By V. Temmy. Louisiana Baptist Universty.

Nongated ion channels are distributed throughout the neu- other action potential because of the inactivation of the ronal membrane; voltage-gated channels are largely re- voltage-gated sodium channels quality himplasia 30caps herbs books. When an action potential invades the nerve terminal cheap himplasia 30caps otc zen herbals, volt- channels predominate on the cell body (soma) and den- age-gated calcium channels open, allowing calcium to en- dritic membrane. Membrane conductance and capacitance affect ion flow in release of neurotransmitter. An action potential is a transient change in membrane po- of neurotransmitters that activate specific receptors on tential characterized by a rapid depolarization followed by their postsynaptic target cells. Most neurotransmitters are stored in synaptic vesicles and activation of voltage-gated sodium channels and the repo- released upon nerve stimulation by a process of calcium- larization phase to an inactivation of the sodium channels mediated exocytosis; once released, the neurotransmitter and the delayed activation of voltage-gated potassium binds to and stimulates its receptors briefly before being channels. Metabolic maintenance of neurons requires specialized hillock is depolarized to a threshold for rapid activation of a functions to match their specialized morphology and com- large number of voltage-gated sodium channels. It activates muscles for move- transmission of information from one cell to another by ment, controls the secretion of hormones from glands, reg- conducting electrical impulses and secreting chemical neu- ulates the rate and depth of breathing, and is involved in rotransmitters. The electrical impulses propagate along the modulating and regulating a multitude of other physiolog- length of nerve fiber processes to their terminals, where ical processes. To perform these functions, the nervous sys- they initiate a series of events that cause the release of 37 38 PART I CELLULAR PHYSIOLOGY chemical neurotransmitters. The release of neurotransmit- structure formed by glial cells (oligodendrocytes in the ters occurs at sites of synaptic contact between two nerve CNS or Schwann cells in the peripheral nervous system, cells. Regular intermittent gaps in the myelin sheath on the postsynaptic cell membrane. The speed with which an axon receptors either excites or inhibits the postsynaptic neuron. The end rotransmitters, and the activation of receptors constitute the of the axon, the axon terminal, contains small vesicles means whereby nerve cells communicate and transmit in- packed with neurotransmitter molecules. When a neuron is activated, an action potential is gen- transmission, and discuss aspects of neuronal structure nec- erated in the axon hillock (or initial segment) and con- essary for the maintenance of nerve cell function. The action potential causes the re- lease of a neurotransmitter from the terminal. These neurotransmitter molecules bind to receptors located on PASSIVE MEMBRANE PROPERTIES, THE target cells. ACTION POTENTIAL, AND ELECTRICAL The binding of a neurotransmitter to its receptor typi- cally causes a flow of ions across the membrane of the post- SIGNALING BY NEURONS synaptic cell. This temporary redistribution of ionic charge Neurons communicate by a combination of electrical and can lead to the generation of an action potential, which it- chemical signaling. Generally, information is integrated and self is mediated by the flow of specific ions across the mem- transmitted along the processes of a single neuron electri- brane. These electrical charges, critical for the transmission cally and then transmitted to a target cell chemically. The of information, are the result of ions moving through ion chemical signal then initiates an electrical change in the tar- channels in the plasma membrane (see Chapter 2). Electrical signals that depend on the passive prop- erties of the neuronal cell membrane spread electrotonically over short distances. These potentials are initiated by local Channels Allow Ions to Flow Through current flow and decay with distance from their site of initi- the Nerve Cell Membrane ation. Alternatively, an action potential is an electrical sig- Ions can flow across the nerve cell membrane through three nal that propagates over a long distance without a change in types of ion channels: nongated (leakage), ligand-gated, amplitude. Nongated ion channels are al- of channel openings and closings in the membrane. They are responsible for the influx of Na and efflux of K when the neuron is in its resting state. Ligand- Special Anatomic Features of Neurons Adapt gated ion channels are directly or indirectly activated by chemical neurotransmitters binding to membrane recep- Them for Communicating Information tors. In this type of channel, the receptor itself forms part The shape of a nerve cell is highly specialized for the re- of the ion channel or may be coupled to the channel via a ception and transmission of information. When chemical trans- neuron is designed to receive and process incoming infor- mitters bind to their receptors, the associated ion channels mation; another is designed to conduct and transmit infor- can either open or close to permit or block the movement mation to other cells.

