By C. Basir. North Central College.
STANFORD INTRODUCTION Release of transmitter from a neuron is triggered by the arrival of a propagated nerve impulse at its terminals cheap 120mg sildigra otc erectile dysfunction from diabetes. This wave of excitation causes the opening of voltage-gated Ca2-channels or mobilisation of Ca2 from intracellular stores (e purchase sildigra 50mg erectile dysfunction 20. As a result, there is a phasic increase in free intracellular Ca2, probably to a concentration of about 0. The subsequent fusion of neurotransmitter storage vesicles with the axolemma, together with the extrusion of their contents into the synapse, is thought to take about 100±200 ms; this cascade is therefore fast enough to effect rapid signalling between neurons. While this chapter is concerned primarily with the neurochemical mechanisms which bring about and control impulse-evoked release of neurotransmitter, some of the methods used to measure transmitter release are described first. This is because important findings have emerged from studies of the effects of nerve stimulation on gross changes in transmitter release and intraneuronal stores. The actual processes that link neuronal excitation and release of transmitter from nerve terminals have been studied only relatively recently. The neurochemical basis of this stimulus±secretion coupling, which is still not fully understood, is described next. The final sections will deal with evidence that, under certain conditions, appreciable amounts of transmitter can be released through Ca2-independent mechanisms which do not depend on neuronal activation. MEASUREMENT OF TRANSMITTER RELEASE ESTIMATION OF TRANSMITTER TURNOVER EX VIVO Until the development of sensitive assays and sophisticated collection techniques, release studies relied on measuring changes in the concentration of neurotransmitters in whole organs, or dissected brain regions, following nerve stimulation. However, under resting conditions, the transmitter content of any given organ or brain region is remarkably constant. The store of classical transmitters (monoamines and acetylcho- line)in nerve terminals is rarely depleted by physiologically relevant rates of neuronal stimulation. This suggests that transmitter synthesis normally keeps pace with release. Although this approach is rarely used nowadays, it is outlined here because it uncovered some important findings which are relevant to current studies of the regulation of transmitter release. Webster &2001 John Wiley & Sons Ltd 82 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION The turnover rate of a transmitter can be calculated from measurement of either the rate at which it is synthesised or the rate at which it is lost from the endogenous store. Transmitter synthesis can be monitored by administering [3H]- or [14C]-labelled precursors in vivo; these are eventually taken up by neurons and converted into radiolabelled product (the transmitter). The rate of accumulation of the radiolabelled transmitter can be used to estimate its synthesis rate. Obviously, the choice of precursor is determined by the rate-limiting step in the synthetic pathway: for instance, when measuring catecholamine turnover, tyrosine must be used instead of l-DOPA which bypasses the rate-limiting enzyme, tyrosine hydroxylase. One limitation of this method is that the specific activity of the radiolabel is progressively diluted as the radiolabelled transmitter is released from neurons and replaced by that derived from unlabelled substrate. This method also assumes that there is no compartmentalisation of the terminal stores, yet there is ample evidence that newly synthesised acetylcholine and monoamines are preferentially released. An alternative approach is to monitor the rate at which the store of neurotransmitter is depleted after inhibition of its synthesis (Fig. It is already evident that the turnover rate of a transmitter is only a crude measure of its release rate. Further limitations are that there is appreciable intraneuronal meta- bolism of some neurotransmitters: notably, the monoamines. Another problem, again affecting monoamines, is that some of the released neurotransmitter is taken back into the nerve terminals and recycled. Despite these drawbacks, studies of turnover rates uncovered some important features of transmitter release. TECHNIQUES IN VITRO In situ preparations Many early studies of transmitter release depended on measuring its concentration in the effluent of a stimulated, perfused nerve/end-organ preparation. This technique is still widely used to study drug-induced changes in noradrenaline release from sympathetic neurons and the adrenal medulla. Monoamines, for instance, are rapidly sequestered by uptake into neuronal and non-neuronal tissue whereas other transmitters, such as acetylcholine, are metabolised extensively within the synapse. Because of these local clearance mechanisms, the amount of transmitter which overflows into the perfusate will depend not only on the frequency of nerve stimulation (i. At normal rates of neuronal activity, endogenous stores of neurotransmitter are maintained at constant (steady-state)levels, indicating that the supply of new neurotransmitter (through synthesis)meets the demand (determined by release and metabolism). Consequently, the rate of the depletion (A) of the endogenous store of transmitter after inhibition of its synthesis indicates turnover rate and is described by the equation: TT eÀkt 0 where [T] is the tissue concentration at time t;[T]0 is the transmitter concentration at time 0; and k is the rate constant for the efflux of transmitter.
