combination of closed sodium channels and open potassium channels leads to the neuron re-polarizing and becoming negative again. The neuron continues to re-polarize until the cell reaches ~ –75 mV, the equilibrium potential of potassium ions; this is the point at which the neuron is hyperpolarized, between –70 mV and –75 mV. After hyperpolarization the potassium channels close and the natural permeability of the neuron to sodium and potassium allows the neuron to return to its resting potential of –70 mV. During the refractory period, after hyper-polarization but before the neuron has returned to its resting potential the neuron is capable of triggering an action potential due to the sodium channels ability to be opened, because the neuron is more negative it becomes more difficult to reach the action potential threshold.
HCN channels are activated by hyperpolarization. Hyperpolarization is a change in membrane potential, neuroscientists measure it using a technique known as patch clamping. Using this method they are able to record ion currents passing through individual channels; this is done using a glass micropipette called a patch pipette, with a 1 micrometer diameter. There is a small patch that contains a few ion channels and the rest is sealed off, making this the point of entry for the current. Using an amplifier and a voltage clamp, an electronic feedback circuit, allows the experimenter to maintain the membrane potential at a fixed point and the voltage clamp measures tiny changes in current flow; the membrane currents giving rise to hyperpolarization are either an increase in outward current or a decrease in inward current. During the afterhyperpolarization period after an action potential, the membrane potential is more negative than when the cell is at the resting potential. In the figure to the right, this undershoot occurs at 3 to 4 milliseconds on the time scale.
The afterhyperpolarization is the time when the membrane potential is hyperpolarized relative to the resting potential. During the rising phase of an action potential, the membrane potential changes from negative to positive, a depolarization. In the figure, the rising phase is from 1 to 2 ms on the graph. During the rising phase, once the membrane potential becomes positive, the membrane potential continues to depolarize until the peak of the action potential is reached at about +40 millivolts. After the peak of the action potential, a hyperpolarization repolarizes the membrane potential to its resting value, first by making it less positive, until 0 mV is reached, by continuing to make it more negative; this repolarization occurs in the figure from 2 to 3 ms on the time scale. Purves D, Augustine GJ, Fitzpatrick D, et al. eds.. Neuroscience. Sunderland, Mass: Sinauer Assoc. ISBN 0-87893-742-0. Basic Neurochemistry Molecular and Medical Aspects by Siegel, et al