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The waves recorded on an electroen- This series of connections from the reticular formation cephalogram (EEG) are described in terms of frequency buy himplasia 30caps without a prescription herbs and pregnancy, through the intralaminar nuclei of the thalamus and on to the which usually ranges from less than 1 to about 30 Hz purchase himplasia 30 caps on line herbals usa, and forebrain is termed the ascending reticular activating system. Since the waves are a summation of activity tems that regulate sleep/wake cycles and consciousness. So in a complex network of neuronal processes, they are highly important is the ascending reticular activating system to variable. However, during various states of consciousness, the state of arousal that a malfunction in the reticular for- EEG waves have certain characteristic patterns. At the high- mation, particularly the rostral portion, can lead to a loss of est state of alertness, when sensory input is greatest, the consciousness and coma. Alpha waves, a rhythm ranging from 8 to 13 Hz, are observed when the person is awake but relaxed Beta (13–30 Hz) with the eyes closed. When the eyes are open, the added vi- sual input to the cortex imparts a faster rhythm to the EEG, ranging from 13 to 30 Hz and designated beta waves. The Theta (4–7 Hz) slowest waves recorded occur during sleep: theta waves at 4 to 7 Hz and delta waves at 0. The characteristic waveform signifying seizure activity is the Seizure appearance of spikes or sharp peaks, as abnormally large spike numbers of units fire simultaneously. Examples of spike ac- tivity occurring singly and in a spike-and-wave pattern are shown in Figure 7. The ascending reticular activating system is pe- wave riodically shut down by influences from other regions of the reticular formation. The EEG recorded during sleep re- 100 µV veals a persistently changing pattern of wave amplitudes 1 sec and frequencies, indicating that the brain remains continu- 0 ally active even in the deepest stages of sleep. The EEG pat- Patterns of brain waves recorded on an tern recorded during sleep varies in a cyclic fashion that re- FIGURE 7. Wave patterns are designated alpha, peats approximately every 90 minutes, starting from the beta, theta, or delta waves, based on frequency and relative ampli- time of falling asleep to awakening 7 to 8 hours later (Fig. These cycles are associated with two different forms pear as many neurons are activated simultaneously. Rapid eye movement (REM) sleep: back-and-forth alertness scale, when sensory input is at its lowest, in deep movements of the eyes under closed lids, accompanied by sleep, a synchronized EEG has the characteristics of low fre- autonomic excitation quency and high amplitude. An absence of EEG activity is EEG recordings of sleeping subjects in laboratory set- the legal criterion for death in the United States. Corpus callosum A normal sleep cycle begins with slow-wave sleep, four Frontal lobe stages of increasingly deep sleep during which the EEG be- Lateral comes progressively slower in frequency and higher in am- ventricle plitude. Stage 4 is reached at the end of about an hour, Cerebral Basal when delta waves are observed (see Fig. The subject cortex ganglia then passes through the same stages in reverse order, ap- proaching stage 1 by about 90 minutes, when a REM period begins, followed by a new cycle of slow-wave sleep. Slow- Sylvian wave sleep is characterized by decreased heart rate and fissure blood pressure, slow and regular breathing, and relaxed muscle tone. Stages 3 and 4 occur only in the first few sleep Anterior cycles of the night. In contrast, REM periods increase in du- Temporal commissure ration with each successive cycle, so that the last few cycles lobe consist of approximately equal periods of REM sleep and The cerebral hemispheres and some deep FIGURE 7. The corpus callosum is the major commissure of the seeming contradictions in its characteristics. The anterior the EEG exhibits unsynchronized, high-frequency, low- commissure connects rostral components of the right and left amplitude waves (i. The cortex is an outer rim of gray matter (neu- cal of the awake state than sleep, yet the subject is as diffi- ronal cell bodies and dendrites); deep to the cortex is white mat- ter (axonal projections) and then subcortical gray matter. Second, the autonomic nervous system is in a state of excitation; blood pressure and heart rate are increased and breathing is irregular. In males, autonomic excitation in REM sleep in- columns perpendicular to the surface. This reflex is used in diagnosing im- neurons give rise to descending fiber tracts and intrahemi- potence, to determine whether erectile failure is based on a spheric and interhemispheric fiber tracts, which, together neurological or a vascular defect (in which case, erection with ascending axons coursing toward the cortex, make up does not accompany REM sleep). A deep sagittal fissure divides the cortex into a usually report dreaming.