Compliance is a problem over a long period if dosing is required more than once a day buy sildigra 25mg low price erectile dysfunction use it or lose it. It is probably acceptable in reality sildigra 50 mg with visa erectile dysfunction water pump, if not scientifically, to divide the drugs into old-established AEDs and new AEDs. Only the latter have been developed chemically to modify the known synaptic function of the amino acids. It was largely replaced in 1932 by phenytoin for the management of tonic±clonic seizures and partial and secondary epilepsy. Carbamazepine followed, then ethosuximide for absence seizures and valproic acid. These remained, apart from the introduction of the benzodiazepines, the mainstay of therapy until the last decade. They were introduced solely on their ability to control experimentally induced seizures. Their mechanisms of action were unknown and no thought was given to the possibility of NTmodification and in fact subsequent research has shown that with the exception of the benzodiazepines none of them work primarily through NTmanipulation. Phenytoin and carbamazepine An effective AED might control seizures and not be too sedative, by stopping a neuron from firing excessively without affecting its ability to respond normally. Studies in cultured spinal cord neurons (Macdonald and McLean 1986) have shown that concentrations of phenytoin equivalent to those occurring clinically do not affect the resting membrane potential or the shape of a single-action potential but reduce the rapid discharge induced by depolarising the neuron, while leaving the first action potential intact (Fig. It is believed to block voltage-dependent sodium channels (not those mediating the synaptic currents) after their activation, i. These effects, which are also shown by carbamazepine, would explain their effectiveness experimentally against maximal electroshock-induced THE EPILEPSIES 343 Figure 16. Antiepileptic drugs either directly affect ion channels to reduce Na (1) or increase Cl7(2) influx, depress glutamate release (3) or its action through NMDA receptors (4), or potentiate the effect of GABA by reducing its destruction by uptake (5) or metabolism by GABA transaminase (6), acting directly on GABAA receptors (7) or potentiating that effect of GABA through an action on benzodiazepine receptors that allosterically alter the GABAA site (8). Currently there are no clinically useful drugs that act as glutamate receptor antagonists seizures and clinically in focal and generalised epilepsy. Also, since they act only on the inactivated channel, they will not affect normal neuronal function, which is why in the experimental study, the first action potential remains unaltered. Neither compound is of any value against absence seizures and may exacerbate them. They have no clear effect on NTfunction although there is some evidence that in vivo they may potentiate GABA-induced chloride currents. Experimentally it has no effect on the voltage-gated sodium channels affected by phenytoin but has been reported to suppress the transient T-type calcium currents in the thalamic neurons which are the origin of the 2±3 Hz spike and wave discharge characteristic of this form of epilepsy (see Mody 1998 for detail). Since these discharges are thought to arise from oscillations in excitability induced by changes in the T-type calcium current (see section above on the origin of absence seizures), this would obviously be a neat explanation of its efficacy in that condition. Unfortunately some workers have not been able to repeat this finding at clinically equivalent concentrations and consider ethosuximide to reduce a special persistent Na channel and a Ca2-activated K channel. Barbiturates and benzodiazepines (B & Bs) As outlined above (see also Chapter 9), these drugs have been found to influence the Cl7 channel of the GABA receptor. Phenobarbitone acts directly to prolong its A 344 NEUROTRANSMITTERS, DRUGS AND BRAIN FUNCTION Figure 16. Note that while the structures of phenytoin and ethosuximide are similar and also close to that of phenobarbitone, they are effective in different forms of epilepsy. Muscimol is a GABAA agonist but is not an effective antiepileptic drug THE EPILEPSIES 345 Table 16. Ineffective in PM (20±80) DIBENZAPINES Carbamazepine GM FE 1 Improves mood. Drug of choice in FE (10±20) SUCCINIMIDES Ethosuximide PM (AS) 1 Drug of choice for PM, with Na valproate (20±60) BARBITURATES Phenobarbitone GM/FE 2 Sedative. Little used (50±100) Primidone GM/PE Works partly by conversion to phenobarbitone in body BENZODIAZEPINES Diazepam SE 8 Given intravenously in SE (5100) Clonazepam ME SE Diazepam largely replaced by clonazepam Clobazam PM Adjunct to other anti-epileptics. Partly as an anxiolytic SHORT-CHAIN FATTY ACIDS Sodium valproate GM PM 6 also 1 Inhibition of GABA metabolism ME (and 2) too slow to explain initial anti- convulsant effect. Increasing use in ME, PM, GM (5±15) Newer drugs Lamotrigine PE GM (AS) 4 (1) Fewer side effects (24) Gabapentin PE GM? Excreted unchanged Vigabatrin PE (GM) 6 Exacerbates AS (PE) Notes: The numbers (1±8) refer to their sites of action as shown in Fig. The benzodiazepines are particularly effective against experimentally induced PTZ seizures. Phenobarbitone may be as effective as phenytoin and carbamazepine in partial and generalised tonic±clonic seizures but its other central effects such as sedation, depression, listlessness and cognitive impairment mar its usefulness.