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Paralysis or weakness of mus- or bowel retraining proven 30caps himplasia elchuri herbals, or the need for indi- cles may also cause vocal cord dys- viduals to wear or utilize a catheter (tube function himplasia 30 caps low price herbs and rye, which in turn can affect voice inserted into the bladder to drain urine). Other motor problems can cause artic- Posttraumatic Seizures ulation disorders in which there is no sig- nificant weakness or lack of coordination Seizures may be experienced in the peri- for reflexive action but rather the inabil- od immediately after the brain damage. In some instances, seizures may be able to scrape a food particle off occurring in the immediate postdamage their teeth with their tongue, but they phase resolve after swelling of the brain may be unable to coordinate the muscles recedes. In many other cases, however, indi- that move the tongue to produce a pho- viduals continue to have seizures, a condi- netic sound. It results from dysfunc- Language impairment may differ tion of the language centers in the brain, depending on the area of the brain dam- rather than impairment in the muscu- aged. Individuals may nitive skills: be able to understand and read simple material; however, as the complexity or • Memory length of the message increases, difficul- • Attention and concentration ty becomes more apparent. Although • Self-awareness they are able to comprehend, they may • Problem solving and decision making have difficulty expressing thoughts in • Information processing and concept speech and writing because of difficulty formation putting words and sentences together log- • Judgment ically. Speech may be labored, slow, and/or difficult to Memory encompasses the ability to understand, and small connecting words, store and retrieve information. Memory and increased verbal output, but with for both new and old information may be reduced information content, so that affected. Immediate memory lasts only sec- Wernicke’s aphasia are typically unaware onds or minutes unless converted in- of their communication difficulties. An example In some instances individuals may ex- of immediate memory is remember- perience global aphasia, in which there ing a phone number long enough to is severe difficulty communicating because dial the number, but then not com- of both the inability to use language (to mitting it to memory for later use. Short-term memory lasts from min- to wipe the same spot on a counter until utes to hours, but it is then lost if not someone intervenes. An Individuals with brain damage may be example of short-term memory may unable to remember skills that were once be learning facts for a test but not very familiar. For instance, they may be committing the facts to long-term unable to complete simple daily tasks, memory for continued use. Long-term memory is memories that to remember the steps involved or are stored and are able to be retrieved because, after completing the first steps of in the future, whether after weeks or the task, they forget their original goal. Memory problems can be the most lim- iting of all of the potential cognitive con- A variety of memory problems may be sequences of brain damage because they experienced after brain damage. Some affect the individual’s ability to learn, individuals may be able to remember facts store, and retrieve information. The abil- but are unable to remember how to do ity to profit from experience is often specific tasks. Consequently, individuals may be able to remember the names and may continue to make the same mistakes birthdates of family members but be un- over and over, since the ability to apply able to remember how to operate a wash- what was learned from past experience is ing machine. The ability to gener- retrograde amnesia in which they are alize from one situation to another may unable to remember things that occurred also be impaired. For example, an ments may have forgotten their own per- individual who has learned a skill in a sonal history so they do not recognize rehabilitation setting may be unable family members, or they may not be able to perform that skill in his or her own to remember what type of work they had home. After brain damage individuals can Attention and Concentration have difficulty remembering or learning new information so that they are unable After brain damage individuals may find to acquire new memories or recall recent it difficult to focus attention and to con- conversations and events. Conse- stances individuals make up answers to quently, they may be unable to follow a questions, or make up situations or events train of thought or perform multiple step (confabulation). They may have difficulty faulty memory but from the tendency to focusing on one task, may be easily dis- juxtapose unrelated memories together. At tracted, or may be unable to “shift gears” other times, in conversation, individuals from one task to another. Individuals with may get stuck on one theme, repeating a brain damage may find it difficult to per- question, phrase, or concept again and form multiple tasks at one time, such as again (perseveration). Perseveration can writing down messages or notes while also pertain to tasks that the individual talking on the phone, or carrying on a repeats over and over, such as continuing conversation while polishing furniture. For example, if they want to visit a friend in another city, they may recog- Individuals with brain damage may nize that they can take a train to get there, have limited ability to recognize or under- but they may not be able to consider how stand the limitations they are experienc- they would obtain money for the train ing. They may lack insight into the fare, how they would obtain a ticket, or appropriateness of their behavior and may how they would get to the train station. There may also be an inability to There may also be lack of ability to ini- monitor and adjust their own actions tiate and sustain activity.

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