The proximal pole is attached to the ends of the tains the cell nuclei (Fig cheap 120mg sildigra free shipping erectile dysfunction cleveland clinic. There are two types of in series with the extrafusal muscle fibers such that con- intrafusal fibers: nuclear bag fibers buy sildigra 120 mg impotence new relationship, named for the large tractions of the muscle stretch the GTO. There are about twice as many nuclear chain eter than the type Ia variety, which innervate the muscle fibers as nuclear bag fibers per spindle. Muscle contraction stretches the GTO and gener- type fibers are further classified as bag1 and bag2, based on ates action potentials in type Ib axons. The GTO output whether they respond best in the dynamic or static phase of provides information to the central nervous system about muscle stretch, respectively. Sensory axons surround both the noncontractile mid- Information entering the spinal cord via type Ia and Ib portion and paracentral region of the contractile ends of axons is directed to many targets, including the spinal in- the intrafusal fiber. The sensory axons are categorized as terneurons that give rise to the spinocerebellar tracts. The axons of These tracts convey information to the cerebellum about both types are myelinated. Type Ia axons are larger in di- the status of muscle length and tension. Alpha motor neurons innervate shaped endings that wrap around the middle of the intra- the extrafusal muscle fibers, and gamma motor neurons in- fusal muscle fiber (see Fig. Cells bodies of both alpha and clear chain fibers are innervated by type Ia axons. Type II gamma motor neurons reside in the ventral horns of the axons innervate mainly nuclear chain fibers and have nerve spinal cord and in nuclei of the cranial motor nerves. This high number reflects the com- ings of both primary and secondary sensory axons of the plex role of the spindles in motor system control. Intrafusal muscle spindles respond to stretch by generating action po- muscle fibers likewise constitute a significant portion of the tentials that convey information to the central nervous sys- total number of muscle cells, yet they contribute little or tem about changes in muscle length and the velocity of nothing to the total force generated when the muscle con- 94 PART II NEUROPHYSIOLOGY A R Ia Response Passive stretch of muscle fibers from resting length Tension Wt. T Passive stretch B Ia response ceases R Stimulate alpha Ia Response motor neuron Tension Wt. T Stimulate C Ia responsiveness is maintained Stimulate alpha R and Ia Response gamma motor neurons Tension Wt. A, The Ia C, Concurrent alpha and gamma motor neuron activation, as oc- sensory endings from the muscle spindles discharge at a slow rate curs in normal, voluntary muscle contraction, shortens the muscle when the muscle is at its resting length and show an increased fir- spindle along with the extrafusal fibers, maintaining the spindle’s ing rate when the muscle is stretched. Rather, the contractions of intrafusal fibers play a spindle were reinstituted, the Ia nerve endings would re- modulating role in sensation, as they alter the length and, sume their sensitivity to stretch. This is accomplished by coordinated activation discharge of action potentials. Contraction of the muscle of the alpha and gamma motor neurons during muscle con- increases the firing rate in type Ib axons from Golgi tendon traction (see Fig. Ia endings report both the velocity and the length of stretch, type Ia endings show a greater firing rate increase, while muscle stretch; type II endings report length. B, With the release CHAPTER 5 The Motor System 95 types of endings, each located distal to the sensory endings on the striated poles of the spindle’s muscle fibers (see Fig. The nerve terminals are either plate endings or trail endings; each intrafusal fiber has only one of these two types of endings. Plate endings occur predominantly on bag1 fibers (dynamic), whereas trail endings, primarily on chain fibers, are also seen on bag2 (static) fibers. This arrangement allows for largely independent control of the nuclear bag and nuclear chain fibers in the spindle. Gamma motor neurons with plate endings are designated dynamic and those with trail endings are designated static. This functional distinction is based on experimental find- ings showing that stimulation of gamma neurons with plate endings enhanced the response of type Ia sensory axons to stretch, but only during the dynamic (muscle length chang- ing) phase of a muscle stretch. During the static phase of the stretch (muscle length increase maintained) stimulation of the gamma neurons with trail endings enhanced the re- sponse of type II sensory axons. Static gamma neurons can affect the responses of both types Ia and II sensory axons; dynamic gamma neurons affect the response of only type Ia axons. These differences suggest that the motor system has the ability to monitor muscle length more precisely in some muscles and the speed of contraction in others. THE SPINAL CORD IN THE CONTROL OF MOVEMENT Muscles interact extensively in the maintenance of posture and the production of coordinated movement.
Because of the structures that pass through the axilla buy sildigra 120 mg free shipping erectile dysfunction psychological, a dislocation causing the pupil to dilate buy discount sildigra 50 mg online erectile dysfunction doctors in alexandria va. Each disc has damage may result in recurrent dislocations of the shoulder, even after minor tough circular fibers which surround and contain a soft jellylike center known as injuries, as well as limited range of motion if rotator cuff injuries occur. In a herniated disc, the soft nucleus pulposus has pushed (herniated) through a weakened area of the fibers to press on a nerve root. A low-density area with peripheral enhancement is seen adjacent to the can also result in numbness/pain and motor deficits specific to the nerve root. By knowing neuroanatomy you can surmise which nerve root is irritated and such as this are suggestive of an abscess. This abscess is closely related to the therefore which disc is herniated: psoas major muscle and does not seem to be related to any of the surrounding organs. These abscesses generally form from hematologic spread Disc Nerve Root Numbness/Pain Motor Deficit Lost Reflex of bacteria secondary to an infection in another part of the body. The iliopsoas test is a physical examination maneuver performed when there muscle is suspicion of an intra-abdominal inflammation. The patient lies on the L4–L5 L5 Superior foot Extensor hallucis None right (unaffected) side and tries to extend her left leg at the hip against the longus muscle examiner’s hand. Abdominal pain upon extension of the left leg occurs L5–S1 S1 Lateral foot Gastrocnemius Tendo calcaneous because of the stretching of the inflammed psoas major muscle. The fibers that surround the nucleus pulposus are naturally weakest in the 3. This is done by inserting a tube into the right and left posterior aspects. This means that herniations usually bulge to abscess and allowing it to drain over several days. Antibiotics must also be the right or to the left, thereby affecting only one leg. This may cause numbness of the perineum and dysfunction of bowel or urinary bladder control. The muscles that extend the knee joint are the rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis. Although several conditions of the lung can look like this on the radiograph, tendon, which extends to the patella. Between the patella and the tibia is this man’s history favors one diagnosis. He is a lifetime smoker, he has the patellar ligament that attaches at the tibial tuberosity. This patient is developed a chronic cough, and he has been losing weight. He most unable to extend his leg at the knee because the tibial tuberosity has avulsed assuredly has lung cancer. Two major muscle groups, the quadriceps femoris and the hamstrings span has actually eroded through the pleura and is invading the plexus, causing the knee. The quadriceps group extends the knee joint, whereas the the arm symptoms. The apex of the lung is also very close to the sympathetic chain in the upper groups are in direct opposition to one another and hold the knee in a thorax and the neck. Tumor invasion in this location afflicts sympathetic neutral, or straight, position through their resting muscle tone. The result is loss of muscle tone in the eyelid, quadriceps femoris can no longer exhibit force on the tibia because of the loss of pupil dilation, and inability of the skin to sweat on that half of the face. Van De Graaff: Human Back Matter Appendix B: Answers to © The McGraw−Hill Anatomy, Sixth Edition Clinical Practicum Companies, 2001 Questions Appendix B Answers to Clinical Practicum Questions 801 Clinical Practicum 14. The patient’s recent change in facial appearance and the change to his 1. Although the patient has an underlying pulmonary infection causing some hands and feet are due to excessive growth hormone. Growth hormone is shortness of breath, the acute worsening of his symptoms is due to a tension released by the pituitary gland. The tension pneumothorax occurred secondary to a (seen on the MR image) that is secreting excess growth hormone, and which bronchopleural fistula resulting from the patient’s underlying infection. A tension pneumothorax is Excess growth hormone during childhood greatly accelerates growth, a type of pneumothorax in which gas collects in the pleural space resulting resulting in gigantism.
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